Galvanic Corrosion between Galvanized Steel and Aluminum

A. Hello, Ronald. To my knowledge the grounding neither accelerates nor retards galvanic action; I think that part is irrelevant as long as you are not introducing additional materials.

A galvanic corrosion cell can be likened to a dry cell battery for ease of understanding:
- The two metals in your construction (aluminum and zinc) act like the carbon and zinc in a battery, with their difference in electrochemical potential providing the driving force. Depending on the particular aluminum alloy, this potential difference between it and the zinc might be around 0.2 volts rather than the 1.5 volts potential difference between carbon and zinc.
- Ambient moisture and accumulated salts play the role of the conductive glop in a battery. Unless bleach is used for cleaning, or some other unusual circumstance, the environment will probably be fairly dry and non-conductive.
- Physically connecting the two metals without insulators between them is like connecting the positive and negative poles of a battery with wire. When it's practical to use insulators you should, and then there is no possibility of galvanic corrosion.

I have seen somewhat similar construction to what you speak of -- galvanized steel and anodized aluminum -- used in outdoor construction with no problem as long as away from the sea, so it should be even less of a problem in indoor use. That is probably because zinc and aluminum are pretty close galvanically and anodized aluminum has a non-conductive anodized film on it which limits the metal-to-metal contact. But where I've seen it, galvanized U-Bolts clamped to the aluminum, so there was less disruption of the anodized film than if you drill it, exposing bare aluminum, and screw it together. But if the bolts are large I would use galvanized bolts rather than zinc electroplated bolts; and if they are small (and consequently affordable) I would use stainless steel bolts.

It's also important to realize that "good enough" always depends on circumstances. Galvanized bolts on aluminum bleachers is one thing, and is probably okay, especially indoors, but galvanized bolts can absolutely not be used to connect aluminum aircraft parts. Good luck.

A. I agree with Mr. Mooney. On indoor applications such as this, galvanized and zinc plated hardware shouldn't pose a real threat. However, if you wanted to go one step further to ensure that galvanic corrosion will not be a problem, I recommend looking into zinc/aluminum dispersion coatings. These thin, dry film coatings protect the steel from red rust while diffusing the bi-metallic cell created when the steel hardware comes in contact with the aluminum.

Q. I realize the battery-like aspects of galvanic Interaction of dissimilar metals. With regards to the grounding I was thinking about the grounding that is used on Microwave towers and the Guy Wires attached to a copper ground. I wonder if grounding a bench would give the current a path to travel instead of sacrificing the metal. The purpose would be to effect the interaction between the galvanized steel and the soil. Guy lines on towers would have more stray current.
I feel confident that the anodized surface would provide enough insulation though.

A. Hi, Ronald.

Different sorts of systems are grounded for various reasons, not necessarily to prevent galvanic corrosion. But you could take a car battery, and ground either pole (but not both), without effecting its functioning, and I think the same is true of a galvanic cell battery.

Regards,

A. Attaching aluminum siding to steel should be done with non-metallic [ed. note: i.e., insulated] stainless steel fasteners, or a specially coated grade 5 fastener to prevent not only normal corrosion of the fastener but also a phenomena called hydrogen assisted stress cracking corrosion. You may consider using an isolator between the aluminum and steel as well, it will serve the dual purpose of being a thermal break should you decide to insulate the wall and condition the enclosed space. You didn't say if the steel is galvanized, or what the finish on the aluminum is to be. All of those make a difference in the answer.

A. Hi, Trice.

I can't answer the question about the PVC fittings, and don't have a clear picture of what you are asking, but maybe another reader will.

Any time two different metals are actually touching each other (metal to metal contact) in a damp environment, you have some potential for galvanic corrosion. However, the seriousness of the situation is widely variable and depends on the application. For example, it would be absolutely unacceptable in a jetliner. But I have seen galvanized and aluminum railings and fittings mixed together in many cases with no serious consequences; I would be very surprised if you got any galvanic corrosion if the poles are touching; if they are not touching, you cannot. Good luck.

Regards,

A. Yes, while aluminum and zinc (the galvanized finish) are fairly close together in electrochemical activity in the seawater series, they are not close enough. Galvanic compatibility with aluminum is one of the things that helped keep cadmium around for so long despite its toxicity.

