Effect of pH on susceptibility to environment-assisted cracking of AZ61 Mg alloy

Abstract

This research aimed to understanding the effect of pH on susceptibility to environment-assisted cracking (EAC) of AZ61 Mg alloy. In order to keep solution pH, borate or carbonate buffer solutions were employed as test solutions. The solution pH was adjusted from 8.3 to 11.2 by controlling concentration ratio of bicarbonate to carbonate. In addition, a borate buffer solution of pH 8.0 was prepared. The specimen and the test solution were set to a constant load EAC testing apparatus and then a dead weight was applied to the specimen. Initial stress was 170 MPa, corresponding to 0.2 % proof stress. During the EAC test, strain, corrosion potential and solution pH were measured spontaneously. The solution pH was constant during the EAC test. The specimens tested in the solutions of pHs from 8.0 to 10.5 exhibited typical creep curves of first, second and third stage, followed by failure due to EAC. The fracture time increased as the solution pH raised to 10.5, and no EAC was obtained in the solution of pH 11.2. The corrosion potentials of all the specimens shifted higher during the test. A final corrosion potential shifted higher as the solution pH raised. Some fine transgranular cracks were observed on the surface, and the crack mode was irrespective to the solution pH.