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Blemishes / scabbing on fresh Galv'd steel

I'm a works inspector on a railway refurbishment and a few of our steel framed buildings are Galvanised. There are blemishes that look like hard algal growths all over the steel (largest 50 cm2 x 10 mm height). These scabs are graphite to black in colour and can be easily dislodged but discolouration remains.

I would like to know:
I)What are the causes of these 'blooms'.
II)Are these patches detrimental / do they reduce overall Galv thickness.
III) Remedial action if detrimental.

Many thanks,

Rob Monje
Works inspector - London, Great Britain
December 18, 2008

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December 20, 2008

Rob,

These lumps could be a number of things, but perhaps some more info will tell.
You say they are "all over" the steel. Are they really all over, or perhaps a few spread about? How far apart and what proportion of the surface is covered?

From your description they could be flux residues. (Flux is part of the galvanizing process, in a similar way to using flux in welding or soldering). If so the galvanizing does not meet ISO 1461 (the standard), but they can be easily removed as you have noticed. The standard calls for no flux residues.

The residual stain (after knocking the lump off) is on the surface of the zinc, so does not compromise the corrosion protection, but doesn't look nice. You could clean this off if you can't live with the ugliness, using a stiff brush and hot soapy water.

Were they there from the start?
Is this new galvanizing or old?
Does the lump dissolve in water after you knock it off?
Did you talk to the galvanizer (if its new galvanizing) about it?

Geoff Crowley
Crithwood Ltd.
Westfield, Scotland, UK