# NEW PIGMENTS AND ADDITIVES FOR CORROSION PROTECTION BY ORGANIC COATINGS

EDITED BY : Flavio Deflorian, Michele Fedel and Victoria Johnston Gelling PUBLISHED IN : Frontiers in Materials

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ISSN 1664-8714 ISBN 978-2-88966-213-5 DOI 10.3389/978-2-88966-213-5

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# NEW PIGMENTS AND ADDITIVES FOR CORROSION PROTECTION BY ORGANIC COATINGS

Topic Editors: Flavio Deflorian, University of Trento, Italy Michele Fedel, University of Trento, Italy Victoria Johnston Gelling, Other

Citation: Deflorian, F., Fedel, M., Gelling, V. J., eds. (2020). New Pigments and Additives for Corrosion Protection by Organic Coatings. Lausanne: Frontiers Media SA. doi: 10.3389/978-2-88966-213-5

# Table of Contents

*04 Editorial: New Pigments and Additives for Corrosion Protection by Organic Coatings*

Flavio Deflorian, Michele Fedel and Victoria Johnston Gelling

Fan Zhang and Jinshan Pan

Ammar Shafaamri, Chiam H. Cheng, Iling A. Wonnie Ma, Shahid B. Baig, Ramesh Kasi, Ramesh Subramaniam and Vengadaesvaran Balakrishnan

*120 Polypyrrole/Metal Oxides-Based Composites/Nanocomposites for Corrosion Protection*

Niteen Jadhav, Subramanyam Kasisomayajula and Victoria Johnston Gelling

*127 Protective and Suppressing Electromagnetic Interference Properties of Epoxy Coatings Containing Nano-Sized NiZn Ferrites* Andrzej Miszczyk

# Editorial: New Pigments and Additives for Corrosion Protection by Organic Coatings

Flavio Deflorian1 \*, Michele Fedel 1 and Victoria Johnston Gelling2

1Department of Industial Engineering, University of Trento, Trento, Italy, 2Department of Coatings and Polymeric Materials, North Dakota State University, Fargo, ND, United States

Keywords: organic coatings, corrosion protection, pigments, nanotechnology, environmentally friendly

Editorial on the Research Topic

#### New Pigments and Additives for Corrosion Protection by Organic Coatings

Corrosion protection of over- and under-ground structures as well as industrial products by protective coatings is one of the most widely employed methods. The durability of the protective coatings is commonly prolonged thanks to the addition of pigments or specific compounds which are able to provide the substrate with improved corrosion protection. During the past 20 years, the field of corrosion protection science and technology has developed, produced, and tested a variety of novel pigments and compounds whose full potential as well as their long-term efficiency are as yet unknown. Pigments in paints are not only added to improve the barrier properties but they can be designed also to provide passivation, galvanic protection, pH buffering, or self-healing of the polymeric matrix. Corrosion inhibition can be achieved by adding to the polymeric matrix partially soluble pigments or stimuli sensitive reservoir of chemical species in which the leakage depends on an electrochemical or chemical trigger. Along with the development of modern paints, technologists have been also focused on new pigments for the development of the so called "smart coatings." These new coatings are to address the environmental concerns related to traditional pigments and compounds (such as chromates and phosphates), to provide self-healing and to improve the longevity of the coatings. This objective can be accomplished by combining the polymeric phase with innovative pigments/compounds with tailored properties. In this frame, there is a significant ongoing research seeking to improve the functionality of protective pigmented organic layers often employing a nanotechnology based approach.

This special issue of the Frontiers in Materials–section Environmental Materials—is motivated by the need to update the state-of-the-art in the development of completely new pigments for corrosion protection and to establish a milestone by taking a snapshot of the efficiency of the current cuttingedge solutions.

The ten papers in this collection give an in-depth picture of the state of the art and future developments in the field of new pigments and additives for corrosion protection by organic coatings, coming from research groups from around the world engaged in research in this area. Two articles in particular are interesting as they present an accurate review of the new generation of nano-composites for corrosion protection (Bao et al.) and on methods of assessment of protective performance in this field (McMahon et al.). In fact, it is important to emphasize that innovation in this area, on one side, involves the development of new materials, but also needs accurate and scientifically rigorous evaluation and verification methods to predict their behavior in the field. Still considering the methods of investigation, the article by Nazarov et al. provides an focus on the use of one of the most powerful electrochemical techniques in this area, namely the use of the Scanning Kelvin Probe.

Edited and reviewed by: Changdong Gu, Zhejiang University, China

> \*Correspondence: Flavio Deflorian<EMAIL_ADDRESS>
#### Specialty section:

This article was submitted to Environmental Materials, a section of the journal Frontiers in Materials

Received: 14 September 2020 Accepted: 22 September 2020 Published: 13 October 2020

#### Citation:

Deflorian F, Fedel M and Gelling VJ (2020) Editorial: New Pigments and Additives for Corrosion Protection by Organic Coatings. Front. Mater. 7:606256. doi: 10.3389/fmats.2020.606256

The other articles are more focused on the development of new protective systems based on organic coatings with new pigments, where nanotechnologies already play a fundamental role, and will be even more relevant in the future.

One of the most advanced technology for corrosion protection by coatings is the use of microcapsules, with inhibitors inside, able to stop the corrosion and having a self-healing capability or containing chemical compounds polymerizing and repairing the scratches. A first example of a protective system based on microcapsules containing chemicals compounds with inhibitory action is the paper by Karaxi et al.

The paper of Cotting et al. represents a study that develops a new coating with self-healing capacity thanks to the chemical properties of microcapsules that polymerize in the event of damage to the coating itself.

Inorganic pigments, which are traditionally used in organic coatings, are showing relevant developments in particular at nano-scale, giving a relevant contribution to innovation in this area, as shown in the case titania nanopigments in the paper of Shafaamri et al. Also conductive polymers, extensively studied in the last decade to develop new composite coatings for corrosion protection, are promising materials as demonstrated in the paper by Jadhav et al.

One of the more effective driving forces for innovation in pigments for organic coatings for corrosion protection are the environmental issues associated with most of the corrosion inhibitors currently used in industry. Several pigments used in the past have a significant impact on the environment and human health and must be substituted. A way to find new environmentally friendly products is to give a look to the natural biological products and use it as additives in coatings. This is the approach of the paper of Zhang et al., using mussel proteins as effective additives in organic coatings.

A further, very relevant, field of innovation is the development of multifunctional coatings. The concept can be considered from different points of view. In some cases, multifunctional coatings can be considered materials using more than one strategy for improving corrosion protection. An example is the paper of Castaneda et al., where both Zn particles and nanotubes are employed in organic coatings to reduce the risk of microbiologically induce corrosion. Moreover, the traditional, and still essential, function of the pigments, to contribute to the corrosion protection action of the organic coatings, is sometime combined with new technological functions. An example of this new trend in multifunctional coatings is the paper of Miszczyk where nano-sized ferrites are studied to improve coating durability, but also to act as microwave absorber.

This collection of works represents an important opportunity to provide an updated view of innovation in the sector of new pigments and additives for corrosion protection with organic coatings. This overview is considering both the point of view of new materials and of investigation techniques and evaluation, highlighting how nanotechnologies on the one hand, and multifunctionality on the other, are the keywords for a future where coatings for corrosion protection will improve our quality of life while taking care of the environment.

#### AUTHOR CONTRIBUTIONS

All authors contributed equally to the Editorial.

Conflict of Interest: The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Copyright © 2020 Deflorian, Fedel and Gelling. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

# Next-Generation Composite Coating System: Nanocoating

Weiwei Bao, Zhifeng Deng, Shaodan Zhang, Zhuoting Ji and Haichang Zhang\*

*School of Materials Science and Engineering, Shaanxi University of Technology, Hanzhong, China*

Nanocoating combines the protective properties of conventional coating system with effects on nanoscale such as high hardness, UV scattering, and uniform dispersing. Their facile and low-cost synthesis, together with superior protective properties and multi-functionalities, makes nanocoating attractive candidates for next-generation coating systems. In this review, a brief introduction regarding mainstream nanocoating and its related challenges including the zinc oxide-, titanium dioxide-, silica dioxide-, graphene-, carbon nanotube-based nanocoating system is presented. Finally, a perspective of the nanocoating is demonstrated.

Keywords: corrosion resistance, protection, nanocoating, nanoparticle, coating system

#### Edited by:

*Flavio Deflorian, University of Trento, Italy*

#### Reviewed by:

*Changdong Gu, Zhejiang University, China Santiago Fajardo, Centro Nacional De Investigaciones Metalurgicas (CENIM), Spain*

> \*Correspondence: *Haichang Zhang<EMAIL_ADDRESS>
#### Specialty section:

*This article was submitted to Environmental Materials, a section of the journal Frontiers in Materials*

Received: *25 February 2019* Accepted: *01 April 2019* Published: *17 April 2019*

#### Citation:

*Bao W, Deng Z, Zhang S, Ji Z and Zhang H (2019) Next-Generation Composite Coating System: Nanocoating. Front. Mater. 6:72. doi: 10.3389/fmats.2019.00072* INTRODUCTION

Nanocoating is defined as having either the thickness of the coating or the second phase particles that are dispersed into the matrix in the nanosized range, which is a trend in the application of nanotechnology for the prevention and control of corrosion. This technology is growing rapidly in the field of corrosion protection and wear resistance of steel substrate materials compared to conventional coating (Bugnicourt et al., 2016). Conventional coating, organic, or inorganic coating, have been widely applied for the protection of metals against corrosion, which has made significant progress in recent years. However, despite significant improvements in coating technologies, problems continue in the long-term protection of metal from aggressive environments (Søresen et al., 2009). For the purpose of addressing this issue, the introduction of nanoparticles, nanocomposites, or nanocontainers in the coating formulation has been developed to enhance the wear resistance, shock, or pressure resistance and improve thermal conductivity of offshore structures, vessels, and drilling equipment (Müler et al., 2017). In addition, nanomaterials in the coating system can enable coating with superhydrophobic performance which endows the nanocoating with multi-functionalities such as anti-corrosions, anti-icing, and anti-fogging etc., (Das et al., 2018). Since nanocoating equips the substrate with special functions, the key results of nanocoating are summarized in several good reviews (Bugnicourt et al., 2016; Müler et al., 2017; Das et al., 2018). However, a comprehensive overview of nanocoating, with special emphasis on the distinct nano-pigment systems and their challenges, has not been yet thoroughly covered by the recent report.

In this review, we will briefly introduce mainstream nanocoating, its related challenges and applications including the zinc oxide (ZnO)-, titanium dioxide (TiO2)-, silica dioxide (SiO2)-, graphene-, and carbon nanotube (CNT)-based nanocoating system. Finally, our perspective on the potential further development trends for the next generation nanocoating is provided.

# DISTINCT NANOCOATING SYSTEM

## ZnO-Based Nanocoating

Nanocoating systems provide corrosion protection of metallic substrates via barrier effect, inhibitive effect, or anodically active metal effect (galvanic effect) (Müler et al., 2017). The Znbased nanocoating protective mechanism belongs to one of the galvanic effects, in which it sacrificially corrodes by the oxygen and moisture from the coating surface and forms a corrosion product such as ZnO. This corrosion product provides protection similar to a barrier effect. Recently, Ramezanza et al. reported different proportion of ZnO nanoparticles (0, 2, 3.5, 5, and 6.5 wt%) into the epoxy-polyamide coating (Ramezanzadeh et al., 2011). The studies showed that the physical interaction between the nanoparticles and coatings resulted in improved resistance against hydrolytic degradation in comparison with the blank epoxy coating. In addition, introducing ZnO-nanoparticles into the coating system could increase the coating curing degree resulting in the ZnO-based nanocoating with lower curing enthalpy. When adding the ZnO nanoparticles, the barrier properties of the coating has been significantly improved due to the increasing crosslinking density of the coating and the formation of corrosion product. The nanocoating with the best barrier properties was observed with ZnO composite of 3.5%. Once the ZnO nanoparticles were increased above 3.5% volume concentration, the barrier properties, typically the corrosion resistance, was decreased due to the wreaked crosslinking density and the high loading aggregations. Another reason might be that the amount of resin was insufficient to wet all the pigments; therefore, the coating did not form a continuous film, which resulted in extensive defects throughout the coating. When constructing ZnO-based nanocoating, the size of ZnO particles considerably affects the nanocoating anticorrosion properties. ZnO particles with 20 nm in the polyurethane coatings showed better protective performance than that of the ones with 400 nm (Yang et al., 2005). This could be explained by the fact that small size particles have greater surface activity, such as borders and corners, which can absorb more resin on their surfaces.

Except enhancing the anticorrosion properties, nano-ZnO in the coating could also improve its UV resistance. When exposing alkyd-based water borne coating under UV-radiation for 110 h, there were several cracks and pinholes on the surface, while for the ZnO-based nanocoating under the same condition, no cracks were observed (Dhoke et al., 2009). Nano-ZnO particles, typically between 20 and 40 nm, could scatter UV radiation between 200 and 400 nm, thereby providing good UV blocking property to the coating system without interacting with the UV radiation. It should be noticed that the size of the nanoparticles plays a key role in enhancing the UV resistance.

