STRESS CORROSION CRACKING OF STAINLESS STEEL IN CONTINUOUS DIGESTERS, 2008 Engineering, Pulping and Environmental Conference

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Stress corrosion cracking (SCC) of the 316L stainless steel central pipe and the 304L stainless steel clad impregnation zone of a kraft continuous digester at a mill in Brazil prompted an investigation into the environmental conditions that could support the SCC of austenitic stainless steels. U-bends of 304L and 316L stainless steels were exposed in synthetic "weak white liquors" of different hydroxide levels (10 to 25 g/L NaOH) and sulfide content (0 to 40 g/L Na₂S) at 165ºC. Four different metallurgical conditions were investigated: annealed, welded, stress relieved, and sensitized. SCC of all specimens was found after 712 hours immersion exposure in weak white liquors having an effective alkali (E.A.) greater than 30 g/L NaOH. No SCC was observed in a weak white liquor of effective alkali (E.A.) equal to 25 g/L NaOH without Na₂S. The SCC in all cases was both transgranular and intergranular. SCC initiated earlier (after 190 hours) in the stress relieved and sensitized specimens than in the annealed or welded conditions. The SCC of type 316L was often more severe than was the case for SCC of type 304L stainless steel.