Corrosion of Carbon and Stainless Steels in Kraft Liquor Which Contains Polysulphides Available to Purchase

Fibre shortages and a move towards closed cycle operation have resulted in a renewed interest in polysulphide kraft pulping. Recently, a process has been developed whereby polysulphide is formed directly in the kraft mills causticizers by the addition of air or oxygen. The purpose of this work was to identify materials of construction for polysulphide enriched kraft liquors and to characterize the effect of polysulphide on existing carbon steel equipment. Weight loss and D.C electrochemical experiments were carried out in white, green and weak-wash liquors, with and without polysulphide. It was found that 304L and 316L stainless steels can experience corrosion rates of approximately 0.004 mm.y⁻¹ in the liquors tested. Carbon steel (ASTM A-285 Gr.C) exhibited distinct active/passive behaviour in all three liquors. It was possible to passivate the carbon steel in green and weak-wash liquor even when the concentration of polysulphide in the liquors was below 2 grammes per litre (g.L⁻¹). Carbon steel was more difficult to passivate in white liquor but 2 g.L⁻¹ polysulphide was often sufficient. The steel nearly always passivated when the liquor concentration was 5 g.L⁻¹ or greater. In a 14 day corrosion test in white liquor with a nominal polysulphide concentration of 8 g.L⁻¹, carbon steel coupons were active and corroded at a rate of 0.5 mm.y⁻¹ while other carbon steel coupons in the same environment passivated.