Effect of Oxide Film Properties on the Kinetics of O₂Reduction on Alloy C-22

Abstract

The kinetics of oxygen reduction have been studied electrochemically on the Ni-Cr-Mo(W) Alloy C-22 in 5 M NaCl at 70 • C using potentiostatic and voltammetric techniques and electrochemical impedance spectroscopy. Oxygen reduction was observed to occur at low potentials on defective oxide films containing a high concentration of anion vacancies and appeared to proceed by a combination of 2-electron and 4-electron transfer processes. At more positive potentials a passive film comprising a Cr III -dominated barrier layer at the alloy/oxide interface and a Mo/W enriched outer layer at the oxide/solution interface was formed, and oxygen reduction was completely suppressed. Oxygen reduction could be revived after transpassive degradation of the oxide film at more positive potentials, and appeared to be catalyzed on the Mo VI /W VI surface film.

Investigating the Influence of Cr and Mo Additions to Commercial Ni-Based Alloys Exposed to Neutral and Acidic Chloride Solutions

“…This indicates that a breakdown event in a passive oxide would retain Mo(W) at the breakdown site at a rate dictated by the Mo(W) content of the alloy. This is consistent with the observations of Maurice et al 14 38 explored the kinetics of the O 2 reduction (in neutral 5 M NaCl at 70 °C) in relation to oxide film properties on alloy C-22. They found that higher rates of O 2 reduction occurred on films which had been damaged and subsequently repaired.…”

“…The increase in R f , accompanied by a decrease in C f , is consistent with an improvement in passive film properties due to its thickening and/or the elimination of point defects, with the latter reflected in a decrease in the dielectric constant of the film. 34,38 Overall, the impedance data presented here suggest that increasing Cr content to >15 wt.% results in only minor improvements in passivity. However, as is clear by inspection of Figs.…”

“…35 More complex circuits involving additional time-constants 2 have been proposed, including the need for diffusional (Warburg) elements to improve the fit of the equivalent circuit. 34,38 However, the inclusion of a Warburg element was not found to significantly improve the fit to the spectra in Fig. 2 and was therefore avoided.…”

Self‐Supported Hierarchical Shell@Core Ni3S2@Ni Foam Composite Electrocatalyst with High Efficiency and Long‐Term Stability for Methanol Oxidation

“…[10] Zhang et al reported that NiS has a high poison tolerance towards MOR in alkaline media due to the direct oxidation of methanol on the pre-oxidized NiS surface. [11] However, the performance of Ni 3 S 2 toward MOR has been rarely investigated. The investigation of Ni 3 S 2 towards MOR in alkaline media is needed.…”

Passivation of Ni-Cr and Ni-Cr-Mo Alloys in Low and High pH Sulfate Solutions

“…The SLDs of the inner layer on both alloys were close to the theoretical SLD value of NiO suggesting the oxide films on both alloys were rich in Ni, similar to previous work. 46,60,61 For Ni-Cr-Mo, the SLD of the outer layer decreased with increased potential suggesting a selective dissolution of Ni(II) in favor of Cr-enrichment as the potential increased (Fig. 12b).…”