RESEARCH ON PITTING CORROSION BEHAVIOR OF COPPER IN THE SOLUTION WITH HCO 3 - AND Cl -

Abstract

The strategy for disposal of high-level radioactive waste in china is to enclose the spent nuclear fuel in sealed metal canisters which are embedded in bentonite clay hundreds meters down in the bed-rock.The choice of container material depends largely on the redox conditions and the aqueous environment of the repository.One of the choices for the fabrication of waste canisters is copper,because it is thermodynamically stable under the saline,anoxic conditions over the large majority of the container lifetime.However,in the early aerobic phase of the geological disposal the corrosion of copper could take place,and the corrosion behavior of copper would be influenced by the complex chemical conditions of groundwater markedly.Pitting corrosion of copper often takes place in power plants or air-conditioning condensate water.The corrosion environment usually contains HCO_3~-, SO_4~(2-)and Cl~-.In the early stage of geological disposal,if the aerobic water with HCO_3~-,SO_4~(2-)and Cl~- immersion repository,the pitting corrosion of copper may occur.Some researchers believed that SO_4~(2-)and Cl~- would promote the occurrence of pitting corrosion of copper,and HCO_3~- will lead to surface passivation and inhibit pitting.It is considered that in the solution with HCO_3~- and SO_4~(2-), HCO_3~- could firstly promote and then inhibit pitting.However,there is no systematic work about pitting in the solution with HCO_3~- and Cl~-.In this work,the cycle polarization behavior and surface morphology of pitting on copper has been investigated in HCO_3~- and Cl~- mixed solution,respectively by electrochemical cyclic polarization test and SEM.The results showed that the circular polarization curves of copper could be divided into four types.The pitting on the surface of copper occurs only in the environment with both Cl~- and HCO_3~-.In the area of active dissolve pitting,the pitting susceptibility increased with the increase of concentration of Cl~-,while it increased then decreased with the increase of the concentration of HCO_3~-.In the area of passive film rupture pitting area,pitting susceptibility increased with the increase of concentration of Cl~- and with the decrease of the concentration of HCO_3~-.