A modified surface charge approach to the film growth and dissolution of iron in concentrated phosphoric acid

Abstract

Rotating ring-disk voltammetric, photo-electrochemical, and impedance measurements were performed to study the film growth and dissolution of iron in concentrated H3PO4. The only species released in the current plateau region is Fe(III), and its release consumes ca. 90 % of the total current passing through the disk. This means that most of the charge passed is consumed for the dissolution of Fe through the oxide film, and the film formation efficiency is very low. Photocurrent spectroscopy allows the identification of the anodic layer with doped ferric oxide. Potential dependence of photocurrent is governed by a Poole-Frenkel mechanism of electronic carrier generation. An updated version of the so-called surface charge approach to the mechanism of conduction of anodic oxide films is proposed to explain the obtained impedance spectra. It is based on the assumptions that interstitial cations are the main ionic charge carriers and that the field strength in the barrier layer is constant. A negative surface charge built up at the film/solution interface via accumulation of cation vacancies accelerates interstitial cation transport, thus explaining the pseudo-inductive behavior of the iron/film/electrolyte system under small amplitude perturbation.