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As an alternative to toxic chromate coating, Mg-rich primers have been designed to protect Al alloys (in particular AA2024-T3 [UNS A92024]) and developed in analogy to Zn-rich primers for steel substrate. The Mg pigment present in the primer can provide cathodic protection to Al substrates and significantly deter Al alloy corrosion when damage occurs. To determine how much the pigment metal alloy composition can be varied without sacrificing corrosion protection. three different primers based on Mg alloy powders as pigments were formulated with an epoxy-polyamide polymer binder at different pigment volume concentrations (PVC). Their behaviors were studied after cyclic exposure in a Prohesion chamber using electrochemical methods. Testing results from electrochemical impedance spectroscopy (EIS) and scanning electron microscopy (SEM) showed that the metal-rich primers with Mg alloys as pigments could provide cathodic protection. and precipitates formed from oxidation of Mg alloy particles were similar to the ones found in the pure Mg-rich primer system. Furthermore, the investigation of the properties of the three alloy pigments gave a good understanding of the effects of particle shape, particle size, particle size distribution, and metal alloy chemical composition. [ABSTRACT FROM AUTHOR]

The standard QUV/Prohesion accelerated testing method simulates weathering conditions by exposing coatings to alternating wet and dry conditions. The degradation of the coatings is influenced by the transport of water and ionic species across the coating to the substrate. Coating properties that influence this transport are diffusion coefficient and saturated water volume condition, both functions of temperature. Experimental results from which these properties were accessed are presented for a standard United States Army vehicle primer. A procedure involving alternate exposure of the coatings to sodium chloride (NaCI) and room temperature ionic liquid (RTIL) media was used to simulate the wetting and drying processes, respectively, at varying temperatures. In situ single-frequency impedance measurements were used to monitor the evolution of water uptake and water loss with the use of the hydrophilic ionic liquid medium, allowing the removal of the water to be monitored. Electrochemical impedance spectroscopic measurements were used to determine the coatings properties after immersion in the media. The calculated capacitance evolution results associated with the wetting and drying processes were used to calculate the diffusion coefficients associated with ingress and egress of water. The capacitance evolutions were consistent with a Fickian distribution and were modeled using a solution to Fick's second law. The influence of temperature was demonstrated by the calculation of the activation energies associated with the ingress of water, egress of water, and the barrier property of the coating. [ABSTRACT FROM AUTHOR]