Stress Corrosion Cracking And Pitting Behaviors Of Stainless SteelsUsed For Heavy Water Collection Tubing In Nuclear Power Plants

L-grade low-carbon austenitic stainless steels are widely used in key structural parts of nuclear power plants due to their high corrosion resistance.This work is based on the corrosion cracking failure of heavy water collection tubing(304L and 316L stainless steel)of heavy water reactor nuclear power plant.The stress corrosion cracking(SCC)and pitting corrosion behaviors of 304L and 316L stainless steels in simulated service environment were studied,including the influences of electrode potential and temperature on SCC sensitivity as well as the effects of temperature and solution on pitting corrosion.The purpose was to provide scientific basis for understanding and controlling of the failures in plants.More detained works performed are as follows.1.The SCC characteristics of the two materials in a simulated solution(containing 0.7mg/L Li+?100 mg/L Cl-)at 250? for service conditions were tested using slow strain rate test(SSRT),and the effect of electrode potential on SCC of different materials was examined.The results showed that different degrees of SCC occur in 304L and 316L,and the fracture morphology was the cleavage fracture.The electrode potential had a significant effect on the SCC of the specimens.With the decrease of the electrode potential,the SCC sensitivity of 316L decreased evidently,and the SCC sensitivity of 304L also decreased slightly.2.The SCC characteristics of the two materials under simulated service conditions of 150? simulated solution were tested by SSRT.The effects of electrode potential and temperature on SCC of different materials were investigated and discussed.The results show that the 304L and 316L stainless steels all exhibited different degrees of SCC.Lowering the temperature,the SCC sensitivities of both materials were reduced.The SCC fracture morphology was quasi-cleavage fracture.The electrode potential had a significant effect on the SCC of the 304L,and the SCC sensitivity decreased significantly as the electrode potential decreased.At a given electrode potential,the SCC sensitivity of 316L was low as compared with 304L.Only a small amount of SCC features were observed on the fracture morphology,showing higher SCC resistance.3.The pitting behaviors of the two materials in simulated solution and 3.5%NaCl solution at 30? and 60? were tested by using anodic polarization and critical pitting temperature(CPT)methods.The effects of temperature and Cl-concentration were examined.The results show that the polarization curves of the two stainless steels in the two solutions were similar in the same environments.The increases of temperature and the concentration of Cl' in the solution reduced the pitting resistance of the materials.Anodic polarization curve test and CPT test results showed that the 304L pitting sensitivity was higher than 316L in the same test environment.The anodic polarization curve in the simulated solution test showed that the potential of the retrace point was high,resulting in crevice corrosion in the specimens,but the measured Eb value of the pitting corrosion was not related to crevice corrosion.4.The anodic polarization curves of the two materials at 150? and 250? for in simulated solution were tested.It was found that the pitting potential of the material at 250? was reduced as compared with to 150? and the protection potential was increased;the pitting potential measured by the second scan was higher than that of the first one.