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ABSTRACT 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.
INTRODUCTION The corrosion rate of kraft digesters and other equipment that come in contact with black liquor varies considerably. Black liquor consists of residual inorganic chemicals after pulping, numerous organic constituents of wood, and other chemical species produced during the pulping process. The composition of liquor is a very important variable in the overall corrosion of mill equipment exposed to black liquor. However, the composition of black liquor varies from mill to mill, and within one mill with changes in chemical charge, cooking parameters, and species of wood pulped. The major inorganic species in black liquor are sodium sulfide, sulfate, sodium thiosulfate, sodium carbonate, and sodium hydroxide. Sodium chloride is also present as an impurity that may either originate from the water supply and/or the wood chips. Whereas, the organic content of the black liquor largely depends on the wood species pulped and on the cooking process used.
The majority of published work on the effects of inorganic chemical concentrations on corrosion in pulping liquors was done for white and green liquors, where the role of constituents is relatively easy to study in a laboratory. The role of each inorganic species in the white and green liquor has been studied, reported, and reviewed by various researchers [ 1-6]. A number of studies [7-17] have shown that the corrosion rate in black liquors does not correlate with their inorganic composition. There are clear indications that organic constituents of black liquor play an important role in determining its corrosivity. However, there are literally hundreds of organic compounds detected in black liquors and they vary from wood species to wood species. For birch kraft black liquor, Niemela [18] discovered around 600 compounds but could only identify -350 organic compounds using gas-liquid chromatography and mass spectrometry (GLC-MS). The role of wood extractives and other organic constituents of black liquor in the corrosion of pulping equipment is not very well understood.
Wensley [13] studied the corrosion behavior of carbon steel in three different liquors, one from a mill pulping hardwood ex
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