Characterization and Applications

#### **Chapter 6**

## An Insight on Corrosion Resistance Ability of Biocompatible Dental Implants through Electrochemical Impedance Spectroscopy

*Duduzile Nkomo and Nomsombuluko Masia*

### **Abstract**

Corrosion resistance is an important requirement in the study of biomedical implants. Implant surface can be modified to provide good adherence and/or optimum biocompatibility with the human body at the bone-implant interface. Titanium alloys are typically used because of their excellent corrosion resistance and biocompatibility. However, to improve these properties, the alloy surface is roughened using alumina (Al2O3). More details on the corrosion resistance of these alloys can be obtained by using electrochemical impedance spectroscopy (EIS) method. EIS is the most suitable method for monitoring corrosion rate values due to its reproducibility, it is non-destructive and has reliable determination of small corrosion rates, much lower than those measured by other techniques. It can also study high-impedance systems, such as coatings and linings, high-purity water, and organic coating/metal systems or corrosion in a low-conductive solution. This method has been used to evaluate electrochemical properties of modified surfaces. This chapter will explore the effectiveness of EIS in studying the corrosion behaviour of machined and surface-modified Pure Ti grade 4 for dental implant applications. The basic EIS concepts are discussed and their derivation thereof to provide information about the corrosion resistance of biomedical implants is explored.

**Keywords:** Biomedical, EIS, Surface modification, Implants

#### **1. Introduction**

There are multiple ways to study the corrosion resistance of biomedical implants. The common laboratory procedures include open circuit potential test (OCP), potentiodynamic polarisation test (PDP) and EIS. The commonly used electrochemical methods present measurements of currents, electrical charges or electrode potentials as a function of time. In the previous years, the use of EIS method was seldom because contrary to other electrochemical methods, EIS presents the signal as a function of frequency at a constant potential. This limits the fundamental understanding of the method because electrochemists think in terms of time and not frequency. Moreover, issues associated with understanding the EIS method are that it requires the knowledge of mathematics, Laplace and Fourier transforms, and complex numbers. However the evolution of this method has developed from using rather simple calculations and has rendered it useful in different applications such as solid materials, corrosion, passivity and batteries [1].

EIS has been used to investigate the corrosion resistance of biomedical implants. Currently we focus on the study of corrosion resistance of biomedical implants for dental applications. Different materials have been used for dental implants. Commercially pure titanium and titanium-aluminium-vanadium alloy (Ti-6Al-4 V) are most widely used biomaterials. They became popular because of their chemical resistance, mechanical strength, low toxicity, lightness and excellent biocompatibility. Ti and its alloys have the tendency to react with oxygen to form a stable oxide layer called titanium dioxide (TiO2). This oxide layer is responsible for chemical stability and corrosion resistance of titanium [2].

The types of corrosive reactions that occur in the oral cavity are electrochemical, where the saliva plays the role. There are various types of corrosion that occurs in the oral cavity such as galvanic, fretting, pitting and uniform corrosion. Titanium implants coexist with different metallic dental superstructures and dental restorations in a patient mouth. This complexity can be linked to the occurrence of galvanic corrosion creating galvanic current, which passes through the surrounding tissue causing sharp pain. Extensive release of metal ions from a prosthetic device can result in adverse biological reactions and can lead to mechanical failure of a device itself. It is therefore important to conduct sufficient corrosion tests in order to avoid such failures and evaluate the efficiency of each corrosion test method used [2].

The advantages of using the EIS method are numerous. It provides a substantial information that can be further analysed in electrochemical properties investigations. Methods such as OCP, PDP and EIS have been used in the current study to determine the corrosion resistance of titanium pure Ti grade 4 in machined (TiGr4M) and roughened (TiGr4R) conditions in different Ringer's lactate solutions.

#### **2. OCP**

To compare the results obtained from each electrochemical testing method, we start with OCP investigations. The OCP test results shown in **Figure 1** for TiGr4 in the machined (TiGr4M) and roughened with alumina (TiGr4R) conditions are explained. OCP is the most common and simplest method used to study the film

#### *An Insight on Corrosion Resistance Ability of Biocompatible Dental Implants… DOI: http://dx.doi.org/10.5772/intechopen.100330*

formation and passivation of metallic structures. The potentials are measured against time and the results obtained reflect whether or not the passive film is stable in the particular environment. Thus, to mimic the nature of human mouth environment, the TiGr4M and TiGr4R samples were tested in four different pH solutions namely, the base Ringer's lactate solution-pH 5.5, Ringer's lactate-pH 2, Ringer's lactate-pH 2 with addition of sulphur and Ringer's lactate-pH 7. Each sample alloy was immersed in a solution for four hours in order to ensure stable formation of a passive oxide film. The onset of passive oxide layer stabilisation can be indicated by constant potential over a period of time.

The OCP plots for TiGr4M in all solutions was observed in the positive potential region compared to TiGr4R. In the case where no external potential applied or external disturbance such as surface defects on the test material, the formed passive oxide film remains attached to the bulk material and thus no further corrosion will be observed. A higher/more positive corrosion potential/OCP is an indication of more stable oxide film in a particular environment. Thus due to surface roughening with alumina on TiGr4R, an unstable passive oxide film formation is observed. This was observed for three different Ringer's lactate solutions except for pH 7 solution. The corrosion potential was measured at negative/more active potentials, indicating the unstable nature of the formed oxide layer. It can be deduced that the OCP test is an important step in electrochemical tests as a method studying the formation of a passive film. However, further information about the stability of the passive film is required in order to qualify new materials for use in corrosive environments such as the human mouth. This information can be obtained by applying external voltage stepwise and recording the resulting current due to anodic reaction taking place at the surface of the test material/working electrode. This has been done by use of PDP tests.

#### **3. PDP**

The PDP tests are ran such that the potential of the test sample is increased stepwise, which triggers the occurrence of either the oxidation or reduction reactions on the test material surface, and as a results the current is generated. The presentation of the potential in the function of current density (or log current density) for each measured point results in obtaining the polarisation curve. In the current presented work each scan was started from 250 mV to +1500 mV versus the corrosion potential/OCP at a scanning speed of 10 mV/min. The polarisation curve was used to determine the corrosion behaviour of the TiGr4 by Tafel slope extrapolation. The PDP plots for TiGr4M and TiGr4R in different Ringers' lactate solutions are shown in **Figure 2**.

The current density measured by Tafel slope extrapolation (cathodic and anodic reaction slope) was determined and subsequently the corrosion rates were calculated. The corrosion rates obtained were very low, less than 0.02 mm/year indicating that the material tested was in the fully passive condition. The passive behaviour of a test material can be studied by observing the increased current density as it is an indication of corrosion. Thus a material with high passive current is more prone to corrosion compared to the one with lower passive currents. Using the PDP plots, it can be observed that TiGr4R (dotted lines) showed higher passive currents than TiGr4M (solid lines) in all solutions, meaning with further exposure into Ringer's lactate solutions, the passive oxide film formed in TiGr4R has a tendency to move from passive-active as was indicated with OCP test results.

The PDP and OCP measurement are more dependent on time and thus limits these methods because with further exposure to corrosive environment, the

**Figure 2.** *Potentiodynamic polarisation plots of TiGr4M and TiGr4R in different Ringer's solutions.*

exposed material can move from passive to active. Furthermore, using Tafel slope calculations for determination of corrosion rate requires that one uses data further away from the corrosion potential for accurate extrapolation and thus incorporating other reactions, which may bring significant errors in the obtained results, especially in this case where an additional phase of alumina has been introduced in TiGr4R. The EIS method can therefore be useful to obtain more accurate results in the study of electrochemical properties of this surface-modified and machined TiGr4 used for dental implant materials.

The use of EIS is highly recommended because it contains information that can be analysed at each frequency. That is, taking impedance at each potential produces series of data at different frequencies. The difference between the commonly used methods and EIS is that, although the polarisation resistance can be obtained from steady-state measurements by a potential-current plot slope, impedance plots provides more information at every point and not just at one value of polarisation potential. That is, it can be used for fine-tuning mechanisms and determining the kinetics of processes, resistances and capacitances, and it allows for the determination of real surface areas in situ. Particularly, EIS has been used for the following studies:

EIS calculations are carried out using mathematical formulations that provide meaningful models and therefore physically relevant. The extraction of information is carried out by determination of model parameters, which directly corresponds to

#### *An Insight on Corrosion Resistance Ability of Biocompatible Dental Implants… DOI: http://dx.doi.org/10.5772/intechopen.100330*

physical phenomena. The model parameters provides information about the measured quantity and shows a physical expected behaviour [3].

