Passivation of Iron in Sulfate, Perchlorate, and Borate Solutions: Role of Borate in the Passivation Process

Abstract

The anodic passivation of Fe has been investigated in borate buffer, sulfate, and perchlorate solutions. In sulfate and perchlorate solutions of either pH 3.0 or 8.4, the passivation process is highly inefficient with very large amounts of anodic charge passing before a Substantial decrease in current is observed. In these solutions, passivation appears to be associated with precipitation of a salt film and/or changes in the solution pH near the surface which permits the formation of a passive oxide. The efficiency for potential-step passivation cannot be significantly increased by changing the concentration of the supporting electrolyte, the pH of the solution from 3.0 to 8.4, or the potential of anodization. However, the addition of <5% pH 8.4 borate buffer to the neutral sulfate or perchlorate solutions results in a dramatic increase in the efficiency, with the anodic passivation charge decreasing by as much as two orders of magnitude. Similar trends are observed in pH 8.4 Na2SO4 solutions with previously passivated Fe electrodes which were cathodically reduced to different extents prior to anodization. The more of the prior film that remains on the surface before the anodic potential step in sulfate, the more efficient is the passivation process. The marked similarity of these results suggests that the beneficial action of borate towards Fe passivation is due to its ability to assist in the formation of the surface oxide film. While the pH buffering capacity of the borate can be playing a role, it is possible that borate buffer contains strongly interacting anions which give it an inhibiting capability. The interaction of these anions with the Fe surface is believed to stimulate formation of a surface oxide film, and this could explain the results in the present work. Passivation of Fe in sulfate and perchlorate solutions would be inefficient because these anions, unlike those derived from borate, do not have inhibitive abilities, nor do their soIutions have buffering capacity. The results indicate the important role played by prior oxide films, and the need for weli+defined starting surface conditions in passivation experiments.Studies of the passivation of Fe have generally been carried out in acid sulfate or neutral borate buffer solutions. During the last quarter century, the latter solution has been preferred because controlled cathodic removal of prior oxide films and highly efficient anodic passivation are possible in this electrolyte (1, 2). Since little or no Fe dissolution occurs during anodic treatment, in borate buffer, roughening of the electrode surface does not occur, making subsequent analysis of the surface (by ex situ techniques such as RHEED, Auger, XPS, SIMS, etc.) much easier. The borate buffer solution has become a standard electrolyte for studying the anodic behavior of Fe. On the other hand, there has been concern about work in sulfate solutions because of the possibility of SO42-adsorption interfering with passivation and the considerable amount of dissolution (as Fe 2+) resulting in ...

Passivation of Permalloy Thin Films: II . In Situ Characterization of the Oxide Layer by Photoelectrochemical and Impedance Measurements

“…Fig. This value increases sharply when adsorption occurs, as is the case for the borate ions (37,45,46). In this frequency range the capacitance is obtained using a parallel plate capacitor model, assuming that the oxide behaves as a pure dielectric with a frequency independent dielectric constant.…”

The Impedance Response of a Passive Film Revisited by a Double Modulation Technique

“…It thus appeared that the MICTF technique should be able to provide information on the nature of a passive film by individualizing the various components of its capacitance. In this work, we report on the response of the passive film formed on iron in phosphoric acid solution, passive iron in acidic media being extensively investigated over the last century by plenty of dc, transient and ac techniques . A specific attention has been paid to the capacitive response of the electrode as a function of the potential so as to be able to provide a detailed analysis of the capacitance response with the MICTF technique and to compare it with the usual Mott‐Schottky plot.…”

Implications of the kosmotrope/chaotrope nature of the anions on the breakdown of passivity of iron by halides

“…The passivation and the growth of passive films on iron have been extensively studied [27,[33][34][35][36][37][38][39][40][41][42][43][44][45]. The choice of a pH 8.4 borate buffer solution has been found to allow the formation of a 'true' passive film [16,45].…”