The effect of chromate addition to a chloride solution on crack growth in pre-pitted samples of Al–Zn–Mg alloy

The effect of chromate addition to a chloride solution on crack growth in pre-pitted samples of Al–Zn–Mg alloy AA-7039 has been investigated using potentiodynamic polarization experiment, open circuit potential (OCP) transient during slow strain rate test (SSRT), scanning electron microscopy (SEM), abrading electrode technique and ac-impedance spectroscopy. The addition of CrO42− ion to NaCl solution raised the OCP in value on the potentiodynamic polarization curve. The OCP transients and SEM fractographs revealed that the initiation of stress corrosion (SC) crack from pits occurs in CrO42− ion-free chloride solution during SSRT, but mechanical crack develops in CrO42− ion-containing chloride solution. From the analyses of data obtained by using abrading experiments and ac-impedance spectroscopy, it was found that the repassivation of newly exposed bare surface is enhanced by adding CrO42− ion to chloride solution, which is ascribed to the formation of protective chromium oxide film on the surface. This is responsible for the hindrance to pit-to-SC crack transition of AA-7039 in CrO42− ion-containing chloride solution. Based upon the experimental results, it is suggested that the occurrence of a stress-assisted dissolution just after the mechanical exposure of the pit bottom at the transition time ttr is a necessary condition for proposed pit-to-SC crack transition mechanism.