Effect of grain refinement on the resistance of 304 stainless steel to breakaway oxidation in wet air

A 30 (mu)m thick nanocrystalline coating (NC) with a composition identical to that of conventional coarse-grained 304 stainless steel (SS) was prepared via direct current magnetron sputtering (DCMS). The DCMS NC exhibited a typical columnar microstructure, with a mean grain size of ~ 12 nm in the plane normal to the growth direction. Comparison of the oxidation of the 304 NC with 304 SS (~ 20 (mu)m in an average grain size) was investigated at temperatures (700-900 °C) in flowing air with 0-40% H2O vapor. Catastrophic breakaway oxidation started from the upstream edge of the coarse-grained sample in wet air after an incubation period, due to the conversion of protective Cr-rich scale into non-protective Fe-rich one as a result of the water-vapor-promoted evaporation of chromia from scale. Increasing temperature or water-vapor concentration accelerated the appearance of breakaway oxidation. In contrast, breakaway oxidation did not occur in wet air for the DCMS NC, where abundant grain boundaries greatly enhanced the Cr diffusion to guarantee the stable growth of the Cr-rich scale.