Water Transport in Multilayer Organic Coatings

Abstract

Protective multilayer coatings are designed, with each layer providing distinctive features to reduce the transport of water, ions, and oxygen from the environment to the substrate. The top coat provides the barrier, whereas the base coat/primer provides adhesion to the substrate as well as galvanic or passivation protection. The change in the coating capacitance during the ingress or egress of water is associated with the change in the dielectric property of the coating and can be monitored using the electrochemical impedance spectroscopy response of a high-frequency signal applied to the coating. The wetting and drying behaviors of epoxy and urethane single-layer coatings and epoxy primer/urethane top-coat two-layer coatings were experimentally measured. A hydrophilic room-temperature ionic liquid was used to simulate drying while maintaining a nonaqueous electrical contact to the coating. Embedded electrodes were used to access the top-coat-primer interface in the two-layer coating. Mathematical models were developed based on Fick's second law for water transport in single-layer and two-layer coatings. Comparison of measured and simulated water uptake demonstrated that the assumptions used in the single-layer model were appropriate while the assumptions used for transport across the interface between the layers in the two-layer coating were inadequate.

A new acceleration factor for the testing of corrosion protective coatings: flow-induced coating degradation

“…[13][14][15][16][17][18][19][20] However, Fick's law of diffusion may not be able to explain the substantial drop in the barrier properties of coatings as we increase the flow rate, since the water concentration in the bulk (dilute) solution is maintained constant despite the different flow rates. The flow rate of the electrolyte solution may have a prominent influence on the diffusion process of ions entering and ions (as corrosion/degradation products) exiting the coating layers, as would be reflected on the values of the coating resistance and capacitance.…”

Predicting water transport in multilayer coatings

“…. Note, that coatings, designed to act as a barrier have permeabilities in order of 10 À9 À10 À10 mol m À1 s À1 [3,17].…”

“…It was shown that water absorption and degradation of the multilayered systems depend on the chemical characteristics of each layer and on the layer position in the system [2]. Allahar et al [3] pointed out that the layer interface can play an important role in transport of water in multilayer coatings. De Rosa et al [4] simulated water transport in two-layered coating as Fickian diffusion in each layer and discussed the influence of ordering of more and less permeable layers.…”

Preparation and properties of epoxy/basalt flakes anticorrosive coatings

“…[3][4][5][6] Epoxy is such a choice of organic coating, and it has advantages over others due to its excellent adhesion, low curing shrinkage, good mechanical properties, and chemical inertness. [6][7][8][9] To further enhance the anticorrosion performance of epoxy coatings, Corning developed epoxy coatings containing glass flakes, in which a so called "maze" effect inhibits the mobility of corrosive species. [5,10] The "maze" effect is realized by increasing the path for corrosive ions diffusion due to the good penetration resistance of the glass flakes.…”