Risk of galvanic corrosion of dissimilar metals in bonding the chassis to Earth?

[SoulFetish]

When I was in the early stages of the amplifier project I'm working on, I had tentatively planned on using all stainless steel fasteners and hardware when the time came to start the construction. I chose Aluminum for the chassis material for a number of reasons. But namely because it's easier to mill and I like the finish over steel.
Anyway, when I started researching fastener types and material for the project, I realized that using stainless steel(or other materials) could present potentially serious problems under certain conditions and I wanted to get some feedback on this.

Here is a simple and quick rundown on what I'm talking about for those who may need it. This is a PDF published by Fastenal Engineering & Design Support:

Article - Corrosion.pdf

One point to note is, that a galvanic reaction will only occur in the presence of an electrolyte. Equipment which will always be maintained and stored in a controlled environment (such as indoors in a residential home), probably won't be affected. So, this will probably never come up in the toaster forum.
But, we build amplifiers for all kinds of musicians. Musicians who live near the coast and high humidity environments, or in cold weather climates where large quantities of salt is used to control on roads and walkways. I've seen amplifiers which have been through floods, fires, or worse - Warp Tour.
Unfortunately, the only way of protecting the metals from galvanic corrosion is electrical isolation. Not an option for connecting the chassis to earth! This is really the point which is of most concern to me. This is where oxide buildup on steel would create an insulating layer of resistance, or the corrosion of aluminum (which is the more anodic metal), would also degrade the contact area we rely on in event of a fault.
So, what's the solution for those of us who use aluminum chassis? Even though stainless steel looks pretty and offers great resistance to corrosion under ideal conditions, Zinc plated steel is probably the safer bet for all conditions. Actually, deposited aluminum coating on steel would probably be the best solution.
Maybe I'm being a little obsessive or paranoid, but if UL 60065 Audio•Video Safety Guidlines, state that the resistance from the earthing conductor terminal to the chassis cannot exceed 0.1 ohms, I can live with being overly cautious.
But, I would love to hear from you guys who have serviced amps for years. Have you seen this come up as an issue? Or is there any long term data on this? (Am I overthinking it?)

[mozwell]

i doubt you would ever see a problem, but you could use an aluminium Penetrox joint compound if you are really worried about it.
There are many other joint compounds out there that will do just as well.
Using zinc plated steel fasteners on an aluminium chassis should be fine, and be sure to use nice thick star washers that cut into the panel for the main earthing point. Adding the penetrox will stop the aluminium forming its oxide layer & keeping the integrity of your main earth up to scratch. Just before you apply the penetrox, brush the area to get rid of the oxide layer, apply the pentrox & make your earth bolt assembly.

All that said, Penetrox is mainly for use on copper / aluminium connections, or for high current connections, but if you are worried, then using a small amount of it wont hurt at all.
A small jar of this will last you for years, so buy the tiniest amount you can get....

[Tom Phillips]

I agree with mozwell that you are not likely to have a problem with consumer equipment used indoors and outside intermittently under reasonable conditions. The article you linked is directed at equipment designed for use outside in the weather. There is much more detailed information on the subject used by shipbuilders and designers of equipment that is deployed in salt water. In those cases the dissimilar metals issue is extremely important. I have seen many examples of 50+ year old electronics equipment constructed with aluminum chassis and various plated steel fasteners that was still in excellent condition corrosion wise.

[Chuck H]

From a strictly experienced layman perspective, it's not uncommon in older electronics for bolt secured dissimilar metals to lose a good electrical connection due to oxide formation, but I haven't seen a case where electrical contact was compromised due to galvanic corrosion. In extreme cases I'm sure it needs to be considered, but most of us aren't working in kilovolts in ocean spray environments

[SoulFetish]

i doubt you would ever see a problem, but you could use an aluminium Penetrox joint compound if you are really worried about it.
There are many other joint compounds out there that will do just as well.
Using zinc plated steel fasteners on an aluminium chassis should be fine, and be sure to use nice thick star washers that cut into the panel for the main earthing point. Adding the penetrox will stop the aluminium forming its oxide layer & keeping the integrity of your main earth up to scratch. Just before you apply the penetrox, brush the area to get rid of the oxide layer, apply the pentrox & make your earth bolt assembly.

All that said, Penetrox is mainly for use on copper / aluminium connections, or for high current connections, but if you are worried, then using a small amount of it wont hurt at all.
A small jar of this will last you for years, so buy the tiniest amount you can get....

Moz! Thanks for the great info and reply.

