Kinetics of iron oxidation, oxygen reduction and hydrogen evolution under controlled conditions

Background

The Tafel slopes and exchange current densities of reduction and oxidation reactions are important parameters in corrosion science. They are used in different contexts, varying from more fundamental electrochemical studies to corrosion modeling. They are generally determined by experimentally measuring so-called polarization curves. However, the scatter of these parameters between studies, for similar metals in similar environments, is significantly large. This applies in particular for iron / steel in pH neutral or alkaline solutions, where iron dissolution, oxygen reduction and hydrogen evolution can occur.

Part of this literature scatter possibly arises from the experimental methodology applied, namely influences of the measurement protocol and experimental conditions. A known limitation of measuring polarization curves is that they are influenced by changes caused by the measurement, for instance, by accumulation of reaction products of the electrochemical reactions at the metal surface, or by changing the metal surface itself. The extent of this effect on the measured polarization curve itself is, however, rarely addressed in the literature.

Aims and objectives

The aim of this project is to deliver reliable and reproducible values for the kinetic parameters of iron oxidation, oxygen reduction and hydrogen evolution under controlled conditions, in particular conditions that are representative for steel corrosion in concrete and soil. We also aim to study the extent of the influence of the experimental methodology on the measurements of the Tafel slopes and exchange current densities.

Methodology

Polarization curves are measured in a controlled environment: We use a rotating disk electrode (RDE) in a buffer solution and bubble the electrolyte with air or nitrogen to keep a stable and well-defined oxygen concentration in the solution. Iron and iron alloys are used as working electrodes, with well-controlled surface conditions.

The experiments are performed for varying rotation rates of the RDE, to analyze the effect of variable transport such as of corrosion products away from the metal surface. By varying the potentiodynamic sweep rate and the scan direction, the direct effect of the measurement method is investigated.