Pickling stains
Pickling stains
(OP)
Hello all,
we are performing pickling on greek ascolloy castings and on some parts on some batches we got stains, 3-4 hundrets of milimeter high, very difficult to remove. Is there any idea what it came from, what is the problem?
Thanks
we are performing pickling on greek ascolloy castings and on some parts on some batches we got stains, 3-4 hundrets of milimeter high, very difficult to remove. Is there any idea what it came from, what is the problem?
Thanks





RE: Pickling stains
RE: Pickling stains
RE: Pickling stains
RE: Pickling stains
There is some usually some very fine metal residue (sub-micron in size and black) that gets redeposited on the surface during pickle. If it isn't forcefully rinsed off before it drys it will not come off.
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Plymouth Tube
RE: Pickling stains
While awaiting your report of the concentrations of HF/HNO3 a lot of times the problem can be related to the following. If you get too aggressive, high HF ratio, you will have fast times along with more attack on the base metal. A higher HNO3 ration will take longer but will leave the metal brighter.
Is the part a high value added piece that has to look good?
RE: Pickling stains
RE: Pickling stains
HNO3 concentration is arround 103g/dm3
HF is arround 2.7g/dm3.
part has to look good.
RE: Pickling stains
Better agitation in pickle, and a high pressure hot water or steam blasting after a dip rinse.
Often you are better off pickling, rinsing, and back into pickle. This will minimize the surface residue.
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Plymouth Tube
RE: Pickling stains
RE: Pickling stains
RE: Pickling stains
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Plymouth Tube
RE: Pickling stains
RE: Pickling stains
These are additional steps in your pickling line and if you have some smaller pieces, like test bars you can test these options out.
The following contain very hazardous chemical so take every precaution while using all personal protective gear.
Immediately after the part is removed from the pickling solution rinse dip it in concentrated HNO3 at room temperature for one or two seconds, remove and rinse. Again if possible test this on a small scale. Ventilation is required as this process may give off copious amounts of NOX fumes.
This is pretreatment is commonly used for scale resulting from high temperature.
This is called an alkaline permanganate bath. Use a solution of 15% by weight of NaOH + 3% KMnO4 by weight. Keeping the solution above 90 degrees C soak for 2 hours. Remove, rinse and start the pickling process. Watch your part in the pickling bath as the times can be much shorter.
One thing that is very important in making this solution is to add the NaOH very slowly to cold water while stirring.
I want to emphasize that you don't attempt these processes if you don't have expertise in handling very hazardous chemicals.
Things to ponder.
If you are near any facility that has molten salt baths it would pay to discuss your cleaning problem with them. I would especially look at the Virgo type salt baths for your material. I would contact Heatbath and see if there is any facilities near you or within trucking range.
RE: Pickling stains
RE: Pickling stains
I may worry too much about this but I've never just told one to go mix up this and that as I've seen too many near hits along with a few hits. I don't exclude anyone even new college graduates who missed general chemistry 101, handling of chemicals and concentrations.
i don't like wash pot chemistry on any scale.
Addendum:
I have told this story many times about one prat our maintenance department cleans parts by soak cleaning in an alkaline bath. i had it setup where there solution was made up by adding two SS buckets of cleaner to the large tank and one to little tank. Safety audit said the the material should be a recipe on the wall weighed so a very high dollar digital scale was added to cleaning room. On every overhaul of a process line I would check on the operation of the leaning. While I was observing the process one of the operators was making a new batch of cleaner for the small tank. He got the bucket of cleaner and put it one the scale took it off and added it to tank. The only problem was there was on weight given and when I asked about it I was told that the scale broke about the second day, but they had still been passing the buckets over the scale.