FTM 186: Can black oxide treated stainless rust?

Fastener Training Minute 186

This edition of the Fastener Training Minute with Carmen Vertullo was originally published March, 16th, 2023 as “Why did screws made of 18-8 stainless steel that were passivated and black oxide finished rust in a salt spray test?” during episode 186 of Fully Threaded Radio.

Hello everybody, this is Carmen Vertullo with your Fastener Training Minute, coming to you from beautiful El Cajon California, and also from the Fastener Training Institute.

Like almost all of our topics, today’s topic comes from a email that actually just came in this morning. I had a different topic planned, but this one just came in and I felt was so compelling that I switched. It has to do with three things, and I kind of like it when we have overlapping issues or processes or materials involved. In one particular Fastener Training Minute the things involved were black oxide finishes and stainless steel in salt spray testing. Now I think we may have had a recent similar discussion on this topic, but this is a really interesting case, and unlike many cases, I was able to come to a pretty quick answer for the person who had the question.

The question was this: we have a customer that is trying to get a screw that we sell them that does not have a 96-hour salt spray requirement, but now they have another application that has a salt spray test requirement. The screws are made of 18-8 stainless steel and passivated, and then they get a black oxide finish. If you look at the pictures from the salt spray test, there is a lot of red rust on the screws. What the hell is going on here?  I thought 18-8 stainless steel could pass a salt spray test. Why is this happening? when we come back I’ll tell you.

Well everybody, welcome back to the Fastener Training Minute. This is Carmen Vertullo and today we’re talking about the combination of black oxide finish on stainless steel and salt spray testing. Well first let’s just pretend like we were putting some fasteners into the salt spray test and they then had a black oxide finish treatment.  If these fasteners were just steel they would not last 15 minutes, they probably would not last even a few minutes because black oxide is not a corrosion resistant coating. Write that down. If you don’t learn anything else from this Fastener Training Minute that would be the best thing to learn. Black oxide is not a corrosion resistant coating. In fact it’s not even a coating, it’s a surface finish treatment. It’s non-dimensional. It essentially takes some of the surface iron and converts it to black iron oxide It’s a very similar type of iron oxide to the iron oxide that is red rust except there are more Oxygen atoms in the formula for black iron oxide.

It just ends up being black iron oxide (Fe3 O4) instead of red iron oxide (Fe2 O3). The way that black oxide normally provides some semblance of corrosion resistance (and I mean that literally semblance, not real corrosion resistance ), is that it is a very good sponge for soaking up oil or wax. So when we put an oil or wax on it and that’s really where most of that corrosion resistance comes from. If you leave it outside, in a short amount of time, that stuff will dry off or go away in pretty soon the bare steel will rust. But now we’re putting this black oxide onto stainless steel.  So in theory, when the stainless steel is exposed after the black oxide is gone, we would expect it not to rust, and outside in the world that’s pretty much what happens. The black oxide stays black. It’s not going to be subjected to what’s underneath it in terms of being a corrosion risk so long as it stays black. We have lots of  stainless steel products that have black oxide on them that works fine.

Set aside for a moment the fact that putting black oxide on stainless steel is a little bit trickier than putting black oxide on steel. The applicators have to know how to do that correctly. Well what happened here?  Well first off what did I do? Well I don’t know what answer to this is, so I turned to Google. Now you have  heard me say many times you have to be very careful about what you find and what you can trust on the internet. You have to know the source, you have to know the author, you have to know whether or not it’s believable. Most of all, does it add up to something that makes sense based on what your current knowledge is, and if it doesn’t, then don’t believe it.

In this case, I found something that was very believable, and it was actually published all the way back in 1999, in a very good publication called Products Finishing. Products finishing online which also has a magazine. And they talk all about coatings and finishings, and the particular title of this article was Fasteners and Finishes to Reduce Corrosion. and provide Cosmetic Finishes on Stainless Steel Fasteners. And that’s sort of what this was about, and they talk about black oxide ,and its color and how it works and so on, but right back into the middle of this article all the way down, this is what it says:

A negative to black oxide finish is the fact that the coating is not a corrosion-resistant finish and it is not suitable in any situation for outdoor use, a fact that negates it’s selection for stainless steel in the first place.  In samples of stainless steel parts tested for suitability with black oxide as a coloring agent, it was found that parts exhibited red rust in unacceptable amounts. Series 400 stainless steel parts rusted more rapidly when black oxide finished than did parts left plain passivated. Series 200 parts rusted faster than did Series 300 parts, and showed more undesirable rust product when oxide, than did plain passivated parts. Series 300 stainless steel, the main material of most threaded fasteners, showed red rust on parts with nominal salt spray hours. An investigation of why this was occurring showed that the nature of the black oxide coating was responsible. The surface layer of the stainless steel had been converted to red iron oxide, another term for red rust, and ferric sulfide smut. This smut was black and colored the oxide layer. While post applied oils and waxes reduced the rate at which the rust finish occurred, the failure was still fairly rapid“.

And it goes on to say a few things about the chemistry behind converting the black oxide into red oxide and it doesn’t always happen. But the bottom line is that you cannot expect a stainless steel black oxide coated part (even though it might work fine in it’s particular application),  to perform well under salt spray testing. And that was a really good answer. To be able to say to my client that they could say to their customer: “bad idea” So let’s find some other way to make your parts black if you want to put them in a salt spray chamber test. Perhaps that will be the topic of another future Fastener Training Minute. “How do you get really good looking black fasteners“.  And there are several ways to do that, and I know most of them.

By the way, if you’d like to have this article, just email me at Carmenv@carverem.com,  I will send you this article, but if you just Google “pfonline.com”, or if you just Google what I Googled which was “fasteners black oxide stainless steel” you’ll probably find it.

Well that has been your Fastener Training Minute.  I hope you learned something and thank you for listening.

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