The role of molybdenum in the dissolution and the passivation of stainless steels with adsorbed sulphur

Abstract

The effects of pre-adsorbed sulphur on the dissolution and the passivation of Fe17Cr12.6Ni (100) and Fe17Cr14.5Ni2.3Mo (100) alloys in sulphuric acid were studied. Sulphur was pre-adsorbed at high temperature in H 2H 2S gas. The surface concentrations of sulphur were measured with radioactive sulphur (the 35S radioisotope). The results demonstrate that adsorbed sulphur catalyses the dissolution of the alloys, but the effect is much less pronounced on the molybdenum-containing alloy. It is shown that the presence of molybdenum induces a desorption of sulphur from the surface at the corrosion potential. The kinetics of this molybdenum-induced desorption of sulphur was investigated. The mechanisms of this effect are discussed and compared with the mechanisms which have been reported previously for NiMo alloys.