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

TLDR: In this article, 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.

Abstract: 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.