Interesting topic. Would the spray foam approach eliminate any chance of "sweating" between the metal siding and the insulation? If so, seems like it would be the way to go unless it contains something that attacks the metal over time.
Do we have any Airframe mechanics on this site? How do they insulate the air
frames on airliners? Certainly can't risk deterioration there for sure.
There is no sweating between foam insulation and metal. All insulated aluminum skinned refrigerated trailers have dry foam after years of use if there are no water leaks from outside intrusion.
There is also no corrosion interaction between foam and steel. I've dismantled hundreds of trailers and scraped the foam off of aluminum, steel, fiberglass, PVC, and wood and have never seen any type of reaction between them.
Reefer trailers have steel unit frames that support the 3500lb refrigeration unit. This frame is covered in foam and does not corrode from the foam on it.
Unless the foam is wet from outside intrusion it is stuck solidly to the aluminum panels and steel unit frame after years of contact with no damage to the metal.
As I stated above the only rot I've seen was leaky sub floor saturated foam and aluminum. There is a high concentration of acid in these areas of a trailer as the truck washers use highly concentrated acid to make the aluminum floor shine so that corrosion is likely acid etching corrosion.
ETA: When I built new insulated walls I would cut wood fir-outs which would screw on to the inside of the aluminum posts. Then new foam was sprayed on the panels. The fir-outs set the thickness of the wall and also isolated heat transfer from outside. In essence the metal panels and posts are 1/2" to 1" away from the inside wall liner. This keeps sweating from occurring as no metal touches the inside wall from the outside.