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The Diffusion Poisson Coupled Model (DPCM) is presented to modelling the oxidation of a metal covered by an oxide layer. This model is similar to the Point Defect Model and the Mixed Conduction Model except for the potential profile which is not assumed but calculated in solving the Poisson equation. This modelling considers the motions of two moving interfaces linked through the ratio of Pilling–Bedworth. Their locations are unknowns of the model. Application to the case of iron in neutral or slightly basic solution is discussed. Then, DPCM has been first tested in a simplified situation where the locations of interfaces were fixed. In such a situation, DPCM is in agreement with Mott–Schottky model when iron concentration profile is homogeneous. When it is not homogeneous, deviation from Mott–Schottky model has been observed and is discussed. The influence of the outer and inner interfacial structures on the kinetics of electrochemical reactions is illustrated and discussed. Finally, simulations for the oxide layer growth are presented. The expected trends have been obtained. The steady-state thickness is a linear function of the applied potential and the steady-state current density is potential independent.

Several indoor atmospheric corrosion layers (0–800 years old) were selected from different localisations in France. Each sample was scrapped from its iron substrate. The resulting powder was mixed with graphite in appropriate proportions and the mixture was pressed onto a stainless steel grid to constitute a composite electrode. The electrochemical responses of the different samples were recorded under galvanostatic regulation, in a near-neutral pH-buffered NaCl solution at 25 °C. The E – t reduction curves allowed the determination of two characteristic parameters, E τ /2 , the potential value obtained at half the transition time, and Q τ , the coulombic charge obtained at the end of the reduction. The diminution of E τ /2 and Q τ with the age of the corrosion layer showed that the “reduction reactivity” decreases with time, suggesting a progressive stabilisation of the corrosion layer. In a second part of the work, we synthesised several common ferric or ferrous/ferric products (goethite, lepidocrocite, magnetite, maghemite, ferrihydrite) and compared their reduction responses (product alone or mixture of 2 or 3 products) to those of corrosion samples.

The electrochemical reduction of lepidocrocite γ-FeOOH was investigated at 25 °C in neutral or slightly alkaline solutions containing chloride, sulphate or bicarbonate anions by means of thin lepidocrocite film electrodeposited on inert gold substrate or graphite/lepidocrocite powder composite electrode. Electrochemical measurements were coupled to in situ electrochemical quartz crystal microbalance (EQCM) and ex situ SEM and FTIR analysis. The reduction of lepidocrocite occurs in all the electrolytes considered here. The initial reduction product is adsorbed ferrous ion, F e ads 2 + . The desorption of F e ads 2 + is promoted as pH increases, leading to an increase of reduction depth. This promotion is related to the formation of secondary FeII-containing species, as revealed by SEM and FTIR. The comparison of γ-FeOOH reduction potentials and iron corrosion potentials let us state that the galvanic coupling is possible.