Effect of Sulfide Ions on Caustic Cracking of Mild Steel

Abstract

Potentiostatically controlled slow strain rate tensile tests were conducted on AISI 1018 mild steel in hot (92°C) caustic solutions of and in a chemically inert cell. Reduction of area was used as a measure of susceptibility to stress corrosion cracking (SCC). The presence of sulfide ions raised the active‐passive transition potential to more noble values, and maximum susceptibility to SCC occurred at potentials near the active‐passive transition in each environment. Cracking at these potentials was further confirmed by conducting tests on prefatigue cracked double cantilever beam specimens at a known stress intensity, and crack fractography was studied by scanning electron microscopy. Crack liquid pH measurements were obtained with pH indicator paper using a freezing technique. A hydrogen embrittlement mechanism of SCC was eliminated from consideration in both environments on thermodynamic grounds. All observations were reasonably consistent with a dissolution mechanism of SCC involving film rupture events.