Electron-Transfer Reactions on Passive Chromium

The kinetics of electron transfer between electrochemically passivated chromium and several outer-sphere redox couples have been investigated as a function of passive-film thickness, d = 5 to 25/~, in 1M H2SO4. The apparent rate of electron transfer decreased exponentially with increasing thickness of the passivating overlayer. Similarly, during potentiostatic film growth, the electron-transfer rate decreased logarithmically with time, t, as a consequence of'the film-growth kinetics, since d varies with log(t). These relationships indicate an electron-tunneling controlled process. However, variations in the film thickness and defects result in electron transfer occurring at distributed sites of enhanced tunneling probability, making a meaningful quantitative analysis difficult. Chromium plays an essential role in the corrosion resistance of many engineering alloys. Protection results from the formation of a thin, -5 -25 /~, passivating oxide film. The electrochemical characteristics of this electrode are determined by reactions involving both ions and electrons. The passive film blocks ion-transfer processes associated with metal dissolution. The kinetics of film formation indicate that ion transport across the protective film occurs by a high field mechanism. 1'~ To sustain the high field condition, the film must be electronically insulating. In contrast, the reversible potential and associated redox kinetics of the 3+ 2+ Fer ~ electron-transfer reaction have been readily measured on a passive chromium electrode. 3 The relatively facile nature of this reaction leads to the contradictory suggestion that the passive film is electronically conducting. '~'4 Theoretical studies address this question of film conductivity by invoking electron tunnel ing through the thin depleted semiconducting oxide Film? 6 Chromium with its native oxide has been used previously to construct solid-state tunnel ing junct ions] Similarly, the tunnel ing process has been used with varying degrees of rigor, to describe redox reactions occurring on oxidized tin, 8 iron, ~ nickel, 'q chromium, 9 stainless steel, 9 and SnO2) ~ However, no detailed electron-transfer studies have been performed on electrochemically passivated chromium. Much effort has been expended to resolve the influence of tunneling in heterogeneous electron-transfer reactions. Many of these studies have been hampered by complications arising from the probing redox couple (e.g., O.2/H20 on Pt ~1) or ambiguities associated with the tunnel ing barrier (e.g., pinhole defects in alkylthiol barriers on gold). ~2 The influence of the tunnel ing process should be most evident for films ranging from 5 to 30/~ in thickness2 '~ This corresponds precisely to the thickness range of the passivating film found on chromium. The rate of charge transfer across this barrier should reflect the electronic properties of the film as well as its thickness. Here we describe the kinetics of electron transfer between several simple outer-sphere redox couples and a chromium electrode as a function of passive film thickness. Overview of the electrochemistry of chromium.---Combined surface analytical and electrochemical studies have * Electrochemical Society Active Member. "Present address: NIST, 224/B166, Gaithersburg, Maryland 20899. b Present address: IGEN, Inc., Rockville, Maryland 20852. generated significant information concerning the passive state of chromiumJ '2 The characteristic i-E curve for chromium in sulfuric acid is given in Fig. 1. The active-passive transition, associated with a five-orders-of-magnitude decrease in reactivity, is apparent at -0.439 V. Active dissolution yields chromous ions, while passivation is associated with the formation of the blocking passive layer. A small cathodic current loop, due to proton reduction on the