FTM 185: approved coatings for structural bolts

Fastener Training Minute 185

This edition of the Fastener Training Minute with Carmen Vertullo was originally published February, 17th, 2023 as “What are the approved coatings for structural bolts during episode 185 of Fully Threaded Radio.

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

As most of our Fastener Training Minutes come, this one comes from an email that I received. It’s actually sort of like a press release. It came from a Fastener Magazine. I won’t say which one, I don’t know if it’s a press release, but it kind of reads like one, and I’m going to read it to you, and when I return we will learn some things from it because I think it’s kind of pregnant with a lot of information that causes us to ask some questions. It’s about structural bolts, it’s about coatings and it’s about hydrogen embrittlement. So I’m going to go ahead and read it to you right now, and when we return we’ll talk about it.

It starts off like this: the title is Hydrogen Embrittlement Protection. Such-and-such company introduced a unique coating system for Tension Control bolts (TC) and Hex Structural bolts providing several benefits compared to Black bolts. According to the company, the new coating far surpasses the performance of mechanically galvanized coatings. In fact it’s achieved extended corrosion resistance of 1000 hours of salt spray, which is at least twice that of mechanically galvanized coatings. Additionally, the coating cannot generate hydrogen embrittlement and offers good short-term protection allowing Tension Control bolts to be inserted into structural connections and tightened at a later date, without the presence of corrosive properties which would typically affect the preload.

And that’s the announcement it’s pregnant with good learning opportunities, more from what it has left out than what it has said. So when we return, we will give birth to those learning ideas, but in the meanwhile if you have a moment and you have access to it, please get out your ASTM F31 and ASTM  F3125M specification which you can find in your IFI book. Or you can find it online with the IFI online standards and a turn to table A1.1, Permitted Coatings. Because we’re going to talk a little bit about that and we’ll be back with the rest of the Fastener Training Minute in just a minute.

Well welcome back everybody, this is Carmen Vertullo with the other side of the Fastener Training Minute. Today we are talking about a press release that I just read to you regarding a new coating for structural bolts. Now the coating advertises as being 1000 hours salt spray tests compliant and it cannot generate hydrogen embrittlement. So if we take a look at ASTM F35 ASTM F3125M it tells us which coatings are permitted to be used and there not very many. The table is divided up into two columns the column down the left is for the 120 KSI bolting assemblies which are what we would used to call A325, now we call them grade A325. And on the right side 150 KSI and those are what we would call the ASTM A490 structural bolts, which are the higher strength structural bolts. Notice there are no tension control bolts mentioned in either one of those and  we’ll get to that in a minute.

But on the lower strength ASTM A325 structural bolting assemblies, and we call them assemblies because we have an assembly when it’s a nut a bolt and a washer together as a lot, tested as a lot, and we use those when the structural engineer cares about the load that the bolt is going to be able to apply to a joint. So it’s a controlled assembly, sometimes in what’s called a slip critical connection or a fully tensioned connection. It’s not just slap together and snug tightened. So for our 120 KSI or ASTM A325 grade, we’re allowed to hot dipped galvanize those. It’s a very thick coating at 50 microns class 55 it’s called, that’s one thing that’s allowed. When we use that coating, by the way, the nuts will have to be over tapped because the coating on the bolt is so thick. Another  coating that’s allowed is an F3393 coating which is a dip spin aluminum zinc coating. And that coating is also allowed on 150 KSI, the higher strength A490 structural bolts the F3393 coating. And those are the only coatings that are mentioned in the Permitted Coatings table. Now there is a note, Note A, and it addresses the twist-off Tension Control bolts. And it says coatings for twist-off style bolt assemblies shall be agreed upon between the producer, supplier and user, and are not permitted except when applied under the direction of the manufacturer. So it’s important to note that under the standard, in general commerce, no coatings are allowed to be used on twist-off Tension Control bolts unless everyone is in agreement. Now obviously the producer of this new coating has tested them with their Tension Control bolts, and if someone wants to use them that is the end-user, the supplier and the producer all get together and say: “we’d like to have that coating”, and then you can do it.

