Corrosion and passivation of a nickel—silicon-base alloy in sulphuric acid solutions

Abstract

A cast alloy containing 9·25–9·75 per cent Si, 2·5–3 per cent Ti, 2–3 per cent Cu and 2·75–3·25 per cent Mo has a low rate of corrosion in boiling sulphuric acid solutions at all concentrations, except within a narrow range between about 47–55 wt-%. Within this range the corrosion rate rises to a peak in 53% solution and declines rapidly again. Below 53% no visible solid corrosion product forms on the alloy surface whereas above 53% scale is formed, the colour and composition of which vary with concentration of sulphuric acid. From electrochemical measurements in boiling sulphuric acid solutions it is inferred that in concentrations below 53% the alloy, although corroding at a low rate, remains active, the corrosion being predominantly under cathodic control. In the range 55–75% the alloy becomes passive after an induction period, the duration of which decreases with increasing concentration within this range. In concentrations greater than 75% passivation is rapid. The changes in the corrosion behaviour of the alloy are associated with changes in the cathodic reactions in sulphuric acid solution with concentration. Up to 53% concentration the acid behaves as a hydrogen-evolving acid and solutions of intermediate concentration, viz 55–75%, also behave in this way during the initial stage of corrosion of the alloy. At a later stage, however, the cathodic reaction in solutions of intermediate concentration changes and hydrogen sulphide and sulphur are the predominant reduction products. The change from hydrogen evolution to formation of hydrogen sulphide and sulphur is catalysed by a surface enriched in copper, formed by dissolution of copper from the alloy during the initial stage of corrosion and subsequent redeposition. The copper dissolves again to react with hydrogen sulphide to form cupric sulphide, which is incorporated in the corrosion product scale on the surface of the alloy. The scale exerts a masking influence over the surface and permits a true passive film to nucleate and grow laterally beneath it. In concentrated sulphuric acid, viz 75% and above, sulphur dioxide is the most probable cathodic reduction product of sulphuric acid and passivation of the alloy is rapidly achieved without the formation of a corrosion-product scale containing cupric sulphide.