The oxygen and water were the main reasons that caused the metal materials to corrode. Thus, the development of superhydrophobic coating, which has nearly non-wetting results with easy rolling of water droplets from the surface, is of utmost important. Superhydrophobic coating with water contact angle (WCA) >150◦ and sliding angle (SA) <10◦ are often difficult to be obtained. Introducing modified nano-ZnO particles into the coating is a simple and inexpensive approach to obtain superhydrophobic coating (Das et al., 2018). Traditionally pure nano-ZnO are hydrophilic due to many hydroxyl groups existing on its surface (Qing et al., 2015). Modification the nano-ZnO particles with stearic acid obtaining hydrophobic nano-ZnO was reported by Qing et al. (2015). Using the modified nano-ZnO particles to construct fluorinated polysiloxane (FPDHS) nano-ZnO nanocoating showed superhydrophobicity with WCA as high as 166◦ **Figure 1a**), while the pure FPDHS coating exhibited hydrophobicity with WCA of only 143◦ . Recently, various approaches to develop superhydrophobic ZnO-based nanocoating have been reported, such as using polydimethylsioxane, hexadecyltrimethoxysilane, and aminopropyltriethoxysilane etc., (Wu et al., 2005; Chakradhar et al., 2011; Li et al., 2015; Velayi and Norouzbeigi, 2018). Appropriate modification ZnOnanoparticles can not only enhance the coating hydrophobicity, but also increase its corrosion resistance, for instance the corrosion rate of modified ZnO-based nanocoating showed 11 fold decrease compared to the pure coating. When constructing superhydrophobic nanocoating, it should be noticed that the coating superhydrophobic improved with the increase in surface roughness, meanwhile introducing small size and appropriate amount of nanoparticles is quite important for forming high surface roughness.

# SiO2-Based Nanocoating

Silicon dioxide, one of the most complex and most abundant families of materials, exists as a synthetic product and as a compound of several minerals. The first commercial-grade surface-modified silica nanoparticles were produced into market in 2002, which were introduced into the epoxy resin by a modified sol-gel process and had a very narrow particle size distribution (Sprenger, 2013). Many metal-O-Si bonds were generated on the surface of substrate, which provided not only the supplement of the good physical barrier effect, but also the improvement of adhesion force between coating and substrate. Silica with inherent strong hardness and resistance against chemicals and alteration characteristics allow the nano-SiO2 particles in coatings to improve various properties such as modulus, stiffens, toughness, scratch resistance, and anticorrosions. Conradi et al. presented a 0.5 kg load on the surface of the pure epoxy and 130 nm SiO2/epoxy coating, separately, which showed the indents in the epoxy coating were significantly large compared to those in the SiO2/epoxy coating, and the hardness was enhanced by almost 40% (Conradi et al., 2013). Improvement of the Vickers hardness could be due to the strong hardness of Nano-SiO2 into the coatings considerably affected the morphology, the area roughness parameters Sa from 16 to 329 nm. This further influenced the surface hydrophobicity with the WCA from 84.6◦ to 94.2◦ . The hydrophobicity affected by the surface roughness could be due to trapped air between the surface and the water droplet, which can minimize the contact area by creating micro/nano structures. Improved hydrophobicity resulted in the nano-SiO2/epoxy coating with enhanced anticorrosion effect.

As discussed in section ZnO-Based nanocoating, coating hydrophobicity plays a key role in the anti-corrosion, hence

modification nano-SiO2, resulting in the nanocoating with superhydrophobicity and improvement compatibility can significantly increase the nanocoating corrosion resistance. Very recently, Ye and colleagues compared the performance of different nano-SiO2 based coating systems such as electrodeposited oligoaniline modified silica/epoxy coating (E-M-SiO2) and electrodeposited pure silica coating/epoxy (E-SiO2). The E-M-SiO2 coating exhibited superhydrophobicity with WCA of 167◦ , while E-SiO2 exhibited hydrophobicity with WCA of 143.5◦ (Ye et al., 2018). Superhydrophobicity could availably inhibit the diffusion of corrosive medium, hence E-M-SiO2 coating showed better corrosion protection compared to E-SiO2. The author also compared the performance between E-M-SiO2 with sol-gel oligoaniline modified silica/epoxy coating (M-SiO2), in which the E-M-SiO2 showed better hydrophobicity and corrosion protection performance (**Figure 1b**). This could be due to most silica coating obtained by sol-gel approach with low surface roughness, which seriously restricts the thickness of coating, thereby the physical barrier of pre-process coating was weak.

For conventional coating, there is no protective effect once it is damaged, protection only lasts for the first damage. Self-healing coatings, possessing the internal capability to repair sustained damage by themselves or with some outside stimulation, is in the trend due to its providing feedback active healing ability to a scratch on metal surfaces for long term protection. Introducing self-healing microcapsules, such as perfluorooctyl triethoxysilane-urea-formaldehyde (POT-UF) shell, ethanolamine-polystyrene (ETA-PS) shell or diethanolamine-polystyrene (DEA-PS) shell, into the modified nano-silica/epoxy nanocoating and enhancing the coating anti-corrosion performance was reported by Kongparakul's group (Kongparakul et al., 2017). Among all the three systems, the POT-UF shell microcapsules performed the best corrosion prevention due to the increased length of the diffusion pathways, as well as decreased the oxygen permeability of the coating. Other approaches, such as using nanocontainers, self-healing additive, layer-by layer deposition, etc. could also obtain self-healing nanocoating (Shchukin et al., 2006; Li et al., 2013). Hydrophobic coating is unstable and easy to lose its ability to resist water (Zhang et al., 2006). Hence, self-healing coating combined with superhydrophilicity behavior are promising due to its long-term protection properties.

# TiO2-Based Nanocoating

Titanium dioxide, one of the most commonly occurring oxides of titanium, received a lot of attention due to its easily available, relatively inexpensive, chemical stability, and harmless nature which was widely applied in several fields such as paints, coatings, inks etc. (Munafò et al., 2015). Introducing nano-TiO2 into coating systems can lead the nanocoating with improvement protection and can even endow it with self-cleaning properties. Shen et al. reported that covering a uniform TiO2 nanoparticles films on the surface of 316L stainless steel could sufficiently improve its corrosion resistance properties, the corrosion resistance increasing over three order of magnitudes (Shen et al., 2005). The protection mechanism of TiO2-based nanocoating is that electron and hole are generated in the coating once exposing nano-TiO2 to UV-irradiation, in which the electrons transferred to the metal substrate thereby making its electrode potential more negative than its corrosion potential, meanwhile, TiO2 does not get consumed. Nano-TiO2-based coating is promising to protect metal with anti-corrosion. However, the protection mechanism suggests that pure TiO2 nanocoating cannot be used under dark conditions due to the photo effect. In addition, once small defects appear in the nanocoating, these nanocoating are no longer able to protect the exposed zone, which means the nanocoating works only until their surface is intact. Numbers of works attempt to address this issue through the modification nano-TiO2 coatings such as noble metal deposition, doping of metallic and non-metallic ions, etc. (Hoffmann et al., 1995; Park and Choi, 2004). Yun and colleagues reported TiO2-nanocoating without photo-effect using doped TiO2 with N, S, and Cl (Yun et al., 2007). Electrochemical polarization was conducted in 0.5 M NaCl solution under dark conditions for the different coating systems. Comparing this to the bare 316L, the pure TiO2-nanocoating slightly improved the corrosion-resistance, while the corrosionresistance performance for the dopant-modified TiO2 coating was significantly enhanced. This could be due to the eliminating photo effect of the modified nano-TiO2 coating (**Figure 1c**). Other research regarding modification nano-TiO2 particles improving the nanocoating corrosion resistance has also been reported, such as polypyrrole/Sn-doped TiO2, thermal oxidation, introducing polyaniline in the coating system etc. (Krishna and Sun, 2005; Radhakrishnan et al., 2009; Mahmoudian et al., 2011).

TiO2 is normally hydrophobic. However, once it is exposed under UV-illumination, it becomes both oleophilic and hydrophilic, which will decrease the organic liquid angle or WCA until there is almost complete flattening of organic liquids or water droplets on the surface. This property prevents the external dirty directly contacting the coating surface and allowing an easier removal of stain. Based on this performance, TiO2-based nanocoating as self-cleaning effect coating was widely used, such as on glass, stone, and photovoltaic module (Munafò et al., 2015). A single-layer (SL) and three-layer (ML) treatment nanocoating were obtained through spraying the nano-TiO2 product directly on the stone surface with different amounts obtained (Goffredo et al., 2013). Under UV illumination, there was no significant degradation of stains on untreated surfaces, while the photocatalytic degradation of the strain was very rapid for the SL and ML coating, the rhodamine B losing almost 50% of its original color after 4 h. TiO2-based nanocoating are transparent, hence it can be applied directly on architectural surface for protection without altering their original aspect. Numbers of works on the TiO2 nanoparticles into different coating systems for optimizing its self-cleaning performance have been reported (Kaplani et al., 2015; Crupi et al., 2018). However, the analyses carried on the TiO2 self-cleaning efficiency up to now still lack a more complete approach. The system studied on the relationship between the self-cleaning and the size or amount of nano-TiO2, the further optimization of the nano-TiO2 particles and extending the lifepath of the coating are still a challenge.

### Carbon (Graphene and Carbon Nanotube)-Based Nanocoating

Nanoscale carbon, graphene and CNT, are potential useful applications as nanocoating in materials protection due to its inherent characters, such as the high stiffness and conductivity capacity (Hu et al., 2014). Chen et al. first time successfully used as-grown graphen to protect the surface of metallic growth substrate of Cu or Cu/Ni alloy away from air oxidation (Chen et al., 2011). The covered graphene resulted in a passivation of metal underneath, which offered an inserted block between the reactants and the protected metal. Except the oxidation resistance, graphene-based nanocoating could also effectively extend materials thermal stability and corrosion-resistance. Kwon et al. used graphene and metal-chloride-doped graphene over layers on 25 um thick copper foil, in which the chlorine evaporation was almost blocked, and maintained the doped state even after thermal annealing (Kwon and Kim, 2014). Yu and coworkers introduced graphene, typically metronidazole modified graphene oxide, on the metal surface which exhibited better corrosion resistance than the bare metal (Yu et al., 2016).

It should be noticed that graphene offers an impressive oxidation barrier for the short-term, however, for long-term scales, graphene promoted the metal corrosion instead of preventing it. Schriver et al. compared the sample surfaces between bare Cu and graphene-coated Cu under ambient conditions for up to 2 years (Schriver et al., 2013). The results showed that slight color changing happened for the bare Cu after 2 years. For the graphene-protected Cu, it was shiny and free of oxide within 1 week. However, the surface stared to tarnish non-uniformly with small patches after 1 month, which gradually caused high oxide after 5 months. Finally heavy oxidization on virtually all of the surface was observed at 18 months (**Figure 1d**). Non-uniform oxidation or defects of graphene will lead to stress and cracking of the oxidation layer, which often opens a new avenue for corrosion. In addition, the monolayer graphene often exhibited wetting transparency (Hu et al., 2014). These characteristics result in monolayer graphene with bad protection properties. To address this issue, multi-layered graphene or nanocomposite such as mixture with epoxy were used, which showed good protective performance (Rafiee et al., 2012; Tang et al., 2013). Yang et al. studied epoxy coating on a copper substrate modified by fluorographene nanosheet, which showed superhydrophobicity with WCA up to 154◦ (Yang et al., 2017). The superhydrophobicity characteristic of this nanocoating was ascribed to the irregular microstructure constructed by randomly stacked fluorographene.

CNT, allotropes of graphene, was used in coating effect protection performance typically on the anti-oxidation and corrosion resistance. Recent studies showed the CNT-based nanocoating exhibited similar protection performance as graphene-based nanocoating (Janas and Koziol, 2013). In many cases, the CNT not only acts as a physical barrier to the corrosion process, but also improves the electrochemical properties to anti-oxidation (Ates, 2016). Epoxy composite coatings filled with different multiwalled CNT contents was reported by Frankel's group, which indicated that multiwalled CNT can influence the coating performance such as improvement adhesion strength, increasing wear resistance and the total impedance enhancement (Khun et al., 2013). Nanocoating containing modified CNT composite should be in the trend.

#### CONCLUSION AND PERSPECTIVES

The ever-growing interest in nanocoating and its fascinating protective properties makes it a promising candidates for nextgeneration protecting system. In this review, we have given an overview of the recently popular composite nanocoating system based on ZnO, SiO2, TiO2, grapheme, and CNT. On the basis of the literature findings above, it is noticeable that introducing the modified-nanoparticles into the conventional coating system could significantly improve the coating protective characters, typically the corrosion-, UV-, and oxidationresistance performance, and even self-cleaning functionality. However, the amount of introduced nanoparticles should be proper, otherwise, it will reduce the protective properties. When using a carbon-based nanocoating system, it should be the CNT or graphene modification, or compositing it with other coating systems to extend its corrosion resistance.

