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 p re-pitted samples of Al-Zn-ME alloy AA-7039 has been investigated using pot entiodynamic polarization experiment, open circuit potential (OCP) transien t 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- io n-free chloride solution during SSRT, but mechanical crack develops in CrO4 2- 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 CrO4 2- ion to chloride solution: which is ascribed to the formation of protecti ve chromium oxide film on the surface. This is responsible for the hindranc e to pit-to-SC crack transition of AA-7039 in CrO42- ion-containing chlorid e solution. Based upon the experimental results, it is suggested that the o ccurrence of a stress-assisted dissolution just after the mechanical exposu re of the pit bottom at the transition time t(tr) is a necessary condition for proposed pit-to-SC crack transition mechanism. (C) 2000 Elsevier Scienc e S.A. All rights reserved.