Effect of strain rate and corrosion products on pre-exposure stress corrosion cracking in the ZK60 magnesium alloy

Several recent studies indicated that the layer of corrosion products deposited on the surface of Mg alloys being immersed in a corrosive solution can be the key reason of the subsequent alloy's embrittlement in air. This phenomenon has been referred to as pre-exposure stress-corrosion cracking (PESCC). To clarify this suggestion and get a deeper insight into PESCC, in the present study, we investigate the effect of strain rate on the mechanical properties and fractographic features of the alloy ZK60 before and after pre-exposure to corrosive media as well as after pre-exposure and removal of corrosion products. It is found that the ductility of pre-exposed specimens increased with strain rate in contrast to that of the reference unexposed specimens. This confirms that the mechanism responsible for PESCC should be time-dependent. Furthermore, it is concluded that this mechanism is associated with corrosion products layer containing sealed corrosive media rather than with diffusible hydrogen, which has been long believed to be the root cause of PESCC. The presence of retained corrosive media sealed within the corrosion products layer is confirmed by the manifestation of corrosion products on the edge of the fracture surface as well as inside the secondary cracks of the pre-exposed specimens tensile tested in air. Mechanisms of the crack growth responsible for PESCC of the alloy ZK60 are discussed comprehensively.