Early stages of corrosion of stainless steel 309 S in geological brines studied by sychrotron radiation photoelectron spectroscopy

Abstract

The chemical transformations of the native oxide on stainless steel 309 S prior to pitting in anoxic near saturated geological Q3-brine were investigated by synchrotron radiation photoelectron spectroscopy (SRPES). A Cr-rich hydrated mixed iron-chromium-oxide exhibiting n-type semiconducting behavior is formed after wet polishing. Surface accumulation of holes in the metastable pitting potential region induces the hydroxylation of oxide bonds and a subsequent selective dissolution of iron. The open structure of the remaining surface chromium hydroxide facilitates the inward access of chloride and water. The inhibiting effect of sulfate is ascribed to the formation of stabilizing O-S-O bridges blocking chloride access.