Application of a Methodology Combining Predictive and Descriptive Models to Describe the Influence of the Temperature on the Passive Film Steady State

Passive materials are working in more and more severe conditions. By example, the development of the Proton Exchange Membrane Fuel Cell – PEMFC, leads to the used of stainless steel in higher temperature to improve the energetic efficiency. The main issue understanding the passivation process lies with the multiplicity of the phenomena modifying the behavior of the film. As describe into the Point Defect Model - PDM, the electrochemical reactions occurs at the metal/oxide interface and the oxide/solution interface. The reactivity is limited by mass transport through the oxide. Although, this phenomenon is slow because occurring into a solid phase, it could be observable by electrochemical impedance spectroscopy at regular frequencies because of a very thin oxide.

A methodology was developed to study this transport phenomenon combining the use of the PDM with a descriptive model to analyze experimental data from electrochemical impedance spectroscopy. The evolution of the main charge carrier density vs. the formation potential of the oxide is studied and according to the PDM the diffusion coefficient of the point defect through the oxide is determined. Knowing the thickness of the oxide, by means of X-ray Photoelectron Spectroscopy, the time constant of the transport is determined. Based on this value, a descriptive model is selected avoiding the over-parameterization of the EIS data.

In this way the combination of a predictive model and a descriptive one is used to objectively take into account the transport phenomena without using an over-parameterized model. This methodology is used in acidic solutions at several temperatures on two materials. Pure chromium is studied as model material and 316L as an industrial one. The conditions which leads to the consideration of the mass transport in the electrochemical impedance analysis are discussed. At higher temperature, the study of the mass transport seems to be necessary.