Corrosion of Mg alloy AZ91 – the role of microstructure

Abstract

The paper presents new findings concerning the role of microstructural variations in the localised corrosion of a magnesium alloy AZ91. The alloy with prior solution treatment (T4 condition), when subjected to aging treatments (T6 condition), developed intermetallic β phase (Mg₁₇ Al₁₂), which is cathodic to the alloy matrix, i.e. the primary α phase. Also, the β phase in the T6 materials was surrounded by Al rich α (often termed eutectic α). Corrosion of the T4 and T6 alloys was characterised by immersion and electrochemical tests. Microscopic and microtopographic examinations of the specimens from immersion tests suggested that Al rich α has an independent electrochemical identity, and undergoes extensive localised corrosion (anodic dissolution) under the cathodic influence of the adjoining β phase.