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Ted Mooney, P.E. RET

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Hydrochloric Acid Concentration in Pickling before Galvanizing?

Q. Dear all,

Please I want to know the concentration percentage for the Hydrochloric acid used in the Pickling tank.

NB: The galvanized metal doesn't need to be hardly cleaned.

Please advice about the best concentration percentage.

Riad ElEzbawy
Project Research Department - Egypt
February 11, 2009

? Hi, Riad. Sorry but I don't understand your situation. Strong hydrochloric acid (say 35 percent) is used in pickling in a steel mill; a lower concentration (say 10 to 25 percent is used in acid activation) before plating or galvanizing ... but it sounds like you want to clean galvanized (zinc coated) metal, and even very mild hydrochloric acid will very quickly remove the zinc.

So I don't quite understand "pickling" a galvanized surface.

Regards,

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

A. Hydrochloric acid is often used for cleaning steel prior to galvanizing, and at typical strengths of 3-15%. Its common to start a new tank at a high strength (say 15%) then as the tank ages the acid concentration reduces as it converts iron oxides to iron chloride.
But HCl concentration is not the only important parameter. Concentration of iron chloride is also important, as well as temperature.
Prof. Kleingarn did a lot of research on this and produced a graph of concentrations of HCl and FeCl2 to enable operators to optimise pickling time.

Geoff Crowley
Crithwood Ltd.
Westfield, Scotland, UK

Q. Dear Mr. Geoff Crowley

At the moment I'm searching for the article of Kleingarn "Pickling in hydrochloric acid" presented at Intergalva 1988, but I can't find anything. Could you tell me where I could found this article?

Thank you!

Luis Xavier
- Lisbon, Portugal
October 31, 2011

A. Hi Luis. Courtesy of Barlow R. Campano, Chemical Engineer HIDADA Galvanizing Plants 1 & 2 Jeddah, Saudi Arabia, we have a good article on that subject in our on-line library. Good luck.
Luck & Regards,

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

A. Sir:
For pickling, commercial concentrated hydrochloric acid is 20 degrees baumé density and is about 35.5%. This acid is usually diluted by about an equal volume of water thus forming about 17% hydrochloric acid on day one. Some iron (+2) is required to speed up pickling in the early part of the bath cycle. As more and more iron (+2) dissolves in the acid the pickling rate gets faster and faster until the hydrochloric acid goes down too low (below about 5%) free acid. This acid can then be used to strip the zinc from rejects until the acid content is about 2%. For proper pickling, temperature and a suitable inhibitor are essential. The inhibitor protects the base steel after the mill scale has been removed. Often some of the old bath is retained so that Fe (+2) is present for the next acid bath cycle.
Regards,

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

Q. Dear Mr. Ted,

Thanks a lot for your valued answer as you have already answered my question because I was asking about the best concentration percentage for the hydrochloric acid inside the pickling bath.

According to that I can assume that 15% is great for (new steel) pickling.

thanks for your support

Regards,

Riad ElEzbawy
- Cairo, Egypt

Q. Dr. Cook,

You talked about the iron content increasing over the life of the bath. At what percentage is it too high?

Tim Williams
- Statesville, North Carolina, USA
March 13, 2009

A. Sir:
I just looked at a graph that I made 30 years ago. With 0% zinc and 10% "free" HCl at 72 F, the maximum solubility of iron(+2) is 12%. When zinc is present in the acid then the iron(+2) solubility is lower.
Regards,

Dr. Thomas H. Cook
Galvanizing Consultant - Hot Springs, South Dakota, USA
March 21, 2009

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