The problem is the copper used to treat the wood causes galvanic corrosion in dissimilar metals.
Ranked by similarity:
Graphite
Platinum
Titanium
Austenitic Stainless (passive i.e. with protective film)
Chromium Steels (11-30% Cr) (passive)
Silver
Inconel (80% Ni, 13% Cr, 6.5% Fe)
Nickel and High Nickel Alloys
Monel (67% Ni, 30% Cu)
Bronze, Gunmetal
Copper
Alpha Brass (70% Cu, 30% Zn)
Naval Brass (60% Cu, 40% Zn)
Tin
Lead
Austenitic Stainless (active i.e. protective film removed)
High Nickel Cast Iron
Chromium Steels (active)
Cast Iron
Mild Steel
Aluminium Alloys
Cadmium
Aluminium
Zinc
Magnesium
From
http://www.reliability.com/articles/guest_article03.htm 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.
