Tafel slopes and exchange current densities of oxygen reduction and hydrogen evolution on steel

The prediction and prevention of steel corrosion in engineering applications rely on the accurate understanding of kinetic parameters, such as the Tafel slopes and exchange current densities. These parameters show a large spread in literature. We investigated the dependency of these kinetic parameters on the measurement methodology for stainless and carbon steels, in a controlled rotating disk electrode setup with a near-neutral (pH 7.5) buffer solution. Consistent results were found for hydrogen evolution on stainless steel, with Tafel slopes of -0.13 to -0.15 V/dec and exchange current densities around 0.01-0.02 A/m². The studied oxygen reduction kinetics showed the largest dependency on the measurement methodology, especially the potentiodynamic scan direction. Supported by active light reflectance spectroscopy, the large observed variations were attributed to the influence of an oxide film, which may overshadow the oxygen reduction at small over-potentials. The obtained variation gives insight on the accuracy of documented and measured values.