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topic 41930 p4

Galvanic Corrosion between Galvanized Steel and Aluminum

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A discussion started in 2006 but continuing through 2019

January 9, 2018

Q. We have traditionally been using SS 18-8 lag bolts for all our roof mount solar applications on asphalt shingle roofs. In the mount design, an aluminum base plate about 3/8" thick is lag screwed into the rafter (through the roof and sheathing). The base plate is flashed and an L-foot bolted to a stud on it. The mount supplier recommends using 5/16" x 5" SS lag screws to affix the base plate to the roof. What is the advantage to using stainless vs 5/16" hot dipped galvanized? Are the former simply stronger and more resistant to breakage during installation than the latter, as some have suggested? Or is there a material compatibility issue to be considered here, between the aluminum base plate and the lag screw? Or is it one of long term corrosion resistance? Note that all our installations are in snow country, far from marine environments.

Bhima Nitta
Installer - North Bennington, Vermont, USA

January 2018

A. Hi Bhima. My wife and I love Bennington & North Bennington and try our best to visit every year or two! I ask for your deference for a moment or two of reminiscing …

Way back when I was an underclassman in Engineering school, we started with courses in statics & dynamics, strength of materials, metals lab and so on, trying to absorb the theory of how to design so things were safe but economically feasible. But as we got closer to graduation we started to be taught about "codes": you don't build a steel frame building based solely on how strong you calculate a beam ought to be and how hard you think the wind will blow based on your own experience, and how many people you guess might stand in one room based on the parties you've attended; rather, design codes are written based on actual experience from successes and failures in hundreds of thousands of buildings. So, in addition to your own calculations, you always consult the building codes.

What I'm getting at is that -- while I'm happy to offer my personal opinion on a one-of-a-kind situation for a hobbyist or homeowner -- in this day & age where we have millions of rooftop solar cell installations which we should be learning from, I'm uncomfortable making suggestions based solely on my own limited understanding of general galvanic theory :-)

Yes, my guess is that 18-8 lag bolts are a better idea galvanically, and longer lasting, and probably of equal strength, and probably less likely to stain a roof if something goes south and they start to rust ... but surely there is a whole world of actual experience out there which should be tapped into somehow. There must be an organization of solar panel manufacturers and/or installers who are publishing journals, sponsoring seminars, and accumulating, tabulating, & publishing information on successes and failures of different materials and fasteners in rooftop solar installations. Good luck!


pic of Ted Mooney
Ted Mooney, P.E. RET
Pine Beach, New Jersey
We need "Aloha" now more than ever

January 10, 2018

thumbs up sign Thank you, Ted. You're probably quite right - there should be resources for solar installers I can tap into. I'll look into them.


PS: When you come up to North Bennington next, come by and check us out! We are Power Guru and we are at 5, Bank St.

Bhima Nitta [returning]
- North Bennington, Vermont, USA

Hot dip galvanized fasteners are rusting

January 23, 2018


We are facing issue of Fastener rusting.
The fasteners are of Hot Dip galvanised and the connecting part is a Galvalume (55% Al+45% Zn) coated steel.

We are using this kind of connections in all the solar fields across India.

I am confused whether it is a Galvanic corrosion or Failure of Zinc coating of Fasteners.


Harshith Vadnala
Quality - Hyderabad, India

41930-3a   41930-3b   41930-3c   (Click pics for larger versions)
February 23, 2018

Q. We have an outdoor shelter in a park that is located about 300 m from a harbour environment and 2.5 km from the ocean.

It has been installed for just over 13 months. The construction is galvanised mild steel RHS framing with powder coat over the top, a corrugated color steel metal roof. The curved sections with no corrosion are Aluminium with powder coat. The fastenings are 304 SS.
Can anyone help explain why there is corrosion on the galvanised frame? But not the aluminium.

Could it be poor pre-treatment before powder coating?
Inferior metals?

When it was installed the period was quite wet and stormy -- what if the uprights that are concreted had partially filled with salt spray / rain water during construction (I don't think there are drain holes) -- could this cause some sort of electrolysis across the framing.

Paul Goodwin
- Melbourne, Victoria, Australia

February 2018

A. Hi Paul. I can't see the roof real well, but it looks like it's fine ... which indicates to me that the pretreatment and powder coating of the steel structure may be of questionable quality. According to the American Galvanizers Association and British Standards Institute at
this installation doesn't seem like it should have been problematic.


pic of Ted Mooney
Ted Mooney, P.E. RET
Pine Beach, New Jersey
We need "Aloha" now more than ever

February 27, 2018

A. Hi Paul, how are you doing? I send a great hello from Continental Europe to South Hemisphere!...

Is the paint flaking or bursting out from the galvanized substrate? It is not really perceptible from the pictures.
And for what understood, no (visible) defects on the aluminium powder coated parts, correct?

