Critical Potentials for Pitting Corrosion of Ni, Cr-Ni, Cr-Fe, and Related Stainless Steels

Critical pitting potentials for the binary Cr‐Fe and Cr‐Ni alloys in become more noble (correspondingly more resistant to pitting) with increasing content, particularly in the region 25–40% and 10–20% Cr, respectively. The potentials for 57.8% Cr‐Fe and pure Cr fall within the transpassive region. Ni containing 3.2% Mo, to the contrary, shows a lower (more active) critical potential; higher per cent Mo‐Ni alloys appear to fall within the transpassive region corroding anodically as plus Ni++ without pitting. Ni alloyed with 15% Cr‐Fe shifts the potential in the noble direction; Mo alloyed with 15% Cr, 13% Ni stainless steel has a similar but even greater effect. At 0°C, 15% Cr, 13% Ni stainless steel exhibits a potential 0.5v more noble than at 25°C, corresponding to greatly increased resistance to pitting. This shift is less pronounced for the stainless steels containing Mo; in fact, at or above 1.5% Mo, the critical potentials at 0°C are below those at 25° C. In , alloyed Mo shifts the potentials slightly in the active direction, contrary to a marked noble shift in . At 0°C the potentials are still more active. These trends correlate with observed pitting for 15% Cr, 13% Ni stainless steel and the similar alloy containing 2.4% Mo in 10% both at 0° and 25°C. Absence of pitting is observed in 10% at 0°C for 15% Cr, 13% Ni stainless steel, but not for the similar alloy containing 2.4% Mo, which pits. The over‐all results are explained on the basis of competitive adsorption at the metal surface and an effect of temperature on the structure of the double layer.