Galvanic action and dissimilar metals

One-line orientation

When two dissimilar metals touch with moisture present, the more active (anodic) metal corrodes to protect the more noble (cathodic) one — the exam tests which metal sacrifices itself and the classic bad pairings (copper + galvanized steel).

Key points

• The galvanic series ranks metals from active (anodic) to noble (cathodic):
  • Anodic end (reactive, corrodes first): aluminum, zinc — and galvanized (zinc-coated) metal. Mnemonic: anodes run “A to Z” (aluminum → zinc).
  • Cathodic end (noble, resists corrosion): copper, bronze, and gold/platinum.
  • Stainless steel’s position in the galvanic series changes mainly with its active or passive surface condition and the surrounding electrolyte.
• Who corrodes: the anodic metal sacrifices itself; the cathodic metal is protected.
• Problem pairings (anodic metal corrodes faster when joined to the noble metal, esp. when wet):
  • Copper + galvanized steel — copper is cathodic; the galvanized zinc corrodes (classic).
  • Brass + galvanized steel — same mechanism as copper.
  • Stainless steel + galvanized steel — stainless is cathodic; galvanized side corrodes.
  • Aluminum with more-noble metals, or in ACQ-treated wood chemistry — aluminum corrodes.
  • Copper + zinc, mortar (alkaline) + zinc — accelerate zinc corrosion.
• Cathodic / sacrificial-anode protection: intentionally attach a more anodic metal (zinc, magnesium, aluminum) so it corrodes instead of the protected structure — used for buried/ submerged steel. Galvanizing applies the same principle as a coating.
• Prevention: isolate dissimilar metals (non-conductive gaskets, coatings, barriers), keep the fastener the noble/cathodic member, and pick metals close on the series.

In a wet metal pair, the more active metal corrodes to protect the nobler one

SCHEMATIC

Position on the galvanic series — not which metal sounds tougher — decides which metal corrodes.

View diagram Hide diagram

Scroll horizontally to explore →

Key

• The anode — the more active metal; it corrodes (blue, pitted).
• Index line — locates each part on the series.

Same rule, twice — a mistake above (copper over galvanized steel), on purpose below (a sacrificial anode): the more active metal corrodes to protect the nobler one.

Ordinal series order (schematic), not measured electrode potentials; steel and stainless positions follow the standard galvanic series.

Confusions / comparison

Pairing	Anode (corrodes)	Cathode (protected)	Note
Copper + galvanized steel	Galvanized zinc/steel	Copper	Classic bad pairing; worse when wet
Brass + galvanized steel	Galvanized zinc/steel	Brass	Same mechanism as copper
Stainless + galvanized steel	Galvanized zinc/steel	Stainless	Stainless acts noble here
Aluminum + noble metal / ACQ wood	Aluminum	The noble metal	Aluminum is reactive
Sacrificial anode (by design)	Zinc / Mg / Al (added)	The structure	Cathodic protection / galvanizing

Related

→ Steel types and production (this module): galvanized vs stainless vs weathering steel · Steel connections (this module): dissimilar-metal fasteners in connections · Plumbing piping materials (Systems module): dissimilar-metal/dielectric unions in piping.