A MEASURING METHOD FOR THE PITTING POTENTIAL AND REPASSIVATION POTENTIAL IN CREVICE CORROSION OF ALUMINUM-ALLOYS

The pitting potential of commercially available alloys has been measured at chloride ion concentrations of 0.01 to 1.0 M at 25-degrees-C. Following is the dependence of the pitting potential at 25-degrees-C on chloride ion concentration for aluminum alloys 1100 (UNS(1) A9100), 3004 (UNS A93004), 5052 (UNS A95052), 5182 (UNS A95182), and 5082 (UNS A95082): [GRAPHICS] The Cu2+ ion method was used to measure pitting potential, and a high degree of correlation was shown to exist with the critical pitting potential obtained by the potentiodynamic method previously used. At chloride ion concentrations below 0.03 M, the values determined by the potentiodynamic method show considerable scatter, but when the Cu2+ method is used, reproducibility is good. Repassivation potential was also successfully measured for crevice corrosion (E(Rcrev)) of commercially available aluminum alloys using a method developed for stainless steel. E(Rcrev) of aluminum alloys was less noble by 200 to 250 mV than the pitting potential, confirming that crevice corrosion is the more frequently occurring type, even in aluminum alloys. With commercially available Al-Mg alloys, both pitting potential and crevice corrosion potential became less noble the greater the amount of magnesium, and it was verified that they were more prone to local corrosion.