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Galvanic Corrosion between Galvanized Steel and Aluminum



Bleachers/benches of combined aluminum and galvanized construction

++++++

Q. We are installing anodized aluminum benches in a basketball gym. The supplier has provided galvanized steel brackets to be attached to the bench and zinc plated steel bolts from the bracket to the concrete.
Questions:
1) Will the galvanized bracket be sufficient to prevent the corrosive interaction between the steel and the Aluminum?
2) By attaching the steel bracket directly to the concrete, will that provide sufficient grounding to prevent the electrolysis?

Ronald McKenzie CEF
consultant - Huntington Beach, California


++++++

A. Hello, Ronald. To my knowledge the grounding neither accelerates nor retards galvanic action; I think that part is irrelevant.

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, 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.

pic of Ted Mooney Teds signature
Ted Mooney, P.E. RET
finishing.com
Pine Beach, New Jersey


++++++

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.

King Drummond
- Cleveland, Tennessee


++++++

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.

Ronald McKenzie CEF [returning]
consultant - Huntington Beach, California


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,

pic of Ted Mooney Teds signature
Ted Mooney, P.E. RET
finishing.com
Pine Beach, New Jersey



+++++++

Q. I have a new 5th wheel RV with aluminum window frames. The screws holding the windows had begun to rust and now there is a crystalline growth extending out from various areas of the track the window slides along. Some screws have literally disintegrated. I also have power loss from my twelve volt batteries. Could this electrolysis be caused from the grounding of the electric circuit or is it likely to be caused by a contact to a live wire, perhaps a screw puncturing a live wire?

Lyle A Balfour
Consumer - Revelstoke, BC, Canada



Aluminum greenhouse on galvanized footers

May 18, 2009

Q. I have an aluminum sided greenhouse to install and my question is this: Can I set galvanized steel footers and bolt the aluminum siding directly to the footers without worrying about corrosion? Is there a better way to make this connection?

Leslie Fay
jeweler/metalsmith - silver spring, Maryland


May 20, 2009

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.

Paul Griese
- Canton, Connecticut



Tent frames of mixed aluminum and galvanized construction

May 10, 2011

Q. I am looking to change out my galvanized tent frames and fittings to aluminum. I will not be able to do them all at once, is there any danger mixing the galvanized fittings with the aluminum poles in the interim? The tents are exposed to both sun and rain.

Also, would PVC fittings be durable and sturdy enough to use with aluminum poles to construct frames supporting up to 30 lbs dispersed over an area of 100-125 square feet.

Trice Ray
- Bridgetown, Barbados

May 12, 2011

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,

pic of Ted Mooney Teds signature
Ted Mooney, P.E. RET
finishing.com
Pine Beach, New Jersey


Electrolysis between aluminum solar panels and galvanized frames

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Q. Will electrolysis occur when aluminum is in contact with galvanized steel?

Craig Horner
solar energy - Chico, California


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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?

pic of Ted Mooney Teds signature
Ted Mooney, P.E. RET
finishing.com
Pine Beach, New Jersey

March 15, 2012

Q. I would like to ask a Question about solar panels. Usually we make a solar installation with a projected lifespan of 25 years; currently we use aluminum framed panels and hot zinc dipped " galvanized " framing to mount the panels on with stainless steel bolts. My question, of course there is some acid in rain, and it rains quite a lot here, also on top of factories other pollutants that are corrosive may be expected.
For a 25 year life span, what would be recommended? Nylon spacers to keep the panels and frame separate? Or are there better materials we can use? Or will it not be an issue?

Thanks for your time!

Arjen Helder
solar panels - Xiamen, China


August 25, 2010

Q. Hi,

We manufacturer a solar racking system that utilizes galvanized (hot dipped or pre-galvanized) Unistrut. The solar panels are anodized (typically black). We use stainless bolts to attach the panels to the unistrut. The bolts attach through the aluminum mounting holes that are exposed aluminum. Will a galvanic reaction occur with this configuration? This is being installed on flat commercial roof space and fully exposed to the elements.

Elie Rothschild
manufacturer - SF, California


August 26, 2010

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,


Harry van der Zanden
- Budapest, Hungary


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 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 be used.

Regards,

pic of Ted Mooney Teds signature
Ted Mooney, P.E. RET
finishing.com
Pine Beach, New Jersey


July 12, 2012

Q. Can anyone help me with this problem: the roofing is only about 18 months old, and there are already black rust spots appearing (between 1/2 " and 3" across) in different areas, the whole length of roofing under the solar panels drip lines also have major white rust or oxidation occurring?