For materials, impedance spectroscopy provides possibilities to characterise the complex electrical behaviour such as resistivity, capacitive and inductive behaviour. Therefore, it is sensitive to both surface phenomena and changes of bulk properties including conduction mechanisms. It allows quantitative characterisation of corrosion processes as well as the evaluation of the anti-corrosion performance of protective coatings on corrosive metals. The use of EIS in corrosion includes detection of surface and material changes due to exposure, prediction of the lifetime of the protective passive film, evaluation of coating systems, identifying corrosion processes that lead to failures and measurement of water uptake by coatings. The EIS method follow the evolution of surface modified materials as they undergo either artificial or natural exposure to conditions that cause corrosion failure to provide information about the corrosion kinetics, which other methods do not offer [3].

Therefore, this study focuses on the use of EIS as the better method to further evaluate the corrosion resistance of TiGr4 in the machined and roughed condition. We look at the information that is obtained from EIS in comparison to that obtained from the OCP and PDP methods.

#### **4. Nyquist plots**

The first concept of EIS to tackle is the Nyquist plot. It has been established that EIS is applied to the characterisation of electrode processes and complex interfaces. Moreover, it studies the system response to the application of a periodic small amplitude of alternating current (AC) signal. Analysis of the system response contains information about the interface, its structure and reactions taking place there. The system response is generated from an electrical circuit, i.e. application of an electrical perturbation (current, potential) causes the appearance of a response. An application of a known potential *E (t)* to a resistance *R* gives a current *i(t) = E(t)/R.* When the same potential is applied to the series connection of resistance *R* and capacitance *C*, the total potential difference is a sum of potential drops of each element. Taking into account that of capacitance, *E(t) = Q(t)/C*, where *Q* is the charge stored in a capacitor, Eq. (1) is obtained [4].

$$\mathbf{E}(\mathbf{t}) = \mathbf{i}(\mathbf{t})\mathbf{R} + \frac{\mathbf{Q}(\mathbf{t})}{\mathbf{C}} = \mathbf{i}(\mathbf{t})\mathbf{R} + \frac{\mathbf{1}}{\mathbf{C}} \int\_{0}^{\mathbf{t}} \mathbf{i}(\mathbf{t})\mathbf{d}\mathbf{t} \tag{1}$$

Eq. (1) can be solved by using what is known as Laplace transform technique. This technique is an integral transform in which a function of time *f(t)* is transformed into new function of a parameter *s* called frequency, *F(s)* according to Eq. (2) [4].

$$\mathcal{H}\left[f(t)\right] = F(s) = \bigcap\_{0}^{\infty} f(t) \exp\left(-st\right)dt\tag{2}$$

The direct application of Laplace transform to Eq. (1), given that L Ð*t* 0*i t*ð Þ*dt* � � ¼ *i s*ð Þ*=s*, gives Eq. (3)

$$\mathbf{E(s)} = \mathbf{i(s)R} + \frac{\mathbf{i(s)}}{\mathbf{sC}} \tag{3}$$

Thus:

$$\mathbf{i(s)} = \frac{\mathbf{E(s)}}{\left(\mathbf{R} + \frac{1}{s\mathbf{C}}\right)}\tag{4}$$

The ratio of the Laplace transforms of potential and current, i.e. *E s*ð Þ/*i s*ð Þ is expressed in units of resistance, Ω, referred to as impedance, *Z(s)* [4]:

$$\mathbf{Z(s)} = \mathbf{E(s)}/\mathbf{i(s)} = \mathbf{R} + \frac{\mathbf{1}}{\mathbf{sC}} \tag{5}$$

The polarisation resistance *R or Rp* value is inversely proportional to the corrosion rate. This means, the decrease in the values of *Rp* with increasing concentration of corrosive ions indicate the increase in corrosion rate. This interpretation can be used to support results obtained from the OCP and PDP tests in terms of the stability of the passive layer [5].

EIS has been used to explain kinetic parameters for electron transfer at the implant/saliva interface and to understand mechanism of corrosion and passivation for metallic materials in various media conditions. We use the study conducted by AlOtaibi *et al*. [2] to simplify the interpretation of Nyquist plots and the information obtained. The typical Nyquist plots obtained after immersion in artificial saliva for different exposure times is shown in **Figure 3**. The plots shown in **Figure 3** can be best fitted to the equivalent circuit model shown in **Figure 4**. The impedance results are best analysed and interpreted in terms of equivalent electrical circuit models

**Figure 3.** *Typical Nyquist plots of dental implants in an artificial saliva [2].*

**Figure 4.** *The equivalent circuit model used to fit the EIS experimental data [2].*

#### *An Insight on Corrosion Resistance Ability of Biocompatible Dental Implants… DOI: http://dx.doi.org/10.5772/intechopen.100330*

obtained by simulating the electrochemical behaviour of alloy-medium interface. The equivalent circuit refers to the presence of an oxide film layer where the inner barrier film is represented by Rp2 and Cd1, but the outer layer is due to the presence of a wall capacitance, presented by Q and polarisation resistance RP1 of the oxide film. The overall solution resistance is presented by Rs. The electrical impedance parameters are therefore measured from this perspective.

Nyquist plots typically consists of capacitive loops at different frequencies. The loop can be at higher frequencies, medium frequencies and inductive loops at lower frequencies. The higher region frequency semicircle corresponds to the charge transfer of the corrosion process and oxide film effects. The medium frequencies semicircle corresponds to the mass transport associated with the diffusion of metallic species through the corrosion product layer. The relaxation of the surface adsorbed species is considered to be the beginning of lower frequency inductive loop. Thus, the diameter of the capacitive loops will decrease with increased concentration of corrosive species such as addition of sulphur in the Ringers solutions or decreasing of pH of the solution from 5.5 to 2 using sulphuric acid in the testing of Pure Ti dental implants [5].

We have established that Nyquist plots are often used in electrochemical literature because they allow for an easy prediction of the circuit elements. However, they do not show all details. Different types of Nyquist plots may be obtained from different values of capacitance. The only difference may be that the point on the semicircle would correspond to different frequencies. It is also possible to observe an incomplete semicircle over all the frequency range indicating a highly resistive surface [6]. This method can be used to determine the electrochemical parameters, which is AC impedance of the material. In the case of a series connection of the resistance and capacitance the impedance is given by Eq. (6). The result may be presented graphically using two plots namely Nyquist plots and Bode plots. The Nyquist plot is a plot of *Z*' (x axis) versus *Z"* (y axis), that is real versus imaginary components plotted for various frequencies.

$$\mathbf{Z}(\text{jo}) = \mathbf{1} + \frac{\mathbf{1}}{\text{joC}} = \mathbf{R} - \frac{\mathbf{j}}{\text{ooC}} \tag{6}$$

Where: Z(*jω*Þ stands for faradaic impedance of the circuit and *ω* stands for angular frequency.

**Figure 5.** *Nyquist plots for TiGr4M and TiGr4R in different ringers solutions.*

In our study, the EIS measurements were performed for each surface treatment of TiGr4. The frequency sweep was applied from 0.01 to 100000 Hz with an AC amplitude of 10 mA. Frequency sweep change according to material characteristics. The Nyquist plot was used to determine the capacitive behaviour, simply indicated by the size of the diameter of semicircle. **Figure 5** shows the Nyquist plots for TiGr4M and TiGr4R in different Ringer's solutions. The plots observed showed a capacitive behaviour, which is an indication of high corrosion resistance associated with incomplete semicircle observed. The increase of the diameter of the semicircle gives an indication of a high passivation of the surface against corrosion. TiGr4M showed the highest corrosion resistance indicated by the largest diameter/open incomplete semicircle. The *R* for TiGr4M is always higher than that of TiGr4R, which is in accordance with the results obtained from PDP tests where higher corrosion resistance for TiGR4M were observed.

#### **5. Bode plots**

The Bode plots have some advantages over the Nyquist plots since frequency appears as one of the axes, this shows that impedance depends on the frequency. The plots uses the logarithm of frequency to allow a very wide frequency range to be plotted on one graph, but with each decade given equal weight. The Bode plots also show the magnitude (|Z|) on a log axis so that you can easily plot wide impedance ranges on the same set of axes.

