Effect of Film Formation Time on the Morphology and Corrosion Resistance of Praseodymium Salt Conversion Film of AZ31B Magnesium Alloy

In order to explore the corrosion protection properties of the new rare earth salt praseodymium salt conversion coating, a dense praseodymium salt conversion coating was prepared by chemical conversion method to modify the surface of the magnesium alloy substrate. This experiment focuses on the surface properties of conversion coatings prepared with different film forming times. Under simulated seawater conditions (3.5% NaCl solution), the corrosion resistance characteristics of the films were evaluated by electrochemical tests (Tafel polarization curve and electrochemical impedance spectroscopy), immersion tests, and titration tests. Scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), X-ray energy dispersive spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS) were used to test and analyze the microscopic morphology and structural components. The wear resistance and surface hardness of the film were analyzed from a mechanical point of view by means of a microhardness tester and a friction and wear tester. After comparative experiments, it was found that too short or too long film formation time is not conducive to the formation of film layers. The Tafel curve shows that when the film formation time is 30 min, the self-corrosion current density is 1.740×10-9 A/cm2, which is 4 orders of magnitude lower than that of the bare magnesium sample; the self-corrosion potential is -0.681 V, compared with the bare magnesium sample. The magnesium sample was shifted by 900 mV. Electrochemical AC impedance spectroscopy showed that the capacitive arc curvature radius of the film was the largest when the film was formed for 30 min, and the charge transfer resistance was also the largest after EIS fitting, which was 45 650 Ω.cm2. The immersion test and saline drip test show that the film has fewer pitting areas and crack defects when the film is formed for 30 minutes, the spotting resistance time is 82.14 s, the film thickness can reach 26.2 μm, and the film is relatively firm, the test results are consistent with the electrochemical test parameters; The composition analysis shows that the conversion coating mainly contains C, H, O, Mg, Pr and other elements, and the main materials of the coating layer are praseodymium hydroxide and praseodymium partial oxide; The addition of rare earth metal praseodymium improves the mechanical properties of the film to a certain extent, and improves the wear resistance and hardness of the film. Finally, the film formation mechanism of praseodymium salt conversion film on the surface of magnesium alloy can be inferred from the composition analysis results, which is consistent with the cathode film formation mechanism. Pr(NO3)3·6H2O as the main film-forming salt can effectively improve the surface properties of AZ31B magnesium alloy. At 30 min, the magnesium alloy praseodymium salt conversion coating can effectively prevent the invasion of corrosive medium, and its corrosion resistance and stability are the best. © 2023 Chongqing Wujiu Periodicals Press. All rights reserved.