So if we take a look at the announcement once again, you will notice it’s billed as hydrogen embrittlement protection. That’s a big issue right now, and it’s been a big issue for a while. But it’s kind of coming to a head. It’s a big issue amongst the ASTM F16 Fastener Technical committee, which oversees F3125 and F3125M. And one of the things that we were trying to protect against over the years with coatings, is to not induce hydrogen embrittlement in  ASTM A490 bolts. Now they’re not hydrogen embrittlement susceptible because their maximum hardness is Rockwell HRC 39, and we only have susceptibility to hydrogen embrittlement above that. Additionally they are double protected because they have a maximum tensile strength as well, and so we really should not ever have a situation if those ASTM A490 are properly produced, where we have hydrogen embrittlement susceptibility. And this is the argument that’s been going back and forth for some time. I suspect at some point we will remove the restrictions for coatings on the A490 bolts once this is worked out, but right now they’re not allowed.

But this announcement is billed as a hydrogen embrittlement protection coating, that’s what it is. However this coating, unless it happens to be, and it could very well be, an ASTM F3393 coating at 1000 hours of salt spray, you could comply with that requirement, and then it would be allowed in the specification. But only for hex structural bolt assemblies, not for tension control bolts unless it is often agreed upon by the parties. So that’s kind of one of the things I think that should have been better addressed. And then it talks about the coating itself only that it’s got a special name. I won’t tell you what it is but that it has 1,000 hours of salt spray performance which is not that hard to get these days. There are lots of proprietary coatings that will give us that specification. But it doesn’t say how thick it is. It doesn’t tell us whether or not the nuts have to be over tapped or whether the nuts if they’re not over tapped, do you have the coating in the threads as well. Now I happen to know a little bit about how Tension Control bolts work. I’ve actually helped a client of ours develop a coating for their nuts, a wax that would help them control the torque tension relationship because a Tension Control bolt is in fact just a one use torque wrench. When that spline breaks off at a certain torque we get a certain tension based on the K-factor or the friction of the bearing surface of the nut and the threads of the nut, and that all depends on the lubricant that’s on the nut. So it’s conceivable and it’s actually probable, that this coating is going to provide that highly controlled coefficient of friction that would be needed in order for theTension Control bolt to work. It goes on to say that this is helpful because it allows Tension Control bolts to be inserted into the structural connections and tightened at a later date without the presence of corrosive properties.

Now it is a relatively common problem on structures where the bolts get installed, the nuts get put on, they maybe even get snug tightened, but they don’t get fully tensioned until sometime in the future. A few weeks, a few months, who knows, maybe six months later, and in the meanwhile we’ve had weather we’ve had rain and there is some rust if the bolts are plain, or even if they’re not plain but hot dipped galvanized even. And now we don’t know whether or not that bolt is going to perform the way it’s supposed to. Well they addressed this for Tension Control bolts, but it really is is just the same exact problem with a hex bolts because when we install the hex bolt we put the nut on, we snug it, we match mark it, and at some point we come back and re tighten it. But it’s susceptible to the same exact two issues that the Tension Control bolts would be susceptible to, especially if it’s coated but also if it’s plain. So putting that against the Tension Control bolt as solving that problem is only half of the issue there. So I’ll be interested to see what this coating is. I’ll be looking it up, and maybe we’ll talk about it more.

But in the meanwhile two things you really just need to be aware of are:

1.        There are no approved coatings for Tension Control bolts, even though it’s pretty common to find them with mechanical zinc,  a couple of different manufacturers make them that way, theoretically because everyone has agreed that we’re going to use them and they work.

2.       The only other coatings that are approved are hot dipped galvanized for ASTM A325 bolts or F3393 dip spin coatings for either A325 or A490 bolt assemblies, not Tension Control bolts.

I know that’s a little bit complicated but it actually is pretty well-thought-out and it’s  really improved and part of the standard, and the standard has a get-out-of-jail-free card if you want to use something different all you got to do is get the producer that supplier and the end-user together and agree that you could coat these bolts with whatever you want, you could use red lipstick, just as long as everyone is in agreement.

Well that’s our Fastener Training Minute for today, along  three of my favorite topics structural bolting, hydrogen embrittlement, and coatings.

I hope you learned something and thanks for listening.

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