Corrosion Behavior Of Nuclear Grade Carbon Steel In Boric Acid Solution

A508-3 carbon steel is the main structural material of the nuclear power plant pressure vessel shell,and is an important barrier to protect the reactor pressure.However,the corrosion failure phenomenon of carbon steel material is an important issue affecting the safe operation of nuclear power plants.As a pressure vessel casing,A508-3 carbon steel usually does not contact the primary circuit boric acid aqueous solution,but when the inner stainless steel or the penetration piece breaks,it will be exposed to boric acid,causing corrosion failure.In this paper,the corrosion behavior of A508-3 carbon steel in different temperatures and different concentrations of boric acid solution was studied.After the A508-3 carbon steel and 304 stainless steel were coupled,the effects of cathode anode area ratio,solution concentration and temperature on the galvanic corrosion were investigated,and the products after corrosion on the metal surface were observed and analyzed.The main conclusions were as follows:At 50 °C,A508-3 carbon steel produced severe corrosion in different concentrations of boric acid solution,and there was no passivation during corrosion.As the concentration of boric acid increases,the surface charge transfer resistance of A508-3 carbon steel becomes smaller and the corrosion resistance decreases.When the concentration of boron in the solution reaches 1200 ppm,A508-3 carbon steel has the highest corrosion current density and low self-corrosion potential,and the corrosion sensitivity is the strongest.The EIS measurement of A508-3 carbon steel in different concentrations of boric acid solution at 50 °C for 7 consecutive days showed that the corrosion resistance of carbon steel became smaller with time.At the same time,the high concentration boric acid solution corresponding to high corrosion sensitivity.The corrosion rate of carbon steel materials at 50 °C,90 °C,130 °C,170 °C,210 °C was calculated by weight loss method.The results show that the corrosion rate increases first and then decreases with the increase of temperature.When the temperature is low,A508-3 carbon steel corrosion is a heat-activated process,and corrosion increases with increasing temperature,and the corrosion rate is affected by the ion composition of the solution.The SEM characterization results show that the increase of temperature can deepen the corrosion degree of carbon steel,and by comparing 170 °C with 50 °C,the high temperature can significantly change the content of O,Fe and other elements,and the corrosion product composition changes.The galvanic corrosion test proves that the A508-3 carbon steel and 304 stainless steel are coupled to each other and accelerate corrosion as an anode.As the area ratio of the cathode anode increases,the galvanic potential of the A508-3 carbon steel and the 304 stainless steel galvanic couple gradually increases,and the galvanic current decreases.If the anode is too large,the system will be stabilized quickly.When the area ratio of yin and yang reaches 1:10,the galvanic current after the system is stable is close to zero.At this time,the anode reaction speed is too fast,and the cathode cannot transfer a large amount of electrons transferred from the anode,causing an increase in reaction resistance.The weight loss test shows that the galvanic corrosion can significantly accelerate the corrosion rate of A508-3 carbon steel,and the corrosion rate of carbon steel increases with the increase of solution concentration.SEM characterization showed that the morphological corrosion of A508-3 carbon steel was different at 120 °C,the oxide particles became larger,the surface of 304 stainless steel was evenly distributed,and the oxide particles were multi-faceted cubic structure.