| Magnesium alloys have become the preferred materials in automotive,aircraft,defense and high-tech industries because of the advantages of abundant resources,lightweight,high strength-to-weight ratio,good castability and so on.Unfortunately,poor corrosion resistance has hindered their widespread use.Much effort on surface treatment has been devoted to the improvement of their corrosion resistance.Chemical conversion treatment is widely used for magnesium alloys because of low energy consumption,low cost and simplicity in operation.Traditional chromate conversion treatment has been prohibited because of its toxicity and carcinogenesis.Thus,there is a real necessity to implement alternative conversion treatments.This paper focuses on the preparation of Ce-V conversion coating on AZ31 magnesium alloy and the revelation of the formation mechanism and protection mechanism of the coating.The main research contents and results are as follows:(1)A new chromium-free Ce-V conversion coating on magnesium alloy was proposed in combination with the protective effect of cerium salt and vanadate conversion coating.The coating preparation process was optimized through the orthogonal experiments and the single factor experiments.The optimum conversion solution is composed of sodium vanadate(NaVO3,19.7mM)and cerium nitrate(Ce(NO3)3,9.21mM)at 50?.The pH of the solution is about 2.5.The treating time is 20 minutes.The color of the Ce-V conversion coating is golden yellow.It is mainly composed of spherical particles with network.It has duplex structure with 1.98 ?m of thickness,which exhibits amorphous structure.The outer layer of coating mainly contains Mg,Ce,V and O.The inner layer is mainly composed of Mg,Al,V,F and O.(2)The influences of parameters on the surface morphology,chemical composition and corrosion resistance were investigated.Moreover,the changes in the electrochemical information during coating formation were also investigated by electrochemical measurements.Then,a formation mechanism was proposed and discussed.The formation mechanism of the Ce-V conversion coating shows the combined coating system of vanadium and cerium species provides synergistic effect for coating formation.The formation process involves the dissolution of MgF2-MgO film and Mg substrate,initial formation of inner vanadate conversion coating and the deposition of CeVO4.The formation factors affected the morphology and composition of the coating through the influence of the chemical reactions on the solid-liquid interface in these three stages,which have had effects on the corrosion resistance of the coating.(3)Evaluating the corrosion resistance of the Ce-V conversion coating was through the neutral salt spray tests,NaCl solution immersion,scratch tests and simulated scratch cell tests.The various characterization methods and electrochemical measurements were adopted to study the corrosion and self-healing behavior of the Ce-V conversion coating in the NaCl solution.The Ce-V conversion coating can endure at least 96 hours of neutral salt spray.The coating increases the corrosion potential by about 157 m V,decreases the corrosion current density icorr by 2 orders of magnitude,and enlarges the capacitive loop by 30 times compared with AZ31 magnesium alloy substrate.During 7 days immersion in a 3.5 wt% NaCl solution,not only the unbroken coating exhibited good corrosion protection,but also the scratched coating was not corroded,which showed the self-healing behavior.The self-healing ability of the Ce-V conversion coating is confirmed from all the immersion tests.The results reveals that the self-healing effect of the Ce-V conversion coating is mainly provided by the release and migration of vanadium compounds,which formed the passivation coating containing the oxides/hydroxides of V and Mg.(4)In order to further enhance the corrosion resistance of the Ce-V conversion coating,stearic acid(SA)was used to modify the Ce-V conversion coating.The modified coating was characterized.The optimum preparation process parameters were determined through the orthogonal experiments.The optimum processing parameters of the SA modified Ce-V conversion coating were confirmed as follows: stearic acid concentration is 10 g/L,treating temperature is 25?,and treating time is 8 hours.The influences of treating time on the surface morphology,contact angle and corrosion resistance were investigated.Evaluating the corrosion resistance of the Ce-V conversion coating was through the neutral salt spray tests and NaCl solution immersion.Moreover,the corrosion behavior of the modified coating in the NaCl solution was study.The results show the impact factor-treating time affects the corrosion resistance of the coating most.The longer time in the treatment,the higher corrosion resistance of the modified coating.After the modification treatment,the coating morphology is turned from the spherical morphology to the cauliflower-like morphology,which effectively repaired the cracks.CH3(CH2)16COO-is successfully grafted to the Ce-V conversion coating.The coating surface property is turned from hydrophily to superhydrophobicity.The contact angle is 154.6°.The modified coating can endure at least 22 days of neutral salt spray.The corrosion potential is ennobled by 149 mV,the corrosion current density decreases about two orders of magnitude and the coating resistance increases 28 times compared with the Ce-V conversion coating.SA modification treatment can obviously develop the long-term protection in 3.5 wt.% NaCl solution compared with the Ce-V conversion coating and has no effect to the self-healing ability of the coating.The above results show that Ce-V conversion coating significantly improves the corrosion resistance of magnesium alloy.Additionally,it can react with organic compounds to further improve corrosion resistance.The study is a important theoretical guiding significance for the design of the magnesium alloy protection. |
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