#### REFERENCES

Nanocoating with a wide branch of genuine properties has been intelligently used by the scientific community and industries. The future research on this field should be focus on further improving its protecting properties combined with multifunctionalities, such as self-cleaning, self-healing, UV-resistance, antibacterial, thermal resistance, color-changing in one single nanocoating. In addition, studies on the relationship between minimization of the size, optimization of the modification and the amount, uniform dispersion of the nanoparticles and the nanocoating performance are important, and finally development of a new nanocoating system. Furthermore, introducing organic nano-pigment or multi-different pigments into one coating system to enhance the coating properties is a growing trend.

## AUTHOR CONTRIBUTIONS

All authors listed have made a substantial, direct and intellectual contribution to the work, and approved it for publication.

## FUNDING

The authors acknowledge the support from the Natural Science Foundation of China, under Grant 21805151, the Natural Science Foundation of Shandong Province, China, under Grant ZR2018MB024, and Special Scientific Research Planned Projects of Education Department of Shaanxi Province (No. 17JK0161).

**Conflict of Interest Statement:** The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Copyright © 2019 Bao, Deng, Zhang, Ji and Zhang. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

# Application of Scanning Kelvin Probe in the Study of Protective Paints

Andrej Nazarov\* and Dominique Thierry

French Corrosion Institute, Brest, France

Industrial coatings are composed of layers of different polymers (top coats, primers) containing pigments, corrosion inhibitors, and fillers as well as additives. For corrosion protection, it is vitally important to preserve the strong adhesion and long-term stability of the metal-polymer interface in corrosive environments. In recent decades, the performance of painted materials increased, which requires the application of advanced methods for quick assessing, ranking and predicting corrosion stability. Scanning Kelvin probe (SKP) is a highly sensitive and non-invasive technique to analyze in situ the metal-polymer interface of high-performance industrial coatings. SKP is able to monitor the adhesion and corrosion underneath different kinds of paints without the need for long-term corrosion tests. SKP is a localized electrochemical technique with a spatial resolution in the range of 70–100µm. Hence, it is possible to obtain information about the intact and corroding portions of the interface at defect sites, corrosion blisters, contaminants, and intermetallics, quality of pretreatments, and the development of galvanic couples that lead to corrosion de-adhesion of the polymeric coatings. This article reviews the application of SKP to the determination of the mechanisms of corrosion deadhesion of model paints, thick marine paints, coatings with zinc rich primers, automotive paints, and coil coatings applied on galvanized steel substrates.

Keywords: SKP, polymeric coatings, corrosion protection, adhesion, pigments, pretreatments

## INTRODUCTION

Historically, polymeric coatings were assumed to protect the substrate from corrosion by acting as a barrier toward water, oxygen and ions. However, later studies found that oxygen and water relatively quickly saturated the coating and could not be rate determining factors in corrosion protection (Leidheiser and Granata, 1988; Thomas, 1991). Kinsella and Mayne (1969) and Mayne and Scantlebury (1970) found that ionic resistance is a key factor in polymer coating stability, known as "resistance inhibition." The relationship between the ionic resistance of a coating and its protective ability has been identified (Kinsella and Mayne, 1969; Mayne and Scantlebury, 1970). Ionic resistance has been highlighted as an important (even critical) property of a coating in modern works (Kendig and Mills, 2017; Mills and Jamali, 2017).

Funke placed great emphasis on the importance of wet adhesion to the substrate, claiming "adhesion is performance" and that it is a key factor in determining the overall coating lifetime (Funke, 1979; Negele and Funke, 1996).

The high importance placed on ionic resistance has been criticized in an article of Sykes et al. (2017). The existence of micro galvanic couples underneath the coating leads to the rapid failure of resistive coatings. Coating failure normally initiates at a local defects, which can be the result of application errors, chemical heterogeneities in the coating

Edited by:

Michele Fedel, University of Trento, Italy

#### Reviewed by:

Ole Øystein Knudsen, Norwegian University of Science and Technology, Norway Wolfram Fürbeth, DECHEMA Forschungsinstitut (DFI), Germany

\*Correspondence: Andrej Nazarov<EMAIL_ADDRESS>
#### Specialty section:

This article was submitted to Environmental Materials, a section of the journal Frontiers in Materials

Received: 28 March 2019 Accepted: 25 July 2019 Published: 08 August 2019

#### Citation:

Nazarov A and Thierry D (2019) Application of Scanning Kelvin Probe in the Study of Protective Paints. Front. Mater. 6:192. doi: 10.3389/fmats.2019.00192

**12**

or other defects, such as bubbles, under film deposits or mechanical damages. The corrosion of steel at a defect can lead to cathodic delamination of the surrounding coating (Leidheiser et al., 1983; Hamade and Dillard, 2005; Sørensen et al., 2010). On the other hand, coating de-adhesion can result from anodic undermining or filiform corrosion (Sharman, 1944; Funke, 1981). Schematics illustrating these two modes of coating de-adhesion are given in **Figure 1**. Leidheiser et al. (1983) described the mechanism for the cathodic delamination of polymeric coatings. The corrosion of steel at a defect due to the presence of an aqueous electrolyte can lead to spreading of the cathodic oxygen reduction reaction to the surrounding interface (**Figure 1A**). Delamination is effective for metals (e.g., Fe, Zn), which are covered by electron conducting surface oxide films. Considering a coated steel substrate immersed in an electrolyte at a near-neutral pH, the half-cell reaction responsible for the coating delamination process is oxygen reduction (Leidheiser et al., 1983). A linear relationship has been found between the diffusion coefficients of the different cations in an aqueous solution and the delamination rate (Leidheiser et al., 1983). The adhesion instability has been explained by the reduction of the metal oxide film at the interface or by chemical degradation of the polymer chains by products of oxygen reduction such as alkali or peroxide species. Three locus of failure have been suggested: in the oxide, in the layer of polymer or the interface (Leidheiser et al., 1983). After a weakening of the interface due to cathodic processes, residual stresses concentrated at the metal-polymer interface can cause peeling of the coating (Negele and Funke, 1996; Hamade and Dillard, 2005).

Anodic undermining or filiform corrosion has been found in polymer-coated metals exposed to humid atmospheres and is enhanced by artificial or natural impurities, such as sulfur dioxide or chloride (Sharman, 1944; Funke, 1981). It is supposed that the galvanic cell governs the growth of the filament (**Figure 1B**). Although filiform corrosion usually occurs on organic-coated aluminum parts, it has also been observed on magnesium and cold rolled steel. Filiform corrosion normally initiates at small, sometimes microscopic, scratches or defects in the coating. In the case of iron and zinc, cathodic oxygen reduction takes place at defect sites on the surface of rust, and anodic oxidation and formations FeCl2 or ZnCl2 develop underneath the coating prior to de-adhesion (**Figure 1B**). There is only one certain way to prevent filiform corrosion, which is to dehydrate the rusted steel surface by reducing the relative humidity of the air to below ∼60% (Funke, 1981). On the other hand, the stability of metalpaint interfaces can be enhanced by treating the metal surface, and these types of conversion coatings have a beneficial effect toward preventing filiform corrosion (Stratmann, 2005).

The electrochemical heterogeneity of the metal surface governs the spatial separation of the cathodic and anodic corrosion reactions. The stability of the coatings to corrosion driven deadhesion is determined by formation of galvanic elements and the local electrochemical methods can obtain spatially resolved information related to metal-polymer interface. The local techniques that have been applied to metals covered by organic coatings have been reviewed (Grundmeier et al., 2000; Ogle et al., 2000; Philippe et al., 2003; Stratmann, 2005; Hausbrand et al., 2008; Rossi et al., 2008; Huang et al., 2011; Upadhyay and Battocchi, 2016). A very important advantage of local scanning techniques is the possibility of comparing the map of the electrochemical parameters with the topographic information (presence of defects or rust clusters), metal microscopy data (microstructure and contaminants), differences in surface treatments, etc. DC current techniques (the scanning vibrating electrode technique and scanning electrochemical microscopy) are mainly applicable for assessing relatively low resistive paths, such as defects in the coating. Techniques based on AC current measurements (local electrochemical impedance spectroscopy and SKP) are effective for obtaining information from the interfaces underneath intact and highly resistive polymeric coatings. The objective of this particular review is to discuss the growing information concerning the application of SKP to analyze the properties of paints and paint additives that influence their stabilities and protective properties. Industrial paints have increased corrosion stabilities, and the application of highly sensitive and noninvasive in situ methods for quick assessment and ranking is highly important.

#### SKP PRINCIPLES AND APPLICATION

SKP is a non-destructive local electrochemical technique that measures the contact potential difference between a working electrode (e.g., the polymer-coated metal) and a vibrating reference electrode (e.g., a CrNi alloy or gold). A schematic of the Kelvin probe is shown in **Figure 2A**. When two metals are in contact, electrons move to reach equilibrium that creates the contact potential difference (CPD). This potential difference (1V, Equation 1) is proportional to the difference in the electron work functions (Φ) of the two materials. Φ is defined as the minimum work necessary to extract an electron from the Fermi level of a metal and transfer it to a point just outside the metal surface (carrying no net charge) (Hölzl and Schulte, 1979).

To measure the contact potential difference, the probe vibrates in the vertical plane, which creates a capacitor with an alternating distance, and an alternating current flows between the probe and the working electrode (Equations 2, 3). The amplitude of this current is proportional to the CPD (Equation 4).

$$
\Delta V = (\Phi\_w - \Phi\_\mathcal{\mathbb{P}}) / e \tag{1}
$$

$$C = \varepsilon \varepsilon^{\circ} A/(d + \Delta d \sin \alpha t) \tag{2}$$

$$I(t) = I\_a \sin(\alpha t) \tag{3}$$

$$I\_a = \,\Delta V \stackrel{\circ}{\text{s}} \, A\alpha/d,\tag{4}$$

where e is the electron charge, C is the capacitance, ε and ε ◦ are the dielectric constants of the surface coating and vacuum, respectively, d is the distance between the probe and working electrode, I(t) is the AC current, Ia is the amplitude of the current, ω is the frequency of the current, t is the time, and A is the surface area of the probe.

The work function (Φw, Equation 1) is a fundamental property of a metal, and it is proportional to the actual potential

FIGURE 1 | Schematics for corrosion-driven coating de-adhesion. (A) Cathodic delamination and (B) anodic undermining galvanic cells and potential distributions along the interface.

of an electron in a metal (Equation 5; Hölzl and Schulte, 1979; Trasatti and Parsons, 1986).

$$
\Phi N = -\alpha = -\mu\_{\varepsilon} + FX\_{\text{w}} \tag{5}
$$

where α is the electrochemical potential, µe is the chemical potential of the electrons, **N** and **F** are the Avogadro number and Faraday constant, respectively, and Xw is the potential difference across the metal-air interface (Trasatti and Parsons, 1986).

According to these definitions, the electron work function or electrochemical potential contains bulk and surface contributions. The bulk contribution is determined by the chemical potential of the electrons. The surface contribution (Xw) includes the number of potential drops related to the surface oxide film and dipole layers of adsorbed molecules (Equations 5–7).

$$X\_{\le} = \frac{\mu\_{\text{ox}} - \mu\_{\varepsilon}}{e} + F\_b + \beta\_{\text{ox}/air} \tag{6}$$

$$X\_{\le} = \frac{\mu\_{\text{ox}} - \mu\_{\text{e}}}{e} + F\_b + \beta\_{\text{ox}/\text{coating}} + \beta\_{\text{coating}/air} \tag{7}$$

For a bare metal surface, the potential drop (Xw, Equation 6) includes the contact potential difference between the bulk metal and oxide film relating to the difference in the corresponding chemical potentials of the electrons in the metal (µe) and in the oxide (µox). The second potential drop (Fb ) relates to the adsorption of environmental components (molecules of oxygen, water, etc.) that bend the conductive and valence bands of the semiconducting oxide film. The electric charges in the oxide are compensated for by the charges of the double electric layer, such as the dipoles of adsorbed species (e.g., O− 2 ions), creating a potential drop (βox/air).

For polymer-coated metals, the SKP measured potential has a potential drop (Xw, Equation 7) that includes two additional potential drops βox/coating and βcoating/air. The first corresponds to charge transfer or the formation of a double electric layer of oriented dipoles between the functional groups of the polymer and the oxide that are related to the adhesive bonds (Evans and Ulman, 1990; Taylor, 2000). The second potential drop relates to the dipoles in the organic chains or charged groups of the polymer (e.g., Donnan potential drop) (Samec et al., 1993; Taylor, 2000).

Initially, the Kelvin probe was extensively applied to investigate the kinetics of metal oxidation in the gas phase, leading to the formation of oxide films. In addition, an important area of study was the adsorption of different gases onto the surface of bare metals or those covered by oxide films. These processes change the potential drop across the metalair interface (Equation 6). Thus, CPD between a platinum probe and an evaporated metal film was monitored during the adsorption of oxygen, nitrogen or water vapor (Hackerman and Lee, 1955). Both reversible and irreversible changes of the Volta potential were found, which corresponded to the formation of either a dipole barrier or an ion barrier that prevented electron transfer. Thus, it was shown that oxygen adsorption increases the electron work function due to electron transfer from the metal to the adsorbed oxygen molecules (Hackerman and Lee, 1955). An increase in the electron work function due to the formation of an ionic barrier related to the oxide film has also been demonstrated (Hackerman and Lee, 1955).

