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 chemica l cerium nitrate treatment of stainless steel has been investigated. T he aim of the work was to study the role of cerium in modifying the ox ide layer properties, especially the kinetics of the cathodic reaction s. For this, electrochemical and surface analytical studies were carri ed out. During exposure to hot (90 degrees C) cerium nitrate solution, oxide film formation by chromium passivation and an accompanying diss olution of iron oxide takes place, leading to an enrichment of chromiu m in the oxide layer. Further, insoluble cerium species are precipitat ed at the cathodic sites of the surface. The oxygen reduction reaction is inhibited on these films. The effect of the cerium treatment canno t be solely attributed to the formation of a chromium-rich oxide layer , since the cathodic reactions are more strongly inhibited on the ceri um-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 electroly te clearly show that the inhibition of the oxygen reduction reaction i s not due to a lower electron conductivity of the oxide layer. The cat hodic inhibition effect can be attributed to a high resistance against reductive dissolution. This is partially due to the chromium enrichme nt and in addition to the cerium precipitation at the weak sites of th e oxide layer which otherwise under cathodic polarization would lead t o reductive dissolution, thus providing current paths for electrons pa rticipating in the oxygen reduction reaction. Treatment parameters suc h as time, alloy composition, solution chemistry and potential during treatment were studied. Clearly, all factors leading to a maximum chro mium enrichment and/or cerium precipitation increase the cathodic inhi bition efficiency. (C) 1997 Elsevier Science Ltd.