Characterization of Oxide Film Formed on Austenitic Stainless Steel by In Situ Micro Raman Spectroscopy

Crack initiation and crack propagation are known as two distinct stages of Environmentally Assisted Cracking (EAC). In the case of smooth surfaces, crack initiation is caused by rupture or degradation of surface film, which is induced by the combined effects of strain and electrochemical reaction with the environment. Therefore, to EAC, it is important to understand the mechanical and chemical properties of oxide film formed on smooth surfaces. Micro Raman Spectroscopy (MRS) was used to characterize the oxide film on deforming surfaces of austenitic stainless steel [304L] during tensile tests in air and high temperature water. A specimen pre-oxidized at 1000DGC was tested at room temperature. According to the Raman spectra, the pre-oxidized film mainly consisted of Cr, i.e., Cr2O3 and (Fe,Ni)Cr2O4. In addition, the Raman peak for Cr2O3 shifted during tensile tests. In the beginning, the wave number of the peak gradually decreasing with increase of strain until 5% strain. After 5% strain, the wave number increased, and then the cycle repeated itself again. On the other hand, oxide film formed in high temperature water mainly consisted of NiFe2O4, and the wave number for the NiFe2O4 peak did not shift. It is thought that NiFe2O4 molecules in oxide film do not deformed because they either delaminate or are loosely bonded. Oxide film that consists of NiFe2O4 might be easier to degrade than Cr oxide under straining conditions. Furthermore, in tensile tests performed in high temperature water, differences in the oxide composition between strained region and unstrained region was detected. The oxide film on strained region consisted of much more alpha-Fe2O3 and Cr oxide than that on unstrained region. In-situ Raman analysis showed that alpha-Fe2O3 and Cr oxide formed at about 0.1 % strain and it is thought that the ability of the oxide film to protect the surface become low because of degradation caused by deformation.