Water content affecting the passivation of nickel in water—methanol solutions

Abstract

The passivation of Ni has been studied in acid deaerated methanolic solutions at different water content employing potentiodynamic and potential decay techniques. The concept of kinetic effective water content was used to describe the relationship between passivation potential, acid concentration and added water. It is shown that the passivation potential and the critical current density can be expressed as: ▪ where C H 2O is equal to ( X H 2O ) 2/( X H 3O + ),and X H 2O and X H 3O + are the water and protons mole fraction respectively. The relation experimentally found for E p well describes also the passivation behaviour of nickel reported in the literature by many authors both in water and water—methanol mixtures of sulphuric acid. By using a potential-dependent passivation reaction in which the free water molecules are directly involved in film formation, it is possible to deduce the experimentally found effective water content dependence of E p. Furthermore, from the open-circuit potential decay curves of passivated Ni sheets in water solution of sulphuric acid at +800 mV vs sce, two arrests of potential were found. The first potential arrest was dependent on the specimen—environment contact time while the second was found stable with time and very reproducible. Similar experiments were also carried out in methanol—water mixtures at the same sulphuric acid concentration. In this last case, only one arrest potential was observed whose dependence on the Kinetic water content was as the relation reported above.