Screw fastener material for aluminum bracket mounted to steel tube

[HouseBoy]

I need to attach an aluminum track overhead to a structural steel tube header.
I need to use #12-24 machine screws
What material should I use (and what materials to avoid)?
This is for folding (exterior) doors, hung from above and guided in a track below. Vertical support is at the top tract only.

I've been told to use 316 stainless. I'm hoping that is right but I'm concerned about the SS threads inside the carbon steel tube - 5/8 wall thickness will be drilled and tapped. Can't get thread rolling screws in the size I need 12-24 x 3.5" long)

Does all this sound correct?
Would zinc coated carbon steel be "better"?
Thanks

 

[CoryPad]

Use carbon steel fasteners with a coating like Magni 565, Doerken Delta-Protekt, Atotech Zintek, etc. These coatings have coatings with excellent compatibility with aluminium, and are much less expensive than stainless steel parts. In the structural arena, they are known with brand names like Stalgard from Elco/Stanley, Climashield from ITW Buildex, etc.

 

[HouseBoy]

CoryPad,
Money is not an object and the question of corrosion is paramount so... I appreciate that CS will be less costly but if we are addressing the question of corrosion only, is your rec less corrosive in the 3 material "sandwich"?
In other words, I'm thinking that maybe I have more of a problem between the screw and the carbon steel tube? Is THAT a factor?

 

[CoryPad]

Galvanic corrosion becomes a problem when you join aluminium to carbon steel with a stainless steel fastener. If you use a coated carbon steel fastener, it will have limited galvanic corrosion between the aluminium and carbon steel parts. If the coating is good enough, then you have good general corrosion resistance and cosmetic appearance.