CORROSION MORPHOLOGIES ON MAGNESIUM ALLOY AZ-91

Initiation and early propagation of pitting and filiform corrosion on bare alloy AZ91 (9% Al, 1% Zn) are investigated by natural immersion c orrosion tests, electrochemical measurements and microanalytical studi es. Initiation sites are few. Corrosion spreads from these sites first in the form of filiform corrosion for a limited period of time and pi tting which later develops into a cellular type of etching..The import ant factors affecting filiform corrosion are temperature, material str ucture and degree of polarization at the anodic sites. Filiform attack on AZ91, unlike the classical mechanisms of filiform corrosion on coa ted metals, is driven by hydrogen evolution reaction on the cathodic s ites of the surface, occurs under significant anodic control, propagat es at a high, constant speed independent of degree of polarization alo ng preferential paths determined by compositional and crystallographic factors, and is a temporary phenomenon under open circuit conditions. Pitting corrosion is more predominant with decreasing anodic polariza tion.