# Study on Heat Transfer Performance and Anti-Fouling Mechanism of Ternary Ni-W-P Coating
## 3 Results  
### Surface Morphology and Microstructure  
As shown in Figure 4a,b, fouling were deposited on the surfaces of the specimens. Compared with the fouling on the surface of the coated specimen, red products on the uncoated specimen were causedby the corrosion products generated on the bare substrate surface after the flow fouling experiment. Therefore, there are not only calcium carbonate fouling but also iron rust formed on the substrate surface. The result conforms to our previous studies [10,13] and proves that Ni-W-P has an admirable effect on anti-fouling performance. After calculating the mass change of the fouled specimens, it was found that there is no obvious rule between the mass change and the W content in the coating, while the certainty is that the mass gain of fouling on all the coated specimen is less than that of the bare substrate surface in the same period. Due to the difference of fouling adhesion, the deposit growth rate is the most significant parameter during the process [40], the thickness of fouling layer changes. As a result, the thermal resistance of the fouling layer also increases notably. In other words, the heat transfer coefficient of the untreated material has been affected partly due to the existence of Ni-W-P coating. As shown in Figure 4, the result reveals that the fouling on the surface of the specimen did not change significantly with the increase of W content. Because of the scouring effect of water flow and the influence of Van der Waals force in the molecules, the deposited fouling could fall off on the surface of the specimens and the mass change shows irregularly. Thus, the phenomenon of mass gain of fouling is inconsistent with our previous study in static fouled solution [10,16]. Whereas, it is certain that the existence of Ni-W-P coating, the surface tension decreases, which makes it more difficult for fouling to adhere and the deposition of fouling has been inhibited. To study the thickness and structure of the fouling deposited on the coated surface, scanning electron microscopy with energy dispersive spectroscopy was employed to test the structure of the deposited Ni-W-P coating. In Figure 5a, we can obviously observe that the surface of Ni-W-P coating is well-organized with cell body lines. The appearance of the globular morphology, which is in the form of the salient of the cell body, can be explained as the uneven deposition of P element. Without P element, Ni would grow orderly in accordance with the structure of face-centered cubic. The P element can densify the cell body of the coating and co-deposit the Ni on the substrate surface. During this process, P atoms will hold the positions of several Ni atoms, thus forming Ni-P supersaturated solid solution. When it increases to a certain stage, the Ni-W-P coating cannot array orderly in accordance with the structure of crystal Ni. Consequently, the Ni-W-P coating shows an amorphous structure, which can be proved by the XRD result. Figure 4: Mass change of coated specimens after 72 h fouling (**a**) macro appearance of specimen with Ni-W-P coating after flow fouling; (**b**) macro appearance of bare specimen after flow fouling. As can be seen in Figure 5b,c, the color of the tissue changes distinctly. It turns out that the globular morphology is filled with Ni[W,P] structure. The thickness of globular morphology is larger than that of the coating in Figure 5c. The possible explanation for this is the local inhomogeneity of the bare substrate surface that leads the excessive deposition of Ni[W,P] in the hole. Meanwhile, we cut the cross sections of several areas in order to calculate the heat transfer behavior of Ni-W-P coating. It was found that the thickness distribution of Ni-W-P coating after 2 h plating is 20 \(\upmu\)m \(\pm\) 2 \(\upmu\)m. According to the Figure 6, there are apparent peaks of Ni, W, and P at the cross section. As the W and P elements were solid dissolved in the Ni crystal cell, the content of Ni element is obviously higher than that of the other two elements.