48252

White hazy finish after descaling Titanium [Canada] 

March 21, 2008

Hello,

I am responsible for the cleaning and descaling of Titanium Heat Treated (950 Deg F for 1 hour) parts. I am currently using an alkaline cleaner with a dechlorinated and DI water rinse prior to descaling. The same cleaning procedure is used prior to the heat treating process.

These parts are being descaled in a 20 - 25% Nitric acid/ 1.5% Hydrofluoric acid solution for a period of 1 - 5 min's, then rinsed in a dechlorinated city water bath (Continuos overflow)and then a final dechlorinated DI water spray rinse.

Sometimes we are seeing a white hazy blotchy finish and other times a uniform nice clean looking finish. The descaling purpose is to remove any free iron left on the parts due to the welding and heat treating process. I checked the white, hazy, blotchy, areas with a Kozlow free iron tester and they are free of iron.

I am new to this type of process (Descaling & Passivation) and I want to make sure things are being done right.

Any suggestions or comments would be greatly appreciated.

Thank you !

Michael Hall
Aerospace technician - Metal finishing - Ottawa, Ontario, Canada

Michael,

the cause of such foggy spots is due to slight damages from previous processes (or some iron remaining on the surface). If surface of titanium parts is uniform, the nitric + HF solution produce uniform clean surface. However if any cracks or uneven alloy structure, or else take place, then there is not enough of nitric in local areas (nitric acid works as passivator), and hydrogen starts to damage the surface.

Our titanium etching gel or liquid electrolyte do not produce white or foggy areas on titanium surface, even if initial surface is not perfect.

Anna Berkovich Russamer Lab Pittsburgh, Pennsylvania

If I remember correctly, the nitric - HF ratio has to be more than 14:1, which if your tank goes low on the Nitric or high on the HF, then it will etch. As Anna said, if it is a rolled )rough) surface it can be attacked.

James Watts
- FL

Thank you for your replies.

I try to maintain a ratio of at least 16:1 Nitric/Hf. However when I am making additions of Hf to the bath, I am doing so because the descaling process is taking longer than usual due to the Hf being consumed (Correct me if I'm wrong). Or is it because the dissolved titanium in the bath is slowing the descaling process ? I make a new bath when the dissolved titanium reaches 10g/L.

I do not add Nitric when adding Hf because my concentration level for Nitric is pretty consistent at 20 - 25%.

Should I be adding Nitric, when adding Hf ?

Sincerely,

Michael Hall
Metal Finishing - Ottawa, Ontario, Canada

Adding nitric would not be required as you add HF. I normally started with the weakest bath allowed by the OEM's and gradually increased the HF as it was used until I got to the max allowed. To keep the nitric-HF ratio proper, I had to add more nitric from time to time.
The problem is several fold. A bit of the HF is being used (tied up as a complex molecule) but you should be titrating for free flouride and not total flouride. The etch of the Ti is reduced by the formation of the complex IE: the more product that you form, the slower the reaction, regardless of concentration of the HF. There are equations for working this out, but it is a chapter in the chemistry book. Adding HF to the limit just allows you to get a bit more time out of the solution.

James Watts
- FL

I do not kow whether this comment will help but I use hydrogen peroxide to remove titanium oxide (and nitride) from titanium. The concentration can vary greatly. I control the pH to 7 - 9 using pyrophosphate as a pH buffer. In my case I have glass and other materials destroyed by nitric and hydrofluoric acids but even if I did not I would still prefer hydrogen peroxide. Dissolution of the oxide takes 1 - 24 hours. You can shorten the immersion time by heating and by raising the concentration of hydrogen peroxide. I usually leave my parts in the solution overnight cold. This process does nothing to brighten a dull surface; the titanium is not attacked at all.

Nick Clatworthy
- Whitstable, Kent, UK