Influence of anodic coatings on stress corrosion behaviour of 7017 aluminium alloy

Abstract

Purpose – The purpose of this work was to study the cracking susceptibility of a 7017 aluminium alloy, after anodising under various conditions. Design/methodology/approach – Slow strain tests in dry air, laboratory air and sodium chloride solution were employed. Anodic oxide films were produced with various applied current densities and thicknesses, in horizontal or vertical orientation of the coatings, at the free corrosion potential and also at various anodic or cathodic potentials. For the interpretation of the results, a metallographic study of the specimens before and after straining to failure was carried out using a scanning electron microscope. Findings – The behaviour of anodic coatings was found to depend very much on the anodising conditions. The coatings reduced the ductility of the alloy in dry air but can actually increase the ductility in laboratory air and in 3.5 per cent sodium chloride solution. In most cases, the ductility of coated specimens was greater in 3.5 per cent NaCl solution than in dry air, possibly due to crack blunting by the aggressive environment. Anodic coatings moved the free corrosion potential of the alloy in the noble direction and both the anodised and the bare alloy generally suffered a reduction in ductility at potentials anodic or cathodic to the free corrosion potential, the fall being more rapid for the anodised alloy. Research limitations/implications – The mechanism causing the increased ductility of coated specimens in 3.5 per cent NaCl solution than in dry air remains yet to be confirmed. Practical implications – The selection of suitable anodic coatings for the protection of aluminium alloys against stress corrosion cracking depends on the anodising conditions. Originality/value – The paper provides information regarding the influence of anodising conditions on the anticorrosive properties of electrolytically prepared anodic coatings on aluminium alloys.