Sound technical content, curated with aloha by
Ted Mooney, P.E. RET
Pine Beach, NJ

Passivation of stainless steel hypotubes

My company currently uses passivation on our hypotubes*. We use a 4 second rinse and a 20 minute bath in 20% nitric acid. The theory is that this will remove tramp metals, cutting oils and possibly leave a stable oxidized layer on the inner surface of the stainless steel hypo tube. We really are not sure if using the nitric acid is really doing anything.

What I would like to know is how effective is passivation, is there any standards dealing with the process, and why it is done.

If you can help me I would appreciate a reply

* -- a hypotube is made in the same manner as a hypodermic needle but it's used in a catheter, with saline being the only fluid in contact with the inner surface.

You have explained passivation quite well, Martin, except with regard to "cutting oils" -- passivation is not a substitute for cleaning; rather the parts need to be well cleaned before passivation.

Yes, it works. However, quoting the ASM Metals Handbook, "In most natural environments, stainless steels will remain in a passive state . . . Change to an active state occurs only where chloride concentrations are high, as in seawater . . ." So, perhaps with saline solution it doesn't actually achieve much, but it is a very accepted and generally useful practice for the removal of tramp iron from the surface, and probably should be continued.

I would suggest that you get a copy of ASTM A380 . However, there probably are additional ASTM or other specs related specifically to your product line; for example, I know that there are specific specs for surgical implants (ASTM F86, for example), so I presume that there probably are others that are applicable to you.

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