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Black spots on galvanized structures

2005

Hi,

I am Quality Manager for a company in England.

We manufacture Lighting Columns and structures and we are recently experiencing 'Black Spot' on our columns.

This Black spot always appears at the drip line and from the top of the column progressing to approximately 1.5 meters down the shaft of the column.

This black spot comes in the form of small areas of missed galv, from very small-1 mm diameter to approximately 12 mm diameter.

The surface condition of the black steel is good, usually new steel with a slight coating of mill surface oil.

I would be very grateful if you could assist me into solving this problem of how it is occurring.

As a lighting column manufacturer, experiencing black spot is a disaster because the column must be sent back to the galvanizers for strip and redip because if we did not, the column would very soon rust in service.

Also. small repairs are acceptable, but a large number of repairs are usually unacceptable to our clients.

Looking forward to hearing from you,

Best Regards,

Keith Allan
- England

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2005

Mr. Allan - This "black spot" you are referring to can be caused by any one or a combination of problems. The first thing to keep in mind is that the steel surface must completely free from any paint, oil, rust, etc. for the zinc to react and alloy to form the galvanized coating. But there are other factors that can play a part,even if the surface is properly prepared, such as;
- Poor fluxing-is galvanizer using a freflux or a topflux?
- Excess aluminum in kettle-sometimes the addition of too much aluminum alloy can cause black spots.
- Excess time between pickling and dipping into zinc bath. This can allow some oxidation which may interfere withgalvanizing.
- Since it seems to be occurring at the drip line, I assume that you mean the lower edge that zinc would drip from as the pole is being withdrawn from the zinc bath. If the angle of withdrawal is not steep enough, then drips can occur all the way down the length if the pole, which would need to be removed by the galvanizer. If these drips are carefully filed, they would be almost unnoticeable. If they are just wacked off, they can remove most of the zinc with them, leaving a bare spot. The oily film could be from post treatment.

Whatever the cause, it sounds like one that your galvanizer should be able to fix quite easily. Good luck! I would like to know what the solution is.

David Jaye
Houston, Texas