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Stress corrosion cracking of 316 stainless steel in caustic solutions Crowe, David Charles


Stress corrosion cracking (SCC) of type 316 stainless steel was studied in hot (92°C) solutions of 3.35 mol/kg NaOH and 2.5 mol/kg NaOH + 0.423 mol/kg Na₂S by means of potentio-statically controlled slow strain rate testing techniques (SSRT). Anodic polarization curves were also determined for the steel, together with those for Ni, Cr and Fe. SCC occurred in the transpassive region in 3.35 mol/kg NaOH, with no detectable tendency to crack in the active-passive region, unless in the sensitized condition. In the NaOH + Na₂S solution, SCC was detected in the active-passive region. Fracture mechanics techniques were used to study the kinetics of stress corrosion crack propagation in 3.35, 8 and 12 mol/kg NaOH, and 12 mol/kg NaOH +0.423 mol/kg Na2S. Cracking was studied as a function of stress intensity (K[sub I]), temperature (T) and potential (E). Crack fractography was studied by scanning electron microscopy and corrosion films investigated by electron diffraction. Region I (K[sub I]-dependent) and Region II (K[sub I]-independent) crack behavior were observed. The results indicated that SCC was associated with potentials at which instabilities occurred in passive films and that the basic mechanism of cracking involved a film rupture and dissolution process, with dissolution processes exerting predominant rate control in Region II. In the case of the sulfide containing solution, hydrogen embrittlement processes could not be eliminated as a contributing factor.

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