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Development of a corrosion testing method for hydrophobic coatings under hydrostatic pressure
AdvisorMenezes, Pradeep Lancy
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Surface coatings used for corrosion prevention and inhibition are subjected to a wide range of dynamic mechanical and chemical conditions. Hydrophobic and superhydrophobic coatings are a class of coatings that have the unique ability to trap air at the interface between the coating and its environment, preventing the transport of corrosive media to the substrate. This is especially useful in submerged environments and in environments with high precipitation, where the composite layer of air can act as an additional barrier between the substrate and the environment. Instruments, cables, and connectors of commercial oceanographic equipment used for observation, sensing, and measurement fail at unacceptably high rates, and replacement or repair of these devices are prohibitively expensive. Similarly, corrosion is a general problem for offshore wind farms and oil and gas structures due to the high precipitation environments where they are used, leading to structural degradation and damage. The force at which liquid droplets impact a coated surface can significantly affect the protective properties of the coating. For superhydrophobic coatings, these two phenomena can cause severe disruption to the layer of air at the interface between the coating and environment, and this can have severely degrade the protective properties of the coating. Even though superhydrophobic coatings have been well-investigated for their ability to reduce the wettability of a surface and thereby inhibit corrosion, their behavior under varying hydrostatic pressures are not well-understood. This research work presents a method by which superhydrophobic coatings on steel substrates can be evaluated for corrosion inhibition while under hydrostatic pressure. Many hydrophobic and superhydrophobic coatings that have been recently developed are easily damaged either from simply touching the surface with bare skin or via an abrasive load. This research also addresses the impact that mechanical loads have on the protective properties of a commercial superhydrophobic coating to determine its industrial viability.Electrochemical impedance spectroscopy (EIS) was used to study the influence of hydrostatic pressure on the composite air layer and the corrosion inhibition properties of a commercially available superhydrophobic coating. To evaluate the impact that the air layer has on the protective properties, tests were performed on the coated surface in the presence and absence of the air layer. The air layer was removed by wetting the surface with ethanol before testing, which forced a transition from the Cassie wetting state into the Wenzel state. Equivalent circuits were applied to the EIS data to separate the contributions of the air layer to the corrosion inhibition properties of the coating. The results demonstrated that the superhydrophobic coating transitioned from the Cassie wetting state into the Wenzel state under hydrostatic pressure. The corrosion inhibition properties of the coating were found to drastically decrease with increasing hydrostatic pressures. The impact of mechanical abrasion of the superhydrophobic coating was evaluated to understand the tribological impact. The coated samples were subjected to increasing abrasive loads and the surface, wetting, and corrosion inhibition properties were evaluated. The surface roughness was found to decrease with increasing abrasive loads, which then resulted in a decrease in contact angles, an increase in sliding angles, and increased corrosion rates.From these studies, a method has been established to study and quantify the influence that hydrostatic pressures and abrasive loads have on the corrosion inhibition abilities of a superhydrophobic coating.