Intergranular Corrosion

Definition - What does Intergranular Corrosion mean?

This is a corrosion type that attacks the boundaries of the metal crystallites, as opposed to attacking the surface of the metal. Intergranular corrosion can also be referred to as intergranular attack under a condition known as grain boundary depletion.

Metals and alloys, like other elements, have micro-structures that can be described as grains. Metals can contain multiple grains, and these are separated by a grain boundary. Intergranular corrosion can be defined as an attack along the boundaries of several grains in the metal or near the grain boundary with the largest portion of the grain remaining unaffected.

Corrosionpedia explains Intergranular Corrosion

The relation of corrosion to the grains can be explained in terms of element segregation. When an element adequate for the resistance of corrosion is lost, either from the boundary or the zone adjacent to it, it creates a condition where the spot becomes an anode with reference to the rest of the grain. Corrosion then proceeds along the affected grain boundary and may cause grains to dislodge due to the boundary deterioration.

Stainless steels and weld decay sensitization are the best examples of intergranular corrosion. Grain boundaries that are rich in chromium elements will precipitate lead. This makes the boundaries very vulnerable to corrosion attacks in various electrolytes. This is caused by reheating the part that has been welded, especially in multi-pass welding.

In the process of intergranular corrosion, a knife-like attack, a form of intergranular corrosion, can occur when carbon reacts with niobium, titanium or the austenitic stainless steels. Carbides form in the areas close the welded part, making it difficult for them to diffuse. This condition can be corrected by reheating the part to enable the carbides to diffuse.

Aluminum-base alloys are mostly affected by this corrosion due to two main reasons. The first is when the phases anodic to aluminum are along the grain boundaries. The second is due to the depleted parts of copper which are adjacent to the boundaries. Other alloys that are prone to intergranular corrosion are those that have elongated and flattened grain micro-structures. These are the heavily worked on alloys or those that have been extruded.

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