# Corrosion Behavior and Conductivity of TiNb and TiNb Coated Steel for Metallic Bipolar Plates
## 3 Results and Discussion
### Surface Morphology  
The SEM images of 316L SS, TiNbN/316L SS and TiNb/316L SS before and after potentiostatic polarization corrosion tests are shown in Figure 2.  
In Figure 2a, the polished SS surface is uneven due to grinding and polishing, and a number of holes are randomly distributed on it. In Figure 2b, the TiNbN surface is much smoother than that of the SS. However, small holes still exist, and at the bottom of the surface valley, some large particles appear. The micrograph of the TiNb coating shown in Figure 2c is smooth and nearly no obvious holes can be seen. Obviously, the deposition of TiNbN or TiNb has smoothened the SS samples. This may also limit the formation of micro corrosion cells. The morphologies of uncoated and coated 316L SS after potentiostatic polarization tests are shown in Figure 2d-f. After corrosion, 316L SS shows several holes (Figure 2d) which are larger than those observed on 316L SS before corrosion. In Figure 2e, localized corrosion is apparent especially for the valley and defect areas on the TiNbN coating while the surface of TiNb/316L SS in Figure 2f shows few large or deep corrosion pits. This is relevant with the coating defects formed on TiNbN before corrosion. For the hole defect, the electrolyte solution  
Figure 2: Scanning electron microscopy (SEM) images of the surface of 316L stainless steel (SS), TiNbN/316L SS and TiNb/316L SS.  
diffusion at the bottom of it is slow. The concentrations of O\({}_{2}\) and ions inside the hole differ from that in solution, resulting in the different electrochemical activities of grains. Due to the lack of O\({}_{2}\) and the dense passive film, the inner region of the hole becomes the anode of a corrosion battery with a smaller area and a higher corrosion current density, compared with other surface zones. Thus the localized corrosion is induced and more defects are formed. Images in Figure 2g,h under higher magnification indicate that the grain size of sputtering deposited TiNbN varies greatly but with a smaller average value compared with TiNb which might be attributed to the incorporation of N. For the TiNbN coating, the distinction in grain size could result in an inhomogeneous distribution of grain boundaries and finally contributing to the non-uniformity of passive layer [27]. Additionally, the SEM results show the particle agglomeration phenomenon of TiNbN is more serious than of TiNb. And the agglomeration may cause the differences in coating's surface structure and composition by which the localized corrosion of TiNbN can be induced.  
In order to characterize the coating composition, EDS was applied and results are shown in Figure 3. Since the coating is thin, the detected images may include intensity peaks from some elements in the substrate, the substrate oxidation layer and adsorbed substances on sample surface. In the film of TiNbN/SS, the atomic ratio of Ti, Nb and N is about 7:22:10 (with the usual reservations regarding quantifications of light elements). For the coating of TiNb/SS, the atomic ratio of Ti and Nb is about 1:4. The EDS results illustrate under the same sputtering power mentioned above, the deposition speed of Nb is faster than that of Ti. Possible reason for this phenomenon is that Nb is a DC power target, and Ti is powered by a RF electric source. For the same material, the sputtering rate of the RF target is lower than that of the DC target at the same set power.