Effect of Black Liquor Oxidation on the Stress Corrosion Cracking Susceptibility of Selected Materials

• Published in: Corrosion (Houston, Tex.) Vol. 62; no. 9; pp. 765 - 772
• Main Authors: Hazlewood, P.E., Singh, P.M., Hsieh, J.S.
• Format: Journal Article
• Language: English
• Published: Houston, TX NACE International 01.09.2006; NACE
• Subjects: Applied sciences; Austenitic stainless steels; Black liquor; Carbon steels; Chemical composition; Corrosion; Corrosion effects; Corrosion environments; Corrosion mechanisms; Corrosion resistance; Corrosion tests; Cracking (corrosion); Duplex stainless steels; Exact sciences and technology; Hardwoods; Hydroxides; Liquor; Materials selection; Metals. Metallurgy; Mills; Odor control; Odors; Odour; Oxidation; Pulp & paper mills; Stainless steel; Stress corrosion; Stress corrosion cracking; Thiosulfate; Thiosulfates; Uniform attack (corrosion); Open circuit; steel; Black liquor; Paper pulp; pulp and paper industry; open-circuit potential; Corrosion rate; Stress corrosion cracking; Potential; Crack susceptibility; Paper industry; Corrosion; Paper; Oxidation; Application
• ISSN: 0010-9312; 1938-159X
• DOI: 10.5006/1.3278301

ABSTRACTDuring the kraft pulping process, wood chips are cooked in white liquor, which primarily contains sodium sulfide (Na2S) and sodium hydroxide (NaOH). In this process the lignin is fragmented into smaller segments whose sodium salts are soluble in the cooking liquor, leaving cellulose and hemicellulose in the form of intact fibers, which can be further processed for papermaking. The resultant liquor with dissolved organics after pulping is called black liquor (BL) as a result of its dark color. BL is concentrated and used in the kraft recovery boiler as a fuel to recover inorganic pulping chemicals and heat energy. During pulping, some hydrogen sulfide (H2S) and organo-sulfur compounds are formed that may result in the distinct smell of pulp mills. Oxidation of BL, an optional recovery stage, was first implemented in the paper industry as a means of reducing sulfur-based odors and the chemical makeup requirements for sulfur.1-2 Today, oxidation of BL is receiving new interest with the realization of decreased viscosity and lower fuel values of the BL. Lower fuel values allow mills to increase pulp production by consuming more liquor in existing recovery boilers, and still recovering necessary pulping chemicals. Changes in the general corrosion and stress corrosion cracking (SCC) susceptibility of used alloys or candidate alloys due to BL oxidation treatment are the main concern addressed by the present study. A key component of oxidized BL corrosivity is the extent to which the oxidation is carried out. Depending upon the extent of oxidation, sulfide in the BL may react to form NaOH and sodium thiosulfate (Na2S2O3), as shown in Equation (1), or complete oxidation may convert the sulfides into sulfates via the reactions in Equations (1) and (2). Oxidation of both sulfide and