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This paper presents a study of the effects of surface chromate passivation treatments on the corrosion rate and appearance of 55% Al-Zn coatings on sheet steel after weathering in an industrial atmosphere for a period of six years. Weight loss measurements were employed to determine corrosion rates, and surface appearance was evaluated by photoelectric reflection meter measurements and visual observations. It was found that increasing amounts of chromium (Iridite 9L6) applied to the surface of 55% Al-Zn coated sheet steel markedly decrease both the corrosion rate of the coating and the rate at which the surface gradually darkens in appearance and loses reflectivity caused by weathering. The bright reflective surfaces observed on six-year-old specimens with 14 mg/m2 chromium on the surface indicate that this level is a suitable minimum for preventing nonuniform weathering appearance problems and for optimizing the corrosion resistance. The applied surface chromium should not exceed about 31 mg/m2 chromium because of yellow surface discoloration at these higher levels.
chromate coatings, weathering, zinc-aluminum alloys, coatings, atmospheric corrosion, corrosion tests
Senior engineer, Columbia General, Inc., Columbus, OH