Study On Passivation And Re - Passivation Of Ultra - Low Carbon New Stainless Steel

Super-low-carbon martensitic stainless steel (SMSS) as a new material of oil and gas pipeline had excellent performance. It is important for our country to research and develop the new martensitic stainless steel stainless steel which has a good corrosion resistance.In this thesis, passivation, repassivation and semiconductor properties of the new super martensitic stainless steels have been investigated in different solution systems (pure acid/alkaline solution, NaCl solution, Na2S2O3 solution), different pH, different passivation potential. The experimental methods were electrochemistry, X-ray photoelectron spectroscopy (XPS) and soaking.The pure acid or alkaline solution:Polarization curves had shown that, in H2SO4 or NaOH solution, SMSS had a stable passivation region and secondary passivity region. With the increase of pH value, peak current density and the passivation current density increased too. But activation of peak did not appear in NaOH solution. SMSS was directly into a state of passivation after activating. And the passivation zone range was very wide.The electrochemical experiments in the pH 1 H2SO4 solution had shown that:The film growth rate of new SMSS increased with potential, and the time of passivation film was shorter. In the passivation region, the current densities of two SMSS were similar. And the passivation film was also stability after the long time polarization. But the impedance of two SMSS passivation film were smaller at 0.6 V. The pitting sensitivity also increased at 0.6 V. By analyzing the semiconductor characteristics of SMSS, we had known that the capacitance decreased with frequency increase; the stable passivation area had P type and N type semiconductor structure in different polarization time. SMSS showed different semiconductor characteristics with improving polarization potential. In addition, adding W could reduce the acceptor and donor density, improved the stability and corrosion resistance of the passivation film.The system of NaCl solution:In alkaline solutions, passivation region width was wider, passivation current (Ip) was smaller, polarization resistance (Rp) was greater. So the passivation film of SMSS in alkaline solutions had better passivation behaviors than that in acid solutions. All results point out that passivation film structure of SMSS1 sample was more stable, and capacity of passivation film was enhancing. Mott-Schottky curve had showed that, in pH 4/10, the slope of SMSS 1 is higher than that of SMSS 2, the concentration of ND or NA is also better than SMSS 2. In alkaline solution, the passivation film surface was mainly composed of iron oxides. The oxide of Fe/Cr ratio was higher in the alkaline solution. It suggested that adding W can also improved SMSS corrosion and passivation ability.About the system of Na2S2O3 solution:The electrochemical tests in different concentrations and pH conditions had show that sulfur ions of solution changed the passivation ability of new SMSS. The passivation region was the widest in the solution of 10-3 mol/L. And the SMSS had a good resistance to passivation ability and stress corrosion. In pH 4-5, stable passivation current density changed little, the passivation ability is better, the passivation film has good stability and protection. But the passivation ability of SMSS were very weak in pH 3.5. The anodic dissolution occurs more easily. It would lead to the passivation film layer to reduce the amount of Cr2O3 and change the structure of passivation film.About the repassivation test in different concentration of chloride solution:The increase of the concentration of chloride ion would not only reduce the repassivation rate, but also could make the stress corrosion sensitivity of SMSS increased. It made the material more easily stress corrosion when SMSS was service. In addition, adding W could make SMSS the repassivation rate increased.The XPS test could determine the W elements affect passivation ability. XPS results not only confirm the above conclusions, but also suggesting that adding W can also promote more Cr-ox and Fe-ox formed on the passive film. It revealed the passive film structure were Fe oxide and Cr oxide mixed layers. Fe oxidation product and Cr oxidation product would help to improve the protective property of passive film, which could promote the formation of a passive film structure more stably and densely.