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Galvanic Corrosion between Galvanized Steel and Aluminum

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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.
Question: 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


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A. Hello, Ronald. I don't think the grounding either accelerates or retards galvanic action.

A galvanic cell is like a battery with a wire connecting the positive to the negative. The two metals in your construction (aluminum and zinc) act like the carbon and zinc in a battery; ambient moisture and salts act like the conductive glop in a battery; and the physical connection of the two metals is equivalent to connecting the positive to the negative with wire.

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 with anodized bolts.

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

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

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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


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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. 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
Brick, New Jersey


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


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
Brick, 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



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
Brick, 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

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

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


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.

Its 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


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
  ^- Privately contact this inquirer -^



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 with the re-roofing. If so, the small cost of a neat job should be accepted. 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 are freely mixed.

Regards,

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

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