During measurement, the Volta potential of the probe (Φp/e) is kept constant. This makes possible to use CPD measurements (1V, Equation 1) to determine the Volta potential of the working electrode (Φw/e) relative to a reference electrode or determine the electron work function Φw relative to the vacuum level. Finally, SKP measures the surface distribution of the electrochemical potential above the metallic substrate or polymer coated metallic substrate (Grundmeier et al., 2000; Hausbrand et al., 2008). To transform Volta potential to electrochemical potential the probe has to be calibrated using a reversible reference electrode (e.g., Cu/CuSO4). Modern SKP instruments measure potential distribution with a spatial resolution in the range 70–100µm. They are equipped with an environmental chamber that enables measurements in situ, in a corrosive environment.

M. Stratmann and H. Streckel introduced SKP to electrochemical corrosion (Stratmann and Streckel, 1990; Frankel et al., 2007). It has been shown that for corroding metals, SKP measures the corrosion potential and can be used for studying the corrosion under very thin electrolytic films (Frankel et al., 2007). In the case of an active surface, the potential drop Xw (Equation 6) related to surface oxide significantly decreases. For metals without corrosion activators, the potential is noble and relates to the potential of the metal under passive conditions (Hausbrand et al., 2008). In this case, the redox properties of an oxide film can influence the measured potential (Grundmeier and Stratmann, 1999). According Nernst equation (Equation 8), the potential of Fe is a function of the activities of the different iron species, Fe3+ and Fe2+, in the surface oxide:

$$
\Delta E = const. \ + \frac{RT}{F} \ln \frac{a(Fe^{3+})}{a\left(Fe^{2+}\right)}\tag{8}
$$

A potential drop in a semiconducting oxide film that does not change the oxidation state, such as in zinc, also significantly contributes to the potential measured by the SKP. An increase in the oxide thickness of a Zn/ZnO electrode increases the potential measured in air (Nazarov et al., 2015). This can be the result of an increase in the thickness of the space-charge layer in thicker oxide films. The adsorption of environmental components onto the oxide film surface also plays a role (Equations 6, 7). Replacing air with nitrogen decreased the potential of passive zinc by 100–150 mV. Thus, the formation of an oxide/O2− dipole layer increased, and oxygen desorption decreased the Zn potential (Nazarov and Thierry, 2007; Nazarov et al., 2015).

Well-oxidized and uniform oxide films create stable adhesive bonds with coatings (Funke, 1979; Kendig and Mills, 2017; Mills and Jamali, 2017). The application of SKP to studies on metal surfaces are based on measuring the conditions of the surface oxide, which are characterized by a potential drop Xw (Equations 6, 7). Intermetallics, contaminations or defects in oxide locally change the potential that can be visualized by SKP (Nazarov and Thierry, 2007). Thus, SKP inspection of an oxide surface before application of polymeric coatings can detect surface defects and areas with non-uniform potential distributions, which can later lead to the formation of galvanic corrosion elements working underneath the coating and accelerating coating deadhesion.

# RESULTS

## SKP Assessment of Cathodic Delamination of Polymeric Coatings From Carbon and Galvanized Steels

Corrosion-driven coating de-adhesion (**Figure 1A**) is the result of a non-uniform distribution of the electrochemical potential across the metal- metal oxide -paint interface and the work of macro galvanic couples. SKP mapping of the potential makes possible to determine the galvanic elements (**Figure 1A**).

Using SKP, M. Stratmann et al determined the electrochemical mechanism for the cathodic delamination of polymeric coatings from steel surfaces (Stratmann et al., 1991; Leng et al., 1999a,b). SKP has the main advantage of being able to measure the potential of a metal underneath a highly resistive, intact and delaminated coating in situ (**Figure 1**). For measurements, an artificial defect in a coating containing a NaCl solution was prepared (**Figure 2B**). A linear relation between the coordinates for the displacement of the delamination front vs. the square root of the time of exposure was found (Leng et al., 1999a,b). These results showed that ion migration (Na+) from the defect along the interface is the rate-controlling process for the cathodic delamination of thin model coatings (**Figure 1A**). The deadhesion rate is determined by the mobility of the cations, but the mobility along the metal-polymer interface is much slower than the mobility of ions in the bulk electrolyte (Leng et al., 1999b). The results of SKP investigations related to cathodic delamination of the different polymeric coatings from iron, steel and galvanized steel substrates have been published in Stratmann and Hoffmann (1989), Stratmann et al. (1991), Nazarov and Thierry (1999, 2004a, 2010), Nazarov et al. (2012), Leng et al. (1999a,b), Furbeth and Stratmann (2001a,b,c), Williams and McMurray (2003), Reddy et al. (2004), Reddy and Sykes (2005), de la Fuente and Rohwerder (2008), Nazarov et al. (2012a, 2018a), and Khun and Frankel (2013).

For example, **Figure 3** shows SKP maps for alkyd coating containing TiO2 pigment deposited on mild steel surface. The delamination starts from the scribe contaminated by NaCl aqueous electrolyte (Nazarov and Thierry, 1999). The potential of the intact interface relates to the steel in the passive state (cathode, 0.5 V vs. SHE), and at the defect site the steel has a low potential of active corrosion (anode, −0.3 to −0.2 V vs. SHE, **Figure 3**). The potential difference of 0.7 V is the electromotive force of the galvanic element leading to cathodic delamination of the coating from the defect. **Figure 4** shows typical SKP profiles

for steel—epoxy system as function of distance for different times of exposure in neutral 0.5 M Na2SO4 solution.

Other factors, such as the roughness of the steel surface and the texture of the surface, influence the cathodic delamination rate of epoxy coatings (Khun and Frankel, 2013). SKP measurements showed that an increase in the steel roughness decreased the rate of delamination. The surface features of the underlying steel substrate also influenced the cathodic delamination of the epoxy coatings. The textured lines (due to abrasion) parallel to the direction of delamination accelerated the failure compared with texture lines perpendicular to the direction of delamination. In this study, a linear dependence of the creep of coating deadhesion vs. the time of exposure was found (Khun and Frankel, 2013).

It is important to note that strong spatial separation of cathodic and anodic reactions with spreading of the cathodic reaction to the surrounding surface occurs also without a coating, due to atmospheric corrosion of steel or zinc locally contaminated by NaCl (Nazarov and Thierry, 2004a; Nazarov et al., 2015). The formation of galvanic couples consisting of anode in the region of the NaCl deposit and cathode on the bare steel surface has been observed by SKP (**Figure 5**). The distance of the cathodic spreading also had a dependence on the square root of time, and the spreading across the bare metal surface was faster compared with that of coated steel (see **Table 1**; Nazarov et al., 2018a). The electromotive force of the galvanic element that is equal to the potential difference between the low-potential anode and the noble potential of the passive steel surface was approximately 0.6 V (**Figure 5A**). This is the thermodynamic reason for galvanic corrosion. On the other hand, the rate of galvanic corrosion is a function of the rates (polarization resistances) of the partial corrosion reactions and the ionic conductivity along the interface (Nazarov and Thierry, 2004a). The area of cathodic spreading increased with increasing NaCl concentrations in the local deposits. The efficiency of the cathodic reaction on the bare steel surface governs the cathodic spreading that can be visualized by SKP (**Figure 5**). The rates of electron transfer from the substrate to oxygen molecules (Equation 10) and the migration of cations along the surface can be function of the surface chemistry. Metal surface pretreatments

TABLE 1 | Rates of cathodic deadhesion (spreading) for square root or linear kinetics during exposure in aqueous electrolytes, type of coating, and pigment, exposure conditions, coating thickness.

(conversion coatings) can be ranked according to their effect on cathodic spreading. Thus, SKP measurements of bare metal surfaces can be useful for choosing the appropriate pretreatment before painting.

SKP is an ideal tool for in situ monitoring of the degradation processes at the buried interface contaminated by soluble salts. The study of stability of the steel/coating interfaces to initial stages of blistering and underfilm corrosion has been carried out (de la Fuente and Rohwerder, 2008; Nazarov et al., 2012a). SKP mapping gives the exact locations of the corrosion of the steel underneath a 150µm thick layer of epoxy-diamine/polyamide paint (**Figure 6A**). This SKP study was carried out to determine the critical amount of NaCl at the interface that can prevent the development of corrosion beneath the paint. For a particular paint system, 5 µg/cm2 was the threshold level of NaCl contamination for the paint to inhibit corrosion in humid air at 95% RH. Thus, SKP can quickly assess paints that can withstand corrosion due to NaCl contamination of the interface (de la Fuente and Rohwerder, 2008; Nazarov et al., 2012a). Corroding locations show low (active corrosion) potentials relatively surrounding intact interface (**Figure 6B**). Cathodic delamination of the coating around these active locations can be expected as the exposure time increases.

Cathodic delamination of the polymeric coating can occur on a metallic substrate that has sufficient electronic conductivity and catalytic activity to support the oxygen reduction reaction under

the coating (Leidheiser et al., 1983). Corrosion of galvanized steel in the defect down to the zinc layer leads to cathodic delamination of polymeric coatings (Furbeth and Stratmann, 2001a,b,c). The propagation kinetics of the cathodic front from the defect is proportional to the square root of the exposure time. In contrast to steel, zinc is an amphoteric metal, and the formation of alkali in cathodic region leads to the growth of an oxide layer at the zinc–polymer interface (Furbeth and Stratmann, 2001a). The cathodic reaction decreases the adhesion of a coating on a steel surface, but it completely delaminates the coating from a zinc surface.

If a defect in the organic coating is prepared through zinc layer down to the steel substrate, the polarity changes, and the defect can serve as a cathode that drives anodic coating deadhesion from the defect. As a result, the kinetics are a complex combination of cathodic and anodic processes that occur at the metal-polymer interface (Furbeth and Stratmann, 2001b).

# SKP Assessment of the Mode of Paint Corrosion Driven De-adhesion

It has been observed that the galvanic elements leading to the cathodic delamination of the coating can reverse their polarity during exposure (**Figures 1A,B**). SKP and scanning acoustic microscopy were applied to study the corrosion underneath a 120µm thick epoxy amide resin coating (Reddy et al., 2004; Reddy and Sykes, 2005). It has been demonstrated that in some instances, the formation of iron rust at the sites of local coating degradation can reverse the potential distribution and the electrochemical activity, with rust serving as a cathodic reactant and new anodes being formed.

Thus, delamination of a thin polyvinyl butyral (PVB) coating (20µm thick) from iron starting at a defect site containing NaCl initially showed cathodic disbondment (Williams and McMurray, 2003). Spreading of the low-potential region corresponding to delamination of the PVB coating during exposure was found. In the second phase, the formation of rust at the defect site switches the cathodic mode of de-adhesion to an anodic mode. Filaments grow from the rust locations. The rust formed on the iron underneath the coating is an effective cathodic component (Stratmann and Hoffmann, 1989). The filaments spreading either under the cathodically delaminated film or under the intact film contain low-potential active heads, where anodic iron oxidation occurs (Williams and McMurray, 2003). The heads mainly contain iron (II) chloride (**Figure 1B**). The tails of the filaments contain hydrolyzed iron-corrosion products, have a positive potential, and can serve as the cathodes in filament galvanic cells.

Contamination of the defect site in the coating by iron chloride instead of NaCl results in only anodic undermining of the coating occurring (filiform corrosion). Iron is not a mobile cation due to hydrolysis, oxidation, and rust deposition. Thus, this contaminant cannot support spreading of the cathodic region underneath the coating (Williams and McMurray, 2003).

Using steel coated by pigmented epoxy-vinyl coating (100µm thick), it was found that the polarity of the galvanic couple and the mode of delamination were functions of the thickness of the electrolyte in the defect (Nazarov and Thierry, 2010). Under a thick electrolyte film, the defect site was anodic that polarizes the surrounding interface leading to cathodic delamination (**Figure 7A**). Under thin films, in atmospheric corrosion conditions, the defect in the same electrode showed a positive (cathodic) potential that anodically polarized the surrounding steel-coating interface and facilitated anodic undermining (**Figure 7B**). In this system, the delamination (anodic undermining) was uniform without the formation of filaments (Nazarov and Thierry, 2010). The defect and the steel underneath of the coating developed the rust with different compositions and degrees of oxidation. The anodic front (**Figure 1B** and area of low potential in **Figure 7B**) is enriched with chloride and contains FeCl2. The Fe2+/Fe3+ redox couple in the rust near the defect site provided effective cathodic oxygen reduction (Stratmann and Hoffmann, 1989).

