Steel/Aluminum Corrosion resistance

[daper]

I work for a Curtain Wall engineering company. We use a lot of aluminum extrusions and often we have to have them reinforced with structural steel angles or rectangular steel tubing. The aluminum shapes are hollow and we put the steel inside, where the aluminum has no protective finish. Some builders specify wrapping the steel with tape or a bituminus coating before anchoring it in place. We usually specify Dril-Flex fasteners through the side of the aluminum extrusion.
Here's my question: We have been using only a red oxide paint coating on the steel for some builders. Is this a sufficient barrier to prevent corrosion of the aluminum?

 

[TVP]

Doubtful, but it depends on some specific details:

1. Are Dril-Flex fasteners steel? If so, what type of coating is applied to them? The geometry of the fastener head has an effect as well.

2. What are the specifics of the red oxide paint coating? It is an epoxy, polyurethane, etc.? How thick? Is the red pigment iron oxide? Does the coating contain any chromates? Is there a pre-treatment applied to the steel prior to painting, such as zinc phosphate?

3. I am not familiar with bituminous coatings, but I would guess that they have some electrical conductivity, therefore making them a poor choice for joining steel to aluminum. The best coatings are electrically insulative; thick organic coatings like polyester powder coats with thickness of ~ 75 micrometers applied over a zinc phosphate pre-treatment are best for the steel parts.