The phase angle plots are used to determine whether or not the film formed on the metal surface is intact and behaving like a capacitor. The representation shows the phase angle versus frequency (f). Frequency is generally presented on a logarithmic scale [7]. The presence of time-constant (or frequency) distribution is frequently modelled by use of a constant-phase-element (CPE). The impedance response typically reflects a distribution of reactivity that is commonly represented in equivalent electrical circuits as a CPE. Different expressions for a CPE have been presented in the literature [8], e.g.:

$$Z(o) = \text{Re} - \frac{j}{(joTo)a} \tag{7}$$

If the phase is greater than 0° it exhibits a CPE behaviour. The magnitude of the complex capacitance would also then vary with the frequency. The phase angle expressed in degrees, is related to the CPE exponent n by the Eq. (8): [9].

$$
\boldsymbol{\Theta} = \mathbf{\mathcal{O}}(\mathbf{1} - \mathbf{n}) \tag{8}
$$

For *n* = 1, the CPE behaves as a capacitor; for *n* = 0, the CPE behaves as a resistor; and for *n* = �1 the CPE behaves as an inductor [8]. CPE behaviour was called frequency dispersion of capacitance because it appears as if the double-layer is behaving as a capacitor whose capacitance depends on the frequency [8, 10, 11]. Generally, the appearance of a CPE is due to the presence of inhomogeneity in the electrode material system and it can be described in terms of a distribution of relaxation times, or it may arise from non-uniform diffusion whose electrical analog is an inhomogeneous distributed RC transmission line [9].

The increased value of the phase angle at its maximum, particularly at low frequency values reveals the increased corrosion resistance [7]. **Figure 6** shows the phase angle plots for TiGr4M and TiGr4R in different Ringers solutions. Phase angles close to 80° were obtained at medium and low frequencies, suggesting a

*An Insight on Corrosion Resistance Ability of Biocompatible Dental Implants… DOI: http://dx.doi.org/10.5772/intechopen.100330*

#### **Figure 6.**

*Phase angle plots for TiGr4M and TiGr4R in different ringers solutions.*

passive behaviour for both TiGr4M and TiGr4R. TiGr4M behaved similar in all solutions. TiGr4R showed an increased value of the phase angle at low frequencies in high pH solutions. Meaning that reducing the pH and addition of sulphide ions decreased the corrosion resistance of the TiGr4R. High values of phase angle at low frequencies for TiGr4M is always higher than that of TiGr4R, which is in accordance with the results obtained from OCP, PDP and Nyquist plots where higher corrosion resistance for TiGR4M were observed.

The modulus plot determine the dissolution kinetics of metallic surface which is represented by the magnitude of charge transfer resistance |Z|/Z'. The magnitude is usually presented on a logarithmic scale as a function of frequency on a logarithmic scale. The slope of the line at low frequencies, therefore, has a value of 1 for the blocking electrode considered [8]. The log |Z| vs. log ω curve can yield values of Rp and RΩ. At the highest frequencies, the ohmic resistance dominates the impedance and log (RΩ) can be read from the high frequency horizontal plateau. At the lowest frequencies, polarisation resistance also contributes, and log (RΩ + Rp) can be read from the low frequency horizontal plateau. At intermediate frequencies, this curve should be straight line with a slope of 1, as shown on **Figure 7**. Extrapolating this

**Figure 7.** *Bode plot for a simple electrochemical system [12].*

**Figure 8.**

*Bode impedance of the interface, |Z|, plots for TiGr4M and TiGr4R in different ringers solutions.*

line to the log |Z| axis at ω = 1 yields the value of CDL from the relationship, where ω =2πF [12]:

$$|\mathbf{Z}| = \mathbf{1}/\mathbf{C\_{DL}} \tag{9}$$

The higher the values of |Z|, especially at lower frequency range represents a higher corrosion resistance of the material [7]. The change of the Bode impedance of the interface (|Z|) with frequency for TiGr4M and TiGr4R is shown in **Figure 8**. The highest values of |Z|, particularly at the low frequency were recorded on both TiGr4M and TiGr4R in Ringer's pH 5.5 and Ringer's pH 7. The highest values of |Z|, at low frequencies for TiGr4M is always higher than that of TiGr4R, which is in accordance with the results obtained from PDP, Nyquist and Phase angle plots. Thus Bode impedance plots further proved higher corrosion resistance for TiGR4M compared to TiGr4R.

#### **6. Conclusion**

Corrosion testing of TiGr4 in roughened and machined conditions has been conducted. OCP tests were conducted for determination of corrosion potential of TiGr4. The passivity behaviour was studied using the PDP method. It was noted that both samples were capable of forming a passive oxide layer. From these tests it was observed that both samples were corrosion resistant, with TiGr4M demonstrating a slightly higher resistance.

A biomaterial used for dental implant application is subject to different corrosion reactions in the oral cavity and these reactions are electrochemical. Thus, we used the EIS models with a prominent parameter of frequency to forecast the lifetime of the passive layer of the proposed biomaterial. The requirement is that the implant should last a patient's lifetime once installed. Using EIS, we confirmed that indeed there is more information that could be obtained regarding the stability of the passive oxide layer formed/corrosion resistance during the exposure to artificial saliva. We used Nyquist plots, Bode plots and phase angle plots to generate information about the resistivity and capacitive behaviour of TiGr4 for both surface conditions in different pH artificial saliva solutions.

It was deduced that with regards to the passive nature of TiGr4M and TiGr4R: All the EIS plots showed that TiGr4M has the highest corrosion resistance in all the *An Insight on Corrosion Resistance Ability of Biocompatible Dental Implants… DOI: http://dx.doi.org/10.5772/intechopen.100330*

pH conditions while TiGr4R has the lowest. This was demonstrated by an increase of the diameter of the semicircle which indicated high passivation of the surface against corrosion. TiGr4M showed the highest corrosion resistance indicated by the largest diameter/open incomplete semicircle. The increased value of the phase angle at its maximum, particularly at low frequency values revealed the increased corrosion resistance. The higher the values of |Z|, especially at lower frequency range represents a higher corrosion resistance of a material.

The analysis of these EIS plots indicated that TiGr4 is a suitable material for dental implant application. Surface-modification with alumina does decrease the corrosion resistance but, insignificantly as the TiGr4R also persistently showed a comparative capacitive behaviour as with TiGr4M. Thus, it is beneficial to modify the surface with alumina as it increases the bone-implant adherence and the corrosion resistance is not utterly compromised. This offers a biocompatible material.

#### **Acknowledgements**

Authors would like to acknowledge Mintek and Southern Implants (PTY) LTD for financial assistance, sample preparation and approval to publish this work. The authors also wishes to appreciate the assistance from the Mintek colleagues especially Ms. Melanie Smit and Dr. Alain Mwamba.

#### **Conflict of interest**

The authors declare no conflict of interest.

#### **Author details**

Duduzile Nkomo\* and Nomsombuluko Masia Mintek, Randburg, South Africa

\*Address all correspondence to<EMAIL_ADDRESS>
© 2021 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

#### **References**

[1] A. Lasia, *Electrochemical impedance spectroscopy and its applications*, vol.<PHONE_NUMBER>. 2014.

[2] A. AlOtaibi, E. S. M. Sherif, S. Zinelis, and Y. S. Al Jabbari, "Corrosion behavior of two cp titanium dental implants connected by cobalt chromium metal superstructure in artificial saliva and the influence of immersion time," *Int. J. Electrochem. Sci.*, vol. 11, no. 7, pp. 5877–5890, 2016, doi: 10.20964/ 2016.07.77.

[3] J. J. Carroll, *Advenced Spectroscopy: Advanced applications: Battery Research, Bioimpedance, System design*. 2010.

[4] A. Lasia, *Electrochemical Impedance Spectroscopy and Its Applications Electrochemical Impedance Spectroscopy and its Applications Andrzej Lasia Département de chimie , Université de Sherbrooke , Sherbrooke*, no. July. 2017.

[5] M. G. Acharya and A. N. Shetty, "The corrosion behavior of AZ31 alloy in chloride and sulfate media – A comparative study through electrochemical investigations," *J. Magnes. Alloy.*, vol. 7, no. 1, pp. 98–112, 2019, doi: 10.1016/j. jma.2018.09.003.

[6] B. S. Gugelmin, L. S. Santos, H. D. A. Ponte, and C. E. B. Marino, "Electrochemical stability and bioactivity evaluation of Ti6Al4V surface coated with thin oxide by EIS for biomedical applications," *Mater. Res.*, vol. 18, no. 3, pp. 602–607, 2015, doi: 10.1590/1516-1439.201514.

