Passivating Stainless Steel Medical & Surgical Instruments

A. Sonali,
Typically, your customer or a manufacturing standard such as from ASTM will give you these requirements.

In general though, there shouldn't be a need for any further cleaning after passivation, other than rinsing the passivation acid off the parts completely.

A. While I like vapor degreasers, your application sounds more like one that is optimally suited for ultrasonics both in the alkaline cleaner/neutralizer and the DI rinse. Since you are doing surgical parts, it would not take a large unit to run your parts unless you had them by the thousands, which I doubt since they are gun drilled (deep and narrow).

A. Dear Jon,

My feeling is that you are right about the chloride contamination. However the passivation would be a hell of a lot better if the items had been electropolished, the surface will be "enriched" with Chrome & nickel, as the Iron will have been preferentially removed and the resultant oxide film will be as complete as it is possible to get.

I can only recommend that you talk with a company in Sydney Australia Called METAGLO Pty Ltd, they have developed a very modern approach to this problem

Regards

A. Jon:

We have had several customers with similar problems caused by chlorinated paraffin inclusions. You might try 5% NaOH at elevated temperatures (up to 180 °F) before the passivation with CitriSurf. This has worked at other places. Ultrasonics also helps a lot if you can use it.

A. Sulton:

Much of the surgical instrument industry has gone to using citric acid based products for passivation. These instruments are commonly 410SS or similar, with laser etching. The laser etching is a very weak point in the corrosion problem and must be passivated to pass the autoclave tests.

The trick is to get the passivation required without removing the laser etch marks that the manufacturer wants to remain. This can be attained with a significant improvement in corrosion resistance over the old nitric acid methods laid out in QQ-P-35C.

adv.
Contact us if we can be of assistance.

Regards,

A. Dear Sultan,
Passivation is removal of free iron and other contaminants from the surface of stainless steel to make it corrosion resistant.
But other factors according to the product and manufacturing practice is also to be considered.
Best Regards,

A. Tara,

Yes 0.0002-0.0004" Is a common removal amount for surgical tooling. Laser mark is a concern for corrosion sites as the laser marking process leaves an iron oxide rich site. New to the market is a "glaze" that can be brushed on the area to be laser marked. The glaze is then fritted by the laser. Corrosion testing on this material is very good. I do not know if it has or needs FDA approval. When we electropolish for passivation on medical parts we use ASTM F86, While it covers marking of surgical implants, it also covers passivation means and includes electropolish. I like citric acid (covered in ASTM A967) for post passivation on 17-4 and 400 series stainless steel. However, it will lighten and sometimes remove laser mark, as will electropolish. It is a funny world some laser mark before passivation, some after, some both. I would like to see more of the fritted glass used with laser mark. Most laser mark o 17-4 will drip red rust in 8 hours. Generally the darker the laser mark the more it will corrode.

A. Jon's comments about electropolishing are good. As far as passivation goes, we get excellent results and an 80% reduction in corrosion problems using the right citric acid products.
These will not remove the laser mark when done right and will still give excellent corrosion protection on these grades.

There are many companies who use varying techniques to passivate surgical instruments, but many have gone to passivation prior to laser marking (to passivate the base metal) and then again after laser marking to remove the ferrites from the etch mark and leave the carbon.
Each laser mark apparatus and procedure gives different results on different alloys. Some are removed very easily and some with great difficulty.
Some use ultrasonics successfully and some have to turn off the ultrasonics to passivate the mark to prevent the removal of the carbon while still removing the ferrites.

adv.
Let us know if we can help. We have many surgical instrument companies using our products for this exact application.

A. citric acid based passivation can and is used for implants as well as instruments. It is also used on many non ferrous alloys just as it is with stainless steel.

On what basis do you expect you must use nitric? If you refer to ASTM F86 spec on implants, there is a clause there that allows any effective treatment as long as you document it. If your customer is specifically requiring nitric, it may be worth the effort to convince them of the benefits in allowing you to use citric.

If you still go with nitric acid, you can refer to a stainless passivation spec such as ASTM A967 and follow the formula and procedure recommended for 316 SS.

adv.
Let us know if we can be of help.

A. Bruce,
You're not really comparing equivalent processes. Bead blasting is usually done to smooth down a very rough surface or to clean off scale. A passivation bath removes iron and improves the protective chrome oxide layer.

If your application is cleaning and repairing surgical instruments, I see no reason why you would want to bead blast and every reason you would want to passivate.

If there is the occasional stain that cannot be removed by a citric bath, the best course would be a brief pickling in a harsher acid (phosphoric or nitric if necessary), rather than a mechanical process like blasting or grinding.

A. We share the same experience. If you are looking for puffy rust after the boil test, it will be rare to find this on 420. (I have not try the boil test on 420F.) You may find rust spots on 440C. The unclear part of the boil test is "shall show no signs of corrosion" (from ASTM F1089). If there is material removal or surface deterioration, it is definitely corrosion. What about discoloration with brown films or staining?

copper sulphate ⇦this on eBay or Amazon [affil links] test gives much easier results to interpret but is prone to false positives.

A. It may be that there is just not enough free iron on the surface to generate red rust. We have often taken fresh machined parts (usually 303, 304, and 17-7) and subjected them to testing (24 hour Humidity and Copper sulphate) with no failures, without any passivation, only degreasing.

The most common source of failure is ferric metal smear or contamination from Metal tools/cutters and sanding/grinding contamination. Using carbide tools and being careful with abrasives can greatly reduce failures. While passivation will remove ferric compounds on the surface that exist as part of the alloy, the material will usually self passivate with time.

To get a failure, you may need to expose the material to iron in processing.

A. Hello Jamshed,
Look in ASTM A380 and AMS2700 for complete listing of bath makeups, temperatures, and recommended alloys they should be used on. By 'cold' I assume you mean 'unheated' or 'ambient temperature'? There are a few options to look at, both Nitric and Citric. AMS2700 Table 4 has a very good listing of alloy and the type(s) of passivation that may be applied. And the ASTM A380 Citric method listed under 'general cleaning' is a room temperature method that is actually effective for passivation of certain alloys, not just cleaning.
You've got several to choose from. Whatever method you choose, I strongly recommend doing a 24 hour water immersion test to be certain that you have been successful using the method you chose, before going into full production.
Last, don't underestimate the importance of getting your items REALLY clean before passivation.

A. Jamshed,
Per ASTM A967, a 4 to 10 wt% citric acid solution for at least 20 minutes at 20 °C. You can read the nitric option there too, or in the old military spec QQ-P-35c, but for that mix of grades you wouldn't be able to use the same nitric formula for everything.