Letter 6885

Passivation of Titanium 

(2000)

Titanium alloy screws (implants) are passivated in nitric acid for a minimum of 35 minutes. Is there a maximum length of time that the screws can remain in the bath without adverse effects? Is there a relationship between growth of the oxide layer and passivation time?

D. Sudduth
- Largo,FL USA

Mr. Sudduth:

"Passivation" of Titanium is totally different than passivation of stainless steel. The use of nitric acid to passivate titanium is only to remove any iron or other exogenous materials from the surface. The oxide layer on Titanium forms very quickly and all by itself when exposed to air. Since there is no iron in the base metal, there is no enrichment of the surface layer like there is with stainless steel, where the surface layer is enriched with chromium.

So, the answer to your question is that it will not help to leave it in the nitric acid for a longer time, but it could etch away some of the Titanium. So, you do not want to leave it in too long. It may be safer and better to use a citric acid based material for the passivation. This will not etch the titanium, but give excellent removal of iron and other exogenous materials.

Lee Kremer Stellar Solutions, Inc. Algonquin, IL, USA

I'd like clarification of the concentrations for both the NH₃ and Citric acid passivation treatments for titanium. Is there a recommended pre-clean such as NaOH?

David R
- Methuen, MA, USA

is there an ASTM,Mil-Spec etc...that covers titanium passivation?

Frank Harris
R&D engineer - Boca Raton, FL USA

^-- I want to contact this person and my company is a supporting advertiser of finishing.com

Hi,
Titanium should not even be passivated, and in citric acid is dangerous for titanium because crack corrosion can building up in the material.

Regards

Anders Sundman     surface finishing engineer Sweden

You can treat titanium in Nitric acid for many hours, and nothing will happen, since titanium is passive in nitric. Speed of oxide layer formation is close to zero. In order to increase thickness of passive film you need to either use stronger oxidizing agents like hydrogen peroxide, persulfates, or switch to electrochemical passivation.

Anna Berkovich Russamer Lab Pittsburgh, Pennsylvania

Frank,
None that I have ever been able to track down.

Again, "passivation" of titanium is for removing contaminant iron that may otherwise turn into surface rust later on, in a similar fashion to the removal of surface iron from stainless steel during passivation. So the same general procedures used for stainless apply to titanium as well.

Citric acid is NOT dangerous to titanium.

Ray Kremer Stellar Solutions, Inc. Algonquin, IL, USA

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