Some observations concerned with the formation of sodium chromite on AISI 316 exposed to oxygenated sodium

Abstract

Exposure of AISI 316 stainless steel, at 450–675°C, to static sodium containing 1–24 ppm of oxygen for times of 48–4400 h caused emergent grain boundaries to become grooved and grain faces modelled. Coupons exposed to high-oxygen sodium formed hexagonal crystals of sodium chromite which smeared to form flakes when rubbed. The minimum oxygen concentration, C E ( ppm), needed to form sodium chromite increased with the exposure temperature, T (K), according to the relationship log C E = (−3450/ T) + 4.87, which can be related to boundary conditions based on thermodynamic and tribological data. The sodium chromite first formed near emergent grain boundaries and increased in amount by a growth process with an apparent activation energy of 150 kJ/mol. The formation of sodium chromite on the surfaces of heavily cold worked coupons was much more profuse than that on the surfaces of annealed coupons.