When two dissimilar materials are connected electrically and immersed in a conductive liquid, an electrolyte, their corrosion performance might differ significantly when compared with the uncoupled metals. As a rule, the less noble material (chemically active), the anode, is more severely attacked, whilst the more noble metal (chemically inert), the cathode, is essentially protected from corrosion. The corrosion attack is normally most evident close to the junction of the two metals. This phenomenon is called galvanic corrosion.
The degree of galvanic effects, as described above, will depend largely on the nature and kinetics of the electrochemical reactions taking place on the surfaces of the two materials forming a galvanic couple. Other factors that influence galvanic corrosion are the difference in the nobility of the two metals, the surface area ratio between the two metals and the conductivity of the solution.
The relative nobility of different conducting materials in a certain environment is indicated by the so-called galvanic series. Such series are valid for one specific environment and are based on measurements of the open circuit potential (OCP) of each material, uncoupled, in that environment. The higher the OCP, the more noble the material. The smaller the difference in OCP between the two metals forming a galvanic couple, the lower the driving force for galvanic attack. It should be noted that changes in electrolyte composition and temperature could cause significant changes in the positioning of different materials in a galvanic series. As long as stainless steels stay passive, they are in most environments nobler than other metallic construction materials and thus form the cathode in most galvanic couples. Galvanic coupling to stainless steels might, on the other hand, increase the corrosion rates of less noble metals such as mild steel, galvanized steel, copper and brass. Galvanic corrosion between different grades of stainless steel is generally not a problem, provided that each grade would be passive if exposed uncoupled in the particular environment. Galvanic corrosion can be prevented by the use of insulated flanges and isolation spools, but such insulators may cause crevice corrosion in chloride solutions.
9. Galvanic series in flowing seawater, 10°C.
A small anode-to-cathode surface area ratio causes an increased corrosion rate of the anode and should thus be avoided. The coating or painting of a less noble material, galvanically coupled to an uncoated stainless steel, should also be avoided, as very high corrosion rates could be obtained on small anodic areas formed where coating defects occur. Coating the more noble metal in a galvanic couple is, on the other hand, an effective way to reduce the risk of galvanic attack.
The conductivity of the electrolyte affects the intensity as well as the location of the attack. A low conductivity tends to reduce the corrosion rate, but the attack can become very concentrated at the area adjacent to the contact site between the two metals.
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