FTM 143: What does Passivating Stainless Steel do?

Chromium makes stainless corrosion-resistant, but oxidation still occurs

Why bother passivating Stainless Steel fasteners?

This edition of the Fastener Training Minute with Carmen Vertullo was originally published August 20, 2019 as “Passivation of Stainless Steel fasteners” during episode 143 of Fully Threaded Radio.

Stainless steel is a corrosion resistant alloy ; this is one of the properties of stainless steel. Stainless Steel is made from a number of elements, one of which is chromium. It’s Chromium that makes stainless steel stainless. When the surface of stainless steel is exposed to air, it develops a very thin coating of chromic oxide. Not sure if that’s the exact chemical but it sounds like that, and once that passive film is on the surface of stainless steel, no further corrosion occurs.

Now sometimes in the processing of stainless steel, we interrupt that passive film or we contaminate it; in particular with elements of iron from cutting tools, from paneling and various other ways. So in order to re-establish the Integrity of that film we passivate the stainless steel.

When we dip the stainless steel in acid, it completely removes that passive film and then once we remove it from the acid, it is exposed to oxygen, which re-establishes a brand-new perfectly filmed passivated surface. The process does not actually clean the stainless steel per se, you need to start off with a clean piece of stainless steel. This is not a cleaning process. It’s a passivation process.

Now most of the service providers that do this for us will do it to various specifications from ASTM or AMS or the federal government, but you must specify the alloy and you must specify what specification and type of acid time and the temperature and so on. This is needed and so for passivation to be effective or to know that it’s affected. We also would want to test it afterwards. Lots of tests Labs do that testing and we do it here. One of the reasons why some test Labs also provide passivation services is because those two services kind of go hand in hand.

So knowing all of that, one of the things that I did not know about passivation was that it conditions the surface of the stainless steel fasteners, so it affects the K-factor. The K-factor is that friction component of the torque tension relationship, and passivated stainless steel fasteners actually have a higher K factor than non passivated stainless steel fasteners, and therefore the torque would be affected.

So if you’re going to passivate a piece of stainless steel, be careful that you understand the K-factor change and often times that’s overcome by another supplemental coating on the stainless steel; lubrication or wax or something like that.

Now all of this information is going to be available in a lot more detail at the Las Vegas Fastener show where we will be presenting at full day program from the faster training institute and AIM Testing Laboratory on plating and coating of fasteners.

We will be spending a lot of time on passivation. They’re doing this actual experiment that I just told you about regarding the K factor in bright before that here in September and the FTI webinar is about plating & Coatings and we’ll talk a little bit about passivation there, more than we did on this Fastener Training. Minute.

It looks like about a 6-minute talk. So that’s the least you need to know about passivation of stainless steel fasteners. To find out the rest of it, join us at the faster training institute in AIM Test Labs program on plating and coating at the Vegas Fastener show this year. This has been Carmen Vertullo at the faster training minute. Thanks for listening.

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