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topic 41930p3

Galvanic Corrosion between Galvanized Steel and Aluminum

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A discussion started in 2006 but continuing through 2019

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
Striving to live Aloha

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
Aloha -- an idea worth spreading

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
Aloha -- an idea worth spreading

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
Aloha -- an idea worth spreading

affil. link
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
Aloha -- an idea worth spreading

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 [affil. link 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
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