Corrosion testing was performed in white, green, and black liquors from a kraft mill. The liquors were modified in composition to simulate conditions of high (40°/0)sulfidity and low (30°/0)sulfidity, and then heated in laboratory autoclaves to the temperatures of the respective tanks from which the samples were taken. Specimens of carbon and stainless steels were exposed under free corrosion potential conditions, and their corrosion rates determined from weight loss measurements. In white, green, 45°/0solids black, and flash tank liquors, active corrosion rates for the carbon steels were typically 20 to 75% higher in the higher sulfidity liquors. In 15?%solids weak black liquor there was no appreciable difference in corrosion rates, with carbon steels remaining passive in both low and high sulfidity. In 26% solids intermediate black liquor there were large increases in the corrosion rates of carbon steel between low and high sulphidity liquors, resulting from a change from passive to active conditions. Stainless steels UNS S30403, S32304, and S31803 had very low corrosion rates in althea liquors tested, regardless of sulfidity.
In the pulp and paper industry, there are several alkaline liquors with colorful names such as white liquor, green liquor, and black liquor. The kraft chemical pulping process takes place in large pressure vessels, called digesters, and is essentially a reaction between wood chips and white liquor, a solution containing principally sodium hydroxide, sodium sulfide, and sodium carbonate. After pulping, the resultant black liquor (hydroxide-depleted white liquor containing dissolved organics such as lignin) is extracted and concentrated by evaporation of the water and burned as a fuel in a recovery boiler where the inorganic chemicals are recovered in molten salt form and subsequently re-dissolved in water as green liquor. Addition of lime from kiln reconstitutes the green liquor into white liquor. Although there have been several investigations of corrosion in alkaline liquors]-]s,and sulfide has been identified as corrosion activate? Has there has never been a systematic investigation of the effects of sulfidity on the corrosivity of any alkaline pulping liquor.
