# Corrosion Behavior and Conductivity of TiNb and TiNb Coated Steel for Metallic Bipolar Plates
## 3 Results and Discussion
### Corrosion Behavior  
The corrosion behaviors of 316L SS, TiNb/316L SS and TiNbN/316L SS were characterized electrochemically by potentiodynamic polarization, EIS, and potentiostatic polarization. Figure 6 displays potentiodynamic polarization curves of bare and coated 316L SS. From the curves, corrosion potentials (\(\phi_{\text{corr}}\)) are obtained. The \(\phi_{\text{corr}}\) is related to coating's thermodynamic stability, and the higher \(\phi_{\text{corr}}\) value normally means better corrosion resistance from the aspect of thermodynamics [3]. In the tests, the \(\phi_{\text{corr}}\) of 316L SS is \(-0.761\) V vs MSE, whereas TiNbN/316L SS has a more positive value of \(-0.744\) V. Compared with the substrate, the TiNb film increases the \(\phi_{\text{corr}}\) by 138 mV. When potentials are swept from negative to positive, all three samples show obvious passivation areas. In the passivation area, as the potential value increases, the current density remains nearly constant. At 0.19 V vs MSE, all the samples are passivated and 316L SS exhibits a current  
Figure 4: X-ray photoelectron spectroscopy (XPS) spectra of (**a**) Nb 3d and (**b**) Ti 2p (original-black lines, fitted-red lines) measured on TiNb/316L SS. Figure 5: X-ray photoelectron spectroscopy (XPS) spectra of (**a**) Nb 3d and (**b**) Ti 2p (original-black lines, fitted-red lines) measured on TiNbN/316L SS. density of 17.3 \(\upmu\)A\(\cdot\)cm\({}^{-2}\), while it is 2.28 \(\upmu\)A\(\cdot\)cm\({}^{-2}\) and 44.6 \(\upmu\)A\(\cdot\)cm\({}^{-2}\) for TiNb/316L SS and TiNbN/316L SS, respectively. The results indicate that the TiNb film has high corrosion resistance at 0.19 V vs MSE. This is owing to the formation of corrosion resistant passive film on TiNb coating obtained from anodic polarization process. Moreover, it is noteworthy that although the TiNbN film is thicker, TiNb alloy establishes better corrosion resistance. This phenomenon is mainly associated with the surface condition and composition of TiNbN film. EIS was performed and Figure 7 shows Nyquist impedance spectra of coated and uncoated 316L SS measured at OCP and 0.19 V vs MSE. In Figure 7a, influenced by surface defects, TiNbN/316L SS shows two time constants at OCP which reflect the impedance information from coating and substrate [12; 39]. In the Nyquist impedance spectra of TiNbN/316L SS at 0.19V, TiNb/316L SS, and 316L SS, incomplete semicircular arcs appear. The depressed semicircular shape of SS is attributed to the roughness of sample's surface. To fit the impedance spectrum of TiNbN/316L SS measured at OCP, an equivalent circuit with two constant phase elements (CPE), shown in Figure 8a [40], is adopted. The CPE is represented by \(Q\). Under ideal conditions, a CPE corresponds to a capacitor [41]. \(R_{\text{s}}\) is the resistance of electrolyte solution between working electrode and reference electrode. \(R_{\text{f}}\) and \(Q_{\text{f}}\) are coating's resistance and capacitance, respectively, and \(R_{\text{ct}}\) is the charge transfer resistance, while \(Q_{1}\) represents the double layer capacitance [42]. The other spectra in Figure 7a,b are fitted by the equivalent circuit in Figure 8b [43]. Table 1 lists the fitted values of \(R_{\text{ct}}\). The higher \(R_{\text{ct}}\) suggests the enhanced corrosion resistance [44; 45]. Figure 6: Potentiodynamic polarization curves of 316L SS, TiNb/316L SS, and TiNbN/316L SS. Figure 7: Nyquist impedance spectra of 316L SS, TiNb/316L SS and TiNbN/316L SS @ (a) open circuit potential (OCP), (b) 0.19 V vs mercuous sulfate electrode (MSE). Comparing \(R_{\rm ct}\) values of different materials in Table 1, it is observed that the charge transfer impedance of TiNb coated 316L SS is much higher than for the uncoated substrate. Part of reasons for this phenomenon is that the surface layer of TiNb could be oxidized into TiO\({}_{2}\) and Nb\({}_{2}\)O\({}_{5}\) spontaneously in air according to XPS measurements, providing a high corrosion resistance.