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topic 41930

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.
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 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.

Ted Mooney,
Teds signature
Ted Mooney, P.E. RET
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.


Ted Mooney,
Teds signature
Ted Mooney, P.E. RET
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.


Ted Mooney,
Teds signature
Ted Mooney, P.E. RET
Pine Beach, New Jersey

Electrolysis between aluminum solar panels and galvanized frames


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

Craig Horner
solar energy - Chico, California


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?

Ted Mooney,
Teds signature
Ted Mooney, P.E. RET
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
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.


Ted Mooney,
Teds signature
Ted Mooney, P.E. RET
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, 2014 -- this entry appended to this thread by editor in lieu of spawning a duplicative thread

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?


Marcus Maedl
solar installer - San Diego, California, USA

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).


Ted Mooney,
Teds signature
Ted Mooney, P.E. RET
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.


Ted Mooney,
Teds signature
Ted Mooney, P.E. RET
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.

Khozem Vahaanwala
Khozem Vahaanwala
Saify Ind supporting advertiser
Bangalore, Karnataka, India

saify logo

June 2014

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


Ted Mooney,
Teds signature
Ted Mooney, P.E. RET
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.

Khozem Vahaanwala Khozem Vahaanwala
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

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
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.

- 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.


Ted Mooney,
Teds signature
Ted Mooney, P.E. RET
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

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.


Ted Mooney,
Teds signature
Ted Mooney, P.E. RET
Pine Beach, New Jersey

October 8, 2014

Q. I recently attended an agricultural machinery field day where a machine was being demonstrated that could erect up to four kilometres a day of six wire electric fence. (Three live wires and three un-insulated earth wires) The fence posts were aluminium driven in to the ground. These were pre-drilled to cater for insulators on the live wires but the earth wires had direct contact with the posts. The wire was 2.5 mm galvanised plain wire. What sort of. Life expectancy could be expected of the un-insulated earth wires in this sort of environment?

Graham Turner
- Grafton, N.S.W. Australia

November 24, 2014

Q. Further to my question posted in October, I have since contacted the manufacturers of the aforementioned electric fence regarding my concerns as to the possibility of galvanic corrosion between the 2.5 mm galvanised wire and the aluminium fence posts. He claims that, because the wire is able to move back and forth (due to expansion and contraction following temperature changes) the wire isn't in contact in one place long enough for the corrosion to occur. Does this sound like a reasonable assumption? I have seen a situation where the un-insulated galvanised wire has actually worn through where it ran through the hole in steel fence posts but this was due the the wires being strained too tight with the result that the wires would hum like a guitar string in strong winds. The vibration caused the wire to wear through.

Graham Turner
- Grafton, N.S.W. Australia

December 2014

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.


Ted Mooney,
Teds signature
Ted Mooney, P.E. RET
Pine Beach, New Jersey

Reverse Galvanic Corrosion?

November 12, 2014 -- this entry appended to this thread by editor in lieu of spawning a duplicative thread

Q. I am a materials and corrosion engineer. The bulk of my experience is with military wheeled vehicles.

I have come across numerous cases of galvanic corrosion between zinc plated carbon steel fasteners and aluminum panels. In this case the aluminum is the anode and the carbon steel is the cathode. The surprising thing is that very often the steel fasteners have a great deal of red rust and the aluminum is fine. From what I understand of galvanic corrosion, I would have expected very little corrosion to either material. The steel fasteners are either Yellow Zinc Plated or Tin/Zinc Alloy plated. My theory is that the zinc plating is acting as an anode to the aluminum and, given the big difference in surface area the zinc is quickly dissolved, which leaves the carbon steel un-coated. Since the aluminum and carbon steel are relatively close in the galvanic series the aluminum provides very little protection for the steel and the steel corrodes quickly due to general corrosion.

Any thoughts?

Peter Kopinski
Materials and Corrosion Engineer - Livonia, Michigan

November 2014

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.


