Metallurgy: mixing stainless and aluminum

I’m building a custom battery/under seat tray. I picked up polished stainless steel for the tray/fender and hardware of aluminum angle stock (angle iron) and rivets.

Then it hit me, dissimilar metals may cause aluminum corrosion.

The bike is stored outside under a cover. Should I expect the rivets to corrode quickly and create problems cosmetically or structurally?

you will be fine. It will eventually cause corrosion but it will take a very long time to do any if that might compromise the structure. The speed of the corrosion is effected by the alloys you are using.

doc_rot said:

you will be fine. It will eventually cause corrosion but it will take a very long time to do any if that might compromise the structure. The speed of the corrosion is effected by the alloys you are using.

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Ok great thank you

jetmechmarty said:

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Interesting chart... is it meant to show degrees of anodic vs cathodic and imply a relative stability (borderline) between the stainless and aluminum? I’m assuming that’s how it ought to read.

It's showing, in the case of galvanic corrosion which is what we're talking about, that one metal acts as an anode and one metal acts as a cathode. There have to be certain other factors involved to cause an issue. The reason it can be an issue when using stainless bolts in a motor is because of heat. Heat = electricity which causes galvanic corrosion in dissimilar metals. Water would be another issue, since it acts as an electrolyte. In battery tray, you are unlikely to encounter either, or at least you can do things so that you don't. You can also seal either the aluminum or the stainless steel to prevent corrosion.

It would last approx. 3 months at sea! On a bike youre good.

You must have an anode, a cathode, and an electrolyte for corrosion to occur. Remove any one and you're good.

jetmechmarty said:

You must have an anode, a cathode, and an electrolyte for corrosion to occur. Remove any one and you're good.

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Yea salty water is one hell of a electrolyte, ask my car

When you look at the chart, it's the anodic metal that gets eaten in the presence of electolyte.

Marty,
That chart looks familiar - is it out of a FAA manual?
Question - aren't there suitable aluminum rivets for the project?
BTW, you folks that bolt carbon fiber parts on your rides without using CRES (corrosion-resistant steel) fasteners or similar (good quality stainless fasteners are probably okay on our stuff) run the risk of the metal - especially aluminum - getting a serious galvanic corrosion problem.
Years ago, some bright spark designed the wings of a naval aircraft with carbon fiber skins and aluminum honeycomb core material without a fiberglass barrier ply between the skins and core and yup, the wings had to be replaced after awhile due to galvanic corrosion. Obviously, salt water is a great electrolyte and accelerated the problem but IMO, would have happened eventually regardless...
Interesting stuff that gets posted on DTT - why I keep browsing the site after all these years.
Pat

pacomotorstuff said:

Marty,
That chart looks familiar - is it out of a FAA manual?

Pat

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I pulled the chart using Google, but yes, aspiring Airframe & Powerplant mechanics are supposed to be familiar with this stuff.

A good example of this is when you remove a bolt from an old motorcycle engine case. The bolt may be covered in white powder. There is no rust on the bolt. This is because the aluminum alloy is anodic to the bolt. The aluminum is corroding, but not the bolt.