Design, Fabrication, and Impedance Characterization of a Capacitance-Based Salinity Sensor for Marine Applications

We report on the fabrication and electrochemical impedance characterization of a parallel plate capacitance sensor for ocean water salinity monitoring. The standard printed circuit board PCB fabricated and assembled sensor shows excellent response to changes in salinity with a sensitivity of 3.3 k/PSU. Use of the standard PCB process, necessitated by the requirement of low cost and ease of fabrication, posed challenges such as electrode corrosion and fouling. We investigated two polymeric materials as possible electrode passivation coatings, Novolac resin polymer Novolac and a proprietary commercial organic polymer Accuflo. By performing electrochemical impedance spectroscopy EIS and electrical equivalent-circuit-based model fitting, we show that Accuflo has better stability and passivation characteristics in comparison with Novolac. The pore resistance of Accuflo was found to be 3 orders of magnitude higher than that of Novolac, which indicates that Accuflo is relatively less porous than Novolac. Optical observations of the Accuflo- and Novolac-coated sensors after 48 h of immersion in ocean water show that Accuflo maintains its structural integrity, whereas Novolac is severely compromised and the underlying electrode has undergone corrosion, in agreement with EIS analysis. This confirms that Accuflo is a better choice as an electrode coating for underwater applications. The oceanic studies began in the 1930s with the search for petroleum, continued with the emphasis for improved naval warfare, and more recently have been driven by a need to understand and protect the ecosystem. Among the different facets of oceanography, physical oceanography relates to the study of physical properties and dynamics of the ocean, the primary interests being the oceanatmosphere interaction, the oceanic heat budget, and the coastal dynamics. No discussion on oceanography is complete without mention of parameters such as salinity, temperature, pressure, and density. Salinity is commonly defined as the ratio between the weight of dissolved material in the seawater sample and the weight of the sample. 1 This ratio is generally expressed in parts per thousand. The dissolved material includes dissolved gases but excludes fine suspended particles and other solids that are in contact with seawater. Salinity is also conveniently referred to as total dissolved salts. Together, salinity and temperature determine seawater density and buoyancy, driving the extent of ocean stratification, mixing, and water mass formation. In this paper we develop a salinity sensor for deployment in ocean water salinity sensing. Particularly, we evaluate two different polymeric materials for their effectiveness as electrode passivation layers for the salinity sensor. The electrode passivation layers serve as barrier layers against electrode corrosion in the ocean environment. The electrode passivation materials used in this study are Novolac resin polymer Novolac and a proprietary commercial polymer, Accuflo, marketed by Honeywell, Inc. These polymeric films were subjected to calibrated ocean water samples and their barrier properties evaluated by electrochemical impedance measurements. The ocean environment is high in chlorine content, by virtue of which the corrosion processes are accelerated. 2 The passivation properties of polymer film are important in long-term survival of the electrode in the ocean environment. Electrochemical impedance spectroscopy EIS was used to study the electrode‐polymer‐ electrolyte interface. EIS has long been successfully employed to study electrochemical kinetics of processes such as corrosion, electrodeposition, fuel cell kinetics, etc.