Changes in the polarity of the galvanic elements were found for bare iron surfaces with local NaCl contaminants (Nazarov and Thierry, 2004a). Atmospheric corrosion and rust formation increased the potential of the contamination, and the potential

with permission from NACE International (Nazarov and Thierry, 2010).

became more positive (left hand side in **Figure 5B**). However, significant anodic spreading was observed only for the coated samples. The coating additionally separates the partial reactions, creating a galvanic couple of different aeration, for which the cathode is in a well-aerated defect site and the anode is underneath the polymeric coating. For different systems, it was found that peeling out and removing the paint from anodic region transforms it into a cathodic surface (Nazarov and Thierry, 2010; Nazarov et al., 2012).

During corrosion exposures, the conditions of the defect site can periodically change, accelerating the cathodic or anodic modes of coating de-adhesion. Depending on the thickness of the electrolytic layer at the defect site, switching of the polarity of the galvanic couple was found for a polymer-coated zinc surface on a galvanized steel substrate (Nazarov et al., 2012). Using a cataphoretic epoxy coating (a system is used in the automotive industry), it was demonstrated that the potential distribution corresponded to cathodic delamination for a thick electrolytic film (**Figure 8A**) and to the anodic undermining (**Figure 8B**) in the case of atmospheric corrosion (Nazarov et al., 2012).

Only anodic de-adhesion mode, with spreading of the chloride ions and formation of anodic blisters contained ZnCl2, was observed by SKP for de-adhesion of the coating from galvanized steel (Zn) from the defect down to the steel surface (Nazarov et al., 2012). A more positive steel substrate serves as a cathode, and a more negative zinc-coating interface serves as an anode. This galvanic couple governs the cut-edge corrosion of polymer coated galvanized steel (coil-coated materials) either under atmospheric or immersed in an electrolyte conditions (Nazarov et al., 2012).

Due to its low electronic conductivity, cathodic de-adhesion is strongly inhibited by substrates with dielectric oxide films, such as aluminum and magnesium alloys. However, anodic (filiform) coating de-adhesion in humid atmospheric conditions can be significant (Williams et al., 2001; Le Bozec et al., 2002; Leblanc and Frankel, 2004; Romano et al., 2009; McMurray et al., 2010). **Figure 9** shows the potential distribution over an Al alloy coated with an electrophoretic epoxy coating after 6 weeks of exposure to air at 85% RH. These systems are applied in the automotive industry. The measurements of Le Bozec et al. (2002) performed in air and argon at 95% RH showed that the exchange of the atmosphere decreased the potentials of the tail and the linear defect (**Figure 9**). Thus, pre-corroded areas of Al are a source of cathodic activity (Equation 10) directed to the anodic location in the filament head. The filament head has the potential to be more active at −0.5 V (SHE).

The stress generated during cross-linking and curing of an electrophoretic coating influences the adhesion and spreading of the filiform corrosion of an aluminum AA6016 (Romano et al., 2009). It was found that an increase in the amount of cross-linking in the epoxy cathaphoretic coating increased the adhesion and the uniform front of spreading transforms to more narrow filaments.

Finally, it is possible to point out that the mechanism of deadhesion is mainly determined by electrochemical conditions of the metal in the defect (Nazarov and Thierry, 2010). However many factors such as the electronic conductivity of the oxide film and the corrosion products, amount of water, degree of aeration, the type and concentration of pollutants are influencing and consequently only careful SKP evaluation can inform about the type of coating deadhesion and predict the coating stability under particular conditions.

#### SKP Assessment of Paint Stability to Delamination. Influence of Loading Pigments and Corrosion Inhibitors Into the Paints

The effects of the thickness of a polymeric film and the addition of pigments on the stability of the interface to de-adhesion have been studied using SKP. As was discussed above, the

FIGURE 8 | Potential profiles (V vs. SHE) of hot-dip galvanized steel samples with linear defects in the electrophoretic epoxy coatings. Deposition of a thick electrolyte layer at the defect site (A) and after drying and exposing to air 95% RH (B). Reprinted with permission from Elsevier (Nazarov et al., 2012).

delamination rate of unpigmented coatings was controlled by cations mobility and delamination was dependent on the square root of the exposure time (Leng et al., 1999a,b; Furbeth and Stratmann, 2001c; Reddy and Sykes, 2005; Bi and Sykes, 2016). The kinetic was only slightly slower for thicker coatings (Bi and Sykes, 2016). However, the pigments can decrease the cathodic delamination rate significantly (see **Table 1**). A linear dependence of the delamination vs. time was detected for pigmented systems (Hernández et al., 2004; Grundmeier et al., 2006; Bi and Sykes, 2011, 2016; Nazarov et al., 2018a). The rate of delamination also depends from mobility of cations along the interface (Bi and Sykes, 2016). The rate of deadhesion of pigmented epoxy coatings decreased in the sequence KCl > CsCl > NaCl > LiCl (Bi and Sykes, 2016). Important parameter for pigmented coatings is number of interaction points (hydroxyl groups) between the coating and the substrate that is controlling interfacial interaction (Sørensen et al., 2010). Thus, oxygen diffusion rate, mobility of cations and adhesion are factors, which control the stability.

SKP was used to study the stability of iron with coatings contained advanced pigments based on graphene nanoplatelets and carbon nanotubes (Khun and Frankel, 2016; Glover et al., 2017, 2018). The delamination rates correlated strongly with a decrease in oxygen permeation through the coating as a function of increasing pigment volume fraction in a polyurethane/ multiwalled carbon-nanotube coating (Khun and Frankel, 2016). The SKP technique was used to measure the propagation of the delamination front underneath the PVB coating. Incorporation of high-density graphene nano-platelet pigment blocked the oxygen and water pathways to the coating-steel interface, which significantly decreased the rate of cathodic delamination by 98.6% (Glover et al., 2018).

In opposite to oxygen accelerating corrosion, another air component such as carbon dioxide can decrease the corrosion rate of galvanized steel. CO2 creates carbonates at the interface with galvanized steel during atmospheric exposure that decrease the cathodic coating delamination (Furbeth and Stratmann, 2001c). Polymeric coatings selective to CO2 were studied. The important work of Shkrirskiy et al. (2019) investigated the relative ability of a polymeric coating to absorb a carbon dioxide. SKP inspections showed that increased permeation of carbon dioxide through coating and access the interface is favorable for stability of coated galvanized steel (Shkrirskiy et al., 2019).

SKP was applied to study the effect chromate inhibitors that could leach from the coating (Williams and McMurray, 2001). On the other hand, in many countries, legislation does not permit the application of chromates. Less hazardous phenyl phosphonic acid was investigated as an inhibitor of corrosion-driven organic coating disbondment from iron surfaces (Glover and Williams, 2017). SKP assessment of corrosion underneath the paint was carried out in situ under corrosive atmospheric conditions. Increasing the loading of phosphonic acid progressively decreased the delamination rate up to 55% (Glover and Williams, 2017). From the delamination kinetics, it was concluded that the species interacting with substrate form an interfacial salt layer that inhibits oxygen reduction beneath the film and cathodic de-adhesion.

SKP was employed by the group of G. Williams and H. N. McMurray to evaluate coating delamination from Al, Fe, Zn substrates after loading of an ion-exchanger, bentonite clay (Williams and McMurray, 2004, 2017; Dodds et al., 2017). Ion exchanger supplies the interface with divalent alkali-earth and trivalent rare-earth metal cations with the subsequent precipitation of sparingly soluble hydroxides. The deposit reduced the ionic conductivity of the electrolyte underneath the polymeric coating and correspondingly decreased the current in galvanic cell. For the best performing divalent cation-exchange pigments, the inhibitory effect was comparable to or provided better protection than that of a conventional strontium chromate pigment (Williams and McMurray, 2004, 2017). Pigments containing Mg2+ prevented delamination of the coating from hot-dipped galvanized steel (HDG). In situ SKP demonstrated that the high level of inhibition of cathodic delamination could be attributed to the deposition of Mg(OH)2 precipitate (Dodds et al., 2017).

SKP was applied to monitor the cathodic delamination of a 300µm thick-pigmented epoxy coating from steel, starting from the defect site after immersion of the sample in an aqueous electrolyte (Nazarov et al., 2018a). Due to high barrier properties, this coating is applied in offshore or marine environments. It was found that at ambient temperatures, the cathodic delamination was very slow and the paint partially retained its adhesion (see **Table 1**). However, under atmospheric corrosive conditions the reduction of oxygen on the rust surface at the defect site led to rapid coating de-adhesion following the anodic undermining mechanism. Thus, anodic de-adhesion under atmospheric weathering conditions is harder to prevent compared with cathodic de-adhesion when metal-polymer joint is immersed in an electrolyte (Nazarov et al., 2018a).

Zinc-rich primers (ZRP) are components of polymeric coatings that provide active corrosion protection to steel structures in heavily corrosive environments, such as offshore. In general, a paint contains different polymeric layers with a total thickness of >400µm that significantly delays the degradation of the paint and decreases the observation of corrosion failures. In situ SKP measurements of a complete paint system showed the efficiency of cathodic protection of a steel substrate by ZRP with different compositions and from different producers (Nazarov et al., 2018b). The electrolyte ingression to the interface from the defect site and activation of the Zn particles in the primer determine the ability of the primer to polarize the substrate and provided sacrificial protection under either immersed or atmospheric conditions. Thus, SKP mapping can determine the activity of the zinc particles at the interface as a function of the distance from the defect sites (scribe and cut-edge). Monitoring the interfacial potential during exposure provided information about the consumption of the zinc particles and the lifetime over which the primer could provide cathodic protection to the substrate. Thus, SKP provides a quick means of assessing the sacrificial steel protection provided by industrial marine paints containing zinc-rich primers under atmospheric or immersed conditions (Nazarov et al., 2018b).

## SKP Assessment of the Metal-Metal Oxide-Coating Interface to Determine the Nature of Adhesive Bonds and Their Stability

Achieving durable corrosion protection with polymeric coatings requires an understanding of the chemical interactions at the hybrid polymer/metal oxide/metal interfaces. It is important obtain information in situ directly from the solid/solid interface. This section shows the possibility of SKP assessment of the metal–coating interface to determine the nature and the stability of the metal–coating bonds (Nazarov and Thierry, 2004b, 2005; Nazarov et al., 2008, 2011). Metal-polymer interactions include hydrogen bonding, ionic polar or acid-base interactions (Brönsted and Lewis kinds of interaction), and van der Waals secondary forces. Formation of the interfacial dipoles are illustrated by Equations (11) and (12) and in **Figure 10** (Nazarov and Thierry, 2004b).

$$\text{OMe}-\text{OH} + \text{H}-\text{O}\text{COR} \rightarrow \text{OMe}^{+-}\text{OCO}-\text{R} + \text{H}\_2\text{O} \tag{11}$$

$$\text{OMe}-\text{OH} + \text{HN}-\text{RR}^{\text{H}} \rightarrow \text{OMeO}^{-+}\text{H}\_2\text{N}-\text{RR}^{\text{X}} \tag{12}$$

Depending on the ionic groups distributed throughout the coating, the charges in the metal and coating layers can be different. These charges can be determined by scanning across different metal-polymer joints (**Figure 10A**).

**Figure 10A** shows the potential distribution for steel substrate sequentially coated by alkyd and epoxy coatings. It is clear that different coatings interact with the steel surface differently. The epoxy coating significantly decreased the potentials of steel or zinc (500 mV), and the alkyd coating increased the potential (100–150 mV) (Nazarov and Thierry, 2004b, 2005). This can be explained by the formation of dipoles according to Equations (11) and (12). Alkyds contain acidic groups, and their interactions with the surface hydroxide OH-Me result in a negative charge in the coating layer and a positive charge on the metal oxide surface. Protonation of the amino groups of the epoxy coating by the OH-Me groups of the surface hydroxides creates a positive

charge in the coating and a negative charge on the metal surface (Equation 12). The potential change across the interface due to the formation of metal oxide-polymer dipoles can be illustrated by the following equation:

$$
\Delta \varphi = -4\pi \alpha \mu N\_{\mathbb{S}}\,, \tag{13}
$$

where ∆ϕ is the potential drop across the interface, NS is the surface coverage, µ is the effective dipole moments of the adsorbed complexes, and α is polarizability (Nazarov and Thierry, 2004b, 2005; Nazarov et al., 2008).

Ingression of the water separates the interface, decreased the dipole moment and the potential drop (Equation 13), and coated surface exhibits the potential close to the values of bare metal surface (Nazarov and Thierry, 2005; Nazarov et al., 2008). However, this process is reversible, and drying the sample restores the dipoles and the initial high potential drop of coated metals with adhesive bonds (Nazarov et al., 2008). The potential changes that occur during water ingression are related to the formation of a thin aqueous film at the interface, which can lead to hydrolysis. On the other hand, a small amount of water at the interface is beneficial for increasing the degree of hydroxylation of the surface oxide (O-Me-OH) and creating anchoring hydrogen bonds that improve the interfacial durability (Nazarov et al., 2008). Aging of the samples decreased variation of the potential during wet-dry cycles and irreversibly decreased the potential drop ∆ϕ (Equation 13). Thus, the chemistry and the strength of interactions at the interface can be monitored during exposure of the adhesive joints.

