Simulation and Experimental Research on Nickel-based Coating Preparedby Jet Electrodeposition at Different Scanning Speeds

In order to study the processing mechanism of jet electrodeposition and explore the influence of different scanning speed on the wear and corrosion resistance of nickel-based coating prepared by jet electrodeposition. The reciprocating scanning motion of the nozzle was used to prepare the nickel-based coating in a specific area. Combined with COMSOL software, the coupling effect of multiple physical fields in the process of jet electrodeposition at different scanning speeds was numerically calculated. Scanning electron microscope, microhardness tester, material surface comprehensive performance tester and electrochemical workstation were used to analyze the surface morphology, section thickness, microhardness, abrasion resistance and corrosion resistance of the nickel-based coating prepared by jet electrodeposition at different scanning speeds. Results show that with the increase of scanning speed, coating grain size decreases, and the coating thickness increases after the first decreases, and microhardness increase after decreases first, abrasion resistance and corrosion resistance were lower after increase first, When the scanning speed reaches 600mm/min, the jet electrodeposited nickel-based coating has the best performance, the maximum thickness reaches 24.83μm, the microhardness reaches 616.86HV, and the wear scar area is 2766.75μm2. In addition, the self-corrosion potential is -0.33V, the self-corrosion current density is 5.16E-7A·cm2, and the equivalent impedance is 4660Ω. The experimental results are consistent with the simulation results, which verifies the accuracy of the simulation model and provides theoretical guidance for further experiments related to jet electrodeposition.


Introduction
Electrodeposition method is a commonly used method to get different metals (such as copper, nickel, cadmium, zinc, etc.) [1][2][3][4] and different coatings (such as alloy coatings, composite coatings) [5][6][7] . The principle is simple: a voltage is applied between the cathode and the anode to produce a current, thus forming a metallic deposit on the surface of the cathode [8,9] . At present, the processing methods of jet electrodeposition are becoming more and more mature. The main focus of jet electrodeposition technology is on the composition of electroplating solution and the research of coating properties [10,11] . In order to better understand the processing process of jet electrodeposition, people still have many difficulties to be solved, so the computer simulation is particularly important.
At present, the electrodeposition processing technology is becoming more and more mature. Due to the superiority and scientificity of simulation research, the research through software simulation has increasingly become the focus of scholars at home and abroad [12,13] . Huang, D [14]  When preparing Ni-SiC composites by jet electrodeposition method, Wei Cui [15]  E. Bahrololoom [16] used COMSOL software to simulate the interference of electric field generated by obstacles of different types and sizes on electrodeposition. Through the study of current density and thickness of deposition layer. The results show that the electric field interference can reduce the thickness of the deposition layer, and the simulation results are in good agreement with the test results. Wang [17] et al.

Scanning jet electrodeposition test principle
The principle of jet electrodeposition is basically the same as that of ordinary electrodeposition, but the difference lies in the different mass transfer process of electroplating solution. The mass transfer form of ordinary electrodeposition is mainly diffusion and electromigration, while the main form of jet electrodeposition is convection, so jet electrodeposition speeds up the material transfer efficiency [18,19] . Jet electrodeposition technology is an outstanding representative in high-speed electroplating [20,21] . Due to its unique fluid dynamics characteristics, the surface of the cathode workpiece under the high-speed impact of electroplating solution is not only mechanically activated to effectively reduce the thickness of the surface layer and the diffusion layer at the negative workpiece, but also the metal grains are refined and the deposition layer is more compact. On the basis of jet electrodeposition, the nozzle is connected to the motion system. Under the control of the numerical control system, the nozzle moves over the workpiece with a fixed scanning speed to form a coating in the scanning area. In this experiment, jet electrodeposition was carried out on a self-built platform to prepare nickel-based coatings, and the scanning jet electrodeposition test platform is shown in

Scanning jet electrodeposition test
The composition of the electroplating solution is shown in  [22] . As a cathode surfactant, sodium dodecyl sulfate can reduce the interfacial tension between electrode and electroplating solution, so that the hydrogen formed is difficult to stay on the surface of the cathode, to prevent pinholes and pitches [23] .

Geometric model analysis
According to the actual processing process and principle, the whole processing process basically remains unchanged along the direction perpendicular to the nozzle movement. Therefore, a simplified 2D model is adopted for simulation to establish the geometric model of scanning jet electrodeposition simulation as shown in Fig. 3, in which the whole geometric region is filled with electroplating solution.
As shown in Fig. 2  greater than the critical Reynolds number, it is turbulent flow [24] , the Reynolds number is defined by Eq. 1: Where I represents the current density on the electrode surface, I0 represents the exchange current density, αa represents the anode transfer coefficient, αc represents the cathodic transfer coefficient, R represents the ideal gas molar constant, T is thermodynamic temperature; η means overpotential, Eeq represents equilibrium potential, E0, eq is the standard equilibrium potential; Tref represents the reference temperature, set as 293.15K.
In the process of numerical calculation, the metal ions on the surface of the cathode undergo reduction reaction, which leads to the change of cathode boundary, that is, cathode growth. According to

Multi-physical field distribution results
As the coordinate X of the center line of the nozzle changes, the distribution of the physical field also       The reason is that when the scanning speed is small, the current action time in the area just below the nozzle is long, and the grains can fully grow, so the grain size is large. With the increase of scanning speed, nozzle directly by the regional electric time is shorter, not fully deposition substrate metal ions, so the formation of the grain size is relatively small, while at the same time with the increase of number of scanning area lead nozzle directly can form more grain, thus increasing scanning speed to the grain refinement. However, when the scanning speed is too high, the change of force on the flow field in the working area is intensified, leading to obvious disorder of the flow field, and the stability of the fluid passing through the surface of the substrate is reduced. Furthermore, various defects are formed on the coating surface when the scanning speed is too high.

Coating thickness distribution results
Therefore, proper scanning speed is beneficial to obtain better sediment quality.  Fig. 10(a) and (c)), the coating surface is relatively smooth; when the scanning speed is large (as shown in Fig. 10(d) and (e)), the coating flatness is low. However, due to the limitation of processing efficiency, the thickness value is different from the simulation result, which is consistent with the processing efficiency predicted above. By comparing the center thickness of the coating in simulation and test, the machining efficiency is calculated to be 77.38%, 78.09%, 82.00%, 80.16%, 76.26%, respectively. Therefore, the overall processing efficiency is 78.78% on this test platform.  The CFT-I material surface comprehensive performance tester was used to carry out the friction and wear tests, and the sectional parameters (including section width, section height and section area) of the wear marks were measured by the LEXT OLS4100 laser confocal microscope. Figure 12 shows the threedimensional section diagram of the wear marks at different scanning speeds, and the specific parameter values are shown in Table 3. the sectional parameters are inversely proportional to the wear resistance. As can be seen from Table 3 Table   4. The results show that the scanning speed has a great influence on the self-corrosion potential and selfcorrosion current density of the nickel base coating.
When the scanning speed increases from 200mm/min to 600mm/min, the self-corrosion potential of the coating increases, and the self-corrosion current density decreases. As the scanning speed continues to increase, the self-etching potential of the coating decreases and the self-etching current density increases.
When the scanning speed is 600mm/min, the selfcorrosion potential of the coating is the maximum (-

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