Marco Antunes
- Hagen, Germany

February 28, 2018

Q. Hi Marco

I have sent through some better images.

You are correct that there is no sign of corrosion on the roof or the aluminium curved section - only the galv.

It appears the powder coating is only failing where it is not exposed to the weather. Attached are 2 photos of the failing in close-up --

41930-4a   41930-4b
(click pic for higher resolution)

Thanks Paul

Paul Goodwin
- Melbourne, Victoria, Australia

March 5, 2018

A. Hi Paul,

I would risk to say that this could be induced by galvanic corrosion, after all you have aluminium and stainless steel all together with zinc - at some part there should be a connection point between these metals that will close the electric circuit and accelerate the corrosion.

Another point leading to this can be also the pretreatment of galvanized previous to painting that may be causing the defect but from this side, I would bet on the galvanic corrosion.

Marco Antunes
- Hagen, Germany

March 2018

A. Hi. I respectfully disagree with galvanic corrosion being the cause of this problem, and believe that the galvanized structure was not properly pretreated and powder coated. The surface area of exposed aluminum and stainless steel is so small that, while it might theoretically cause localized pitting & corrosion near the joints and directly contacted area, I can't see it causing broad swaths of galvanized surfaces many feet away corroding, especially phosphatized and powder coated surfaces.


pic of Ted Mooney
Ted Mooney, P.E. RET
Pine Beach, New Jersey
We need "Aloha" now more than ever

March 6, 2018

A. I agree with you Ted.
It certainly looks like inadequate or inappropriate pre-treatment of the HDG.
To find that the areas which are not concealed do not fail so badly is quite understandable as these would have the benefit of natural washdown from rainfall.
I also wonder if the Al & HDG were cured simultaneously (i.e., in one oven cure cycle)? If that is the case it is quite possible that the heavier HDG sections did not achieve full cure whilst the lighter Al sections might well have.
If the HDG sections are not fully cured then the polymers will not have completely cross linked and that would also contribute to poor corrosion resistance.
I suspect it to be a combination pre-treat and cure induced failure.
Hope this helps.

william doherty
trainer - newcastle nsw australia

March 12, 2018

Q. I'm designing a steel-framed house (cold formed steel) for a client who wants to use clear anodized aluminum windows. The house is in a hot and humid climate, ten miles from the sea (near Jeddah, Saudi Arabia).

I can insulate the window frame from the metal studs with Bituthene or similar rubber-like flashing, but the self-tapping screws through the aluminum nailing fins will still create direct (local) contact between the aluminum and the galvanized metal studs.

Will a certain type of screw material alleviate this problem? Or screws with a special coating?

Thank you for your help!

Caspar Mol
- Jeddah, Saudi Arabia

July 19, 2019

A. Caspar Mol wrote asking about galvanic concerns when fastening aluminum windows to a steel-framed structure. One answer is elastomeric memory-foam products (an "expanding foam tape") "Hannoband" that fills the void between the window's frame and the rough opening. Relatively few screws are needed, perhaps one each side. If you use a stainless steel fastener, the point of contact with the aluminum will be much less likely to led to corrosion. Furthermore, since the window will lack the conventional nailing flange, there will be no galvanic contact there. Also, the method has superior air tightness, leading to less infiltration, a plus.

Arthur Ogawa
TeX Consultants - Three Rivers, California, US

March 14, 2018

Q. Our neighbor's contractors put a hole in our aluminum fence; they offered to patch it with galvanized steel fencing. Won't these metal interact/rust quickly?

valerie veniot
- halifax nova scotia canada

March 2018

A. Hi Valerie. It's obviously difficult to accurately predict exactly what will happen and when, but we do know that it's not advised to repair aluminum with galvanized metal this way, so it doesn't sound unreasonable of you to require that the repair be done with aluminum (unless we don't have enough of the story to comment appropriately).


pic of Ted Mooney
Ted Mooney, P.E. RET
Pine Beach, New Jersey
We need "Aloha" now more than ever

June 15, 2018

Q. WOW I'm so impressed by the availability of expert opinions.
My issue: I have purchased a slide on camper constructed from galvanised steel panels, it is to sit on my aluminum truck tray.
I just learned about galvanic reaction and as the camper and tray will be exposed to rainwater and sea spray, I am hoping that you can provide a simple solution to prevent erosion.

