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topic 56364

Hydrofluoric / Nitric acid solution for pickling PH 15-7 Mo stainless steel

Current question and answers:

April 13, 2021

Q. Hello fellows,

I am facing a problem of black staining on A286 (precipitation hardening) stainless steel after pickling. Pickling is being performed in mix acid of HF and HNO3 followed by rinsing and warm water splashing. But the black spots or stains appear on the surface. Does anybody have seen the same problem or any suggestion?

Kind Regards,
Ridwaan

Ridwaan Ali
- Karachi, Pakistan
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April 23, 2021

A. Hello Radwaan, I have seen this issue before when pickling different materials and traced it back to work hardened area that happened during the machining process but wouldn't show up until after pickling because work hardened area doesn't etch the same and generally turns black from my experience.

mark battles
Mark Battles
Best Technology Inc.
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Plymouth, Minnesota
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Previous closely related Q&A's starting in:

December 17, 2010

Q. Hi,
I need a pickling bath consisting of hydrofluoric acid and Nitric acid for PH 15-7 Mo steel. Do any of you have any experience with this solution for etching steel? I have done some experiments with the solution but have some problems with Intergranular Attack in my samples.
Hope you can help, Thanks

Tommy_Damborg Jensen
Engineer, Chemical - Grenaa, Denmark
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simultaneous December 22, 2010

The first question is why are you pickling this part?
Many years ago I tried to do some analysis on 718SS but was not good enough or did not have the right equipment to be able to positively find the depth.
Did some activation of 17-4PH without any long term problem, but the much higher Mo content will almost certainly drive the attack at the grain boundaries.
Just a wild guess, based on Ti that increasing the nitric:HF ratio will help some.
The most obvious is to reduce the time in the solution to an absolute minimum.
It will be interesting to see what the real metallurgists have to say.

James Watts
- Navarre, Florida
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December 22, 2010

A. Tommy,

Can you explain your situation with a little more detail. At the moment all we have is an abstract idea of etching using a HF/Nitric acid solution.

What concentration limits are you using?
What are your pre-cleaning steps?
How much material are you trying to remove?
How long is your immersion time? Do you know how long the parts are in the bath before they start to show inter-granular attack?
Why are you etching? Is it to remove bulk material or to remove a contaminant such as heat treatment scale?

With this sort of information I'm sure someone can provide you with a solution to your problem.

Brian Terry
Aerospace - Yeovil, Somerset, UK
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December 23, 2010

A. Once you sort out what Brian said, remember that you will need to titrate for free fluoride. It is not terrible, but it takes a bit of learning and can be frustrating.

James Watts
- Navarre, Florida
^- Reply to this post -^


January 4, 2011

Q. Hi Brian,

Here are the answers to your questions:
- The concentration limits are not certain but the experiments I have made were done at a 1:10 ration for HF:HNO3 %vol. So I have tried different concentrations at this ratio.
- Pretreatment of the samples are cleaning in a Turco soap to insure that the samples are clean. A water break test is made to insure the test pieces are clean.
- The amount of material to be removed are 10 um
- The immersion time is being determined by an etch rate test of the bath.
- The etch is done as to prepare the parts for penetrant inspection.

Thanks for the responses.

Tommy_Damborg Jensen [returning]
Engineer, Chemical - Grenaa, Denmark
^- Reply to this post -^



Maintaining acid levels in Nitric-hydrofluoric pickling process

April 5, 2019

Q. We pickle 304 stainless in a solution that is Nitric and HF. Our targets are 15%-20% Nitric and 1%-8% HF.

We are looking to get control of the acid levels by type so that we can establish more consistent dunk times and plan more effectively. We currently measure and use Baumé to determine where the acid level is. From there we make additions of Ammonium Bifluoride and Nitric. The method for choosing how much is essentially undefined.

I am looking for a better measurement method and a scientifically grounded process for maintaining the acid levels.

Andy Lemke
- Seattle, Washington
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