Different epoxy coatings interact differently with the metal surface and create dipoles with different coverage that can be used for SKP ranking according to the adhesive force and interfacial interactions (Nazarov et al., 2011). It has been found that epoxy hardeners containing diamino groups, which are used in twocomponent epoxy coatings (EP1), could dissolve surface oxides (ZnO), creating stable complexes and an interphase with a mixed organic-inorganic nature. This interaction created a large potential drop across the interface and provided the highest adhesion and water stability to the metal-epoxy polymer (EP1) joint (**Figure 11A**; Nazarov et al., 2011). In contrast, another epoxy coating (EP2), which has a low adhesion to metal, showed a low potential drop (**Figure 11B**; Nazarov et al., 2011).

These points are in agreement with SKP measurements of the potential shifts of iron surfaces with adsorbed carboxylic acids, amines and amides (Wielant et al., 2008, 2010; Nazarov et al., 2011). The adsorption of amines can simulate the epoxy/amine coating systems described above. The adsorption of carboxylic acids shifted the potential in the positive direction (Equation 11), whereas adsorption of amines and amides induced a negative potential shift of 400–500 mV (Equation 12). Brönsted acid–base interactions change the overall dipole moment. The potential shift increased with the degree of protonation on the surface and increased with the amount of surface hydroxide groups (Wielant et al., 2008, 2010). These results are in line with the SKP study on the adsorption of amines and an epoxy coating on Al hydroxylated surfaces (Salgin et al., 2013b). It has been shown that the interaction of N-methyldiethanolamine in the epoxy coating with hydroxide groups on the Al surface decreased the potential of the epoxy-coated Al. The value of the potential drop could be controlled by the amount of hydroxyl groups in the oxide. In addition, the amount of interfacial dipoles can be increased by modifying the chain of an adsorbed amine with hydroxide groups (Wielant et al., 2007). Thus, the application of SKP enables new possibilities for controlling the metal-coating interface with the aim of increasing the adhesion and stability to hydrolyze of the metal-polymer bonds.

## SKP Assessment of Metal Surface Pretreatments and Improvement of the Stability of the Interface to Corrosive De-adhesion

The surface energies of metallic substrates are much higher than the surface energies of polymers. This difference decreases the energy of the interfacial interactions and the stability of the contacts. Thus, we have to modify metal surfaces and create the interphases with intermediate properties to obtain good interactions with the polymer. The role of these pretreatments is to increase metal-polymer adhesion under dry and wet conditions, obtain an interface with high dielectric properties that inhibit oxygen reduction, and provide correspondingly high stability against corrosion driven delamination. In addition, the

interface has to decelerate the migration of ions along the interface, which circulate in the galvanic cells.

The correlations between the oxide surface energy of the substrate, the adhesion force of the polymer film in corrosive environments and the delamination rate from the defect site as a function of the pretreatment was shown (Wielant et al., 2009). The SKP technique has normally been used to detect the displacement of the de-adhesion front under the polymeric coating from the defect site, as discussed above. Steel surfaces were pretreated in different ways, creating surface hydroxides with different surface energies and polar components (Wielant et al., 2009). It was shown that cathodic delamination decelerates with increase of polar oxide surface energy and the adhesion interactions at the oxidecoating interface.

Additionally to adhesion, interfacial ion mobility is a key parameter in the work of surface galvanic elements on coated galvanized steel and aluminum alloy (Klimow et al., 2007; Posner et al., 2009, 2013, 2014; Salgin et al., 2013a). Cations move during the spreading of the cathodic reactions, and anions migrate across the interface during anodic undermining (**Figure 1**). The fixed surface charges (ion-exchange groups) determine the ion mobility that can be determined using SKP. Thus, controlled modification of the surface chemistry could selectively hinder or activate the mobility of the desired charge carriers to better control the electrochemical processes occurring on the oxide surface (Posner et al., 2009; Salgin et al., 2013a).

SKP can be used to study the properties of thin conversion films on steel, aluminum or galvanized steel surfaces without the deposition of a top coat. SKP was applied to phosphated zinc surfaces (galvanized steel) to study their ability to reduce oxygen in a humid atmosphere (Nazarov and Thierry, 1998). It was shown that the phosphated surfaces contaminated with carbon microparticles increased the transients of potential during exchange air to argon atmosphere. The cathodic particles were localized in the SKP maps as locations with more positive potentials relatively to the background level of phosphated zinc. Complete phosphating and replacing of the surface zinc oxide by phosphate decreased the surface potential. Thus, the potential measurements can be used for determination of the quality of phosphatation and the presence of cathodic contaminants (Nazarov and Thierry, 1998).

Investigation of the Zn/ZnO electrode using SKP (Nazarov et al., 2015) showed that cathodic spreading is effect of the semiconducting ZnO film participating in the oxygen reduction reaction. A decrease in the ZnO thickness decreased the potential of Zn and inhibited cathodic spreading. From these results, was concluded that effective pretreatment has to decrease the potential of Zn due to the formation of more dielectric and less catalytic deposits (Nazarov et al., 2005). Chromate conversion coatings have shown excellent inhibition of the corrosion. Normally, chromating creates Zn surfaces with extremely negative potentials (−0.7 V vs. SHE) (Nazarov et al., 2005). To replace chromates, different compositions have been developed, and SKP has been useful for selecting the right compositions on the base of vanadate and magnesium phosphates (Nazarov et al., 2005; Gao et al., 2018). **Figure 12** shows SKP potential transients as a result of exchanging the air atmosphere for an argon atmosphere. The magnitudes of the potential variations are proportional to the inhibition ability of the pretreated surface to adsorb and reduce oxygen. Thus, the lowest potential deviations during exchange of the atmosphere were seen in compositions containing magnesium vanadate and magnesium phosphate that were comparable with the effect of SrCrO4 species (Nazarov et al., 2005). The results showed that the addition of magnesium ions was important for inhibition of oxygen reduction on the zinc surface that is in line with the studies of ion-exchange pigments leaching magnesium cations (Dodds et al., 2017; Williams and McMurray, 2017). Additionally, using SKP and XPS, V(IV) species were seen to be cathodic inhibitors of zinc corrosion. The protection was related to the formation and enrichment

of the coating by V(IV) species over increasing exposure times (Gao et al., 2018).

Currently, Zr- and Ti-containing compositions were formulated to achieve better corrosion protection of zinc alloys and steels (Khun and Frankel, 2015; Lostak et al., 2016; Sababi et al., 2017; Milosev and Frankel, 2018). SKP measured the potential distribution and delamination kinetics in the vicinity of artificial defects (Milosev and Frankel, 2018). It was shown that a Zr-based treatment could decrease the electromotive force in cathodic galvanic elements and correspondingly the delamination rates of different coatings (Khun and Frankel, 2015; Lostak et al., 2016). Another silane-Zr-based conversion layers decelerated the electron transfer rate and coating delamination from galvanized steel due to the electron insulating properties of the ZrO2 conversion coating (Lostak et al., 2016).

Important SKP works investigated the stability of polymer coated and pre-treated steels, which are applying to packaging materials (Wint et al., 2015, 2016; Edy et al., 2019). Thus, this technique was used in situ to compare the stability of a polyvinyl butyral coating deposited on steel substrate that was pre-coated by chromium/chromium oxide layers deposited from bath containing Cr(VI) or Cr(III) species (Wint et al., 2016). It was found that the system containing Cr (VI) species was fully resistant to all kinds of deadhesion. The conversion coating deposited from electrolyte contained Cr (III) species showed cathodic and anodic disbonding. However, cathodic delamination rate of polymeric coating pretreated by Cr (III) compositions significantly decreased after developing of the coatings (Edy et al., 2019). **Figure 13** shows the cathodic delamination rates as a function of the formulations. It was found that the conversion coating diminished electrocatalytic activity for cathodic oxygen reduction process that correlate with a decrease in the delamination rate (Edy et al., 2019). Similar SKP study of

FIGURE 13 | Delamination distance vs. time of delamination of polyvinyl butyral coating on chrome-chrome oxide Cr(III)—steel substrate as function of formulation of conversion coating (Edy et al., 2019).

delamination of organic lacquer from tin-iron packing materials was carried out (Wint et al., 2015). It was found that tin and iron-tin intermetallics decrease the rate of cathodic oxygen reduction that also improved the coating stability to cathodic disbonding.

Surface modification of aluminum alloys is important for creating stable metal-polymer interfaces. Normally, deadhesion of the epoxy coating from an untreated Al substrate starts immediately after the ingression of water to the interface (Nazarov and Thierry, 2004b, 2005). This is the result of a low surface density of hydroxide groups in Al2O3, which cannot sufficiently bond with the functional groups of the coating (Salgin et al., 2013b). To solve this problem, SKP was successfully applied to find the correlations between type of Al surface modification, the density of the surface hydroxide groups and the Volta potential (Özkanat et al., 2012).

To have high stability of electrophoretic epoxy coatings on Al alloys substrates is an important issue for the automotive industry. SKP study (Nazarov et al., 2012b) compared the development of filiform corrosion as function of different solgel pretreatments (amino-silanes, silane-ZrO2 (Oxsilan), and sol-gel nanoclays). It was found that some amino-silanes can accelerate anodic undermining of electrophoretic coating. Amines have a positive charge due to their protonation, which decreases the potential of the interface. After exposure, SKP found extended interfacial area that was activated by migration of the chloride ions from the defect site along the interface. Fast corrosive deadhesion of the coating can be an effect of the positive amino groups, which accelerate

the migration of negative chloride ions according to the ionexchange mechanism. Other compositions decreased the area of activation and the rate of filaments spreading from the defect site (Nazarov et al., 2012b).

#### SKP Assessment of the Corrosion Stability of Coil Coated Hot Deep Galvanized Steels

Highly stable interface for polymer coated galvanized steel is important issue for automotive and building industries. SKP was successfully applied to study these novel Zn-Mg and Zn-Mg-Al alloys for galvanized steel with polymeric coatings (Hausbrand et al., 2003, 2009; Prosek et al., 2008, 2015; Vimalanandan et al., 2014; Davies et al., 2015). Due to the selective oxidation, the oxide film in the alloys consists of MgO, which is a perfect dielectric material and has poor electrocatalytic properties for reducing of oxygen (Prosek et al., 2008). In addition, due to the enrichment of the oxide film by MgO, the material obtains a low potential and small electromotive force for galvanic cells driving cathodic coating deadhesion (Hausbrand et al., 2009; Vimalanandan et al., 2014). Thus, alloying of zinc by Mg inhibits the cathodic delamination of polymeric coating significantly (Hausbrand et al., 2003, 2009). The strong inhibition of spreading of cathodic reaction was observed for Zn-Mg alloys with small additions of Cr, Zr, and Ti (Nazarov et al., 2017). However, Mg is remaining main component in the alloy inhibiting work of cathodic galvanic element.

Cut-edge corrosion is kind of degradation of coil coated galvanized steels. It is anodic undermining mode of coating deadhesion that is result of work galvanic element consisting from steel substrate (cathode) and the interface Zn/ZnO/polymeric coating (anode) (**Figure 14**). The electromotive force (0.4–0.5 V) between anode and cathode governs coating anodic deadhesion. Different industrial coil coatings were studied using SKP to find the effective pigments, pretreatments, composition of alloys, which decrease the rate of cut edge corrosion (Prosek et al., 2012, 2015, 2016a,b; Nazarov et al., 2017). Thus, these SKP studies were important for developing of new more corrosion stable coil coated products.

# CONCLUSIONS

1. A review of the literature shows that scanning Kelvin probe is a highly effective technique for assessment of corrosion of metals underneath of dielectric polymeric coatings. Nonuniform distribution of electrochemical potential leads to work of galvanic elements and to coating deadhesion. SKP measures the potential distribution along the interface that helps to predict the coatings failure. Cathodic delamination and anodic undermining modes of corrosion driven de-adhesion can be distinguished.

2. The rate of propagation of the de-adhesion front from the defect site can be determined. The pigments in coatings decrease the rate of oxygen diffusion and inhibit cathodic disbondment. Conversion coatings, adhesion promoters and corrosion inhibitors increased the stability of an interface to anodic undermining. SKP can be used to find the effect of pretreatment on ionic conductivity along the interface controlling acceleration/deceleration effect of galvanic elements.

3. SKP is perspective technique that can determine in situ the nature of adhesive bonds and their stabilities. It can be used for engineering of the developed coatings and interfaces.

4. SKP technique helps quickly select most promising and long lasting protective industrial paints. The technique analyses the interface underneath the multilayer coatings in situ. Typically, only short-term corrosion exposure has to be employed to predict coating stability.

Thick marine paints and compositions containing zinc rich primers for marine and offshore application can be quickly ranked. Electrophoretic paints on galvanized steel or aluminum substrates used in automotive industry can be evaluated as function of curing or pretreatment chemistry. Coil coated materials used in automotive and architecture industries can be investigated in order to choose effective pretreatment, pigment and alloy composition.

# AUTHOR CONTRIBUTIONS

AN was expert in application of electrochemical techniques to study corrosion and degradation of metal–polymer joints. DT was expert in theory of corrosion and electrochemistry, corrosion testing, corrosion protection by polymeric, and metallic coatings. The article was written in close collaboration.