Would a rubber mat, i.e., such as commercial kitchen flooring mat on the tray be a prevention solution?


wendy kurka
- Malvern, Victoria, Australia

June 2018

Hi Wendy. If the galvanized camper is not bolted to the aluminum truck body, a flooring mat to keep them from touching would be a complete solution. But if there is a metallic path from one to the other (bolts with no insulation for example), galvanic corrosion is possible, especially near the bolts.


pic of Ted Mooney
Ted Mooney, P.E. RET
Pine Beach, New Jersey
We need "Aloha" now more than ever

Best U-bolts for Aluminum Boat Trailer

August 12, 2018

Q. My situation: I need to replace the springs connected to the galvanized steel axle on my aluminum i-beam tandem boat trailer. While I am at this, I want to also replace some rusty U-bolts holding the 3" square cross members to to the i-beam frame. The trailer currently has 1/2"x3"x5" galvanized square u-bolts. The trailer is about 12 years old, threads are rusted on the galvanized bolts. The trailer goes in an out of salt water about 10 times/year and I typically rinse it off in a freshwater pond after use. I have not cut off the u-bolts but I do not see too much corrosion on the i-beams or cross members where the galvanized bolts contact the aluminum, a bit of light chalking.

My choices for replacement U-bolts are zinc plated, Galvanized, or Stainless. What do you all think?

Ralph Webby
- Cape Cod, Massachusetts / USA

August 2018

A. Hi Ralph. Zinc plating is not suitable for saltwater exposure, probably not even for a freshwater only trailer.

Galvanized worked originally and will work again, but make sure they are actually galvanized (hot dip galvanized) by looking for the characteristic mottled crystalline look or dull drips. If they are shiny, smooth, drip-free and not mottled, they are probably not actually galvanized (some people started calling zinc plated fasteners "electrogalvanized", and then hungry marketers started feeling justified in shortening this to " galvanized"). If a standard nut fits them, they are not hot dip galvanized.

Stainless would be okay too but probably prohibitively expensive.


pic of Ted Mooney
Ted Mooney, P.E. RET
Pine Beach, New Jersey
We need "Aloha" now more than ever

August 15, 2018

Thank you Ted for the quick response!

The stainless is a bit more expensive but not prohibitive, so I may go with it. I was also a bit concerned about the strength of the stainless vs galvanized. Can I mix all three?

Ralph Webby [returning]
- Cape Cod, Massachusetts

August 2018

A. Hi again, Ralph. You already know, if only from reading this thread, that you ideally shouldn't mix any metals unless they are insulated from each other … the more so on a salt water trailer. But, although aluminum coated bolts are used on aluminum airplanes, they are probably not available, affordable, and practical for an aluminum trailer, so you are forced to do at least a little mixing.

I would pick either galvanized or stainless fasteners and stick with it rather than tossing a salad. Good luck.


pic of Ted Mooney
Ted Mooney, P.E. RET
Pine Beach, New Jersey
We need "Aloha" now more than ever

February 13, 2019

Q. Will my anodized aluminum solar panel frames face galvanic corrosion?
Hi, my company is planning to build up a small solar park on the roof, but my company is close to an iron mine (undergoing one). The material of the frames are aluminum alloy coated with 15 microns type II anodized coating. Since the iron mine will cause some iron residue (either in the form of atoms, ions or oxidized compounds) to stay on the surface of the anodized frames, which might jeopardize the formation of rust, and possibly galvanic corrosion where anodized surface is incomplete or scratched. Considering the life of solar panels are at least 25 years long, will the frames face critical issues in the long term? Is there any way to solve the problem in that case? (e.g., increasing the thickness of anodized coating?) Thank you very much and I appreciate your opinions!

Claire Chen
- Henan, China

March 18, 2019

A. Firstly I don't think that any air contamination from the Iron mine is of any real danger to your Aluminium frames.

You could powder coat/paint the Aluminium with UV resistant material, it will only extend the life.

Khozem Vahaanwala
Khozem Vahaanwala
Saify Ind
supporting advertiser 
Bengaluru, India
Saify Ind

July 21, 2019

Q. Hello,
I am building an observatory from shipping containers. It's 11' by 20' with a 6' high wall around it and it is on top of the containers. It's going to have a roof 11' by 20' that will open 90 degrees (11' radius) with 2 linear actuators.
I'm trying to keep the roof weight down and would like to know if it would be ok to use aluminum as the framework and metal corrugated roof sheeting instead of steel as I had originally intended. I have three 6' long heavy duty steel piano hinges that will be welded to the existing steel framework. I could bolt the hinges to the aluminum, perhaps with some type of material between. There will be rubber gaskets between the existing steel framing and the roof framing to keep out dust, etc.
It doesn't have to last forever but I don't want galvanic action to corrode things in a few years. Thanks for any advise you can offer. I might add that the location is in the Nevada desert, very dry.

Bill Johnson
- Jean, Nevada, USA

July 2019

A. Hi Bill. You can't have galvanic corrosion without a conductive liquid. It doesn't sound to me like you have anything to worry about.


pic of Ted Mooney
Ted Mooney, P.E. RET
Pine Beach, New Jersey
We need "Aloha" now more than ever

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