## 4 Results and Discussion
### Potentiostatic Tests
#### 4.2.2 Carbon Coated with SBR-based Material (S4)  
The SBR-impregnated carbon S4 has a smooth and glossy surface before polarization. Overall, the surface remains glossy but is no longer smooth but rough. The specimens of levels 2 and 3 are mostly mat and rough, there are only a few glossy surfaces left. In addition, an increased resolution of the impregnation in the area of the intersection points can be observed (Figure 9).  
Figure 10 shows the SEM images of the SBR-impregnated carbon fibre S4, the surface of the reference (Figure 10a) is mostly smooth apart from impurities. Sometimes the carbon fibres shine through; there are randomly arranged pores of different sizes. In the reference specimen, there are also imperfections of different sizes from 20 um, at which the carbon fibres are exposed. Rubber is known to be non-conductive. The pores and imperfections in the impregnations as well as the poor adhesion between SBR impregnation and carbon fibre explain why the SBR-impregnated carbon fabric also shows polarizability. After the polarization at level 1, destructions are already visible at isolated points where carbon fibres are exposed (Figure 10b). Around these areas is a white coating that looks different. According to EDX analysis, this consists mainly of carbon and oxygen and low intensities of calcium and silicon can also be detected. Overall, significantly more carbon fibres are visible due to the impregnation, which suggests that the impregnation dissolves over a large area. A destruction of the SBR impregnation therefore already occurs in panel area 1.  
Figure 9: From left to right: S4 anodes of stages 1 to 3 after polarization.  
With increasing polarization both, the amount of deposited material and the removed surface of the impregnation increase. The impregnations already show imperfections and pores before polarization, so that a liquid electrolyte can enter the roving. Due to the poor adhesion between SBR and carbon fibre, this is particularly pronounced for the SBR-impregnated material and explains its good polarizability compared to carbon with epoxy resin impregnation. By destruction the impregnations, underlying carbon fibres come into contact with the electrolyte and increase the effective surface area with increasing dissolution. In Figure 10c a filament can be seen, which has become free and transverse cracks are recognizable, which can be either in impregnation or in carbon fibre. Figure 10d also shows a filament, which is free of impregnation. On the basis of this picture one can say exactly that with anodic polarization this filament is attacked here and cracks developed, which do not come from the production.  
Dissolution of the carbon fibres could not be achieved by the potentiostatic tests. Before the carbon fibres are dissolved, the impregnations and the sizing are first destroyed. This can be explained by the fact that the carbon fibre consists of mainly covalent carbon compounds in contrast to the impregnations and the size and the covalent bond has a higher bonding energy. Since no optical changes have occurred with E4 and S4 despite significant decomposition of the impregnation materials, destruction of the impregnation materials to carbon dioxide and calcium carbonates can be assumed. Visible decomposition particles have only formed as a result of the decomposition of the sizing of the unimpregnated carbon fibres. However, the exact composition of these particles could  
Figure 10: SEM images of S4: unpolarized reference (**a**), after polarization at level 1 (**b**), level 2 (**c**) and level 3 (**d**).  
not be determined, since the freeze-dried residual solution consisted mainly of calcium carbonate. Calcium carbonate is formed by the reaction of calcium hydroxide with carbon dioxide:  
\[\mathrm{Ca(OH)}_{2}+\mathrm{CO}_{2}\rightarrow\mathrm{CaCO}_{3}+\mathrm{H}_{2}\mathrm{O} \tag{3}\]  
However, the carbon dioxide can be a product of carbon from the dissolution reactions as well as from the air.