Initiation and growth of corrosion fatigue crack for AVPP treated stainless steel.

The influence of material surface condition, notably passive film condition and surface residual stress and the effect of applied surface stress on the development of corrosion pits has been assessed in relation to their role on the fatigue behaviour of stainless steel materials, in particular the early stages of corrosion fatigue. Several experimental techniques have been used to quantify changes in material response during stress and corrosion interactions. The original surface film of 316L austenitic stainless steel was modified electrochemically using an alternative voltage passiviation process (AVPP) to study the effects of this modification on the mechanical and corrosion properties. The composition, structure and thickness state of the original and modified oxide films were analysed applying X-Ray photoelectron spectroscopy (XPS) technique. Chromium enrichment in the modified passive film surface was observed, as was increase in the passive film thickness for the modified surface.Potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) tests were performed in artificial seawater on the original and the modified passive film, in order to evaluate the differences in corrosion behaviour of the material. Results show that the pitting potential Ep of the modified film is higher than the original film. Also EIS results show improvement in the corrosion resistance in material surface with the modified film. Potentiostatic measurements using the Scanning Droplet Cell (SDC) system were used to investigate the effects of surface finish and the electrolyte flow rates on localised corrosion (pitting). The localised corrosion rate increased as the flow rate and surface roughness increased. Changes in surface residual stress state of the native and modified oxide films during cyclic loading were studied using X-Ray Diffraction technique. Surface compressive residual stress was found to decreased as a result of the introduction of a new oxide film.Nanoindentation tests using a Berkovich diamond indenter were performed to explore the differences in mechanical properties between 316L SS oxide film before and after AVPP treatment. The load-depth curves of the native oxide film surface were compared with those of the modified oxide film surface to investigate the differences in the mechanical properties. It was found that the 316L oxide-film has a higher hardness compared to the AVPP oxide-film. Tensile tests were conducted to investigate the tensile strength of the material, results show that the yield strength and the tensile strength of 316L SS samples were about 300 MPa and 600 MPa respectively. The effect of corrosion pits on the fatigue behaviour of stainless steel materials in particular the short crack regime was investigated using the SDC in combination with a fatigue rig. The mechanism of corrosion fatigue crack growth, which consists of, pit nucleation, pit growth, transition from pitting to fatigue crack initiation, and short crack growth were studied. This system consists of a fatigue rig, and electrochemical droplet cell. Results showed improvement in the corrosion fatigue life in the AVPP modified 316L SS surface compared to the native surface due to a delay in pit initiation.