A surface analytical and electrochemical study on the role of cerium in the chemical surface treatment of stainless steels

The mechanism of oxide layer formation and modification during chemical cerium nitrate treatment of stainless steel has been investigated. The aim of the work was to study the role of cerium in modifying the oxide layer properties, especially the kinetics of the cathodic reactions. For this, electrochemical and surface analytical studies were carried out. During exposure to hot (90 °C) cerium nitrate solution, oxide film formation by chromium passivation and an accompanying dissolution of iron oxide takes place, leading to an enrichment of chromium in the oxide layer. Further, insoluble cerium species are precipitated at the cathodic sites of the surface. The oxygen reduction reaction is inhibited on these films. The effect of the cerium treatment cannot be solely attributed to the formation of a chromium-rich oxide layer, since the cathodic reactions are more strongly inhibited on the cerium-treated stainless steel than on passivated pure chromium. Moreover, the cerium treatment is efficient in retarding the cathodic kinetics on pure chromium. Studies with a redox couple present in the electrolyte clearly show that the inhibition of the oxygen reduction reaction is not due to a lower electron conductivity of the oxide layer. The cathodic inhibition effect can be attributed to a high resistance against reductive dissolution. This is partially due to the chromium enrichment and in addition to the cerium precipitation at the weak sites of the oxide layer which otherwise under cathodic polarization would lead to reductive dissolution, thus providing current paths for electrons participating in the oxygen reduction reaction. Treatment parameters such as time, alloy composition, solution chemistry and potential during treatment were studied. Clearly, all factors leading to a maximum chromium enrichment and/or cerium precipitation increase the cathodic inhibition efficiency.