Hi All,
If you guy's are going to mate two different metals mechanically, go here...
PRC-DeSoto & Pro-Seal Aerospace Products or here
Mastinox Aerospace Coatings - Interturbine Logistik - The One Stop Source for Aviation Materials
They can supply the coatings you need to do the job.
If these products are used correctly, this interface between steel and aluminum will never corrode.
Regards,
Scott