#### REFERENCES

concentration of cations on delamination, chemical analysis of the interface. Corros. Sci. 41, 579–597. doi: 10.1016/S0010-938X(98)00167-X

of organic functionality and oxide surface chemistry. J. Phys. Chem. C 112, 12951–12957. doi: 10.1021/jp802703v

**Conflict of Interest Statement:** The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Copyright © 2019 Nazarov and Thierry. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

# Recent Development of Corrosion Protection Strategy Based on Mussel Adhesive Protein

Fan Zhang\* and Jinshan Pan

Division of Surface and Corrosion Science, Department of Chemistry, KTH Royal Institute of Technology, Stockholm, Sweden

Many of traditional anti-corrosion approaches using chromate are effective but hazardous to natural environment and human health, so development of green and effective alternatives is desirable. One of the mussel adhesive proteins derived from mussel byssus presents extraordinary adhesion to steel surface and exhibits film-forming and corrosion inhibition properties. Novel strategies for enhancing the corrosion inhibition of steel by the protein have been demonstrated recently. The protein together with ceria nanoparticles presents a great potential for the development of new corrosion inhibitors and thin films that are "green" and "effective," and have "smart" protection properties.

Edited by:

Flavio Deflorian, University of Trento, Italy

#### Reviewed by:

Benjamin Salas Valdez, Universidad Autónoma de Baja California, Mexico Changdong Gu, Zhejiang University, China Isolda Costa, Instituto de Pesquisas Energéticas e Nucleares (CNEN), Brazil

> \*Correspondence: Fan Zhang<EMAIL_ADDRESS>
#### Specialty section:

This article was submitted to Environmental Materials, a section of the journal Frontiers in Materials

Received: 28 June 2019 Accepted: 12 August 2019 Published: 27 August 2019

#### Citation:

Zhang F and Pan J (2019) Recent Development of Corrosion Protection Strategy Based on Mussel Adhesive Protein. Front. Mater. 6:207. doi: 10.3389/fmats.2019.00207 Keywords: mussel adhesive protein, film-forming corrosion inhibitor, corrosion inhibition, self-healing (self-repairing), Mefp-1

# INTRODUCTION

Metal corrosion is a serious material degradation problem from both economic and structural integrity standpoints. Carbon steel is the most widely used metal material, and corrosion of carbon steel occurs in almost all practical environments, but it can be largely controlled by suitable strategies. Because of hazardous nature of traditional corrosion inhibitors, development of green and effective alternatives is desirable.

Mussel adhesive proteins (MAPs), derived from Mytilus foot, have been suggested as a basis for environmentally friendly adhesives. Chemical characterization of MAPs started in early eighties (Waite, 1983a). So far at least six adhesive proteins have been extracted and identified from several species of mussel, and named as Mytilus foot proteins (Mfp, plus a number indicating the chronological order of identification; Zhao et al., 2006). These proteins present a basic isoelectric point due to the high content of posttranslational modified amino acid residues (Waite and Tanzer, 1980; Waite, 1983b). They are classified into three groups according to their function in the mussel foot: cuticle protein, adhesive protein, and structural protein. The 3, 4-dihydroxyphenylalanine (DOPA) has been recognized as the primary residue response for both adhesion and cohesion properties of the proteins.

# DIFFERENT MUSSEL ADHESIVE PROTEINS

#### Cuticle Protein

Mfp-1 is the only protein associated with the protective outer cuticle of byssal threads including plaques (Sun and Waite, 2005). It is the largest foot protein that has a molecular weight of about 108 kDa (Filpula et al., 1990; Holten-Andersen et al., 2009). It's molecule structure is open and extended in the acidic solution with minimized secondary structure (Williams et al., 1989), and consists of 75–80 repetitive decapeptide units with about 15 mol% of DOPA in the sequence (Waite, 2002).

**29**

#### Structural Protein

Mfp-2, 4, 6 serve as structural proteins in mussel foot. Mfp-2 is the structural component of the plaque matrix. It has a molecule weight about 42–47 kDa and rich in disulfide containing cysteine of about 6 mol% in repetitive sequence (Rzepecki et al., 1992). Pure Mfp-2 is prone to form aggregates upon prolonged storage (Rzepecki et al., 1992). Mfp-4 is the junction between byssal plaque and thread. It has a mass of about 93 kDa and contains an N-terminal histidine-rich domain and a C-terminal domain, which can bound strongly to Cu2+, and calcium-binding plaque proteins, respectively (Zhao and Waite, 2006). Mfp-6 acts as a conjunction of the interfacial proteins and the plaque matrix protein, and antioxidant of the interfacial proteins to maintain adhesive property of the mussel plaques in oxidative environment (Nicklisch et al., 2016). It has a mass of 11.6 kDa, and contains a high level of tyrosine (20 mol%) and cysteine (11 mol%). All these structural proteins contain limited amount of DOPA about 2–3 mol% (Nicklisch et al., 2016).

## Adhesive Protein

Mfp-3, 5 are identified as interfacial proteins present at the plaque-substrate interface. They form a primer-like layer at the plaque-substrate interface providing adhesion to the substrate. Mfp-3 is the smallest one with a mass of about 6 kDa (Papov et al., 1995; Warner and Waite, 1999). Most polymorphic mussel foot protein analogous to Mefp-3 have been identified from mussel species (Floriolli et al., 2000). The variants contain high level of DOPA (20 mol%) and rich in lysine and glycine. Mfp-5 is also a small protein with a molecular mass of 9 kDa (Waite and Qin, 2001). It has a homogeneous primary sequence that contains the highest content of DOPA (about 30 mol%) amongst all foot proteins, is also rich in lysine (20 mol%) and glycine (15 mol%), and has a significant number of phosphoserine residues in the sequence. It is found that over 75% of the DOPA in Mfp-5 is adjacent to lysine, which can remove hydrated cations from the adhesion interface and promote the approaching of catechol of DOPA to the mineral substrate (Maier et al., 2015). The phosphoserine residues contribute to an enhanced mineralbinding property of Mfp-5 to calcareous surfaces (Long et al., 1998; Silverman and Roberto, 2007). Therefore, Mfp-5 shows the greatest adhesion than other reported foot proteins. Mfp-3 and Mfp-5 present in a small amount in the mussel plaque (Lee et al., 2011).

Mefp-1, 3, and 5 extracted from Mytilus edulis have shown corrosion inhibition properties for metallic materials. Most studies were conducted with Mefp-1 as corrosion inhibitors in different forms. Only two works have been reported on Mefp-3 and Mefp-5, which were evaluated as dissolved inhibitor against flash rust (Nelson and Hansen, 2016; Hansen et al., 2019). Mefp-3 and Mefp-5 show high auto-oxidation rate in bulk solution due to the short chain (Haemers et al., 2003). These factors make them unfavorable materials for development of corrosion inhibitors. One the other hand, Mefp-1, the best-characterized mussel adhesive protein, naturally functions as a protective cuticle of byssus. It is abundant in mussels and contains high level of DOPA, which provides the molecule controllable adhesive and cohesive properties. The oxidation rate is low at acidic condition (Haemers et al., 2003, 2005). All these facts make Mefp-1 the most promising candidate for the development of bio-based corrosion protective strategies, and a range of Mepf-1 inhibitors have been developed in recent years. Therefore, this review will be focused on Mefp-1.

# SURFACE CHEMISTRY OF MEFP-1

#### Adsorption

Mefp-1 can adsorb strongly on metal and metal oxides and form films on the surface. Mechanisms of the strong adhesion are attributed to displacement of surface-bound water molecules from the target surfaces, and simultaneously formation of catechol-metal coordination and hydrogen bond between the catechol of DOPA and metal atoms of the substrate (Wei et al., 2016). It shows a saturated adsorption of Mefp-1 due to the electrostatic interactions, the maximum adsorption amount is almost independent to the protein concentration, but determined by the aggregation degree (Haemers et al., 2001). The adsorption rate of the protein is highly related to the hydrodynamic radius (Krivosheeva et al., 2012). Therefore, non-aggregate Mefp-1 adsorbs faster than Mefp-1 aggregates. For anticorrosion property, multilayer adsorption and film compaction are desired, which may occur on aggregated Mefp-1. This occurs only when the adsorbed layer has undergone a conformational change (Haemers et al., 2002). The adsorption amount of aggregated protein is higher, and the formed film is thicker as compared with the non-aggregated one (Krivosheeva et al., 2013).

#### Oxidation

DOPA undergoes auto-oxidation in presence of oxygen (Haemers et al., 2003), or can be oxidized by addition of chemical oxidants or enzyme. Oxidation of DOPA results in formation of oxidative conversions of DOPA-quinone, leading to intermolecular crosslinking (Yu et al., 1999). The quinone reversely dismutates to aryloxy free radicals, which then couple and re-oxidize to form coupled diphenols (Burzio and Waite, 2000). The coupling of diphenols results in polymerization of the protein. The polymerization property of DOPA enables the development of highly effective anticorrosion adhesives (Hansen et al., 1998), which can be achieved by adjusting pH, and adding catechol oxidase chemical or enzyme.

Film-forming corrosion inhibitors require both strong affinity to the substrate and low permeation of the corrosive media. Since adhesion and cohesion properties of Mefp-1 are dependent on DOPA in reduction and oxidation states, respectively, the enhancement of the cohesion property is obtained on the expense of the adhesion property, therefore, a too extensive oxidation of Mefp-1 film may lead to interfacial failure (Waite, 2002). The crosslinking level of Mefp-1 must be controlled optimally to ensure a good balance between the adhesion and cohesion properties of the inhibitor film.

#### pH-induced Oxidation

The pH of the buffer solution has a strong influence on the aggregation degree of Mefp-1 molecules and the film formation rate on surfaces (Haemers et al., 2001). At higher pH the auto-oxidation rate of Mefp-1 is higher, which results in a higher aggregation rate (Haemers et al., 2005). Moreover, at higher pH the formed protein film appears to be more compact (Zhang, 2013), which can be explained by the enhanced oxidative crosslinking of the DOPA segments in the film. The nearneutral pH has been demonstrated as the optimal pH for Mefp-1 film deposition.

#### Chemical- or Enzyme-Induced Oxidation

NaIO4 is suggested to be a suitable oxidant to induce crosslinking of Mefp-1 (Hedlund et al., 2009). Treatment of the Mefp-1 film in 10 mM NaIO4 solution for 10 min leads to oxidative conversion from DOPA to DOPA-quinone, followed by diphenols coupling (Höök et al., 2001). Therefore, the film becomes a thinner, denser and more uniform (Zhang et al., 2012). Enzyme catalyzes crosslinking of DOPA functional groups in the same oxidative mechanism as the chemical oxidants (Fant et al., 2000). Mushroom tyrosinase has been used to treat the adsorbed Mefp-1, giving an increased corrosion resistance (Nelson and Hansen, 2016).

## Complexation

DOPA can chelate Fe3+ to form complexes (Zeng et al., 2010). In situ observations of Mefp-1 on carbon steel indicate an enhanced adsorption of Mefp-1 at local corroding sites (Zhang et al., 2011). The enrichment of Mefp-1 is related to the interaction between DOPA and Fe ions released from the substrate. DOPA is a bidentate through which Mefp-1 can form strong complexes with Fe3+ in mono-, bis-, and triscatecholate- Fe3+ forms, depending on the pH and catechol/Fe3+ molar ratios.

The tri-DOPA/Fe3+ complex formation causes a compaction of and removal of the coupled water from the Mefp-1 film. Different types of metal ions can induce metal-ligandcomplexation, whereas Fe3+ is the most investigated, especially with aspect to corrosion inhibition. Since Fe3+ ions can be released from the corroding steel substrate, the complexation with Mefp-1 offers the promise of the protein film with the intrinsic self-healing property (Holten-Andersen et al., 2011).

# Electrochemical Tuning

Mefp-1 adsorption and the film conformation can be reversibly controlled by applying an electrochemical potential (Zhang et al., 2017). The study with Pt substrate has shown that Mefp-1 adsorbs to metal via electrostatic and non-electrostatic interactions. At low potential (hydrogen adsorption–desorption region), Mefp-1 adsorption increases with the increasing potential, leading to higher protein coverage of the surface. At medium potential, the pre-adsorbed Mefp-1 film becomes less compact with increasing of potential, forming a thicker layer with higher water content. At high potential (Pt oxidation region), the pre-adsorbed Mefp-1 undergoes oxidative crosslinking, resulting in a film compaction. Moreover, the adsorbed protein film presents a blocking effect on hydrogen adsorption/desorption and Pt oxidation/reduction. The reversible electrochemical reactions of Mefp-1 provide strategies to control corrosion inhibition properties of the protein film.

## Combination With Ceria Nanoparticles

The thin film of ceria (CeO2) has shown an enhanced corrosion resistance to metallic substrates (Zhong et al., 2008). Nanoparticles of ceria present more special properties, such as oxygen storage and oxidation power due to the redox reaction (Preda et al., 2011). In the regard of the impact to human health, ceria nanoparticles are not cytotoxic but instead protect cells from oxidative insult (Xia et al., 2008). Moreover, negatively charged ceria nanoparticle can facilitate the multilayer deposition of the positively charged Mefp-1.

