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 flowin g sodium in a loop at different temperatures (623, 723, and 823 K in t he hot zone (upstream) and 623 and 583 K in the cold zone (downstream) positions). Leaching of elements fi om the surface of the steel by th e flowing sodium was observed. Al 823 K, the leaching of nickel result ed in destabilisation of the austenitic structure and the formation of a ferrite phase. At lower temperatures (583-723 K), the I,ate of leac hing was too slow to cause any observable effect on stainless steel. T hermal aging at 823 K caused precipitation of carbide and the generati on of a ditch structure (a structure with continuous precipitation of carbide at the grain boundaries). Analysis by X-ray photoelectron spec troscopy of the nature of the corrosion product formed on the surface exposed to sodium excluded the possibility of formation of ternary com pounds of sodium. Compounds containing Cr-O and Fe-O bonds were reveal ed in the downstream region. Segregation of carbon on the surface of s tainless steel was observed at 623 K in the upstream region. Analysis of the carbon profile by secondary ion mass spectroscopy indicated a r egion 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 co ntent because of the slow rate of diffusion at low temperatures. (C) 1 997 The Institute of Materials.