Application of Magnetoelectropolishing on the Oxidation Resistance Improvement of Stainless Steel Cladding in High-Temperature Water

Abstract

Mechanical polishing (MP), electropolishing (EP), and magnetoelectropolishing (MEP) were applied on 308L stainless steel (SS) weld cladding. The surface characteristics after surface treatment and the oxides after immersion in the primary water of a pressurized water reactor were studied. XPS results showed that the passive film of surface-treated MEP 308L SS had higher Cr/(Ni+Fe) than MP and EP 308L SS, and reported surface-treated MEP 316L SS. Transmission electron microscopy results showed that duplex-layer oxide films consisted of Fe-rich spinel outer crystals and Cr-rich inner layers were formed on all surface-treated 308L SS cladding after immersion. The inner oxide layers on austenite were Cr-rich spinel dominant oxides and much thicker than the chromia-dominant oxides on ferrite. MEP decreased the inner oxide layer thickness on austenite of oxidized 308L SS. The oxidation resistance of the ferrite matrix with higher Cr content was not obviously impacted by surface treatment. A higher Cr/(Ni+Fe) in the preformed passive film on the austenite was mainly responsible for the better oxidation resistance of MEP 308L SS cladding. The improvement of oxidation resistance by MEP on dual-phase 308L SS cladding was different from that on single-phase 316L SS, suggesting that a local heterogeneous passive film on surface-treated MEP 308L SS was more likely degraded to a duplex-layer oxide film compared to the single-layer one on MEP 316L SS.