.316 Stainless Steel, The Critical Pitting Temperature And Ion Synergy Study

As one of the most important failures of stainless steels, pitting corrosion was further investigated. Immersion experiment, dynamic potential methods and constant potential CPT test are the mainly three methods to evaluation the pitting resistance. The immersion experiment is weak in precision and concise and the dynamic potential methods is not applicable for low temperature passivation at most case. So the constant potential CPT test becomes the most applicable evaluation methods This method is well-developed in west country but few research in this field in our country. In this paper pitting corrosion behavior of 304 and 316 stainless steel was studied in the constant potential CPT test by testing the relationship of critical pitting temperature and chlorine ions, sulphate ions concentration, analyzing the mechanism of temperature and concentration of chloride ions impaction on the pitting corrosion; revealed the reason of sulfate and chloride ions synergies process affecting on the pitting corrosion. The results can be helpful to understand the mechanism of pitting corrosion, the main findings are as follows:(1) Temperature and concentration of chlorine ions effect on the corrosion behavior of stainless steel mainly by effecting on the dissolution rate of stainless steel passive film, and the dissolution rate of stainless steel passive film decides the anodic reaction speed. The relationship between the anodic reaction speed which leads to the passive film growth and temperature meets with the Arrhenius law.(2) Under Certainly applied voltage, the relationship between critical pitting temperature of 304,316 stainless steel and Cl^- concentration meet with the empiricalformula: CPT =α+γlnc_Cl^-(3) In the solution of certainly Cl^- concentration, SO₄^2- affects on the stainlesssteel corrosion behavior mainly by impacting the Cl^- adsorption on stainless steel surface, resulting to the passive film dissolution rate changes.(4) In the solution of certainly applied voltage and Cl^- concentration, there is adsorption competition between Cl^- and SO₄^2-, and in different concentrations of SO₄^2-the competitive adsorption result in different outcomes, the critical pitting corrosion temperature of stainless steel is lower or higher.