Research On Pitting Behavior Of 904L And 254SMO In High Chloride-containing Acid Environment

Condensed water from blast furnace gas pipeline in steel industry is a high choride-containing acid solution, pipeline materials need to have excellent corrosion resistance, but 316L stainless steel used in the condensate pipelines is difficult to meet the service requirements, corrosion perforation phenomenon occurs in a short service period, so it is necessary to substitute super-austenitic stainless steel for 316L stainless steel. Super-austenitic stainless steel contains a large amount of Cr, Ni and Mo elements, has great corrosion resistance. It is used in some harsh working conditions, so it becomes the perferred material to replace the ordinary austenitic stainless steel.In this paper, 904L and 254SMO super-austenitic stainless steels were studied as the main objects, the composition of corrosive medium was similar to the composition of condensed water in steel industry. The critical pitting temperature in simulated solution and the pitting susceptibility of two super austenitic stainless steels in simulated solution of different temperature, Cl-concentration, pH were investigatd using different electrochemical techniques such as EIS, potentiostatic polarization and Mott-Schottky curve. The surface corrosion morphology, pitting density and the maximum pitting depth were studied using metallographic microscope and stereo microscope. The result of this study list as follows:(1)With the increase of temperature, the pitting potential Eb, protective potential Ep, resistance of passive film in the simulated environment decrease,indicating the stability of passive film deteriorate. Potentiodynamic polarization and EIS analysis show that the pitting corrosion resistance of two super-austenitic stainless steels is equivalent below 45?; the pitting corrosion resistance of 254SMO is better than 904L above 45 ? . Potentiostatic polarization analysis shows that the critical pitting temperature of 904L and 54SMO in simulated environment are 47.8? and 71.2?, respectively.(2)With the increase of Cl- concentration, the pitting potential Eb, protective potential Ep, the impedance values and current density Iss of two super-austenitic stainless steels in the simulated environment decreased,demonstrating the stability and self-repairing capability of passive film decrease. Mott-Schottky curves indicate that the donor density ND and flat band potential E,b of passive film increase with increasing Cl- concentration,indicating vacancy number of passive film increases. The pitting density and maximum pitting depth of two materials after soaked increase as Cl-concentration increases.The maximum pitting depth of 904L and Cl-concentration are related in quadratic function, the maximum pitting depth of 254SMO changes linearly with Cl- concentration. Comparative study proves that Cl- concentration has great Influence on the pitting sensitivity of 904L,when Cl- concentration is higher than 78500mg/L, which has reached the critical Cl- concentration, pitting corrosion resisitance of 904L decreases obviously, the pitting corrosion resistance of 254SMO has no significant change.(3)With the decrease of pH, the pitting potential Eb, protective potential Ep,the impedance values and current density Iss of two super-austenitic stainless steels in the simulated environment decrease, the pitting incubation period shortens. Mott-Schottky analysis shows that the donor density ND and flat band potential Efb of passive film increase as pH decreases, leading to the decresase of film resistance. The pitting density and maximum pitting depth of two materials after soaked increase as pH decreases.The maximum pitting depth of 904L and 254SMO is linear with pH, but the pitting depth of 904L is more obviously influenced by pH. Comparative study proves that pH has great Influence on the pitting sensitivity of 904L, when the pH of solution is below 6.00, which has reached the critical Cl- concentration, the corrosion tendency of 904L increases significantly, while the pitting corrosion resistance of 254SMO has no obvious change.The passive film of 254SMO partially dissolves in pH3.00 simulated solution, but the stable pitting is not observed.(4) Comprehensive analysis shows that the pitting corrosion of 904L and 254SMO in condensed water mainly depends on the variation of corrosion medium. Under the condition of high temperature, high chloride or low pH condensate environment, 904L may suffer pitting corrosion. In contrast, the pitting sensitivity of 254SMO in this environment has no apparent change.