Corrosion of AISI Type 316 Stainless Steel in Sodium and the Assessment of Long-Term Tensile Behaviour

Abstract

Austenitic stainless steel of the type AISI 316 was exposed to flowing sodium in a loop at 823 K for 57.6 Ms (16000 h). The specimens were examined for changes in microstructure, and tensile properties. A specimen was also subjected to thermal-ageing under identical conditions to facilitate comparison and resolve the effects of sodium-exposure from mere thermal-ageing. The investigations revealed that, exposure to sodium caused surface morphological changes. A modified layer of 10-15 μm thickness was formed on the surface. This layer was observed to contain ferrite which was attributed to the preferential leaching of nickel. The bulk matrix revealed the generation of a continuous carbide precipitate at the grain boundaries (ditch structure). The thermally-aged specimen also revealed a ditch structure. Tensile tests showed that both the sodium-exposed and thermally-aged materials increased their yield strengths. Analysis of the carbon profile by Secondary Ion Mass Spectrometry (SIMS) indicated that the sodium-exposed surface was depleted in the carbon (to a thickness nearly corresponding to that of the degraded layer). The austenite phase inside the surface degraded region was carburized and a maximum content of the carbon (0.135 mass%) was noticed at a depth of 43 μm from the surface. From the data on the carbon profile, the effective diffusion coefficient of the carbon in the steel was evaluated to be 3.75 x 10 -18 m 2 .s -1 (at 823 K). Analysis indicated that the bulk carbon content increased to 0.089 mass% from 0.054 mass%. Based on the results obtained in the present investigation and data available in the literature, the consequences of corrosion by sodium on long-term exposure [(946.7 Ms (30 years), the normal design life of reactor components)] were analyzed. The carbon profile for long duration of exposure to sodium was evaluated by the application of equations based on diffusion laws. The average bulk carbon content after exposure to sodium for 946.7 Ms was estimated to be 0.125 mass%. The influence of carburization on ductility was estimated. The room temperature ductility of 316 Stainless Steel was estimated to be reduced to 39% (from 69% for the as-received material). At elevated temperature (850 K), the ductilitv was estimated to be reduced to 18%.