48741

Passivate Titanium per ASTM F86-04 [New York] 

May 8, 2008

One of our customers asked us to quote price on passivating ASTM F136-92 titanium TI-6AL-4V-ELI per ASTM F86 [link is to spec at TechStreet]-04. They would like citric passivation. Can we do it in Citric or Nitric Acid?

Nancy Zapata
Electropolishing shop - Brooklyn, NY, USA

Take a look at letter 6885 on the site here for a variety of opinions regarding this subject.

Without trying to appropriate their information, what I get is that the "passivation" of Ti is something of a misnomer; it is a very different material from stainless steel, and the purpose of treatment via citric or nitric acid is very different.

I will look up the ASTM standard next time I go to the library, and try to be of further help, if no one else weighs in first.

Dave Wichern - Bronx, NY, USA

I did some reading on this subject. Although I cannot pretend to be an expert on the subject, most of what I found is in accord with what Mr. Kremer said in the letter thread referenced above.

In the ASM Handbook, there is a tabulation of corrosion rates of titanium in a variety of chemicals, including citric acid at a number of concentrations. The corrosion rates in all the solutions tested were close to nil, except one; this was a supersaturated citric acid solution at 150+ degrees C. Calling it a "solution" is a bit of a stretch, because it was only about 30% water. This liquid attacked the Ti, vigorously. Cautions regarding pitting, crevice corrosion, etc., mainly mentioned reducing acids, such as hydriodic and phosphorus acids, and, of course, chlorides.

It also seems to me that Ti spontaneously forms a passive oxide layer as long as oxygen is available, and that no additional chemistry is required. In fact, I found few references to the "passivation" of titanium; mainly the processes decribed in ASTM F86 [link is to spec at TechStreet]-04, and ASTM B600-91 [link is to spec at TechStreet] are referred to as "cleaning", "acid pickling" or "descaling." The purpose of all of these is generally given as the removal of foreign substances and iron inclusions.

One caveat: all of this data applies to pure Ti, not the alloy that Ms. Zapata is working with. Alloying elements may affect the corrosion chemistry in a number of unpredictable ways.

Dave Wichern
- Bronx, NY, USA

I am in agreement with Dave. The term "passivation" is generally misused by the industry, even with stainless steel the true passivation occurs when the iron-free chromium enriched surface reacts with oxygen in the air (though a bath containing oxidizing chemicals can put a jump start on the process). The purpose of the so-called passivation bath, nitric or citric, is only to remove the surface iron in order to allow the actual passivation to occur.

The same holds true for several other alloys, merely substitute the appropriate metal for chromium, and with the iron coming only from surface contamination and not also from the alloy itself. Nonferrous alloy parts that are not in danger of surface iron contamination shouldn't need an acid bath "passivation" treatment at all.

I hadn't heard that an oversaturated citric bath does indeed damage titanium, but a standard citric passivation bath is only 4-10% citric so there is no problem.

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

We also agree with Dave, however we believe that the passivation of titanium to improve its corrosion resistance IS POSSIBLE. We are talking about possibility of formation of less defective and thicker oxide film on the surface of titanium. Such passivating film can better protect titanium from corrosion. By improving structure, composition and thickness of passivating film, we can improve corrosion resistance of titanium alloys. Improved and thick passivating film can be obtained chemically or electrochemically in solutions that slowly dissolve titanium oxide.

Anna Berkovich Russamer Lab Pittsburgh, Pennsylvania