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Best sulfuric acid concentration and iron content for pickling

April 12, 2012

Q. Thank you for all your information.
I kindly ask you to tell me what is the best sulfuric acid concentration (g/l) and iron Fe2+ (g/l) to use when working with cold pickling bath without heating (ambient temperature).

Best regards

Mondher CHERIF
galvanizing plant - Tunisia

April 14, 2012

A. Sir:

There seems to be some confusion here. Sulfuric acid pickle is normally used at about 150 °F., NOT AT ROOM TEMPERATURE. At room temperature, sulfuric acid pickles TOO SLOWLY to have a practical use. The usual concentration for sulfuric acid is 8% to about 12% and a good inhibitor at the correct concentration is absolutely essential. The lower the iron (Fe+2) the better and especially the lower the zinc (Zn+2) the better. To initially make-up the pickle tank 6 vertical inches of conc. H2SO4 are used for every 100 inches of tank. Be sure to add the acid after the water is in the tank.

Hydrochloric acid is normally used at room temperature 70 °F to about 85 °F. The concentration is made up using about 40% water and 60% conc. HCl by volume. This gives an HCl concentration of about 18%. HCl will not pickle properly until there is some Fe+2 dissolved in it. The first few days of pickling will be slow. Zinc ion in the HCl slows pickling very much. A good inhibitor used at the right concentration (by testing) is required.

35 years ago I tested 50 inhibitors for H2SO4 and 50 inhibitors for HCl. Only one of each worked properly. During this testing I used a concentration of 3 parts (by volume) of inhibitor to 10,000 parts of mixed acid. Only about 5 inhibitors in each group worked properly. Then I tested for thermal stability (in the sulfuric acid case) and only two worked well. Then I tested pickling speed and only one in each case worked well. Many inhibitors had very bad side affects including a tar ring in the beakers which I could NOT REMOVE and had to discard the beakers. On my recent trip to three plants, one of these plants used 5 barrels of inhibitor 6 months ago and stopped all pickling in three of the four pickle tanks.


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

March 13, 2014

Q. Great response about the inhibitor Dr. Cook, I appreciate your insight and experience. Can you direct us to any published references about acid inhibition? Thanks!

Rick Morgan
- Seattle, Washington

March 14, 2014

A. I wonder how many of the inhibitors tested 35 years ago are still available in the same formulation?
Here in UK, I can't get any of those available 20 years ago. They've all been replaced by better formulations, and now most of them work very well. Used to be some rubbish available, now quite rare to find a poorly performing formulation.

Geoff Crowley
- Glasgow Scotland

March 16, 2014

I presume you are an employee of the new galvanizing plant in Seattle. My article on inhibitors is:

Metal Finishing (ISSN 0026-0576)

Metal Finishing, pages 15-18, OCT.1982

I think Metal Finishing is no longer published. You should be able to get a xerox copy of my article via interlibrary loan from a large library. My article only tells how I tested inhibitors. None are named.

The best sulfuric inhibitor is no longer available as it had ethylene glycol (antifreeze) in it which is toxic. The best HCl inhibitor is available, however the owner of the company hesitated when I asked him if it is the same formulation. In North America there are many acid inhibitors that are rubbish.


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

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