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Passivating NiTi for a medical implant [California] 

April 2, 2008

Hi,
I am using Nitinol for an implant that is submerged in blood, so I want to make sure there is a surface oxide layer that prevents corrosion. What is the best way to achieve it? Does Nitinol automatically form an oxide layer when exposed to air? When heat treated? Also what are good passivation methods for Nitinol?

Tenny Calhoun
product designer - Mtn View, CA, USA

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

Tenny,
Most non-ferrous metals do automatically form an oxide layer, yes. I've seen a video with a block of aluminum cut in half where the oxidation visibly takes place in mere seconds.

Corrosion on non-ferrous metals and alloys is usually due to contamination iron that has been deposited on the surface during the manufacturing process. "Passivating" such parts means getting rid of that iron. Since stainless steel passivation also is all about surface iron removal, the same solutions and procedures apply to these non-ferrous applications. I.e. nitric or citric acid based formulas may be used.

Let us know if we can be of help to you.

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

Ni-Ti - stents after standard treatment (oxide removal, electropolishing, rinsing), have the potential of puncture Ev, measured in accordance with the test ASTM F2129 [link is to spec at TechStreet], equal to + 0.5 - +0.7 V. It is much less than point of oxygen formation, which indicates the possibility of stents pitting corrosion inside the human body. The lack of corrosion resistance of electropolished stents is due to imperfect structure and composition of passive films produced in electropolishing and rinsing. Special passivating solution remove imperfect passive film formed during electropolishing and replace it by passivated perfect film structure and composition, resulting in increased corrosion resistance of stents.

Tests on passivated stents does not show puncture potential Ev at all. And potential increase is not followed by increased current (punctures) up until oxygen formation at potentials higher than +1 V.

Microscopic observation confirmed the existence of pitting corrosion on non-passivated stents that passed the tests of ASTM F2129, and complete lack of pitting corrosion on passivated stents.

Passivating effect is due to a change of film structure and composition: nickel oxide is removed from th surface, while passivated film is enriched in TiO2 titanium oxide. Such passivating film becomes porous less.

According to our tests Nitric acid penetrate into the nitinol surface too deep and create porous passivating film.
We have conducted numerous tests (described in ASTM F2129 specifications) and selected optimal passivating solution for composition 50%Ti-50%Ni alloy.

Anna Berkovich Russamer Lab Pittsburgh, Pennsylvania