The problem is the copper used to treat the wood causes galvanic corrosion in dissimilar metals.
Ranked by similarity:
Austenitic Stainless (passive i.e. with protective film)
Chromium Steels (11-30% Cr) (passive)
Inconel (80% Ni, 13% Cr, 6.5% Fe)
Nickel and High Nickel Alloys
Monel (67% Ni, 30% Cu)
Alpha Brass (70% Cu, 30% Zn)
Naval Brass (60% Cu, 40% Zn)
Austenitic Stainless (active i.e. protective film removed)
High Nickel Cast Iron
Chromium Steels (active)
Note copper and zinc ("galvanized" or "plated") are quite far apart on the table.
I'm pretty sure the newer wood treatment (which is rusty in colour) contains no copper.