| Phosphate chemical conversion coating on the surface of metal is dense and fine,which is mainly used for corrosion prevention and coating base for metal material,etc.But so far,there have been a very few studies in field of phosphate chemical conversion coating on the surface of titanium or titanium alloy.In this paper,we prepared the zinc phosphate chemical conversion coating on titanium alloy(TC4)by adopting the method of chemical conversion at normal temperature with galvanic coupling.Research was mainly focused on the effects of the different coupling materials,coupled area ratios,contact area ratios and time.The structure,morphology,adhesive strength and corrosion resistance of the chemical conversion coating were analyzed by X-ray diffractometer(XRD),Thermal Field Emission Scanning Electron Microscope(SEM),Scratch spectrometer and Electrochemical workstation.Then the related chemical conversion mechanism of galvanic coupling was discussed.The results revealed that there was hardly any crystal on the surface of TC4 without galvanic coupling,no matter how long it was treated,while a layer of crystals were formed on the surface of TC4 by galvanic coupling.The main component of the coating was Zn3(PO4)2·4H2O.The coating was finer and denser with no substrate bared by coupling with magnesium alloy.In the range of testing,the coating was fine and dense with a best adhesive strength and corrosion resistance when the area ratio of TC4-MZG(Mg-1.75Zn-0.55Ga magnesium alloy)coupling system was 1:1.When the TC4-MZG alloy coupling system was fully in contact,the size of coating grain decreased obviously,and its critical load during scratch test could reach 24 N.Galvanic coupling technique could initiate the chemical conversion reaction,accelerate the rate of coating-forming and shorten the time of transformation.The initiation of the conversion time was about 15 s?30 s.The number and size of grain increased with the prolonging of time,while this did not mean that the conversion time was longer and then the effect was accordingly better.The coating was finer and denser with no substrate bared for 20 min.The grains were in between the state of continuous dissolution and secondary precipitation.Coupling with AZ91D magnesium alloy(MA)had an obvious effect on the phase and morphology of the chemical conversion coating formed on TC4.Chemical conversion coating was mainly composed of hopeite and zinc in the form of particle.The content of Zn phase increased with the increase of the coupling area ratio.The coating was piled up by particles,which was different from other coupling systems.The effect on the coating formation in the chemical conversion by galvanic coupling was not an accidental phenomenon.A uniform and dense layer of chemical conversion coating formed on copper alloy and SS which was similar to titanium alloy by galvanic coupling at normal temperature.The coating was mainly composed of Hopeite(Zn3(PO4)2·4H2O).Galvanic coupling could shorten the induction time,promote the formation of the early point of incipient precipitation,and prepare the conversion coating with excellent structure and performance.By analyzing the potential-time curve of the coating-forming,we could conclude that the formation of the conversion coating was divided into five stages,namely the dissolution process on anode,early nucleation stage on cathode,the growth and redissolution stage of crystal nucleus or grain on cathode,the rapid growth stage of crystal and the steady growth stage of conversion coating on cathode. |
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