# Study on Corrosion Behavior of Waterborne Polyurethane Coating with High Thermal Conductivity
## 1 Introduction  
With the rapid development of the power industry, the service scope of power transmission and transformation equipment gradually expands, even including the high salt spray, heavy sand, and strong sunshine environment. In this situation, the corrosion reliability problem is particularly prominent [1]. For the power equipment, such as transformers, some coatings are generally used to improve their corrosion resistance and extend the service life. However, it is noteworthy that the majority of currently commonly used coatings are solvent-based products. The emission content of volatile organic compounds (VOCs) of these coatings is around 300-500 g/L, which seriously endangers human health and regional ecological environment safety [2]. Thus, the environmentally friendly anti-corrosive coating is the current main trend. Among them, the waterborne anticorrosive coating, such as waterborne polyurethane, has recently obtained increasing attention due to the low VOCs emissions [3, 4]. In terms of transformers, in addition to corrosion reliability during service, they usually also face serious temperature rise problems [5, 6]. The too high hot-spot temperature will accelerate the aging process of the insulation and markedly decrease the transformers' life expectancy [7, 8]. The waterborne coating promotes the environmental protection performance of anticorrosive coatings for transformers; however, their thermal conductivity is still poor, which cannot meet the heat dissipation requirement of the transformer. Especially for high-power transformers serving in the tropical climate, the over-heating faults frequently occur. Based on this situation, apparently, it is very necessary and urgentto develop a new type of low pollution, high thermal conductivity anti-corrosion coating for the power industry protection field. On the aspect of improving the thermal conductivity of the coating, adding fillers with high thermal conductivity to the matrix is a convenient and effective method [9; 10; 11]. [PERSON] [12] et al. studied the thermal behavior of epoxy resin added with AlN and BN, and the results showed that the AlN-filled coating exhibited a higher thermal conductivity than BN composites. Compared with the blank sample (0.168 W/m\(\cdot\)K), the thermal conductivity of the coating with 10 wt % AlN reached 0.1987 W/m\(\cdot\)K. [PERSON] [13] et al. reported the synergistic effects of the BN and Al\({}_{2}\)O\({}_{3}\) on enhancing the thermal conductivity of epoxy resin, and it was found that a composite with high thermal conductivity of 1.182 W/m\(\cdot\)K was obtained after adding 22.5 wt % BN and 7.5 wt % Al\({}_{2}\)O\({}_{3}\). In addition to these inorganic additives, the graphene and carbon nanotubes gained the increasing attention from academic and industrial fields due to its excellent thermal conductivity properties and good compatibility with polymer matrix [3; 14]. [PERSON] [15] et al. investigated the effects of graphene/carbon nanotube on the thermal conductivity of acrylic resin. They concluded that the coating modified with 9.9 wt % graphene nanosheets and 0.1 wt % carbon nanotubes displayed a high thermal conductivity of 2.97 W/m\(\cdot\)K, which is related to the formation of honeycomb structure and decreasing of the interface heat resistance. Obviously, carbon nanomaterials show great advantages in improving the thermal conductivity of coatings. It is noteworthy that the expense of the graphene and carbon nanotubes is very high, and the dispersion process of single-component graphene in coatings is very difficult and usually needs some pre-processing technology [16], which greatly limits the application of graphene coatings in the power industry. Considering the above situations, exploring a low-cost coating with high thermal conductivity is of great significance for engineering applications. In this paper, a low-cost commercial mixture of carbon nanotubes, carbon nanohorns, graphene, and ultrafine graphite powder is used to modify the thermal conductivity of waterborne polyurethane coating.