Anodizing of hot-dip aluminized mild and stainless steel improves corrosion and heat resistance

Abstract

Anodizing of aluminized ferrous materials is feasible taking care that during pre-treatment too much of the aluminium outer layer is not dissolved in the NaOH cleaning solution. Oxalic acid and tartaric acid solutions in distilled water have been found to be suitable as electrolytes. Suitable thickness of oxide coating can be grown in 2 to 10 minutes in oxalic and 2 to 40 minutes in tartaric acid baths. Anodizing from oxalic acid bath gives pale oxide films while that from tartaric acid bath gives irridescent bluish-yellowish-greenish hues depending upon the angle from which the sample is viewed. Metallographic examination of transverse sections shows uniform anodic oxide film formed on the outer aluminium layer produced by aluminizing the ferrous materials. The anodizing of hot-dip aluminized mild and stainless steel samples has been found to improve salt spray corrosion resistance and also heat, oxidation and scaling resistance of the aluminized steel. Anodized hot-dip aluminized mild steel can withstand long exposure at 800°C and the anodized hot-dip aluminized stainless can be employed at 1100°C. Shri T.L. Sharma, Senior Scientific Assistant; Shri A. Nag, Senior Laboratory Assistant; Shri S.M. Arora, Scientist; Dr. Y.N. Trehan, Scientist; and Prof. V.A. Altekar, Director; National Metallurgical Laboratory)