Corrosion control in portable gear.

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dalesql

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I have a bunch of portable power distribution equipment that gets rented out to clients for trade shows, fairs and festivals. It's pretty much all cord and plug connected gear, end users are usually not electricians, and the gear suffers a fair amount of heavy use. I'm not really concerned with normal wear and tear on the gear. We keep lots of replacement parts, and repair anything bad that we see upon return from a customer.

My specific question is we have cord sets for portable power distribution that have cast aluminum boxes and covers, and inside are steel parts and screws, and the copper ground wire to the grounding screw inside the box. As I work on these things, I'm seeing quite a lot of corrosion of the aluminum boxes, as the box is acting as a sacrificial anode for the steel and copper metal parts.
The corrosion is not developed to the point where I'm worried about structural integrity to the boxes, but the corrosion is worst where the steel screws go into the aluminum threaded holes, and the corrosion is causing the paint on the outsides of the boxes to flake off.

Right now, it's mostly a cosmetic issue, but I figure down the road another couple of years and my screw holes will be losing enough thread that covers are gonna start falling off on me, exposing the energized guts of the boxes to my customers. I'd like to avoid that, and I'd like to keep the boxes looking nice.

I'm playing with the idea of attaching some kind of magnesium sacrifical anode to the exterior of the box, but have not found something that doesn't require custom magnesium parts at a prohibitive cost.

Any better ideas out there? I really don't want to have to replace all the aluminum boxes every few years. Lots of labor and parts for just a cosmetic issue, and I don't want to lose the gear, or any of my customers to corrosion faults.
 
Stainless steel screws might help but dissimilar electrometallic corrosion tends to concentrate at the junctions of the different metal, so I don't thing it will do much. As for using zinc, when I finally found a link on the web that showed which metal I use to protect what metal, it said protect aluminum with magnesium, protect steel with zinc or magnesium.

Most of the magnesium stuff I'm finding on the intertubes is boat and bridge corrosion control anodes. Large lumps of magnesium with steel straps through them to bolt onto steel structures and streamlined lumps of magnesium to attach to boat parts. But I found one local wholesaler and I'm sending an e-mail off to them in hopes that they can help me.

Another user privately suggested changing out the aluminum boxes for non-metallic boxes rated for the use. That is actually the long range plan, but the non-metallic boxes are even more expensive, and we have a fairly large stock of existing aluminum boxes in use, and on the shelf as spares. I'm lazy, I'd much rather spend some time on corrosion control now to save me the repair time later and reduce customer problems.

I should have also mentioned that cosmetic appearance for some of our clients is important. The multi-million dollar wedding event last summer used a lot of these stringers for the interior and exterior lighting on the tents, and power for the caterers and entertainment acts. Also, we get a lot fewer hassles from the various local electrical inspectors on the gear if it looks nice and spiffy.
 
Have you considered nylon screws w/ nylon washers to insulate the parts at the screw connections? This would of course require bonding jumpers, but I would think a jumper w/ a SS ring connector and SS stud may be far less likely to corrode? Plus you could place the jumper in a less visible location?

My 2?
Doug S.
 
Assuming the corrosion is being hastened by moisture, I would look at ways to moisture proof the boxes and coat the dissimilar metal joints. There are some waxy grease compounds or rust proofing sprays that you could use on the joints and some silicone sealant as a gasket on connectors and box seams should do a lot to keep out moisture. Any chance of a photo?
 
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