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The concept of a critical potential below which pitting of 18–8 and other passive alloys does not occur in aqueous Cl− media is affirmed. Increasing Cl− concentration shifts the critical potential to more active values. The potential is shifted to more noble values by presence of other anions, e.g., ClO4− , SO4−− , NO3− , OH− , sufficient concentrations of which act as pitting inhibitors. Lowering of temperature similarly enobles the critical potential. The shift at 0°C exceeds the oxidation‐reduction potential for Fe++→Fe++++e accounting for resistance of 18–8 to pitting in FeCl3 solutions at ice temperature but not at room temperature. The critical potential is not affected appreciably in the acid pH range; it moves markedly in the noble direction in the alkaline range corresponding to observed resistance to pitting in alkaline Cl− media. These results are interpreted in terms of competitive adsorption of Cl− and other anions for sites on the alloy surface. Only at a sufficiently high surface concentration of Cl− is oxygen, making up the passive film, displaced locally, and passivity thereby destroyed resulting in a pit. The special behavior of NO3− inhibition and factors affecting reproducibility of measurements are discussed.

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Critical pitting potentials for the binary Cr‐Fe and Cr‐Ni alloys in 0.1N normalNaCl become more noble (correspondingly more resistant to pitting) with increasing normalCr content, particularly in the region 25–40% normalCr 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 MoO4−− 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 0.1N normalNaBr , alloyed Mo shifts the potentials slightly in the active direction, contrary to a marked noble shift in 0.1N normalNaCl . 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% FeBr3 both at 0° and 25°C. Absence of pitting is observed in 10% FeCl3 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.

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