The corrosion rate of carbon and stainless steel equipment, like digesters, black liquor storage tanks, and other equipment that comes in contact with black liquor, varies from one mill to another and sometimes within the same mill. Generally, corrosivity of black liquor is known to change with the wood species pulped. Corrosivity of black liquor does not correlate with their inorganic constituents. Pulping conditions vary from mill to mill, leaving different amounts of residual inorganics, solids
content, etc., in resulting black liquors. Therefore, it is difficult to evaluate the effect of organic constituents of black liquor on its overall corrosivity. The present study focuses on establishing relative corrosion susceptibility of carbon steels and other alloys in black liquors from different wood species which were pulped under similar cooking conditions, leaving similar amounts of residual inorganic chemicals in the resulting black liquors. Five different wood species (two softwoods and three hardwoods) were used in this study. Results from this study show that the tested black liquors from softwood species are more corrosive than the black liquors from the hardwoods tested. However, the
corrosivity of black liquors from hardwoods depend upon the wood species used. No correlation was found between the major inorganic constituents of the black liquor and the corrosion rate of steel alloys
in these liquors. This study clearly demonstrates the important role of organic constituents of black liquors in determining their corrosivity.
Keywords: Black liquor, corrosivity, softwood, hardwood, digester, wood extractives, organic constituents, inorganic constituents, wood species