Exfoliation Corrosion of AlMgSi Alloys in Water

Abstract

An investigation of the causes and mechanisms of severe exfoliation corrosion damage in aluminum pipe irrigation networks utilizing fresh water has been conducted, making use of electron microscopy and microanalysis techniques, together with chemical and electrochemical methods. Pipes were made of (6000) series alloys, manufactured by hot extrusion processes, water cooled, and cold drawn. The exfoliation corrosion was a two‐stage process: an initial stage of pitting corrosion starting at the material surface and a propagation stage of the corrosion process parallel to the surface along the extrusion direction. The corrosion processes involved electrochemical anodic and cathodic reactions that were found to be governed by material factors of alloying and impurity element concentrations (e.g., copper, manganese, and iron) and of the presence, distribution, and composition of intermetallic particles in the alloy matrix. These particles (up to 10 μm in size), found in arrays or streaks along the extrusion direction of the pipes, served as efficient cathodic sites to the aluminum matrix, resulting in enhancing the initial stage of pitting, followed by preferential propagation of anodic corrosion paths (second stage), leading to directional attack of the aluminum matrix pipe.