# Corrosion Behavior and Conductivity of TiNb and TiNb Coated Steel for Metallic Bipolar Plates
## 3 Results and Discussion
### Corrosion Behavior
At 0.19 V vs MSE, the high potential and the continuous supply of oxygen could passivate TiNb and lead to the formation of dense passive film which is corrosion resistant and could act as a barrier to inhibit the bulk solution from approaching the sub-layer directly. In comparison with TiNb, the \(R_{\rm ct}\) value of the TiNbN film is much smaller, either at OCP or 0.19 V vs MSE. Two major factors may contribute to this phenomenon. From a morphology point of view, the particle clusters and pores observed on the TiNbN coating can cause localized corrosion. The other reason is that the bonds between nitrogen and metallic atoms may impede the formation of more homogeneous and corrosion resistant passive layer. According to the above analysis, from the EIS and potentiodynamic polarization test results draw one can the conclusion that TiNb has good corrosion resistance while TiNbN and bare 316L SS are likely to dissolve in the aggressive cathode environment. To evaluate the stability of the materials, potentiostatic polarization experiments were conducted at 0.19 V vs MSE. The results are shown in Figure 9. In the beginning, the current density of each coated 316L SS sample declines while that of 316L SS first increases sharply, then decreases gradually. After 2000 s, 316L SS experiences an obvious fluctuation due to the surface states changes affected by electrochemical dissolution, but throughout this period, its average corrosion rate is increasing. As for TiNbN/316L SS, the time needed for total passivation is about 3000 s, and its stabilized current density is approximately 6-7 \(\mu\)A\(\cdot\)cm\({}^{-2}\). Although TiNbN/316L SS and 316L SS show comparable current density values and variation trends at the initial stage of tests, the TiNbN coating is much more stable than bare 316L SS, judging from the smoothness of the curves. That is because the coating deposited by magnetron sputtering is uniform and has less defects than the substrate. The current density of TiNb/316L SS decays rapidly first and then stabilizes at 0.9-1 \(\mu\)A\(\cdot\)cm\({}^{-2}\). This demonstrates that the TiNb coating can be easily passivated and keep a low corrosion current density for a relatively long period, influenced by the high potential and the oxygen saturated acid solution, which shows the good corrosion resistance and stability of the TiNb coating. From the potentiostatic polarization tests above, it is assumed that TiNb/316L SS has the potential to work as a PEMFC cathode flow plate material for a long time. Figure 8: Equivalent circuits used for the Nyquist impedance spectra of **(a)** TiNbN/316L SS @ OCP; **(b)** TiNbN/316L SS @ 0.19 V vs MSE, 316L SS, and TiNb/316L SS. The electrochemical testing shows that both TiNb and TiNbN coatings can form steady passive films at 0.19 V vs MSE, and the long-term corrosion current density of TiNb/316L SS is lower than 1 \(\mu\)A\(\cdot\) cm\({}^{-2}\), illustrating the high corrosion resistance of TiNb coating. Compared with TiNb/316L SS, TiNbN/316L SS is of poorer corrosion resistance.