Corrosion Behaviors Of Reactor Pressure Vessel Low Alloy Steel And Stainless Steel Cladding In Simulated Pressurized Water Reactor Primary Water Environments

Low alloy steel?LAS?is used to fabricate reactor pressure vessels in pressurized water reactor?PWR?nuclear power plants.The inner surface of the reactor pressure vessel is usually cladded with stainless steel?SS?.Reactor pressure vessel steel in PWR pants doesn't contact with primary water under normal condition,but could be exposed to the leaked primary water as a result of the damage of the pressure boundary materials in the primary loop.Studying the corrosion behaviors of low alloy steel under the conditions of PWR primary water leakage can provide basis for the reliability evaluation and lifetime management of reactor pressure vessels.The oxidation behaviors of stainless steel cladding in simulated PWR primary water were studied.The surface treatment for improving the oxidation resistance of stainless steel cladding in simulated PWR primary water was investigated.The effects of temperature,solution compositions,dissolved oxygen and galvanic coupling with stainless steel on the corrosion behavior of A508? LAS in ambient pressure and high temperature/high pressure boric acid solutions were investigated.Corrosion immersion tests,electrochemical measurements and characterizations of the surface oxides with scanning electron microscopy,transmission electron microscopy and X-ray photoelectron spectroscopy etc.were performed.The oxidation behaviors of316L SS and 308L SS cladding in high-temperature water after mechanical polishing,electropolishing and magnetoelectropolishing were compared.The main conclusions are:1.The open circuit potential states of A508? LAS changed from passivation state to active dissolution state with the increase of boric acid concentration at 25?.When boric acid concentration was 0.2 mol/L or higher,the open circuit potential of A508? LAS was stabled at a low potential in the active dissolution region of the Fe-H₂O E-pH diagram and a high corrosion rate.When the concentration of boric acid solution was 0.1 mol/L or leass,the open circuit potential of A508? LAS was randomly represented at three levels:high potential,low potential and intermediate potential.A high open circuit potential corresponded to the oxide stable region and a low corrosion rate.The corrosion rate of A508? LAS in boric acid solutions with ferric ions was mainly determined by the concentration of ferric ions.2.The corrosion rates of A508? LAS in aerated and deaerated boric acid solutions with lithium hydroxide showed different temperature dependencies at 25-95?.The corrosion rates in aerated solutions showed a peak value at approximately70-75?,while the corrosion rates in deaerated concentrated solutions increased monotonically with increasing temperature.The dissolved oxygen in aerated solutions facilitated the formation of a protective film thus reduced the corrosion rate.3.For the A508? LAS and 316L SS galvanic couples,the galvanic effect on the corrosion behaviors of A508? LAS in deaerated boric acid solutions with lithium hydroxide was related to the controlling processes of metal corrosion rate at different temperatures.For the temperature lower than 100?,the galvanic coupling accelerated the active dissolution corrosion of low alloy steel.At 110? and 150?,the galvanic coupling accelerated the dissolution and film formation of low alloy steel.At 250? and 310?,the galvanic coupling accelerated the film formation of low alloy steel.4.Magnetoelectropolishing surface treatment significantly improved the oxidation resistance of 308L SS cladding and 316L SS in simulated PWR primary water.After exposed in high temperature water,the surface of 316L SS with magnetoelectropolishing retained its original brightness and the thickness of the surface oxide film was only a few nanometers.Duplex-layer oxide films in a sub-micron thickness were formed on 308L SS cladding surfaces with mechanical polishing,electropolishing and magnetoelectropolishing.The thickness of the inner oxide layer on the exposed 308L SS with magnetoelectropolishing treatment was the thinnest,which was related to the high Cr/?Fe+Ni?in the oxide film on the as-treated stainless steel with magnetoelectropolishing.