Kinetics of the Fe+++/Fe++ Reaction on Fe-Cr Alloys
TYCO LABS INC WALTHAM MA
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The electrode kinetics of the FeFe couple on passive Fe-Cr alloys were studied in acid solutions of constant ionic strength M MgSO4. Tafel behavior is generally observed with exchange currents of the order of 10 to the -6th power ampsq cm, a cathodic transfer coefficient of 0.40, and an anodic transfer coefficient of 0.36. The electrochemical reaction order is unity both for oxidation of Fe and reduction of Fe the heat of activation at the reversible potential is 10.8 kcalmol. The results show that an apprecible fraction about 0.4 of the total potential drop on anodic polarization occurs within the passive film. This potential drop is probably associated with changes in the average oxidation state of cations between the inner and outer layer of the surface oxide. The electrochemical characteristics of the film depend on the pH of the solution, probably because of the migration of protons into the passive film. A change in the kinetics of Fe oxidation which occurs in the transpassive region for the Fe-Cr alloy is probably due to the production of Fe3 or Cr6 ions in the inner oxide layer.
- Electrochemical Energy Storage