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topic 39277

Why do we use 15-18% HCl for pickling

A discussion started in 2006 but continuing through 2019


Q. Hi. I am a Chemical Engineer who just completed my Masters from SUNY-Buffalo. am currently working with an Indian company who manufactures fume extraction system for the pickling industry. I would like to know, why do we use around 18% HCl for pickling. What will happen if the concentration is too high? How will the finishing be affected if we vary the concentration? Also how was this concentration arrived at, i.e., by experience or experiments or any theory.

Chirag Thakkar
fiberglass industries - Mumbai, Maharashtra, India


A. Hi Chirag,
The 18-22% value of HCl in pickling solutions was adopted long ago in the metal finishing industry (not sure of the exact year). My guess would be it was the result of research which involved trial and error. If the HCl concentration in pickling solutions is too high, the metal to be treated could be attacked, worker safety is jeopardized, plant structure and equipment is at major risk. That's where you come in with fume scrubbers!

Best Regards,

Mark Baker
process engineer - Malone, New York


thumbs up signThanks Mark for your answer.

Chirag Thakkar [returning]
- Mumbai, Maharashtra, India


A. Dear Mr Thakkar,

HCl contains Hydrogen, which leads to Hydrogen embrittlement effect after pickling. Hence For high tensile applications it is generally HCl pickling is avoided, or the concentration is reduced.

Second major reason is Fuming characteristics of HCl. HCl Fumes usually damage the surrounding construction and fabricated structure. To avoid excessive fuming, the concentration is kept on lower side.

Third reason is health related. However the best is your fume extraction system, the fume escapes and creates health hazard. Hence the concentration is kept at lower.

Contrarily, if we keep the concentration too low, then the pickling does not take place, and increases the processing time and cost.

One need to strike a balance between the two and thus this concentration is arrived

v g rajendran
V. G. Rajendran
- Trichy, Tamilnadu, India


A. I agree with V. G. Rajendran and want to add more. Several acids alone or in combination are generally used for pickling. These acids, however, also attack base metal. Thus acid corrosion inhibitors are used. The optimum condition for pickling baths depend on various factors such as concentration of acid, quantity of inhibitor/s, pickling bath temperature, pickling duration etc. A lower acid concentration can be worked out following above parameters.

- Pune, India


A. Dear Mr. Thakkar,

In HCl the chlorine remains in a very unstable form. That why it loses its strength even if it is not in use in open bath.
Being a chemical engineer you understand chlorine in the contact of metal form salts, which act as an electrolyte and promote corrosion.

Navin Maheshwari
- New Delhi, India


A. Sir:
If one uses "straight" HCl acid the fumes are extreme for workers and the roof of the factory is quickly destroyed in one year.
If one uses weak HCl then the acid bath lasts only a short time and large volumes of "spent" acid are produced which is very costly.
Good HCl inhibitors reduce fumes for the workers but more importantly protect the base steel after mill scale removal. I published an article in Metal Finishing (magazine) to help people chose a good inhibitor.
The literature indicates that HCl produces less hydrogen embrittlement than sulfuric acid. I have never experienced hydrogen embrittlement with acid pickled steel.

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


Dear Mr.Chirag,

If we use commercial Grade HCl ( 30 - 32 % ) directly for the acid pickling tank for Galvanizing plant, black Materials which is to be galvanized may damages over and above, surrounding area of the Galvanizing plant may with full of Acid fumes and its very difficult to work with this kind of environment. plus structure of the plant may damage. that's why we prefer to use pickling of materials with 18 - 22 % HCl.

Ilesh G Vyas
Gunatit Builders
supporting advertiser
Manjalpur, Vadodara, Gujarat
gunatit builders

May 17, 2008

A. Dear's

Chloride containing agent such as HCl, since there is an obvious risk of pitting corrosion, so it is used in that much of 15-18% only.


August 25, 2011

Q. We are manufacturer of steel line pipe and GI pipe. We have a problem with acid fumes. I have heard that there are some chemicals which if mixed with pickling solution reduce fumes. Please guide.

Muhammad Ramzan Noor
Employee - Pakistan

April 23, 2012

Q. I am a trainee Engineer at a cold-rolling complex and looking after commissioning activities at Push-Pull Pickling line of the mill. My pickling line will soon enter into its production phase. 18% HCl will be used for pickling steel strips. I am keen to know what are the factors that effect the pickling time? I.e., the optimum time for which a unit area of the strip should be in contact with acid bath to achieve pickling.
Is there any relation or calculations which can be used, or any thumb rule?

Arham Saleem
- Karachi, Pakistan

April 26, 2012

A. Time is a function with temp and concentration PLUS metal condition. You may require different times with different lots from the same mfgr.
Typical would be 10 to 30 min with 20 min a common time. It still depends on the variables in the first line.

James Watts
- Navarre, Florida

May 18, 2012



June 6, 2012

A. Part of the commissioning exercise is to determine the envelope parameters for your process line. You should have an idea of what they are from the original specification, but you need to confirm them under real life conditions. I suggest you run a small pickling bath hold the acid content and temperature constant and run the pickle until the quality of the steel is unsatisfactory. As you do this, I suggest you monitor the total metal content of the pickle acid - remember that pickling HCl often has a base metal content already in it - pure concentrated HCl is about 36%, but industrial concentrated HCl is much cheaper and is 32% and is often contaminated with other metals. I would expect your acid pickle to need regenerating after the metal content increases to between 50-100 g/l and the acid content falls to about 50-70 g/l. You will find that the metal content is more important and this is subject to how free from scale your feedstock metal is. it will also depend on the type of steel being pickled - mild steel will have totally different characteristics to stainless steel. If you are pickling stainless steel, I would suggest using nitric acid and/or hydrofluoric acid.
Increasing the concentration of HCl will cause other issues, for instance, if it is too strong, the fumes will make using it almost impossible unless you have very good extraction. This will of course be complicated by increasing the temperature, as the hotter the solution, the greater the amount of acid vapor...

trevor crichton
Trevor Crichton
R&D practical scientist
Chesham, Bucks, UK

August 16, 2012

Q. Hello,
I am a new chemical engineer in the metal finishing industry. I would like to know why HCl or sulfuric acid is more preferred over say phosphoric acid for a zinc phosphate coating process for steel.

