Galvanizing Pot Temperature -- Ideal zinc temperature for galvanizing?

A. Sir,

Most North American galvanizers use about 840 °F [ed. note: 449 °C] for hot dip galvanizing. This can vary depending on a number of things.

Regards,

A. In Europe, 450 °C [842 °F] seems to be the average, but I've seen as low as 440 and up to 460.
Of course at these temperatures the precision of the instrument should not be overlooked. Is 450 indicated exactly 450?

A. Dear Sir,

The recommendations in my previous post is for General Galvanizing. Temp recommendation for Continuous Line will be different.

Regards,

A. For a steel kettle the temp is likely to be set at 450 °C ± 5. (445-455). Much higher than this (>460) and your kettle life will be dramatically reduced. You should expect approx. 10 years or 100,000 tonnes from a kettle. If you ran your zinc in a steel kettle at say 470 °C, you might half the life, 500 °C it might do well to last a few weeks!
Zinc melting point is 419 °C. You need enough heat to heat the steel (don't want to immerse a load and freeze the whole kettle), and to retain some fluidity on withdrawal.

Ceramic kettles are required for higher temperatures.

But in all this remember to take account of the accuracy of your measuring system. If it say ± 1%, then a reading of 450 °C might really be between 445 and 455 °C.

A. Firstly the melting point of zinc is about 419 °C, so clearly to immerse steel in zinc the temperature must be higher than that. In practice not many operate at less that about 435 °C.
Galvanizing isn't just a dip coating, rather its an alloying reaction. An alloy (a mixture of zinc and iron) forms on the surface of the steel. The rate the alloy forms is dependent on several factors, but primarily the chemistry of the steel (especially Si and P), and the temperature of the zinc.
Hotter = faster generally, but it's not linear. There's a spike of reactivity at about 530 °C where the reaction takes place several times faster.
Now from that you might think speed is good, but consider this.. The zinc must be contained in something. That is a steel tank, externally heated. And that tank is also reacting with the zinc, forming alloy, and that consumes the steel tank slowly.
So at 530 °C, a tank that starts at 50 mm thick might last a few weeks until it's too thin to be safe (perhaps 25 mm). But at 450 °C it should last 12 years or 120,000 tonnes processed whichever comes first.
So, while the standard allows for a wide variation in temperatures, practical considerations mean a more likely range of 440-460 °C.
It is possible to contain zinc in a ceramic tank, but heating the zinc is a lot less efficient both is terms of energy consumption, and in oxidation losses of the zinc, so that method is rare now, and becoming more rare.

A. A very late response to the question.
I am not exactly sure why the standard warns about temp, but a guess is that its a warning of very variable and unpredictable outcomes.
A standard tries to get a uniform outcome. Any encouragement to use conditions that result in variability isn't a good outcome.

A. The reactivity between zinc and steel increases with temperature. Get the steel too hot and you get thicker coating.