Pickling of Steel Q&A's

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

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.

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 in dissolving steel instead of rust. Using an inhibitor is how you deter the iron content from rapidly building up. Good luck.

Regards,

A. Hi Abbay. Do you feel that this decline is due to less steel production, less pickling of the steel, or a switch to substitute pickling agents? Please share what you believe you know on this subject.

My experience on this site is that if you tell people what you think you learned and where you learned it, they will usually be more willing to comment on it.

Thanks!

Regards,

I believe it is due to greater regeneration and use of spent acid. Do you think it is causing a decline in the use of virgin HCl for steel pickling? Also, are there any new/recent alternatives to HCl that are gaining significance in steel pickling?

A. Hi again. I (and most of our readers) am from the metal finishing industry rather than the steel industry. My direct experience in steel pickling is rather limited, but I think that a couple of decades ago a strong movement from hot sulfuric acid to cold hydrochloric acid took place and that people are not going back ... and I know of no 3rd choice. I suppose it's possible that scale-breaking machines have improved, allowing better mechanical removal, which reduces the need for chemical removal though.

I also believe your hypothesis that regeneration and better acid management science has reduced the need for virgin material, and suggest that you contact companies like Scanacon for the perspective they may be able to offer on this factor. Good luck.

Regards,

A. Hi Gee. "EPC" = Engineering, procurement and construction? It sounds like there is or will be a plant with a lot of stainless steel piping that must be pickled, passivated, or in some way cleaned & prepared for service? Your client is certainly wise to want to see the effect of the procedure on a sample of the pipe before exposing all of the pipe in the plant to an untested procedure :-)

But there are specialist firms which travel from plant to plant to undertake this cleaning/pickling/passivation before commissioning and it might be a good idea to retain one rather than hazarding the plant to inexperienced hands. I don't personally know the procedures though.

Regards,

A. Hi Dan. Certainly acids and lubricants can change the surface color, but no, they do not penetrate the steel (I'm not sure if I understood the question).

Regards,

I run a pickle line that uses hydrochloric acid to pickle. I would be happy to answer any questions on the process as I understand it. Our line was manufactured in the 60's or 70's but we do have some modern upgrades. I would like to compare our process with other HCl pickle lines that are more modern.

A. Hi Harshal. I think you should buy pickled and oiled steel rather than trying to start with steel which has hot rolling scale on it. Steel mills usually include mechanical scale-breaking operations before pickling, and it may not be practical to remove such scale by chemical means alone regardless of which acid you use :-(

Good luck.

Regards,

A. Harshal, on the pickle line at my plant we use a flattener to break mill scale and correct poor shape. If the guage is under .170, we also use a tension leveler. The HCl alone will not remove all mill scale. Our flattener fractures it so the acid can get underneath. The main purpose of the HCl is rust and carbon removal.

A. Hi Steve. The "oil" would most likely be a hydrocarbon oil, and oil is poisonous. But I don't see any reason you couldn't patiently remove the oil with hot water, detergent, pumice ⇦ this on eBay or Amazon [affil links] , and a scrub brush ⇦ this on eBay or Amazon [affil links] .

When it is free of oil it will be "waterbreak-free", i.e., you'll see a smooth layer of water over the whole piece, with no beading or dry areas. After cleaning and testing for a waterbreak-free surface, you can oil it with mineral oil.

Of course, going to a plating shop and getting the piece nickel plated or tin plated would be even better^^{possible as well} :-)

Regards,

A. Good day Steve.

Ted has given very good advice on cleaning the steel, but I disagree with the nickel plate, as I think it will eventually wear/flake off based on the baking/temperature/use cycling. Why not "season" the steel as you would a cast iron frying pan?
Buon appetito!

Regards,

Hi Eric. On second thought, I'd agree that seasoned steel would probably be all around nicer than nickel plating.

