Anticorrosive behavior of steel coated with epoxy.

Organic coatings that use epoxy resins are widely used to protect steel and metal against weathering and corrosion.

Researchers at National Technical University in 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 resistance to corrosion (Pigment & Resin Technology, Vol. LO4 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, labeled, S1-A, S1-B, S1-C, S2-A, S2-B, …, S4-C, were prepared for each coating system. The characteristics of the four coating systems are listed in the following table.

Coating System First Layer Second Layer S1 Zinc dust Epoxy paint, 100 micrometers thick S2 Zinc phosphate Epoxy paint, 100 micrometers thick S2 Zinc phosphate with mica Finish layer, 100 micrometers thick S4 Zinc phosphate with mica Finish layer, 200 micrometers thick Each coated panel was immersed in deionized and deaerated water and then tested for corrosion. Since 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 for 60 days, and one for 120 days, in random order.
The design is illustrated in the following diagram.
Exposure Time Coating System/Panel Exposed 24 Hours S1-A, S2-C, S3-C, S4-B 60 Days S1-C, S2-A, S3-B, S4-A 120 Days S1-B, S2-B, S3-A, S4-C Following exposure, the corrosion rate (in 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?
Exposure Time System S1 System S2 System S3 System S4 24 Hours 6.7 7.5 8.2 6.1 60 Days 8.7 9.1 10.5 8.3 120 Days 11.8 12.6 14.5 11.8