Jesse Willetts
- Daintree, QLD, AUSTRALIA

June 25, 2014appended

Q. We have been mounting solar panels of various kinds (typically anodized aluminum framed) on standard, off the shelf, galvanized Unistrut channel. This allows for very cost effective, structurally sound installations.
Recently, a concern about the materials mix was brought to my attention. I have never seen any problems in the field, but our oldest installations are barely 10 years in service.
Any words of wisdom on the subject?

Thanks

Marcus Maedl
solar installer - San Diego, California, USA
  ^- Privately contact this inquirer -^



March 31, 2012

Q. I am starting to manufacture a mild steel product that will be zinc, nickel or powder coated. Will the nickel or zinc coating be strong enough to resist corrosion when it is placed up against an aluminum support? We are leaning towards nickel. Will the corrosion be there still but over a longer period of time? The other reason we are leaning towards nickel is that it in exposed to road salt in parts of the country. Thanks

Bruce Robinson
New Manufacturer - Regina Saskatchewan, Canada

April 4, 2012

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,

pic of Ted Mooney Teds signature
Ted Mooney, P.E. RET
finishing.com
Pine Beach, New Jersey


Galvanized nails with zincalum roofing

August 5, 2012

Q. I have zinc alum roofing sheets. We have access to galvanized steel nails for fastening the sheets to the rafters. Is this going to cause trouble soon? What trouble? Would it be better to try to buy zinc alum nails or some other metal?

Marilyn Stein
- Waiyevo, Taveuni, Fiji

June 2014

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,

pic of Ted Mooney Teds signature
Ted Mooney, P.E. RET
finishing.com
Pine Beach, New Jersey



September 4, 2012

Q. My question is regarding a project I am currently doing, I would like to fasten Corten Steel to an aluminum frame, the steel will be the cladding for my garage door and the aluminum is the support frame holding the cladding. As Corten is a naturally corroding steel will it be an issue to fix this to the aluminum or is the concern regarding the bolts, if so should I use Stainless or galvanized bolts to mitigate this?

David Latimer
- Melbourne, Victoria, Australia

November 12, 2012

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.

khozema Khozema Vahanwala
Saify Ind
 
Bangalore, Karnataka, India


June 2014

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

Regards,

pic of Ted Mooney Teds signature
Ted Mooney, P.E. RET
finishing.com
Pine Beach, New Jersey


December 25, 2012

Q. Hi, would there be any corrosion problems using hot dipped galvanised steel stair stringers with aluminum step treads bolted on? Which bolts would be the best to use?

Barb Stevens
- Montville, QLD, Australia

December 28, 2012

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

khozema vahanwala Khozema Vahanwala
Saify Ind 
Bangalore, Karnataka, India


August 29, 2013

Q. We are a commercial builder putting up pre-engineered metal buildings with zinc-aluminum coated alloy metal 26 ga. panels. A customer recently said he "heard" finish would leach off the panel and pollute the earth.

Any truth to this ?

John Morrissey
building systems - Mattapoisett, Massachusetts, USA
  ^- Privately contact this inquirer -^


August 31, 2013

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.

Geoff Crowley
galvanizing &
   powder coating shop
Glasgow, Scotland



"Galvanize spray" on aluminum

September 15, 2013

Q. Hello Folks .

I am just an old retired fisherman with a large problem. I have a 16-foot aluminum fishing boat with a really bad transom, after so many attachment having been installed over its lifetime. The original aluminum looks like a colander or sieve. It's an a 1984 Klamath Alascan.

The wood, which was on the inside of the transom, just rotted out, so I got into it and removed the old wood and replaced it with new.

I then went ahead and put the new piece of aluminum onto the old with lots of Silicone sealant and screwed and bolted in place. Unfortunately that did not work to well and water came in as soon as I launched the boat. This was a blessing in disguise!! Because when I removed the new piece of aluminum so that I could get it welded like I should have done in the first place I found Electrolysis had occurred between the old and new. I had sprayed the old original aluminum with Yellow Chromate and the new piece with Galvanize spray. Please what should I have done?? And what should I now do before the Welder comes to weld it into place. Thank you for your time.

Brian M DAVIS
- Concord, California, USA


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,

pic of Ted Mooney Teds signature
Ted Mooney, P.E. RET
finishing.com
Pine Beach, New Jersey



Aluminum gutters touching galvanized gutters

November 14, 2013

Q. I was just told by a roofing contractor that my galvanized gutters were sheared off from my house when the new aluminum gutters were installed and now the galvanized/aluminum contact will corrode my new gutters. Of course he is recommending that he removes the galvanized attachment from the roof when he re roofs for an additional cost. My question is it this true and should I spend the extra money to save my new gutters?

Amy Smith
- Deerfield, Illinois, USA
  ^- Privately contact this inquirer -^


November 15, 2013

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,

pic of Ted Mooney Teds signature
Ted Mooney, P.E. RET
finishing.com
Pine Beach, New Jersey
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