Microstructure Of Zn-Cu-Ti-M Alloys And Coatings And Their Corrosion Behavior In NaCl Environment

Steel materials are the most widely used metal materials in the world.They have deep applications in the fields of transportation,construction,petrochemicals,etc.However,when serving in a marine environment with high humidity and high Cl ion content,chemical reaction will inevitably happen between steel and corrosive medium.As a result,steel will be corroded.Many materials,such as Al,Zn and Zn-Al alloys,have been widely used in the surface protection of steel,but these materials all have relatively obvious shortcomings.Recently,much work indicated that Zn-Cu-Ti alloy not only has good comprehensive mechanical properties,but also has excellent corrosion resistance,showing the prospect of being used as coating material for steel.Hence,this paper designs and melts three series of Zn alloys:Zn-(0.3?0.9)Cu-0.3Ti,Zn-0.6Cu-0.3Ti-(0.3?0.6)Ce and Zn-0.6Cu-0.3Ti-(0.1?0.3)Mg alloys(Referred to as Zn-Cu-Ti-M alloy).In addition,arc spraying technology was used to prepare alloy coating on Q235.The structure and corrosion performance of the alloys and coatings were investigated and their corrosion mechanism was explored.The solidification structure of the Zn-Cu-0.3Ti alloy was studied,and it was found that the precipitated phases in the alloys are Cu Zn5,Ti Zn3 and Ti Zn15.Cu Zn5nanoparticles with size of 15nm-50nm are uniformly dispersed in Zn matrix,and the Cu Zn5 micron particles with size of 0.8?m-2.5?m exists near the phase interface,and the strip-shaped Ti Zn15 phase exists at the grain boundary.With the increase of Cu content,the Zn matrix evolves from underdeveloped dendrites or equiaxed morphology to equiaxed morphology,and the precipitated Cu Zn5 nanoparticles increase significantly.After the addition of rare earth Ce,Ce Zn5 micron particles precipitate in Zn-0.6Cu-0.3Ti alloy,which are distributed inside the Zn matrix or at the phase interface.Moreover,Ce addition promotes the growth of the Zn matrix in a dendritic manner,and can refine the Zn dendrites and secondary dendrite.After a small amount of Mg is added to Zn-0.6Cu-0.3Ti alloy,long strip-like Mg Zn2 phase precipitate at the grain boundary and the growth mode of Zn matrix changes,which grows in an equiaxed crystal manner.The structure characteristics of the Zn-Cu-Ti-M alloy coating were analyzed,and it was found that the thickness of the alloy coatings is uniform,about 110?m.The coating is bonded to the substrate well.Compared with Zn-Cu-Ti-M alloy,the microstructure of the coating is very similar to that of the corresponding alloy.Ti Zn3particles precipitate in Zn-Cu-Ti alloy coating.After Ce addition,Ce Zn5 is formed in Zn-Cu-Ti alloy coating.Mg addition leads to the formation of thin strip-like Mg Zn2phase in the coating.The difference on the structure of Zn-Cu-Ti-M alloys and their coatings is that Zn-Cu-Ti-M alloy coating shows finer microstructure,that is finer precipitation and finer Zn grain.The polarization curves of Zn-Cu-Ti-M alloys were investigated.It's found that the addition of rare earth Ce and Mg enhances the self-corrosion potential of Zn-Cu-Ti alloy as a whole.Ce and Mg addition can reduce the self-corrosion current density of the alloy.After adding 0.3%Ce,the self-corrosion current of Zn-0.6Cu-0.3Ti alloy decreases rapidly from 2.76×10^-3A/cm²to 5.85×10^-4A/cm².After adding 0.1%Mg,the self-corrosion current of the alloy reduces to34.52×10^-5A/cm²,which is mainly due to the refinement of the Zn matrix and the precipitation of the insoluble Ce Zn5 and Mg Zn2 phases.Adding too much Ce or Mg cannot exert a positive effect on the corrosion resistance of the alloy.With the extension of corrosion time,the self-corrosion current of Zn-Cu-Ti-M alloy gradually decreases.After 20 days of corrosion,the self-corrosion currents of Zn-0.6Cu-0.3Ti-0.3Ce and Zn-0.6Cu-0.3Ti-0.1Mg alloys respectively decrease to26.06×10^-5A/cm²and 19.25×10^-5A/cm².It's found that the longer the corrosion time,the weaker the influence of Cu,Ce and Mg content on the corrosion performance of Zn-Cu-Ti-M alloy.This is mainly due to that long-term corrosion promotes the formation of denser corrosion products on Zn-Cu-Ti-M alloy surfaces.Compared with Zn-Cu-Ti-M alloys,Zn-Cu-Ti-M alloy coatings show lower self-corrosion current,and the self-corrosion current of Zn-0.6Cu-0.3Ti coating is only 15.48×10^-5A/cm²,which is an order of magnitude lower than the corresponding alloy.This is mainly owing to the finer structure of the coating.The addition of Ce or Mg further reduces the self-corrosion current of the alloy coating,and the increase of Ce or Mg content helps to improve the corrosion resistance of the coating.The possible reason is that Ce or Mg have higher solid solubility in the Zn matrix solidified under rapid cooling condition.With the extension of the corrosion time,it is found that the corrosion current density of Zn-Cu-Ti-M alloy coatings generally shows a trend of first decreasing,then increasing and then decreasing.The impedance spectrum of Zn-Cu-Ti-M alloys and coatings have only one capacitive arc.Adding Ce and Mg elements and prolonging the corrosion time can increase the resistance value of the corrosion product layer.After adding 0.3%Ce,this resistance value increases from 75.6?.cm² to 87.39?.cm² in Zn-0.6Cu-0.3Ti alloy;after adding 0.1%Mg,this value increases to 92.16?.cm².Similar rules are found in Zn-Cu-Ti-M alloy coatings.However,compared to the alloy,the corrosion product layer resistance of the coating is higher.The corrosion product resistance of the Zn-0.6Cu-0.3Ti alloy is 1678?.cm².This is attributable to the small amount of O element generated during arc spraying and fine structure of the coating.In the initial stage of corrosion in 3.5%Na Cl solution for Zn-Cu-Ti-M alloys and coatings,the products formed are Zn(OH)₂ and a small amount of Zn₅(OH)₈Cl₂·H₂O.After corrosion for 5 days,the products evolve into Zn₅(OH)₈Cl₂·H₂O and Zn O.In addition,there are also traces of oxides or hydroxides of Ti,Cu,Ce and Mg.Single corrosion product is not a pure phase,but a composite phase,which is composed of Zn₅(OH)₈Cl₂·H₂O,Zn O,etc.However,the ratio of Zn O in the corrosion product is quite different.The ratio of Zn O in the corrosion product layer of the Zn-Cu-Ti-M alloy is low because Ce and Mg inhibit the transformation of Zn(OH)₂ to Zn O.The morphology of the corrosion product evolved from the initial flake shape to a stable spherical shape.The composition and morphology evolution of corrosion products of Zn-Cu-Ti-M alloys and coatings under dynamic corrosion condition are very similar to those of static corrosion.In addition,it is found that both Zn-Cu-Ti-M alloy and coating show similar corrosion process.