Nanoindentation evaluation of passive film stress and growth kinetics

Load controlled nanoindentation in conjunction with a potential step method were utilized for the investigation of precipitated iron sulfate film stresses and growth. Two types of experiments have been undertaken on thin sheet samples of Fe 3% Si single crystal in IM H 2 SO 4 . Samples were either allowed to deflect in the indentation direction or constrained. A distinctive difference between the indentation curves for the above two types of tests allowed separation of the effects of film stresses and local electrochemical processes. A proposed theoretical model accounting for both electrochemical and mechanical effects allowed modeling of the indenter tip motion following a potential step. Within the scope of the model, the time dependent film thickness (3.5 μm at maximum), electrostrictive film stress (330 MPa at maximum) have been determined.