## 4 Discussion  
By employing optical coherence tomography as a new powerful tool for the non-destructive testing of metal coatings, additional valuable information about the sample is available. In contrast to electrochemical impedance spectroscopy, OCT provides an imaging modality. Moreover, lateral information about the sample with \(\upmu\)m resolution is available. Further studies with artificial degradation will be performed to evaluate if this lateral resolution is sufficient to estimate the durability of the coatings.  
We have proven that the determined thicknesses of the coatings are in good agreement with the theoretically expected values and with the MI measurements. We did not have to change the parameters for different layer thicknesses. However, if a different coating material were to be investigated, these parameters might be adjusted. When analyzing thicker coatings, the standard deviation for the MI measurements tends to rise, but not for the OCT measurements. The differences between the theoretical values and the OCT measurements come from a non-uniform distribution of the material, which cannot be considered at all in the simple theoretical estimation. Another explanation for the uncertainty could be a change in the refractive index, caused by a density change. This deviation appears in particular for thicker samples. Furthermore, a cross-sectional microscopic image would be beneficial to validate our findings. However, the preparation of a cross-sectional sample profile image is difficult to achieve without compromising the sample. During the different preparatory work steps (e.g., cutting, embedding (curing process), and grinding at the end), high temperature can be reached. This is typically destructive for products with low temperature stability, such as Paraloid B48N (glass transition temperature, \(Tg=50\)\({}^{\circ}\)C). Though we have shown that our OCT measurements are already superior to the common MI technique, future applications may induce further demands in terms of axial resolution, field of view, or image acquisition speed:  
In our study the thickness of the transparent protective coatings was always more than 6 \(\upmu\)m, so the axial resolution of our OCT system was fully sufficient to differentiate between the different layers. Nevertheless, if thinner coatings would have to be analyzed, OCT systems with 1 \(\upmu\)m axial resolution have already been demonstrated [37].  
In our study, we had a field of view of 3.5 \(\times\) 3.5 mm\({}^{2}\) in which we were able to see the different regions of interest (i.e., corrosion and metal with and without protective coating). However, in future applications, a considerably larger field of view may be required. For that purpose, an OCT system with an enormous field of view in the meter range has been recently reported by [PERSON] et al. [38].  
The A-Scan rate of our system was 29 kHz. The time needed to scan an area of 3.5 \(\times\) 3.5 mm\({}^{2}\) was approximately one minute, which is much faster than an analysis with common EIS systems. However, much less acquisition time may be desired when large objects are investigated. In that case, high-speed OCT systems with scan rates in the MHz range may be used [39].  
We are confident that in the future OCT will enable the investigation of real mining objects that have to be conserved, and that all requirements for that purpose will be covered.