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What is "Pickling" of Steel?

Q. 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 you,

Luke Pdeleted
- Strasburg, Colorado, America


A. Hi Luke. If you picture the making of steel, you have a red-hot metal that you are forming, rolling, forging, or whatever. The surface of the hot 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 (acid) removal of scale.

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Ted Mooney, P.E. RET
Brick, New Jersey

Regeneration of HCl Pickling Liquor

Uses and Composition of Pickling Inhibitor


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

trading co - Jaipur, India


A. Hello, Ishaq. Hydrochloric acid is used to remove rust and scale from steel, Ishaq. But hydrochloric acid will also attack and dissolve the steel itself. Pickling inhibitors are interfacially active compounds which bind to the steel as monomolecular layers, and shield it, reducing the attack on the steel. This saves steel, and lengthens the life of the acid.

One way of answering your question about their composition is to say they may contain both organic and inorganic ingredients (according to Surtec, they commonly contain alkenes, alcohols, amines, polymerized aromatics, and heteroaromatics), but they are not a generic chemical formulation; they are proprietary products available under such trade names as Rodine, Akzo and Stannine LTP. Rodine alone has about a dozen different formulations for different situations. Good luck.

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Ted Mooney, P.E. RET
Brick, New Jersey

Can pickling be done in the field?


Q. 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 (i.e. on a job site)?

Sam Johnson
- Cotapaxi, Colorado, USA


A. Hi Sam. "Pickling" is not actually a trade name, it is the name for the generic industrial process of 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 before the steel mill ships the material to you.

People adopt shortcuts in their speech, so galvanizers and electroplaters sometimes call the rust removal or activating acid dip before their process "pickling", but it's a semantics issue; it's different and milder than what steel mill personnel would call 'pickling'.

It is possible to do field application of metal cleaning and processing chemicals and there are companies like Astropak, that specialize in it. Good luck.

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Ted Mooney, P.E. RET
Brick, New Jersey


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

Chris M.
student - California, USA


A. Phosphoric acid is sold commercially as Rust Converteramazoninfo or Naval Jellyamazoninfo, Chris. This is best for deterring future rust. The more powerful acid for quickly dissolving rust is hydrochloric acid, sold commercially as muriatic acidamazoninfo, but this leaves the surface extremely active and prone to flash rusting. Sulfuric acid is also widely used, but at room temperature it is only a mild activator; it needs to be very hot to actually function for pickling. Good luck.

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Ted Mooney, P.E. RET
Brick, New Jersey

Environmentally friendly pickling

August 3, 2008

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



Chi Leary
Artist - C Spgs., Colorado, USA

August 4, 2008

A. Hi, Chi. 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" -- maybe "remove scale"?

You can get to raw steel via mechanical methods like sandblasting or polishing if you want to avoid acids. But steel is a reactive surface; you can get it shiny momentarily but it is going to rust fairly quickly if not protected. Good luck.


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Ted Mooney, P.E. RET
Brick, New Jersey

Best sulfuric acid concentration?

September 14, 2008

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

Ahmed Y Musa
- Malaysia

September 14, 2008

A. Hi, Ahmed. The sulfuric 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.


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Ted Mooney, P.E. RET
Brick, New Jersey

September 20, 2008

Q. 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 choose 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

September 25, 2008

A. Hi. I have heard that everything from 5 percent to 40 percent has been used. But most typical would probably be about 25 percent. Again, some people start out at higher concentration, then add water to get additional life.


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Ted Mooney, P.E. RET
Brick, New Jersey

Pickling vs. passivation

November 6, 2008

Q. What is the difference between passivation and pickling of steel?

Jerry Jenkins
Aerospace - Ogden, Utah, USA

November 6, 2008

A. Hi, Jerry. The page 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. Passivation also supposedly chromium enriches the surface of the stainless. The purpose is to prevent rusting and improve corrosion resistance. It's most commonly done by immersion in nitric acid or citric acid.


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Ted Mooney, P.E. RET
Brick, New Jersey

Dimension reduction from pickling

January 16, 2009

Q. 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

January 24, 2009

A. 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 to you. So the user is not involved with scale removal, and some will say the user is not involved with pickling.

