A contribution to the study of the electrochemical behaviour of nickel in a methanol-water-sulphuric acid system

Abstract

The electrochemical and corrosion behaviour of nickel in a methanolic environment has been studied by many authors. By inspection of the results reported in the literature a comparability as well as reproducibility is hardly evidenced and no explanations are so far reported. On the other hand such discrepancies could be possibly interpreted in terms of a different electrode surface pretreatment used by those authors. The aim of this paper is to ascertain the influence of the experimental procedure as well as experimental conditions on the electrochemical behaviour of nickel. Experiments were performed on nickel sheets in MeOH + 0.1 N H 2SO 4 with a water content of 1%, 3% and 10%, respectively, as function of: • metal purity grade (99.97 and 99.999% nickel samples were employed) • sample surface preparation • fluid-dynamic conditions. The results obtained have shown that the experimental procedure and conditions affect the electrochemical behaviour of nickel. As an example it is evidenced that: • the purer the nickel sample, the worse the passivation features • as far as passivity is concerned, no improvement was shown as a function of electrode stirring rate • the passivation is concerned, no improvement was shown as a function of electrode stirring rate • the passivation potential values increase as the stirring rate increases. Furthermore potentiodynamic curves drawn on nickel sheets polished under water up to the 1000 grade emery paper are affected by the subsequent rinsing operation. For instance the 99.97% nickel rinsed with methanol shows a decrease in the passivation potential value in MeOH + 0.1 N H 2SO 4 + 3% H 2O under stagnation conditions compared to that obtained on specimens rinsed with water. Finally it was observed that nickel washed with water either passivates or not depending on the stirring rate while nickel sheets washed with methanol never passivate regardless of the stirring rate.