Long Period Grating-Based pH Sensors for Corrosion Monitoring

Abstract

Corrosion related deterioration of aging aircraft has proven to cause reduced flight availability, service lifetime, costly repairs, and if undetected, can result in potentially unsafe operating conditions. The purpose of this research is to develop, fabricate and test optical fiber-based chemical sensors for monitoring corrosion from early stages through the entire corrosion event. Although there are several preventative methods under development to address the problem of corrosion degradation, new techniques are still needed that are cost-effective and reliable to ensure an acceptable health status determination of aging aircraft and civil infrastructure. In using optical fiber-based sensors to detect corrosion precursors such as moisture, pH, nitrates, sulfates, chlorates and corrosion related metal-ion by-products the severity of the corrosive environment can be determined allowing predictive health evaluation of the infrastructure. The long period grating (LPG) element is highly sensitive to refractive index changes and with appropriate design geometry a variety of target molecules can be detected. Optical fiber long period gratings are designed to act as spectral loss elements that couple a discrete wavelength out of the optical fiber as a function of the surrounding refractive index. By applying special coating that change refractive index with absorption of target molecules to the LPG surface, it becomes a transducer for chemical measurement. Presented in this research is the incorporation of pH-sensitive hydrogels with long period gratings for the development of a fiber optic-based pH sensor. Optical fiber-based pH sensors offer numerous advantages in wastewater monitoring, blood diagnostics, bioremediation, as well as chemical and food processing. Specifically this research focuses on pH sensors that can be multiplexed with other chemical sensors for a complete chemical analysis of the corrosive environment.