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Achieving dull and blackish surface finish in hot dip galvanizing  

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One of our clients want dull and blackish uniform surafce appearance after hot dip galvanizing process. Steel which is being galvanized has a chemistry as C%- less than 0.25%, Si - .15 to .25%, P- 0.02 to 0.045% and Mn- 0.6 to 1.3%. With this chemistry how to achieve dull blackish colour in hot dip galvanizing process.Galvanizing coating thickness shall be in the range of 100 to 125 microns. Request to share your knowledge and experience for mass production.

Jayant
Transmission towers hot dip galvanized steel items - Maharashtra, India

Galvanizing is a series of layers of Zinc metallurgically bonded to the steel. But the outermost layer is most commonly pure zinc, which is coloured (initially) silvery grey. Thats the colour of zinc. As it oxidises though, it will turn dull grey. (Under some conditions of steel reactivity it will be dull grey right from galvanizing, as its alloy of Fe and Zn on the surface).

Your client is asking for a natural element to be an un-natural colour. Thats not an unusual request, but most commonly satisfied by another coating put there for this purpose.

Your options are: paint (solvent based), powdercoating, or similar overcoating of the zinc.
You could react something with the zinc, such as zinc phosphate, which will leave a dull nearly black colour, but this is not a permanent finish and with time will erode to leave dull grey again.

It would be interesting to know why they want this color finish. That might lead you to suggest a better solution.

Geoff Crowley    galvanizing &    powder coating shop Glasgow, Scotland