[7] K. Habib and F. Al-Sabti, "Electrochemical impedence spectroscopy versus optical interferometry techniques during anodization of aluminium," *Opt. Lasers Eng.*, vol. 35, no. 4, pp. 225–232, 2001, doi: 10.1016/ S0143-8166(01)00013-6.

[8] M.E Orazem and B. Tribollet, "Electrochemical Impedence Specroscopy: EIS", 2008.

[9] R. Srinivasan and F. Fasmin, "An introduction to Electrochemical Impedence Specroscopy", 2021.

[10] E. Barsoukov and J. R. Macdonald "Impedance Spectroscopy Theory, Experiment, and Applications", 2005.

[11] S. Tamilselvi, V. Raman and N. Rajendran, "Surface modification of titanium by chemical and thermal methods – electrochemical impedance spectroscopic studies", Corrosion Engineering, Science and Technology, vol.46, no. 4, pp. 585-591, 2011.

[12] Application Note AC-1, Subject: Basics of Electrochemical Impedance Spectroscopy, Princeton applied research.

#### **Chapter 7**

## Hot Corrosion and Oxidation Behaviour of TiAl Alloys during Fabrication by Laser Powder Bed Additive Manufacturing Process

*Ntebogeng Mogale, Wallace Matizamhuka and Prince Cobbinah*

### **Abstract**

This research paper summarises the practical relevance of additive manufacturing with particular attention to the latest laser powder bed fusion (L-PBF) technology. L-PBF is a promising processing technique, integrating intelligent and advanced manufacturing systems for aerospace gas turbine components. Some of the added benefits of implementing such technologies compared to traditional processing methods include the freedom to customise high complexity components and rapid prototyping. Titanium aluminide (TiAl) alloys used in harsh environmental settings of turbomachinery, such as low-pressure turbine blades, have gained much interest. TiAl alloys are deemed by researchers as replacement candidates for the heavier Ni-based superalloys due to attractive properties like high strength, creep resistance, excellent resistance to corrosion and wear at elevated temperatures. Several conventional processing technologies such as ingot metallurgy, casting, and solid-state powder sintering can also be utilised to manufacture TiAl alloys employed in high-temperature applications. This chapter focuses on compositional variations, microstructure, and processing of TiAl alloys via L-PBF. Afterward, the hot corrosion aspects of TiAl alloys, including classification, characteristics, mechanisms and preventative measures, are discussed. Oxidation behaviour, kinetics and prevention control measures such as surface and alloy modifications of TiAl alloys at high temperature are assessed. Development trends for improving the hot corrosion and oxidation resistance of TiAl alloys possibly affecting future use of TiAl alloys are identified.

**Keywords:** titanium aluminides, oxidation, hot corrosion, additive manufacturing, laser powder bed fusion

#### **1. Introduction**

Titanium aluminide (TiAl) is a member of group material referred to as intermetallics, consisting of various metals resulting in ordered crystallographic structures formed when the concentration of the alloy exceeds the solubility limit [1]. Properties as low density, high strength and elevated temperature properties

make TiAl replacement candidates for nickel-based superalloys used in the aerospace and automotive industries [2–4]. One such alloy tried and tested by General Electric [5] for commercial turbofan engines is Ti-48Al-2Cr-2Nb. Despite the attractive high-temperature properties attained in research to date, the inherent poor ductility of TiAl at ambient temperatures remains a concern [6]. Over the past 20 years and recently, much work has been devoted to material tailoring through compositional variations and alloying aimed at improving room temperature ductility [7–11].

Phase evolution in TiAl alloys governs the mechanical and physical properties to be obtained. Primarily, two ordered structures exist, namely, γ-TiAl (L10) and hexagonal α2-Ti3Al (D019), resulting from different thermo-mechanical treatments. Furthermore, the mechanical properties to be obtained are dependent on the microstructure. Three microstructures exist, namely, equiaxed single γ phase, fully or near (γ/α2) lamellar and duplex (consisting of colonies of lamellar γ/α2 and pure γ phase grains). The achieved microstructure is significant for its mechanical properties, especially in structural applications. Duplex microstructures with enhanced ductility measures such as fracture strength, yield strength and strain have been reported [12–14]. Fully lamellar structures, in particular, have shown the best creep performance as contrasted to other microstructural modifications [15–17].

For the intended application, considering the inherent brittle nature of TiAl alloys, material tailoring through microstructural evolution is often necessary. Additionally, the low ductility and brittleness of TiAl alloys at ambient temperatures make their processing using conventional methods difficult. To overcome problems associated with conventional processing, such as microstructural inconsistencies inherited from solidification and phase evolutions resulting in the scattering of mechanical properties, heat treatment cycles are often designed [18–21]. Traditional methods requiring post-treatment are time-consuming, labour and capital intensive, waste a lot of start-up material, and require unnecessary production costs. Therefore, there is a need to manufacture TiAl alloy components without the above-mentioned technical deficiencies and limitations and satisfy industrial needs for component fabrication [22].

For the last decades of the 20th century [23], the Additive Manufacturing (AM) method has been employed to obtain objects by the subsequent material supply. AM mainly aims to complete a collection of traditional subtractive manufacturing practices while avoiding and limiting the need for post mechanical processing such as machining. Laser powder bed fusion (L-PBF) is an AM technique, historically referred to as Selective Laser Melting (SLM) developed by F&S Stereolithographietechnik GmbH with Fraunhofer ILT [24], where a component is manufactured by melting a powder bed in a layer-by-layer sequence employing laser beam irritation [25]. The L-PBF process is initiated by creating a 3D digital part model (usually scan data or a CAD file), followed by slicing the model into thin layers using special software. The powder bed is achieved by spreading powder onto the substrate surface. The powder bed is selectively melted through cross-sectional scanning generated from the 3D part model by applying a laser beam. After crosssection scanning, powder bed layering is achieved by sequentially adding layers one after the other repeatedly until the part is complete. Recent studies [25–29] have shown that L-PBF is an innovative and efficient process employed to manufacture TiAl alloys compared to historically employed traditional manufacturing processes such as casting [30–32], ingot metallurgy [33–35], or even solid-state powder sintering [36–38]. The benefits of L-PBF include short production cycles and cheaper production costs. Also, parts produced are of high quality and have been found to exhibit desirable performance [39].

Exploring AM technologies to improve on properties of TiAl and its alloys is essential. As such, mechanical properties like compressive and tensile ductility

*Hot Corrosion and Oxidation Behaviour of TiAl Alloys during Fabrication by Laser Powder Bed… DOI: http://dx.doi.org/10.5772/intechopen.100345*

measures [40–42], wear resistance [43, 44], elevated temperature creep and oxidation resistance [45–48] superior to those processed by conventional means have been reported. Operation temperatures in new-generation gas turbines have fast-tracked progress in material development in the aerospace industry.

The dual combination of high temperatures and contaminant-containing aircraft environments shifts focus to hot corrosion and oxidation. Hot corrosion and oxidation can lead to catastrophic failures through material consumption at an unpredictably rapid rate. Much work has been devoted to understanding the hot corrosion and oxidation of TiAl alloys already [49–53]. As such, this research paper serves as a summary of the laser additive manufacturing of TiAl alloys. Particular attention is also given to the mechanisms, kinetics, prevention control and recent developments in hot corrosion and oxidation of TiAl alloys.

#### **2. Titanium aluminide (TiAl) alloys: phase, microstructures and mechanical properties**

#### **2.1 Phase and microstructural evolutions**

#### *2.1.1 Phase evolutions*

The three main phases of the Ti-Al system consist of various TiAl compounds, namely, γ-TiAl, α2-Ti3Al and TiAl3 [1]. Of the three phases, only γ-TiAl and α2-Ti3Al have shown to be of engineering significance [54] with outstanding properties. They are lightweight and can be implemented for structural parts, automotive and elevated temperature aerospace applications. The γ-TiAl phase is a face-centred tetragonal ordered phase with an L10 structure. It consists of atomic layers at 90° to the c-axis [55] with lattice parameters a = 0.4005 nm, c = 0.4070 nm and a tetragonality ratio (*c*/*a*) of 1.02 [56, 57]. The compositional range of the γ-TiAl phase is from 48.5 to 66.0 at.% of Al. The α2-Ti3Al phase has a hexagonal DO19 structure with a compositional range from 20 to 38.2 at.% of Al.

