Relation of Electron Configuration to Passivity in Cr-Ni-Fe Alloys

Abstract

Critical current densities for passivity were measured for single phase Cr-Ni-Fe alloys as a function of Cr content for 20%, 40%, 50% and 60% Ni compositions. Major discontinuities in slope approximate the ratio 12/88 for Cr to Fe in the 20 and 40% Ni alloy series, and 14/86 for Cr to Ni in the 50 and 60% Ni alloy series. These ratios correspond to observed critical compositions for passivity in the Cr-Fe and Cr-Ni binary systems. Additional discontinuities occur at the critical binary Cr to Fe ratio in the 50 and 60% Ni series. This behavior is interpreted in terms of separate electron interaction in the ternary system between Cr and Fe distinct from interaction between Cr and Ni. Flade or activation potentials become more active with increasing Cr content, corresponding to increased stability of passivity. In accord with greater stability, average time for breakdown of passivity increased exponentially with Cr content.

A Direct Measurement of the Activation Potential of Stainless Steels in Nitric Acid

“…5 seem to be very similar to what was found in other electrolytes. 11,16,18 Therefore, the E a of stainless steel in very concentrated nitric acid is comparable to sulfuric acid in terms of proton activity dependence.…”

“…Activation potential depending on pH.-Previous measurements of the activation potential demonstrated a proportional relationship between E a and the pH in sulfuric acid for several materials from pure iron to various types of iron-based alloys [12][13][14][15][16]19,29 AESEC measurements of E a for the Si-rich SS in different concentrations of nitric acid enabled assessment of this relationship. Measurements of the activation potential of the Si-rich SS are shown in Fig.…”

“…16,19 Once again, this method cannot be applied to nitric acid because the sample is spontaneously passive. It must also be noticed that using a voltammetric linear scan may have an impact on the mechanism of passivity breakdown in the case of chromium rich passive layers.…”

“…16,17 They lead to an establishment of a linear dependence between the activation potential and the logarithm of the proton activity. These measurements have mostly been performed in sulfuric acid using linear sweep voltammetry.…”

Anodic pickling of stainless steels in sulphuric acid

“…The second method consisted of evaluating the passivity retention through break-down times [8,9] of the passive film. Namely, the steel electrode was allowed to assume the free active corrosion potential in a deaerated solution of HzSO4 whose concentration was chosen as a function of steel reactivity: 0"01 N for Type 410, 4.6 N for Type 430 and 18 N for the austenitic grades.…”