###### Abstract
OTFTs rely on carbon-based organic semiconductors and functional surface layers to perform as chemical sensors capable of targeting and detecting specific environmental conditions and chemical species responsible for and indicative of corrosion onset. Compared to other emerging sensing techniques, such as fiber optics and electrochemical sensors, OTFT chemical sensors are more advantageous in terms of robustness, small footprint, low-cost, and energy-efficient fabrication, which is adaptable for large scale production. Furthermore, they exhibit higher sensitivity, faster response time, and low power consumption [10]. Organic semiconductors have been the focus of intensive interest in both the academic and industrial sectors, owing largely to their abundance, solution processability, large-area manufacturing capabilities, mechanical flexibility, and comparable electronic properties compared to their inorganic counterparts. While p-type organic semiconductors have had enormous success in various applications, n-type semiconductor progress has been substantially lagging owing to its environmental instability and low carrier mobility stemming from electron trapping. The performance of n-type OTFTs can be improved by both molecular design and device optimization in term of deposition techniques, passivation, and encapsulation [11]. As a chemically sensitive functional layer, modified graphene has attracted significant attention in recent years owing to its high electrical and thermal conductivity, high carrier mobility, and mechanical flexibility [12]. The solution processability and ease of chemical functionalization of graphene allows for low-cost, large-scale fabrication of thin films with high sensitivity to target analytes. There have been little to no attempts thus far at designing chloride ion sensors using thin film sensors for structural health monitoring. Our reported work is motivated by this scarcity of reported chloride ion sensors designed specifically for monitoring aircraft corrosion, and utilizing advantages offered by graphene-derived materials in corrosion monitoring. In our work, OTFT humidity and chloride sensors are fabricated using reduced graphene oxide (rGO) and hydrogenated graphene coatings, respectively. rGO is the product of the reduction of graphene oxide, by which GO is partially reduced by the controlled removal of oxygen-containing groups from the surface of GO layers using simple treatments, namely chemical, thermal, and photochemical reduction [13]. This process restores the inherent electrical conductivity of graphene while maintaining the solubility in water and in organic solvents and chemical reactivity characteristic to GO [14]. The sensitivity of rGO to humidity stems from the presence of functional groups at the edges of the graphene layers that bind with water molecules and enhance the conductivity of rGO [15]. Additionally, chemically functionalized hydrogenated graphene coating is used for Cl\({}^{-}\) ions sensing. There are conceptually two main methods for the synthesis of hydrogenated graphene, namely gas phase approaches, which include plasma hydrogenation and thermal cracking, and wet chemical approaches, such as Birch and electrochemical reduction [16; 17]. The hydrogen atoms disrupt the structure of the graphene matrix leading to the hybridization of sp\({}^{2}\) bonds into sp\({}^{3}\) and the formation of C-H bonds [17]. Although the chemical affinity of hydrogenated graphene has not been considerably investigated, studies suggest that the C-H bonds in hydrogenated graphene may exhibit chemical reactivity and display selectivity toward halide ions, most specifically to Cl\({}^{-}\) ions [18; 19]. The rGO-coated OTFT humidity sensor is designed for qualitative measurements of surface moisture levels, and it is used as a peripheral sensor to ensure the hydrogenated graphene-coated OTFT Cl\({}^{-}\) ion sensor operates in relatively dry conditions.