Corrosion Monitoring of Steel Bar in Mortar using Fe-C Coated Long-Period Fiber Gratings

In this study, the corrosion process of steel bars embedded in mortar cylinder and immersed in 3.5 wt.% NaCl solution was monitored with long period fiber gratings (LPFG) sensors. Each LPFG was first deposited with a thin layer of silver (600 nm or 1000 nm thick) and then electroplated with a thick layer of Fe-C film (6 um or 10 um thick). The coated LPFGs were attached to the surface of a steel bar that is then embedded in mortar cylinder. Twelve mortar cylinders were cast, cured for 28 days, and tested in 3.5 wt.% NaCl solution for 105 days. The corrosion evolution of steel bars in mortar cylinder was monitored with electrochemical impedance spectroscopy (EIS). The wavelength change of the corresponding LPFGs was measured with an optical spectrum analyzer (OSA). Results showed that the effects of both silver and Fe-C film thickness on the change of resonant wavelength were insignificant in the test range. The wavelength of a coated LPFG sensor was first observed to drop suddenly in the first 7 days due to penetration of NaCl solution through mortar cover and Fe-C and silver layers, corresponding to a passive state of steel bar. The slow increase over time after 7 days was due to the initiation of corrosion and buildup of corrosion products. A linear relationship was observed between the corrosion mass loss and the increase of resonant wavelength, indicating Fe-C coated LPFG sensors can effectively monitor the corrosion evolution of steel in mortar