The Corrosion Resistance Of Silane Film On Magnesium Alloy Fabricated By Electrochemically Deposition

Magnesium alloy is the lightest structural material in engineering applications,with high specific strength and low density,making it an ideal lightweight material.However,the active chemical properties of magnesium,low density of oxide,and poor corrosion resistance of magnesium alloys limit the application range of magnesium alloys.Silanization treatment technology can provide relatively reliable corrosion protection for magnesium alloys.The silane film obtained by the traditional technology is thin and uneven,and the protective performance is weak.In this paper,a silane film with certain corrosion resistance was deposited on the surface of AZ31B magnesium alloy by immersion method,and then electrochemical deposition was used to improve the silane film forming conditions,increasing the thickness of the silane film on the substrate surface,and improve the protective performance of the silane film.Then use the same method to prepare TEOS-BTESPT hybrid film with excellent hydrophobic properties doped with tetraethyl orthosilicate(TEOS);cerium salt-BTESPT hybrid film was prepared by electrochemical deposition which added cerium nitrate.The test showed that CeG+can generate insoluble oxides to delay the corrosion rate of magnesium alloys during the corrosion process.First,bis-(y-triethoxysilylpropyl)tetrasulfide(BTESPT)was used as the silane agent,ethanol and deionized water were used as the hydrolyzing agent.The silane film was prepared on the surface of the AZ31B magnesium alloy by immersion.In order to explore the influence of solution pH,silane concentration,and alkane-water ratio on the quality of silane film formation,orthogonal experiments were designed,which obtained the best immersion method film formation process parameters.The test proved that silane film samples had good corrosion resistance.The pitting time of the immersed silane film sample was extended to 39 h from the substrate 10 min,in 3.5 wt%NaCl solution at room temperature.There were uniformly distributed white bumps on the surface of the magnesium alloy by the scanning electron microscope(SEM),and which contained Si and S elements after energy-dispersive X-ray spectroscopy(EDS)analysis.X-ray photoelectron spectroscopy(XPS)and Fourier transform infrared spectroscopy(FTIR)analysis showed that-Si-O-Si-valent bond existed.These phenomena proved that the silane film was formed on the surface of the magnesium alloy.FTIR indicated that there were obvious-Si-O-Si-,-CH2,and-C-S-structures on the surface before corrosion,and almost no absorption peak was detected after corrosion.Polarization curve and electrochemical impedance spectroscopy(EIS)showed that the corrosion current density of magnesium alloy samples were reduced by 1 to 2 orders of magnitude,high frequency impedance was increased by 1 order,and low frequency impedance was increased by 2 orders,which demonstrated the corrosion rate was slow down.In order to increase the thickness and coverage of the film,the electrochemical deposition method was used to form the silane film under the optimal process parameters of the immersion method,with the graphite electrode as the anode and the magnesium alloy as the cathode.The deposition potential was set to 3 V,4 V,and 5 V,and the deposition time was 5 min,10 min,and 15 min,which explored the influence of deposition potential and deposition time on the corrosion resistance of silane film.Under 4 V,10 min conditions,the corrosion resistance of magnesium alloy samples reached to the longest 50 h,which was significantly improved compared with the immersion method.Electrochemical testing revealed the corrosion potential of the sample was positively shifted,the overall corrosion current density was obviously lower than that of the substrate,and the low-frequency impedance was increased by 3 orders,showing good capacitance characteristics.SEM observation declared that the thickness and surface coverage of the silane film of the electrochemical deposition method are extremely greater than that of the immersion method.At the same time,because of low negative deposition potential,visible pores appeared on samples surface,hindering the further improvement of the protective performance of the film.In order to decrease the porosity of the film,TEOS and Ce(NO3)3 were added to the silane solution respectively,and the hybrid film was prepared by electrochemical deposition.The salt water immersion test illustrated that the two dopants have improved the corrosion resistance of the silane film.The addition of TEOS made the film have better hydrophobic properties,even the contact angle reached 126°.The silane film was observed to be net-like under SEM.The structure had micron-level dot-like protrusions.This microstructure improved the hydrophobicity of the film and further improved the corrosion resistance of samples.The hybrid silane film was prepared by electrochemical deposition in a Ce3+concentration of 0.05 mol/L silane solution.SEM found that the flaky Ce-containing regions were evenly distributed on the sample surface.After the salt spray test,S and Si elements content decreased in the flaky regions.And the relative content of Ce did not decrease or even increased slightly.It may be that the cerium salt formed insoluble oxides or hydroxides during the corrosion process of the matrix,which can compensate for the pitting holes in the initial stage of corrosion and delay corrosion.