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"Phosphating instead of HCl Pickling before Galvanizing"



March 8, 2016

Q. Good morning

My name is Rob Watchorn and I currently operate a Hydrochloric (HCl) Acid Regeneration facility in South Africa.

At present, all the galvanizers use HCl to pickle their steel prior to galvanizing.

I have been requested to investigate the feasibility of replacing the HCl with a Phosphate based pre-treatment.

I have some concerns relating to how the galvanizing process may be affected by the phosphate coating on the steel.

Any advice or information will be greatly appreciated.

Thanks

Rob

Rob Watchorn
HCl acid regenerator - Johannesburg, South Africa
^


March 12, 2016

A. I don't think phosphate coating will work at all.

However if your interest is simply to stop the use of HCl, then phosphoric acid will probably work, however:

You will probably need longer contact time = lower line speed.

Phosphoric is more expensive.

Sulfuric acid could probably also work, with the same problems as phosphoric.

jeffrey holmes
Jeffrey Holmes, CEF
Spartanburg, South Carolina
^


simultaneous March 13, 2016

A. Sir:
I believe I visited your plant about 1985, had dinner with one of your executives, and discussed a possible HCl reclaiming plant near Houston. Because the price of virgin HCl in South Africa was very high your plant was doing very well financially. In Houston HCl was cheap and a deep well disposal was nearby (near Houston), an HCl recovery plant was not financially reasonable.

HCl, H2SO4, and HNO3 are the three "strong" acids. HCl and H2SO4 are reducing acids, whereas HNO3 is an oxidizing acid.
Thus for pickling mild steel, HCl and H2SO4 are normally used. There are many organic weak acids (e.g. formic, acetic, etc.). H3P04 (phosphoric acid) could be classified as a medium strength acid which would have too slow of pickling (cleaning off mil scale off steel) to be useful. In addition, once the phosphoric acid became "spent" (saturated with iron and zinc phosphate), how would it be disposed of?

In the USA HCl and H2SO4 are both used for steel pickling. The "spent" HCl usually goes to deep well. The "spent" H2SO4 is cooled and crystals of Iron and Zinc heptahydrate are crystallized out, and the recovered acid is re-used again and again. These crystals are often used in plant fertilizers. I enjoyed the dinner, thank you.

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


March 15, 2016

A. Galvanizing isn't a coating(in the usual sense of the word), it's an alloying reaction. It will only take place between iron and zinc, and only when the iron (steel) is chemically clean.
Acid is used to clean the steel.
The type of acid depends on what is best cost. HCl is perhaps most common, as it's low cost, works at ambient temperature, and is relatively safe.
H2SO4 is next most popular, often due to cost locally, but usually needs heating, and "over pickling" starting to dissolve the steel - can occur.

Depositing something like phosphate (you didn't say which phosphate) onto steel won't leave a chemically clean surface for the zinc to alloy with.

If the steel isn't clean, then all you'll get is a hot piece of steel.

geoff_crowley
Geoff Crowley
Crithwood Ltd.
supporting advertiser
Westfield, Scotland, UK
crithwood logo
^

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