The Study Of Local Corrosion Behavior Of Two Stainless Steels In Simulated Marine Environments

The implementation of the marine development strategy of China requires marine engineering equipment to provide quality assurance,which needs high-performance materials as a support.In this work,the localized corrosion behaviors including pitting corrosion,crevice corrosion and stress corrosion cracking(SCC)in simulated marine environments of two kinds of stainless steel materials,austenitic stainless steel Z2CND18-12 N and duplex stainless steel S32760,were studied mainly with electrochemical methods.The simulated environments were artificial sea water prepared according to ASTM standard,and various NaCl solutions with different concentrations and at different temperatures.The selected different temperature conditions were 3?,30? and 60?.The selected NaCl concentrations of the solutions were 3.5% NaCl for normal concentration in the sea water and 10% Na Cl to simulate concentrated condition due to evaporation.The main work performed and conclusions obtained are as follows:(1)The anodic polarization behaviors of the two steels in artificial seawater and 3.5% NaCl solution were studied at first.S32760 exhibited similar polarization behaviors in both solutions,and showed wide passivation electrode potential zone,and high pitting potential above +1000mV(vs saturated calomel electrode,SCE,the same for the potential values below).The polarization behaviors of Z2CND18-12 N in the two solutions were also similar,showing apparent metastable fluctuation and then pitting breakdown.The pitting behaviors of the two materials at 3?,30? and 60?,respectively,in 3.5% NaCl and 10% NaCl solutions were studied.S32760 showed excellent pitting resistance with pitting potentials around +1000mV,but there was some metastable pitting at 60?.Z2CND18-12 N showed excellent pitting resistance close to S32760 at low temperature of 3?.However,the increase of temperature led to a significant decrease in pitting resistance for this steel,as obvious pitting breakdown appeared in the polarization curves and pitting corrosion morphology on the specimens when the temperature was up to 30? and 60?.Z2CND18-12 N showed weaker pitting corrosion resistance than S32760 duplex stainless steel at relatively high temperatures.When the concentration of NaCl solution increased,the pitting potential decreased.(2)For comparison,the pitting corrosion behaviors of the materials were studied by using methods of potentiodynamic polarizing,critical pitting temperature testing and corrosion morphology observation.The results showed that the comprehensive evaluation of different methods should be more reasonable.For example,the pitting potential at 60? of S32760 duplex stainless steel was close to +1000mV,similar to those at lower temperatures,but metastable pitting morphology could be observed on the specimens with scanning electron microscopy,so it had pitting tendency under this condition.The critical pitting temperature testing can well reflect the pitting resistance and sensitivity to temperature of high corrosion resistance materials.(3)The crevice corrosion resistance of S32760 duplex stainless steel was significantly higher than that of Z2CND18-12 N.The repassivation potential of S32760 was higher than + 800 mV at 3.5% and 10% NaCl solution at 3?,30? and 60?.The crevice corrosion resistance of Z2CND18-12 N stainless steel was relatively poor,although it did not show crevice corrosion at the low temperature of 3? in both NaCl solutions and at 30? in 3.5% NaCl solution.It was sensitive to crevice corrosion in 10% NaCl solution at 30? and in the solutions at 60?.Compared with the results of pitting,crevice corrosion resistance of Z2CND18-12 N was not as good as its pitting corrosion resistance,and when the solution environment became harsher,it was prone to crevice corrosion at lower potential.(4)The effect of potential on SCC in the environment of 3.5% NaCl solution at 30? was studied by using slow strain rate testing(SSRT)method combined with electrochemical monitoring/controlling method.Z2CND18-12 N austenitic stainless steel showed SCC at the positive potential +100mV and above,and severe anodic dissolution at +400mV and +600mV accompanied by SCC cracking.The SCC mechanism of Z2CND18-12 N should be anodic dissolution type.SCC of S32760 duplex stainless steel occurred at the negative potential,with a critical potential range from-800 mV to-900 mV.SCC occurred in the potential of-900 mV and below-900 mV.When the potential was + 600 mV,S32760 did not show SCC.The SCC mechanism of S32760 should be hydrogen induced cracking.