Assessing residual stresses in the surface layer of the ZK60 alloy after an exposure to corrosion solution

Magnesium alloys preliminary immersed in a corrosion solution suffer from embrittlement, referred to as pre-exposure stress corrosion cracking (PESCC). It was suggested that PESCC can be attributed to the corrosion product film-induced stress (CPFIS), which is known to be responsible for SCC in many alloys. However, the internal stress associated with the formation of the corrosion products (CP) layer on Mg alloys have not been investigated as yet. Thus, in the present study, the internal residual stresses of the first and second kinds were assessed in the alloy ZK60 exposed to the corrosion solution, using the deflection of the thin plate and by the X-ray diffraction technique, respectively. It is found that the deposition of CP on the surface of the alloy ZK60 creates the compressive internal stresses of both kinds — I and II. The macro residual stress of the kind I in the thin plate is found to be not exceeding 3 MPa, while the micro residual stress of the second type in the surface layer of 20 – 30 µm can be as high as 290 MPa and cause plastic deformation of the bare metal with the internal stress which cannot be relieved by the removal of CP.