What is "Pickling" of Steel?

Letter 12028

I am a student from Strasburg high in Colorado and I was wondering what is pickling steel? I have been looking all over the internet and you are the closest I have gotten.

Thank u,

Luke P
- Strasburg, CO, America

If you picture the making of steel, Luke, you have a red-hot metal that you are forming, rolling, forging, or whatever. The surface of the steel is going to react with the oxygen and water vapor in the air, forming something similar to a very heavy rust. This is called 'scale'. The scale needs to be removed, and 'pickling' is usually the name given to the chemical removal of scale.

Ted Mooney, P.E.  finishing.com Brick, New Jersey

I want to know the uses and composition of Pickling Inhibitor used in the pre-galvanising process of cold rolled steel.

Ishaq
trading co - Jaipur, India

Hydrochloric acid is used to remove rust and scale from steel, Ishaq. But hydrochloric acid will also attack and dissolve the steel itself. The purpose of a pickling inhibitor is to reduce the attack on the steel. This saves steel, and lengthens the life of the acid.

One way of answering your question about composition is to say it's not a basic generic chemical formulation, it's a proprietary product. Good luck.

Ted Mooney finishing.com Brick, New Jersey

I work for a general contractor and I am reviewing the specifications for metal fabrications. The specs call out pickling before galvanization. I understand a little that pickling is a trade name for a chemical used to remove rust. How exactly is this process done in the factory? Can pickling be done in the field (ie on a job site)?

Sam Johnson
- Cotapaxi, Colorado, USA

"Pickling" is not a trade name, it is a generic industrial process for removing mill scale. Often there is a mechanical 'scale breaking' step first, which passes the product through rollers designed to fracture the brittle scale to mechanically flake off what you can, and to offer more surface area for quicker chemical reaction. Then hydrochloric or sulfuric acid dissolves and removes the heavy mill scale.

You can field apply some acids to structures if an occasion warrants, but as a semantic issue I don't think people would call this 'pickling'.

Ted Mooney finishing.com Brick, New Jersey

Sir, I am a student carrying out a project work on PERFORMANCES OF VARIOUS PICKLING SOLUTIONS ON RUSTY MILD STEEL. Pls, assist me with the various types of pickling solutions for removing rust on mild steel including there compositions.I will be grateful.thank you

Chris M.
student - California, USA

Phosphoric acid is sold commercially as Rust Converter [link is to product info at Amazon] or Naval Jelly [link is to product info at Amazon], Chris. This is best for deterring future rust. The more powerful acid for quickly dissolving rust is hydrochloric acid, sold commercially as Muriatic Acid [link is to product info at Amazon], but this leaves the surface extremely active and prone to flash rusting. Good luck.

Ted Mooney finishing.com Brick, New Jersey

Is there an easy environmentally friendly way to remove pickling from steel to achieve a shiny surface?

Thanks!

Chi

Chi Leary
Artist - C Spgs., CO, USA

Hi, Chi. As already mentioned, pickling is the removal of scale. It's a process, it's not a surface coating, so I don't know what you mean by "remove pickling".

You can get to raw steel via mechanical methods like sandblasting or polishing. But steel is a reactive surface; you can get it shiny momentarily but it is going to quickly rust.

Regards,

Ted Mooney finishing.com Brick, New Jersey

Dear Sir, I am willing to use H2SO4 as pickling solution for mild steel, what is the best concentration of H2SO4? Rgds

Ahmed Y Musa
- Malaysia

Hi, Ahmed. The acid needs to be heated, of course, so the first question back to you if what temperature are you trying to operate this pickling process at? Some people believe the best economy is achieved by pickling at a relatively high concentration and as the metal builds up, dilute it. It is claimed that the reduction in dissolved metal by dilution overcomes the dilution effect of the acid, so some aggressiveness is restored simply by adding water. Hopefully we can steer you to appropriate books about pickling, if you can tell us a little more about your situation.

Good luck.

Regards,

Ted Mooney finishing.com Brick, New Jersey

Thanks a lot for quick reply, what I tried to do is to use a novel inhibitor in pickling process. Different temperatures and different inhibitor concentrations will be carried out in the test. I was choose the H2SO4 because it's very common in industry. If I use high concentration of H2SO4 then the more O2 dissolved while low concentration, means little effect of H2SO4 in pickling process, Please advice which concentration of H2SO4 I should use? Regards,

Ahmed Y Musa
National University of Malaysia - Bangi, Malaysia

Hi. I have heard that everything from 5 percent to 40 percent has been used. But most typical would probably be about 25 percent.

Regards,

Ted Mooney finishing.com Brick, New Jersey

What is the difference between passivation and pickling of steel?

Jerry Jenkins
Aerospace - Ogden, Utah, USA

Hi, Jerry. The page repeatedly explains what pickling is, so I won't repeat that part. Passivation is never to my knowledge employed on steel, but you may be thinking of stainless steel. In that case passivation is the removal via acid of any remnants of plain steel that may have contaminated the stainless surface during fabrication operations. It also supposedly chromium enriches the surface of the stainless.

Regards,

Ted Mooney finishing.com Brick, New Jersey

Hi. When hot rolled steel is pickled to remove rust, how much of the surface is removed? Is it a measurable amount or even noticeable? Many thanks

Albert Weatherill
Surveyor - Hull, Yorkshire, UK

Hi, Albert. Again we have semantics issues that may lead to misunderstanding. The steel mill removes the scale before they stock or ship the steel. So the user is not involved with scale removal, so many would say the user is not involved with pickling.

But during shipping and storage before and after shipping, rust will start to build up on the steel and you wish to remove this. Generally an acid dip to remove rust will involve timing the immersion proportionally to the amount of rust, plus the use of inhibitors to limit the attack on steel. I can't give you a dimension of how much steel is removed, but it isn't much -- plating shops do rust removal of stampings and fasteners before plating on a commodity level, and it rarely causes any dimensional problems.

Regards,

Ted Mooney finishing.com Brick, New Jersey