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, SI, S2, S3, and S4. Three panels were prepared for each coating system. (These panels are labeled Sl-A, Sl-B, Sl-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.
Exposure Time Coating System/Panel Exposed
24 hours................................S1-A, S2-C, S3-C, S4-B
60 days................................. Sl-C. S2-A.S3-B, S4-A
120 days................................ Sl-B, S2-B, S3-A, S4-C
Following exposure, the corrosion rate (nano amperes 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 System System System System Time S1 S2 7.5 91 10s S3 S4 24 hours 60 days 120 days 6.7 8.7 118 12.6 145 11.8 6.1 8.3 8.2 Coating System 1st Layer 2nd Layer S1 S2 S3 S4 Zinc dust Zinc phosphate Zinc phosphate with mica Zinc phosphate with mica Epoxy paint, 100 micrometers thick Epoxy paint, 100 micrometers thick Finish layer, 100 micrometers thick Finish layer, 200 micrometers thick