The effect of additions of carboxylic acids to the anodising bath on the corrosion and SCC of anodised aluminium

Purpose - The purpose of this paper is to study of the effect of carboxylic acids additions to the anodising bath on the subsequent corrosion and stress corrosion cracking (SCC) tendencies of anodised 1050 Al-Alloy in 3M NaCl solution. Design/methodology/approach - The study was carried out using SCC tests and electrochemical cyclic potentiodynamic measurements at a high or a slow scan rates. The anodic coatings were prepared electrolytically in a bath of 4 M H2SO4, with and without additions of 0.015 M oxalic, malonic, tartaric, maleic, or citric acids. The consequent thicknesses and packing densities of the coatings were measured. Findings - The SCC behaviour was found to vary with both anodising conditions and stress level. The addition of carboxylic acids in the anodising bath increased the protective properties of the coating. In corrosion conditions without stress, the addition of the carboxylic acids decreased the susceptibility to pitting corrosion, the effect depending on the presence or absence of corrosion products. The addition of the carboxylic acids during anodising leads to the formation of less porous and more compact oxide layers and increases the anticorrosive properties of the coatings. The benefits were most pronounced for the coating prepared in the presence of maleic acid. Research limitations/implications - The belief that the effect of carboxylic acids on the corrosion and SCC behaviour of Al-Alloys is due to the absorption of these compounds on the metal surface and the formation of complexes on it, depending on the structure and carbonic chain of these compounds, remains yet to be confirmed. Practical implications Anodic coatings prepared in an electrolytic bath containing carboxylic acids can be used for the protection of aluminium alloys against corrosion. Originality/value - The paper provides information regarding the improvement of the anticorrosive properties of electrolytically prepared anodic coatings on aluminium alloys.