Re: dissolving Ti but not tool steel

Thank all for all of your suggestions.

My rather late response means that the some of the messages in the thread have expired.
I would appreciate it if Uncle Al could repeat his response because to me it was quite promising but I did not get a chance to save it.


The nozzle is for spraying Ti powder onto a substrate. Warm (~350°) gas travelling at supersonic speeds carries the powder. The powder is impinging on the walls and at these temperatures begins to stick (surprisingly). The nozzles are 200 mm long with a tapered bore that is 2 mm at one end and about 4 mm at the other. The inside is polished because scratches disturb the gas flow and increase the likelihood of fouling. The amount of fouling varies but the thickness of the titanium about 1 mm.

When this occurs with aluminium the nozzles can be quickly and easily rescued by throwing them in a bath of caustic soda. However, I have not found a chemical that will dissolve titanium but not tool steel. I am trying hydrogen peroxide but I have doubts and safety concerns. It has been suggested elsewhere to heat the clogged nozzles in a hydrogen atmosphere but that may effect the surface finish and distort the nozzle. similarly heating the nozzles to use the difference in thermal expansion.

Other suggestions sound complicated like electrochemical polishing and electromilling. I have done a lot of electrochemical polishing for TEM specimen preparation. I know how tricky it is. This will be even more so with bits of Ti stuck half way done a 200 mm long tube. There may well be a chemical solution in combination with the correct V-I that attacks the Ti and not the Fe matrix, but what amount of experimentation will take to find that. Electrochemical milling and polishing are a last resort.


Unless I cannot find a simple chemical dissolving route that works, I am then going to try grinding out the titanium using a long tapered mandrel and fine polishing media. Not easy but hopefully a bit straightforward.

Regards,
Steven

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