True compatibility is achieved by making it all zinc coated or all aluminum, but good anodization of the aluminum will help a lot. The $64,000 question, of course, is what are the details of the application?

A. Dear Elie Rothschild,

The answer is: YES.

The only question mark is the time span in which this will occur. I wouldn't be surprised if within a year or a few years the connection is lost, but I have no practical experience with your actual situation(s). The Al will dissolve preferentially.
If you have the possibility to isolate the RVS from the Aluminium, e.g. by using a teflon seal, that will already greatly reduce the chance for a devastating galvanic couple to occur.
If you could use fasteners from the same material, that would even be a better solution, although still a potential difference will occur, due to the fact that the two surfaces are never equal and due to crevice corrosion.

Best regards,

A. Hi. I agree with Harry, but only partially...

You need a conductive liquid for galvanic corrosion to occur and I have heard several stories that when the only liquid that the joint is exposed to is rainwater, it's really not much of a problem. Of course, this could be tempered by whether the installation area is near the sea or an industry that could create strong acid rain.

If the holes were predrilled and were anodized, rather than being bare as you mention, I think the problem would be very minor. In view of the exposed aluminum in the holes, I think insulating bushings and washers should probably be used.

Regards,

A. Aluminum PV on Galvanized or Aluminum framing will require to be electrically grounded per the requirements of NEC 690.47
The typical solution is WEEB washers to mechanically cut through the anodized alum surface to the base metal to ensure electrical contact. Stainless fasteners are permitted, and likely the best choice.
RBL.

A. Thanks, Toronto. This is an excellent point! While it's fine to be concerned about potential galvanic corrosion as a generality, an installation must comply with applicable codes for the industry and the locale, especially with regard to potential hazards from poorly grounded electrical equipment.

It sounds like aluminum panels, galvanized framing, stainless steel hardware, and maybe copper wire will be galvanically connected on solar panel installations. Fortunately, rainwater (away from the sea and industrial pollution) is relatively non-conductive.

Luck & Regards,

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

A. Hi Bruce.

When dissimilar materials are in contact with each other, it becomes a matter not only of picking corrosion resistant finishes, but finishes that are galvanically compatible with each other. For this reason, zinc would probably be better than nickel when in contact with aluminum, since they are reasonably close together in the galvanic series.

A thin zinc plating will not be enough. I would suggest either zinc plating plus powder coating (which may reduce or eliminate the galvanic corrosion), or hot dip galvanizing (i.e., a thick coating of zinc).

Regards,

A. Hi Marilyn. I have no personal experience with that, but I have heard that the zinc alum panels are pretty good at "diffusing" any galvanic issues with galvanized materials.

Regards,

A. Hello Mr Latimer,

As its been 2 months since you posted your question, I do hope that you used Stainless Steel bolts and not the zinc coated ones as the zinc could possibly play a spoilsport somehow. SS is more like the Corten Steel in a way of its alloying elements and the zinc would be inviting trouble.

A. Hi. I think I'd want to use fiberglass screws/bolts or insulators on metal bolts in this case.

Regards,

A. Hot dip Galvanized Bolts would be the best in your case.

A. I've heard similar questions / statements before - that zinc as a sacrificial coating will leach off and cause pollution.

But what alternatives?
Paint: flakes off and goes to earth.
No protection. Rust flakes off and goes to earth where it came from, as iron oxide.
Zinc coatings: oxidize and goes to earth where it came from.

It's a bit hard to define pollution to someone who wants to be emotional about it. Zinc is a natural element, comes from the earth, protects steel (which in turn saves energy and air pollution), and returns to the earth from whence it came.

A. Hi Brian. Yellow chromate is fine for aluminum although it's an environmental hazard and I'm a bit surprised you were able to buy it. "Galvanized spray" (zinc-rich paint) is for protecting steel, and should not be applied to aluminum.

Regards,

A. Hi Amy. It's not clear to me if you are getting new gutters yet again with the re-roofing. If so, the small cost of a neat job should be accepted because the remnants of the old gutters will rust up some day. If you are just getting new shingles, and leaving the gutters, then leave them. Although there is somewhat of an incompatibility, a gutter is not an airliner. I see stadium bleachers at high school football games all the time where galvanized and aluminum materials are freely mixed.

