Organic coatings that use epoxy resins are widely used for protecting steel and metal against weathering and corrosion. Researchers at National Technical University (Athens, Greece) examined the steel anticorrosive behavior of different epoxy coatings formulated with zinc pigments in an attempt to find the epoxy coating with the best corrosion inhibition (Pigment & Resin Technology, Vol. 32, 2003). The experimental units were flat, rectangular panels cut from steel sheets. Each panel was coated with one of four different coating systems, S1, S2, S3, and S4. Three panels were prepared for each coating system. (These panels are labeled S1-A, S1-B, S1-C, S2-A, S2-B, . . . , S4-C.) The characteristics of the four coating systems are listed at the bottom of the page.

Each coated panel was immersed in deionized and deaerated water and then tested for corrosion. Because exposure time is likely to have a strong influence on anticorrosive behavior, the researchers attempted to remove this extraneous source of variation through the experimental design. Exposure times were fixed at 24 hours, 60 days, and 120 days. For each of the coating systems, one panel was exposed to water for 24 hours, one exposed to water for 60 days, and one exposed to water for 120 days in random order. The design is illustrated in the accompanying table.

Following exposure, the corrosion rate (nanoamperes per square centimeter) was determined for each panel. The lower the corrosion rate, the greater the anticorrosion performance of the coating system. The data are shown in the next table. Are there differences among the epoxy treatment means? If so, which of the epoxy coating systems yields the lowest corrosion rate?

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Exposure Time 24 hours 60 days 120 days Coating System/Panel Exposed S1-A, S2-C, S3-C, S4-B S1-C, S2-A, S3-B, S4-A S1-B, S2-B, S3-A, S4-C