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Titanium chemistry in HF and HNO3 chemical milling

Q. To effectively evaluate potential titanium removal techniques from a HF/HNO3 chemical milling operation, I am looking to obtain information about the basic titanium chemistry that occurs. Any feedback would be most appreciated.

Richard Poduska
consultant - Rochester, New York
November 20, 2008

A. The etching chemistry is rather straightforward. The HF is used to dissolve the titanium for form titanium fluoride. The nitric acid is used to dissolve the vanadium and aluminum found in most alloys.

The dissolved titanium is difficult to remove. I am currently seeking a patent on a process to economically remove the titanium to extend bath life nearly indefinitely.

Best of Luck.

Ira Donovan, M.S.F.
Kansas City, Missouri
November 25, 2008

A. The etching chemistry is NOT rather straightforward.

The process of dissolving of Ti in HF/HNO3 mixture depend on changes in concentration of HF and HNO3, and the extent of the depletion of the solution, and the temperature: depending on these conditions etching process may be in active or passive stage. In the first case - the process is accompanied by gaseous hydrogen embrittlement, in the second - titanium dissolves in the passive state, with good surface strength. It is possible to distinguish the two types of processes and prevent titanium surface damage by control of the potential of titanium electrode on an appropriate model. We always put such controls.

Anna Berkovich
Russamer Lab
supporting advertiser
Pittsburgh, Pennsylvania
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December 1, 2008

A. I must side with Anna.

Not too long ago, I was involved in a project that entailed a nitric/HF mixture used for removing "alpha case" from Ti alloys after heat treating. As I recall, the ratio of the two acids was more critical than the absolute concentration of either.

I believe that the Ti is present in the spent acid mixture as hexafluorotitanate ion. Titanium is an odd beast in aqueous solution; it acts more like a non metal.

dave wichern
Dave Wichern
Consultant - The Bronx, New York
December 2, 2008

Q. Can anyone provide the balanced chemical equations for chem milling titanium with hydrofluoric acid? I had an internship where they used an HF chem mill to remove the alpha case and about a hundredth of an inch from parts made of Ti.

now I'm curious how the whole thing really worked.

Kyle Valentine
- Ann Arbor, Michigan, USA
January 14, 2009

A. Here's a WAG (wild ass guess):

Ti + 6HF --> H2TiF6 + 2H2

dave wichern
Dave Wichern
Consultant - The Bronx, New York
January 21, 2009

simultaneous replies

A. I would not bet a free cup of coffee on it, but I think that the product in Dave's equation is TiF6 for the complex molecule Ti hexafluoride, and 3H2's.
Note also that the equation for the rate of reaction is reactant 1 times reactant 2 (which is raised to the 6th power) divided by the product. Therefore, the reactions proceeds extremely rapidly at first and then is slowed down as the amount of product is formed and eventually becomes too slow to be useful.

James Watts
- Navarre, Florida
January 23, 2009

January 24, 2009

A. Titanium forms a very stable complex with hydrogen fluoride:

2 Ti + 10 HF --> 2 [TiF6]2- + 3H2 + 4 H+


2 Ti + 12 HF --> 2 [TiF6]3- + 3H2 + 6 H+

The HNO3 reacts with the H2 forming NO and water:

3H2 + 2 HNO3 --> 4 H2O + 2 NO

HF dissolves the Ti and HNO3 prevents hydrogen embrittlement.

Antonio Poell
- Brilon, Germany, EU

Titanium etch chemical ratios

November 28, 2017

Q. Hi all.

I am required to carry out an etch on a titanium component prior to conducting a fluorescent penetrant test. The maintenance manual that I am using details which chemicals to use; however I'm finding it difficult to decipher. The data I have is as follows:

MATERIAL                          CONTROL RANGE
Nitric Acid (HNO3, 42 BE, 70%) 4.5%
Ammonium Bi-Fluoride 6.4 oz/gal.

It's obvious that from the data above that I need a 70% concentration nitric acid and an ammonium bifluoride of 6.4 oz/gal. I am then required to mix the two together to create the etch solution, however what is the mix ratio? Or do I have it wrong and I have to mix 6.4oz of ammonium bifluoride per gallon of 70% nitric acid? I also don't understand the "CONTROL RANGE" heading, what is the 4.5%?

Any help appreciated; I don't have any experience with this kind of thing. Please don't post alternate etch solutions, I must use the chemicals as stated in the maintenance manual I am using.


Anthony Doherty
Non-Destructive Testing - Perth, Western Australia, Australia

A. Hello Anthony!

To me, it appears you buy 42 degrees baumé nitric acid, which is a standard reagent available from many chemical supply houses. You mix that standard concentration of nitric acid with water to make a 4.5% solution of nitric acid in water. Then you add the ammonium bi-fluoride at the 6.4 ounces per gal ratio.

Good luck!

lee gearhart
Lee Gearhart
metallurgist - E. Aurora, New York
January 18, 2018

Q. Could I use fluoboric acid (HBF4) instead of HF acid to perform chemical milling of titanium?

Ellis Burke
- Dorset, Christchurch, United Kingdom
April 27, 2018

A. Hi Ellis
Your challenge is not the etchant which will probably work but with finding a resist that will withstand the aggressive chemistry. Undercutting at the edges is likely unless the application is correct - speak to the resist supplier.

Please be sure that you fully understand the hazards of HF and have appropriate first aid facilities at hand.

geoff smith
Geoff Smith
Hampshire, England
April 28, 2018

thumbs up sign Hey Geoff, cheers for the help, we are currently looking for different solutions to etch and/or mill titanium (T-40 & T-A6V)

Ellis Burke [returning]
- Dorset, Christchurch, United Kingdom
May 8, 2018

A. We use alternative (milder) etching solutions as preparation for titanium color anodizing, titanium blackening and titanium type II anodizing.

Contact me for consultation.

Anna Berkovich
Russamer Lab
supporting advertiser
Pittsburgh, Pennsylvania
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May 9, 2018

July 22, 2019

Q. Antonio Poell mentioned that:

Titanium forms a very stable complex with hydrogen fluoride:
2 Ti + 10 HF --> 2 [TiF6]2- + 3H2 + 4 H+
2 Ti + 12 HF --> 2 [TiF6]3- + 3H2 + 6 H+
The HNO3 reacts with the H2 forming NO and water:
3H2 + 2 HNO3 --> 4 H2O + 2 NO
HF dissolves the Ti and HNO3 prevents hydrogen embrittlement.

What is the effect of maintaining a minimum concentration of titanium in the bath (2 pounds per 100 gallons) as some aerospace primes propose? Does it affect hydrogen pickup in the titanium casting/part?

Steve Kennerknecht
Investment Casting - Montreal, Quebec, Canada

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