Stress variations during polarization of iron thin film electrode in pH 8.4 borate buffer solution

The stress variations during polarization of iron thin film electrode in pH 8.4 borate buffer solution have been measured by a cantilever bending method to investigate the correlation between stress evolution and electrochemical reaction of the iron electrode. The stress variations toward compressive direction during active dissolution of iron result from the relaxation of the residual tensile stress due to the decrease in thickness of the iron electrode. The stress variations toward tensile direction in the pre-passive region are brought by the anodic deposition of ferrous ions once dissolved into solution. The successive potential steps for each interval of 1 h in the passive region between 0.20 and 0.90 V induce the stress variations toward compressive direction at potentials more positive than 0.30 V in connection with the growth of the inner (barrier) layer in the passive film. The electrostriction stress in the inner layer of the passive film formed on iron at 0.90 V has been estimated from the stress relaxation in the cathodic potential sweep after 1-h passivation at 0.90 V. In addition, the stress variations toward compressive direction during cathodic reduction of the passive film are attributed to the volume expansion due to the formation of iron (II) hydroxide at the intermediate step of the cathodic reduction at pH 8.4. Furthermore, the rapid stress variations toward compressive direction during hydrogen evolution at potentials more negative than 0.52 V are caused by the volume expansion in near the surface region of the iron thin film electrode due to hydrogen adsorption or absorption.