But during shipping, and during 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 the use of inhibitors to limit the attack on steel, and timing the immersion proportionally to the amount of rust. I can't give you a dimension of how much steel is removed, but it isn't much -- plating shops do rust removal from fasteners before plating them, and it rarely causes any dimensional problems even on fine-threaded small diameter fasteners.

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Ted Mooney, P.E. RET
Brick, New Jersey

November 10, 2010

Q. I am in the rollforming industry although we typically rollform painted or galvanized steel coil sometimes we roll uncoated steel. When we rollform uncoated steel the progressive bending process produces a fine dust we call mill scale. Does that mean all the mill scale is not completely removed during the Hydrochloric acid removal process or should I seek a new supplier?

Gary Jensen
buyer - Liberty Lake Washington



Folks! Although it's human nature for more people to ask for help that to offer it, this is a public forum where people help each other out, not a place where I pose as an expert on every situation. I've been the sole responder on 9 questions on this page. If you found this page, you know at least something about the subject -- please try to help the other people out. Thanks!
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Ted Mooney, P.E. RET
Brick, New Jersey

Ah, but Ted, you are responding so well to the questions. What more could I add? You are indeed a good expert!
The only comment I would add is around the topic of acid types & concentrations. HCl/muriatic or H2SO4 could both be used to pickle carbon/mild steel. The concentration ranges are usually kept low (mostly for fuming and health reasons), possibly between 5 - 25% (wt/vol).
As metals build (and discolor the solution), the reaction rate will begin to slow. Raising the temperature for any of these acids will increase the reaction rate (think basic chemistry). These parameters, plus the time in solution, can yield a good, clean, and bright surface. If the parameters are out of control, you might get a non-uniform appearance, etching, or even a dark/smut layer that is unappealing.
Asking for a specific concentration range, temperature, etc is only helpful if you supply the type of steel (and shape/form) and more info. You might be better off to try a few conditions in a lab first.
Amanda Glass
- Springboro, Ohio, USA

thumbsup2Thanks for the help, Amanda! Regards,
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Ted Mooney, P.E. RET
Brick, New Jersey

July 16, 2011

Q. We have thick walled MS pipes (ID 4, 6, 8 inch & length 20 ft) which have mill scale that must be removed. I would like to know what would be the best concentration of either H2SO4 or HCl (whichever is the best suitable for the application) to get the job done with minimum fumes (even after using the pickling inhibitors & wetting agents). Thanks.

Sachin Kanse
- Belgaum, Karnataka, India

August 10, 2011

Q. Does anyone know how large the market for pickling might be for the US?

Doug Vail
Student - Atlanta, Georgia

January 10, 2012

Q. I do have a question about the potential downsides of pickling. We've recently changed our suppliers of P&O A1011 hot rolled steel that we use exclusively for manufacturing purposes. However in doing so we've run into a problem in which the surface inclusions (craters) of the steel seem to resist the painting process we use, which has never been a problem in the past. We've had substantial rejection of product due to the steel rusting underneath the paint. Would it be a waste of time to investigate the possibility of the pickling process of this new manufacturer somehow chemically attacking our paint? Thanks for any help.

Kevin Salenski
- Sacramento, California, USA

January 18, 2012

Q. Hello, my question is if I'm pickling STEEL with a cathodic process with H2SO4 70% - 80% to high temperature like (80° C) (30 - 60 seconds) is it possible to change physical properties of material, basically TENACITY? Thank you!

Carolina A
- Colombia

January 19, 2012

A. Hi, Carolina.

Unfortunately, the word "tenacity" did not translate well, so I'm not understanding your question. Pickling is basically a surface phenomena which should not have too much effect on the underlying steel, with one exception that I can think of: Pickling, especially cathodic pickling, will introduce lots of hydrogen, and can cause hydrogen embrittlement of high strength steels -- even catastrophic hydrogen embrittlement.


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Ted Mooney, P.E. RET
Brick, New Jersey

February 15, 2012

Q. Hi, I am currently doing project of scale flakes on hot rolled coils. The scale normal appeared after some few days after temper mill or when the coils are store at the stockyard. does the chemistry have an influence in the formation of this scale flake?

