HF nitric pickling: avoid black staining with a smart fix

Summary for hf nitric pickling Discussion

Hydrofluoric nitric acid pickling on precipitation-hardened stainless steel is discussed as a cause of black staining and inconsistent attack. Ridwaan Ali asked about black spots on A286 after pickling in HF/HNO3 followed by rinsing and warm water. James Watts said he had seen similar blackening on work-hardened areas that did not etch the same, and suggested the first question is why the part is being pickled at all. He also warned that higher molybdenum content can drive grain-boundary attack, and that reducing immersion time to the absolute minimum may help. Brian Terry asked for more detail on concentration, cleaning, material removal, immersion time, and the reason for etching, noting that those details are needed to diagnose the problem. James later added that free fluoride should be titrated. Tommy Damborg explained his own process: about 1:10 HF:HNO3 by volume, Turco cleaning, a water-break test, 10 µm removal, and etching for penetrant inspection. A later question asked how to better control nitric-hydrofluoric bath levels for 304 stainless.

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
April 13, 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
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Best Technology Inc.

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Plymouth, Minnesota
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April 23, 2021

⇩ Similar, related, Q&As — oldest first ⇩

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
December 17, 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
December 22, 2010

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

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
December 23, 2010

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

Maintaining acid levels in Nitric-hydrofluoric pickling process

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
April 5, 2019