ROLE OF ADSORBED SULFUR IN THE DISSOLUTION AND PASSIVATION OF NICKEL AND NICKEL-SULFUR ALLOYS

The influence of sulphur on the dissolution and passivation of nickel in acidic medium was investigated. The amounts of sulphur in the bulk and on the surface of the metal were measured by using 35S radiotracer. A possible influence of the crystallographic orientation of the metal on the phenomena under investigation could be revealed by using nickel single crystals. On pure nickel the highest rate of dissolution was observed on the (110) face, and the lowest rate on the (111) face. The residual current after passivation was lower on the (111) face, in relationship with the epitaxial feature of the oxide NiO formed. The presence of a monolayer of adsorbed sulphur on nickel increases the rate of dissolution in the active region and extends the active region towards more anodic potentials. The formation of the passive layer requires the partial desorption of the adsorbed sulphur. When sulphur was homogeneously dissolved in the bulk of the metal, the following experimental results were obtained: (1) There is a critical sulphur concentration above which the active-passive transition disappears. This concentration depends on the crystallographic orientation. The higher the rate of dissolution of a face, the lower the critical concentration. It is ranging from 30 to 50 × 10-6 at a potential sweeping rate of 1 V/h. (2) The metal dissolution brings about a surface enrichment with sulphur, responsible for the inhibition of the passive layer formation. This enrichment is further developed beyond the monolayer coverage, until a thin film of nickel sulfide with a stationary thickness is formed. The metal dissolution takes place through this non-protective layer with a remarkably high current density. (3) Under certain conditions reported here a localised corrosion with epitaxial pits was observed on the low index faces. This corrosion is attributed to a preferential dissolution of the metal at the structural defects resulting in a sulphur enrichment that inhibits locally the formation of the passive layer. The formation of the passive layer NiO can also be inhibited when sulphur is no longer introduced into the metal but into the electrolyte, in the form of a soluble sulfide. In all studied cases the passivation of the metal can occur only if the sulphur concentration on the surface is below the monolayer coverage. © 1979.