Electrochemical and Stress Corrosion Cracking Behavior of Digester Steels in Kraft White Liquors

Stress corrosion cracking (SCC) of ASTM A516 Grade 70 and A285 Type C steels was found to occur in a simulated white liquor containing 90 g/L NaOH + 35 g/L Na2S. Cracking in constant potential slow strain rate (SSR) experiments was found to occur at potentials near the active-passive transition of the steel at 90 and 110 C. Cracking was also observed for both steels at a potential of −1200 mVSCE. Criteria developed by Parkins20,21 from fast and slow scan polarization curves to predict the potential range of SCC were found to be inappropriate for use in simulated white liquor. Two distinct potential regions of passive film formation were identified from anodic polarization curves. The formation of sulfide-containing films was associated with potentials near −900 mVSCE. and the formation of hydroxide-containing films was associated with potentials near −1100 mVSCE. Cracking in SSR experiments occurred within the former region, but not in the latter. Additions of NaCl, Na2CO3, and Na2S2O3 had little measurable effect on the anodic polarization behavior of the steels, nor were any differences noted between curves produced with ASTM A516 Grade 70 and A285 Type C steels or with AWS(1) E7018 weld metal.