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Compatibility of 400 Series Stainless Steel with Aluminum

I'm evaluating the compatibility issues associated with using a 400 series (410) stainless steel (S.S.) screw used to fasten a Zamak 3 casting to a powder-coated, extruded aluminum component (6005-T5 or 6061-T6) for outdoor applications. The Aluminum Design Manual allows 300 series (austenitic) S.S. to come into contact with aluminum, but it does not permit 400 series (ferritic) S.S. to come into contact with aluminum. My question is this: will a yellow zinc dichromate coating of the 400 series S.S. screw provide adequate corrosion resistance?

Justin Mann
Evaluation Specialist - York, Pennsylvania, U.S.A.
November 24, 2010

Stainless steels, including Type 410, cannot be zinc plated. And even if one could, this would not be a good application for it due to the multiple dissimilar metals in contact in an outdoor environment. Is there a reason you don't want to use a standard 300 series fastener from Elco or SFS intec? If the reason is cost, you would be better off using a carbon steel fastener with a good multilayer coating (Stalgard or similar).

Toby Padfield
Automotive module & component supplier - Michigan, USA
November 29, 2010

I'm evaluating a product that has already been tested, and it used a 410 stainless steel screw. Because the testing is time-consuming, I was hoping to find an alternative to retesting. It's funny that 400 series stainless steel can't be zinc-plated, because that's what I'm told this screw was. Thank you for your time.

Justin Mann
Evaluation Specialist - York, Pennsylvania, U.S.A.
First of two simultaneous responses -- December 1, 2010

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Cadmium plating is a reasonable material for SS/Al interface, but cad is out of favor by most companies.
I would differ with Tony in that 410 can be plated with zinc, but it will require a nickel strike and or a cyanide zinc.
All of the above is not really a factor as it is cheaper to just use a 300 SS in the first place and avoid all of the side issues. If it is not strong enough, go to the next size bigger fastener.

James Watts
- Navarre, Florida
Second of two simultaneous responses -- December 1, 2010

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Justin,

I was a little hasty with my previous reply, and should not have stated than martensitic stainless Type 410 cannot be zinc plated. Some manufacturers of self-drilling screws do offer Type 410 stainless steel with a subsequent zinc plating, but they are meant for thin sheet metal applications, like roofing, with coated steels, and not multiple non-ferrous joint materials. Also, the zinc is often applied by mechanical plating rather then electroplating. While mechanical plating has the advantage of not inducing hydrogen embrittlement in hardened screws, the coating has relatively poor corrosion resistance when compared to state-of-the-art electroplated zinc. You would be better off with one of the choices I mentioned previously.

Toby Padfield
Automotive module & component supplier - Michigan, USA
December 8, 2010