Fasteners to connect aluminum parts

[jimmytwotimes]

Corrosion potential due to the use of dissimilar metals is a common concern in the cladding industry. There are lots of factors: interior vs. exterior exposure, anodized vs. painted aluminum, prime painted vs. galvanized steel vs. stainless steel, steel fasteners vs. stainless fasteners, etc...I feel this is a topic that engineers often do not have a solid stance on - myself included.

I want to consider a simple case: two aluminum components need to be connected with a bolted connection (ie. clear holes in aluminum, fastener to consist of nut and bolt). Industry standard seems to call for the fastener to be stainless steel. This makes sense knowing the aluminum will corrode preferential to the fastener, but the rate of corrosion will be reduced since the anodic area is >> cathodic area. A bit of local corrosion around the fastener is probably acceptable.

What if the fastener was zinc plated steel? My thought would be the zinc corrodes at an increased rate, which leaves the steel exposed. Although the raw steel would likely be cathodic wrt to the aluminum, the raw steel would be susceptible to atmospheric corrosion, and that is not good practice.

In reality I feel there are a lot more factors that come into play, so I was looking if anyone could share their experience or thoughts. Zinc plated steel fasteners are more economical, but I'm hesitant to go against what I've seen to be industry standard, and my logic says is that way for good reason.

 

[phamENG]

Aluminum is an expensive building material when compare to steel. It is much more flexible (E[sub]Al[/sub]/E[sub]Steel[/sub]=0.35+/-) and much weaker (strength varies from "no minimum yield" up to the upper 20ksi/ low 30ksi range for specialty structural products), so you need more of it from a volume perspective to build a comparably strong and stiff structure. It's also very light and very corrosion resistant. These benefits, combined with the larger sizes needed, combine to produce a generally higher cost for an equivalent structure. So, it's clear that it must be used for a good reason or reasons. Corrosion resistance is usually chief among them. You can put it up and not worry about rust. It might get a little dull as it forms its patina, but it'll go on looking good for a very long time as long as you clean it. That's one big driving reason for using stainless steel fasteners. As you say, zinc will eventually be consumed and the steel will corrode, leaving rust streaks on your aluminum. Stainless doesn't have that problem.

Other than that - not much of a reason that I know of. If you look at Table 2-7 in the Steel Construction Manual (page 2-51 in the 14th edition), you'll find the "Metal Fastener Compatibility to Resist Corrosion" table. Galvanized Fasteners with Aluminum is actually one of the best combinations from a galvanic cell perspective.

Another option is aluminum fasteners. I've never designed anything with them, and I'm not sure of their availability, but I know the ADM has a section on bolting and they talk about aluminum structural bolts.

 

[brut3]

This excerpt it taken from the commentary of the 2015 Aluminum Design Manual - Section A.3.7 Screws

"This Specification addresses aluminum screws only. There are no ASTM specifications for aluminum screws. The material for screws used to connect aluminum parts is selected to meet strength and corrosion resistance considerations.Steel screws with a Rockwell hardness of C35 or greater may suffer hydrogen-assisted stress corrosion cracking(HASCC) where exposed to certain dissimilar metals, moisture, and tensile stress due to installation or loading. Aluminum and austenitic stainless steel screws do not experience HASCC. When fasteners will not be exposed to contact with liquid water or humidity near the dew point, certain other steels, with appropriate hardness, and appropriately coated and/or plated are often used. An example is 430 stainless steel, which has a nominal composition of 16% chromium. The AISI North American Specification cation for the Design of Cold-Formed Steel Structural Members provides design rules for steel screws."

300 series stainless is the industry standard and what I specify in my designs; however, based upon my interactions with fabricators and installers of aluminum products this is what I most typically see:
- Fasteners are completely exposed to the elements - 300 Series Stainless fasteners are used
- Fasteners are concealed or indirectly exposed to the elements - Galvanized steel fasteners meeting the Rockwell hardness requirements as specified above are used (Reference Elco Dril-Flex or ITW Buildex TEK Select)