Fe(C)-coated optical fiber sensors for corrosion alarm monitoring

Steel corrosion in concrete leads to severe destructions of the civil engineering structures. The detecting of the early corrosion is especially essential for steel-based structures. This paper summarized a series research works on optical fibre corrosion sensors, based on Fe(C)-coated Fibre Bragg Grating (FBG) and Fe-coated optical fibre polarizer. Three types of optical fibre sensors are presented. Type 1 and type 2, Fe-C coated FBG sensor and Fe coated etched FBG sensor, are both based on Fe(C)-coated FBG. The volume expansion and the RI variation of the coating lead to the FBG central wavelength shift respectively. By monitoring the wavelength shift, the corrosion status is evaluated and monitored. Type 3, Fe-coated optical fibre polarizer, is fabricated by side-polishing a single mode optical fibre and depositing a Fe-film on the polished side-face. The birefringence characteristics of the sensor will be reduced after being corroded, which is used for the corrosion status indicating. The fabrication processes of the three types of sensors are introduced. By investigating the experimental results of corrosion test in NaCl solution, the performance of the sensors are discussed. The experimental results show that the proposed sensors are proved to be sensible of early corrosion.