Preparation And Microstructure Of Zn-Cu Alloy Coating On Low Carbon Steel Surface

Zinc coating is widely used for corrosion protection of metal materials.In the zinc coating process,sherardizing coating has more excellent comprehensive performance than other zinc coatings and its preparation process is simple and environmentally friendly.At present,sherardizing coating is mainly used for anticorrosion in atmospheric environment,but the anticorrosive effect in extreme environments such as ocean is very limited.In order to improve the anti-corrosion ability of the coating,the anti-corrosion performance of the coating under special circumstances is often improved by alloying.researches shows that copper has a good effect in the application of marine anti-corrosion coatings.In this paper,Zn-Cu co-deposited by pack cementation,hot-deposited copper-sherardizing and electroless copper-sherardizing process were studied in an attempt to introduce copper into sherardizing coating to improve the corrosion resistance of sherardizing coating,extend its service life and expand its range of applications.The as-prepared coatings were characterized by scanning electron microscopy?SEM?,energy-dispersive X-ray spectroscopy?EDS?and X-ray diffraction?XRD?.The corrosion resistance of the coatings were investigated by a potentiodynamic polarization test in a 3.5wt.%NaCl solution.In addition,the evolution mechanism of the coating was analyzed in detail.The microstructure and potentiodynamic polarization curves of Zn-Cu co-deposited coatings formed at 400,420,440 and 460?were studied.The results show that when other conditions are the same,the increase of copper in the pack powder will lead to thinner coating thickness and crack increase.When the copper content in the pack powder is constant,the thickness of the formed coating is insensitive to temperature changes in the range of 420-460?.During the co-deposited process,copper does not enter the deposited layer and reacts with zinc in the pack powder to form a compound..The Zn-Cu co-deposited process did not improve the corrosion resistance of the coating.Then the microstructure and dynamic potential polarization curves of the Zn-Cu alloy coating prepared by the hot-deposited Cu-sherardizing and the electroless Cu-sherardizing process were studied.In the hot-deposited Cu-sherardizing process,the sample was first subjected to hot-deposition Cu treatment at 460?for 6h,then the obtained sample was powder sherardized at 400?for 3h.In the electroless Cu-sherardizing process,copper plating treatment was firstly carried out at 30?for 3to 5 minutes using a copper plating agent diluted at a certain ratio,and the obtained samples were subjected to sherardizing at 380,400,430 and 460?for 3 hours,respectively.The results show that the coating prepared by the lectroless Cu-sherardizing process has better bonding strength and surface quality,and the hot-deposited Cu-sherardizing process is easy to introduce oxides,resulting in reduced coating adhesion and surface quality.The coatings prepared by the two methods at 400?are composed of CuZn₄,?-phase(FeZn₁₀)and?-phase(Fe₁₁Zn₄₀),wherein CuZn₄ is located at the outermost layer of the coating.In addition to forming CuZn4 in combination with Zn,copper is also dissolved in the?-phase(FeZn₁₀).Electroless Cu-powder Zn infiltration experiments show that when the temperature is increased to 430?,CuZn₄ will no longer be contained in the coating,and the solid solubility of copper in the?-phase(FeZn₁₀)decreases with increasing temperature.The electroless Cu-sherardizing coating has improved corrosion resistance compared to the powder sherardizing coating.