But nickel plating will never flake off in an oven if done properly. Continuous casting molds for red-hot steel are nickel plated, and flaking of the plating would cause a seizing failure that can be a dangerous calamity costing hundreds of thousands of dollars, so they make sure it doesn't happen :-)

Regards,

Good day Ted. It pleases me to know that I have my head screwed on! Yes, you are correct regarding adhesion, if done correctly. I have processed SS automotive bulb shields with Wood's Ni, Semi Ni, bright Ni, microporous Ni, and chrome. Adhesion was always an issue with 55 ft2 work load, loose contacts, etc. We baked 100% @ 650 °F for three hours to check for blisters/ adhesion, chrome show, etc. It is of my opinion that Ni for oven racks is suitable, because the food for baking is not in direct contact with the grill. But the pizza would be. Good idea? Happy Holidays and best wishes for the New Year! Regards, Eric Bogner, Lab. Tech Aerotek Mfg. Ltd. - Whitby, Ontario, Canada Hi Eric. Merry Christmas. It's harder to get good adhesion on stainless than on steel, but Dini's "Electrodeposition" shows some fabulous adhesion even on stainless. Although I designed & started up many hundreds of plating lines, and probably visited well over a thousand plating shops over the decades, I never personally plated anything beyond pennies & keys for kids' science classes :-) I'm pretty much limited to book knowledge on plating processes :-) Regards, Ted Mooney, P.E. RET Striving to live Aloha finishing.com - Pine Beach, New Jersey

A. Hi Arijit. In a metal finishing shop, degreasing always precedes acid activation (assuming sandblasting is not done as a substitute for both). The reason is simply that the parts can be expected to be dirty and oily, and acid immersion does not clean them properly.

When you speak of 20 to 40 minutes pickling time, it sounds like you are talking about scale removal at the steel mill (which I'm not very familiar with) rather than removal of light rust, and acid activation in a plating, galvanizing, or metal finishing shop because your immersion times are 10 to 50X as long as I'm accustomed to.

But if you are free to vary your pickling time, it seems that starting with a short immersion time in strong acid, then lengthening the immersion time as you dilute the acid through use would make the most sense. I'm not aware of subtle deleterious effects from too short or too long immersion times as long as the pickling is sufficient and there is no etching/pitting.

Regards,

Thought for the day: Rule #7, Give Back to Society -- Ratan Tata

A. Hello Mr Arijit,

Why degreasing? Because acid does not dissolve grease or oils, and they (oils and greases) BLOCK the acid chemical process in the parts we put in there. They act as masking agents, so we could get some black spots in the next process because the pickling process was not rightly done, the metal was not activated, etc.

We do not know which process you do to the metal sheets, but I assume it is phosphatizing or galvanizing/electrodeposition. All need pickling to remove scales from the first rolling, and if the rollers were lubricated, you need to remove the oils and greases.

Hope I answered your question!

Best regards,

Daniel

A. We always use DI water in the final rinse. It is important that the chloride concentration is less than 5 mg/L to prevent flash rusting.

A. Hi William. Pickled & oiled steel can certainly rust and corrode. Whether it can "re-scale" is a semantics question -- I would not call it scale no matter how corroded it got, because I personally reserve that term for what happens when red hot steel is exposed to the atmosphere -- although I suppose that others might use the word in other senses. But please don't keep it an abstract question -- tell us your situation and what happened or what are actually concerned about. Thanks.

Regards,

A. Hi Durai. 7.3 times :-)

Please take a minute to read my request, immediately above, about abstract questions being unanswerable, and tell us your own actual situation. Thanks!

Regards,

A. Hi James. Yes it is really necessary. Myself and Daniel already offered our answers to that question on this page. Please refer to them and expound on your question if your think our answers didn't quite cover your situation. Thanks.

Regards,

A. Hi again James. Scrub a few with a pumice solution and a tampico brush (or toothbrush depending on size of the parts) and let us know what happens when you pickle them.

In general though, I don't think I'd be surprised to see carbon spots on cast iron after pickling.

Regards,

A. Hello James!

I think you can try the following:

- Take one or two already shot blasted parts.
- Degrease them with some solvent (acetone, MEK, or anything like that).
- Pickle them with the process you already are using for every part.
- Inspect them for black spots.

If the spots disappear, you have grease or oil in the pores of the material, I think you need some degreasing agent between shot peening and pickling... IF you NEED to do acid pickling (if you don't, don't pickle!)

If the spots don't disappear, grease is not the problem. Maybe, as Ted said, you have some carbon deposition in the surface (laminar or nodular graphite) and acid is making them appear, but they would be very very small, so I think the most possible cause is grease in the surface.