Kent Reid
- Columbus, Ohio, USA

September 2013

A. Hi Kent. I may be misunderstanding you, but I think that is incorrect. HCl or heated H2SO4 are used for pickling because they are more aggressive than phosphoric acid -- but phosphoric acid is used in phosphating processes because the intention is to leave a phosphate conversion coating on the surface.


Ted Mooney, Teds signature
Ted Mooney, P.E. - Pine Beach, New Jersey
Striving to live Aloha

September 17, 2013

Q. Why does HCl acid strength reduce drastically when we pickle mild steel or heat treated angles (example : from 18.5% to 7% in 2 days)

Premprakash Singh
- Maharashtra, India

September 19, 2013

A. Hi Premprakash. There are probably two causes, actual chemical consumption and vaporization. HCl is consumed in dissolving rust, so if the components are scaly or rusty, you'll use a lot of acid. The acid also attacks the base steel and is consumed by doing that -- proper inhibitors can significantly reduce that wasted consumption.

HCl also has a very high vapor pressure, that's why it smells so strongly. If quiescent, the evolution of gas will be slower; if sloshed around, the evolution of HCl gas will be extreme. Good luck, and feel free to supply additional details for a more quantitative answer.


Ted Mooney, Teds signature
Ted Mooney, P.E. - Pine Beach, New Jersey
Striving to live Aloha

September 20, 2013

A. In addition to the acid consumed in dissolving iron, and iron oxides, and acid lost to vaporization, as mentioned by Ted, there is also the drag-in of water and drag-out of acid into the rinses.

In some cases such as barrel plating or continuous sheet steel pickling, drag-out can be the largest loss of acid.

Lyle Kirman
consultant - Cleveland, Ohio

September 23, 2013

thumbs up signThanks Lyle. That's something I realized but totally forgot to think about :-)


Ted Mooney, Teds signature
Ted Mooney, P.E. - Pine Beach, New Jersey
Striving to live Aloha

February 28, 2014

Q. Hi
My name is Howard working as Process Technologist for a pickling and galvanising plant.

We are currently using acid life extender and 12% HCl to pickle our wire before galvanising; the problem we are facing now is iron is constantly increasing as the acid decreases. We are running at 100 g/L Fe and 105 g/L HCl -- wanted to find out if we should be worried or we have reached Fe & HCl saturation?

Howard Bosae
wire industry - vanderbijlpark,Gauteng, South Africa

March 5, 2014

A. It would appear that your pickle inhibitor is not as effective as it should be, either because it is inherently ineffective or it is being used at too low a concentration.

Cal/OSHA is proposing to set an airborne permissible worker exposure level (PEL) of 0.3 ppm time weighted average (TWA) for hydrogen chloride (HCl). If passed, this would be a lower level than 0.4 ppm recently set in California for hydrogen fluoride (HF). Most platers are well aware of the dangers of HF or hydrofluoric acid. This would be a difficult standard for many platers to meet.

For details, see
which I lifted some of the above text from.

Tom Rochester
Plating Systems & Technologies, Inc.  
supporting advertiser
Jackson, Michigan, USA
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Why do we want fluorine in HCl pickling acid, and what problems does it cause

April 3, 2014

Q. What is the importance of Fluorine in Pickling Acid such as HCl, and what problems occur when fluorine is present in HCl?

Sam Parulekar
- mumbai, Maharashtra India

April 2014

A. Hi Sam. Fluorine usually does not belong in pickling acid for steel, although it has some applicability for stainless steel, aluminum, or other metals (usually cast) which contain silicon because fluorine is required for removal of silicon.

thumbs up signThe hypothetical question of what problems might occur if you add inappropriate chemicals to pickling baths is impossible to answer (the work can be spoiled, the tank can be destroyed, the fume extraction system can be ruined, the employees can be hurt, the wastewater treatment system can malfunction and cause a fish kill ... the list is interminable); please try your best to explain your actual situation. Thanks!


Ted Mooney, Teds signature
Ted Mooney, P.E. - Pine Beach, New Jersey
Striving to live Aloha

April 6, 2014

Q. My question was if, in manufacturing of HCl has some fluorine or fluoride content, would it cause any problem to the HOT ROLL COIL or the pickling bath?
Or Fluorine (F) and Fluoride has no importance in pickling?
One more question, if temperature of HCl increases its performance improves -- but if temperature increases beyond its specified degree, what problem will it cause?

Sam Parulekar [returning]
- mumbai,Maharashtra India

April 18, 2014

A. Fluorides are often used in pickling formulations to remove silica from the surface of the particle being activated or plated. I recently started reading Per Moller's two-volume Advanced Surface Technology" (I haven't finished it yet) which is quite comprehensive. In Chapter 3, under 'Activation' he mentions that "...iron oxides are complexed with fluoride making it possible for a plated material to be deposited..."

Tom Rochester
Plating Systems & Technologies, Inc.  
supporting advertiser
Jackson, Michigan, USA
plating systems & technologies banner ad

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