Generation of surface degraded layer on austenitic stainless steel piping exposed to flowing sodium in a loop: intercomparison of long-term exposure data

Exposure of austenitic stainless steels to high temperature sodium causes leaching of the constituent elements. Different elements are leached to different extent on account of the difference in their solubility in sodium. Nickel is having the highest solubility and hence would be leached out preferentially. The leaching of nickel causes the destabilization of the austenite and promote the generation of ferrite phase. The layer containing ferrite is referred to as `surface degraded layerʹ. This layer may have undergone the leaching of other elements like chromium also. The paper compares the thickness of surface degraded layer reported by different investigators. An ambiguity in the literature in explaining the observed corrosion behaviour and need to distinguish between complete metal loss and generation of degraded layer has been clarified in this paper.