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Unread 04-13-2009, 08:22 PM   #1
beeper
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Rust due to metal on metal

I was telling someone how bad my gas tank skid plate was rusting on the passenger side and they said that it was because there are two pieces of metal in contact there. They went on to explain that when two pieces of metal are touching the rust gets really bad because salt and everything else just sits between the two pieces. If this is true, then whats the difference between those two pieces of metal and hinges or, more importantly for me, rocker guards? I know that rubber is sometimes used between metal, but why don't rocker guards such as these cause a lot of rust to form? Also, does it matter if the peices are painted prior to being put together?

edit: Just came across this. Is this the solution to the metal on metal rust problem?

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Unread 04-13-2009, 08:55 PM   #2
prairefire
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The place I heard this was on a home reno show: Steel should never touch copper, it will cause oxidization.

Not sure why, never took Chem in high school.
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Unread 04-14-2009, 06:38 PM   #3
lupinsea
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What prairefire is talking about is called a galvanic reaction. I forget the chemestry behind it but it operates sort of like a battery in that when the two dissimilar metals touch eachother it creates an electrochemical reaction that causes the electrons to migrate from one metal to the other (I think) which causes oxidation (rusting).

Ah, found this.

The idea is that on the galvanic scale, metals that are further apart on the scale will have faster corrosion rates. Metals that are closer together will have slower rates of corrosion.

Sometimes the galvanic reaction can be used to improve corrosion resistance such as with galvanized steel. The layer of zinc plating corrodes instead of the underlying steel (I think).

With all this being said, i can't figure out what different metals your gas tank is reacting too. It's a steel gas tank mounted on a steel chassis with steel nuts and bolts.

It sounds like it's a simple case of the contact rubbing off the paint coating and exposing bare steel to the elements.
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Unread 04-14-2009, 08:47 PM   #4
beeper
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Very informative replies. Has anyone heard of people having problems with rust after putting on rocker guards? I'd like to buy some but not if it increases the chances of the body rusting. It doesn't seem like this is a problem, but I wanted to make sure before I purchased a pair.
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Unread 04-15-2009, 11:16 AM   #5
lupinsea
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Rust under rocker guards can happen. Get some grit or mud back there, then any slight movement between the body and the rocker guard causes things to rub and grind off the paint. Get some moisture back there where you don't have the air circulation to dry it out . . . To say nothing of any metal-on-metal rubbing if there's nothing between the rocker guard and the body tub.

The link in your first post is a good product and what I went with on my rocker guards. But you can also use other gasket-like material. Someone suggested some cheap vinyl flooring, or some thin rubber sheet trimmed to fit. You still would have any water issues but the gasket would act as a cushion to prevent the metal-to-metal contact that could rub off the paint. With the gasket material on there it would be a similar condition to the plastic fender flares and how they contact the body and "trap" moisture. After 8 years being on my Jeep I pulled off one of my flares. . . . no rust, no paint rubbing issues at all.
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Unread 04-15-2009, 11:24 AM   #6
Exparrot
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I used rubberized undercoating on both my rockers and my corner guards to keep a little buffer between the metal and the tub. Essentially it does the same thing the "sticker" does and it's a lot cheaper than the alternative.
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