[SoulFetish]

I agree with mozwell that you are not likely to have a problem with consumer equipment used indoors and outside intermittently under reasonable conditions. The article you linked is directed at equipment designed for use outside in the weather. There is much more detailed information on the subject used by shipbuilders and designers of equipment that is deployed in salt water. In those cases the dissimilar metals issue is extremely important. I have seen many examples of 50+ year old electronics equipment constructed with aluminum chassis and various plated steel fasteners that was still in excellent condition corrosion wise. Also keep in mind that you will have other items such as pots & tube sockets that you will need to attach to your chassis. If you try to specify specific metals for those items you will be going down a difficult path.

All fair points. My main concern was with the earth/chassis connection and looking long term ie. 15-20+ years. As far as mounting fasteners go, I'm thinking zinc plated steel will do the trick. But I appreciate the feedback, it's helpful.

[Chuck H]

I assume you know this, but it wasn't mentioned, so... There are anti corrosive "grease" products for such connections too. They have a high melting point and dielectric properties. I see the stuff still in place and doing it's job in better products from fifty years ago! I haven't used it, but they should be readily available. I know Loctite makes one.

[Richard]

Would you use the dielectric grease on an electrical connection?

[SoulFetish]

Would you use the dielectric grease on an electrical connection?

I've never wondered what it would be like to make a fastener capacitor until just now.

[Chuck H]

I've never wondered what it would be like to make a fastener capacitor until just now.

The audiophools will love that idea. It's just a way to minimize oxide formation. And as far as I can tell it works. Marshall never used it on their aluminum chassis and there are occasional problems with the bolted ground connection in those amps. I think anyone would consider "a few decades" enough longevity for build quality, but if you're serious about using aluminum AND avoiding the problem the grease can't hurt.

[eschertron]

There's a conductive grease that I've used on some auto headlamp connections in the past, called Ox-Gard. Not sure how many similar products are out there, but it's the one I've found that fits the bill. Ox-Gard Anti-Oxidant Compound | Gardner Bender

Yes, I know I'm not having fun building capacitors out of erector-set parts.

I wonder if I could build a working capacitor out of a peanut-butter and (dielectric) jelly sandwich?

[Tom Phillips]

Would you use the dielectric grease on an electrical connection?

I believe that one of the functions of that type of grease is to keep the moisture and air away from the contacts. The actual conductive path could be the points of a star washer pushing into the chassis and the ring contact attached to the wire. Given that I'd say yes, you could use the dielectric grease on an electrical connection. I have seen it done on outside connections. Silicone grease filled wire nuts are sold for applications such as outside sprinkler valve wire connections. The grease itself does not need to be conductive in that case.

[Chuck H]

I believe that one of the functions of that type of grease is to keep the moisture and air away from the contacts. The actual conductive path could be the points of a star washer pushing into the chassis and the ring contact attached to the wire. Given that I'd say yes, you could use the dielectric grease on an electrical connection. I have seen it done on outside connections. Silicone grease filled wire nuts are sold for applications such as outside sprinkler valve wire connections. The grease itself does not need to be conductive in that case.

Exactly. The real question then shouldn't be "Would you use the dielectric grease on an electrical connection?". A better question is, would you use a conductive and viscous solution in proximity to other electrical contacts in a chassis that gets fairly hot? You clean the surfaces with deox, make the connection and then slather on the grease. I suppose you could use a bob of silicone too. I'd have more confidence in the grease. Come to think of it I HAVE used the stuff. It came with my new headlamps. Try to keep the stuff off your hands or your grip will be "lubricated" all day.

[SoulFetish]

It should be stated that you would never, EVER, under any circumstances use a dialectric compound between the physical connection of your chassis and Earth!

[Ken Moon]

The problem with connections between copper (or stainless steel) and aluminum (or silver) is the formation of copper oxide and/or aluminum oxide, which can be lessened by the use of an antioxidant paste containing zinc particles, like Penatrox. The zinc acts as a "sacrificial" anode to protect from the galvanic action between the dissimilar metals.

If you like to use stainless steel hardware on aluminum chassis (I do), you can place a zinc washer between the aluminum chassis and the stainless washer/bolt/nut, and it should last even longer.

The formation of copper and aluminum oxides is greatly enhanced by the presence of oxygen, so sealing the connection (with clear coat paint or even liquid plastic or rubber) is the last line of protection, even if you use zinc to reduce the corrosion.

Do all three of these things, and you can play your amp outdoors at the beach for decades