Godfrey Matshili
- South Africa

May 17, 2012

Q. Related topic, but a unique situation.
We use ~35% HCL heated to 135F with Enthone Actane 32 Inhibitor to remove a burn mark left after heat sealing a glass to to a Nickel/Iron alloy wire (electronic device)
We typically need 45-60 min with agitation to achieve a bright finish on the base metal. We keep the pickle for about 10 days and then decant and refresh, trying to keep some metal ions in solution rather than start from scratch. Recently we have had poor removal rates. We possibly have seen some re-rusting, which I am trying to sort out (maybe rinse water to acidic). Looking to evaluate the HCL concentration of the pickle. It starts at about 1.16 s.g. cold. Can't use this method after the dissolution of metals, right? Also, I have read that titrating for acid concentration is not so simple due to the Ferric Chloride. Would an ISE be the right tool for quick and accurate monitoring? Etch rate test maybe? This process is used prior to Tin plating.
Thanks for your thoughts.

Mike Russo
- Lake Mills, Wisconsin, USA

August 11, 2012

Q. Dear Sir,
We have steel pickling plant using HCl. We have water carry-over problem on both edges of the sheet. We have replaced squeeze rolls many times but failed to remove water contents. Please guide me how can I resolve this problem (the hardness of squeeze roll rubber is between 65 to 70 shore).

Zafar Majeed
- Karachi, pakistan

12028 August 17, 2012

A. Dear Forum persons, regarding the post asking for help in removing water from the plate after pickling with HCl, we got some problems with this and was solved by reducing the concentration of HCl to 35% and adding some small amount (less then 1% volume of the tank) of a product that the market here calls "Lauril" its kind of a chemical used in soap and shampoo making. it helps in flowing the water but it was in our small metal shaping shop. really don't know if it will help in a steel mill.
Congratulations! for me this one is the best resources in finishing in the web !
Regards from Brazil!

Celio Santos
Arte Na Lata - Aruja, São Paulo, Brazil

November 7, 2012


IMPORT/EXPORT - laredo, Texas

November 7, 2012

Hi, Luis. It could be anything -- including a simple typo. Slang and acronyms are not specifications, and if they don't have a general meaning that is instantly and widely understood, and saves us time, then they have no meaning at all. If a couple of minutes of googling gets you nowhere (and it got me nowhere), the acronym is meaningless and must be spelled out. Good luck.


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Ted Mooney, P.E. RET
Brick, New Jersey

April 23, 2013

A. Luis, I can't swear to it but likely HRPA = Hot Rolled Pickled Aluminum Killed.

Kevin Carroll
- Long Beach, California

November 15, 2012

Q. Hi,

I learnt that chelating agents like EDTA and oxalic acid are able to dissolve iron (III) oxide. Using this concept, I intentionally create a dark layer of oxide scale on the surface of a low alloy steel specimen by cooking it in an oven at 650 °F for 2 hours. Thereafter, I carried out several pickling tests by dipping the scaled surface with aqueous solutions of EDTA and also with aqueous solutions of oxalic acid, but the iron oxide scale still remained and do not seem to be dissolved in the same way as when dipped by a hydrochloric acid solution.

I had tried several concentrations of oxalic acid and EDTA until the saturation point, but to no avail. Do I need to modify the test such as by heating up the aqueous solution or by adding some form of catalyst which may help to lower down the activation energy before the iron oxide can be dissolved?

Many Thanks for your advice.


Lim Ronnie
- Singapore

November 23, 2012

Q. I wanted to know the composition of waste pickle solution. Is it mostly comprised of Fe2+ or Fe3+?

Prabath Gunasekara
- Colombo, Sapugaskanda, Sri Lanka

November 26, 2012

A. Iron dissolves as Fe 2+, but can be slowly oxidized by atmospheric oxygen to Fe 3+. However, as long as the bath is still being used for pickling, any Fe 3+ will be converted back to Fe 2+ in a reaction that dissolves more iron.

Lyle Kirman
water treatment systems - Cleveland, Ohio

December 11, 2012

Q. Dear sir,
I am doing a project to find out the retention time for pickling with HCl. I am considering concentration, weight and section in this.
Do I have to consider any other parameters?
Is there any instrument or method to find out surface cleanliness?
Is there any instrument or method to find out quantity of rust on a material?
Is there any projects , paper or any kind of literature about retention time for pickling which would help?
Please help.
Thanks in advance.

