On the Nature of the Occluded Cell in the Stress Corrosion Cracking of AA 7075-T651—Effect of Potential, Composition, Morphology

The SCC of AA 7075-T651 has been investigated in various electrolytes at different applied potentials. The rate of crack growth in the precracked, double cantilever beam, specimen was measured and related to the aluminum ion concentration and pH within the crack as well as the morphology of the crack wall surface as viewed with a scanning electron microscope. The rate of crack growth, the composition of the solution within the crack, and the morphology of the crack wall are determined mainly by the anion in solution and this in terms of the aluminum-anion complexes formed during the reaction. In sodium chloride solution, the pH at the crack tip was acidic at the open circuit and in the anodic potential range while, in the cathodic range, it was basic. At anodic potentials, the aluminum ion concentration reached 0.4M within the crack. In Na₂SO₄ solution, crack propagation was very slow at the open circuit and at anodic potentials even though the pH at the crack tip was acidic. However, when the potential was shifted into a cathodic range, the crack began to propagate at an appreciable rate. In NaNO₃ solutions, crack propagation rate increased in the cathodic range due to the formation of ammonia within the crack. Very slow crack growth was observed in the anodic range. The analysis of the solution within a simulated crevice indicated that the composition of the solution in a simulated crevice and an actual stress corrosion crack were quite similar.