Corrosion And Passivation Behavior Of Super Martensitic Stainless Steel 15Cr

In this paper, corrosion and passivation properties of two kinds of supermartensitic stainless steels with W (No.1) and without W (No.2) have been investigated based on semiconductor electrochemistry and corrosion electrochemical principle, using an electrochemical test methods, Mott-Schottky method and SEM. The resistances to pitting corrosion performance and passivation performance of supermartensitic stainless steels have also been analyzed in different pH and Cl-concentration, under different temperature conditions.Pitting occurred in both samples in3.5%NaCl solution with different pH (1-4) values. With the increase of pH, the pitting potential increases while the passive current density decreases. Under the condition of-0.1V open circuit potential, the electrochemical impedance spectroscopy has similar trend. The capacitive reactance of the electrochemical impedance of the arc increases with the increase of pH value, i.e. the resistance value increases, and the impedance after polarizing for1hour under the conditions of-0.1V is higher than the impedance at the open-circuit potential.It illustrates that the corrosion resistance of samples No.1and No.2enhanced with the increase of pH value in3.5%NaCl solution, and the passivation film obtained by high potential polarization has a higher stability and protective ability. Both samples soaked in solution with pH value of1and4for72h are analyzed, and it is found that when the pH value is1, the surface passivation film of two specimens is unstable, forming and dissolving quickly, which results in the formation of nonuniform passivation film with poor stability. While the pH value is4, surface passivation film of two samples are relatively stable after soaking for36h, and there is not much change, which indicates uniform and stable surface passivation film. The Mott-Schottky curve analyses are conducted to both samples under the condition of-0.1V and-0.2V. Below the flat band the two passivation of the surface of the specimen film was p-type semiconductor, and the at pH1and2the slope of the p-type semiconductor is significantly higher than in other conditions, which indicates a higher content of Cr oxide. Exceed the flat band was n-type semiconductor, and the slope increased with the increase of pH value. It shows that with the increase of pH value, the defect density in the passivation film decreases, while the content of Fe oxide increases and the stability of the passivation film are higher.From the polarization curve, it can be seen pitting in excluding Cl-solution does not occur spontaneously in both samples in different types of Cl-solution, but in a solution containing Cl-pitting occurred, with the concentration of Cl-increasing, corrosion resistance of No.3and No.4samples reduced. The electrochemical impedance of the two types of samples under the open-circuit potential shows similar trend. Impedance capacitive arc reduced with the Cl-increase, and the resistance value decreases, the sample surface passivation film with Cl-rise become porous, and less protective. Both samples are soaked in the solution with Cl-contents of0%and5%for24h analysis, we found electrochemical impedance of two samples stabilized after3h’soaking for0%Cl-solution, while in solution with5%Cl-, the electrochemical impedance will become stable after9h, and gradually increased after9h.it indicates that in Cl-free solution, the sample surface passivation film is formed in short time, and relatively stable. In the impedance, the impedance of0%Cl-solution is much lowers than the5%Cl-solution, indicating that the presence of Cl-for specimen surface makes passivation film loose and porous and reduce protection performance. Mott-Schottky curve analysis is conducted to the sample under-0.1V. Passivation film of the two samples formed in different Cl-solution is n-type semiconductor, and with the increase of Cl-concentration, the defect density increases, while protection performance decreases.In3.5%NaCl solution under the different temperature condition, from the of polarization curves, pitting corrosion occurred to both samples, and pitting corrosion resistance reduces with increasing temperature. Both samples have a similar trend of the electrochemical impedance of the open-circuit potential, i.e. full impedance decreases with increasing temperature. The elevated temperature accelerates the corrosion of the specimen surface. The specimen surface passivation film is thin and nonuniform, thus reducing protection performance.