Galvanizing Vats Overheating ... Quantifying Damage to Kettle

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A. Hi Utane.

The way to share additional photos is to attach them in e-mail to mooney@finishing.com and we will post them here. Thanks

Luck & Regards,


Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com - Pine Beach, New Jersey

A. Utane,
There are some questions and clarifications required in your post, but based on what we know so far after Google translation and Ted Mooney efforts to interpret. (thanks!)

It sounds like your kettle in electrically heated? Correct?
Lead is much more dense than zinc. I cannot float to the top.
Also the limit of solubility of Pb (lead) in Zinc (Zn) is about 1.1%. No more will dissolve in the zinc, it will just settle to the bottom of the tank as liquid Pb, a lot hotter than its melting point.
It sounds like your thermocouple failed and your zinc overheated.
The normal wear of a kettle is about 2mm/year. It can vary between 1mm and 3mm under normal conditions and operating temperature of the Zn of 450C. If the zinc is heated hotter, the rate of wear on the kettle reaches about 10mm / day (thousands of times faster). In all these cases the steel of the kettle is dissolving in the zinc. When it dissolved it forms dross, an alloy of iron (Fe) and Zn, usually a crystalline formation, but the crystals are very brittle and break for make what looks like grains of sand. This dross is slightly heavier than zinc, so sinks to the bottom. It is lighter than Pb, so it sits on top of the Pb if there is some liquid Pb on the bottom. The ratio of Fe to Zn is about 1:19. In other words a kilogram of Fe makes 20 kg of dross, and to do so consumes 19 kg of Zn.
The zinc turning blue is an indication of very hot zinc.
You cannot "precipitate" lead from zinc easily, and there's only maximum 1% Pb dissolved anyway. That was not your problem.

Your pictures show other problems than just kettle erosion. There is excessive build-up of crusty ash/zinc near the top that will prevent good heat transfer and so allow the kettle steel to get hotter than the zinc, causing faster erosion. The dross (you have called it zinc sludge),will sink to the bottom where it can also prevent heat transfer into the zinc and cause accelerated wear. I have seen too many kettles with holes caused by these two reasons.
You said you removed 6 tonnes of zinc sludge (dross, the Fe/Zn alloy like sand), but we don't know your kettle size to get any idea if that much is a problem or not. What we can say is that it all came from dissolving steel (Fe) somewhere. Removing dross should be done every 2 weeks, or 1000 tonnes of production - whichever comes first. Any less often leads to danger in the kettle of a hole forming, and of the dross going hard making its removal harder still. Normal dross production is about 0.6% to 0.9% of tonnes galvanized. (6-9kg dross per tonne steel galvanized). More than this indicates problems somewhere.

The limit for a zinc tank is often 30mm thick. It starts at 50mm thick (most often, some are thicker) and should be replaced when the thinnest part is 30mm left. It is possible to measure the thickness of the remaining steel of the kettle with the zinc still in and still liquid. It takes specialist equipment, but this is available along with experienced technicians to do the job. Typically takes about 4-8 hours. Most kettles live for anything from 6-15 years, but this is very dependent on how much work they do. At 10,000 tonnes a year, expect about 10 years life. At 60,000 tonnes a year maybe 6-8 years. Not many kettles last more than 15, but a small number have done 20 years. These numbers are for well made kettles. For kettles from some suppliers, divide these numbers by 3. Changing kettles is a long job and costs a lot.

I only supply knowledge, and am independent of manufacturers, chemical suppliers etc. I do know most of the better known suppliers and can comment privately on their products. I do wonder though why you need some of the equipment you have specified? Its is quite feasible to run a hot dip galvanizing plant without sophisticated analysis instrumentation. Most Galvanizers will have a hydrometer, thermometer, burette, a few bits of glassware for titrations, some reagents and that's enough.
Consumables like zinc, pretreatment chemicals alloys etc have many suppliers.