A SS duct liner w/ a carbon steel outer framework is a very standard situation. The carbon stiffners can even be Epoxy coated to further resist corrosion. The carbon welds easily to the SS, either 304 or 316. Hope this helps.
The person that spec'd. the Galv. bolts with the Alum. didn't know what he was doing. The SS is the proper fastener. Either T304 or T316 or even 16-6 will work.
The 316 is the better for corrosion resistence. Never let carbon or Galv. parts interact with the Alum. Hope this helps.
Hi! Corten is basically a 572-50 steel w/ weathering features IE: it is meant to NOT be painted (at least I haven't seen a paint that will wear well). The cracking problem is probably due to a brittleness. A Sharpy V-notch test will tell if it is a "fracture-critcal" problem. Hope this helps.
Stainless fasteners for use on Galv. connections are O.K., as long as the total fastener is SS & not a SS bolt w/ a Galv. Nut. Any joint corrosion will be the Galv. parts, not the SS items. Of particular note are Cooling tower bases & structures that are continually damp. These will hold up...
