Corrosion Resistance And Thermal Control Property Of Phosphate Conversion Coating On Mg-Gd-Y Magnesium Alloy

Regarding as the lightest structure material,magnesium alloys are widely applied in contemporary society in industries.Owing to its poor resistance of corrosion,the application of magnesium alloys is limited.Chemical conversion coating is one of the most promising surface treatment techniques for its low coasting and easy operating.Even though traditional chromate conversion coating could dramatically improve the corrosion resistance of magnesium alloy,the toxicity and carcinogenicity of Cr?VI?drive scientists to find out a non-chromate conversion technique.Phosphate conversion coating is one of the most attractive technologies.However,the mechanism of the formation phosphate conversion coating on magnesium remains to be unknown.Furthermore,blaming to huge heating effect of works in aircraft or 3C industries,coatings are supposed to own a good corrosion resistance equipped with thermal control properties.1)The effect of Volta potential between two different phases in conversion processIn this study,we adapted two series of magnesium alloys,Mg-Gd-Y-Zr?Mg-RE?and AZ91D to investigate the formation mechanism of phosphate conversion coating.Via XRD,SKPFM et al,It was found that the Volta potential between two different phases performed a crucial role in the deposition of coating and generation of hydrogen bubble.In Mg-RE,Rare earth?RE?elements mostly precipitated in second phase(Mg₂₄?Gd,Y?₅)which owns the larger Volta potential between?-phase?320 mV?comparing with AZ91D(Mg₁₇Al₁₂)?140mV?.In this situation,this phenomenon leaded to the formation of denser and more protective conversion coating on Mg-RE compared to AZ91D.2)The effect of nucleation pre-treatment of corrosion resistance of conversion coatingA newly designed pre-treatment aimed to enhance the density of nucleation during the beginning of conversion period.For one thing,Nano-SiO₂ particles adhering with agent coupling KH550?N?NCH₂CH₂CH₂Si?OC₂H₅?₃?could obviously accelerate deposition rate and refine the grain size of the final coating.For another,Ti/Zr pre-treatment could also provide more nucleation sites.Via nucleation pre-treatment,the corrosion resistance of coating improved up to 170%comparing to the original technique.Furthermore,Ti/Zr Pre-treatment take advantages of easy operated.3)In situ added technology to produce a corrosion resistance and thermal control conversion coating.To obtain a combination of coating with corrosion resistance and thermal control properties,Nano-SiC particles were in-situ added into conversion solution.SiC particle is a material with large banding gap.It was found that in-situ added SiC particle could improve the corrosion resistance of phosphate conversion coating and performed a good thermal control function as well.Final,set a connection between the concentration of added SiC with the corrosion resistance and thermal control properties.