Pickling Bath for Stainless Steel S32304 (Duplex) 2

[debut78]

Hi there,

I have a question for about pickling bath for a stainless steel duplex! I know that the solutions of nitric (HNO3) and hydrofluoric (HF) acids are used but i have a doubt about the % of each that must be used! i searched the web and i found some information but isn't enough explicit to begin with the process! Also because of the type of steel and the conditions of the bath, is there someone with experience in this process that could help me?

Thanks!

 

[EdStainless]

We are using a standard 20% Nitric 6% HF for almost all SS grades. Usually the temp is between 120F and 140F depending on how aggressive we want to be.
For high Cr alloys that have a very tight scale it helps to pickle in sulfuric/HF first. Typically 15% sulfuric 3% HF would be used at similar temperatures.
The metal will come out black and require a short N-HF pickle to clean it up.

= = = = = = = = = = = = = = = = = = = =
Rust never sleeps
Neither should your protection

http://www.trent-tube.com/contact/Tech_Assist.cfm

 

[debut78]

In this case the % of Cr is between 22-24%. I will use the TIG weld process!
So for this type of SS i should use first the HF & sulfuric (15% & 3%)and then N & HF? But in this way i must use 2 baths?
Thanks

 

[EdStainless]

If you are concerned about production, then look into using two baths. You can still just use Nitric HF, it will just take a bit longer.

I presume that the parts will be annealed in air after welding and prior to pickle.

= = = = = = = = = = = = = = = = = = = =
Rust never sleeps
Neither should your protection

http://www.trent-tube.com/contact/Tech_Assist.cfm

 

[debut78]

The pickling is for parts that will be produce! If with 2 baths we can reduce the time of pickle then its an option! The time here is an important factor! So if using the 2 baths, do you have an idea of the time for each bath considering the temperature of 120F? And for the 2nd bath, do you know which is the % of N and HF?

Yes, after welding the parts they will be annealed in air and then will go after to pickle, but i don't know how much time they will be before the pickling!

Thanks for your help!

 

[EdStainless]

The time will depend on the acid temperature and how tight the scale is. You may need the sulfuric/HF bath hotter than 120F. You are going to want to use some part agitation in order to get uniform pickling. I would look at parts every 10min. I would estimate that you will get good pickling in 30-50 min. You will probably only need 10-15 min in the nitric/HF (20% + 6%). If this gives you too much etching you can lower the temp and/or lower the HF to 4%)
Do you have permits for all three acids?
Do you have a way to test for acid strength?
Scanacon makes testers to do this. You can check both acids and the metals content. You will want to keep the acids and metals in a controlled range to get uniform pickle response.

= = = = = = = = = = = = = = = = = = = =
Rust never sleeps
Neither should your protection

http://www.trent-tube.com/contact/Tech_Assist.cfm

 

[debut78]

For this specific SS (duplex) with 23% of Cr the pickleability is difficult (is 3 being 4 the max).
To get an uniform bath we must use a part agitation to do that, i agree. So overall the time to pickle should be at the maximum around 65min. Its many time to pickle one piece!
I don't have the acids yet! I think when i bought them, will give the permission (i never bought acids)! Also i don't have any way to test the acid strength. That's another problem to solve.
One thing, the % of the acids that you post, doesn't change with the temperature? The temperature only affects the time of pickling? Is that right?

With one bath only (Nitric + HF and some sulfuric) do you know how much time it will take to pickle in the same conditions of temp.? and the % of the acids?

Usually how big are the bath containers?

Thanks!

 

[EdStainless]

By using two baths you can have parts in both at the same time.
If the sulfuric/HF time is 45 min., then you will get a load out of your system every 45 min. once you get running. (the first lot will take 65 min)
If you used just N+HF I would guess that you would have 60-90 min. to pickle. And the parts would not come out as uniform, they would have spots.

We steam heat our tanks. They have poly liners in them, not exactly sure which plastic grade.
Our tanks are large, 50' long and 4,000 gal, just right for tubing.
The work is hung from nylon slings. They hold up fairly well.

= = = = = = = = = = = = = = = = = = = =
Rust never sleeps
Neither should your protection

http://www.trent-tube.com/contact/Tech_Assist.cfm

 

[debut78]

so, using only H+NF the result won't be the same as using 2 baths! that's understandable! I will present the 2 options and then will see!
About the tanks! its huge, indeed! 15m³ it's a lot! with that i surely could pickle almost all the parts only in one time!
I'm talking about parts that they volume its around 0,1 to 0,5m³!

