Fastening Aluminum Cladding to Building Substrate - Dissimilar Metals and Galvanic Reaction

[RFreund]

I've gone down a hole and am trying to come out the other side with understanding "best practices" for fasteners used in exterior applications (not necessarily in a marine environment. Generally speaking I'm referring to aluminum cladding components (i.e. clip angles) that are part of a rainscreen system that attach to a cold formed metal stud or wood stud wall. For most aluminum to steel connections I like to use a screw that is similar to the DeWalt BiFlex screw. It is made of stainless steel and has a galvanic barrier coating with a self-drilling tip. My understanding was that stainless steel is desirable because if the coating (which I believe has AL flakes in it) were compromised at least the fastener is protected (More Noble/cathodic than AL). However, when looking for a screw with a wood drilling tip, I couldn’t find one that was stainless steel and had a coating. I can find many that are made of carbon steel and have a coating or stainless steel with no coating. At first I thought that carbon steel base material was bad as it was less noble than AL, but I now I’m thinking that might be wrong. They appear to be very similar but AL is slightly less noble. So now I’m questioning what my best option is: (1) a stainless steel fasteners with no coating or (2) a carbon steel (base material) with zinc plating and a galvanic barrier.

Info from several sources to also consider:

ETA-10/0200 fastening screws for metal members and sheeting lists many screw combinations for fasteners attaching roof and wall panels to metal or wood substrates. However, they appear to only use stainless steel with aluminum. Typically with a EPDM washer.

Metal Construction Association also recommends a (300 series) stainless steel fastener.

Based on the two documents above it appears that an uncoated stainless steel fastener is acceptable in an exterior application, but is that best? Can a harden zinc plated carbon steel fastener be used? If not, why?

 

[TugboatEng]

Stainless Steels are incompatible with coatings as coatings cause crevice corrosion of the stainless steel screws.

The key is to have a high ratio of aluminum to stainless steel. There will be galvanic corrosion but it will be distributed over a large area of aluminum.

Aluminum is also subject to crevice corrosion so do ensure that there is an air gap between the aluminum and wood. Seal any areas that have contact.

Modern coatings (inorganic zinc) on steel screws perform quite well. Coated steel or uncoated stainless with sealing washers are both good options.