The kinetics of hydrogen evolution and oxygen reduction on Alloy 22

The kinetics of hydrogen evolution and oxygen reduction on Alloy 22 in 5 M NaCl + xHCl + yNaOH solutions as a function of pH at temperatures ranging from 20 to 95 °C have been investigated. The hydrogen evolution reaction proceeds via two basic mechanisms; the reduction of H+ at pH < 4 and the reduction of H2O at higher pH values. Analytical expressions for the exchange current density for the hydrogen evolution reaction on Alloy 22 are also developed for the two mechanisms, in a form that they can be used in corrosion models for assessing the performance of this alloy in high level nuclear waste (HLNW) repositories. The kinetics of oxygen reduction have also been explored over wide ranges of pH and temperature. However, this reaction is complicated by the formation of H2O2 as an intermediate in a two-electron transfer reaction. Nevertheless, kinetic parameters have been obtained for this reaction and an expression has been developed that allows calculation of the exchange current density over a wide range of conditions. Finally, the kinetic data are used to identify probable mechanisms for hydrogen evolution and the reduction of oxygen on Alloy 22.