Materials are subjected to corrosive environments in many engineering applications. Among the most successful techniques employed to minimize material degradation in such environments is the use of protective coatings, which may be non-reactive or serve as sacrificial barriers to a particular corrosion process. Coating deposition methods include liquid solidification deposition (hot-dip), vapor deposition (pack cementation), and liquid droplet consolidation (spray) processes. Galvanneal, chromized, and electric arc spray are respective examples of these coating processes. All of these coatings have been used extensively and have proven successful at reducing substrate corrosion in various service environments.
The coating structural characteristics that may influence coating performance include coating thickness and thickness variation, volume percent porosity or voids or both, volume percent flaking or cracking or both, and the presence and distribution of different phases. Such characteristics have been examined traditionally by direct inspection techniques that are not often accurate, time-efficient, or reproducible. The ability to gather more reliable measurements is necessary to further enhance the understanding of these coatings and lead to the establishment of structure-property relationships.
The techniques of computer-automated image analysis permit materials scientists and engineers to quickly evaluate a coating structure. Relevant measurements that can be tailored to individual coatings are readily obtained in an accurate and precise manner with the use of such quantitative systems. This paper discusses critical microstructural characteristics for galvanneal, chromize, and thermal spray coatings. Measurement procedures for such characteristics with the use of an automated quantitative image analysis system are discussed.