Density Function Theory calculations show that catechol is energetically favored to bind strongly to ceria nanoparticles (Zhang et al., 2013). Therefore, ceria nanoparticles can be irreversibly incorporated into the Mefp-1 film, to facilitate a continuous build-up of the multilayer of Mefp-1/ceria composites (Sababi et al., 2012). Mefp-1 adsorbs in an extended conformation that allows significant hydrodynamic coupling to the solvent, whereas the incorporation of ceria nanoparticles induces the compaction of the protein layer. As shown in **Figure 1**, the deposited composite film on carbon steel is heterogeneous with some micro-domains showing coherent enrichment of Ce (from ceria) and C (from Mefp-1), as well as existence of tri-Fe3+/DOPA complex.

# MUSSEL ADHESIVE PROTEIN INSPIRED CORROSION PROTECTION

#### Mefp-1 as Dissolved Corrosion Inhibitor

Mefp-1 has been demonstrated as an effective corrosion inhibitor to carbon steel when dissolved in corrosive solutions (Zhang et al., 2011). The corrosion inhibition mechanism varies depending on the exposure condition. Under acidic condition, the initial corrosion inhibition efficiency is higher in the solution with higher NaCl concentration. In this situation, the autooxidation of Mefp-1 is retarded or proceeds very slowly; therefore, the oxidative crosslinking is negligible. The higher inhibition efficiency is mainly attributed to the enhanced DOPA-Fe3+ complexation, due to a higher amount of released Fe3+ in the solution with higher NaCl concentration. The longer-term corrosion inhibition of Mefp-1, e.g., 7 days, increases with increasing pH, and the efficiency is higher in the solution with less concentrated NaCl (Zhang, 2013). Clearly, the higher autooxidation level of Mefp-1 at higher pH is the dominant factor for the enhanced corrosion inhibition. At near-neutral pH, the inhibition efficiency increases with exposure time (Zhang et al., 2011). Both the oxidation level of Mefp-1 and the amount of Fe3+ released from the substrate increase with time. Therefore, both the oxidative crosslinking and the DOPA-Fe3+ complexation processes enhance the corrosion inhibition.

Furthermore, Mefp-1 has also been reported to provide a certain corrosion inhibition to 304 L stainless steel and Al, retarding the metal dissolution and pitting corrosion (Hansen et al., 1995; Hansen and McCafferty, 1996).

# Pre-formed Mefp-1 Film

Owing to the good film-forming capacity, Mefp-1 has also been investigated as a pre-formed corrosion inhibition film (Zhang

FIGURE 1 | Characterization of the Mefp-1/ceria composite film deposited on the carbon steel. (A) BSE-SEM image and corresponding elemental distribution of Ce, C, and O. (B) AFM topographic image of a compact area (with a size of 500 nm) of the composite film, and the corresponding phase image showing two distinct phases. (C) Raman spectrum of the Mefp-1/ceria composite film on the carbon steel, and the reference spectra of the pure Mefp-1 deposited on glass, ceria nanoparticle deposited on glass, and the complex of Mefp-1 with Fe3+.

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et al., 2012). The pre-formed Mefp-1 film provides a certain corrosion protection for short-term exposure, and the corrosion protection can be significantly enhanced by proper oxidation treatment due to the increased compactness. However, the longterm exposure results indicate that the pre-formed Mefp-1 film may degrade with time under the exposure condition. Therefore, for long-term applications, it is desirable to develop other types of Mefp-1 films that are more protective and stable during long exposure.

## Mefp-1/Ceria Nanocomposite Film

Nanocomposite films of sub-micron thickness composed of Mefp-1 and ceria nanoparticles have been deposited on carbon steel using alternative immersions and one-stepdipping methods (Sababi et al., 2012; Chen et al., 2016). The nanocomposite films provide excellent corrosion protection for carbon steel for long-term exposure, and the inhibition efficiency increases with exposure time. The increase of the inhibition effect is mainly due to the integration of the corrosion products into the film, as well as enhanced complexation between Mefp-1 in the film and Fe ions released from the steel surface. Moreover, the Mefp-1/ceria nanocomposite film possesses a certain self-healing ability on carbon steel (Chen et al., 2016). The self-healing ability is attributed to the DOPA of Mefp-1, and the healing process is explained by the fact that Fe3+ released from the surface defects promote the formation of DOPA-Fe3+ complexes in the nanocomposite film, which retards further dissolution and thus the localized corrosion.

## Synergistic Effect of Phosphate and Mefp-1/Ceria Composite Film

Addition of a small amount of Na2HPO4 into the NaCl solution results in an enhance corrosion inhibition effect of the Mefp-1/ceria composite film in both short-term and long-term exposures (Zhang et al., 2013). In the phosphate containing solution, the composite film deposited on carbon steel sample presents a two-layer like structure, with the composite film as the inner-layer and a vivianite deposit layer as the outer-layer.

The addition of the phosphate results in ca. one order of magnitude increase of the short-term corrosion resistance of composite film. Moreover, the corrosion resistance of the composite film further increases drastically upon the formation of a continuous phosphate deposit layer that fully covers the surface. In this case the Fe ions released from the steel surface are mostly trapped inside the film due to blockage by the deposited outer layer, and this leads to enhanced complexation between the protein and Fe3+. This consequently causes the compaction of the film and hence a significant increased corrosion protection. Thus, phosphate ions in the solution and the Mefp-1/ceria composite film on the surface have a synergistic effect on the corrosion protection for the steel.

Based on the above-mentioned observations, a nanocomposite film composed of Mefp-1, ceria nanoparticles and phosphate has been developed (Chen et al., 2017), which provides a self-healing effect against pitting corrosion of carbon steel. Moreover, it is found that the inhibition efficiency can be further enhanced by thermal treatment, which leads to an enhanced oxidative crosslinking of Mefp-1 in the composite

film. As a result, the film presents a highly compact structure. Furthermore, industrial applications of the nanocomposite films in reinforced concrete structure has been demonstrated (Zhang et al., 2019).

# CONCLUSION

The unique adhesive and cohesive properties of the mussel adhesive protein have inspired scientific research efforts of investigation and development of a range of new "green" corrosion inhibitors and protective thin films for carbon steel. As summarized in **Figure 2**, the corrosion inhibition efficiency of the Mefp-1 can be enhanced by means of adjusting pH, ion-induced complexation, and oxidative crosslinking. The adhesion and cohesion properties of the Mefp-1 film can be reversibly tuned via electrochemical potential. Moreover, Mefp-1 can be combined with ceria nanoparticles to form a composite film on steel surface, showing higher corrosion protection efficiency and self-healing properties. Incorporation of phosphate and thermal treatment of the composite film result in a further enhancement of the corrosion inhibition properties.

Based on the fundamental understanding of the chemistry and the obtained corrosion inhibition properties, the mussel adhesive protein presents a great potential for the development of new environmentally friendly and sustainable corrosion inhibitors and thin protective films.

## AUTHOR CONTRIBUTIONS

FZ contributed with the writing of the draft. JP revised it critically for important intellectual content.

# REFERENCES

## ACKNOWLEDGMENTS

The authors greatly acknowledge the financial supports from the Swedish Foundation for International Cooperation in Research and Higher Education (STINIT project for Swedish-China collaboration, grant no CH2017-7255). Moreover, FZ acknowledge a seed funding from KTH for allowing collaborative research in the field.

**Conflict of Interest Statement:** The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Copyright © 2019 Zhang and Pan. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

# A Review of Modern Assessment Methods for Metal and Metal-Oxide Based Primers for Substrate Corrosion Protection

Matthew E. McMahon\*, Raymond J. Santucci Jr., Carol F. Glover, Balaji Kannan, Zachery R. Walsh and John R. Scully

*Department of Materials Science and Engineering, Center for Electrochemical Science and Engineering, University of Virginia, Charlottesville, VA, United States*

#### Edited by:

*Flavio Deflorian, University of Trento, Italy*

#### Reviewed by:

*Guido Grundmeier, University of Paderborn, Germany Ricardo M. Souto, Universidad de La Laguna, Spain Fatima Montemor, Higher Technical Institute, University of Lisbon, Portugal*

> \*Correspondence: *Matthew E. McMahon<EMAIL_ADDRESS>
#### Specialty section:

*This article was submitted to Environmental Materials, a section of the journal Frontiers in Materials*

Received: *26 April 2019* Accepted: *24 July 2019* Published: *06 September 2019*

#### Citation:

*McMahon ME, Santucci RJ Jr, Glover CF, Kannan B, Walsh ZR and Scully JR (2019) A Review of Modern Assessment Methods for Metal and Metal-Oxide Based Primers for Substrate Corrosion Protection. Front. Mater. 6:190. doi: 10.3389/fmats.2019.00190* Pigmented coatings developed for the corrosion protection of engineering alloys have been successful in the field of corrosion control, and thus are ubiquitous in application. Supporting this success are an array of methods to diagnostically assess these systems and measure their state of degradation. These methods assist the development of next generation coatings, in-service evaluation, and post-mortem analysis of performance. This review comprehensively explores these modern assessment methods, used to screen candidate corrosion prevention pigments, assess pigment protection efficiency, quantify overall pigment effectiveness, and/or predict coating service lifetime. Coating service life assessment is conducted based on the class of coating considered, as categorized by (1) chemical inhibition, (2) galvanic protection, (3) high impedance ion barrier, or a combination of these protection mechanisms. Exciting new *in-situ* and operando methods are growing in use to enable high fidelity measurement of chemical inhibitor release and deposition, such as micro and *in-situ* Raman spectroscopy and inductively coupled plasma atomic emission spectrometry (ICP-AES), to name a few. In the galvanic protection coating class, service life and the state of coating performance are now being quantified through methods such as galvanostatic pulse and accelerated cycle testing. Galvanic throwing power is assessed by scanning vibrating electrode technique (SVET), multi-electrode array (MEA), and scanning kelvin probe (SKP). The performance of high impedance ion barrier coatings has traditionally been understood through use of electrochemical impedance spectroscopy (EIS) and equivalent circuit modeling, which have been applied to all coating classes. Finite element analysis (FEA) models the combined effects of coating resistance, water layer thickness, and pigment potential on galvanic coupling and/or inhibitor species release as well. The development of effective corrosion prevention pigments and the in-service evaluation of their efficiency/effectiveness demands a broad collection of techniques. These methods now include new advances in thermodynamic modeling via chemical stability diagrams to

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assess pigment function in specific conditions, *in-situ* pH measurement, as well as energy dispersive (EDS) and x-ray photoelectron spectroscopic (XPS) analyses to determine elemental distribution and corrosion product formation. These techniques and more are explored in this review to document the state-of-the-art in the field.

Keywords: metal-rich primers, zinc, magnesium, chemical inhibition, SVET, chemical stability modeling

# INTRODUCTION

The cost of corrosion was estimated to be 3–4% of each nation's gross national product (GDP) in 2013, or a global cost of approximately US\$2.5 trillion per year (Koch, 2017). The global use of effective corrosion control practices (protective coatings, considerate design strategies, regular preventative maintenance, etc.) is estimated to reduce this cost by 15–35%, or as much as US\$875 billion annually (Koch, 2017). One means of achieving this savings is through the effective design and application of protective coatings such as metal-rich primers, which are now ubiquitous across many engineering sectors to prevent corrosion and extend service life. Metallic corrosion requires thermodynamically viable anodic and cathodic reactions, as well as electrical and ionic conduction between anodes and cathodes. Pigmented coatings such as metal-rich primers, which are composed of sacrificial pigments embedded within an adhesive resin, operate through a variety of means, such as chemical inhibition, galvanic protection, and high impedance barrier protection. These coatings can be designed to alter the dominant anodic/cathodic reactions that occur, limit one or more of the corrosion cell requirements, or even to introduce a new sacrificial anodic reaction to afford corrosion protection to the substrate. The means of protection is dependent upon the type of sacrificial pigment present, the connectivity of the pigment, and the resin type/porosity, to name a few factors. These coatings do not necessarily function in exactly the same manner as metallic coatings, which are thin metal/alloy layers often electro- or vapor-deposited on an alloy surface, where the entirety of the layer is barrier and/or sacrificial and no binding resin is present. The same sacrificial metals may be utilized in both coating types, though the metal-rich primer design offers certain benefits, such as the ability to delay primer activation through the resin or topcoat and reduce the self-corrosion rate by isolating a portion of the particles, as well as the versatile ability to be spray-coated on nearly any free surface (though proper adhesion can be elusive). A key aspect of metal-rich primer (MRP) performance is the ability of the primer to protect latent or operational defects such as scratches as well as the substrate under the intact coating. Effective design of metal-rich primers to achieve such performance requires specific tuning for each chosen alloy substrate, and this tuning is informed by a variety of characterization methods (Bierwagen et al., 2007; Yan et al., 2010; King and Scully, 2011; King et al., 2014a, 2015; Bastos et al., 2016; Santucci et al., 2017a, 2018a).