FTM 109: Zinc Flake Coatings and Galvanic Compatibility.

Fastener Training Minute 109: Galvanic compatibility as it affects zinc flake coatings

This edition of the Fastener Training Minute with Carmen Vertullo was originally published September 22, 2016 as “Zinc Flake Coatings and Galvanic Compatibility” during episode 109 of Fully Threaded Radio.

Well, hi everybody, this is Carmen Vertullo with the Fastener Training Minute coming to you from the Carver FACT Center here in El Cajon, California, and the Fastener Training Institute. Today’s topic has to do with corrosion, specifically, what are the things that a supplier or even an end user must consider when they choose to change a coating or even specify a new coating for fasteners in an assembly.

I’ve discovered some very important things just recently that I want to share with you and maybe ask you for some help. So when we come back, we’ll talk more about corrosion and coatings and fasteners.

I laid the groundwork for our talk about coatings and corrosion and fasteners, and specifically the topic of galvanic compatibility. What galvanic compatibility means is that when we choose any type of metal which is going to be in contact with another metal, whether it’s a base metal or a coating, those metals could live well and play well with each other, or they may not be very friendly. For example, Aluminum and Stainless Steel are on the opposite ends of the galvanic compatibility chart, which means one or the other of them are going to suffer. In this case, it would be the aluminum. For example, if we were to bolt some aluminum plates or sheet metal parts together with stainless steel fasteners, the stainless steel may be quite happy but the aluminum could corrode out from around it.

So recently I had a situation, actually three of these now in the last 2 years, and another one came to light recently where a supplier used a coating (it’s one of our best coatings), that’s what we call the aluminum zinc dip spin coating from ASTM F1136. You know this coating from a variety of different trade names such as Magni, Geomet, Dacromet and others. They are great coatings and they are affordable, they give us great torque tension control and they’re very corrosion-resistant.

The fact of the matter is, though, that these coatings have aluminum in them. So when we use them with stainless steel assemblies we may suffer the consequence of having some corrosion because of the galvanic incompatibility of the aluminum in the coating, with the stainless steel components that we are bolting together.

Now, I’ve seen this three times it obviously doesn’t happen all the time. And there probably are some good ways that we can use these coatings with stainless steel if we learn how to insulate or isolate the fastener from the stainless steel component.

So I had told you I was going to ask you for some help on this. I would just like to know if anyone has seen this issue before. The specifications nor the manufacturers of the Coatings give any particular warnings or cautions against using stainless steel, but I’ve seen it three times now, so I’m trying to put it out there and see if we can learn more and maybe keep this problem from happening in the future.

I’m bringing this to your attention because I do need some help with the problem and if you want to chat with me about it go to my website to contact me. There is plenty of contact information there and I’d be happy to chat with you about it.

This has been Carmen Vertullo with the Fastener Training Minute.

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