Study Of Chromium-free Passivating Process And Performance On Magnesium Ally

At present the most widely passivation technology of magnesium alloy is chromatesalt passivation,but there is toxic fatal weakness on the traditional chromate saltpassivation. Therefore, the study of chromium-free passivating technology which isfriendly to environment with excellent performance has important economic and socialsignificance. Molybdate passivation technology and tin acid salt passivation technologywere studied mainly on AZ91D magnesium alloy in the paper.Through the single factor experiment and orthogonal optimization method todetermine the best of molybdate passivation technology for Na₂MO₄45g·L^-1，NaH₂PO₄35g·L^-1，KMnO41.0g·L^-1, pH3.0, time25min, temperature45℃; With drip timeexamining the corrosion resistance of passivating film, the best technology ofpost-processing for molybdate passivating film is NiSO43.5g·L^-1,CoSO40.7g·L^-1,NaAc2.0g·L^-1,H3BO35.5g·L^-1, pH5.0, temperature30℃, time15min. Molybdate passivationcoating performance were studied with the methods of SEM, EDS, tafel curves and EIS,The results show that molybdate passivation coating present dry riverbed shapestructure and there are many mesh cracks and oxide particles on it. Passivation coatingwas formed mainly by the elements of Mg, Mn, Mo, P. The corrosion potential ofmagnesium alloy with molybdate passivating film increases about0.4V, corrosioncurrent reduces the2orders of magnitude, linear polarization resistance and exchangeimpedance also increase.Research has identified the best process for tin acid salt passivation: Na₂SnO₃50g·L^-1,Na₄P₂O₇45g·L^-1,C₂H₃O₂Na8g·L^-1,C₂H₃NaO₂6g/L,pH3.0, time25min,temperature45℃; By the electrochemical performance, tin acid salt passivating filmadditives were studied, The results show that the suitable additives and their content fortin acid salt passivation coating are Na₂SiO₃4g·L^-1,NaF1g·L^-1,NaWO₃4g·L^-1,Na2B4O74g·L^-1, among them, with Na2B4O7the passivation film performance gets best improvement. Microstructure analysis results showed that the tin acid salt passivatingfilm was piled up by the false ball particles of approximately spherical, which areclosely spaced with uniform size. Elements and XRD analysis shows that thepassivation coating was mainly formed by the elements of O, Mg, Al, Zn, Sn, its maincomposition is MgSn（OH）₆. Corrosion resistance test showed that drop time ofmagnesium alloy with tin acid salt passivating film was significantly longer, corrosionpotential increases about0.5V, corrosion current reduces the3orders of magnitude,linear polarization resistance increased more than100times and exchange impedancesignificantly enhanced.Through the accelerated corrosion test,a comparative study of the corrosionresistance was done between two chromium-free passivating films and chromium-freepassivating film, and the results show that the corrosion resistance of tin acid saltpassivating film is slightly better than chromate salt passivation film.