Thanks for your precious help!

 

[EdStainless]

You can buy perforated plastic baskets to hold parts. Then you need a mechanism to agitate them up and down.
We have two rinses after pickle. The first is regular water and the second is DI. Both are hot so that parts dry fast.

Start with small tanks, you can always add more later.

= = = = = = = = = = = = = = = = = = = =
Rust never sleeps
Neither should your protection

http://www.trent-tube.com/contact/Tech_Assist.cfm

 

[MikeHalloran]

You might consider putting the pickling operation in a building or shelter made of plastic structurals. The fumes will eat steel structurals, and hoists...



Mike Halloran
Pembroke Pines, FL, USA

 

[kenvlach]

Mike's comment reminds me of how some large holes (eaten away by acid) in a metal ceiling and walls were once explained to me:
”That's our natural ventilation system.”

debut78, Nitric, hydrochloric and hydrofluoric acids have high vapor pressures, increased by heating, and can be very dangerous to both workers and building + hoist (metal + electrical). HF also has the unusual property of attacking peoples' bones. Sulfuric acid has a low vapor pressure, but an acidic mist is created due to gas bubbles formed on the parts surfaces. Acid fume scrubbers are required. Consult with a chemist or ChemE and an Environmental Health & Safety (EHS). Check with government agencies for necessary permits for air emissions and wastewater treatment, along with worker training & other safety requirements.

Additionally, the acid costs and disposal of metals-contaminated spent acids are considerable, so large operations recycle acids via distillation or acid dialysis.

Maybe EdStainless can gives some practical details on how Trent Tube handles the above issues (BTW, I have family to the east, in north central Racine County).

If you intend large, ongoing operations, have professionals such as SMS Demag design the whole plant.

http://www.sms-demag.at/index.htm

 

[MikeHalloran]

There has to be a market for a small pickling machine, on the scale of your typical salt- spray cabinet, with acid recovery, iron removal, and automatic cycles.



Mike Halloran
Pembroke Pines, FL, USA

 

[unclesyd]

I completely second kenvlach's recommendation on getting someone well versed in the art and science to do the design. A turnkey operation will be the way to go.


EdStainless,
Are you sure that you use nylon straps in the vicinity of the pickling vats. On our plant site the gals can't were skirts with pantyhose as the little NOX in the air at times will cause extreme runs in the hose. Nylon jackets are a no-no also because it's going with one drip from a pipe. It used to be worse before all the vent scrubbers.

 

[kenvlach]

Perhaps polypropylene (reinforced to prevent creep) rather than nylon? I recall having to replace all the nylon inserts in a large lot of SS parts passivated Type II per QQ-P-35C.

I've used vinyl-coated & polypropylene-impregnated SS wire rope for an electropolishing line.

 

[debut78]

Hi again.

I want to ask one more thing:
I want to use a SS duplex piece to an exposition (show it to our costumers) and i wonder if there is any product that can be used to brigth the piece after pickling, to make it more pretty! Something that can be add to the bath or used after the bath process.
Thanks again!

 

[MikeHalloran]

Most 'marine exhaust' shops can mechanically buff duplex to a nice mirror finish. The labor charge will run about an hour per square foot.

Or get it electropolished. It looks pretty zoomy, and way different from a mechanically polished surface. If the part has internal cavities or crevices that don't drain fast, you may get streaks from this process. Or maybe that was just some particular electropolishers..





Mike Halloran
Pembroke Pines, FL, USA

 

[EdStainless]

There are chemical brightening solutions on the market. I can't think of a source right now but I'll check for you.

= = = = = = = = = = = = = = = = = = = =
Still trying to help you stop corrosion.
formerly Trent Tube, now Plymouth Tube
eblessman@plymouth.com
or edstainless@earthlink.net

 

[debut78]

Hi there Ed,

Do you have anything about the brightening solutions? Is there a "solution" to use with the bath? maybe it will be use in the process so if we can make it easy and spend less time the better. Is one operation less. But we want a good finish / bright in the piece, so if you have any source i will be greatful.

Thanks!

 

[EdStainless]

As I recall these are phosphoric/sulfuric acid mixtures. They are similar to the solution used in electro-polishing.

= = = = = = = = = = = = = = = = = = = =
Still trying to help you stop corrosion.
formerly Trent Tube, now Plymouth Tube
eblessman@plymouth.com
or edstainless@earthlink.net