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topic 41930, p2

Galvanic Corrosion between Galvanized Steel and Aluminum

1       2

A discussion started in 2006 but continuing through 2017

November 12, 2014

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.
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.
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.
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.
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.
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).


pic of Ted Mooney
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 ... :-)


pic of Ted Mooney
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

A. Hi David. These magnetic drain plugs are widely used, and are available with either anodized aluminum or stainless steel bodies. There is no real 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"

November 1, 2017

Q. Hi,
I have a situation where I am mounting a Hot-dip galvanised cable tray to aluminium struts. I can separate the tray from the struts using rubber however I am not sure what screws I should then use to fix the tray to the struts as I am afraid that they may rust and the tray will eventually become loose. I originally thought that stainless self drilling screws would be ok but have since read that these would probably be worse than a galvanised screw. The cable tray is on a roof and will be exposed to water however it was about 30-40 km from the ocean so not much salt in the air. Any solutions for a self drilling fastener? Thanks

Andrew Leversha
- Melbourne, Victoria, Australia

simultaneous November 2, 2017

A. Hi Andrew!

You should get zinc rich painted self drilling fasteners. I don't know if in Australia you can find them, but we manufacture them here in Argentina so I can imagine they must be there somewhere.

We manufacture this finish to be applied with aluminum-magnesium substrates and for other uses for hot dip galvanized plates, so they work for both ways. I don't think you will have problems with that.

Best of lucks!

Daniel Montanes
TEL - N FERRARIS - Canuelas, Buenos Aires, Argentina

November 2, 2017

A. Australia has in many ways lead the world in the development of self-drilling fasteners. Australian Standard AS 3566.2 [link by ed. to spec at TechStreet] defines demanding requirements for coatings for self-drilling fasteners. These standards were developed by BHP Billiton (now Bluescope) to provide protection equivalent to their Galvalume roofing material. These fasteners are mechanically plated with a zinc-tin alloy (no hydrogen embrittlement) and topcoated with proprietary topcoats. One Australian supplier is Buildex (in Australia, formerly Deutscher). Other manufacturers meeting AS3566 are also satisfactory.

Tom Rochester
Plating Systems & Technologies, Inc.  
supporting advertiser
Jackson, Michigan, USA
plating systems & technologies banner ad

Galvanized vs Stainless Lag Bolts for Solar Applications

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
Striving to live "Aloha"

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
Striving to live "Aloha"

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
Striving to live "Aloha"

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

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
Striving to live "Aloha"

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
Striving to live "Aloha"

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
Striving to live "Aloha"

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
Striving to live "Aloha"

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