The α2-phase has high hydrogen and oxygen absorption rates and suffers from severe embrittlement, though it exhibits optimum high-temperature strength. The γ-phase has low gaseous absorption rates, outstanding oxidation resistance and poor room-temperature ductility. To maximise engineering benefits, dual-phase TiAl alloys consisting of γ + α2 phase are used. These alloys show excellent ductility [13, 58] at room temperatures due to the availability of refined lamellar colonies aiding γ-phase deformation [54, 59, 60]. The most known dual TiAl alloys with outstanding tensile properties are referred to as duplex alloys of the nominal (at.%) composition of Ti-(46–49) Al.

#### *2.1.2 Microstructure-mechanical property relations*

The four significant microstructures which may result in a Ti-Al system are namely, duplex (DP), near-gamma (NG), nearly lamellar (NL) and fully lamellar (FL). The obtained microstructures are greatly dependent on the processing route, Al compositional variations and thermo-mechanical treatments employed. Of the four, only fully lamellar and duplex have been considered necessary in engineering applications [54]. The evolutions (in **Figure 1**) of the microstructures mentioned above were be summarised in works by Cobbinah et al. [6] and Clemens et al. [61].

NG microstructures are obtained via thermal treatments slightly above the eutectoid temperature (Teu), while DP microstructures are achieved between Teu and α-transus temperatures. The thermal treatment implemented significantly

#### **Figure 1.**

*The central portion of the binary Ti-Al phase diagram together with microscopic optical (left) and backscattered scanning electron (right) images showing NG, DP, NL/NL*γ *and FL microstructures achieved via heat-treating within* α *and (*α *+* γ*) phase-field. The phases obtained are identified using contrast, where a light contrast is representative of* α*2-Ti3Al and* γ*-TiAl of a darker contrast [61].*

affects the volume fraction of lamellar grains present. As a result, NL microstructures are obtained at (Teu) and Tα relative temperatures, slightly under Tα. NL microstructures exhibiting a specified globular γ-grain volume fraction are shown as NLγ. FL microstructures are achieved by thermal treatments above Tα. Generally, the obtained properties compensate for other properties [22] as represented in **Figure 1** and should be considered when the material is designed for structural applications. Furthermore, the microstructure-property relationship in TiAl alloys makes it easier to modify the material for the anticipated application.

#### **3. Additive manufacturing (AM) of TiAl**

#### **3.1 Process overview**

Additive manufacturing (AM) presents an opportunity to manufacture TiAl alloys with minimal processing difficulties compared to those experienced during conventional processing, such as near-net-shape forging or investment casting [62]. For tailoring TiAl alloys with optimum properties, laser powder bed fusion (L-PBF) and electron beam melting have been considered suitable [63–66]. Recently, the production of TiAl alloys using L-PBF has gained special attention [29, 67–71] owing to the benefits offered. Some of these benefits [6] complex geometry formation, ease of part dimension control, production of highly defined parts with orifices, mass customisation and material flexibility. Furthermore, during local melting of the powders, high solidification rates are obtained. These result in more refined microstructures.

The component is manufactured (in **Figure 2**) by melting a powder bed in a layer-by-layer sequence employing laser beam irritation [25]. The process is initiated by creating a 3D digital part model (usually scan data or a CAD file), followed by slicing the model into thin layers using special software. The powder bed is achieved by spreading powder onto the substrate surface. In preparation for part

*Hot Corrosion and Oxidation Behaviour of TiAl Alloys during Fabrication by Laser Powder Bed… DOI: http://dx.doi.org/10.5772/intechopen.100345*

**Figure 2.** *Graphical representation of laser powder bed fusion method [72].*

manufacturing, powders are preheated below their melting temperatures to promote bonding and minimise distortion [6]. L-PBF part manufacturing is executed in an inert gas (preferably argon) sealed environment to prevent reactive powder oxidation.

#### **3.2 Research milestones**

The need to replace previously used Ni-based superalloys in aerospace components has fast-tracked research and development of lightweight and cost-efficient TiAl alloys. To date, Ni-based alloys still outmatch TiAl alloys in fabrication costs and mechanical performance. This is mainly due to the poor room temperature ductility of TiAl alloys and the delay in engineering design practices for low ductility materials [54]. Additionally, the high part fabrication costs involved in producing TiAl alloys are related to the knowledge that low ductility fabrication processes, which also produce high melting point alloys, are unavailable. As such, there has been much investment in exploring complex part fabrication techniques, requiring minimal post-processing steps such as L-PBF.

The evidence of many research breakthroughs concerning the production of TiAl alloy parts using L-PBF does not make the processing technique immune to limitations. Efforts have been invested in overcoming processing limitations such as part cracking, micro-pore formation and uneven powder deposition through processing parameter optimisation [73]. Processing parameters can be varied to develop TiAl alloys with excellent mechanical properties in application. Some of these properties are beam size, laser power, scanning speed, scan hatch spacing and powder layer thickness [74].

Polozov et al. [75] confirmed that TiAl-based alloy crack-free samples could be built via L-PBF processing with a high-temperature platform preheating of 900°C. Fully densified samples (highest relative density of 99.9%) were attained at volume energy density 48 J/mm3 . The refined microstructure consisted of equiaxed grains, lamellar α2/γ colonies and retained β-phase. As compared to conventionally produced TiAl alloys, high ultimate compressive strength and strain values were obtained.

Process parameters can be optimised to aid the fabrication of TiAl specimen, and unfortunately, the resultant part still shows pores, cracks and low densities. One needs to understand the crack and pore formation mechanisms and the defectprocess parameter relationships in such a case. Shi and associates [70] investigated

optimal L-PBF process window and the effect of substrate preheating. Moreover, the relationship between crack formation, pore formation, and the process parameters was studied and the crack propagation discrepancy with an increase in the number of deposition layers. It was concluded that crack formation was related to process parameters and the number of deposition layers. The cracks initiated in the 3rd layer are accounted for by residual stress accumulation and the deviations in the composition of Ti-47Al-2Cr-2Nb deposition layers. Furthermore, substrate (Ti-6Al-4 V) preheating at 200°C alleviated cracking. Finally, a good metallurgical bond between the substrate and Ti-47Al-2Cr-2Nb deposition layers was found.

The addition of yttrium (Y) to TiAl alloys (specifically class TNM) and process parameter optimisation dramatically affects the formability, and ultimately the cracking behaviour and control of L-PBF produced components. Gao et al. [76] fabricated TNM alloys with varying Y contents (0, 1, 2, 3, 4 wt.%) and investigated the mechanism of improved formability, cracking sensitivity, cracking behaviour and control mechanism by Y additions. Improvements in the formability of Y added-TNM alloys were assigned to lower melt viscosities and good laser energy absorption. The addition of 2, 3 and 4 wt.% Y to the TNM alloys coupled with a laser energy density greater than 7.00 J/mm2 formed crack-free samples. The obtained microstructure and phase constituents were reported to contribute to microcrack formation and control significantly. Lower Y additions resulted in coarse columnar grains, oxygen segregation at the grain boundaries with dominating brittle B2 phase with poor ductility. In contrast, higher Y additions (2–4 wt.%) refined equiaxed grains, enhanced the oxygen-scavenging effect (through the presence of Y2O3 particles), and decreased brittle B2 phase content at higher Y additions significantly improve the ductility.

Finally, adding Nb to γ-TiAl alloys was also reported to account for improved mechanical properties based. Ismaeel et al. [77] produced Ti-Al–Mn–Nb alloys on a TC4 substrate and studied the effects of different Nb contents on the microstructure and properties of the alloys. The phases obtained consisted of γ-TiAl and α2- Ti3Al and a consecutive microstructural change with increased Nb additions from near full dendrite to near lamellar. Also, adding 7 at.% of Nb resulted in improved alloy's hardness, strength and plastic deformation. Moreover, the elevated temperature oxidation resistance and tribological properties were significantly improved.

#### **4. Hot corrosion**

#### **4.1 Definition**

Hot corrosion can be defined as a chemical degradation on the metallic surface of materials operating at high temperatures, enhanced by the presence of molten ash and gases containing elements such as sulphur (S), chlorine and sodium [78]. Such environmental elements during fuel combustion promote damage to the protective oxide film by forming contaminants such as V2O5 and Na2SO4 [79]. This degradation form was initially identified in the early 1950s on combustion engines and boilers [80] and has been explored in numerous research works [50, 81–87].

