Caustic Stress Corrosion Cracking Studies at 288 C (550 F) Using the Straining Electrode Technique — Comparison of Alloy 600, Alloy 800, and Type 304 Stainless Steel^★

Constant pull rate tests were conducted on tensile specimens of Inconel⁽¹⁾ Alloy 600, Incoloy⁽¹⁾ Alloy 800, and Type 304 stainless steel in deaerated 10% NaOH solution at 288 C (550 F) with a cover gas of 5% H₂ in N₂. The pull rate used for most experiments was 3.3 × 10⁻⁶ cm/s, which corresponds to an initial strain rate of 3 × 10⁻⁶ s^−1. The electrical potential of the specimens was controlled by a potentiostat using a nickel wire as a hydrogen reference electrode. Under open circuit conditions, Type 304 stainless steel specimens cracked rapidly, but Alloys 600 and 800 specimens exhibited only ductile fracture. However, cracks readily formed in Alloy 800 specimens at potentials in the +50 to +300 mV range and in Alloy 600 specimens at potentials in the +150 to +250 mV range. Scanning electron microscope (SEM) photographs of some of the cracked specimen surfaces showed the transition from ductile to brittle fracture as a consequence of changes in the specimen's electrical potential. Longitudinal metallographic cross sections also revealed the grain structure and the mode of cracking. They showed that the cracks were intergranular in Alloy 600 and Type 304 stainless steel specimens, and were transgranular in Alloy 800 specimens.