Sharath Bhat
- Jaipur, Rajasthan, India

January 19, 2013

Q. Suppose 23t (avg.) hot rolled coils are pickled in a push pull type continuous pickling line. The coils are mainly for auto industries. When should the acid be changed for good surface finish and better quality? I want to know what should be the frequency to change or top up acid or fresh addition of acid should take place in the acid tank of pickling line. e.g. after 8 coils or 10 coils or 12 coils, etc. Is there any thumb rule? Please help me.

Siddhartha Mukherjee
- Kolkata, West Bengal, India

January 22, 2013

Q. I want to know when should we change the pickling liquor? Is there any clear way to check the quality of pickling liquor?


Anuj Roperia
student - Chandigarh, India

January 1, 2013

A. Hi Anuj. I could say "don't ever change your pickling liquor". But then you would say "but it gradually loses effectiveness". Then I could say "okay, then change it when it loses too much effectiveness". Then you would say "how much loss of effectiveness is too much?" Then I could say "how much is too much for you?" ...

As you can see from the banner at the top of the page, there are companies which focus their whole business on trying to answer your question, and there are books that try to help you answer the question. It's a difficult question! Short of ordering an automated acid management system or studying books about the topic, I think the only other answer is that you have to start acquiring data comparing pickling time to rejects to costs of regeneration or replacement of acid and adopt a hypothesis and strategy, and refine it over time. Sorry that there is probably no quick answer.


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Ted Mooney, P.E. RET
Brick, New Jersey

January 20, 2013

Q. Hi,
in MS pickling process, will we have any change in the hardness of the surface?

- Ahmedabad, Gujarat, India

January 21, 2013

A. Hi Dipak. To my knowledge mild steel is not hardened or hardenable, and hardened steel is not pickled, so I don't quite understand. Certainly, the mill scale and garbage on an unpickled surface would lead to unreliable hardness readings. Please try your best to explain your own actual situation rather than posing your question in the abstract, with its numerous "ifs ands & buts" :-)



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Ted Mooney, P.E. RET
Brick, New Jersey

January 26, 2013

Q. In mild steel pickling, we use hot rolled coil. I want to know is there any change of hardness of coil before & after pickling?

In our process, when sheet is passed through concentrated HCl tank, it shines; but after passing through water tank, sheet is yellowish. How can we handle this problem?

- ahmedabad,gujarat,India

A. Hi Dipak. I already tried to answer your first question, and I'm still not quite understanding. Why don't you simply measure the hardness before and after pickling and you'll know the answer for your situation for sure rather than asking for continuing guessing. Further we still haven't been told "why" you ask this question, which limits people's ability to read between the lines to help you out.

I suspect that your pickling acid leads to very rapid flash rusting, accounting for the yellowish appearance. I believe you should have inhibitors in the pickling acid, and then almost instantly get the steel to an alkaline state. Good luck.


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Ted Mooney, P.E. RET
Brick, New Jersey

May 11, 2013

Q. Thanks for your answers. It is refreshing to find real answers, so many sites just have half baked opinions.

I have a small project, steel weldments about 1 ft x 1 ft x 2 in, 20 pieces. Thin material is 0.065in. These will be used in a wet environment, so I want to pickle before applying a good coating.

I plan to dip in HCl solution, followed with a tri-sodium phosphate neutralizing dip. Are there any readily available inhibitors available to protect the steel? Do I need to use inhibitor? Any suggestions would be useful.

Thanks for your help. I am no chemist, but an established Mechanical Engineer, so if you ever need anything in that arena please ask.

Fred Rosse
steam engineering - Princeton, New Jersey, USA

May 13, 2013

A. Thanks Fred. You are sort of on the right track but not precisely. If you were doing this commercially, you would want to apply an iron phosphate or zinc phosphate coating before painting. It is the phosphating, not the acid dip portion of it, that affords the corrosion protection.

For a simple small-volume one-time operation, I'd use the tri-sodium phosphate first, as a cleaning solution. If you can scrub with a tampico brush rather than just dipping, that would be better. Then rinse and apply a consumerized iron phosphate coating like Osphoamazoninfo.


