A STUDY OF DIFFUSIONAL ZINC AND ALUMINUM-BASED AND ZINC-BASED COATINGS ON STEEL

This article presents a study of aluminum (Al)- and zinc (Zn)-based di ffusional metallic coatings on small, irregularly shaped pieces of car bon Steel (CS). The Zn ''sherardized'' coating seas applied in a purpo se-built rotary oven, while the Al coating was laid on by cementing wi th halide-activated powders (pack cementation). The Al samples were co ated with Zn to produce the Al-on-Zn (Al/Zn) coatings. Several techniq ues were used to characterize the coatings, such as: metallographic an alysis, semi-quantitative morphological analysis of the different comp onents, and thicknesses using scanning electron microscopy and micro-h ardness tests. A comparative study of resistance to atmospheric corros ion was also carried out on the coatings. The samples that were expose d in a Venezuelan coastal testing station were catalogued as having on e of the most aggressive atmospheres in the world (>C5 as per ISO 9223 for CS). It was found that the best iron-on-Zn coating (Fe/Zn) teas p roduced at 350 degrees C with a 3 1/2-h application time. The optimum temperature for the Al/Zn coating was 360 degrees C with a 4-h applica tion time. It was also determined that Zn and Al/Zn diffusional coatin gs on steel contribute significantly to its resistance to atmospheric corrosion, compared with galvanized steel.