Analysis of oxide film on stainless steel via position-sensitive atom probe

Abstract

Thermal oxide films formed on the surface of a type 316 stainless steel were atomically characterized by a recently built position-sensitive atom probe (POSAP). Oxidation on the clean field evaporated surface was performed at temperatures of 350 °C for 5 min, 475 °C for 1, 5, and 15 min, and 600 °C for 5 min in oxygen at a pressure of 10−4 Torr. The POSAP analysis has clarified that the oxide–steel interfaces at 350 °C for 5 min, and 475 °C for 1 and 5 min are very sharp, while the interfaces at 475 °C for 15 min and 600 °C for 5 min have subnanometer level roughness. In spite of the difference in oxidation temperature and time, the oxygen concentration in the oxide films is in the range from 35 to 50%, and Ni and Mo tend to segregate to the oxide–steel interface. Furthermore at 475 °C for 15 min and at 600 °C for 5 min, Ni and Mo segregate to the top surface region as well as to the interface. The oxide film has a triplex structure consisting of the top surface region being chromium rich oxide, the inner region being iron rich oxide, and the interface being chromium rich oxide. The Mo segregation is localized in a few nanometers in size and appears to inhibit the oxide formation in those segregated regions.