Electrochemical Corrosion Behaviors In Normal Temperature Environments Of Important Structural Materials Used In Coastal Nuclear Power Plants

The safe operation of nuclear power plants is significant. The corrosion problems of structural materials in the construction and operation periods can seriously affect the safety performance. This study mainly focused on electrochemical corrosion behaviors in normal temperature environments of six typical important structural materials used in coastal pressurized water reactor (PWR) nuclear power plants, including the polarization behaviors, pitting and galvanic corrosion. The six materials are low alloy steel 16MND5, stainless steel weld metal 308L and stainless steel base metal Z2CND18-12N, which are mainly used in the PWR of China Guangdong Nuclear Power Croup, as well as low alloy steel A508?, nickel base weld metal 52M and stainless steel 316L, which are mainly used in other PWR nuclear power plants in our country. Three kinds of experimental solutions were used in this work:?PWR primary water solution, which simulated the environment of primary loop opened in shutdown condition;?PWR primary water solution contaminated with chloride ions, which simulated the PWR primary water polluted by chloride caused by marine atmosphere diffused into the houses of plants in the shutdown period with the loop opened;?3.5%NaCl solution, which simulated seawater environment. The main research work and results are as follows:(1)The polarization behaviors of low alloy steel 16MND5, stainless steel weld metal 308L and stainless steel base metal Z2CND18-12N in above three kinds of test solutions were studied. The results showed that, in the PWR solution, three materials exhibited passive tendencies and the self-corrosion current density values were the minimum as compared with those in other solutions, indicating the minimum corrosion susceptibility in the working coolant. When the PWR solution was added with 10 mg/1 (ppm) of chloride ions, corrosion behaviors of two kinds of stainless steel Z2CND18-12N and 308L had little changed, while the self-corrosion rate of low-alloy steel 16MND5 significantly increased. When the test solutions contained high dissolved oxygen, the self-corrosion current density of 16MND5 increased, which should be caused by oxygen depolarization resulting in self-corrosion rate increases. Maximum corrosion susceptibilities of three materials were exhibited in 3.5% NaCl solution, only stainless steels showed passivation properties. The passivation area of Z2CND18-12N steel was wider than that of 308L, indicating that its passive film had better protection and stability than that of 308L.(2) The polarization behaviors of low alloy steel A508?, nickel base weld metal 52M and stainless steel 316L in the three test solutions were studied. The results showed that:the material exhibited the minimum corrosion susceptibility in the PWR solution than in other two solutions. Passivation area of 316L in 3.5% NaCl solution was not stable, which indicated that the corrosion of high concentration of chloride ion on 316L was great. The stability of passivation film on 52M was also fine in 3.5% NaCl solution. The polarization curves of 52Mwere similar in PWR and PWR+10ppmCl- solutions, meaning that the corrosion susceptibility was not changed greatly by the contamination.(3)The pitting potentials of 16MND5,308L, Z2CND18-12N determined from anodic polarization curve measurements according to national standard, as well as the effects of surface status and the direction of weld microstructure were studied. The results showed that:the pitting potential of Z2CND18-12N was high, above 400mV (SCE); there was no significant difference between pitting resistances of 308L with test surfaces perpendicular and parallel to the weld direction respectively; the anodic polarization curve of low alloy steel 16MND5 showed continuous activation polarization control, the current density increased, until the anodic polarization curve slope increased, indicated that the polarization control changed.(4)The galvanic corrosion behaviors of following six couples were studied: 16MND5/308L,308L/Z2CND18-12N,16MND5/Z2CND18-12N, A508 ?/52M, A508?/316L and 52M/316L in the three solutions. The results indicated that:?the galvanic current density values of the six couples were weak in PWR solution, no obvious galvanic corrosion tendency, since the small potential differences.?galvanic current density values of couples 16MND5/308L,16MND5/Z2CND18-12N, A508?/52M and A508?/316L were relatively high in PWR+10ppmCl- solution with higher potential difference, indicating obvious galvanic corrosion tendency. However, galvanic current density of each couple changed with time in different ways during the experimental period. ?the galvanic current density values of couples 16MND5/Z2CND18-12N?A508?/52M and A508?/316L were the highest in 3.5% NaCl solution among the three solutions, indicating the galvanic corrosions were the most serious.?The weak (close to zero) galvanic current density of two relaticely low potential difference couples of 308L/Z2CND18-12N and 52M/316L indicated that no galvanic corrosion occurred in three kinds of solution.?In the serious galvanic corrosion systems, the corrosion of two low-alloy steels 16MND5 and A508 ? as the anode were accelerated, and showed general corrosion status, while the corrosion of stainless steel 308L,316L, Z2CND18-12N and Ni-based alloy 52M as the cathode did not happen apparently.