Shadow corrosion

An electrochemical mechanism, based on energetically favored complimentary reduction and oxidation reactions, operating in conjunction with radiolysis is proposed for explaining the shadow corrosion phenomenon observed under BWR conditions. The electrochemical reaction on platinum and nickel alloys (Inconel and X-750) is the oxidation of H2O2 to produce a localized enhancement in the concentrations of HO2 and O2. Energy level of the conduction band of ZrO2 matches well with that for the reduction of HO2 and O2 regenerating H2O2. This reduction of the powerful oxidants, stimulates electron emission in ZrO2 which then is balanced by increased oxidation of zirconium to generate additional electrons and hence also anion vacancies. A coupling between Zircaloy and platinum or nickel alloy is provided by H+ transport, the source for initiating shadow corrosion, to Zircaloy-2 (Zircaloy-2 negative relative to platinum or the nickel alloy) in the gap between the materials. An enhanced localized corrosion of Zircaloy-2 occurs, its incidence dependent upon the transport of HO2, O2, H+ and H2O2 in the coolant in the gap.