Research of Corrosion Behavior and spraying preparation process of Zn-Ni Coating
Wang Yingfa
CNKI
Abstract
Zn-Ni alloy is used in surface protection of high strength steel commonly,because it has many excellent features such as high anti-corrosion, low hydrogen embrittlement, weldability and machinability and so on. The Zn-Ni alloy not only provides a good shielding effect for substrate, but can play a role of cathodic protection. At present, the Zn-Ni alloy is prepared by electrodeposition generally.Numerous studies showed that the anti-corrosion of Zn-Ni alloy was greater as compared with that pure zinc coating. In terms of the plating process, it has some defects. On the one hand, the thickness of coating has a certain restrictions, usually in about a dozen microns. On the other hand, the plating bath pollutes the environment.
Compared with plating process, Zn-Ni alloy coating prepared by spraying process has many advantages such as simple process, saving material, small environmental pollution and so on. The spraying process can increase the thickness of the coating to improve the anti-corrosion. The research of Zn-Ni alloy with high nickel content prepared by spraying process is few. The main reason is that the melting point of nickel is much higher than that of zinc, and with the nickel content increasing, the brittle and melting point of alloy become greater. So, the preparation of the alloy wire and alloy powder with high nickel content which used in the spraying process is very difficult.
In order to obtain the homogeneous Zn-Ni alloy coating with high nickel content,the alloy powder was prepared by mechanical alloying technology. But the alloy powder, which was prepared by the mechanical alloying technology, was not suitable for thermal spray directly because of the refinement and high activity of alloy powder.Therefore, this paper presents two approaches about the research of Zn-Ni alloy.Firstly, the Zn-Ni composite coating was prepared by oxyacetylene flame spraying process, which was simple process and inexpensive cost, and its anti-corrosion performance was studied. Secondly, the Zn-Ni alloy coating was prepared by cold spraying process and the anti-corrosion of coating was studied. The alloy powder was prepared by mechanical alloying technology.
The anti-corrosion performance of Zn-Ni coating with different nickel content in seawater was tested by the means of potentiodynamic polarization and electrochemical impedance, and with the analysis of SEM， EDS and XRD. The protection performance and corrosion mechanism of the Zn-Ni coatings under the two conditions were studied and analyzed. The results show that the thickness of Zn-Ni composite coatings prepared by thermal spraying process can be about 300jxm, and the coating can play the role of long-term protection to substrate. Secondly, after 12 hours high energy ball milling, part of the Zn-Ni mixture powder has achieved alloying. The alloying degree of powder in the cold spray process can be further improved. However, due to the powder particle size is too small , the thickness of coating can only reach 15?19|im. Thirdly, the zinc of coating prepared by the two spraying processes first was consumed. The attachment of corrosion products to the coating surface and the formation of nickel-rich layer below the product film can slow down the corrosion rate of coating ultimately. With the nickel content increasing,the corrosion potential Econ- of coating becomes more positive. The nickel can enhance the stable potential of coating and decrease the corrosion electromotive force.At the same time, nickel can make the oxygen reduction overpotential of cathodic reaction rise. But when the nickel content is too high, the coating will occur the corrosion micro-cell function and have a negative impact on the coating. What's more,the corrosion behavior of coating prepared by the two spraying processes has some significant differences. In terms of Zn-Ni composite coating prepared
by thermal spraying process, the coating porosity is larger. The corrosion products, accumulated within the coating, have a significant impact on the anti-corrosion of coating and decrease the corrosion current continuously. However, with regard to the Zn-Ni alloy coating prepared by cold spraying process, the structure of coating is dense and the presence of nickel can play a role that restrain the dense Zn(OH)2 to be converted into the loose ZnO. So， compared with thermal spraying coating, the corrosion morphology of coating prepared by cold spraying process is more uniform, more dense. Finally, for the thermal spraying coatings, the anti-corrosion properties of the coating with 20wt% Ni is best. The stable corrosion current Icorr of the coatings range from 1.24jiA/cm to 1.87jiA/cm , while the stable corrosion potential Ecorr range from -0.98V to -0.95V. Regarding the cold spraying coatings, the anti-corrosion properties of the coating with 15wt% Ni is best. The stable corrosion current Icorr of the coatings range from 0.64)iA/cm to 0.94pA/cm ,
while the stable corrosion potential Econ-range from -0.86V to -0.84V. The Zn-Ni coatings, which are prepared by the two spraying process, meet the requirements of the cathodic protection. However, the corrosion resistance of the cold spraying coating is superior to thermal spraying coatings.
Key words: Oxyacetylene flame spraying;Cold spray;Mechanical alloying;Zn-Ni coatmg;Corrosion resistance
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