Oxidation of SS304 and SS316 and the formation of diffusionally isolated regions leading to localised premature breakaway oxidation

The results are presented on the oxidation behaviour of two chromia forming steels, over the temperature range 800 to 900 °C, regions of premature localised breakaway oxidation were observed. SiO₂ formation along emergent grain boundaries encapsulated smaller grains, acting as diffusion barriers to the replenishment of chromium from deeper in the alloy. Rapid depletion of chromium from these isolated grains continued with increasing time until the concentration was insufficient to maintain a protective Cr₂O₃ resulting in the localised chemical failure and formation of breakaways oxides. A finite difference model was used to demonstrate the effect of SiO₂ formation. It was also found that the low concentration of silicon restricted the encapsulation process to only the smaller grains at the surface and protective oxidation was re-established underneath at the alloy surface. The predicted lifetime of components with wall thicknesses of 2 mm was calculated to be in excess of 30 years.