What is "Pickling" of Steel?

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.

A. Hello, Ishaq. Hydrochloric acid is used to remove rust and scale from steel. 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.

A. We have been regenerating "all" of our acids and rinses chemically for the last two years. You can see the article that describes this in "Products Finishing" magazine (November 2002) page 82.

I do not comprehend why in this day and age companies dump acid because of metals and organics saturation.
1. Put 1% PRO-pHx [acid extender] into the acid tank that you wish to eliminate the disposal of.
2. Filter your acid with any type of filter you chose. Small tanks often use a FloKing style pump with a 20 Micron cartridge to capture the organics and metals.
3. Check your acid strength at least weekly. Add new acid to keep the bath strength at optimum. Add 1% PRO-pHx based on the volume of acid added.

A. Hi Sam. "Pickling" is not actually a "trade name" (which I consider to be like a trademark but without discussing the legality of the claim to ownership of the word), 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 and ATP Results who specialize in it. Good luck.

A. Phosphoric acid is sold commercially as rust converter [on eBay or Amazon] or naval jelly [on eBay or 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 [affil links], 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.

A. Sounds like excessive attack of the wire by the acid (instead of just taking off the scale), followed by inadequate rinsing, thus leaving a film of ferrous sulphate ferrous sulphate [affil links].

You don't mention an inhibitor, which would minimise attack of the steel itself. Henkel has an inhibitor Rodine 85 which we have found excellent in 8-12% sulfuric running at 70 °C in a new hot-dip galvanizing plant.

I suggest an inhibitor to minimize metal attack, and improved rinsing as an added precaution.

A. Can't see any way ferrous sulfide could be present. Sulfur in the steel is present as manganese sulfide inclusions which are grey-coloured and microscopically small - there shouldn't be any iron sulfide at all. The sulfuric will generate hydrogen sulfide gas from the small amount of sulfur in the steel, and that gas simply escapes into the air.

Given that you use an inhibitor, and assuming that it works OK (I don't know it, so can have no opinion on its effectiveness), there will be little or no reaction on the surface of the steel as you come out of the pickle, so we're left just with a rinsing problem. Can't see your contaminant as being anything other than ferrous sulphate ferrous sulphate [affil links] on the surface - problem is where does it come from? If you use a tank for rinsing, is the acid concentration in your rinse a bit high? Whether you use a tank or a spray rinse, if you leave some acid on the steel after the rinse then the inhibitor concentration in that residual acid will be too low to prevent reaction between the acid and steel, and you could form ferrous sulphate ferrous sulphate [affil links] while the steel is drying.

Lots of suppositions and possibilities, I know, but maybe some help in deciding where to run specific tests.

A. Sir,
I prefer Rodine 95 for sulfuric acid. The concentration is normally between about 1 part per 10,000 to 3 parts per 10,000 of mixed acid. Additionally you need a simple inhibitor test to know what is in solution. This test appears in an article of mine, now published in the journal, Metal Finishing.
Regards,

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.

Regards,

A. Hi, Ahmed. The sulfuric acid needs to be heated, of course, so the first question back to you is 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,

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.

Regards,

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 .

Regards,

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.

A. Hi Doug.

Do you understand exactly what you are looking for? Do you mean in steel mills only, or do you also mean at galvanizing shops and plating shops; how about for field application of pickling solutions to tanks, boilers, and piping in-situ? Pickling paste for weld areas? Are you including pickling of stainless steels, titanium, refractory metals, cast iron and aluminum castings, or just steel? Do you expect it to be expressed in gallons of HCl & H2SO4, or dollars spent in purchases of pickling solutions, or in square feet of product, or in man-hours expended?

I'm not trying to give you a hard time, Doug, just emphasizing a lesson for students that if you don't understand the teacher's question perfectly & exactly, you must ask your teacher to clarify it ... posting it and hoping that people's answers will clarify the question virtually never works, it only embroils you in Chinese Whispers. Good luck!

Regards,

A. Hi Kevin. I find that unlikely. However, I think it is very possible that your in-house pretreatment system is not able to deal with steel which has been less adequately prepared at the mill. For example, if you are using only a 3-stage iron phosphate before painting, it is possible that the process was adequate for the previously supplied steel but not the presently supplied steel.

Regards,

A. Hi, Carolina.

Unfortunately, the word "tenacity" did not translate well, so I'm not quite understanding your question. Might you mean "ductility"? 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.

Regards,

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 on the web !
Regards from Brazil!

A. 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, thus saving 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.

Regards,

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

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.

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?" ...

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.

Regards,

A. Hi Dipak. To my knowledge mild steel is not hardened or hardenable, and hardened steel is not pickled nor called MS, so I don't quite understand. Certainly, the mill scale and garbage on an non-pickled surface would lead to unreliable hardness readings. But please try your best to explain your situation more explicitly rather than posing your question in the abstract, with the countless "ifs ands & buts" that result :-)

Thanks.

Regards,

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?

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!

Regards,

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 Ospho [affil links].

Regards,

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.

adv.
Contact us for suggestions on how to clean/etch carbon steel.

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.