Chapter C.3 - Intergranular corrosion

Abstract

Corrosion inside the metal can propagate in two ways: By transgranular corrosion where there is no preferential corrosion of grain boundaries; and following preferential paths, corrosion progresses along the grain boundary, which is called intergranular corrosion (IGC). Unlike transgranular corrosion, intergranular corrosion consumes very little metal, so weight loss is not a good indicator of this form of corrosion. Intergranular corrosion is not always detectable with the naked eye. A micrographic examination is required. When it penetrates deeply, it adversely affects mechanical performance, in particular elongation, and can even cause parts to break. The corrosion located along the grain boundaries results from the difference in composition and structure between them and the matrix, because they do not have the same electrochemical properties. Intergranular corrosion is due to a sufficient potential difference between the grain itself and the grain boundary zone. The precipitate hardened aluminium 2XXX aluminium–copper and 7XXX aluminium–zinc–magnesium–(copper) series alloys are the most sensitive to intergranular corrosion. This is also the case for aluminium–magnesium 5XXX series alloys when the magnesium content is greater than 3 wt.%. Generally speaking, the susceptibility of heat-treatable alloys to intergranular corrosion depends on their microstructure and metallurgical states.