CORROSION - The worst enemy

This page covers the following subjects: Causes of corrosion 
Types of corrosion 
Protection against corrosion 
Electrical potential of the metals 
The correct positioning of the anodes 
Many people think that with the use of stainless steel and reinforced plastic hulls now there is no need to worry any more about their vessel. But the modern materials have not managed to defeat our boat’s worst enemy, corrosion.

Causes of corrosion

In reality corrosion is a superficial deterioration which metals suffer due to electrical, chemical or mechanical phenomena.

The potencial difference between the metals and the existence of an electrolyte in contact with them causes one of the metals to act as an anode, suffering from ionization or deterioration. The deterioration, which is produced in a metal, is proportional to the flow of electrons it receives, and this in turn depends on the potential and the resistance of the metal. The most important agent of corrosion between two metals in contact, submerged in an electrolyte, is the potential, which depends on the natural chemistry of the anode and the cathode, the former being attacked or dissolved while the latter remains unaffected.

The surroundings, in this case seawater, can contain a number of chemical products more or less aggressive which, when combined with the change in temperature, attack various materials, corroding the surface or the structure. We find ourselves, therefore, faced with a chemical type corrosion which we must combat with the appropriate protection. The fatigue of the pieces, due to rubbing, abrasion or movement, reduces the protective layer and they deteriorate. This is also known as mechanical corrosion.

Types of corrosion

Not all corrosive phenomenons are identical due to the fact that there exist various types of corrosion. So we can distinguish:

Global corrosion

This type attacks all of the surface of the piece in a uniform manner. In general it is not a serious form of corrosion as the rust produced serves a protective layer to stop the corrosion advancing and weakening the attacked piece. 

Porous corrosion

This type generally attacks determined areas of the piece, forming cracks or fissures. It has to be classified as serious when the crack is produced in certain parts of the piece and especially if it appears in joints or grooves as it produces a fatigue in the metal which can cause its breakage.

There is no doubt that this is the most serious as it is impossible to know the depth the corrosion has reached in the damaged area.

Selective corrosion

Defects or defects in the alloy, causes the piece to become porous and finally yield. This type of corrosion is more commonly found in melted metals or alloys.

Protection against corrosion

All the problems related to corrosion must be considered from three fundamental aspects:

The first, and undoubtedly the most important, is the one concerning the plans, as the corrosion usually starts in the same moment as the design and construction of the vessel, in its different integral parts. So therefore it is important to oversee the positioning of the different metals, making sure that the correct screwing method is used and avoiding different metals from coming into direct contact.

Also the motors and all types of electrical installations demand the correct connection to the bulk. It is necessary that the radio equipment has its own earth connection consisting of a special plaque because it is a serious error to use a passing valve or other means whose deterioration could be vital for the security of the vessel. Another important factor is the adequacy of the paint and antifoulings with the metal of the hull and the stern-drives.

The second aspect to bear in mind regarding the corrosion, is the cause of the corrosion. In general terms, the causes are always due to the failure in compliance with the norms concerning the electrical installations of the boat, which permit the passage of current through vital parts of the vessel. The first thing to do is to check the bulk polarity of the diverse circuits, then it is necessary to verify if current is lost when various devices are used, measuring with a tester if the consumed current is superior than the current that should be used by each device.

You have to make sure that there are not any different metals in contact on board. Something that is rarely taken into consideration, but is very important, is the effect of the ventilation differential produced by humidity in hidden areas, which is a phenomenon that causes a high level of corrosion. This type of corrosion is frequent in the screws that go through wood or plastic and which allows humidity to penetrate due to a lack of adequate waterproofing.

Lastly, the third aspect is the protection. This protection is especially important in hulls where it has not been possible to avoid some of the causes of corrosion originated from the construction, for example the necessity to put two different metals very close to each other. The methods of protection against corrosion are based on the appropriate selection of an alloy (pure metals or with chrome and inhibitors) and an adequate structure (by reason of thermal treatment which eliminates internal tensions and homogenizes solid solutions), and also covering the surface with special materials.

For years the covering has been applied to the pieces susceptible to be attacked, such us the galvanization, but the treatment must be very well done in order to offer the necessary protection and besides, rubbing or knocks can deteriorate the layer leaving the corrosion to act under it, which is frequently more serious.

Another type of covering is paint which, if it is applied correctly and regularly, is one of the best protectors. However, for certain defined pieces, the anodic protection is the most efficient. As we will explain, it consists in sacrificing metals which can be controlled and substituted in favour of those metals to be protected.

Electrical potential of the metals

Electrical potential of some metals in salt water at 25 °C

Electrical potential in V

Sodium -2.71 
Magnesium -2.38 
Aluminium -1.67 
Manganese -1.05 
Zinc -0.76 
Chrome -0.71 
Iron -0.44 
Cadmium -0.40 
Nickel -0.25 
Tin -0.14 
Lead -0.13 
Hydrogen 0 
Copper +0.35 
Silver +0.80 
Mercury +0.85 

As we have seen, the most important corrosive phenomenon is the electrical type. It is especially with the use of modern metals that practically all corrosions that have to be combated in the sea are of this type.

When two metals are in contact via a liquid, a galvanic, or electrical corrosion is produced. The degree of this corrosion depends fundamentally on the difference in electrical potential existing between the two metals in contact. 

The lower (more negative) the potential of the metal, the easier it is corroded; equally, the greater the difference in the potential between the two contacting metals, the greater the galvanic corrosion between them, the metal with the lower potential being damaged.

The table accompanying this text indicates the electrical potential of the more interesting metals, which is produced when they are submerged in salt water at a temperature of 25°C.

The correct positioning of the anodes

To protect a certain piece we use metals with a negative electrical potential, and lower than the electrical potential of the metal of the piece, placed in contact with the metal to be protected. These metals that are used for protection, receive the name of ANODES.

Anodes are fabricated in different shapes and sizes, specifically built for use in vessels. As a general norm, zinc or aluminum are used for anodes to be used in salt water and magnesium alloys are used for boats navigating on fresh water or brackish water. All of the metal parts of the vessel must be in contact with an anode for which we use bolts and flexes or connection cables directly with the piece to be protected. These connectors have to have a cross section of at least 4 ó 5 mm.

Something to bear in mind, is the position of the anodes. The anodes must always run parallel to the longitude of the boat, because only in this way the maximum performance is obtained. The anodes and the points of contact with the pieces should in no circumstances be painted. Bolts or screws should be used to attach them to the objet to facilitate their replacement.

Amongst the elements which require special protection are:

Propeller and Transmission shaft: In the case of the steering shaft, a special anode must be used which has is to be placed about 3 or 4 mm from the support pad of the propeller. But, if the horn is metallic, the anode has to be placed near it; if it’s made from a non conducting material (nylon, rubber), the fixing bolt of the anode must be connected with the motor block.

Metallic rudders: These require a circular anode to be fixed in the center of the blade.

Metallic keels: To protect this part of the boat, an anode has to be placed on each side, held with bolts screwed into the keel.

Flaps of motor boats: An anode is fixed onto the surface of each flap, always longitudinally with respects to the boat. If the flaps are made of Aluminum the screws used to fix the anodes must be galvanized.

Any anode must be replaced without delay as soon as it shows signs of wear and it is essential that they are replaced when they reach 20% of their original weight.