Which screw for aluminum?

[nattugglan11]

Hi,
I've heard that aluminum rusts when in contact with steel, is this just talk? I'm thinking of fastening a U-profile in aluminum, but with which screw?

 

[KnockOnWood]

Use stainless acid-resistant (A4) screw.
Preferably lubricate the screw with some kind of grease or wax.
This prevents moisture from accumulating in the space between the aluminum and the screw.
Alternatively, use a plastic washer and/or plastic bushing.
The greatest risk of corrosion is if you cut threads in the aluminum. This creates a large exposed surface that tends to corrode.

DO NOT use fasteners containing copper, such as brass.

 

[ordain]

Hi,
I've heard that aluminum rusts when in contact with steel, is this just talk?

If the steel isn't completely stainless/acid-resistant, then the aluminum will corrode, yes.

The aluminum part will become a large sacrificial anode for what would normally rust.

If, for example, you drive an iron nail into an aluminum boat that is exposed to the weather, the iron nail will oxidize (would normally rust), but since aluminum is more oxidation-prone than iron and electrons can move freely between the metals, as long as they are in contact, the iron will steal electrons from the boat to compensate for its loss. The nail won't rust until the boat has dissolved enough for the nail to fall out.

That's why sacrificial anodes made of zinc, for example, prevent iron-based boats from rusting. As long as there is zinc left, it will oxidize instead of the iron.

 

[nattugglan11]

Thank you for the response. Is it more appropriate to use regular square tubing (steel) as a threshold instead of aluminum?