Measurement of the moisture and corrosion rate within occluded regions is difficult because of their inherent restricted access. The objective of this work was to simultaneously measure in situ the ingress and egress of water and the corrosion activity within simulated aircraft lap joints. Fiber optic moisture sensors monitored the presence of the aggressive aqueous environment within pristine and corroded joints whereas the relative corrosion rate inside the joint was monitored using a SQUID magnetometer, which measures the magnetic fields associated with the small electrical currents produced from the corrosion reactions. Water ingress was very rapid (~ 1 mm/min) whereas egress was much slower (< 0.01 mm/min) and spatially non-uniform. Corroded joints dried slower than pristine joints due to the finely porous, hydroscopic nature of the corrosion products. Corrosion activity correlated with moisture in a complex manner. Wetting caused a substantial increase in corrosion activity within the joint in comparison to the dry baseline condition. During drying, a transient increase prior to cessation in corrosion activity occurred due to concentration and increase in aggressiveness of the solution within the joint. This work highlights the efficacy of simultaneously monitoring moisture and the corrosion activity within occluded regions by employing small profile fiber optic sensors and SQUID technique.
A fiber optic-based irreversible moisture sensor that can be used to determine if a location has ever been wet during a given monitoring period is presented. The irreversible sensor offers a simple means to detect moisture when more complex dedicated continuous monitoring is not feasible and/or where traditional moisture sensors are not applicable, such as in confided spaces. The irreversible moisture sensor has three response states that indicate if an area is actively wet, has been wet but is currently dry, or has never been wet. The irreversible moisture sensor is based on long period grating fiber optic technology. The functionality and survivability of the sensor to exposure in typical atmospheric environments has been evaluated. Testing consisted of long-term exposure to low and high humidity levels (<10 to 97 % relative humidity), and exposure to freezing and elevated temperatures (-10 and 60 °C). The sensors were monitored periodically to examine their functionality. The sensors exhibited rapid irreversible response to water even after extended exposure to the simulated service conditions, and thus are suitable for long-term service in typical atmospheric environments. Potential applications for this sensor include remote, confined or difficult to access locations, aggressive environments where electrical-based sensors perform inadequately, or where approval for fielding instrumentation for continuous monitoring maybe prohibitive, such as on aircraft.