Research On Preparation And Corrosion Resistance Of Composite Coatings Based On Layered Double Hydroxide Coatings On AZ31 Magnesium Alloys

Magnesium alloys are widely used in many fields due to their unique performances,such as good mechanical properties,low density and good biocompatibility.However,magnesium alloys have active chemical properties,poor corrosion resistance and short service life,which limits its use greatly.Hence,it is important to prolong the service life of magnesium alloy.In this paper,the composite coatings were prepared by various surface modification techniques to protect the magnesium alloys,which improved the performance of magnesium alloy.Layered double hydroxide(LDH)coatings were obtained on the magnesium alloy,providing a stable surface with hydroxyl groups.Subsequently,LDH coatings were modified with the methyltrimethoxysilane(MTMS)solution,MTMS/Ce(NO3)3 solution and polylactic acid(PLLA)solution.Effect of hydrolysis degree of MTMS solution on corrosion resistance performance was investigated.In addition,corrosion resistance of MTMS coatings were influenced by different concentrations of the added cerium nitrate.It was also found that the dipping time of magnesium alloys improved corrosion resistance of the samples.The compositions,morphologies and corrosion resistance were investigated using XRD,XPS,FT-IR,FE-SEM and electrochemical tests.Finally,the mechanisms for coating formation and corrosion process were discussed.The results showed that silane coatings have different surface morphologies.The surface morphologies of the composite coatings were influenced by the hydrolysis degree of silane.The composite coating had the lowest current density(5.537×10-9 A·cm-2)when the ratio of ethanol to water was 1:2,.which indicated that it had the best corrosion resistance.In addition,the coatings had superhydrophobic performance,and contact angle was 150.5°.Incomplete hydrolysis of the MTMS resulted in uneven surface of the coatings.Additionally,superhydrophobic performance of the coatings was invisible.The LDH coatings modified with the MTMS/Ce(NO3)3 had the better corrosion resistance compared to LDH coatings modified with sole MTMS.The results indicated that the appropriate addition of cerium nitrate(10-3 mol·L-1)resulted in a more compact coating and best corrosion resistance.The current density of composite coating was 1.217×10-10 A·cm-2.An excess of cerium nitrate(10-2 mol·L-1)resulted in uneven surface and higher current density(5.498×10-10 A·cm-2)of the composite coating due to the decomposition of silane chains.MTMS/Ce(NO3)3 coatings had superhydrophobic,self-healing and self-cleaning ability.The LDH/PLLA coatings had different complete self-degradation time.In coating formation process,LDH coatings were prepared by steam method,and the PLLA coatings were obtained by dipping method.The coating process was simple and efficient.The corrosion resistance of the composite coatings was decreased with the prolongation of dipping time(1 min,5 min and 10 min).The current densities were 9.280×10-7?2.568×10-7 and 1.008×10-7 A·cm-2,respectively.In addition,long dipping time resulted in thick coating and dense surface.The pH curve of hydrogen evolution experiment proved that the pH of PLLA coatings were close to 7 after self-degradation process.Therefore,the dipping time of PLLA coatings could be selected according to the practical application.