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topic 48391

Compatibility of aluminum and galvanized steel


A discussion started in 2008 but continuing through 2018

April 3, 2008

Q. I would like to ask if I still need a nylon separator between hot galvanized steel and aluminum surface where there is a direct contact between the two? where in fact there is a expected water running through it.

I am reviewing a desing drawing with the above condition. this is for the facade shop drawing. please help.

Mai Saxor
engineer - Philippines


June 4, 2008

Q. Mai
I'm an architect and asking a similar question here in UAE of anodized aluminium supported on galvanised steel frame. Here it's hot and often dry heat but summer creates high humidity and the site is right on the Arabian Gulf.

I think a nylon separator is necessary but the question may be a bigger one than only contact. I understand that rain water can connect the two metals and still produce corrosion.

Can anyone confirm or refute this? Should I be using powdercoating instead of galvanising?

Regards

Matt Brindley

Matt Brindley
architects - Al Raha, Abu Dhabi, UAE


June 4, 2008

A. Hi, Matt. Galvanic corrosion is not the only kind of corrosion, so avoiding galvanic corrosion doesn't guarantee that a material will last indefinitely in an exposure condition, but it certainly helps.

You will not get galvanic corrosion if the parts are not metallically connected. That's my understanding anyway.

A galvanic corrosion cell works like a battery connected to a lightbulb. Two different metals like in the battery, salty conductive water or moisture like the conductive glop in the battery, and a metallic connection like the wires and light bulb. A battery may last only a short time powering the light bulb, as metal moves from anode to cathode to produce the power; but the battery will last a long time on a shelf because the metal cannot move from anode to cathode without the wiring.

Positively charged metal ions can travel though watery solutions but electrons don't; and metal won't electrochemically move from one place to another without both a path for the positively charged ions (moisture) and a path for the electrons (metallic connection).

Regards,

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


April 19, 2011

Q. Dear Sir,

I am metal roofing designer. In rainwater gutter we are using 2 mm thick aluminium gutter. At gutter end laps we are using butt joint. but client is not agreeing to using aluminium butt joint. Instead of aluminium now we are using galvanised steel butt joints. What will happen if aluminium and galvanised steel directly contact each other?

Will it rust fast? Gutter is inside the building, and building is closed from all the four sides.

Thank you.
Sachin

Sachin Kamble
designer - Mumbai, Maharashtra, India

A. Hi Sachin. Galvanizing and aluminum are not completely compatible, so you would not use such a combination on a critical structure like an airliner. But on the other hand they are fairly close galvanically, and rain water is 'distilled' and fairly non-conductive. My guess is that it's probably not going to be a major issue.

Regards,

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



August 4, 2018

Q. What insulation should be used between powder-coated aluminium crestings & finials on a galv iron roof ridge, and what type of fixing screws should be used?

Dianne Kowalick
- Adelaide, South Australia, Australia


August 2018

A. Hi Dianne. Ideally the fixing screws would be galvanized and would be insulated from the aluminum, but if they are not insulated from the aluminum it makes no difference what insulation you use because the galvanized surface and the aluminum surface remain metallically connected. Fortunately, rainwater is relatively non-conductive, so galvanic corrosion may not be a major issue.

Regards,

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



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