# Study on Corrosion Behavior of Waterborne Polyurethane Coating with High Thermal Conductivity
## 3 Results and Discussion
### EIS Results and Analysis  
To further study the effects of composites filler on the corrosion resistance of waterborne polyurethane coating, the EIS diagram is obtained in 3.5 wt % NaCl solution, as is shown in Figure 5. Generally, larger capacitive arcs and diameters of the Nyquist diagram denote much better protective performance [21, 22]. In the case of a waterborne polyurethane coating (Figure 5a), the diameter of capacitive arcs depicts a drastic down-trend as the salt spray time extends, indicating the rapid decline of the protective performance for the substrate. Two time constants are observed in this period, representing that the corrosion reactions start at the interface between the substrate and coating. After 10 d of salt spray tests, the sample suffers from severe corrosion damage (Figure 3h), and a lot of yellow rust emerges on the sample surface. The capacitive arc significantly decreases (Figure 5a). For the modified coating with 2 wt % composites filler, the diameter of the capacitive arcs in the Nyquist diagram (Figure 5c) is clearly larger than it in Figure 5a, indicating that the modified coating possesses more excellent anti-permeability to the corrosive medium. The modulus value of the impedance (\(\left|\,Z\,\right|_{0.01\,\mathrm{Hz}}\)) at the low frequency (0.01 Hz) can be used for evaluating the impermeable and shielding properties of the coating [23, 24]. Obviously, the \(\left|\,Z\,\right|_{0.01\,\mathrm{Hz}}\) of the waterborne polyurethane coating gradually declines from 2.85 \(\times\) 10\({}^{7}\)\(\Omega\cdot\)cm\({}^{2}\) to 3.24 \(\times\) 10\({}^{4}\)\(\Omega\cdot\)cm\({}^{2}\) (Figure 5b) with the extension of the salt spray time, suggesting that this coating has almost already lost the role of shielding electrolytes. In comparison, the coating samples with 2 wt % composites filler display the higher impedance modulus at 0.01 Hz during the entire salt spray test stage, and the \(\left|\,Z\,\right|_{0.01\,\mathrm{Hz}}\) value of modified coating is still up to 1.31 \(\times\) 10\({}^{7}\)\(\Omega\cdot\)cm\({}^{2}\) after 10 d of salt spray tests. These results demonstrate that the modified coating can provide more excellent protective capability for the substrate steel. The EIS results are fitted for further elaborating the protective mechanism of the modified coating by Zsimpwin software. The equivalent circuit in Figure 6a is used to fit the EIS data of the coating samples. For the EIS curve of coating without a composite filler after 10 d of the salt spray experiment, the equivalent circuit in Figure 6b is employed  
Figure 4: The adhesion strength of the modified waterborne polyurethane coating. due to the obstruction effects from the rust layer to the ions [25; 26]. In the equivalent circuit, \(R_{\rm s}\) is the solution resistance, while \(\mathit{CPE}_{\rm c}\) and \(R_{\rm c}\) represent the coating capacitance and pore resistance, respectively. \(\mathit{CPE}_{\rm dl}\) and \(R_{\rm ct}\) correspond to the double-layer capacitance and charge transfer resistance. \(Z_{\rm w}\) depicts the Warburg resistance caused by the diffusion process in the electrochemical reaction. The polarization resistance \(R_{\rm P}\) (\(R_{\rm P}=R_{\rm c}+R_{\rm ct}\)) can be used to present the corrosion protective performance of the coating [27; 28]. In general, the higher \(R_{\rm P}\) value represents a much better corrosion resistance property. Figure 7 depicts the change trend of \(R_{\rm P}\) of the coating at different stages of the salt spray test. Overall, the \(R_{\rm P}\) value of the coating without composite filler shows a decreasing trend with the lengthening of salt spray time. Especially for the sample after 10 d of salt spray tests, \(R_{\rm P}\) decreases to 3.24 \(\times\) 10\({}^{3}\)\(\Omega\)-cm\({}^{2}\). In this case, the protection function of the polyurethane coating is nearly ineffective.