FTM 184: Why do we salt spray test stainless steel

FTM 184: Why do we salt spray test stainless steel

This edition of the Fastener Training Minute with Carmen Vertullo was originally published January, 17th, 2023 as “Why do we salt spray test stainless steel” during episode 184 of Fully Threaded Radio.

Well hello everyone this is Carmen Vertullo with the Fastener Training Minute coming to you from beautiful El Cajon California, the location of the AIM Testing Laboratory,  and also from the Fastener Training Institute.

Today’s topic as usual comes from a client of ours who had a question regarding two different things which seemingly have no overlapping technical issues. But it turns out they do. Those two things are stainless steel and salt spray testing. You will recall that in a recent Fastener Training Minute we talked about stainless steel and passivation and there’s a little bit of passivation in here as well. Technically we would think there would never be a reason to salt spray test stainless steel, because stainless steel does not corrode in the normal environment either 316 or 304 stainless. So we’re going to find out that there could be a reason to salt spray test stainless steel when it’s requested, and when we come back we’ll tell you what that is.

Well, welcome back everybody to the Fastener Training Minute. We’re talking today about salt spray testing of stainless steel stainless steel and general stainless steel fasteners, most of our questions revolve around fasteners, but in this case it could be any stainless steel.

Years ago I had a client and we sold lots of fasteners to a company that made diving equipment such as masks and buoyancy compensators and regulators and the things that scuba divers use. They were salt spraying the stainless steel hardware that we sent them which I thought was kind of silly, because typically stainless steel does not corrode in a salt spray chamber, no matter how long you leave it in there, because in that environment it should not corrode. However what they were testing was not the stainless steel, they were testing the effectiveness of the passivation. Now there are other tests for effectiveness of passivation that would typically be done by the processor doing the passivation.

If you recall from our previous Fastener Training Minute, passivation is a process we do with stainless steel that is a surface conditioning process. Actually the process removes free ferrites, free iron from the surface, so that we don’t get annoying rusty spots on the stainless steel. That’s as much as I’ll tell you about that process right now. Salt spray testing, by the way, is a process that we use to normally test coatings and plating and things like that on fasteners and other things, where we put it in a chamber with a salt fog for some amount of time. Usually it could be a few hours up 2000 hours, or even thousands of hours. So if we’re in the fastener industry, we are used to asking what is the salt spray rating for that coating in hours. With simple things like zinc it could be as little as 24 hours and maybe 96 hours, and depending on the top coat it could get better, and some of our really great coatings like Geomets and Magnis, and I probably should be fair and say all of them, but whatever they are, like the dip-spin coatings can go for a thousand  hours or more.

So the salt spray test is relatively simple it’s just put it in the chamber wait some time and look for corrosion. Sometimes it’s white corrosion, sometimes it’s red corrosion. sometimes there are two ratings: so much time for white stuff, which means the coating is corroding, and then so much time until you get to red rust, which means that what’s underneath is corroding. However there’s no red rust with stainless steel, so you can put it in the salt spray chamber for a long time and not get red rust.It turns out that there are a couple of standards that will allow us, or  allow for testing your stainless steel fasteners in as salt spray chamber. One of them is ASTM A967, one of our very common passivation specifications, and it allows for several different methods to test the effectiveness of the passivation. And one of them is sea salt spray test. And the number of hours for that is only I think 24, I don’t know exactly what it is I should have figured that out, but it’s not very many. It could be as few as two hours. NASA has a verification passivation process which is only a two-hour salt spray, according to the same standard that we use for testing coatings in the salt spray chambers, which is ASTM B117. Typically the passivator is going to use something that’s much faster than that, it only takes a few seconds and that’s the copper sulfate test. So there are other standards that allow for using salt spray testing to check the effectiveness of passivation on stainless steel. So if you ever find someone wanting a reason to salt spray test stainless steel fasteners, there is a reason. It’s not necessarily the best, most effective, or most cost-effective method for testing,  but it certainly does work, and it is well documented, and it is authoritative.

So now you know why we might find ourselves testing a product for rust that normally it would never rust.

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

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