# Preparation of Co-Ni Alloy Coating with Stable Composition by Jet-Electrodeposition
## 3 Results and Discussion
### Surface Morphology of the Coatings  
Figures 4-7 show the surface morphology of the coatings under different solution flow rates. It can be seen that the grain sizes on surface decrease with the increasing flow rate. This change is more dramatic at higher M values. When M is 2:1 or 1:1, the surface of the coating is generally uniform and angularly conical tapered crystal at lower flow rates. As the flow rate of the plating solution increases, the grain size of the coating surface decreases slightly, and the surface becomes smoother. This trend is more obvious when M is at 1:1 than at 2:1. Since the Co content in the two plating baths is basically not affected by the flow rate of the plating solution, the plating solution flow rate has a direct effect on the plating surface. This is because the flow rate of the plating solution reduces the range of the diffusion layer on the surface of the cathode, which increases nucleation during deposition and leads to fine grains. In addition, at the same flow rate, the grain size of Co-Ni coatings obtained with M at 2:1 is smaller than that at 1:1. When M is at 1:2 or 1:3 in the plating solution, the variation of surface grain structure is more significant as the flow rate of the plating solution increases. This is because the Co content of coatings increases with the increasing flow rate, leading to decreasing grain size. Meanwhile, the increase of the flow rate reduces the thickness of the diffusion layer. At the same time, the kinetic regime may switch to a mixed or activation-controlled regime because of reducing mass transport resistance, and the surface of the Co-Ni coatings are further refined.  
Figure 4: Surface morphology of coatings obtained with M = 2:1 in the plating solution at different flow rates: (**a**) 2.0 L/min, (**b**) 2.5 L/min, (**c**) 3.0 L/min, (**d**) 3.5 L/min, (**e**) 4.0 L/min, (**f**) 4.5 L/min.  
Figure 5: Surface morphology of coatings obtained with M = 1:1 in the plating solution at different flow rates: (**a**) 2.0 L/min, (**b**) 2.5 L/min, (**c**) 3.0 L/min, (**d**) 3.5 L/min, (**e**) 4.0 L/min, (**f**) 4.5 L/min.  
Figure 6: Surface morphology of coatings obtained with M = 1:2 in the plating solution at different flow rates: (**a**) 2.0 L/min, (**b**) 2.5 L/min, (**c**) 3.0 L/min, (**d**) 3.5 L/min, (**e**) 4.0 L/min, (**f**) 4.5 L/min.  
The variation of the coating surface with flow rate can be explained by mass transfer theory. According to diffusion mass transfer theory [21], the diffusion thickness can be expressed as:  
\[\delta=D_{i}^{1/3}\mu^{1/6}y^{1/2}u_{0}^{-1/2} \tag{1}\]  
where \(\delta\) is the thickness of the diffusion layer, \(D_{i}\) is the diffusion coefficient, \(u_{0}\) is the tangential initial velocity of the liquid flow, \(\mu\) is the dynamic viscosity coefficient, and \(y\) is the distance from a point on the surface of the cathode to the impact point \(y_{0}\). For the same plating solution, increasing the flow rate of the plating solution can reduce the thickness of the diffusion layer, which is beneficial to the limited current density of the electrodeposition and also the cathode overpotential [22]. The size and number of nuclei are controlled by overpotential and can be expressed by [PERSON]'s electrochemical formula [23]:  
\[\tau=\frac{2\varepsilon V}{Ze_{0}\big{|}\eta\big{|}} \tag{2}\]  
where \(\tau\) is the radius formed by the critical nucleus, \(\varepsilon\) is the surface energy, \(V\) is the atomic volume in the crystal, \(Z\) is the elementary charge, and \(\eta\) is the overpotential. It can be seen from the Kelvin formula that when there is a high overpotential, a small crystal nucleus can be formed. Therefore, an increase in the flow rate of the plating solution facilitates the formation of a small crystal nucleus, whereby the size of the crystal grains is reduced, and the structure is refined.