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Ted Mooney, P.E. RET
Brick, New Jersey

May 15, 2013

Q. After pickling mild carbon steel, it has a red tint to it. It doesn't affect any further processing operations, but some customers don't like the fact that it looks different. Any idea what could be causing it?
We use HCl pickling solutions as well as H2SO4. It happens more on the HCl line. We try to re clean and it usually helps but would want to eliminate this problem. On rare occasions if HCl doesn't clean it the sulfuric would do it.
Also one grade in particular (52100) almost all of the time turns out to have a greenish surface.
Thanks for your help.

Jake Zervas
- Chicago Illinois
  ^- Privately contact this inquirer -^

July 2, 2013

Q. I am an Electrical Engineer at a Steel Mill and I was given a project pertaining to pickling with HCl. What are the causes of flash rust/stain directly after the strip exits the acid tanks.

Thank you,

John Dockson
- Joliet, Illinois, United States
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First of two simultaneous responses -- July 4, 2013

A. Jake, John,

The discoloration (red spots) appear because the HCl is an active etcher (etching goes in active phase). That is why not only oxides are removed from the surface, but the iron from the steel is impacted, and the red iron oxides are formed. It is not good for the steel part, because besides local formation of the rust, hydrogen (from the reaction) penetrates inside the steel surface, and cause local weak points for the metal strength.

"Passive" etching means that only oxides on the surface are impacted, and the actual metal does not react to the etching solution. And the hardness is not changed.

Anna Berkovich
Russamer Labs
Pittsburgh, Pennsylvania

Second of two simultaneous responses -- July 5, 2013

A. Hi John. Flash rusting is the natural state of affairs . . .
First, the acid and rinse water has iron dissolved in it which cannot remain dissolved as the solution evaporates, but must become rust. Second, the surface is extremely active, in acid condition, and activated by corrosive chlorides. As a minimum you must immediately rinse well and get the material into rust preservative or, at the least, into an alkaline condition. Good luck.

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Ted Mooney, P.E. RET
Brick, New Jersey

August 17, 2013

Q. Hi,
After pickling of Hot Rolled Coils with HCl and rinsing, we want to dry the HRC before re-coiling. What should be the max. temperature of the air that we will use to dry the surface of the Hot Rolled Coils?

- Bucharest, Romania
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August 31, 2013

A. Hi Haken
If you use heat you will have to heat up the whole of the metal strip. This is very expensive and slow.
You should consider passing the strip through an air knife to blow off the water.
However you dry the steel it will be very susceptible to surface rust. You may wish to consider a light coating of a rust inhibiting oil if the coils are to be stored for even a short time

Geoff Smith
Hampshire, England

September 4, 2013

A. In order to dry the steel, it is usually easier to heat the final rinse and then dry it with unheated air, than it is to dry it with hot air. The high heat capacity of the metal and low heat capacity of air makes drying with hot air difficult.

In order to avoid "flash" rusting, the final rinse must have a very low chloride concentration. Some companies specify < 5 mg/L of chlorides. Others add a corrosion inhibitor to the final rinse.

Lyle Kirman
water treatment systems - Cleveland, Ohio

January 15, 2014

Q. Hi,

We are using HCl in pickling and have problem of the bath getting ineffective quickly over time, as the iron content increases. Usually our practice is to get fresh bath done with 30% HCl, added with water in 60:40(acid) proportion .
Also problems faced are heavy fumes (we have avoided inhibitors as we are told that it slows pickling). Our product is 12 to 20 mm thick. Initial pickling time is 10 minutes which goes up to 90 minutes - which is unacceptable to our cycle time

How can we achieve fastest pickling by avoiding iron content generation which spoils bath?

Sameer Nayak
- Belgaum, INDIA
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January 17, 2014

A. Hi Sameer. I think you may be misunderstanding the word "inhibitor". This is not a fume suppressant, but a material that helps prevent the acid from being wasted dissolving steel instead of rust. Using an inhibitor is how you deter the iron content from rapidly building up. Good luck.


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Ted Mooney, P.E. RET
Brick, New Jersey

February 3, 2014

Q. After pickling steel with hydrochloric acid, would a solution of methylated spirits and lime be suitable for removing any unspent pickling solution from the steel and stopping flash rust?

Ilk Fish
- brisbane, queenlsand, australia

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