Ted Mooney,
Teds signature
Ted Mooney, P.E. RET
Pine Beach, New Jersey

December 22, 2014

Q. I have been involved in the replacement of a cooling tower made of galvanized steel. I noticed during the tear out that the 8 inch piping which is insulated with fiberglass and covered with an aluminum skin has extreme rust and corrosion where the pipe turns down and connects to the galvanized steel top of the cooling tower. The cooling water is treated with chemicals. The insulation is often soaked and I have noticed that the iron pipe supports where they protrude through the insulation and aluminum skin are extremely rested as well. The new structural I am making to support a new tower of the same type is to be hot dipped galvanized.
I am wondering if I should coat the galvanized structural at the points of contact with the aluminum to prevent corrosion I am wondering if the final piece of insulation metal skin where it contacts the galvanized metal of the cooling tower should be galvanized sheet. The elbow covering is plastic and there would be no direct contact between the aluminum skin and the galvanized sheet at this point.

Thomas Rockriver
welding & maint. svcs. - Chapel Hill, North Carolina USA

January 2015

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.


Ted Mooney,
Teds signature
Ted Mooney, P.E. RET
Pine Beach, New Jersey

February 24, 2015

Q. We have an aluminium frame with an aluminium tray running salt water over the tray panel for evaporation and producing distillate .
The panels are connected by aluminium clamp and galvanised bolt to galvanised support beams.
Even though the support beams are insulated from the ground by wooden posts we see sporadic corrosion of the Aluminium tray?
Can you explain what could be happening?

If we isolated the panel from the galvanised rail with nylon insulators do you think the problem is resolved?

Peter Johnstone
Water processing - Melbourne, Victoria, Australia

February 25, 2015

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).

Brian Terry
Aerospace - Yeovil, Somerset, UK

March 2, 2015

Q. We have completed construction of a home that has an external aluminum stairway supported by primed steel struts sticking out of the building at the bottom and top landing. The struts are directly bolted to the aluminum landings with galvanized bolts. There is not any material separating the steel from the aluminum.

Aluminum staircase a Aluminum staircase b

The home is located 15 miles from the ocean in an area that gets about 20 -30 inches of rain a year during the winter. Is there a potential corrosion issue that can cause a structural problem? Can a sacrificial anode be used as on a boat? Can I obtain cathodic protection by connecting the aluminum to the negative pole of an exterior DC voltage source? How do I do this correctly? Removing the stairs to put a barrier between the steel struts and the aluminum stairs would be a big job.

Glenn Fricker
- Sebastopol, California

March 3, 2015

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.

trevor crichton
Trevor Crichton
R&D practical scientist
Chesham, Bucks, UK

March 13, 2015

Q. Hey guys,

I'm looking at buying a builders trailer 8 x 5. A few of the trailers I have looked at, the base is hot galvanize dipped and the top is checker plate aluminium.

Does hot galvanize dipped and checker plate aluminium react with each other? The suppliers of the trailers insist that they have no problems with it. but I wasn't sure!

Is there something I should be checking for/asking that will prevent the reaction?

Thanks in advance for the advice!

Brandan Holas
- brisbane, queensland, australia

March 2015

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.


Ted Mooney,
Teds signature
Ted Mooney, P.E. RET
Pine Beach, New Jersey

March 16, 2015

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


Brandan holas
- Brisbane, queensland

March 2015

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.


Ted Mooney,
Teds signature
Ted Mooney, P.E. RET
Pine Beach, New Jersey

April 1, 2015

Q. Hi, I'm repairing some camper roof damage from where a tree fell on the top. It is an aluminum frame with aluminum rafters. Couple rafters were bent slightly (about 2") in a few spots - they are actually hanging about 2 inches low but top of rafters for most part are fine. Plan is to place 1 1'2 (3/16th) angle iron alongside of each rafter which will also sit on top of the wall frame on each side, and bolt to the rafter every 15 inches after jacking back into original position. Span is only 8 ft. What type of bolts or tapping screws would you recommend so we don't have issues? Should never be exposed to water or other liquids unless there is a leak. Are bolts the best to use or should we use self tapping? Also, we hope to have the angle iron powder coated if that helps. Thanks!

Chris Carda
- Sioux Falls, South Dakota, US

March 2015

A. Hi Chris. Although steel and aluminum are far from being galvanically compatible, galvanic corrosion only occurs in the presence of a wet and electrically conductive liquid like saltwater. Inside a camper I can't picture it being an issue.

It's a good idea to powder coat the angle iron because steel rusts. Zinc plated or galvanized hardware ought to be fine, and you'll probably find nuts & bolts easier to work with than self-tapping screws.


