# Study on Corrosion Behavior of Waterborne Polyurethane Coating with High Thermal Conductivity
## 3 Results and Discussion
### EIS Results and Analysis
In the meantime, the corrosive ions penetrate into the interface between the coating and substrate metal and result in the severe corrosion process, which is also confirmed by the topography in Figure 3h. It is noteworthy that for the coating with 2 wt % composite fillers, the \(R_{\rm P}\) value  
Figure 5: Nyquist and Bode curves of the coating sample, (**a**,**b**) the coating without composites filler, (**c**,**d**) the coating with 2 wt % composites filler. Figure 6: The equivalent electrical circuit for the coating samples in different stages. is around 1.15 \(\times\) 10\({}^{8}\)\(\Omega\cdot\)cm\({}^{2}\), which is an order of magnitude larger than that of the coating without mixture fillers. Similarly, as the salt spray time further increases, the \(R_{\rm p}\) value also shows a downtrend due to the infiltration effects of electrolyte. However, it is still up to 1.31 \(\times\) 10\({}^{7}\)\(\Omega\cdot\)cm\({}^{2}\) for the samples that underwent 10 d of salt spray tests, implying that the modified coating still has a good physical barrier and anticorrosion capability. In addition, the literature reported that the addition of carbon materials (carbon nanotubes, carbon nanohorns, graphene, etc.) could enhance the hydrophobicity of coating, which will further impede the penetration process of aggressive medium and promote anticorrosion effects [29]. The coating porosity (\(P\)) directly affects the shielding properties and the penetration behavior of the electrolyte. It can be calculated according to Equation (1) [30; 31], where \(R_{\rm pt}\) represents the coating resistance when the porosity of the coating tends to infinity. It can be obtained through Equation (2). Here, \(d\) (\(\mu\)m) is the coating thickness. \(A\) (cm\({}^{2}\)) describes the sample area. \(k_{0}\) is the electric conductivity of the electrolyte. For the 3.5 wt % NaCl solution, it is around 3 S/m. \[P=\frac{R_{\rm pt}}{R_{\rm c}} \tag{1}\]  
\[R_{\rm pt}=\frac{0.01d}{A\cdot k_{0}} \tag{2}\]  
Figure 8 displays the change trend of coating porosity with the salt spray time. It can be found that the coating porosity gradually increases with the increasing of salt spray time. Overall, the porosity of the modified coating with 2 wt % fillers is lower than that of the blank coating, implying that composite fillers can improve the compactness of coating and enhance the shielding property to the electrolyte. Moreover, as the salt spray test time extends, the porosity of the blank sample markedly increases, while it still remains at a low level for the modified coating sample. This phenomenon is attributed to the severe corrosion caused by penetration of an electrolyte. In comparison, there is an absence of obvious corrosion for the modified coating sample, which is verified by the topography in Figure 3d. Thus, the modified coating sample still retains excellent protective performance after 10 d of salt spray tests. Figure 7: The change trend of the polarization resistance (\(R_{\rm p}\)) of the waterborne polyurethane coating.