Semiconductor Electrochemical Research On Mechanism Of Corrosion Resistance Of Oxide Films On Fe-base Alloys

Fe-base Alloys are main construction materials used in the primary circuit of nuclear power reactors.However,corrosion of Fe-base Alloys has become major problem for the secure and economic operation of nuclear power plant.Therefore,investigation for the corrosion mechanism and technology of corrosion protection is of great theoretical significance and application value for the enhancement of safety and economy of nuclear power reactors.In this thesis,the object is the oxide films formed on Fe-base Alloys.Methods of electrochemical,semiconductor photoelectrochemical and surface analysis are used to investigate the composition,corrosion resistance and semiconductor properties of oxide films.The enhancement mechanism of zinc ion and aluminum ion on the corrosion resistance of Fe-base Alloys is investigated.Main contents and conclusions of this thesis are summarized as follows:(1)Zinc injection is widely used in the primary circuit of nuclear power reactors to reduce radiation fields and to reduce the degradation of coolant system materials.The enhancement for the corrosion resistance of stainless steel and Ni-base Alloys results from the formation of oxide films with double layer structures,i.e.,ZnCr2O4 in the inner layer and ZnFe2O4 in the outer layer.It is generally considered that ZnCr2O4 improves the corrosion resistance of oxide films on stainless steel and Ni-base Alloys due to its better thermodynamic stability.Investigation for the impacting mechanism of ZnFe2O4 on the corrosion resistance of oxide films is less.In this thesis,we modify the oxide film on carbon steel through zincizing treatment to make sure that ZnFe2O4 is contained in the oxide films while ZnCr2O4 is not.Semiconductor properties are investigated through electrochemical polarization,Mott-Schottky techniques and photocurrent response.It indicates that ZnFe2O4 is beneficial to the enhancement of corrosion resistance of oxide films formed on carbon steel.Through analyzing the thermodynamic properties and the equilibrium between dissolution and precipitation of related phases,it reveals the enhancement mechanism of ZnFe2O4 on the corrosion resistance of Fe-base Alloys.(2)To further investigate the functional mechanism of ZnFe2O4 on the corrosion resistance of construction materials in high temperature water with zinc injection,the solubility of ZnFe2O4,Fe3O4 and Fe2O3 in high temperature water are compared in this thesis.It indicates that the solubility of ZnFe2O4 is several orders of magnitude smaller than that of Fe3O4 or Fe2O3 when the pH value is above 6 at the temperature of 373K.When the temperature increases to 423K,the pH range for which the solubility of ZnFe2O4 is the lowest is enlarged.In the temperature ranges from 473K to 623K,the solubility of ZnFe2O4 is much lower than that of Fe3O4 or Fe2O3 in the whole pH ranges.It shows that with the increase of temperature,the gap between the solubility of ZnFe2O4 and that of Fe3O4 or Fe2O3 is enlarged.Moreover,the solubility of ZnFe2O4 changes little with the increase of temperature.Therefore,the lower and stable with temperature changes of the solubility of ZnFe2O4 is beneficial to sustain the better corrosion resistance of construction materials in nuclear power plants with the application of zinc injection technique.(3)When injecting zinc into the primary circuit of nuclear power reactor,Zn-64 contained in natural zinc could turn into Zn-65.It could emit gamma-rays,which had a negative impact on the reduction of buildup of radiation fields.Therefore,depleted zinc is used.However,it is expensive.In order to enhance its economy,the technique of zinc injection is improved in this thesis.It suggests simultaneous injection of zinc and aluminum could be applied.With this method,some of the zinc ion was replaced by aluminum ion and its negative impact on reduction of buildup of radiation fields is weakened.Corrosion parameters of the oxide films formed on stainless steel are investigated through electrochemistry technique.It indicates that the corrosion resistance of the oxide film is significantly enhanced through simultaneous injection of zinc and aluminum.To further investigate the enhancement mechanism,photo electrochemical approach and XPS are applied.It suggests that additional phase a-FeOOH and ZnAl2O4 are contained in the oxide films on stainless steel in high temperature water with simultaneous injection of zinc and aluminum.The solubility of ZnAl2O4 is calculated.Moreover,it proposes the possible reactions between the ion in solution and the phase contained in the oxide films.Through analyzing the thermodynamic properties and the equilibrium between dissolution and precipitation of related phases,it reveales that the enhancement mechanism of corrosion resistance is that ZnAl2O4 with lower solubility and more thermodynamically stable properties is contained in the oxide films formed on stainless steel in primary circuit water chemistry with simultaneous injection of zinc and aluminum.The research results lay a theoretical foundation for improving the safety and economy of nuclear power.