Corrosion of Carbon and Stainless Steels in Condensed Black Liquor Vapors

Condensate of black liquor vapors is known to cause corrosion in pulp mill equipment like digesters, flash tanks, storage tanks, evaporator domes, and other equipment handling black liquor condensate. This type of corrosion typically occurs in parts of equipment where the black liquor vapors can condense on a metal surface. Generally organic acids and other wood extractives in the condensate are thought to be responsible for the vapor phase corrosion of pulp mill equipment. Different wood species have different extractives at varying concentrations. Black liquor from softwood and hardwood were used in this study to characterize the corrosive constituents in different condensates. Seven alloys were tested in this study. Carbon steel corrodes at higher rates in the softwood condensate compared to the hardwood condensate. Austenitic stainless steel coupons experienced localized attack in both condensates. Chemical analysis of condensates have shown that although the hardwood condensate had more total organic acids than softwood condensate, the pH of the softwood condensate was always lower than that of the hardwood. Significantly higher concentrations of thiosulfate ions were present in the hardwood condensate than in the softwood condensate. This study indicates that presence of the organic acids is not be the only reason for the overall corrosivity of black liquor condensates.