Preparation Of Zn-Co Composite Coatings By Low Pressure Cold Spray Technology And Research On Its Properties

The zinc is used as one kind of protective materials for steel substrates in view of its lower corrosion potential than iron and strong anti-corrosion performance in seawater,which is the reason that it has attracted more attention for corrosion and protection of scholars.Currently,the zinc-aluminum alloy coatings are the most common materials in zinc alloy coatings.In this system,the Cr⁶⁺is generally added in to improve its anti-corrosion performance,which will cause serious environmental pollution.As for zinc-nickel coatings,the stability of corrosion products still needs to be improved.Therefore,considering the nature of cobalt element in this study that it has better corrosion resistance and good abrasion resistance theoretically,zinc-cobalt system was applied in the corrosive experiment and the insulating corrosion products can be stabilized by rich cobalt area and approach better corrosion resistance effect.To solve the problems that the components are difficult to control and it leads to environmental pollution easily in the plating process,the coatings was prepared by low pressure cold spraying technology instead of plating.In this paper,five kinds of zinc-cobalt composite coatings whose composition is?Xvol%Zn+Yvol%Co?+30vol%Al₂O₃ were prepared by the low pressure cold spraying?LPCS?equipment which is called DYMET-413.The surface and section morphology of coatings and its phase structure are characterized by three-dimensional video microscope,SEM and XRD.The physical properties of coatings are tested by micro hardness tester,electronic tensile tester and friction and wear testing machine.The anti-corrosion performance and corrosion mechanism of the LPCS zinc-cobalt composite coatings in the 3.5wt%sodium chloride solution were investigated by using electrochemical testers and neutral salt spray accelerated tester.The study has made innovative achievements are as follows:?1?The analysis of coatings structure show that:the thickness of coatings is between 200?m and 300?m;the roughness can be improved by adjunction of cobalt element;surface of coatings is flat and the cobalt elements are distributed homogeneous;the coatings content wasn't oxidized or phase changed before and after spraying;?2?The analysis of coatings physical properties shows that:all the hardness values of coatings ware above 65HV_0.2;the data of coatings porosity indicated that all the porosity values of coatings ware below 8%;all the values of bonding strength reaches between 16 and 22 MPa and it achieved the maximum values of 21.77 MPa as the cobalt content is 15vol%.?3?The white corrosion products were homogeneously distributed on the surface of LPCS zinc-cobalt composite coatings and by contrast,larger fall appeared on the surface of pure zinc coatings.The corrosion products tended to be well-proportioned as the cobalt content increase in a certain range and no obvious cavity or crack existed;The XRD results of corrosion products showed that,in the early stage of corrosion,the anodic dissolution of active Zn happened,the corrosion products such as ZnO,ZnCl₂·4Zn?OH?₂·H₂O and Zn₄CO₃?OH?₆·H₂O generated on the surface of coatings.The presence of Ni in coatings play an important role that restrain Zn?OH?₂to be converted into ZnO which is very adverse for extension of the anti-corrosion life of the coating.The self-corrosion potential of coatings tended to be stable after immersion test,the polarization resistance of zinc-cobalt composite coatings with15%wt cobalt content reach maximum values and the change of open circuit potential became flat,current I_corr reduce,R_c and R_ct increase,which reduced the corrosion rate enormously.?4?Combined with the neutral salt spray accelerated test,the result showed that the corrosion resistance of zinc-cobalt composite coatings has been greatly improved in comparison to pure zinc coatings,it still can maintain good corrosion resistant effect after three-month salt spray accelerated test.The zinc-cobalt composite coatings of different cobalt content were prepared by cold spraying technology in this experiment,and its properties were characterized by series of experiments.In addition,the influence of cobalt element was verified and the anti-corrosion performance and corrosion mechanism of the LPCS zinc-cobalt composite coatings were studied,which made a reference value for preparation of metal anti-corrosion coatings and made a significant meaning of anti-corrosion materials.