A study of the stress corrosion cracking of mild steel in alkaline and alkaline sulphide solutions

The stress corrosion cracking (SCC) of an At ST C-1018 mild steel was investigated in three solutions, composed of 12.5 mol/kg NaOH, 3.35 mol/kg NaOH and 2.5 mol/kg NaOH + 0.42 mol/kg Na₂S, respectively. The potential of maximum susceptibility to SCC of steel in the latter two solutions was assessed by a slow strain rate technique. It was found to be slightly higher than the active-passive transition in each solution (-1.00 Vsce in 3.35 mol/kg NaOH and -0.88 Vsce in 2.5 mol/kg NaOH + 0.42 mol/kg Na₂S). A fracture mechanics technique, utilizing fatigue precracked double cantilever beam specimens, was then used to study the effects of stress intensity, temperature and electrochemical potential on crack velocity in all three solutions. Both stress intensity dependent (region I) and stress intensity independent (region II) crack velocity behavior was found. Apparent activation energies for region II of ~ 24 kJ/mol were determined at both Ecorr and -1.00 Vsce in 12.5 mol/kg NaOH. Crack velocities of the order of 10⁻⁹ m/s were measured at Ecorr in 12.5 mol/kg NaOH and at -1.00 Vsce and -0.88 Vsce 3.35 mol/kg NaOH and 2.5 mol/kg NaOH + 0.42 mol/kg Na₂S, respectively. The crack velocities measured at -1.00 Vsce in 12.5 mol/kg NaOH were of the order of 10⁻⁸ m/s. The fractography of the cracks was transgranular in 12.5 mol/kg NaOH at Ecorr. A mixed intergranular-transgranular fractography was observed at the active-passive transition in all three solutions. The results of the two techniques were compared and discussed, as was the role of stress intensity and passivation rate in fracture mechanics experiments. Anodic dissolution, hydrogen embrittlement and adsorption mechanisms were considered. It was decided that the results at Ecorr in 12.5 mol/kg NaOH could best be accounted for by a hydrogen embrittlement mechanism, perhaps assisted by anodic dissolution. Hydrogen embrittlement was eliminated as a possible mechanism at the active-passive transition in all the solutions. The most likely mechanism was thought to be one involving mixed activation-diffusion controlled dissolution. Applications of the results to the pulp and paper industry were considered.Applied Science, Faculty ofMaterials Engineering, Department ofGraduat