Computational modeling and design of multilayer corrosion coatings for galvanic protection of steel

Abstract

Steels represent an economically vital class of alloys for use in structural applications, due to low cost and high strength and toughness, but often suffer from high susceptibility to corrosion in relevant environments. Use of metallic coatings, particularly zinc alloys, has long been a widely employed method for corrosion protection of steel, by acting both as a physical barrier to the aggressive environment, and providing sacrificial protection due to the preferential dissolution of the coating. Recent advances in processing techniques has permitted the efficient deposition of multilayer metallic coatings, which offer tremendous potential for dramatic improvements in performance relative to single layer coatings. However, development of multilayer corrosion coatings is hampered by a number of obstacles, in particular the lack of theoretical or computational tools to predict the corrosion behavior of multilayer coating structures. While existing numerical models for corrosion are well validated for simple geometries and short timescales, there are no models with demonstrated ability to be applied to composite materials such as multilayer coatings, or to incorporate the effects of corrosion damage over time on the effectiveness of the coating. This thesis seeks to address this deficiency through development and validation of two corrosion modeling techniques. The first modeling technique uses standard techniques for numerical modeling of galvanic corrosion to produce time-dependent corrosion simulations for multilayer or compositionally graded coatings, under the assumptions of completely generalized corrosion. The second modeling technique attempts to capture the effect of localized corrosion on multilayer coatings by treating the coating material as a porous electrode with properties calculated through an effective medium approximation. The output of the corrosion models is validated through comparison to a number of quantitative and qualitative corrosion tests on a variety of coatings, and is demonstrated to accurately capture a wide range of phenomena relevant to corrosion of multilayer thin films. Finally, this thesis demonstrates the potential application of the developed corrosion models as a design tool, by applying optimization techniques to determine coating configurations with maximized protective ability.