Effect Of PH On Microstructure And Corrosion Behavior Of Ce(Ⅳ)-Doped Trivalent Chromium Conversion Coating

In order to enhance the corrosion resistance of galvanized steel parts in humid environments,surface treatment is required.The traditional hexavalent chromium conversion treatmet of galvanizing layer is highly toxic and is gradually eliminated.The new trivalent chromium(Cr(Ⅲ))-based(TCC)conversion coating is used as a replacement for the hexavalent chromium conversion coating,and the corrosion resistance has yet to be improved.In the TCC treat process,different pH values have a greater impact on the corrosion resistance and microstructure of the TCC.In this paper,chromium chloride is used as the main chromium salt,sodium nitrate is used as the oxidant,malonic acid,citric acid,and thiourea are used as complexing agents,sodium dihydrogen phosphate and ferrous sulfate are used as auxiliary film-forming agents,and a small amount of cerium salt is doped.NaOH Adjust the pH control conditions of TCC liquid at room temperature with HNO3.Using electrochemical methods such as polarization curve and impedance spectroscopy(EIS),the TCC film modified by doped cerium(Ce(Ⅳ))with pH change(1.5-3.5)was studied in sodium chloride solution and borate buffer solution.The influence of electrochemical corrosion behavior;The surface microscopy results and phase composition of the modified TCC film were analyzed using surface analysis results such as scanning electron microscopy(SEM-EDS),X-ray diffraction(XRD),and assembly scattering energy spectrum analysis.The experimental results show that the increase in pH results in the corrosion resistance and protection of the oxide film on the TCC surface being strengthened first and then decreased,and the protective structure of the modified TCC at pH=2.2 is the most complete.From pH 1.5 to 2.2,the Ce(Ⅳ)-doped TCC surface film structure changes from porous to dense and smooth.The Cr(Ⅲ)-rich oxide layer plays a key role in shielding the depolarizing agent from corroding the inner zinc coating.Then continue to increase the pH value,the adhesion between the inner layer of the TCC and the Zn coating decreases,resulting in poor stability.The formation mechanism of Ce(Ⅳ)-doped TCC in different pH environments was discussed from the perspective of E-pH diagram phase stability region and membrane structure model.The effect of pH change(1.5-3.5)on the corrosion behavior of TCC modified by doped cerium(Ce(Ⅳ))was investigated by neutral salt spray test and week immersion test.All can withstand 72h salt spray without white rust.The order of determining the corrosion resistance according to the blackened area of the film layer is as follows:pH2.2 sample>pH2.0>pH2.5>pH3.5>pH 1.5.By fitting the corrosion loss thickness-time of the modified TCC under different pH conditions in the week immersion test,it was found that the modified TCC was nearly uniformly corroded in NaCl solution.