Super Martensitic Stainless Steel Corrosion Influence Factors

At normal temperature and pressure condition, the corrosion behavior of the super martensitic stainless steel (SMSS) were surveyed for several heat treatment conditions. The relationship between organization and corrosion reactions was explained. The impact of the compositions of SMSS(No.1:W,Cu-free;No.2:W,Cu-containing) was surveyed, too. The corrosion behavior was discussed under different Cl- concentrations, temperatures, CO2 conditions. The stable passive film's role in the corrosion process was discussed. After polarization the passivation properties of the sample was analyzed.Obtained by metallographic and XRD analysis, the grain size of No.l and No.2 is larger when the quenching temperature increases; while the tempered martensite bundles in the SMSS grains become dense when the tempering temperature increases. Be tempered at 650℃, it also contains reverse austenite.After treated at different temperatures, the No.l and No.2 steel were measured with cyclic polarization curve, we find that the quenching temperature has its impact on the pitting corrosion, the one quenched at 1050℃is better than that of 950℃, and the worst is that of 1000℃; The impact of the tempering temperature is more significant on the pitting performance of SMSS, the one tempered at 550℃is better than that of 650℃, and the worst is that of 750℃. The corrosion resistance of the organization under quenched is better than SMSS which is tempered.Integrated all the experimental data under different heat treatment, the denser the martensite bundles are, the less corrosion performance the SMSS is. When quenched after solution treatment at 1050℃for 0.5h, and then tempered at 550℃for 2h, the SMSS which is gained has the best pitting resistance.Do in-depth analysis for the No.l and No.2 steel which has the best heat treatment. Compared the polarization curve with the impedance at different environments, the pitting potential becomes more negative when the Cl- concentration increases, the passive current density shows an increasing trend, and the impedance values decrease. It indicates that the existence of Cl- ion reduces the pitting potential; Compared with in the NaCl solution without CO2, the steel in the NaCl solution with CO2 significantly increased the pitting potential, and when do experiment in the solution without CO2, the formed passive film is not entirely, which indicates that CO2 can promote the passivation. As the solution temperature increases, the pitting potential shows a decreased trend first and then increases, and it indicates that there are different corrosion mechanisms in different temperature ranges. Above 60℃the formed passive film turns to be dense, the matrix is protected effectively, and it reduces the sensitivity of pitting.Do immersion experiment in CO2-saturated and NaCl solution with its Cl- contents:2.1%, we find that as time goes on, the value of impedance has a certain fluctuations in different times, then the passive film becomes stable; and from the final impedance, we could find that the value of No.2 was much larger, which indicates that the addition of W, Cu improved SMSS's pitting resistance; by the circuit simulation method we can also find that, the resistance is infinite when ions through the passive film for No.2 in the simulated circuit, the passive film has more excellent positive ion selective permeability.In the NaCl solution as used in the immersion test, do passive polarization to SMSS. The two kinds of steel can be passivated in the interval between the corrosion potential and the pitting corrosion potential. We find that the film protects the matrix better from the simulated impedance datas, and the No.2 steel has a better passivated effect.