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?
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Stainless Mixed Acid Pickling (Nitric/HF) 2
- Thread starter ChemBros
- Start date
You've asked one of the tougher questions for materials guys. Pickling has mostly beena black art in most steel mills. I can only contribute that the HF is consumed as an iron complex and precipitates.
Michael McGuire
Michael McGuire
- Thread starter
- #3
Do you have any idea what the reason is for the mixed acid? In my opinion, the Nitric is a strong oxidizing acid and actually attacks the metal oxides. These metal oxides are now in solution. In order to maintain the forward reaction rate, the metal oxides are removed by complexing with the fluoride anions.
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?
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?
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- #4
EdStainless
Materials
The Nitric alone will do almost nothing. This is all about solubilities and equilibrium. Consider that a bath with some metals in it (2-4%) will pickle faster than a fresh charge of acid.
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
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
- Thread starter
- #5
ok, when modelling this pickling process I need reaction kinetic data. My thermodynamics is very rusty, how would I estimate the activation energy of a specific reaction to use in an Arrhenius equation? Is my thought direction correct? What else can I do to model the reaction? is there any info available on mixed acid pickling of Stainless? I am struggelig to find any on the net.
EdStainless
Materials
You may need a better search system than the internet. I would wager that there have been a number of PhD thesis written on the topic. Try some major university library systems.
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Plymouth Tube
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Plymouth Tube
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- #7
nottoobright
Industrial
Hi All
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.
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.
- Thread starter
- #8
In my experience there is no difference between the pickle characteristics of 304 and 304L.
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?
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?
EdStainless
Materials
If "not too bright" really needs the lower ferrite (it is probably strain transformation martensite and not ferrite) in the finished wire then he really has no option. The only other way to do this is to go to a grade with higher Ni (ouch).
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
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
nottoobright
Industrial
Good to hear from you EdStainless, and you also Chembros.
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.
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.
The annealing step you added is the culprit. Without anneal the stainless is not attacked by the pickling acid, except at the very small resistance weld zone. But if you anneal, then the entire surface becomes oxidized. This causes the entire surface to roughen from the acid attack. Note that the underlying reason pickling works is that oxide formation depletes the surface of chromium allowing it to be dissolved. Get rid of the anneal.
Michael McGuire
Michael McGuire
nottoobright
Industrial
Thanks Mcguire, BUT, I can't get rid of the annealing step. Without it my ferrite readings go to 10%++. When the inspectors visit the factory they'll make a big pile of scrap, and I mean a really big pile. I need to anneal and pickle.
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.
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.
Let me add something. If the dissociated ammonia is not causing oxidation of the surface then there is no need for pickling. The prior weld oxide should be reduced. But, I really think that the furnace atmosphere is insufficient to avoid oxidation and the etching that results after the ensuing pickle.
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
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
nottoobright
Industrial
After annealing we then profile the wire in 3 more steps using profiling wheels. After that the wire is wrapped and resistance welded into a screen. The resistance welding has a fairly high temperature which causes some discoloration, scaling. This is why I need to pickle as well as to get rid of any mild steel contaminants that we might have picked up along the manufacturing process. Ie; the forming wheels, straightening machinery.
EdStainless
Materials
You are cold forming the wire and not shaping it? I thought that everyone had gone to using narrow slit strip from sheet and cutting with shaping tools for this application.
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
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
nottoobright
Industrial
Ed Stainless (and others) you hit the nail on the head! When cold forming the wire we use a hydraulic oil for lubrication. This was not properly cleaned before the annealing and this caused some sort of reaction between the stainless steel and the oil. I've annealed a clean batch and no problem. Anyone who can give me a chemical equation for the reaction that occured?
Thanks again for the help!
Thanks again for the help!
EdStainless
Materials
The oil breaks down in the anneal and the carbon in the oil reacts with the SS. This is classical sensitization.
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
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
nottoobright
Industrial
Thanks EdStainless! Can you give me any more details on the hot alkaline cleaning? We're running the wire through some solvents now but I still worry about small amounts of contamination.
Alkaline Cleaning Guide from Products Finishing Online:
Regards,
Cory
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.
Regards,
Cory
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.
nottoobright
Industrial
Hello Everyone
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
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
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