Galvanic corrosion issues: aluminum and stainless steel

A. I think there's a 99 percent chance, Saibal, but maybe someone will offer encouragement.

A. Ted is correct, check out the EMF series, you'll see that aluminum is more active than the stainless (iron) so it will corrode preferentially.

A. Dead certain for corrosion. You may be able to prevent it by putting an insulator between the two. Try neoprene or something similar. It must not adsorb any moisture however.

A. Epoxy paint the faying surfaces and pull the rivets wet (soaked, including inside) with zinc chromate primer. Not ideal but good enough for government work.

A. Aluminum in a seacoast environment is not a long term proposition. However, you may want to consider using stainless steel fasteners. The rate of galvanic corrosion is related directly to the ratio of cathode area (stainless) to anode area (aluminum). In other words a relatively small anode area like an aluminum pop rivet will be attacked much faster than a large one like the 1/4" plate. Good luck.

A. Try contacting High Energy Metals. They make metallurgically bonded structures of dissimilar metals, specifically for sea coast environments where corrosion due to dissimilar metals in contact is a threat.

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The explosive bonding sounds like a good method to bond dissimilar metals in some cases, Jim, but how does the fabrication method have any impact on galvanic corrosion? I thought it depended on factors (the emf difference between the two metals, a metallic connection, an ionic connection, and the relative surface areas) that have little to do with the connection method. What am I missing? What material would you explosively bond onto the aluminum pop rivets?

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
2004

Explosion bonding creates a covalent (metallurgical) bond between the dissimilar metals. There is no GALVANIC reaction because the metals behave as one. This does not mean that there will be no breakdown of, say, the aluminum in a corrosive environment. Secondary zincate or chromate should help.

Jim Petri
- Melbourne, Florida
2005

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2005

I think we're on different wavelengths, Jim: if Mr. Sengupta pop rivets an aluminum sheet to a stainless sheet with aluminum pop rivets it's not going to work :-)

Electroplating and galvanizing and some other metal finishing processes also can alloy metals together in the same way. The galvanic corrosion issue arises not in the alloy or the coating or the substrate, but if the substrate becomes exposed due to incomplete coverage, or a scratch through to the base metal because you then have a galvanic battery between two metals.

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey

A. Why can't you put a piece of rubber or fiberglass as a insulator so they don't touch.Use a sleeve for the bolt.

A. I'll certainly be happy to try, Fred.

Picture a conventional dry cell battery still sitting in it's original packaging. The shell is made of zinc while the center rod is carbon/graphite. The two dissimilar materials are not touching each other, but there is an electrolyte, a salty liquid path between them (the black glop). Nothing is happening so far, and the battery will have a long shelf life.

Now take the battery out of the package and, with two pieces of wire, connect a flashlight bulb to the battery. We know that the bulb will light. Here is what is happening: electrons flow out of the negative pole of the battery (the zinc side), through the wire and bulb, causing it to light, and back to the positive pole (graphite) of the battery. Inside the battery, the zinc metal at the surface has become positively charged zinc ions (zinc atoms from which one or more electrons have been removed) and they dissolve and start traveling through the electrolyte glop and meet up with their electrons once again at the graphite rod and become metallic zinc once again, depositing on the carbon. There is a balance here that allows current to flow: the wire connection provides a path for the electrons, the salty glop provides a route for the positively charged zinc ions.

Note what was required for the battery to function: the two different materials for the two electrodes (zinc and graphite), an electrical path (the wiring) and an ionic path (the salty glop). You can also make a battery using aluminum for the negative pole, stainless steel for the positive pole, rainwater contaminated with road salt as the electrolyte through which ions can travel, and a physical connection between the aluminum and stainless for the electrons to travel. If you have all of those conditions you have a battery or a 'galvanic corrosion cell'.

If you can keep the aluminum from touching the stainless, as Tim C suggested, you have no path for the electrons and the corrosion stops, similarly to removing the wires from the battery. If you can keep the area totally dry and absolutely free of salt, you have no ionic path and the corrosion stops. But what usually happens with the stainless rivets in aluminum is you have everything you need for the aluminum to corrode away into the salt water as ions.

A. I would doubt that Loctite would give you a reliable isolation of the two metals because the threads may break through and touch, even if you are able to apply a pinhole-free layer. However, I have read sales claims from manufacturers of marine sealants that this approach works. I guess I'd say if it is for a personal item it is worth trying but if it's an engineering question for a product line, then no. Just an opinion.

Hi, Richard. My experience has been that not even gold plating will make aluminum pop rivets into a satisfactory fastener for stainless steel at all, let alone in the "seacoast environment" that Saibal S speaks of. If you were referring more specifically to Ronald R's posting, coatings of that sort do sound more promising.

Still, if you do sell your coating process to an aluminum pop rivet supplier, I'd love to track how it actually worked. Thanks.

Regards,

A. Hi, Sami. The general principals discussed here are that aluminum and stainless steel are not actually galvanically compatible because they have significantly different potentials, but the passivity of both can reduce the problem to non-problematical for non-critical applications if the area of the aluminum is greater than the area of the stainless steel (stainless fasteners on aluminum architectural features usually aren't a significant problem, but aluminum pop rivets on stainless steel sheets would be).

So I would expect that stainless fasteners on aluminum flanges would not be a significant issue. Stainless wire welded to aluminum pipes, however, sounds a bit problematical to me because there is no opportunity at all for electrical insulation, and the welding process will probably generate several different types of areas that lack any passivation.

Regards,

A. Hi Khalid.

To prevent galvanic corrosion the only thing necessary is to electrically isolate the two metals. For that, neoprene or polysurlyn, or almost any type or thickness of plastic will do fine.

However, the purpose of Polysurlyn (as I understand it) goes well beyond simple electrical isolation: it is intended to be a waterproof, weatherproof coating to prevent the elements from reaching the metal at all.

Regards,