Regards,

A. Hi. No, that explanation doesn't sound plausible to me, but ... An electric fence is not an airplane, and galvanic corrosion issues are not nearly as big a deal. There are lots of examples of aluminum in contact with galvanizing where it is not a serious issue. Rain water is essentially non-conducting, and if this area is free from use of road salt and fertilizers, there should be essentially no galvanic corrosion. It's nearly impossible to predict life expectancy when we don't know the environment, the thickness of the galvanizing, the anodizing (if any) on the aluminum, etc. -- but galvanic corrosion probably does not seriously impact it in this case. Good luck.

Regards,

A. Hi Peter. Galvanic actions don't always proceed in the direction we immediately anticipate because of aerobic vs. non-aerobic conditions, localized phenomena, etc. One reason cadmium plating was such a popular finish for decades is its proven galvanic compatibility with aluminum for critical applications. If you are okay with cadmium plating despite its toxicity, it is an immediate answer to the problem.

If you are required to, or desire to, avoid cadmium plating, one expensive solution is aluminum coated hardware (Ivadized, or electroplated from molten salts, or from organic liquids with the proprietary Sigal / Alumiplate process). Good luck.

Regards,

A. Hi Thomas. As you know, freestanding steel will rust; galvanic incompatibility is not the only cause of corrosion. For galvanic corrosion you need a liquid connection and a metallic connection. So, if there is "no direct contact" between the aluminum and the steel/galvanized components, there is no galvanic problem. So if I am properly understanding you, your plan for "the final piece of insulation metal skin" to be galvanized sounds good.

Regardless, it sounds like the "soaked" insulation is the principal problem. Maybe a change of insulation type to some kind of closed-cell foam is in order. Good luck.

Regards,

A. Hi Peter,

Insulating the galvanising from the aluminium won't hurt, but to be honest I don't think galvanic corrosion is your issue, I think it is simple atmospheric corrosion. If you aluminium is completely unprotected the aggressive nature of salt solutions probably means the natural oxide layer is insufficiently adequate to protect it. Consider having the sheet anodised, this will give better corrosion protection (although this will reduce thermal conductivity).

It is always unwise to put dissimilar metals in direct contact with each other and in an environment where corrosion could occur. In the case of aluminium, it is best to use anodised aluminium, as the hard anodised layer (oxide) is electrically insulating and will help prevent galvanic corrosion. If it is not possible to use anodised material, then you need to use an insulator between the two metals - this can be as simple as a plastic (or rubber) washer or insert. Simply, if the two dissimilar metals are not in direct electrical contact with each other, there can be no galvanic corrosion.

A. Hi Brandan. If the aluminum is not metallically connected to the galvanizing you can't have galvanic corrosion. If they are connected, then we're into the zone of everything is relative. If it was a boat trailer and the aluminum/galvanized junction could get wet, I would not go for it. If it's a builders trailer that doesn't get salt water on it, then you need to look at how bad the road salt situation might be for your geography and usage. The thing is, the two metals are not grossly incompatible, like galvanized and copper would be, but they're not compatible either.

Regards,

Q. Hi Ted,

Thank you for your assistance, what do you mean in terms of metallically connected?

The aluminium top is on plastic or rubber packers but is bolted down to the trailer with galvanised bolts and nuts and also the front and rear ladder racks are galvanised and in direct contact with the ally checker plate top. Will this be a problem and if so will plastic spaces fix the problem and is there an alternative to galvanized bolts that would reduce reaction?

I currently live in the city so it wouldn't be likely to be exposed to salty environments on a frequent basis.

I just don't want to spend all the extra money to prevent rusting of a zinc primed trailer and create more problems for myself in doing so.

Thanks very much in advance

Regards,
Brandan

A. Hi again. Metal conducts electricity whereas plastic & rubber don't. So by "metallically connected" I mean any metallic path from one to the other, not interrupted by plastic or rubber. If the bolts are sleeved with plastic washers and bushings, the parts are not metallically connected. If the bolts are metal and touch both the aluminum and the galvanized surfaces, they are metallically connected.

But the unfortunate truth is that it's very difficult for a consumer to determine the corrosion resistance or suitability of materials from any sort of first principles. That's why we must rely on the reputation that a manufacturer has earned, testing by Consumer Reports, Better Business Bureau files, etc.

Regards,