The authoritative public forum
  for Metal Finishing 1989-2024
  

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Agitation of Galvanizing Flux

Q. I am privileged to visit many galvanizing plants worldwide, and see many practices, most of them good ones.
Recently I have noted some countries' galvanizers follow the practices of others without understanding the implications. In this case it is air agitation of flux tanks.
Usually I ask whey they do it, and the most common answer is to keep the flux dissolved. I contend that this is a bad practice with one occasional exception.
For normal operations, most flux tanks develop red mud. This iron hydroxide complex will be a suspended solid, slightly heavier that the flux solution, and if not agitated it sinks to the bottom of the tank.
But many galvanizers install air agitation and so this iron bearing oxide is a suspended solid, leaving the flux red-brown-orange colour. Every piece of steel dipped in it gets coated not only with flux but also with iron hydroxide.
Why is it bad?
* Air is expensive. Compressors are very inefficient, about 90% heat produced, and 10% energy makes compressed air.
* coating steel with iron hydroxide increases the free iron to the zinc making more dross.
* More ash is produced.
* Iron hydroxide makes the steel harder to dry (its hygroscopic), and wet steel splashes zinc creating a safety issue, and more ash from the zinc so higher consumption.

Allowing the red mud to settle to the tank bottom puts less of it on the dipped steel.
The flux does not need stirring to keep it dissolved. At normal operating strength of about 450 g/Litre, it dissolves easily. Its possible to buy flux in aqueous form, at about double the strength of operating, and that doesn't crystalise out, needs no stirring.

The exception?
During longer down time (longer holiday weekend or similar), bubbling air is a "poor mans" way of oxidising iron dissolved in the flux so that it will separate out as a solid which can be settled. After this operation, allow the tank to settle.

So, do you air agitate your flux?
If so what's your reason?

Geoff Crowley, galvanizing consultant
Crithwood Ltd.

Bathgate, Scotland, UK

September 27, 2024