Electropolishing effects on corrosion behavior of 304 stainless steel in high temperature, hydrogenated water

The corrosion rate of electropolished 304 stainless steel surfaces (UNS S30400) is found to be lower by more than a factor of three relative to that determined previously for machined surfaces in mildly alkaline, hydrogenated water at 260 °C. This favorable result is attributed to significant changes in nanocrystallinity of the corrosion oxide layer caused by the removal of surface microstrain, which had been imparted during the machining process. In the absence of microstrain, a low-porosity, protective, corrosion layer forms that is composed of extremely small and uniformly-sized spinel oxide crystals. Application of scanning electron microscopy (FEG-SEM), X-ray diffraction and X-ray photoelectron spectroscopy (XPS) in conjunction with ion milling and target factor analyses, found the corrosion layer to consist of micrometer-size crystals of a ferrite-based spinel oxide (non-protective) over-laying nanometer-size crystals of a chromite-based spinel oxide (protective). Composition of both phases is unchanged from that previously observed on corroded, machined surfaces and is representative of solvus phases in the immiscible Fe(Fe1−nCrn)2O4 spinel binary. The smaller size (10 vs. 26 nm) and greater surface density (∼10,000 vs. 835 μm−2) of the chromite-based crystals relative to those formed on machined (i.e., cold-worked) surfaces, however, is consistent with the absence of preferred high energy nucleation sites on strain-free surfaces. Therefore, electropolishing, which removes surface microstrain induced by cold-working, represents a preferred reference surface condition.