Objective

Nearly every coatings test and qualification organization has documented instances where the corrosion performance of a series of coating systems in accelerated tests (such as ASTM B117) do not correlate with the rank order of performance in an outdoor exposure environment. This disparity has become more pronounced with the introduction of nonchromate-based corrosion inhibitors, since the B117 protocols were developed around quality control of chromate systems. Previous work has documented cases in which uncoated metal substrates do not show the same corrosion products in laboratory tests as they show in outdoor exposure. Clearly, there are fundamental chemical and thermodynamic differences between these two environments. The first step towards developing better accelerated test methods is to analyze and accurately reproduce these environments in a laboratory setting. The second step in this process is to accelerate the kinetics of the simulated environment to enable performance evaluation in a reasonably short period of time to predict long-term outdoor performance. Properly accelerating the kinetics involves knowledge of both the interaction of corrosive species as well as the effects within the polymeric coating system. These effects are specifically related to the diffusion rate of electrolyte into primer which contains inhibitor species, dissolution rate of the inhibitor, and transport phenomena and kinetics of inhibitor ions to suppress corrosion in damage and scribe sites. The method used to accelerate total corrosion performance must equally accelerate all these factors; otherwise, both false positives and false negatives are possible. A successful true accelerated test method would provide accurate predictable results for any substrate with any type of protective coating present.

The objective of this project is to develop a comprehensive test protocol to accurately predict all aspects of the performance lifetime of Department of Defense (DoD) coatings. The resultant test protocol should ideally be independent of substrate composition and accurately rank the performance of any coating system equivalent to the results obtained by long duration outdoor and field exposure. DoD service environments are variable in nature (e.g., beachfront versus desert), and the test protocol should either be specific to a particular service environment or should be dynamically tunable to match the particular service environment in which the coating is intended to be used. The test protocol should also provide accurate results for particular material configurations such as fasteners and lap joints, which can create concentration cells and galvanic couples. The test method should allow a reasonable prediction of performance lifetime based on a relatively short time frame accelerated test.