Ceramic coatings can effectively prevent the corrosion of steel bars in marine environments. In this study, we prepared phosphate ceramic coatings on the surface of carbon steel. X-ray diffraction and X-ray fluorescence were used to analyze the phase structure and composition of the ceramic and the results show that the main crystal composition of ceramic is P₂O₅ and SiO₂. Scanning electron microscopy was used to characterize the morphology of the surface and the section and results showed that the surface was cracked, and the thickness of ceramic was 349 μm. Meanwhile, a number of high-resolution images of internal structure of solids were obtained by non-destructive X-ray computed tomography (X-CT). Matlab and Mimics software were used to conduct the three-dimensional reconstruction of the CT images. Matrix and holes are distinguished by threshold segmentation in grayscale images, and the porosity of the ceramic coatings was calculated to be 14.0%. In addition, mercury intrusion porosimetry was used to verify the calculation results. Therefore, X-CT can be a useful and reliable tool for the visualization of the internal structure of ceramic coatings.