Carbon Steel to Aluminum connections

[cmbyrd77]

I have a structure that I am designing where the majority of the roof structure is aluminum sections, however the columns are carbon steel. I plan on specifiying that the columns and any carbon steel connection elements be hot dip galvanized. Is there any fastener precautions I need to take to avoid galvanic corrosion? Should I specify stainless steel or aluminum fasteners over galvanized carbon steel fasteners? Thank you very much for the help.

 

[BAretired]

I am no metallurgist, but I think you must separate different metals using non-reactive materials. Stainless steel does not do the job if in contact with a different metal.

BA

 

[cmbyrd77]

Yeah, I agree with you, and everything I've read in the Aluminum Design Manual seems to state that if you galvanize your carbon steel you should be okay.

 

[MiketheEngineer]

Should be until moisture destroys the galvanizing - like maybe 40 years from now - we will all be dead by then. Might consider some type of gasket material between faces to insure no problems.

 

[Ron]

Galvanizing does not guarantee no issue. Isolate the aluminum from the steel with gaskets and use stainless steel fasteners.

 

[JStephen]

It depends on the conditions- dry dissimilar metals in contact are usually no problem, and that's done all the time- in your car, for example. It might help to have some input from the manufacturer of the aluminum items.

 

[shobroco]

More detail is required for a proper answer. As JStephen said, dry conditions permit lots of things (& make the galvanizing a waste of money) but wet conditions can make many situations recipes for disaster. Where are the connections? Are they intended to be dry but could possibly get wet with condensation or inadverent leakage? Are they actually exterior & frequently wetted? If there is any possibility of moisture, isolate, isolate, isolate!

 

[Klunker]

Use isolator material between the steel and aluminum with stainless steel fasteners only. Rubber or vinyl washers will work. For larger areas a cut to fit piece of EPDM works as well.

 

[TXStructural]

Careful with the use of rubber gaskets or washers. Some may allow or cause corrosion.
The reason you would galvanize is to remove the drive for the steel to oxidize, but pure aluminum will oxidize preferentially to zinc, based on electrochemical series. The best solution in most cases is to use corrosion resistant primers on all components. Never paint the most exposed parts and not the others, this sets up for focal macrocell corrosion, where one part which is remote but connected sacrifices to prevent corrosion of another - like cathodic protection - but the area of metal sacrificing is small relative to the "protected" material. Best success is probably achieved by priming everything after fabrication but before assembly.
The use of stainless hardware will remove corrosion at the fasteners, but do nothing for cathodic reactions away from the fastener, or between the connected metals.