Protection of Aluminium Alloys against Stress Corrosion Cracking in Saline Water by Properly Oriented Anodic Coatings: III. Influence of γ 1,2 -Al 2 O 3 and γ 2 -Al 2 O 3 oxide thickness;correlation between sorptive (and/or catalytic) and protective properties of the various oxides

Abstract

This work is a continuation of Parts I (BCJ, 1975, 10,17) and II (BCJ, 1978, 13,28) published under the same title, in which a method was suggested for protecting Al-2·5 wt-% Mg alloy specimens against SCC in saline water by using suitably oriented (in a direction normal to the future stress direction) anodic films of γ₁-Al₂O₃ and its hydrate in the thickness range 27·1 μm down to 3·4 μm. The study has now been extended to cover thicknesses between 27·1 and 51·0 μm and 3·4 and 1·7 μm. The curves, thickness–time to break, for all stress levels between 27·1 and 36·0 μm and 3·4 and 1·7 μm (γ₁-Al₂O₃ are a continuationof those found in Part II; between 36·0 and 38·0 μm the curves are anomalous due to the formation of γ_1,2- Al₂O₃, which accelerates SCC; between 38·0 and 39·0 μm (γ₂-Al₂O₃ protective properties are shown but from 39·0 up to 51·0 μm (γ₂-Al₂O₃) corrosion begins to accelerate, slightly increasing with thickness; between 3·4 and 1·7 μm the protective properties of γ₁-Al₂O₃ decrease. The protective properties ofthe three oxides, which decrease in the order: γ₁-Al₂O₃ > γ₂-Al₂O₃ > γ_1,2-Al₂O₃, are inversely related to their sorptive and/or catalytic properties. Thus γ₁-Al₂O₃, prepared under strict anodising conditions (suitable orientation, 6·0 A/dm², 3·4 μm thickness), remains the best coating to give protection against SCC.