Localized Corrosion of Amorphous Fe-Ni-Cr-P-B Alloys

Experiments were performed with a series of amorphous Fe-Ni-Cr-P-B alloys of varying Cr content to characterize their susceptibility to crevice corrosion. Potentiodynamic and potentiostatic polarization of freely exposed specimens was performed in simulated crevice solutions (1 M NaCl at pH 1-7, and 6% FeCl₃ at pH 1.4). Crevice corrosion experiments were conducted in an artificial crevice cell. Polarization in the simulated crevice electrolytes indicated that alloys containing 2 to 16 At.% Cr can passivate in even 1M NaCl, pH 1, and they do not experience significant pitting at potentials active to that at which oxygen evolution occurs. In agreement with previously published results, only a few At.% Cr were needed to confer passivity; even the alloy containing 2% Cr achieved a passive current density of only 10^-1 A/m² in 1M NaCl at pH 1, and it did not pit below about 1.20 V(SCE). Results indicated, however, that the alloys meet certain requirements for susceptibility to crevice corrosion and occluded cell corrosion in general; that is, they passivate at sufficiently anodic potentials, and the intensity of active dissolution increases with decreasing pH. The amorphous alloys were considerably more inert to corrosion than T304 and Incoloy 800 steels, suggesting a lesser susceptibility to crevice attack. Experiments with the artificial crevice cell confirmed that the amorphous alloys can indeed be forced to crevice corrode, but only at very anodic (transpassive) potentials. They are considerably more resistant to crevice attack than the crystalline Cr-containing alloys.