Study on heat transfer performance and anti-fouling mechanism of ternary Ni-W-P coating

Since the formation of fouling reduces heat transfer efficiency and causes energy loss, antifouling is desirable and may be achieved by coating. In this work, a nickel-tungsten-phosphorus (Ni-W-P) coating was prepared on the mild steel (1015) substrate using electroless plating by varying sodium tungstate concentration to improve its anti-fouling property. Surface morphology, microstructure, fouling behavior, and heat transfer performance of coatings were further reported. Also, the reaction path, transition state, and energy gradient change of calcite, aragonite, and vaterite were also calculated. During the deposition process, as the W and P elements were solids dissolved in the Ni crystal cell, the content of Ni element was obviously higher than that of the other two elements. Globular morphology was evenly covered on the surface. Consequently, the thermal conductivity of ternary Ni-W-P coating decreases from 8.48 W/m•K to 8.19 W/m•K with the increase of W content. Additionally, it goes up to 8.93 W/m•K with the increase of heat source temperature 343 K. Oxidation products are always accompanied by deposits of calcite-phase CaCO₃ fouling. Due to the low surface energy of Ni-W-P coating, Ca²⁺ and [CO₃]^2- are prone to cross the transition state with a low energy barrier of 0.10 eV, resulting in the more formation of aragonite-phase CaCO₃ fouling on ternary Ni-W-P coating. Nevertheless, because of the interaction of high surface energy and oxidation products on the bare matrix or Ni-W-P coating with superior W content, free Ca²⁺ and [CO₃]^2- can be easy to nucleate into calcite. As time goes on, the heat transfer efficiency of material with Ni-W-P coating is superior to the bare surface.