Influence of seawater composition on SCC behavior of anodized 1050 Al-alloy

Purpose - The purpose of this paper is to study the stress corrosion cracking (SCC) behaviour of anodized 1050 Al-alloy in marine environments at different concentrations of sulphate ions. Design/methodology/approach - The SCC experiments were performed by measuring the time to failure in 3.5% NaCl solution, or in the presence of three different concentrations of sulphate ions under conditions of applied anodic current. For the interpretation of the results, changes in potential during SCC tests and optical microscope micrographs of stress corrosion tested specimens at various periods of time, were obtained. Findings - The influence of seawater composition on the SCC behaviour of the anodized 1050 Al-alloy depends on the concentration of sulphate ions, the oxide thickness and the stress level. At the higher stress levels and low concentrations of sulphate ions, increased times to failure were observed. These results were attributed to the inhibitory action of sulphate ions. At a low stress level and higher concentration of sulphate ions, the increased times of exposure and the more intensive corrosive environment led to partial destruction of the anodic coatings and a decrease in the time to failure. Better protective properties were observed at an oxide thickness of 10 mu m. The thicker oxides did not protect so well because they were more brittle, cracking under strain and allowing corrosive species to reach the metal surface. Research limitations/implications - The hypothesised mechanism of the effect of seawater composition on the SCC behaviour of anodized Al-alloys depended on the concentration of sulphate ions and the stress level 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 composition of the marine environment. Originality/value - The paper provides information regarding the influence of sulphate ions on the anticorrosive properties of electrolytically prepared anodic coatings on aluminium alloys.