Hot corrosion behaviors of Super 304H austenitic stainless steel pre-coated in Na2SO4–25%NaCl mixture salt film

Abstract

Hot corrosion rates of Super 304H austenitic stainless steel pre-coated in Na2SO4–25%NaCl mixture salt film in air at 650 °C and 750 °C have been investigated by corrosion kinetics curves. The corrosion products are examined by means of X-ray diffraction and scanning electron microscopy coupled with energy-dispersive spectrometer, and electron probe micro-analyzer to analyze the scale structure and element distribution of the cross section. The results indicate that the kinetics curves are similar and display parabolic growth, and the mass gain obviously increases with the increasing temperature. A two-layer oxide scale composed of Fe oxides containing CuFe2O4 and Cr2O3 forms on the surface of the alloy and easily exfoliates with extending corrosion time or with improving corrosion temperature. Moreover, a corrosion-affected zone with micropores and micro-cracks appears beneath the oxide scale. It is concluded that the selective oxidation occurs and a protective oxide scale forms at the early corrosion stage. The melt salts destroy the integrity of the oxide scale and accelerate hot corrosion of the alloy by the cyclical oxidation–chlorination during the further corrosion processing. In addition, internal sulfidation also contributes to the corrosion of the alloy.