THE EFFECT OF POTENTIAL ON THE STRESS CORROSION CRACKING OF ALUMINUM ALLOY 7075

The effect of the potential on the stress corrosion cracking of an aluminum alloy was interpreted in terms of the potential of zero charge of the metal. Two techniques, the determination of the isoelectric point and capacitance measurement, were used to measure the potential of zero charge on aluminum in various electrolyte solutions. Anion adsorption onto the hydrated alumina surface to form a complex ion shifts the isoelectric point of alumina to the acidic range. The capacity values in sodium salt solutions were stable and low due to the presence of the oxide film. Double bolt loaded cantilever beam specimens were used to study the anion effect and electrical effect on stress corrosion cracking (SCC) of AA 7075-T651. Increasing chloride ion concentration up to about 0.6M increased the SCC plateau growth rate, presumably by increasing the activation of the aluminum surface. At higher concentrations the cracking was lower, perhaps because of lower solubility of oxygen in the solution. 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 aluminum alloy 7075 to ammonia, though this is slow in concentrated nitrate solutions; perchlorate ion was not observed to be reduced to soluble chloride. In general, the substances which activated 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 effect of electrical potential on SCC of AA 7075 in 1N NaCl solution yields a minimum crack growth at -0.900 V/SCE. This is probably due to the formation of the stable but soluble complex ions in the vicinity of the isoelectric point. 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.