Method for Measuring Water Diffusion in a Coating Applied to a Substrate

Water at the interface is often the main cause of corro. sion, blistering, and disbandment of organic coating/substrate systems. Consequently, the transport of water from the environment to the coating/substrate interface is the subject of great interest in many industries such as organic coatings, asphalt pavements, and electronic packaging. There is no suitable method available to measure the transport of water in an organic coating applied to a substrate (applied film). The majority of information available in the literature on the transport of water in organic coatings is from studies of free (detached) films. 1-3However, the characteristics of water migration through a free film and an applied film may not be the same. For example, Holtzman4 observed that water permeabilities of applied films were slightly lower than those of free films. Results of a sorption study by Rosen and Martin5 also showed the same trend for the diffusion coefficient of water in alkyd coatings. However, they found the diffusion coefficients of water in epoxy coatings applied to steel substrates were almost three times greater than those in the free films. Discrepancies in the transport of water between free and applied films were also found in studies of water uptake in organic coatings. Several workers6”9reported appreciably higher water uptake in applied films than in free films while othersl”,l 1 found the reverse; that is, the amount of water sorbed in the applied films was less than that in the free films. These results raise serious doubts about the extrapolation of data from the transport of water in free films to the applied fdms used in actual practice. The two techniques that have been used to provide information on water transport in coatings applied to a substrate, namely, capacitance and gravimetry,s are based on the amount of water taken up by a coating attached to a substrate. Due to the reactivity of the coating/metallic interface and the oxides on the substrate surface, water diffusivity cannot always be deduced from the capacitance data.12 However, the main drawback of both the gravimetric and capacitance techniques is that they do not discern the water accumulated at the coating/substrate interface from that sorbed in the bulk of the coatings. If water is adsorbed at the coating/substrate interface, these methods would incorrectly estimate the rate of water transport in a coating applied to a substrate. The accumulation of water at the coating/substrate interface is inevitable when an organic-coated specimen is exposed to water or high relative humidities. This is because the bonds