Sorry for repeating myself, but: If you don't need to do acid pickle, don't do it. Shot blasting does a very good job cleaning the surface, and pickling activates the surface so rust appears very fast.

Regards, and hope you can solve this issue!

A. Hello again James!

Please tell us which kind of stainless steel (AISI grade, or something like that) is the one with problems. And if you have the same problem with another steel parts.

How is the process made? Pickling, rinsed and dried? Or is another process made after the pickling?

Hope we can solve this!

Regards,

A. Hi James,

When you say Acid Pickling, which formulation do you use? Acid pickling is, for most cases in carbon steel, just hydrochloric acid in water. But for SUS304, you must use other formulations, more aggressive and oxidizing than hydrochloric.

And, can you take a photograph and show the black spots?

Thank you! Regards,

A. James,
Steel shot blasting is bad for stainless. Use stainless shot or non-ferrous media like glass beads.

Daniel Montañés is right about not using hydrochloric, that is very bad for stainless. Stainless pickling is generally a nitric/hydrofluoric mix.

A. Hi James! Long time no see! I was in a trip with almost no time to check finishing.com Q&As as usual :P

I suppose you've read ASTM A380 thoroughly, if not, I would.

I can relate some of this issues, and I would look for:

- Depleted acid bath (iron concentration in pickling bath?)
- The first image looks like smut, if it can't be removed by a pickling bath, it can be removed by hot alkaline potassium permanganate ⇦ this on eBay or Amazon [affil links] , it is risky and adds a step, but works wonderfully with smut.
- See if you have free iron in a part you have already rinsed before and after pickling (ferrocyanide or copper sulphate ⇦ this on eBay or Amazon [affil links] method).

So, I'm sure you are squeezing your brains to solve this, I just hope I can help you from here!

Regards,

A. Hi wagar. There are more economical reagents to neutralize HCl, including lime or magnesium hydroxide; both are also safer than caustic, but each has limitations which must be worked around.

There are also ways to extend the life of the HCl bath, including better inhibitors (so less iron is consumed), and "acid extenders" (which precipitate the iron so the bath does not need to be dumped).

Regards,

A. Hi Wagar. Sorry, although I would expect that the Rodine continues to provide useful inhibition, so acid isn't wasted on raw steel, I don't really know.

In as much as both the Rodine and the acid life extender are secret proprietaries, it may be hard for anyone but the supplier to offer answers about their compatibility and symbiotic effects. I'd suggest that you ask the manufacturer of the acid extender. Good luck.

Regards,

A. There are several acid purification technology that can reduce the amount of caustic needed by 75-90%. You may want to investigate diffusion dialysis, acid retardation, and evaporation as a solution to this problem.

A. Hi Anna

Before asking for outside help you should always visit the tank and view the problem for yourself.
Even experienced operators can give a misleading description of a problem and you may spot some other factor that he did not think important.

For example...

Are you sure that the tank is thoroughly stirred before taking your sample for analysis? Witness it done or do it yourself.

A. Thanks for the kind appraisal, Shauna. I don't know of such a bible although another reader might suggest one. But I will mention 4 points which I think people should not miss for a general understanding of the pickling of steel ...

1. In steel mills mechanical "scale breakers" are used before pickling to reduce the amount of scale/rust which must be dissolved in the acid and the time required. I've never seen such a device in a plating shop, but sand blasting is sometimes used for mechanical removal of rust.
2. Using inhibitors rather than straight HCl is vital. We want the acid to attack rust, but we don't want to waste it dissolving steel. I'm not a pickling chemist, but I understand that the principle is that these organic materials bind to bare steel surfaces helping shield them from acid attack.
3. There are products called "acid extenders" which can precipitate the iron or other metal out of the solution, reputedly greatly extending the life of the pickling solution.
4. The Kleingarn chart/principle which reveals that sometimes even just adding water can speed up pickling or extend the bath's life as it becomes saturated with metal. Barlow R. Campano has been generous enough to supply an excellent on-line article about it.

Happy studies, and thanks again.

Regards,

A. Hi Modibedi. The short answer is that iron readily dissolves in HCl, so you should expect a pretty quick buildup of iron. But if two baths have been operating for the same amount of time and the same amount of production, and one has a lot more iron than the other, its inhibitors or acid extenders are apparently not working as well.

If you're using lined steel tanks, maybe there's damage to the lining?

Regards,