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51318-10701-Stress Corrosion Cracking of Stainless Steel in Heavy Black Liquor - Mill Experience.

This paper describes the cracking failure of a UNS S30403 (304L) stainless steel 72% heavy black liquor storage tank at a Canadian bleached kraft pulp mill.

Product Number: 51318-10701-SG
Author: Stephen Clarke / Walter Bursey / J.D. Irving
Publication Date: 2018
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This paper describes the cracking failure of a UNS S30403 (304L) stainless steel 72% heavy black liquor storage tank at a Canadian bleached kraft pulp mill. The tank was installed in 1995 and operated reliably for 20 years when leaks were found in the upper part of the cylindrical shell. Samples of the cracked area were removed for laboratory examination. Investigation showed that the cracking initiated on the inside surface of the tank in the “tidal” zone where the liquor level fluctuates during normal operation of the mill. The laboratory investigation showed that the cracking was chloride stress corrosion cracking. This failure is consistent with findings at other mills in that high chloride was found in the crack tips even though the bulk chloride content of the liquor is much lower (of the order of 0.28 % on a dry solids basis which equates to 0.39% in the “liquid” 72% black liquor). Analysis of the organic residue on the corroded surface indicated that the organic species in the black liquor were oxidized in the tidal zone, and formed organic acids, which combined with the concentration effect allowed stress corrosion cracking to occur.

Key Words: Black liquor, stress corrosion cracking, SCC, stainless steel

This paper describes the cracking failure of a UNS S30403 (304L) stainless steel 72% heavy black liquor storage tank at a Canadian bleached kraft pulp mill. The tank was installed in 1995 and operated reliably for 20 years when leaks were found in the upper part of the cylindrical shell. Samples of the cracked area were removed for laboratory examination. Investigation showed that the cracking initiated on the inside surface of the tank in the “tidal” zone where the liquor level fluctuates during normal operation of the mill. The laboratory investigation showed that the cracking was chloride stress corrosion cracking. This failure is consistent with findings at other mills in that high chloride was found in the crack tips even though the bulk chloride content of the liquor is much lower (of the order of 0.28 % on a dry solids basis which equates to 0.39% in the “liquid” 72% black liquor). Analysis of the organic residue on the corroded surface indicated that the organic species in the black liquor were oxidized in the tidal zone, and formed organic acids, which combined with the concentration effect allowed stress corrosion cracking to occur.

Key Words: Black liquor, stress corrosion cracking, SCC, stainless steel

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