Pickling stains

[AleksandarLPO]

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

 

[cloa]

How did you pickle?

 

[AleksandarLPO]

Well, after sandblast, degrease in trichloroethylene, then cleaning in Turco and finally pickle in HNO3+HF bath.

 

[cloa]

What concentrations?

 

[EdStainless]

My guess would be either some areas didn't pickle well (not enough agitation?) or they weren't rinsed well enough.
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

 

[unclesyd]

Adding to the post by EdStainless wet stream is one of the best approaches to remove residue after pickling. As stated you cannot allow the part to dry.
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?

 

[blacksmith37]

Haven't heard that alloy name in 50 yr. At that time we used glass bead blasting to remove descoloration/oxides from investment castings (following a molten caustic dip ).

 

[AleksandarLPO]

Thanks for help and answers.

HNO3 concentration is arround 103g/dm3
HF is arround 2.7g/dm3.

part has to look good.

 

[EdStainless]

And I presume that your pickle is hot?
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

 

[AleksandarLPO]

No pickle is room temperature, alkaline cleaner is hot

 

[cloa]

I think Edstainless is really asking how hot? How agitated?

 

[EdStainless]

Are you sure that they are pickle stains and not simply residual oxide that didn't pickle?

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Plymouth Tube

 

[AleksandarLPO]

Well, I am not sure.

 

[unclesyd]

Again if this a very valued part and you have the facilities along with an expert knowledge of handling harsh chemicals here are several things to try.
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.

 

[cloa]

He is already handling pickling solution containing hydrofluoric acid which is of course extremely hazardous.

 

[unclesyd]

There is big difference between handling chemicals on routines basis where there is definate protocol than handling them on a one off basis. I setup a couple of cleaning stations using Alkaline permanganate and HNO3/HF that with a few simple instructions almost anyone can operate it. I would not put any of the operators of this process out there with a drum of NaOH, a drum of KMnO4 and tell them to give me certain mixture. The pickling procedure we use routinely is setup where the mixture is controlled by using 3 bottles of this and one bottle of that. The concentration is controlled by an overflow. When the water is turned on there is three minute delay and the pump comes on. One safeguard on the system is when the NaOH is added there is basket in top of holding tank to prevent the NaOH from accumulating and causing a steam blow back.

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.