Ted Mooney,
Teds signature
Ted Mooney, P.E. RET
Pine Beach, New Jersey

April 7, 2015

Q. Hello All
I picked up on this very pertinent and interesting thread via the internet , my question is as follows
Here in South Africa there is a boom in PCV solar installations, our company is a balance of system supplier including roof mounting structure for solar panels on all kinds of roofs, including roofs made from galvanised steel.

We have products which are galvanised steel, i.e., galvanised steel strut going onto galvanised steel roofs, but also we have lots of enquiries for aluminium strut to be used to do the same job which is to go across the galvanised steel roof to be able to clamp and locate the solar panels. Starting from the roof, here is the order of the installation: GALVANISED ROOF SHEET, ALUMINUM STRUT, ZINC COATED CLAMPS TO CLAMP THE SOLAR PANELS TO THE STRUT, GALVANISED BOLTS TO BOLT DOWN THE STRUT. These installations are guaranteed for 25 years. You can imagine in a developing country like South Africa there are thousands upon thousands of installations taking place, so this subject has become very important and I would appreciate all comments and advice.
Many thanks.

Mike Kirby
- Johannesburg , Gauteng , South Africa

April 13, 2015

A. I don't know if this helps you, Mike, but about 10 years ago I was able to purchase (very cheaply) 3 solar hot water systems. These had been on the roof of a motel right next to the salt water reach of the Nambucca River in New South Wales. The owners were taking advantage of a government subsidy to upgrade to heat exchanger type heaters. The alumimium absorber panels were mounted on a galvanised angle iron frame so as to give them the appropriate angle of slope for the thermo-syphon principle to work.These panels were held in place on the frame with galvanised self-tapping screws. The panels had been manufactured in 1994. In order to remove them from the roof, I needed to dismantle the whole apparatus. The only sign of corrosion visible was some white powder around the site of the screws. In 2011 I installed one of these systems on my own roof and a recent inspection has shown no sign of further corrosion. I must admit that I live about 80 kilometers from the ocean so salt isn't a problem but average annual rainfall here is around 1000 milimeters (39.37 inches)
I hope this helps,
Graham Turner

Graham Turner
- Grafton, N.S.W., Australia

April 15, 2015

thumbs up signHi Graham, thank you for your information; it is extremely valuable. Nothing beats a physical report from a site that is several years old. For your information we have 4 branches of our company here in SA , but the one I work in is 6 hours from the nearest sea; we here in Gauteng on the South African Highveld at altitude have a very dry climate (vehicles of very advanced years show no signs of rust). Many companies are using aluminium frames on galvanised sheet roofs for mounting solar panels mostly due to weight reasons, but we also have had produced a very light galvanised steel strut and the only aluminium we have involved is the frame of the solar panel in 4 very small areas where each panel clamps to the strut.
Best regards

Mike Kirby
- Gauteng South Africa

May 5, 2015

Q. Hi everyone,

I have read lots of threads and I find your discussions very interesting. However, since I am not an expert in coatings / plating procedures (I am a civil engineer), I would appreciate your advice regarding the following problem, which -- I believe -- is relevant to this thread:

I own a house which is about 40 m from seashore. Thus, bear in mind that there is a lot of "salt spraying" - "electrolytic environment". The windows of this house are made from aluminium which is painted through electrostatic process. The aluminium seems to behave exceptionally (it has been 8 years since I installed the windows). The problem comes with the hinges and their mounting parts (screws).

41930-2a  41930-2b  41930-2c  41930-2d  41930-2e  41930-2f

Problem No. 1: There seems to be no aluminium hinges to fulfill my functional needs. At the moment I use Roto Fentro hinges, which are of unknown material and coating (the company doesn't answer my e-mail), but have definitely shown both white and red corrosion products after 3 years of exposure.

Problem No. 2:The mounting of the hinges has to be with DIN 7983 (ISO 7051 - UNI 6956) PH oval screws. I believe that the screws that were used, are just zinc plated. This may have "saved" the aluminium windows from galvanic corrosion, but couldn't stop the screw corrosion which has shown extensive red corrosion on the head and white corrosion on the main body.

As you can understand, the above problems sum up to the fact that I have to replace about 1200 corroded screws along with new hinges. Since it will be an expensive and tiring job, I need to find some materials that are more resistant in corrosion under "marine environment", and - at the same time- won't cause any damage to the aluminium through galvanic corrosion.

