Kerry N. Allahar

Thermal Cycling of Epoxy Coatings Using Room Temperature Ionic Liquids

Nonaqueous room-temperature ionic liquids (RTILs) offer a method of using electrochemical impedance spectroscopy (EIS) to analyze coatings without the confounding effects, such as plasticization, associated with an aqueous working fluid. Thermal cycling has been used as an accelerated testing method for ranking the corrosion protection of coatings. The influence of thermal cycling on the degradation of a coating was investigated using a hydrophilic RTIL as the electrolyte for the drying cycle of EIS experiments. The change in diffusion coefficient, saturated water volume fraction, and activation energy for the absorption and desorption of water by the coating was obtained.

Simulation of Wet-Dry Cycling of Organic Coatings Using Ionic Liquids

The capacitance evolution of an organic coating undergoing cyclic wetting and drying conditions was monitored by single frequency electrochemical impedance spectroscopy. Monitoring of the drying condition was possible with the use of a hydrophilic room temperature ionic liquid and the methodology employed is presented. Experimental results associated with cyclic dilute NaCl wetting and ionic liquid drying are presented for an epoxy coating on an AA 2024-T3 substrate. The calculated capacitance evolutions associated with wetting and drying were generally consistent with Fick’s second law. The calculation of the water ingress and egress diffusion coefficients using a short-time approximate solution and a series solution to Fick’s second law are presented. The latter solution is shown to address the capacitance evolution better than the former with the ingress coefficient larger than the egress coefficient for a given exposed coating surface. There was agreement between the calculated diffusion coefficient ingress values for coating areas exposed to cyclic NaCl wetting—ionic liquid drying and cyclic NaCl wetting—natural drying conditions. Comparison of the impedance spectra for test areas indicated that the use of the ionic liquid as a drying medium influenced the electrochemical properties of the coating only after a number of cycles.

Modeling of Electrochemical Impedance Data of a Magnesium-Rich Primer

The application of Mg-rich primers MRPs for the protection of aluminum structures represents an attractive alternate to the environmentally unfriendly Cr-rich primers that are presently used. The protective modes of MRPs are similar to those of Zn-rich primers ZRPs on steel and include cathodic protection driven by the more active Mg particles compared to the Al substrate and a barrier-type protection due to the insulation of the substrate from the environment. Interpretation of ZRP electrochemical impedance spectroscopy EIS data has been accomplished using a transmission-line model that accounted for the contact impedance between the zinc particles, the impedance associated with the zinc dissolution, and the percolation resistance of the coating. EIS experiments results are presented here for an MRP on a gold substrate under immersion in dilute Harrison’s solution. The data were analyzed using the measurement model technique to determine the consistency with respect to Kramer–Kronig relationships. The transmission-line model was used to analyze the data to demonstrate their applicability for analyzing the protection afforded by the MRP. Gold was used as a noble substrate and to promote that the electromotive degradation of the MRP as the mixed potential of an MPR/gold system was more positive than that of an MRP/Al system.

Monitoring of the AC-DC-AC Degradation of Organic Coatings Using Embedded Electrodes

Abstract The application of embedded sensors is presently being developed to serve as a reliable corrosion sensing technique used for structures protected by organic coatings. Results are presented...

Real-Time Monitoring of a United States Air Force Topcoat/Mg-Rich Primer System in ASTM B117 Exposure by Embedded Electrodes

Abstract The protection of metallic structures by organic coatings has traditionally been as a barrier, reducing the transport of water, ions, and oxygen from the environment to the substrate. Orga...

Corrosion Assessment and Characterization of Aerospace-Bearing Steels in Seawater and Ester-Based Lubricants

The corrosion behavior of bearing steels was screened using potentiodynamic scans in seawater. The results of electrochemical testing provided a relative ranking of the bearing steels when tested in aqueous chloride-containing solution. The corrosion behavior of bearing steels in the lubricant environment has been observed to be quite different than in aqueous solution. Both the amount of water contamination in oil and chloride content of the water impact the observed corrosion rates in oil-water mixtures. All steel compositions tested demonstrated localized corrosion damage when exposed to oil with water added; however, inherently less corrosion-resistant alloys had more widespread shallow attack, whereas higher Cr-containing alloys displayed more localized severe attack. The mechanism of corrosion in the two-phase, oil plus aqueous phase system appears to be controlled by an aqueous corrosion process dependent on steel microstructure and emulsified or free water and likely limited by oxygen availability...

Characterizing the Relaxation of the Open-Circuit Potential during an AC-DC-AC Accelerated Test

Abstract The non-conventional accelerated alternating current-direct current-alternating current (AC-DC-AC) testing method for organic coatings induces degradation by promoting cathodic reactions a...

In Situ Embedded Sensor Monitoring of a United States Air Force Primer Beneath a Topcoat Exposed to Atmospheric Humidity and Thermal Conditions

Abstract Two-layered United States Air Force organic coatings are designed with the topcoat functioning as a barrier against the environment and the primer providing adhesion between the topcoat an...

Electrochemical Noise Monitoring of the Cathodic Protection of Mg-Rich Primers

Abstract Mg-rich primers provide cathodic protection of aluminum substrates similar to that provided by Zn-rich primers for steel substrates. This protection is due to the mixed potential that deve...

In Situ Monitoring of Organic Coatings Under QUV/Prohesion Exposure by Embedded Sensors

Abstract Organic coatings represent the most widely used means of mitigating the corrosion of metallic structures. A coatings primary role is as a physical barrier, reducing the transport of water...