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Black spot on ground area after galvanizing

Dear sir,

I have find some material seen to be black on the area where it has been ground with a sandpaper and the fact is that we can achieve the coating on that area.
please let me now why that particular area is seen to be black.

Tanveer Ahmed
employee - Duabi, UAE
February 7, 2011

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February 7, 2011

The sanded area is grey, not exactly black.
This is because the sanding has removed the top layer of the galvanized coating, that part is nearly pure zinc and for a few months after galvanizing this layer is bright shiny silver.

Under this top layer is a series of alloys of zinc and iron, and these are all dull grey colour. The sanding has revealed these layers.

There is no difference in the corrosion protection given by either shiny silver zinc or alloyed grey coating. (The life expectancy is according to the thickness not the colour or alloy composition). Removing some zinc make the total coating thinner, and so has reduced the life expectancy. How much reduced depends on thickness.

This difference is largely aesthetic, not functional.

It is common in all galvanizing to sand (sometimes called fettling or linishing) rough surfaces.

Given a few months in normal climates outside in the weather you will not see any difference.

Geoff Crowley
Crithwood Ltd.
Westfield, Scotland, UK