Stainless Mixed Acid Pickling (Nitric/HF)
Stainless Mixed Acid Pickling (Nitric/HF)
(OP)
I want to model or at least closely approximate the dominant reactions envolved in the mixed acid pickig process (Nitric/HF). In the litreature I found a list of about 25 reactions including al the flouride/metal complexing reactions. Where do I get all the rate constants or how can I approximate them? What is the reaction mchanism of a mixed acid pickling solution?





RE: Stainless Mixed Acid Pickling (Nitric/HF)
Michael McGuire
http://stainlesssteelforengineers.blogspot.com/
RE: Stainless Mixed Acid Pickling (Nitric/HF)
Another version is that the HF is the actual oxidising agent and the Nitric has more of a passivating function.
Can you maybe confirm any of these mechanisms?
RE: Stainless Mixed Acid Pickling (Nitric/HF)
Do you want another twist, in high Cr alloys you get better pickle in Sulfuric/HF, but in order to get a clean surface you have to go into Nitric/HF.
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Plymouth Tube
RE: Stainless Mixed Acid Pickling (Nitric/HF)
RE: Stainless Mixed Acid Pickling (Nitric/HF)
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Plymouth Tube
RE: Stainless Mixed Acid Pickling (Nitric/HF)
It's maybe a little off the theme of this thread but here goes. I'm having a problem with grey discoloration on my stainless steel screens after pickling. I have never experienced this problem before but now there are a couple variables.
Previously we have made our wedgewire screens using (for example) a SS304 wire, diameter 4.3 mm, hardness before profiling 110 kg/mm2. After profiling into a triangular shape and then resistance welding it into a screen it's cleaned (detergent) and then pickled/passivated in Hydroflouric Acia / Nitric Acid solution for approx 15 minutes. The weld scales dissolve but no apparent greying of the stainless steel occurs. It keeps it's semi gloss finish.
Now we are using an SS304L (lower carbon) and adding an intra anealing step after the first profiling. (complete profiling to triangular shape requires 4 steps). The reason for the anealing is to reduce the levels of ferrite caused during the profiling.
Using the same pickling solution for only 5 minutes the result is a horrible grey, uneven discoloration.
Please note the surface of the profiled wire is a little rougher probably due to the softness and, of course, the wire is softer. (and it's 304L)
Any help would be appreciated. I've got a huge order to produce and all the raw mats purchased but I can't send like this.
RE: Stainless Mixed Acid Pickling (Nitric/HF)
I need some more info. Is the annealing of your 304 L carried out in an lectric or gas furnace? Is the surface oxidised when it exits the annealing furnace? If that is the case, you removed the passive layer of Chrome oxide while annealing and why the pickling process is that much faster.
In the first instance with plain 304, the steel structure, except the weld, were still passified from the pickling process. The mixed acid thus attacked the oxidation in the vacinity of the weld, but the general steel surface was only mnimally attacked due to the passivating oxide layer.
I am not a metallurgist, but do you really need the second annealing step?
RE: Stainless Mixed Acid Pickling (Nitric/HF)
My hunch is that you have some surface contamination going into the anneal. This gives you some sensitization that then pickles faster in the final form.
Do you really need the final wire non-magnetic? Why?
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Plymouth Tube
RE: Stainless Mixed Acid Pickling (Nitric/HF)
The SS304L is required by the specs of the tender, it's not feasible to go higher nickel. No one would purchase from me.
Allowable ferrite reading (using a Ferritescope) is 5% on the wire surface. I can't get this low a reading unless I anneal after the first profiling step.
The annealing is done at an outside supplier so I'm sending some wire to a second annealer to see if there is any difference in the results. I'm also thinking there may be contamination on the surface but the annealing facility assures me this is not the case. They are annealing with an inline furnace, controlled environment with cracked amonia.
I'm not sure of the concentration of my pickling solution, we've never had any problems before so we've always just topped it up depending upon the results. I use the same solution tank with a harder SS304 profiled wire and with a non annealed 304L wire from the same supplier. No noticeable color change. I'm leaning towards soft, rough surface reacting more easily with the pickling solution or contamination in the annealing. If it's contamination in the annealing I'm not sure how to test for this, nor am I sure how to rectify the situation.
RE: Stainless Mixed Acid Pickling (Nitric/HF)
Michael McGuire
http://stainlesssteelforengineers.blogspot.com/
RE: Stainless Mixed Acid Pickling (Nitric/HF)
Is there some way I can get rid of the oxidation before the pickling step. I've found that if I polish the screen with an abrasive, no color change occurs if I pickle it later. This however, is forbidden by the inspectors as they believe it will hide cracks in the surface of the wire.
I'm beginning to lean towards the annealing as being the culprit but I know there has to be a way that the two processes can co-exist. These specs have been around longer than I have.
RE: Stainless Mixed Acid Pickling (Nitric/HF)
If the weld oxide is an issue, pickle first and then anneal in dissociated ammonia, then use a simple nitric ( no HF ) post anneal rinse. This is what stainless producers do after bright anneal.
Michael McGuire
http://stainlesssteelforengineers.blogspot.com/
RE: Stainless Mixed Acid Pickling (Nitric/HF)
RE: Stainless Mixed Acid Pickling (Nitric/HF)
I see two issues.
One is residual oil on the surface prior to anneal. You must clean better.
Two is that DA is not a good enough atmosphere. You want pure, dry hydrogen.
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Plymouth Tube
RE: Stainless Mixed Acid Pickling (Nitric/HF)
Thanks again for the help!
RE: Stainless Mixed Acid Pickling (Nitric/HF)
Chrome carbides form in the grain boundaries sucking Cr out of the near material significantly reducing it corrosion resistance.
Do you use a hot alkaline for cleaning? That is one of the best ways.
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Plymouth Tube
RE: Stainless Mixed Acid Pickling (Nitric/HF)
RE: Stainless Mixed Acid Pickling (Nitric/HF)
http://www.pfonline.com/articles/119908.html
Regards,
Cory
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RE: Stainless Mixed Acid Pickling (Nitric/HF)
Just when I thought all was well the grey wire reared it's ugly head again. Problem, the cleaning process is not sufficient, still some hydraulic oil left on the surface when it's sent for annealing. I need a good solvent that can be used inline. Any suggestions?
For reference we are taking a SS304L cold drawn wire and profiling it to a triangular shape. Lubricant/coolant used for the forming is simply an hydraulic oil. The size of the profile that needs cleaning is approx 2.5mm x 3.0mm. I'd like to have inline running through a bath as opposed to putting a complete coil in a large tank. Each coil is quite long and around 200 kgs.
Thanks, Kirk
RE: Stainless Mixed Acid Pickling (Nitric/HF)
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Plymouth Tube