In Situ Micro Raman Spectroscopy for Characterization of Oxide Film Formed on the New Surface and for Measurements of the Stress of Oxide Film Formed on 304L Stainless Steel

Abstract

Environmentally assisted cracking (EAC) consists of two distinct events viz., i) crack initiation and ii) crack propagation. On a smooth surface, the EAC initiates by the rupture or by the degradation of the surface film due to the combined action of stress and an electrochemical reaction of the materials with the environment. The mechanical properties of the surface oxide films are also important considerations when determining the susceptibility to EAC. In this research, Micro Raman Spectroscopy(MRS) was applied for in-situ oxides characterization and for in-situ measurements of the stress in oxide film formed on the surface of 304L stainless steel during the scratching electrode and the slow strain rate test (SSRT), respectively. The passive oxide film growth formed on the bare surface was continuously monitored by MRS as a function of time. For stress measurements, Cr2O3 was focused on and Raman shift at Cr2O3 peak of Raman spectrum was measured continuously. The strain rate was 8.2×10-7 /sec. . In the initial stage of SSRT, the Raman shift of surface film decreased gradually with strain. At 5% strain, the Raman shift of surface film increased rapidly to around the initial value of Raman shift. It is considered that the surface film was ruptured at this time. At 5% strain, the shift value of Raman peak of Cr2O3 reached to 5cm-1. This value (5cm-1) corresponds to 1.2GPa which value of tensile stress is calculated from reference data. These characteristics of oxide film will be implemented into the theoretical formulation of EAC and their implication to EAC growth rate will be discussed.