Water concentration distribution in coatings during accelerated weathering protocols

The long outdoor service life of painted metal and wood objects, often measured in decades, requires the use of accelerated weathering protocols to predict the long-term efficacy and performance of new barrier protective coating candidates. The mechanism of damage imposed on a coating system in accelerated weathering tests must have parity with the damage mechanisms occurring within the system as it undergoes natural, in-service weathering. Similarly, the physical conditions involved in the accelerated exposure regime, including water distribution as it is defined both temporally and spatially, must show equivalency to the natural exposure regime. An added complication in accurately accelerating coating damage is that the wet and dry cycles of accelerated weathering and corrosion test protocols are often performed at different and elevated temperatures from in-service exposure, which results in altered bulk transport and diffusion rates of water during each phase of the weathering or corrosion protocol, when compared to those rates experienced in field or in-service conditions. In this study, Fick’s law of diffusion is applied to coating surfaces exposed to surface water concentrations at cyclical intervals for a set of two-stage accelerated weathering protocols. The governing equations are solved using Laplace transforms followed by time rescaling to account for the variation in temperature, which impacts the diffusion coefficient between the wet and dry portions of several accelerated weathering protocols. This serves to predict the asymptotic average water concentration within the coating as well as at the coating–metal interface, based on the diffusion and temperature parameters of the coating and the parameters of the accelerated weathering protocol, including the duration of each imposed wet or dry stage. The analytic solution to Fick’s second law also allows the water concentration at the substrate–coating interface to be predicted based on Arrhenius parameters of the diffusion process. This resultant average time of wetness and water concentration variance at the coating–substrate interface can be used to more directly compare corrosion and adhesion loss between various accelerated weathering protocols and natural weathering conditions. The numerical average and range of water concentration as a function of coating depth can then be used to interpret coating damage distribution with depth, and enable more insightful comparison of the results of various accelerated weathering protocols. The simulations indicate that good barrier systems may actually exhibit increased barrier properties in accelerated protocols as compared to systems with mediocre or poor barrier performance as determined by global diffusion coefficients. The good barrier coating imparts substantially less exposure challenge to the interface than the mediocre or poor barrier coatings in accelerated weathering protocols. Therefore, in industry-standard tests, the observed performance gain of good barrier coating systems over poor systems may be unduly exaggerated. However, the overall exposure challenge to the interface of a good barrier coating is likely to be larger in service or field conditions due to the much extended time of wet exposure. Thus, laboratory-accelerated weathering cycles that do not consider the impact of temporal and spatial diffusion could potentially misrepresent the quality of a coating, in most cases overestimating the performance of coatings with good baseline barrier protection for adhesion or corrosion protection. In other words, the accelerated weathering protocols do not equally challenge the interface of different coatings operating in the same in-service conditions.