Protection of Aluminium Alloys against Stress Corrosion Cracking in Saline Water by Properly Oriented Anodic Coatings: IV. Influence of the temperature of the anodising bath

This paper is a continuation of work published as Parts I–III (BCJ, 1975, 10, 17; 1978, 13, 28; 1980, 15, 41) where a method was suggestedfor protecting Al–2·5 wt% Mg alloys against sec in saline water, using suitably oriented (normal orientation of the cells to the future stress direction) electrolytically prepared (in 15% H₂SO₄ at 25°c) γ₁-Al₂O₃(I). The results were optimised using a c.d. of 6 A/dm² to prepare γ₁-Al₂O₃ (I and II) and a thickness of 3·4 μm (III). In III a correlation between sorptive and/or catalytic properties of the γ₁ ⁻,γ_1,2 ⁻ and γ₂-Al₂O₃ and their protective properties was found. In the present work the anodising bath temperature was varied and maximum protective properties were found at 25°c for 3·4 μm and 13·5 μm thick γ₁-Al₂O₃ and at 40°C for 23 μm thick γ₁-Al₂O₃. Thus the optimum mean increase of the time to failure for 13·5 μm thick γ₁-Al₂O₃ is 72%, for 23 μm 100%, andfor 3·4 μm 220% compared with that for the unanodised surface. Explanations for this are suggested, based on the differing secondary structure of the oxide at different temperatures, as revealed by SEM.