Stress Corrosion Cracking of Carbon Steel in Caustic Aluminate Solutions—Slow Strain Rate Studies

Stress corrosion cracking (SCC) susceptibility of 0.18% carbon steel in hot (92 C) caustic aluminate solutions was studied by potentiostatically-controlled slow strain rate testing techniques. Three principal solutions that had similar hydroxyl ion concentrations were used: 2 molai (m) NaOH, 4m NaOH + 1m Al₂O₃, and an industrial solution containing 3.6m NaOH + 0.67m Al₂O₃ + impurities. The most useful parameters for assessing SCC were the number of cracks present in sectioned specimens and the apparent crack velocity determined from the longest crack. In general, at anodic potentials, cracking was most pronounced near the active-passive transition region. The presence of dissolved AlO₂⁻ species promoted cracking in the passive region at potentials where SCC was not observed in the simple NaOH solution. The effects were interpreted in terms of the film rupture-dissolution mechanism of cracking and the effect of AlO₂⁻ on repassivation rates.