Smut On Titanium Parts

Q. I use a Nitric/HF bath to etch titanium. Afterwards I desmut in 45% by volume nitric acid solution. Recently I have been noticing smut on parts. Both the etch and desmut baths are operated at room temperature. The acid concentration is fine. I analyzed the solution and found 400 ppm titanium, 20-40 ppm of sodium, aluminum, vanadium and other alloy constituents. I also found 90 ppm of calcium. The bath is eight years old. Any suggestions? M.R.

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A. I am not sure what concentration you are operating your etch bath at or how you are measuring concentration of each acid constituent (nitric and hydrofluoric). You mention the acid concentration is fine, but if that is determined by a simple acid-base titration or (worse) a simple pH measurement that would not tell you the entire story of what is going on in the tank. The hydrofluoric acid would be the component that would be doing the majority of the etching of the titanium, not the nitric acid. The nitric acid will buffer pH and dissolve some of the other alloying constituents.

So based on your description, I suspect there could be a few potential problems with the bath. One could be that the hydrofluoric acid concentration has crept up from where it normally is or should be. As that concentration gets higher, there will be more etching of your titanium and the nitric desmut tank will have a more difficult time dissolving the residual alloying elements if the time, temperature and concentration remain the same.

The second source of the problem could be a weak desmut tank. You mention what I believe are the conditions of the etch tank, however, a weak nitric acid tank would be much less effective at smut removal. I would expect that if the tank drops below 25%, the ability to remove smut will drop off significantly. Also, you don’t mention a dump schedule or analytical results for it, but that can also build up a significant amount of dissolved metals. That would probably be the first tank to focus on and eventually dump if it cannot return to making smut-free parts.

The last item could be that the etch tank just needs to be dumped and recharged. Although the chemistry you mention did not seem to have a very high dissolved metals content given the age, eight years is a very long time to run any tank, especially one that is meant to dissolve metals by etching.