# Study on Heat Transfer Performance and Anti-Fouling Mechanism of Ternary Ni-W-P Coating
## 3 Results
### Surface Morphology and Microstructure
Here, we calculate the heat transfer of the Ni-W-P coating uniformly without local distinction. As shown in Figure 7, SEM tests were performed on specimens after 72 h of fouling. Figure 7a,b exhibit the surface morphology of the uncoated mild steel. It is notable that there are a large number of fouling deposits on the surface in the form of massive and columnar particles, as well as agglomerates. Columnar fouling is also surrounded by loose-cotton-flocculent fouling. Based on the previous results, uncoated mild steel is primary composed of massive calcite and columnar aragonite, while calcite-phase CaCO\({}_{3}\) fouling takes up a high proportion. A considerable amount of fouling indicates that the induction stage of the fouling deposition process on the bare substrate was ended and the heat transfer of the specimen surface could be severely affected by the rapid growth of calcite-phase and aragonite-phase fouling. The deposited products on the surface of Figure 7c are principal acicular aragonite mixed with a small amount of massive calcite and oxide [41,42]. The grain sizes of aragonite were estimated to be significantly smaller than calcite grains [43]. In the vicinity of calcite-phase CaCO\({}_{3}\) fouling (Figure 7e), oxidation products also deposit nearby. However, no oxidation products were found near the aragonite-phase CaCO\({}_{3}\) fouling (Figure 7d). At the same time, we also carried out a typical observation at the edge of the specimens. It can be found that the edge of the specimen is covered by the calcite and oxide. No aragonite deposited on their margins. On the one hand, the fouling solution will periodically form the double-line vortices arranged regularly in opposite rotation  
Figure 5: **(a)** Surface morphology of specimen 1 after electroless plating; **(b)** Surface morphology of nickel accumulation; **(c)** Cross-sectional morphology of nickel accumulation; **(d)** Back scattered-electron cross-sectional image of Ni-W-P coating of specimen 1; **(e)** Scattered-electron cross-sectional image of Ni-W-P coating of specimen 1.  
direction when passing through the edge of the specimens. The formation of Karman vortex street after the non-linear effect, leading to the increase of the pressure of fouling deposition on the edge of the specimen, as well as the difference of nucleation energy of the fouling lattice [44]. On the other hand, when the vortex appears, the fluid will produce a periodic alternating transverse force on the edge of specimens. The natural frequencies of similar phase of CaCO\({}_{3}\) fouling nuclei keep close to each other, leading to resonance of the crystal nucleus and then the selective deposition of the corresponding phase fouling, as well as the nucleus growth. Further study on nucleation energy barriers and bonding changes of different crystal-phase CaCO\({}_{3}\) fouling will be reported in Section 3.3 See Figure 8, the XRD patterns of fouling of coating and bare substrate show different. There is an amorphous peak due to the penetration of X-ray in thin fouling layer, which is according with the results of Figure 8. It also can be noticed that there are both diffraction peaks of calcium carbonate and amorphous peaks of Ni-W-P coating showing on the coated sample. However, as the fouling layer on the surface of the bare substrate is relatively thick, the X-ray is reflected when it reaches the surface of the fouling layer. Compared with the sole crystal form of CaCO\({}_{3}\) on the bare surface, it indicates that more aragonite deposit on the surface of ternary Ni-W-P coating. Figure 6: (**a**,**b**) Energy dispersive spectroscopy results of Figure 5e. Figure 8: X-ray diffraction pattern of the fouling on specimen surfaces. Figure 7: Surface morphology of the specimen after being fouled (**a**,**b**) uncoated specimen (**c**–**f**) specimen 1.