De‐alloying and dissolution induced cracking of the Zinc‐iron δ phase

Abstract

Abstract The preferential anodic dissolution of zinc from the brittle zinciron δ phase has been investigated in acidified sulphate solutions. From electrochemical measurements, light optical and scanning electron microscopy, electron microprobe analysis and X‐ray diffraction experiments it is deduced that this process results in the formation of considerable concentration gradients within the alloy phase that set up a local state of stress and thus induce the formation of cracks. By the interplay of de‐alloying and crack propagation the corrosion degradation of the δ phase is significantly enhanced. Diffusion driven phase transformations to form the iron‐rich intermediate phases, Γ 1 and Γ, were not observed. The ultimate corrosion morphology that results from the de‐alloying of the δ phase is rather a sponge structure which appears to consist of porous ferrite with a zinc concentration that considerably exceeds its room temperature solubility limit.