The Study Of Pitting Corrosion Of Austenitic Stainless Steel

The technique based on Potential Dynamic Polarization , Electrochemical Impedance Spectroscopy (EIS) and Electric Speckle Pattern Interferometry (ESPI) were applied to study the pitting corrosion behavior of austenitic stainless steel.An ESPI based technique for in situ observation of pitting susceptibility of metals was reported in this paper.The main results are as following:The pitting susceptibility of 1Cr18Ni9Ti,304 and 316 austenitic stainless steel in 3.5%NaCl solution was detected by Potential dynamic polarization , EIS and ESPI.Results of Potential dynamic polarization showed that the values of Eb for 1Cr18Ni9Ti, 304 and 316 stainless steel were 0.085V , 0.465V and 0.573V respectively.Results of EIS showed that the 1Cr18Ni9Ti stainless steel came into pitting induction period after just beginning immertion in the NaCl solution and the 304,316 stainless steel came into pitting induction period after 28h and 72h immertion in the NaCl solution respectively.Results of ESPI indicated that the values of pitting induction time (τ) for 1Cr18Ni9Ti and 304 stainless steel were 1s and 9s.The value of pitting induction time (τ) for 316 stainless steel was more than 120s.The pitting susceptibility of 1Cr18Ni9Ti stainless steel was the worst of the three types of materials,and the 316 stainless steel was the best.The result of ESPI was in agreement with that of potential dynamic polarization and EIS.The effects of concentration for Cl-,SO42-and S2O32- on pitting susceptibility of 304 austenitic stainless steel were studied by ESPI and electrochemical methods.The results showed that the capacity of resisting pitting for 304 stainless steel decreased with the value of Cl- concentration increasing.Different SO42- concentration had different effects on the corrosion behavior of 304 stainless steel in 3.5% NaCl solution.Low SO42- concentration (0～1%)would cause the concentricity of Cl- on the 304 surface which leaded to the decreasing of the corrosion resistance for the material.When the SO42- concentration was 0.5%,the capacity of resisting pitting was the worst.High SO42- concentration (1%～4%)would reduce the effective Cl- concentration on the material surface which increased the corrosion resistance.The capacity of resisting pitting for 304 stainless steel decreased with the value of S2O32- concentration increasing.When the S2O32- concentration was low(0～0.5%),the corrosion resistance decreased quickly.When the S2O32- concentration was high(0.5～3.5%),the corrosion resistance decreased slowly.The effect of the value of pH on pitting corrosion behavier for 304 austenitic stainless steel was studied by Potential dynamic polarization and ESPI.Results of Potential dynamic polarization showed that the value of Eb for 304 stainless steel in 3.5%NaCl(pH=14)was higher than that in 3.5%NaCl(pH=7).The value of Eb in 3.5%NaCl(pH=3)was the lowest of the three kinds of solution.Results of ESPI indicated that the value of pitting induction time (τ) for 304 stainless steel in 3.5%NaCl(pH=14,pH=7, pH=3)were 28s,9s,0.5s respectively.Two methods had the same result,which was the 3.5%NaCl(pH=14)solution possessing the best corrosion resistance and the 3.5%NaCl( pH=3 ) solution possessing the worst corrosion resistance.The SEM morphology of 304 stainless steel surface after polarization showed that the material in 3.5%NaCl(pH=3)and in 3.5%NaCl(pH=7)solutions had the similar etch pit feature,but a lacy cover etch pit was produced in 3.5%NaCl(pH=14)solution.