Surface Oxide Layers on 316L Stainless Steel Formed in 561 K Pure Water at Different Potentials

Abstract

Surface oxide layers on stainless steel were formed in 561 K pure water at different potentials. To understand the oxide's properties in terms of their potential dependence, cross-sectional views of the oxide layer were analyzed using an electron microprobe technique and potential–solubility (equilibrium concentration of ionic species) diagram. In the potential range investigated, duplex oxide layers composed of mono- and bimetallic oxide were formed. Both the structure and composition of the oxide layer were affected by solubility of oxides. At low potential (−0.40 to −0.02 V (SHE)), the most thermodynamically stable (least soluble) Fe-based oxide was FeCr2O4, followed by NiFe2O4 and Fe2O3. FeCr2O4 was formed as the inner oxide layer, and NiFe2O4 and Fe2O3 were formed as the outer oxide layer. The FeCr2O4 layer was thermodynamically stable but did not act as a barrier preventing further oxidation. Its thickness decreased with increasing potential (≈150 nm at −0.40 to −0.32 V, ≈20 nm at −0.14 to −0.02 V, both after 100 h of exposure). This is attributed to the decreasing solubility of Fe2+ (in equilibrium with NiFe2O4 and Fe2O3) with increasing potential. At high potential (0.05 to 0.21 V), the Cr components were thermodynamically unstable because of transpassive dissolution of Cr(III) to Cr(VI). A relatively Cr-depleted inner oxide layer was formed except underneath the NiFe2O4 outer oxide layer.