Microstructure and corrosion resistance of a type 316L stainless steel

Characterization of the microstructure and chemistry of the oxide film of an air-oxidized type 316L stainless steel was carried out by an energy filtering transmission electron microscope equipped with an electron energy loss spectroscopy detector, and the corrosion resistance of the oxidized steel in a hot boiling sulfuric acid solution was evaluated by using an apparatus outlined by the International Standard Organization. It is demonstrated that the corrosion resistance of the steel in a 105 degrees C, 30% sulfuric acid solution can be improved by an oxidation pretreatment of the steel in air at 500 degrees C for 5 min, which produced an oxide film of similar to 70 nm thick. The oxide film has a multilayered microstructure in which the topmost layer is composed of nanoscale gamma-Fe2O3 grains of size similar to 4 nm, followed by a mixture of alpha-Fe2O3 and Fe3O4 phases of grain size ranging from 20 to 75 nm. Chemical analysis of the oxide film across thickness direction using electron energy loss spectroscopy revealed that the O content in the oxide film decreases from the oxide surface toward the oxide/steel interface. In addition, it is observed that an alloying depletion of Cr and Mn exists in the oxide film, compared with the alloying elements in the steel. (C) 1998 Elsevier Science Ltd. All rights reserved.