Effect Of Zinc And Aluminum Simultaneous Injection On Corrosion Behavior Of Structure Materials In PWR Primary Circuit

The development of PWR primary circuit zinc injection technology has become increasingly mature.But depleted zinc is used in traditional zinc injection technology,which leads to high operating costs.The zinc and aluminum simultaneous injection technology proposed in this paper aims to reduce the amount of zinc injection dose and to further improve the overall corrosion resistance of PWR primary circuit equipment structural materials.It has important application value and guiding significance for improving the safety and economy of primary circuit operation.In this paper,the high strength low-alloy carbon steel,stainless steel and iron-nickel base alloy,which were commonly used in the primary circuit,were used as the research objects.By simulating the water operating environment of PWR primary circuit,the effect of zinc and aluminum simultaneous injection on the corrosion behavior and semiconductor properties of metals were comprehensively studied.The morphology and composition of the passive film were compared and analyzed.And the improvement effect and mechanism of the corrosion resistance of different metals by zinc and aluminum simultaneous injection were discussed from the perspectives of thermodynamic property,point defect model,crystallographic property,and solution deposition balance.The main research contents and conclusions were as follow:(1)In the simulated PWR primary circuit water environment,zinc and aluminum simultaneous injection technology was used to prepare the passive film on the surface of HSLA carbon steel A508-3,stainless steel 304L and iron-nickel-based alloy Incoloy800.Potentiodynamic polarization curve,electrochemical impedance spectroscopy and Mott-Schottky curve were used to study the corrosion behavior and semiconductor properties of samples.The research results showed that compared with zinc injection technology,zinc and aluminum simultaneous injection technology effectively improved the corrosion resistance of the iron-based alloy A508-3 steel and 304L stainless steel,the corrosion current density of these two metals were reduced and the impedance of passive film were increased.And the injection of 20ppb Zn+20ppb Al could achieve a significant corrosion inhibition effect.The effect of improving the corrosion resistance of the iron-nickel-based alloy Incoloy800 after zinc and aluminum simultaneous injection was similar to that after zinc injection technology.In addition,zinc and aluminum simultaneous injection reduced the carrier concentration in passive films formed on these three metals,and affected the semiconductor properties of passive films in different ways.Therefore,compared with the traditional zinc injection technology,zinc and aluminum simultaneous injection technology reduced the zinc injection dose while effectively improving the corrosion resistance of the entire equipment structure material in the PWR primary circuit water chemistry.(2)Surface analysis experiments were carried out on the passive film formed on A508-3 steel,304L stainless steel and Incoloy800 alloy in the simulated primary circuit water environment by photocurrent response spectroscopy,SEM,XPS and XRD.The results showed that zinc and aluminum simultaneous injection made the passive film more dense;ZnAl2O4 spinel was detected in the passive film formed on iron-based alloy A508-3 and 304L.The mechanism of zinc and aluminum simultaneous injection to significantly improve the corrosion resistance of iron-based alloy was that the ZnAl2O4 spinel phase was formed in the passive film under the condition of simultaneous injection of zinc and aluminum.The Gibbs free energy change of the formation reactions of ZnAl2O4 under high temperature was calculated by thermodynamic analysis,which proved that ZnAl2O4 was easily formed in the passive film on the surface of iron-based alloy under high temperature.The point defect model was used to discuss the formation mechanism and required conditions of ZnAl2O4 on the surface of iron-based alloys from a microscopic view.And the reason for the decrease of carrier concentration could enhance the stability of passive film was explained through the calculation of point defect diffusion coefficient.However,in addition to ZnAl2O4,NiAl2O4 and FeAl2O4 were formed in the passive film of Incoloy800 alloy,which lead to a weaker effect of zinc and aluminum simultaneous injection technology on the corrosion resistance of Incoloy800 alloy.(3)The oxide film formed on the surface of three metals in high temperature air atmosphere was modified with zinc-aluminum treatment using salt bath quenching.The electrochemical experimental results of the modified oxide film were consistent with the experimental conclusions in the simulated primary circuit water environment:zinc-aluminum treatment improved the corrosion resistance of three metals in order of A508-3>304L>Incoloy800,which proved that zinc and aluminum simultaneous injection technology was effective in improving the overall corrosion resistance of equipment material in PWR primary circuit.The results of surface analysis experiments showed that zinc-aluminum treatment made the oxide film formed on three metals more smooth and dense.By analyzing the elemental composition of the spinel substance in the oxide film,the formation of ZnAl2O4 on the surface of metals was confirmed.And the amount of ZnAl2O4 generated in oxide film greatly affected the corrosion resistance of alloy.According to the ion distribution law in spinel crystallography,due to the high anion preference energy of Ni2+,the Ni2+existing in the passive film on the iron-nickel base alloy surface could consume part of the metaaluminate to form the inverse spinel NiAl2O4,which hinderd the formation of normal spinel ZnAl2O4 to a certain extent,and affected the effect of zinc and aluminum simultaneous injection technology to improve the corrosion resistance of iron-nickel base alloy.(4)The dissolution and deposition equilibrium reactions of ZnAl2O4,NiAl2O4 and FeAl2O4 in aqueous solution were proposed,and the solubility of these spinel phases in high-temperature aqueous solution was calculated.The calculation results showed that the solubility of ZnAl2O4 under the actual working conditions of the primary circuit was much lower than that of NiAl2O4 and FeAl2O4,which proved its extremely high stability.Therefore,through the simultaneous injection of zinc and aluminum,ZnAl2O4 which could stably exist under high temperature conditions,was generated in the passive film on the iron-based alloy,thereby improving the corrosion resistance and stability of iron-based alloy.Although NiAl2O4 in the passive film on iron-nickel base alloy hinderd the formation of ZnAl2O4 the high solubility of NiAl2O4 made it difficult to stably exist in high temperature environment,resulting in a weaker corrosion resistance improvement effect for iron-nickel base alloy.