#### **4.2 Characteristics**

Hot corrosion exists as Type I (known as High-Temperature Hot Corrosion) or Type II (Low-Temperature Hot Corrosion), with the former occurring above 800–950°C and the latter at 600–750°C [88, 89]. The occurrence of either attack form is dependent on several parameters such as the composition of the alloy,

*Hot Corrosion and Oxidation Behaviour of TiAl Alloys during Fabrication by Laser Powder Bed… DOI: http://dx.doi.org/10.5772/intechopen.100345*

contaminant, and gas. Furthermore, other vital parameters are temperature and temperature cycles, erosion processes and gaseous velocity [90, 91]. The main difference between high-temperature hot corrosion (HTHT) and low-temperature hot corrosion (LTHC) is the morphologies thereof. HTHC is distinguished by the occurrence of a non-porous protective scale, internal sulphidisation and chromium (Cr) depletion.

#### *4.2.1 High-temperature hot corrosion (HTHC)-type I*

This form of attack, also referred to as molten salt-induced corrosion, comprises a liquid-phase salt mixture deposit observed at high temperatures at the start of deposition [92]. Traditionally, according to Nicholls and Simms [93], HTHC has been detected in a temperature array between the surface deposit melting point and vapour deposition dew point for the deposit. Above this suggested temperature band, instability of dew point deposit exists, resulting in evaporation. A series of chemical reactions occur, initially attacking the oxide film and progress to deplete Cr present in the substrate [94]. Oxidation of the base material is then accelerated by Cr depletion, promoting a porous oxide scale formation.

An example of this could be the formation of thermodynamically unstable liquid sodium sulfate (Na2SO4) deposits. The marine environment mainly sources such deposits in sea salt form, followed by atmospheric contaminants such as volcanic discharges and fuel. During combustion, the present Na2SO4 can combine with pollutants present in air or fuel (such as chlorides, V and Pb) to form a blend of low melting temperature salts, further broadening the temperature range attack [94]. In the presence of sodium chloride (NaCl), the following reaction after combustion can be observed:

$$2\text{NaCl} + \text{SO}\_2 + \text{O}\_2 = \text{Na}\_2\text{SO}\_4 + \text{Cl}\_2 \tag{1}$$

HTHC can be classified into four stages from initiation up to failure [95]:

This corrosion damage is characterised by a uniform attack, internal sulphide phases, depletion zone beneath a relatively smooth scale–metal interface [80, 96].

#### *4.2.2 Low-temperature hot corrosion (LTHC)-type II*

Type II corrosion has been reported [97–102] as a liquid-phase deterioration by a blend of molten nickel (Ni) or cobalt (Co)-containing sulphates such as Na2SO4- CoSO4 or Na2SO4-NiSO4 accountable for corrosion initiation and propagation. The corrosion initiation is achieved through oxide layer fluxing, while propagation is accelerated by the mass movement of reactive elemental components through liquids present in the corrosion pits [80]. Studies [103–106] have shown that conversion from CoO and NiO occurs when SO3 in the gas reacts with the sulphates, attributing to the extensive usage of mixed Na2SO4-NiSO4 in recent LTHC research studies.

LTHC can be found in coated or uncoated compressor and turbine parts. For instance, the sometimes turbine blade's uncoated internal cooling systems operating at temperatures of about 650–750°C may be prone to this corrosion type [107]. The external rim of uncoated turbine blades reaches temperatures of 400–800°C [108]. LTHC is distinguished by the pit's appearance and the absence of a sulphide zone at the corrosion front, consuming all the S [96].

#### **4.3 Mechanisms**

Two HTHC mechanisms have been proposed, namely sulphide-oxidation and salt fluxing mechanisms [94]. Acidic and basic fluxing reactions, presented initially by Goebel and Pettit [109, 110], may be obtained and rely on the compositions of the alloy, oxide and underlying coating [93]. According to this model, fluxing occurs due to the decomposition of oxides into corresponding cations and O2− (known as acidic fluxing) or oxides with O2− forming anions (referred to as basic fluxing).

In acidic fluxing, oxide ions are donated to the deposit melt through dissolving the oxide scale [93]:

$$\text{MO} = \text{M}^{2+} + \text{O}^{2-} \tag{2}$$

Acidic environments in molten deposits can be developed through two main processes, namely, alloy-induced and gas-phase acidic fluxing. Basic fluxing is achieved through the production of oxide ions in a Na2SO4 deposit. Such is obtained by removing S and oxygen from the residue through reactions with the alloy or underlying coating. Subsequently, the oxide scales (e.g., MO) produced can react with the oxide ions through reactions [93]:

$$\text{MO} + \text{O}^{2-} = \text{MO}\_2^{2-} \tag{3}$$

A conventional model for LTHC was proposed by Luthra [111]. As suggested by the model, LTHC follows two stages, namely, formation of liquid-form sodiumcobalt sulphate and attack propagation through SO3 migration through the liquid salt. In nickel-based alloys, the mechanism suggested by Shih and associates [112] for LTHC is sulphidisation.

#### **4.4 Laboratory testing techniques**

An alloy's resistance to hot corrosion can mainly be determined using four standard tests: the electrochemical, crucible, accelerated oxidation, and burner-rig [94, 113]. The crucible tests remain the most highly ranked test for hot corrosion, simply consisting of either suspending, depositing, or completely immersing the testing sample in molten salts at elevated temperatures, as presented in **Figure 3**.

*Hot Corrosion and Oxidation Behaviour of TiAl Alloys during Fabrication by Laser Powder Bed… DOI: http://dx.doi.org/10.5772/intechopen.100345*

As far as TiAl alloys are concerned, less work has been carried out to understand the hot corrosion behaviour of such alloys [114–116].

Gas turbine environments can be precisely simulated by employing burner-rig tests [117, 118], shown in **Figure 4**. The salt is in aerosol or fog form and fuel oil/ air is introduced into the testing chamber to generate the test environment [119]. Simmons et al. [120] indicated that hot corrosion is an electrochemical process since hot corrosion consists of electrochemical reactions in which the molten salt acts as the conductive media or electrolyte.

#### **4.5 Prevention methods**

Some of the approaches used to prevent hot corrosion include maintaining both fuel purity and composition, properly selecting structural alloys, employing coatings, cleaning hot parts and air filtering [94].

#### *4.5.1 Fuel purity and composition*

Initiation and propagation of hot corrosion are greatly affected by impurities such as vanadium (V), S, and various alkali earth metals [121]. This can be controlled by adding magnesium (Mg), Cr, barium and calcium to the combustion fuel

#### **Figure 3.**

*Configurations used in hot corrosion crucible testing [114].*

#### **Figure 4.**

*Burner rig hot corrosion test schematic representation where (a) is an illustrates the experimental setup for Na2SO4(g) exposure, (b) is an image of the specimen plate for Na2SO4(g) tests in a crucible with the salt container and (c) is an ex-situ salt hot corrosion schematic diagram setup for experimental studies [119].*

to decrease corrosion rate. The presence of zinc (Zn) in the form of anodes in the combustion fuel or as part of the protective coating can significantly reduce the occurrence of LTHC. According to Hancock and associates [122], Zn drastically reacts with chloride ions (i.e., when excess NaCl is available) and transfers the chloride to the gas-salt interface to transform to chloride gas via sulphidisation.

#### *4.5.2 Proper alloy selection*

The addition of Cr to superalloys has effectively reduced the occurrence of hot corrosion [123]. Historically [121, 124], Cr (15 wt.% for Ni-based and 25 wt.% for Co-based alloys) has been added to superalloys to reduce HTHC. Much related to TiAl alloys, Garip and Ozdemir [125] studied the effect of Cr, Mo and Mn on the cyclic hot corrosion behaviour, and subsequently reported the beneficial effects of Cr and Mn additions on the hot corrosion properties of the investigated samples. Cr's effect on corrosion resistance is attributed to the ability of Cr to form Cr2O3, stabilising the chemistry melt, preventing reprecipitation of the protective oxide scale. Contrarily, increased Cr additions to superalloys can compromise the hightemperature strength and ductility [113] by forming TCP phases. The alloy and oxide film adhesion has been reported to be improved by the addition of zirconium, yttrium, scandium, cerium and lanthanum [113]. Silicon (Si), platinum (Pt), hafnium, Ti, Al, and Nb [126] were also found to increase resistance to hot corrosion.

