| An in situ study of the films formed on stainless steel at 288°C was performed using Surface Enhanced Raman Spectroscopy (SERS). By monitoring changes in the structure of the film as the interface potential of the sample changed, it was possible to suggest how the surface film affects Intergranular Stress Corrosion Cracking (IGSCC). Samples included 304 stainless steel, chromium depleted alloys, iron, nickel, and chromium. Observations of the films on iron, nickel, and chromium were compared to calculated Pourbaix diagrams. All of the iron based samples were found to passivate by the formation of alpha-Fe2O3 at --400 mV SHE. Below this potential, the surface film consisted of Fe3O4. On the iron-chromium alloys, gamma-Fe2O3 was found to form at intermediate interface potentials. These results suggest a reason for the well-known observation that IGSCC only occurs at potentials above --230 mV. Namely, when the potential outside the crack is --230 mV, the potential at the tip of the intergranular stress corrosion crack is --400 mV, the value above which all iron base alloys regardless of their chromium concentration are passive. Decreased chromium concentrations decreased the ability of alpha-Fe 2O3 and Fe3O4 to protect the sample. In addition, decreased chromium concentrations increased the potential at which gamma-Fe2O3 formed. Ex situ scanning electron microscopy was used to show that the films formed on the iron based samples were a double layer film consisting of an inner layer of conformal Fe3O4 and an outer layer of ∼1um sized grains of Fe3O4. Upon passivation, the alpha-Fe 2O3 formed a thin layer over all surfaces. The transformation of Fe3O4 to alpha-Fe2O3 takes place by a solid state reaction. The majority of outer layer formation occurred before the formation of the passivating alpha-Fe2O3 layer. Growth of the outer layer is limited by the electrochemical reduction at the film/water interface and not by transport of iron cations through the inner layer of Fe3O4. It is proposed that the ability of gamma-Fe2O3 to help initiate passivation plays a major role in the stress corrosion cracking of sensitized 304 stainless steel. |