From what I have read so far, I have concluded that for screws I could use SS 316, with Dacromet or Geomet Plus coatings. Is that correct or do I have to look for Dykor, Xylar, Xylan or Delta coatings? Is the combination of SS 316 and Geomet possible or am I totally wrong?
Furthermore can any of the above coatings comply with my expectations regarding "extra corrosion resistance and minimum galvanic corrosion"?

For the hinges, I have been on a dead end since I cannot find SS316 hinges (only SS304), and I have excluded bronze and any other copper alloys in fear of galvanic corrosion.

Any help would be most apreciated.
Thank you in advcance.

Anthony Levogiannis
- Athens, Greece

August 18, 2015

Most of your comments show great understanding of the issues. In my opinion, you can't leave a seacoast structure for long periods without maintenance. From the photos it almost looks like salt encrusted on some of the surfaces.

The aluminium is performing well only because it is well coated. Zinc in contact with aluminium is sacrificial as you mention, but you really don't want them to have the chance to exchange electrons. The fasteners and hinges should be fully coated after installation except where there is moving contact. Anytime screws and other fasteners are installed they will damage the coatings, and I would suggest paint/coating after installation. Any place that other materials contact the coated aluminium, re-coat to prevent any moisture to the extent possible. Most coatings are slightly moisture permeable but do the best that you can. Where there is moving contact, use a lubricant and periodically renew it to exclude moisture. And inspect the aluminum frames for any coating failures or possible sources of moisture ingress, re-coating as needed.

316 stainless will still pit unless kept clean or coated. The GeoMet that you mention seems to be a line of products but most of them seem to contain zinc flake. It's not worth protecting stainless steel with zinc coatings.

Some copper alloys/bronzes will form a patina that will stop further corrosion, but you will have to use compatible bronze fasteners for best results. And coating/sealant where in contact with aluminium. If a patina is acceptable visually, bronze would not require coating, only lubricating for moving parts.

Seacoast is the worst environment other than actually aboard a ship, for corrosion, because a ship flexes more. The expression, "you pay me now or you pay me later" applies here. Either you apply and maintain coatings, sealants, and lubricants regularly, or eventually you replace all hinges and 1200 fasteners again.

paul tibbals
Paul Tibbals, P.E.
    gas & electric
San Ramon, California, USA

(My opinions are not related to nor a statement of my employer's)

Options for mounting solar panels on residential metal roof

October 16, 2016

Q. Need to mount 36 solar panels (36x66") on my house steel-panel roof. Solar panels will be mounted in 3 rows of 2 panels each (covering about 16 x 40'). For the metal framing I'm considering...

Option 1. Galvanized Unistrut steel channels lag screwed thru the painted steel roof to rafters below with zinc plated or galvanized hardware. Unistrut to unistrut connections would be made with zinc plated nuts and bolts. With zinc anodic to steel, I'm concerned about corrosion between the lags and the steel roof.

Could that be a long term life (20+ years) problem in the "mild" environment of middle Tennessee?

If I understand the corrosion mechanism correctly, wouldn't the lag screw's anodic zinc around the hole where the lag penetrates the steel roof be sacrificial? - eventually eroding the zinc to expose the underlying lag screw steel thus SLOWING corrosion at the (steel to steel) hole?

Option 2. Aluminum U-channels screwed thru the painted steel metal roof to rafters below with galvanized lags. Aluminum to aluminum connections would be made with zinc plated nuts and bolts. With zinc anodic to both aluminum and steel, I'm concerned here about corrosion between the lags and the steel roof as well as between the zinc fasteners and the aluminum channels and brackets (3 metals).

Where are the main corrosion threats for this option, and what do you recommend for improvement while keeping the cost down?

Thanks much,

Jack McCarron
home owner installing solar panels - Jamestown, Tennessee, USA

October 2016

A. Hi Jack. I have heard, and I believe it, that rooftop residential installations away from the ocean are not particularly conducive to galvanic corrosion because rainwater is relatively non-conductive. I think you'll be best served with the galvanized Unistrut and galvanized hardware rather than introducing a third material of construction (aluminum).


