Mass transfer and morphological changes in AISI 316 stainless steel in high temperature flowing sodium

Stainless steel (AISI type 316) sheet specimens were exposed to flowing sodium in a loop at different temperatures (623,723, and 823 K in the hot zone (upstream) and 623 and 583 K in the cold zone (downstream) positions). Leaching of elements from the surface of the steel by the flowing sodium was observed. At 823 K, the leaching of nickel resulted in destabilisation of the austenitic structure and the formation of a ferrite phase. At lower temperatures (583–723 K), the rate of leaching was too slow to cause any observable effect on stainless steel. Thermal aging at 823 K caused precipitation of carbide and the generation of a ditch structure (a structure with continuous precipitation of carbide at the grain boundaries). Analysis by X-ray photoelectron spectroscopy of the nature of the corrosion product formed on the surface exposed to sodium excluded the possibility of formation of ternary compounds of sodium. Compounds containing Cr-O and Fe-O bonds were revealed in the downstream region. Segregation of carbon on the surface of stainless steel was observed at 623 K in the upstream region. Analysis of the carbon profile by secondary ion mass spectroscopy indicated a region of low carbon (corresponding to the depth transformed to ferrite) followed by carburised austenite in the specimens exposed at 823 K. At all other temperatures there was no significant change in carbon content because of the slow rate of diffusion at low temperatures.