Caustic Stress Corrosion Cracking of Mild Steel

The stress corrosion cracking (SCC) behavior of cold worked mild steel in hot, aqueous, 33 pct NaOH solutions was studied with prefatigue cracked double cantilever beam specimens. SCC kinetics were studied under freely corroding potentials (E _corr ≈ −1.00 V_SHE) and potentiostatic potentials of −0.76 V_SHE near the active-passive transition. The pH of the liquid within the crack was determined and fractography was studied by scanning electron microscopy. Cracking was transgranular atE _corr, intergranular at −0.76 V_SHE, and produced no detectable change in crack liquid pH from that of the bulk solution. Crack rates were dependent upon temperature, potential, and stress intensity (K ₁). The apparent activation energy in Region II, where crack growth rate was independent ofK, was ∼ 24kJ/mol for both cracking modes. This was considered to be due to mixed rate control involving activation polarization and mass transport processes. The mechanism of cracking was entirely consistent with metal dissolution at –0.76 V_SHE and may involve hydrogen embrittlement and/or dissolution effects atE _corr. DOUGLAS SINGBEIL, formerly Research Student, University of British Columbia, is Research Scientist, Pulp and Paper Research Institute of Canada, 570-Blvd. St. Jean, Pointe Claire, Quebec, Canada H9R 3J9.