Fiber-Optic Sensors for Online Detection of Corrosion Degree of Stone Artifacts

To realize online noncontact detection of the degree of chemical corrosion of stone cultural relics, we developed a reflective fiber-optic sensor, and a theoretical model was established. The sensor comprised incident and receiving coaxial fibers located in the middle and inner and outer layers of the coaxial fiber, respectively. We investigated the effects of the number of receiving fibers in the outer layer and the distance between the end face of the sensing probe and the detection object on the relative light intensity of the sensor output. The response characteristics of the sensor to sandstone samples with different degrees of corrosion were examined, and the change in the material composition of the sandstone after acid corrosion was studied. For a fiber diameter of $400 \mu \text{m}$ , the numbers of outgoing, inner receiving, and outer receiving fibers are 6, 1, and 6, respectively. Furthermore, the spacing of the outer receiving fiber was $100 \mu \text{m}$ , and the relative light intensity of the sensor output reached the highest $\sim $ 90%. Following the start of corrosion of sandstone, with a decrease in the pH of the corrosion solution, the sandstone becomes more seriously corroded, and the decrease in the iron ions and changes in the surface microstructure lower the quality of the spectral output of the sensor. The reflectance spectrum (400 nm) of the sensor reached the highest value of $\sim $ 90%, the sensor reflectance spectrum and sandstone iron ion content exhibited a linear relationship, and the sensitivity reached 36.591 per percentage content change.