Lead (Pb) in Hot Dip Galvanizing

A. Hot-dip Galvanizing is based on alloying Iron with zinc. Traces of iron introduced into the zinc kettle(carry-over with the material to be galvanized from wet pre-treatment, from the steel zinc-kettle-wall and the material to be galvanized will alloy with the zinc too, thus forming zinc-iron alloy particles called dross. This dross is some heavier than the zinc and will slightly sink to bottom of the kettle. Lead has two functions: 1. dissolved lead in the zinc-melt has influence on the fluidity of the zinc and will help dross particles sink to the bottom - floating dross particles will cause roughness on the material to be galvanized(bad quality) 2. The lead layer on the bottom of the kettle will prevent dross will be stuck to the iron kettle-bottom and makes it possible to remove the dross from the bottom of the kettle, because lead is heavier than the dross and will not alloy with the dross and zinc - so dross floats on the lead-layer.

A. DEAR SIR,

IN HOT DIP GALVANIZING, PURE LEAD CHARGES IN POT SAVE THE LIFE OF POT, LEAD BEHAVES AS A CUSHION EFFECT.

A. Coming back on your second question I don't know the reason why you'd like to change lead for something else, but in Scandinavia lead will be forbidden for environmental reasons.

Bismuth or Tin could be elements to replace the function of dissolved lead. These elements cause a high fluidity of the zinc and a good wetting of the steel. De-drossing of the kettle is hardly a problem. Cominco Canada made investigations adding Bismuth to Zinc for hot-dip galvanizing.

Regards,

A. Hi, Jaydip. Environmentalists think the ideal amount is zero :-)

You can search the site for "lead-free" galvanizing. Good luck.

Regards,

A. Sorry Jaydip, I can't answer that. But if you patiently search the site, similar subjects have been discussed at length by true experts several times. Good luck!

Regards,

A. Lead ideal amount is 1.3%.

A. Its interesting that someone is forcing you to reduce Pb content. Can you say why they want that?

I think you could perhaps pump the Pb rich Zn off the floor of the tank and so reduce it that way. If you get the free Pb out, and then only replace with Zn without Pb, then it must come down. At about 1% Pb in solution, you probably have some free Pb on the bottom. Getting that out first would help.

Is there a time limit by which you must do this? If not, I think you could get it down in 6 months or so without a complete pumpout. But I'm guessing this, I don't know of anyone trying it.

A. Sirs:

Lead solubility in zinc depends on temperature:

800 °F 1.0% Lead (Pb)
820 °F 1.2% Lead
840 °F 1.4% Lead
860 °F 1.6% Lead

Calculation to determine lead content in zinc as it is used and replaced by SHG zinc:

(Fraction Changed/Day)Raised to Power,Days Times Orig. Pb%

Example:

500,000 pounds zinc in kettle.
5,000 pounds zinc consumed each work day.
0.11% Lead Original Lead percent.

(500,000-5,000)/500,000 = 0.9900

(0.99)Raised to power 20 days times 0.11% equals 0.090% Pb
(0.99)Raised to power 60 days times 0.11% equals 0.060% Pb
(0.99)Raised to power 80 days times 0.11% equals 0.049% Pb

Notes: "days" are work days. Use "x raised to power y" on scientific calculator. Thus put in 0.99 hit enter and then put in 20 (days) then hit "=" Then multiply by 0.11. At 0.11% lead there would be no benefit to pump from the kettle bottom. Be sure to use "brightener bar (10% Al 90% Zn) that DOES NOT CONTAIN LEAD.

The real issue here is that zinc with no additives does not work as well as zinc with lead. I am working on this issue just now with excellent success and no need for lead.

Regards,

A. On our last kettle pump out, which started with a dissolved Pb content at 1.0%, the Pb content of the bottom zinc was about 80% and we got 2 ingots out like that.
So while the theory might say its not worthwhile, the practical experience suggests that a significant reduction might be possible way to hasten the dilution process.
We have not used the technique to take Pb out, but have taken Pb out as an unintended consequence.

Geoff,

In your case with 1% lead, you obviously had liquid lead under the dross. In the other case with only 0.11% lead it is obvious that there is no liquid lead under the dross.

Regards,

A. Apologies: I mis-read the Pb level in the original post.

Yes, reducing from 0.11 to under 0.05% is another matter entirely.

A. Hi Kadva. Apologies but I don't understand your question. Hopefully someone who is more familiar with this may understand what you are trying to ask.

Regards,

A. Dear Mr. Suprabhat,

It is advisable to add lead after 10 days of complete melt out of ZINC in new kettle. Do not operate with more than 50 mm of lead in bottom of kettle.

A. DEAR KANTILAL
TIN AFFECTS GLOSSINESS OF THE SURFACE. THERE IS NO SPANGLE CREATION BY ADDING THE TIN. YOU CAN USE LEAD AND ANTIMONY FOR SPANGLE.

REGARDS,

A. Sirs: Tin in the zinc is known to cause the galvanized steel to crack. In the USA a stressed steel bridge was destroyed by tin. In the UK a stadium had its roof support structures crack due to tin in the zinc. Too much bismuth causes white rust.

Lead in the zinc increases the reactivity of malleable cast iron and produces coating that side off the castings. In Ireland only one galvanizer is lead-free and it is known for good galvanizing of castings.

Lead in the zinc does not protect the kettle.

Regards,

A. Concerning lead in the kettle.
Pb has maximum solubility in Zn at about 1.2% at 450 °C. Further additions than this just accumulates on the kettle bottom as it is the most dense among Zn, dross and Pb.

Zeta dross crystals are not very dense compared to Zn, and the addition of Pb causes them to sink as they combine with Pb in solution, become heavier and sink. More floating dross is found in Pb-free kettles than those with 0.6% Pb or more.
Pb-free is rare (as in 0%) as much Zn supplied has some Pb in it; the ores are found together in nature, so they end up being processed together, and only by active separation is "pure" Zn made.
Pilling (perhaps worlds premier kettle maker) in their book "Galvanizing Kettles - recommendations for the user" recommend having minimum 0.6% Pb.

Sn in the melt is another matter. Rather than actually CAUSE cracking, it aggravates an existing problem. The most recent extensive research on this shows that the galvanizing isn't the cause of the cracking, even in Sn containing melts, but it reveals and exaggerates cracks that are there in the first place but undetected.

A. Dear Shahriar,

It would be extremely easy to have a melt with content 0.95% lead and find areas on an SEM analysis that would show 4.30% lead - this is because EDS/EDX is only a localised measurement, even before you take in all the potential error sources. It is not a bulk measurement of composition, and therefore I do not think it is suitable for this testing. I would be very surprised if any of the relevant standards mentioned it.

Atomic absorption spectrometry ⇦ this on eBay or Amazon [affil links] to ASTM E536 is suitable for both aluminium and lead in zinc and is referenced in ASTM B6-23 (Standard Specification for Zinc)