Why do we use 15-18% HCl for pickling

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,

Thanks Mark for your answer.

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

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.

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.

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.
Regards,

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.

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.

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.

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...

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.

Regards,

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.

Regards,

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.

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

Regards,

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
http://bfksolutions.com/toxic-baby-burp/
which I lifted some of the above text from.

A. Hi Mr. Lamvo. I am not familiar with the distribution channels in Vietnam, and we don't print commercial responses in this forum (for one thing, it leads to unmanageable spam), but any of the worldwide plating supply companies like Chemetall, Macdermid, Atotech, Enthone, or Henkel can probably help if a local plating supply distributor can't. Good luck.

I think you should probably have a local fume extraction system (exhaust hoods) and fume scrubber as well as the inhibitor.

Regards,

A. The percentage for 15 to 18% is due to the resulting concentration after regenerating or recovering acid from the pickling process in another process with a fluidized bed regeneration plant or another process. This percent is enough in order to get the result for removing the oxide on the layer. The temperature used should not be greater than 88 °C. If you use a greater concentration with the same temperature, you will face HCl loss by evaporation immediately.

A. Hi Doshi,
If you can't get Dr. Cook's article in the August 2000 issue of Metal Finishing magazine (or at https://www.sciencedirect.com/sdfe/pdf/download/eid/1-s2.0-S0026057600827257/first-page-pdf), you can Google for Rodine, Akzo Armohib (maybe Armohib 28), Columbia Pickle Pal, RI-inhib, Activol 1803, and Stannine LTP for further info. I'm sure there are several other suppliers of pickling inhibitors. Good luck.

Regards,

A. Hi Shishir! I think this type of counter current can make good results, but I suggest that you, in order to maximize your pickling line productivity, check:

1- Bath temperature: Below 18 °C you will experience reductions of the reaction speed. We have teflon heat exchangers (steam heating system) for that (we have today 4 °C outside so it's a real problem!).

2- Iron content/Concentration: I think all these trials are already made and their results are available in charts. You can check your results with the Kleingarn curve (check https://www.finishing.com/library/campano/kleingarn.pdf). If you have some doubts and an available tank and all you need for your upscaling trial, go ahead!

3- Gap between coils: One of the most difficult problems to solve is the rust between coils. You need to move the coils to make acid clean all the gaps between them, and/or get good wetting agents to improve this.

I think those lines of work, and how you manage to get your best parameters in every one of them, will get you where you want to go in production and quality. They are my three pillars in pickling for our wire process :)

Best of luck! Tell us if you have some interesting results there!

A. Hi Shishir!

I would recycle spent or acid from the previous tanks to increase iron content. You're worried about other heavy metals, are they alloy elements? If the answer is yes, I wouldn't worry about them.

I would worry if you have large quantities of zinc, if you are doing some rework of galvanized wire, but if you don't I would make this spent acid recycling to increase pickling speed.

About pickling speed around 35-40 °C, they are obviously higher than at room temperature, but optimal concentrations don't change too much. They change with solution activity and I would take this curve as valid with a small correction of speed because of the change in temperature.

This values and curves are measured in laboratory, with controlled solutions and so, and your industrial pickling line, I suppose, has many many more variables in it. Don't take the curve or any other book value as written in stone. You always need to measure what you get and make some experiments in your line to get your optimum parameters, as there are variables not taken into account in these charts.

Sorry if there's something not clearly formulated! It's a complex subject and my technical english isn't good enough!

Best of luck!!

 Hi Daniel

Thanks a lot for your advise. I am now engineering the recycling option.
Yes all other heavy elements are alloy metals. Cr, Ni and Zn are prominent. We don't rework galvanised products. So it looks like a good solution to recycle a stream from higher Fe content tanks.

I shall keep you update with the results.
Thanks again.

Shishir

A. Hi Vinay. That's the title and the subject of this page. Please try to digest it and request clarification of any points you didn't understand. And please explain your situation -- have you never done zinc plating before and are in the startup phase, or are you already doing zinc plating and you wish to change the percentage of HCl you are using for some hopefully interesting reason? You say "pickling of metal", but be aware that it can depend on what kind of metal you are pickling or acid dipping. What inhibitor are you using/planning to use? Thanks!

For a deeper understanding you may want to try the search term 'Kleingarn' on this site or elsewhere.

Regards,

A. Hi Shishir!

Scale conditioning bath with KMnO4 and NaOH hasn't so many alternatives. I would try to make this scale removable in HCl, or make the HCl solution more aggressive in order to do that.

We had some problems with some steel grades, but usually we can process without any problem with HCl pickling with some acid degreaser and wetting agents.

Is this an option? If not, you don't have (as my knowledge let's me see) option for this kind of bath.

Best of luck!