###### Abstract  
Corrosion monitoring and management has been at the center of structural health monitoring protocols due to its damaging effects on metallic structures. Current corrosion prevention and management programs often fail to include environmental factors such as Cl\({}^{-}\) ions and surface wetness. Early detection of these environmental factors can prevent the onset of corrosion and reduce repair and maintenance-related expenses. There is growing interest in creating solution-processed thin film environmental sensors with high sensitivity to corrosion precursors, low-cost fabrication, and small footprint, rendering them viable candidates for investigation as potential corrosion sensors that could be easily integrated into existing structures and screen printed or patterned directly into surface coatings. In this work, we have implemented C\({}_{60}\)-based n-type organic thin film transistors (OTFTs) with functionalized graphene oxide for humidity sensing and functionalized graphene nanoparticles for Cl\({}^{-}\) ion detection, using low-cost solution processing techniques. The reduced graphene oxide (rGO)-coated OTFT humidity sensor is designed for the qualitative estimation of surface moisture levels and high levels of humidity, and it exhibits a relative responsivity for dry to surface wetness transition of 122.6% to surface wetness, within a response time of 20 ms. We furthermore implemented an in-house synthesized hydrogenated graphene coating in conjunction with a second OTFT architecture for Cl\({}^{-}\) ions sensing which yielded a sensitivity of 4%/ppm to ultrafine ionic concentrations, over an order of magnitude lower than the range identified to cause corrosion in aircraft structures. corrosion precursors; OTFTs; fullerene C\({}_{60}\); organic semiconductor; hydrogenated graphene; environmental sensing +
Footnote †: journal: Applied Sciences  
detected corrosion. They may detect corrosion damage, but they fail to identify early stages of corrosion and factors responsible for corrosion events, namely changes in pH, metal ions (Fe\({}^{2+}\), Co\({}^{2+}\), Al\({}^{3+}\), Cu\({}^{2+}\)), changes in humidity, and chloride ions (Cl\({}^{-}\)) [1,2]. Reliable structural health monitoring requires continuous and early on detection of environmental factors responsible for corrosion events using in situ, non-destructive sensing mechanisms able to detect minute changes in levels and concentrations of the aforementioned environmental conditions. Frequent exposure to chlorides and humidity provides the essential electrochemical conditions favorable to corrosion events [4]. Chloride ion concentration thresholds for corrosion initiation has been identified to be within the range of 100 ppm and 3500 ppm depending on the type of metal alloy, temperature, and pH [5,6]. As such, there is an urgent need for Cl\({}^{-}\) ion sensing in the aviation industry. Timely detection of chloride ions below corrosion initiation thresholds would allow for the implementation of preventive measures and avoid severe and costly damage. Most Cl\({}^{-}\) ion detectors predominantly use silicon-based potentiometric sensors, such as ion selective electrodes (ISE) and electrochemical field effect transistors (ChemFET), which rely on ionophore membranes and electrolytic solutions [7,8]. Although such sensors have been able to reach detection limits as low as 3.5 ppb [7], their implementation in environmental sensing, especially in aircraft corrosion monitoring, remains challenging. The main challenge to integrating potentiometric sensors in corrosion monitoring is to maintain the environmental stability of the aqueous/electrolyte solution or the membrane [8,9]. Organic thin film transistor (OTFT) sensors have the potential to fill in the gap in corrosion prevention strategy, improve corrosion management approaches, and safeguard the integrity of metallic structures while reducing repair and maintenance costs.