## 1 Introduction  
In built heritage conservation and for museum objects, long-term conservation (e.g., in old mining facilities) is crucial. Ranging from small instruments to large buildings such as winding towers, protective coatings have to be applied. Therefore, common non-transparent protective coatings are not applicable, because the historical character of these cultural heritages must be maintained. Moreover, further degradation has to be hindered in order to retain the cultural heritage for future generations. To evaluate the long-term capability of different coatings, it is essential to determine the layer thickness of the coatings after they are applied on the sample as well as their durability over time. Additionally, early detection of impurities is necessary to prevent further damage. Moreover, it is desired to differentiate between corroded and non-corroded material. However, so far no perfectly satisfying technique for this analysis is available. Optical coherence tomography (OCT) might be a promising tool to fulfill the mentioned needs. Until now, the acceptance of OCT during the last two decades results from its great benefit in biomedical applications such as ophthalmology and dermatology [1; 2; 3]. Nevertheless, many application capabilities of OCT have not yet been exploited. For non-destructive testing, only a few application examples have been published so far; for example, the investigation of art [4; 5; 6], polymers [7; 8], subsurface defect detection [9], strain field mapping by using polarization-sensitive OCT [10], material observation by en-face OCT [11], pharmaceutical film coatings [12; 13], and the investigation of silicon integrated circuits [14; 15; 16]. In this case we apply OCT to metal objects with transparent protective coatings in order to obtain information that the actual common practice electrochemical impedance spectroscopy (EIS) cannot deliver [17; 18; 19; 20; 21; 22; 23; 24; 25; 26; 27; 28; 29]. In EIS, an AC voltage is swept, and the current is measured. Assuming different layers to be either resistive or capacitive, a Bode or Nyquist plot can be made [30]. From these plots, an educated guess about the functionality of the protective coating can be done. This works very well for layer thicknesses in the atomic scale, but no actual thickness value is provided. However this metrology suffers from the drawback of insufficient lateral resolution due to the fact that the signal is integrated over an area of about 1.5 cm\({}^{2}\), which is a typical size of an EIS probe. Additionally, a long duration for the data acquisition, the questionable accuracy of the equivalent circuit diagram, and the uncertainty about the actual layer thickness are fundamental disadvantages of this method [30]. Furthermore, no imaging capability is typically given. Because we are analyzing the depth profiles of our OCT measurements in order to determine a coating layer thickness, the comparison with EIS measurements is not very reasonable. Therefore, another metrology has to be evaluated. Currently, a fast and non-destructive method to determine the actual coating layer thickness on steel or iron is to use a magnetic inductive (MI) thickness measurement system [31]. By measuring the retarded magnetic field of a sample covered with a protective coating layer, the thickness can be estimated by comparing it with a reference sample of bare metal. However, this technique allows only a point-wise layer thickness measurement. Moreover, it can only be applied for non-corroded material, because a corrosion layer superimposes with the coating layer, leading to an inaccuracy of the measurement [32]. In a feasibility study, [PERSON] and co-workers proved that OCT is capable of monitoring protective metal coatings [33]. Here we use OCT and apply post-processing algorithms to detect the coating layer and introduce a scattering layer for the underlying material (metal or corrosion). Moreover, we determine their thicknesses, distinguish between the materials, and unveil breaches of the coating.