Polishing & Passivation of Titanium

A. Not sure of your question. Most medical inplants are processed smooth to eliminate porosity of the surface metal for a couple of reasons. If this is the case, the most common procedure is to use high energy centrifugal force type equipment in a 2 or 3 step process to achieve 12 RMS or finer. If you are asking about a one sided polish, then you must consider some type of wheel system.

A. The medical implants made from cobalt, titanium, stainless steel, Nitinol - are best to electropolish by specially designed high-voltage environmentally safe polishing equipment and electrolyte. In comparison to mechanical polishing this process increases strength of the material surface. You save on time of processing (5-10 minutes), do not need to adjust your equipment to complex shapes, you obtain surgically clean surface without any impurities, and you do not need to passivate surface after polishing.

A. Shaun,
Polishing of all sorts of implants with difficult shapes can be achieved with a six axis robot and polishing wheels.
Tom

A. Those sections describe various testing requirements such as frequency of testing and test selection (water immersion, high humidity, salt spray, copper sulphate, etc.). It is curious that your customer would be specifying that passivation be performed according to ASTM A967, which is not appropriate for titanium alloys. ASTM B600 is the proper standard for cleaning and descaling titanium alloys.

A. ASTM A967 is often specified for passivation of titanium, although it is a misnomer for sure. What you are removing by "passivation" of titanium is any iron or iron oxide that may have been put on the surface in the fabrication process of the parts. Even though you do not truly passivate the titanium metal itself, you need to remove any iron from the surface so that it does not rust. If you have not put ANY iron or other contaminants on the surface in the fabrication process there should not be a need to "passivate".

Testing of the parts per ASTM A967 would verify whether you have an iron contamination problem.

A. Change your heat treat process to a vacuum furnace or a reducing atmosphere and you will not get the oxide coating.
Abrasive blasting is a lot quicker and probably cheaper than scotch brite after the initial cost.

tuo Mr. Watts,

We tried as you suggested with the abrasive blasting and the passivation process is working quite nicely now. It is a lot quicker and with a more uniform finish.

Thank you !

A. What were you intending to do with passivation? If it's simply to remove embedded iron one could do a copper sulphate ⇦ this on eBay or Amazon [affil links] verification (reddish metallic copper immersion forms on iron) or salt spray test -- would only need a couple of hours to verify. Titanium is a pretty passive metal as is without passivation! You might want to review a white paper on this site entitled "Why Coat Titanium?" which tells a little more about "coating" titanium.

A. Disregard my question. I believe I found my answer. ASTM F86 calls out the "final surface treatment" for metal medical implants (which titanium would fall into). This gives the possibility of "passivation" (misnomer as many apparently consider it) of said metal implants without the use of hydroflouric acid

"Immerse in 20 to 45 volume % nitric acid (specific gravity 1.1197 to 1.285) at room temperature for a minimum of 30 min. For an accelerated process, a 20 to 25% acid solution, heated to 120 to 140 °F (49 to 60 °C), may be used for a minimum of 20 min. (See Specification A967 and Practice ASTM A380 ). This treatment provides passivation by surface oxidation and can dissolve certain foreign material that might be present from previous operations"

A. Steve,
It's a complex issue. A967 is written for stainless steel, so a lot of people might resist accepting it as an authority for nonferrous metals. The recent revision of B600 includes "surface iron removal", which is a way of saying passivation without using the word itself, and procedures copied straight out of A967.

F86, which takes precedence when the parts are implants, includes, among other things:

"5. Cleaning
5.2.3 An acidic cleaning process may be used. For titanium, titanium alloys, and tantalum, some possible cleaning processes may be found in Guide B600."
"6. Final Surface Treatment
6.1 Implants shall be given a final surface treatment before
they are packaged. Anumber of different surface treatments are acceptable, including acid treatment, electropolishing, anodizing, and oxidation. The following surface treatments should not be considered restrictive:"

6.2.1 is what you quoted. I know a number of places that are doing a final passivation with citric acid instead of nitric under the auspices of this clause:

"6.5 If acceptable alternative surface treatments for implants are used, these treatments should be specified in the production procedure documentation."

It really all depends on what your goal is. If you want an electropolished or anodized surface, do that. If the surface is already looking how you want it and you just need to ensure there is no iron contamination on the surface, then "passivate" with nitric or citric acid .

A. As always, we need to carefully define "passivation" here so that everybody is on the same page.

When we talk about dipping stainless steel in acid to passivate, we are removing iron from the surface. Much of the iron is inherent to the alloy, but it can also be contaminant iron introduced from external sources. The remaining iron-depleted surface is high in chromium (and sometimes nickel also) which reacts with air to form stable (passive) metal oxide layer.

Titanium alloys do not contain iron, and therefore the surface does not need an acid treatment to enable formation of a passive metal oxide layer. Unless! If the surface is (or may have been) contaminated with iron from an external source, then that iron can become rust down the road if it is not removed. Since it's fundamentally the same task as passivating stainless steel (removing iron from the surface) we use the same process (nitric or citric acid treatment) and tend to use the same terminology. (Though note that ASTM B600 specifically avoids using the words passivate or passivation even while describing treatments borrowed directly from ASTM A967!)

I hope you can see now that putting it in terms of "undoing the passivation" or "negating the passivation" is not really accurate. For a non-ferrous alloy like titanium, it doesn't really matter what the history is. If you have an iron-free surface, and then contaminate it with iron, you create the possibility of surface rust unless that iron is removed via an acid passivation treatment.

It is extremely destructive to use a carbon steel wire brush on anything that isn't also carbon steel. In your case, even a stainless steel wire brush, regardless of whether or not it sets up a rust problem, would be contaminating your titanium surface with not-titanium material.

Since you did not describe your application or the nature of the foreign object damage that needs to be cleaned, I cannot say what is or isn't potentially a problem for you here.

The FOD is non-metallic bonding material that nestles in some of the bores as part of the assembly process. This material is quite robust in that in some cases it is the only means of securing the details to the assembly. The assembly is a non-critical flight part that is ITAR controlled. I understand that the "passivation" cannot be undone, but it seems to me that introduction of a steel, whether it is carbon, or stainless might contaminate the fittings to the extent that they no longer would meet the requirement of the customer that the titanium be "passivated" certified at the time that the customer takes possession. Thank you for your expertise in this subject matter and of course, your time.

Darryl,
Well, again you use the term "undoing the passivation". If you want to think of it that way, you can. It's sort of similar to saying that your pet dog "undid his bath" by rolling in the mud. Most people would just say he "got dirty".

So, if I can make sense of all that properly, this is a titanium part that is getting glued into place, and some of the glue is getting into some holes, so they clean out the holes with a steel wire brush.

I would think this to be one of those ounce of prevention vs pound of cure situations. Could an addition be made to the standard assembly procedure involving some sort of temporary plugs to keep the holes clear and clean? Assuming of course that it would be futile to merely give a dire warning against using too much glue near the holes.

• Topic 6885 "Problems & Solutions in Passivation of Titanium"