## 4 Results and Discussion
### Potentiostatic Tests
#### 4.2.3 Unimpregnated Carbon with Sizing (UC)  
On most specimens, optical changes could already be detected after polarization. Photos of all specimens from the potentiostatic tests with unimpregnated carbon fibres are shown in Figure 11. The unimpregnated carbon fibres were glossy and soft, comparable to a brush before the test was carried out. After polarization at level 1, the carbon fibre is still soft but less shiny. This could be explained by the deposits of calcium carbonate. The solution of the tests at levels 1 and 2 showed no difference. The solution of the experiment at level 3, on the other hand, was discoloured brown and small black particles floated in the solution. The particles deposited on the ground after a few days. These sediments were freeze-dried with a small remainder of the solution and identified as calcium carbonate by Fourier transform infrared spectroscopy. Since the substance, unlike pure calcium carbonate, was not white but grey, further components must be present. However, these could not be detected by infrared spectroscopy. Figure 12 shows the SEM images of UC specimens. The unimpregnated carbon fibres show no damage after polarization at level 1 (Figure 12b). Slight longitudinal grooves can already be observed at the reference (Figure 12a) and no consequence of polarization. Differences in contrast, especially in bright white areas, are caused by charges from the electron ray with insufficient surface conductivity. After polarization at level 2, the sizing of many fibres is decomposed (Figure 12c). This can be seen in most fibres through the formation of deep but narrow longitudinal furrows. Cracks in the radial direction also occur in some fibres, as they have already been detected in the potentiostatic tests. This means that an anodic polarization on level 2 decomposes the size of unimpregnated carbon fibres. On level 1 no destruction of the coating was observed. The change in the slope in the current potential diagram in the phase of the transition range at approximately 900 mV versus NHE is therefore probably due to the reaction that sets in. The sizing of most carbon fibres polarized at level 3 shows very clear destruction phenomena in the form of radial cracks (Figure 12d). The decomposition of the sizing of  
Figure 11: From left to right: specimen P1 of levels 1 to 3 after polarization. the unimpregnated carbon fibres also took place inside the roving. It can therefore be assumed that all carbon fibres conduct electricity and are in direct contact with the electrolyte. The effective surface area of the unimpregnated carbon fibres should raise after destruction of the sizing. In Figure 12d it is questionable, whether this destruction is in carbon or in sizing. Because the damaged layer looks thick and the residual cross section of carbon small, this question had to be examined more closely. Three further methods were used to investigate it in more detail. (1) Examination of the average diameter of fibres in a roving  
The examination of the diameter of unimpregnated carbon showed that the fibres have a diameter between 5 and 10 \(\upmu\)m. Figure 12d shows that the diameter of carbon, under the deposited film around the fibre, is 6 \(\upmu\)m and therefore in the range of diameters of unpolarized samples. Hence, no destruction of the carbon fibre is observed. This has been confirmed with several samples. (2) EDX examination of the deposited film  
EDX analysis was performed on an unimpregnated carbon (UC), in a region, where the deposited film was partly flaked off. So both sides of the film could be analysed. Figures 13 and 14 show that the inner part of the destroyed layer consists mainly of carbon, which could be part of the sizing. The outer layer indicates the presence of calcium, carbon and oxygen. These are the deposits formed due to chemical reaction between calcium of the solution and carbon from the sizing. These deposits can be found more strongly on the surface of the carbon as the polarization potential increases.