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topic 56573

Galvanizing Pot Temperature -- Ideal zinc temperature for galvanizing?



A discussion started in 2009 & continuing through 2017

April 8, 2009

Q. Sir, I am facing a huge problem to set zinc melted temperature for galvanizing how much temperature should be there for zinc according to you for hot dip galvanizing.

Manish Gupta
shop - Delhi, India


Hot Dip Galvanizing

May 6, 2009

A. Sir,

Most North American galvanizers use about 840 °F for hot dip galvanizing. This can vary depending on a number of things.

Regards,

Dr. Thomas H. Cook
Galvanizing Consultant - Hot Springs, South Dakota


May 8, 2009

A. In Europe, 450 °C 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?

Geoff Crowley
Geoff Crowley
galvanizing & powder coating shop
Glasgow, Scotland




To minimize searching and offer multiple viewpoints, we've combined multiple threads into the dialog you're viewing. Please forgive any resultant repetition or failures of chronological order.



January 20, 2011

Q. Dear Sir,

Please tell me why we maintain pot temperature 460 °C only? what happens if I increase or decrease pot temperature?

Prashant Khirwadkar
Steel Employee - Ras-Al-Khaimah, U.A.E.


simultaneous January 20, 2011

A. Dear Sir,

From what I've gather in this forum, there are a few recommendations for optimum temp (to achieve minimum thickness).
i) 442 °C
ii) 443 °C
iii) 445-450 °C
I believe that these variations may be due to the difference in kettle design, location of the temp sensor and other factors. Any temp below or above these range is expected to cause an increase in coating thickness.

Regards,

CS Leong
- Selangor, Malaysia

January 21, 2011

A. Dear Prashant.

You have to maintain your temperature in between 450 - 560 to achieve better quality as well as control the zinc coatings. Over and above life of kettle also depend on the zinc temp as well as also effect on Dross formation.

Ilesh G Vyas
Gunatit Builders
supporting advertiser
Manjalpur, Vadodara, Gujarat
gunatit builders


January 21, 2011

A. Dear Sir,

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

Regards,

CS Leong
- Selangor, Malaysia

January 25, 2011

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, you might half the life, 500 it might do well to last a few weeks!
Zinc melting point is 419. 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 might really be between 445 and 455.

Geoff Crowley
Geoff Crowley
galvanizing & powder coating shop
Glasgow, Scotland




September 28, 2017 -- this entry appended to this thread by editor in lieu of spawning a duplicative thread

Q. Dear Sirs,

In the standard, EN ISO 10684: Fasteners-Hot dip galvanized coatings.
Its stated in p5.5 that "galvanizing shall not be carried out at bath temperatures between 408^480 and 530 degree Celsius".

Why? What happens to the steel?

Kind regards,

Ulf Larsson
- Sweden


October 1, 2017

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.

Geoff Crowley
Geoff Crowley
galvanizing & powder coating shop
Glasgow, Scotland



October 3, 2017

Q. Hi Geoff,
Thanks for your answer.
I noticed that i wrote wrong figures in the text, i mean temperatures between 480-530 degree Celsius.
We galvanize in both low and high temperature, and we are aware of what "wrong" temperature can do to the steel so we use both ceramic and steel kettle.
But why does the standard warn us for galvanize in that temperature range?
Over 480 the zinc becomes very reactive/aggressive to the steel in the kettle, but that can't be the reason for the warnings?

There must be some other disadvantage buy doing that? or ??

kind regards,

ulf

Ulf Larsson [returning]
- Sweden, Sweden



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