# Study on Heat Transfer Performance and Anti-Fouling Mechanism of Ternary Ni-W-P Coating
## 1 Introduction  
The fouling problem, which is regarded as \"the main unresolved problem in heat transfer\", is mainly generated by the deposition of particulates, impurities, salts, and various organic species onto heat transfer surfaces [1; 2; 3]. It can be seen from Figure 1 that the fouling of calcium carbonateand magnesium carbonate accumulate on the tube surface of solar-energy and water-inlet of solar collectors. Due to the corrosion caused by long-term use of water delivery pipes and valves, iron oxides and copper oxides accumulate on the outlet of insulating water tank. Furthermore, the influence of corrosion products on the heat exchanger surface results in further corrosion of metal. After long periods of use in the circumstance of hard water, the solar collector has to be cleaned due to the serious blocking and the reduced efficiency. Fouling inevitably increases the fouling resistance and reduces the heat transfer effectiveness, thus bringing about higher fuel consumption, maintenance costs. Since the 1980s, electroless plating has been treated as an operational technology of the surface modification, attracting widespread attention [4; 5; 6; 7; 8]. The results prove that electroless Ni-P-based coating is an advantageous method for fouling inhibition [9; 10; 11; 12; 13]. The mass gain method [10; 11; 14], the calcium ion loss method [12], and the fouling resistance method [15] were commonly used for measuring the deposition process of calcium carbonate fouling on materials surface. The surface is easy to be corroded and the oxidized surface is prone to form a \"transition interface\", which works as a \"bridge\" to connect the matrix and fouling. The \"transition interface\" consists of corrosion or oxidation, and the lattice of the \"transition interface\" layer can be easily matched with the lattice of fouling. Therefore, the adhesion of fouling on the corrosion interface seems to be easier [13; 16; 17]. Simultaneously, compared with uncoated pipe, the tube with electroless coating can significantly improve the property of condensation heat transfer, showing the coexistence of dropwise and filmwise on most working conditions [18; 19]. Calcium carbonate is the principal component of fouling in industry. It is easy to adhere to the surface of the heat transfer wall to form fouling, affecting the heat transfer efficiency. The compactness and adhesion strength of fouling on the surface of heat exchange wall depends on its crystal structure [20]. Meanwhile, the evidence demonstrates that the induction period of fouling can be effectively prolonged by the surface free energy of coating. Solubility of calcium carbonate reduces with the up of solution temperature, and the studies prove that the dissolved metal cation can further accelerate the deposition process of fouling [2; 21; 22]. From the preceding decades, the nucleation of CaCO\({}_{3}\) molecules have been studied theoretically based on molecular dynamics (MD) and density functional theory (DFT). Based on MD, the grain boundary energy was computed to investigate the change of energy in the aggregation process of Ca\({}^{2+}\) and [CO\({}_{3}\)]\({}^{2-}\). When the distance of interfaces between CaCO\({}_{3}\) particles came close to colliding, the dielectric constant of  
Figure 1: Fouling on the pipes of solar collectors after long periods of use. water became small [23]. Jun, [PERSON] simulated calcium carbonate at high pressure and the phase transition between calcite and aragonite, and results show that in the 300-800 K temperature range, calcite transforms to aragonite at a pressure of around 8 GPa [24]. Additionally, when heated under high pressure, the first-order unsymmetric phase transition of calcite phase occurs first, and then the second-order transition of calcite and aragonite phases happens at a higher temperature [25]. [PERSON] proved that the aggregation of primary particles leaded to the formation of calcite clusters. Because positive and negative charges attract each other, many particles form clusters in the same direction [26].