White hazy finish after descaling Titanium

A. I suggest you try straight 2% HF. Don't exceed 25 seconds. start etching from 10 seconds and find the optimal time by increasing the time you are etching for a second interval for every try.

A. To get a look at the oxide layer, you probably are going to have to plate it with something like copper with little activation or cleaning. This will be rather non adherent, but it will be enough to protect the oxide layer. Dilute your etch by half and swab it on for a couple of seconds at a time. Rinse very well so the vapors do not attack your microscope. Repeat etch as many times as required. When you have your photomicrograph of the oxide, etch the Ti in the full strength as long as is required.

A. Michael,

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

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

A. 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.

A. 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 fluoride and not total fluoride. 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.

A. I do not know 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.