In order to further improve the corrosion resistance of S32750 super duplex stainless steel, the passivation treatment was carried out by nitric acid passivation. The corrosion resistance of S32750 super duplex stainless steel was investigated by electrochemical polarization curve, electrochemical impedance spectroscopy, Mott⁃Schottky curve and immersion method,and the corrosion surface morphology of the sample surface before and after corrosion was observed. The results show that the nitric acid passivation time has a significant effect on the corrosion resistance of S32750 super duplex stainless steel passivation film. When the passivation time is less than 120 minutes, the longer the passivation time, the smaller the corrosion rate is, and the better the passivation film corrosion resistance is.

The corrosion behavior of 2205 duplex stainless steel (DSS) in hydrofluoric acid (HF) was affected by the chlorine ion. The methods including immersion, potentiodynamic polarization and electrochemical impedance spectra (EIS) were used to understand the effect. The results showed that while the NaCl concentration was added into HF solution from 0.015 mol/L to 0.030 mol/L, the corrosion potential got more positive, and the corrosion current density and average corrosion rate increased with the increasing NaCl content. It was because that NaCl decreased the pH of HF solution.

The immersion and electrochemical methods has been used to study the corrosion behavior of automotive steel Q255 in different content of snow melting salt (NaCl) solution. Immersion test show that, the snow melting salt solution is more corrosive than the natural snow solution. The corrosion rate of Q255 steel in the natural water water is smaller than that of the snow melting salt solution, with the corrosion rate of 0.021 07 mm/a after the immersion of 168 h.The corrosion rate increased with the increase of NaCl concentration of the snow solution. The corrosion rate of Q255 steel could reach 0.039 71 mm/a in 3%NaCl solution, the value of which was almost double to the natural snow solution. Electrochemical experiments show that, the presence of chloride ions accelerated the dissolution and mass transfer process of F e 2+ corrosion products, subsequently reduced the adhesion of the corrosion product. Accordingly, the corrosion products would not be able to protect the steel surface, thus the corrosion rate accelerated.

 

The effect of soaking temperature and stirring rate on the corrosion behavior of Q235 and Q345 steel in simulated seawater environment (3.5%NaCl) was investigated by using soaking experiment. The surface morphology and chemical composition of corrosion products were analyzed by means of SEM and EDS. The results show that the corrosion rate of Q235 and Q345 steel has little difference and increase with increasing temperature in static seawater environment, reaching approximately 0.28 mm/a at 40 ℃. The stirring rate has a significant effect on the corrosion behavior of Q235 and Q345 steel and the corrosion rate accelerates with increasing stirring rate. When stirring rate is 300 r/min, the corrosion rate reaches 1 mm/a that is about 3.5 times as big as temperature effect. Uniform corrosion was observed on Q235 steel in simulated seawater. But the local corrosion morphology is found on Q345 steel and the main corrosion product is iron oxides. The application of Q345 material should be cautious compared with Q235 material in the same seawater environment.