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Teds signature
Ted Mooney, P.E. RET
Pine Beach, New Jersey
Striving to live "Aloha"

Aluminium shade battens screwed to hot dipped galvanised steel frame

October 17, 2016 -- this entry appended to this thread by editor in lieu of spawning a duplicative thread

Q. I am designing the components of a simple sunshade structure - galvanised steel frame with aluminium RHS battens between the frames. I am concerned about the risk of corrosion between the two dissimilar metals and also the screws to be used to fix the battens to the frame.

Seeking advice on best way to prevent / minimise corrosion is this situation. Structure is located around 1.5km from a (partly) salt water lake (but not breaking waves).

Damian Crozier
Structural Engineer - Bairnsdale, Victoria, Australia

May 31, 2017 -- this entry appended to this thread by editor in lieu of spawning a duplicative thread

Q. Good morning,

We manufacture aluminium deck substructures. For connecting the extruded lengths together, we use 3 mm thick folded aluminium connector brackets and stainless steel 304 A2/316 A4 self-drilling screws.

The limitation of the folded aluminium brackets is that they are not as strong as steel, and do occasionally fail. Does using aluminium brackets offer significant advantage over galv steel brackets from a galvanic corrosion point of view? We would consider using steel brackets for their superior strength, but not if there is a high risk of galvanic corrosion between the aluminium and steel?

Appreciate help on this.

Kind regards,

Anton White
Product Development - Ellon, Aberdeenshire, UK

May 2017

A. Hi Anton. As you see, even just this one thread has dozens of similar questions, generally answered with something like "it's a theoretical problem, but whether it's a real problem or not depends" :-)

I think the bottom line is that for those who are looking for the robust answer or are dealing with a truly critical issue such as on an airframe, you need to have the same metal surface on the components or you need to insert insulators in such fashion that the aluminum is not electrically connected to the galvanized steel.

Anything less offers some potential for galvanic corrosion which will depend upon whether the aluminum is anodized, whether the joining is via u-bolts which don't penetrate the surface oxidation on the aluminum vs. lock washers which do, how effective any thread lubricant is in sealing out moisture, how close the installation is to the sea or an industry which could make the low conductivity rainwater more conductive, whether there is any chance some customer may apply rock salt or fertilizer to melt ice on the decks, the relative surface area of the aluminum vs. the zinc, etc. Small amounts of the more active material, like galvanized bolts on aluminum platforms, are a bigger problem than larger amounts; so if you've been using galvanized bolts, then galvanized brackets should even slightly improve the situation rather than making it worse.

A final consideration is that all construction metals eventually corrode anyway, so it's only a matter of whether the galvanical acceleration is a major life-shortener. The American Galvanizers Association website says: "Under atmospheric conditions of moderate to mild humidity, contact between a galvanized surface and aluminum or stainless steel is unlikely to cause substantial incremental corrosion. However, under very humid conditions, the galvanized surface may require electrical isolation from the aluminum or stainless steel." That, vague as it may be, is as close to a definitive answer as you are likely to get :-)

Luck and Regards,

pic of Ted Mooney
Ted Mooney, P.E. RET
Pine Beach, New Jersey
Striving to live "Aloha"

Anodized magnetic drain plug

June 23, 2017

Q. Can I use an anodized aluminum drain plug on my steel oil pan without creating a galvanic reaction?

david kron
- us

June 2017

A. Hi David. I don't know what kind of 'oil pan' you are thinking of. The oil pan on your car's engine? Just get a steel plug.

Personally I avoid, to the maximum extent possible, the use of any aluminum threaded stuff. It always corrodes and locks up. By my age I've removed numerous stuck light bulbs with pliers, wasted hours hacksawing off aluminum hose fittings, replaced several locks that had aluminum keys stuck in them ... :-)


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Ted Mooney, P.E. RET
Pine Beach, New Jersey
Striving to live "Aloha"

Magnetic Drain Plug

June 25, 2017

Q. Ted thanks for your reply on the drain plug question. I intend to put oil on the thread of the plug before threading it into the pan. It will be changed every 9 months. the advantage to this anodized plug is that it has a steel magnet at the tip to catch the engine shavings. What do you think? Thanks David.

David kron
- teaneck New Jersey

June 2017

Hi David. These magnetic drain plugs are widely used, and are available with either anodized aluminum or stainless steel bodies. There is no potential for corrosion inside the engine because the environment is oil not water. Seems like a pretty safe application :-)


pic of Ted Mooney
Ted Mooney, P.E. RET
Pine Beach, New Jersey
Striving to live "Aloha"

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