FTM 161: Passivation, what is it?

What is passivation

This edition of the Fastener Training Minute with Carmen Vertullo was originally published February 18, 2021 as “Passivation. What it is, what it isn’t, why it matters to us” during episode 161 of Fully Threaded Radio.

Well, hello everyone. Welcome to the Fastener Training Minute. This is Carmen Vertullo coming to you from the Fastener Training Institute and AIM Testing Laboratory in beautiful. El Cajon, California.

As is is often the case, today’s Fastener Training Minute comes as a result of an email that I received recently. That email actually turned into a little bit of a consultation job and the topic has to do with stainless steel, and in particular the rusting of stainless steel. In this case they were stainless steel fasteners because we’re in the fastener business, but this particular concept we’re going to talk about and the process you all know as passivation, actually applies to any stainless steel item, not just stainless steel fasteners.

When we come back, I’m going to tell you some things that are going to be very helpful to know about passivation of stainless steel. What it is, what it isn’t, why it matters to us, and why sometimes an authoritative citation can be a very powerful tool in your bag to help guide your customers out of a problem.

Most people that are in the fastener industry that sell or deal with stainless steel fasteners are very familiar with passivation. You probably know that it’s a relatively simple process that’s not very expensive. The stainless steel fasteners are immersed in an acid solution (usually an acid solution at some temperature some specified concentration and for some specified time). That acid is usually nitric acid, but it can also be other kinds of acids. The stainless steel fasteners are then taken out out of the acid, exposed to air, rinsed, and now the surface of the stainless steel has gone from what formerly would have been classified as possibly active, to a surface that’s passive. That is, there is no activity going on on the surface that would lead the stainless steel to corrode.

So you might ask what the heck was wrong with the stainless steel in the first place that would need us to do this passivation process to it? Well, in the processing of stainless steel components, it doesn’t matter if they’re fasteners, knives, forks, spoons, pots, or pans, whatever you may be making, the the tooling of the machine making the stainless steel parts leaves behind on the surface of the stainless steel what’s called free iron or iron particles. These are steel particles of carbon steel ferrite, whatever you want to call it on the surface of the stainless steel.
Now under normal conditions it probably would not ever make any difference. But if that stainless steel is exposed to moisture or any kind of an electrolyte, we have an opportunity for those particles that have iron in them to begin to corrode and it looks like our stainless steel is rusting. The stainless steel actually is not rusting, what’s is the stuff left behind on the surface of the part.

So the passivation process does two things: first, it removes all the steel particles left behind by the machine/tooling process. Now, it won’t remove everything, so it’s very important before we go into the passivation process, that our product is clean. If you have scale on it, for example from the hot heading process that scale should be removed either through a much stronger acid process or possibly through an abrasive cleaning such as sand blasting or bead blasting the head of the fastener, or even machining it off. Any oil or grease or left behind goop from the machining process needs to be removed.

Then we put it into the passivation fluid which is acid. Now the company doing the passivation processing is not going to want to put your dirty tarts in their clean passivation tank, because this will contaminate their tank, so they’re probably going to do some kind of cleaning first. But if your parts are excessively dirty, they’re not going to like it. So we start with a clean part, put it in the passivation acid, and take it out. So First, the passivation acid will have dissolved those free iron particles on the surface and removed them, and second, once the stainless steel is exposed to oxygen in the air, a very nice fresh thick (thick is relative because it’s microscopically thin), coating or layer of chromium oxide is formed on the fastener. And that chromium oxide is what makes stainless steel non-corrosive. It’s the secret sauce of stainless steel.

So from time to time, and this is what precipitated this particular Fastener Training Minute, a customer receives some heavy bolting or fasteners. They put them away in some place, and lo and behold they rust. They call the supplier and they complain. “Hey, this stainless steel is not really stainless. How do I know? because it has rusted.”

In this particular case some very large diameter fasteners conforming to ASTM A193 B8 or B8M. You know what that is, right? ASTM A193 B8 is 304 stainless, ASTM A193 B8M is 316 stainless. These large hot formed bolts were sold to someone who put them in some kind of a pipeline. I don’t know what kind because I just saw a photograph of a large flange with the heads of the bolts rusting. So the customer complains to the supplier who becomes my client.

The client calls me, and I say “well were they passivated?” They said no. I asked “were they cleaned on the head?” They said no. So it turns out that ASTM A193 specifically says that if you want clean bright shiny heads, you have to request that on the purchase order. Otherwise, you could get black hot form heads on any product, stainless steel or otherwise. Secondly ASTM A193 does not even provide a provision for passivation even, in the supplement. You have to go to its companion specification ASTM A1962 and A1962M, by the way “M” means they include metric product. In that standard, there is a supplement at the very end, (it’s big number like supplement 70 or 71 or something), where it says if you’d like this product to be passivated call out this supplement and it’ll be passivated to an ASTM standard.

None of that was included in the ordering information of this product. So technically there was no cause for rejection of the product, and that ASTM A1962 standard was the authoritative citation that we used. We ended up writing a nice technical opinion around that issue. Hopefully even though the customer probably will not be happy, at least they have a technical opinion which also states, by the way, that there are no detrimental effects of that surface rust. It’s not going to hurt anything. It’s not going to go in deep, and it doesn’t mean that the fastener itself is ever going to corrode. Assuming it’s in a normal circumstance where it would otherwise be fine. It’s just unsightly. If you don’t like it, clean it off. If it comes back, paint over it. There are ways around that. So those are things that are very helpful to know.

And actually what’s most powerful, is being able to provide that answer in a quick scenario under which the customer is sitting there waiting around and asking.

“Can I use these parts?”, “Do I have to remove them and replace them?”, “Give me something that I can use to understand that there really is not a problem here, so the next time I order this product I will know how to order it so as to prevent my beautiful B7 and B8 stainless steel Bolts from corroding in the application.”

Well now, you know about as much as you need to know about stainless steel passivation when it comes to fasteners, but what’s most important is that you’re able to apply it to solve the problem.

This has been Carmen Vertullo with the Fastener Training Minute. Thanks for listening.

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