Material in contact with aluminum - stainless or galvanized?

[SolarWeenie]

Hi,

Need some advice. We are specifying a racking system for PV modules (sheltered outdoor application). The basic construction is aluminum blocks (~1"x2"x2&quot bolted to a rail. The question is whether to use a HD galvanized rail or a stainless rail. The environment is rural but might have elevated SO2 levels because this is on the roof of a winery. The galvanized rail obviously will not last as long as the stainless (though the Zinc Coating Life Predictor indicates adequate life) but I am concerned about the galvanic couple. Also the aluminum block has two large (3/8&quot 18-8 bolts through it, the manufacturer claims this will not be a problem.

Thanks for the help!

 

[TVP]

Your 18-8 bolt manufacturer is either stupid or unprofessional. Stainless steel in contact with aluminum and an electrolyte (water, sulfuric acid, etc.) will cause galvanic corrosion. Magnesium and zinc are more anodic (less noble) than aluminum, and everything else is cathodic (more noble), with 18-8 quite a bit more noble. You definitely need to take this into account when designing this structure.

 

[Metalguy]

Since your racks are sheltered (and presum. dry most of the time) the galvan. steel should work OK.

Also, while SS vs. alum/zinc/steel couples are generally to be avoided like TVP points out, your area rations are helpful (huge anodes and small cathodes), so I wouldn't expect quick corr. failures. But if you can insulate the SS it's better.