The Chemical Nature of Aluminum Corrosion: IV. Some Anion Effects on SCC of AA 7075-T651

Double bolt loaded double cantilever beam specimens were used. Increasing chloride ion concentration increased the stress corrosion cracking (SCC) plateau growth rate up to about 0.6M, presumably by increasing the activation of the aluminum surface; at high concentrations the cracking was slower, perhaps because of lower solubility of oxygen. No analyses or potential measurements were made inside the crack, but nominal external potential and pH values especially for benzoate and ammonium hydroxide indicated that a large potential drop must be obtained within the crack if a hydrogen mechanism is involved, at least in those solutions. The activity of water in aqueous solutions of various salts did not correlate with the SCC rates, indicating that specific ion effects dominated. The nitrate ion is reduced by 7075 to ammonia, though this is slow in concentrated nitrate solution; perchlorate ion was not observed to be reduced to soluble chloride. In general, the substances which activate the aluminum surface as by forming soluble complex ions with aluminum caused rapid cracking, though there were exceptions such as benzoate, which inhibited corrosion but promoted SCC. The rate of SCC in various commercial plates of 7075 was similar in some environments, but very different from plate to plate in other environments.