Tint etchants for Ti-6-4

[EngineerDave]

I will be getting a book on Metallography soon that will probably discuss this, but in the meantime, I'd like to figure this out before then.

We are working with Ti-6-4, annealing it to deliberately bring out a grain pattern and after that point we want to further bring out the grain structure.

Using conventional ethchants (Krolls and other HF based nasty acids), the surface only gets lighter.

For this cosmetic finish, we'd like to darken the surface of the metal to bring out grain. Do you have any recommendations on tint etchants?

 

[NickE]

I've got a LECO book on metallography. for macro etching they reccomend:

HCl (conc) -- 20ml
HF (48%) -- 40ml
H2O -- 50ml

(ow looks nasty)

For micro-etching they reccomend:

1-3ml -- HF
2-6ml -- HNO3
100ml -- H2O

Swab 3-10 sec. or immerse 10-30 sec.
Note: HF attacks Ti and HNO3 brightens the surface. Make concentration changes on this basis.

Other etchants they reccommend are:

Keller's -- 2ml HF -- 3ml HCl -- 5ml HNO3 -- 190ml H2O
^^^^^^^^ -- says w/ immersion 10-20 sec and warm water wash will reveal general structure.

For electrolytic etching---> (keep cool when mixing)

perchloric acid -- 10ml
2-butoxyethanol -- 10ml
ethanol -- 70ml
H2O -- 10ml
30-65V for 10-60sec

perchloric acid -- 3ml
2-butoxyethanol -- 35ml
methanol (absolute)-- 60ml
60-150V for 5-30sec

perchloric acid -- 5ml
acetic acid -- 80ml
20-60V for 1-5min

NOTE: I have no exerience with these etchants. Just what my LECO metallography guide suggests.

DISCLAIMER: I am NOT responsible for any effect that may occur due to the use of the above formulae. Nor is anyone else that I can tell. Especially immune to responsibility is my employer.


nick

 

[TVP]

Struers has an excellent summary of metallographic preparation of Ti on their website. Use the following link for more information:

http://www.struers.com/resources/elements/12/7525/Application Notes titanium English.pdf

 

[EngineerDave]

Thanks for the recommendations guys.

I've tried some pretty strong ones. I don't know about the 48% HF though.

On the surface of the coupon we hope to etch the surface for a cosmetic effect and literally turn it black if possible. I don't even know if you can find an etchant to blacken the surface, however it is something I'm trying to find out.

All the etchants I've tried so far lighten the surface to a whiter color.

Overall, to give you a brief idea of what we are doing, we are annealing the Ti-6-4 to grow the grains to a large visible size. This leads to a cosmetic grain pattern on the surface of the part. At this point out of the furnace, the surface of the material is dark, but it would be great if we could further darken it to bring out the grain pattern. (My thought is more time in the furnace should help). But I was asked to investigate etchants. At these point unless something else comes up, I will tell them that all the etchants will do is lighten the color of the surface, which is precisely what they are doing. (Under the microscope is a different story of course but we are looking for the macro effect)

 

[NickE]

not sure if this helps but:

my watch band (Its Ti but Ive never run XRF/XRD to find composition) will blacken a bit in soap. (rubs off thou)


nick