#### *4.5.3 Protective coatings*

Such as diffusion, overlay and thermal barrier (TBCs) coatings can be used on relatively resistant alloys to combat hot corrosion. An alloy's surface enrichment by Al, Si or Cr achieves diffusion coatings. Various aluminide diffusion coatings (i.e., PWA70, MDC3V, PWA62, TEW LDC2, Elbar Elcoat 360 and Chromalloy RT22) have been developed and can be alloyed with Pt to improve cyclic oxidation at high temperatures [127]. Overlay coatings, commonly referred to as M (base metal)–Cr–Al–Y coatings, are designed for LTHC and HTHC surface protection. Overlay coatings with low Cr-high Al coatings are used for HTHC protection, while high Cr-low Al coatings are used for LTHC [94]. TBCs protects the substrate from gaseous flow caused by heat and consist of an external ceramic usually zirconia) and an oxidation-resistant bondcoat overlay. Other coatings include intelligent coatings like RT22 (Pt-aluminide) and Sermetal 1515 (a triple-cycle Si-aluminide treatment), have been reported [127].

Inexpensive alternatives include oxide-based glass and glass–ceramic coatings [128, 129]. Oxide-based glass and glass–ceramic coatings exhibit a remarkable combination of properties such as excellent chemical inertness, high-temperature stability and superior mechanical properties, which effectively can mitigate deterioration caused by hot corrosion. The introduction of halogens on the surface of the alloy encourages the preferential formation of aluminium halides at elevated temperatures. The aluminium halides are then converted to thin, continuous, and protective alumina oxides. Fluorine provides the best oxidation protection [130]. Further examples of surface modifications coating and methods studied on γ-TiAl alloys include magnetron sputtering [131], laser cladding [132], sol–gel [133], pack cementation [134], chemical vapour deposition [135], slurry [136], ion implantation [137].

#### *4.5.4 Cleaning hot parts and air filtering*

Motoring washes can be flooded with plain water [121] to prevent hot corrosion using specified procedures in the maintenance manual for the specific engine model. Also, high-efficiency filters can be used to filter out air containing high sodium contents [138].

*Hot Corrosion and Oxidation Behaviour of TiAl Alloys during Fabrication by Laser Powder Bed… DOI: http://dx.doi.org/10.5772/intechopen.100345*

#### **4.6 Hot corrosion studies for TiAl alloys**

Although much work has been devoted to understanding the hot corrosion kinetics of Ni-based and Co-based superalloys, TiAl alloys emerged to have sparked much interest in recent years [1, 56, 57, 139]. Historically, reported works utilised alloys produced using conventional methods; however, more attention has recently shifted to AM routes [70, 73, 140–146]. Despite much devotion to improving structure–property relations of TiAls, little work has been reported on the hot corrosion of additively manufactured TiAl.

Garip and Ozdemir [147] produced an alloy to the nominal at.% composition of Ti-48Al-10Cr using electric current activated sintering and studied the hot corrosion kinetics of the alloys in Na2SO4 salt for 180 h at 700–900°C. A severe hot corrosion attack was observed at 900°C (refer to **Figure 5**), with a porous and loose layer consisting of Na2Ti3O7, TiO2, Al2O3 traces of TiS phase.

In a study led by Xiong et al. [67], bare alloys TiAl, TiAlNb, and Ti3AlNb, were severely damaged after exposure at 750°C in (Na, K)2SO4 + NaCl melts as compared to those coated with enamel or TiAlCr. The corrosion mechanism was described to be much related to self-catalysis of sulphidisation and chlorination of metallic components. The initial mass loss observed is due to chloride volatility via metallic component chlorination. Of the alloys investigated, TiAlNb exhibited the best corrosion resistance due to adhesive Al2O3 enriched scale formation. Lastly, the degradation acceleration of sputtered TiAlCr coating was reported to be due to the chlorination of Cr and Al.

Additions of Nb and Si to traditional TiAl coatings were found to improve the hot corrosion resistance of a Ti-6Al-4 V alloy. In the stated work, Dai et al. [148] investigated the corrosion mechanisms on a mass loss basis following exposure at 800°C in a 75 wt.% Na2SO4 + NaCl salt mixture. Increasing single Nb additions deteriorated the hot corrosion resistance of the coating. Comparatively, increasing single Si additions continued to improve hot corrosion resistance. However, additions of both Nb and Si simultaneously showed better resistance to corrosion than single element additions. The corrosion protection of both Nb and Si (as seen in **Figure 6**) was related to SiO2 and Al2O3 formation in the initial stages of hot corrosion. Secondly,

#### **Figure 5.**

*Cross-sectional SEM images showing oxide scale microstructures with EDS analysis points represented in at.%, after hot corrosion exposure at (a) 800°C and (b) 900°C for 180 h [147].*

**Figure 6.**

*Representative hot corrosion model of TiAl-xNbySi coating where (a) illustrates TiO2 and Al2O3 formation and (b) shows an acidic dissolution of TiO2 to form sodium titanates including NaTiO2 and Na2TiO3 [148].*

Si additions were reported to promote the formation of a Na2O-Al2O3-TiO2-SiO2 enamel, hindering contact between the corrosive media and the oxide scales.

Tang et al. [149] studied the effect of enamel coatings on γ-TiAl against hot corrosion at 900°C. The enamel coating remained stable in the (Na,K)2SO4 melts, thus effectively protecting it against hot corrosion attack. Silicon-based coatings have also been shown to protect TiAl alloys. Rubacha et al. [150] evaluated the hot corrosion resistance of silicon-rich coated Ti-46Al-8Ta (at.%) alloy in NaCl, Na2SO4 and a mixture of the two salts. The formation of an amorphous SiO2 layer with TiO2 (rutile) and α cristobalite crystals enhanced the hot corrosion resistance of the TiAl alloy. Furthermore, Wu and colleagues [151] studied the hot corrosion resistance of a SiO2 coated TiAl alloy in 75 wt.% Na2SO4 + 25 wt.% NaCl salt mixture at 700°C. The enhanced hot corrosion resistance of the TiAl alloy was attributed to the formation of a compact and adherent amorphous SiO2 embedded with Na2Si4O9 and cristobalite. The incorporation of Si in aluminide coatings has also provided longterm oxidation protection of γ-TiAl alloys at temperatures of 950°C by forming a continuous and uniform α-alumina oxide scale [152].

#### **5. Oxidation**

#### **5.1 Definition**

When metallic materials are exposed to elevated temperatures in air, oxidation occurs, resulting in the formation of oxide scales. The crystal structure of the individual metals significantly affects the oxidation rate of high-temperature applicative parts [153, 154].

#### **5.2 Oxidation behaviour in TiAl alloys**

The following reactions may occur when TiAl alloys are subject to an oxidising environment:

$$\mathbf{1}\bigvee\mathbf{2}\mathbf{T}\mathbf{i}\_{(\iota)} + \mathbf{O}\_{\mathbf{2}(\varrho)} = \mathbf{T}\mathbf{i}\mathbf{O}\_{(\iota)}\tag{4}$$

$$\text{TiO}\_{\text{(s)}} + \frac{1}{2} \Big/ \text{O}\_{\text{2(g)}} = \text{TiO}\_{\text{2(s)}} \tag{5}$$

*Hot Corrosion and Oxidation Behaviour of TiAl Alloys during Fabrication by Laser Powder Bed… DOI: http://dx.doi.org/10.5772/intechopen.100345*

$$\text{2Al}\_{\text{(s)}} + \text{\textsuperscript{3}\text{O}} \Big/ \text{O}\_{\text{2(g)}} = \text{Al}\_{\text{2}} \text{O}\_{\text{3(s)}} \tag{6}$$

The ultimate oxidation resistance of alloyed TiAls is achieved by forming protective Al2O3, Cr2O3 and SiO2 scales due to their outstanding thermal stabilities. In contrast, the unfavourable formation of porous TiO2 with a high crack tendency is often observed [153]. Cobbinah et al. [155] found that 4 and 8 at.% Ta additions to Ti-46.5Al alloy promoted the significant formation of a consistent, non-porous Al2O3 layer at the metal-oxide boundary. Additionally, the layer operated as a diffusion barrier and preceded to outstanding oxidation resistance of the TiAl alloys.

In a study by Pan et al. [156], a comprehensive understanding is provided of the role of alloying on the oxidation resistance of TiAl alloys. Protection was related much to the formation of Ti3Sn layer diminishing oxygen diffusion inwardly, promoted by Sn additions. Moreover, spallation resistance was enhanced by the Al2O3 oxide pegs providing a mechanical locking. The effect of cathodically electrodepositing a SiO2 film on the oxidation resistance of a TiAl alloy was studied [157]. After 900°C exposure in air, the resultant alumina- and silicon-enriched glass-like oxide scale (in **Figure 7**) was reported, preventing oxygen diffusion leading to remarkably decreased alloy oxidation rates.

