Inhibition of chloride pitting corrosion of Zircaloy-4 alloy in highly radioactive water by radiolytic nitrate and hydrogen peroxide

Abstract

We report an investigation of the pitting corrosion susceptibility of Zircaloy-4 alloy in presence of radiolytic chloride, nitrate and hydrogen peroxide. The electrochemical behavior of Zircaloy-4 was essentially studied using cyclic voltammetry and electrochemical impedance spectroscopy to provide an indication of mechanisms and oxide layer modifications. The experiments have shown that the pitting corrosion behavior is dependent on the concentration of these radiolytic species in tritiated water. Nitrate shows pronounced inhibitory action due to adsorption of the ammoniac formed on the passive oxide layer buffering the pH, which stops pit initiation and assists repassivation of the oxide surface. The presence of both hydrogen peroxide and NO3 − produces other effects. The passive oxide layer is thicker and its characteristics change in the bulk. Also, pit initiation is stopped whereas it is more difficult to obtain repassivation of existing pits. This can be explained by the capability of 3H2O2 to oxidize the surface and thus enhance passive oxide formation. But, in this case, NO3 −/N3H+ 4 adsorption should be hindered by the 3H2O2 present and consequently there is less buffering of the surface pH limiting repassivation of the existing pits. However, with these two combined effects: pH kept constant on the Zircaloy-4 surface and enhancement of the oxide layer intrinsic characteristics, less pitting is observed than in presence of chloride ions alone.