Electrochemical contact corrosion – what are the causes of corrosion of joints of different types of steel?

Contact between two types of metals with different electrochemical potential may lead to the so-called contact corrosion, also called galvanic corrosion. This phenomenon occurs when stainless steel is combined with a material that is less noble and more susceptible to accelerated corrosion – e.g. aluminum or carbon steel. Corrosion phenomena occur in areas where the electrolyte is present, with the strongest impact in places exposed to moisture. In practice, they lead to local oxidation of the weaker material, and thus its corrosion at the point of connection.

Electrochemical corrosion

Contact corrosion is one of the varieties of electrochemical corrosion, i.e. caused by electrochemical processes. Its occurrence is determined by the occurrence of various potentials on the surface of the corrosive object located in the electrolyte environment. Corrosion cells on the metal surface with a lower potential are anodes, where oxidation and transition to solution take place. The so-called reduction reactions on the cathodes of corrosion cells are responsible for a depolarizer, which may be molecular oxygen or hydrogen ions that can be reduced to hydrogen gas. Differences between the potentials of fragments of the corroding surface depend on the chemical composition of the connection, the type and density of defects, the type and amount of stress and the chemical composition of the electrolyte. Other factors, including physical and microbiological interactions, also influence the rate and direction of changes.

Galvanic corrosion

Different types of steel and their alloys are characterized by different electrode potentials. When two of them come into contact in an electrolyte, the more reactive metal acts as the anode and the less reactive metal acts as the cathode. Due to the electropotential difference between the reactions at the two electrodes, there is an accelerated attack on the anode metal and its dissolution in the electrolyte. As a result, corrosion of the metal in the anode occurs faster and corrosion at the cathode is inhibited. For contact corrosion to occur, both metals must be electromechanically different and must be in electrical contact with each other and with the electrolyte. The behavior of materials can be predicted by determining their corrosive potential. For example, a large anode surface in the form of carbon steel with a small cathode surface in the form of stainless steel means no corrosion at the connection, as well as relatively low corrosion of carbon steel. In the opposite situation, significant galvanic corrosion of the connector may occur. For this reason, it is not recommended to use carbon steel connectors in stainless steel elements. The large surface area of the former material relative to the latter will cause significant contact corrosion and thus accelerate corrosion of the joint. As a consequence, the corrosion resistance of the carbon steel element decreases significantly.

Examples of contact corrosion

Electrochemical contact corrosion very often occurs in multi-element industrial equipment, such as ship installations, water systems, bridges or traction networks. If their elements are located on damp ground, air or directly in water and are made of different metals, a contact link is formed at the joints. In such a situation, the less noble metal is destroyed, especially in the immediate vicinity of the connection. Screws, rivets or welds, which are made of a less noble alloy than the connected steel elements, can also become anodes of the cell. This phenomenon often occurs in the case of parts of structures made of the same materials, e.g. ship plating sheets or pipeline sections. The small surface of the connection elements in relation to the surface of the cathodes causes the current to be exceptionally high and accelerates the dissolution of the joint. If the bonding parts are made of a more noble metal, they become the cathodes of the cell. In the case of anodes with a very large surface, the rate of corrosion is lower.

Prevention of galvanic corrosion

To prevent contact corrosion, appropriate methods should be used to isolate metals with different electrochemical potential from each other. For this purpose, special plastic washers, insulating pastes and painting the elements with additional protective coatings are used. The galvanic potential is also reduced by corrosion inhibitors such as sodium molybdate or sodium nitrite. However, their use should be carefully monitored as they can increase the conductivity of the water in the system and increase the potential for corrosion. Factors taken into account when combining steel with other materials also include their acidity and alkalinity, i.e. pH. If its coefficient is incorrect, galvanic corrosion may accelerate. In most district heating, heating, ventilation and heating systems such connections are not used in gas plants – they significantly increase the risk of corrosion and also lead to the release of particles causing mechanical damage to various components – e.g. heat exchangers or circulation pumps.

Sources:

https://www.worldstainless.org/Files/issf/non-image-files/PDF/Euro_Inox/Contact_with_Other_PL.pdf

http://www.e-instalacje.pl/a/korozja-kontaktowa-3280.html

https://pl.wikipedia.org/wiki/Korozja_elektrochemiczna