Stress Corrosion Cracking of Powder Metallurgy (P/M) AlZnMg Alloys in Deaerated 3.5% NaCl

Stress corrosion cracking (SCC) of three wrought powder metallurgy (P/M) AlZnMg alloys differing in Zn content and heat-treatment was studied in deaerated 3.5% NaCl solution over a wide range of applied potentials, using the slow strain rate tensile technique (SSRT). Hydrogen absorption measurements, fractographic and metallographic observations, and energy dispersive spectroscopy (EDS) analyses of corrosion products were performed. In the range of anodic polarization potentials, the resistance to SCC, as expressed by the ratio of the elongation to fracture in NaCl to that in air, was less than in the range of cathodic potentials, and a slight cathodic polarization resulted in a decrease of the susceptibility to SCC, relative to that at the corrosion potential. At potentials more positive than the pitting protection potential, correlation was found between the susceptibility to SCC and pitting, as expressed by pit density. At high cathodic potentials, the resistance to SCC increased with decreasing Zn contents in the alloy.