Surface modification of TiAl alloys via anodising has sparked interest in many high-temperature oxidation studies [158–161]. For instance, the oxidation behaviour and protection mechanisms of a TiAl alloy were studied by anodising in a methanol/NaF solution and produced an aluminium (Al)-and fluorine-enriched anodic film [162]. After 100 h exposure at 850°C, no evidence of cracking and spallation was displayed on the surface. The enhanced high-temperature oxidation resistance is mainly attributed to the halogen effect, generation of Al2O3 and oxidised Al–F species inhibiting external oxygen diffusion. Much effort has been devoted to developing coatings for γ-TiAl alloys, summarised in an evaluation by Pflumm et al. [130]. Amongst many available coating methods, Si-modified aluminide coatings produced via pack cementation have gained popularity. One such study [81] demonstrated that a continuous α-Al2O3 scale remained adherent after exposure to a temperature of 950°C for 3000 h.

#### **5.3 Oxidation kinetics of TiAl alloys**

When a metal operating at elevated temperatures is exposed to air, an oxide scale forms. As oxide scale formation proceeds, the metal's weight change can be plotted against time. Several laws such as linear, parabolic, logarithmic or cubic can be observed when studying oxidation kinetics [163]. In as far as TiAl alloys are

**Figure 7.** *Representation of a* γ*-TiAl alloy coated with E-SiO2 film (a) and after thermal oxidation test (b) [157].*

concerned, either linear or parabolic oxidation kinetics prevail. While the former offers no protection against high-temperature oxidation, the latter promotes diffusion-controlled oxide scale formation, improving much on the oxidation resistance of the base material. Parabolic oxidation follows and obeys the following law:

$$\left(\stackrel{\Delta m}{\bigvee A}\right)^2 = k\_p t \tag{7}$$

where ∆*m* = change in weight (in mg), *A* = surface area (in cm2 ), *t* = time (in sec) and *kp* = parabolic oxidation rate constant (in mg2 .cm−4.sec1 ).

The optimum oxidation protection governed by the parabolic law often results in a thick and continuous TiO2 and Al2O3 scale. As such, Swadźba et al. [48] investigated the short-term oxidation behaviour of a TiAl 48–2-2 alloy produced by AM at a temperature range of 750–900°C in air. At 900°C, a non-porous scale consisting of TiO2, Al2O3 and nitrates, exhibiting parabolic oxidation (in **Figure 8**), was observed.

Garip [164] likewise studied the oxidation kinetics at 900°C in air for 200 h for TiAl alloys produced via pressureless and resistance sintering. Both alloys exhibited a nearly parabolic oxidation response, with oxidation rate constants of the pressureless sintered alloy of 0.6391 mgn cm−2n h−1, 1.8 times higher than that of the alloy compacted using resistance sintering. Multi-layered oxide scales consisting of TiO2 and Al2O3 were obtained.

#### **Figure 8.**

*Mass change against time plots for (a) oxidation rate constant of the AM produced TiAl 48–2-2 alloy and (b)–(c) the power-law constant – n extrapolation [48].*

*Hot Corrosion and Oxidation Behaviour of TiAl Alloys during Fabrication by Laser Powder Bed… DOI: http://dx.doi.org/10.5772/intechopen.100345*

#### **5.4 Effect of alloy modifications on the oxidation resistance of TiAl**

Oxidation protection offered by forming a continuous Al2O3 scale followed by a multilayer of TiO2 + Al2O3 is limited, unfortunately, to the maximum service temperature of ~830°C. Above this temperature, the protection potential presented by the oxide scales formed severely deteriorates, limiting the high-temperature application potential for structural components [165]. The current trend in research is to improve the oxidation resistance of TiAl through alloy modifications.

Nb is one element used in many research works [86–90] to improve the oxidation resistance of TiAl alloys. Al activities are promoted by Nb additions and accelerate protective Al2O3 oxide film formation, limiting oxygen diffusion into the alloy [166]. Also, the α2 phase present in TiAl alloys is significantly decreased by Nb additions, decreasing its oxygen solubility [54]. Although Nb was primarily used for improving oxidation resistance [167], other high-temperature properties such as strength and creep resistance have been enhanced by the presence of Nb.

The creep resistance and the oxidation resistance of TiAl and its alloys can be enhanced by adding Si. The oxidation improvement is said to be achieved through the refinement of TiO2 particles, inducing refined and compact TiO2 scales on the surface [165]. Moreover, Si promotes Al diffusion into the oxide scale, stabilises Ti, reduces Ti4+ ions and impedes external Ti4+ ions diffusion [168].

The effect of adding molybdenum (Mo) alone to TiAls to improve on hightemperature oxidation is minimal. The protection of Mo-containing TiAl alloys is through the formation of inner oxide layers of TiO2 and Ti2AlMo near the substrate surface [165]. Unfortunately, Mo additions cannot alter the external oxide film formed (i.e., comprises of loose and porous TiO2 scales) and its characteristics. It is recommended in practice that the improvement of high-temperature oxidation cannot be derived from adding Mo alone; instead, the combination of Mo with other alloys can have a beneficial effect on the alloys' resistance to oxidation [169].

Cr additions promote the formation of Cr2O3 oxides, which act as mass ion transport barriers [170], enhancing oxidation resistance. In addition, the Al content existing in the alloys can be significantly suppressed by Cr additions, promoting the formation of Al2O3 scales. Oxygen diffusion at elevated temperatures can be accelerated by Cr3+ ion doping in titanium oxide, improving oxygen vacancy concentration. Contrarily, the doping effect may impair the TiAl alloy's oxidation by making Ti4+ interstitially occupying TiO2 sites, improving the potential energy with a noticeable decrease in diffusion activation energy, encouraging the diffusion of Ti4+ in TiO2 [171].

Zirconium (Zr) additions can also enhance oxidation properties by altering the characteristics of the oxide formed during the primary stages of oxidation and promote oxide grain nucleation [172]. As a result, the refinement of the oxide particles occurs, which can hinder oxygen diffusion. Rare earth metals have been reported to enhance the oxidation resistance of TiAl alloys. As discussed in detail in a research paper by Dai et al. [165], the protection mechanism is contributed by grain refinement, substrate purification, oxide adherence improvement and promotion of Al selective oxidation.

#### **6. Conclusions**

The need for materials to give excellent mechanical properties under high temperatures and extreme conditions such as TiAl is in demand. The use of such alloys would mean a reduction in pollution and noise levels for aero-based engines due to improved thermal efficiencies. There are challenges in producing such alloys using

the conventional arc and induction melt casting techniques due to the extremely high melting temperatures of the alloys. The AM route, particularly L-PBF, presents an opportunity to produce such alloys. What is critical in such trials is the operating parameters during processing. This has a direct influence on the performance and mechanical properties of the alloys so produced. Hot corrosion and oxidation of TiAl alloys are of great concern in gas turbine engines. Hot corrosion can be classified into HTHC and LTHC, with particular reference to mechanisms and characteristics. Protection control methods may result in fewer catastrophic failures. The hot corrosion process must be either totally prevented or detected early to avoid catastrophic failure. A sound understanding of oxidation mechanisms and kinetics of TiAl alloys makes it easier to tailor oxidation-resistant alloys by alloy modifications.

### **Acknowledgements**

This research work is based on the research supported wholly/in part by the National Research Foundation of South Africa (Grant number 130004).

### **Conflict of interest**

The authors declare no conflict of interest.

### **Author details**

Ntebogeng Mogale\*, Wallace Matizamhuka and Prince Cobbinah Vaal University of Technology, Johannesburg, South Africa

\*Address all correspondence to<EMAIL_ADDRESS>
© 2021 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

*Hot Corrosion and Oxidation Behaviour of TiAl Alloys during Fabrication by Laser Powder Bed… DOI: http://dx.doi.org/10.5772/intechopen.100345*

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### *Edited by Fahmina Zafar, Anujit Ghosal and Eram Sharmin*

Corrosion in materials is responsible for huge direct as well as indirect losses around the world. To address corrosion, a combinational approach involving molecular simulations of natural inhibitors, pre-structural designs, and the development of traditional but functional polymeric nanocomposites is recommended. This book presents the basics of corrosion from thermodynamic and kinetic points of view, discusses the major driving force behind corrosion, and provides insight into possible remediation techniques.

Published in London, UK © 2022 IntechOpen © z1b / iStock

Corrosion - Fundamentals and Protection Mechanisms

Corrosion

Fundamentals and Protection Mechanisms

*Edited by Fahmina Zafar,*

*Anujit Ghosal and Eram Sharmin*