Galvanic Corrosion

How galvanic corrosion works

Put a copper plate and a zinc plate in a glass of salt water and connect them with a wire, and you have just built a battery — the same arrangement Alessandro Volta stacked into the first electric pile in 1800. Galvanic corrosion is what happens when that battery forms by accident, inside a bolted joint, a boat hull, or a heat exchanger, with no one wanting it. The two metals are the electrodes, the salt water is the electrolyte, and the metal-to-metal contact is the wire. Once the loop closes, current flows, and current flowing out of a metal into an electrolyte means metal atoms are dissolving.

Every metal in contact with an electrolyte sits at its own electrode potential — a measure of how badly it wants to give up electrons. When two metals with different potentials are wired together, electrons flow from the more active (less noble) metal to the more noble one. The active metal becomes the anode: oxidation happens there, atoms lose electrons and dissolve as ions, and the metal physically wastes away. The noble metal becomes the cathode: a reduction reaction consumes the arriving electrons, and the cathode is protected, corroding even slower than it would in isolation. The half-reactions for a zinc anode and a steel cathode in aerated seawater:

Anode (oxidation, zinc dissolves):
    Zn  →  Zn²⁺ + 2e⁻              E° = −0.76 V (vs SHE)

Cathode (reduction, oxygen consumed):
    O₂ + 2H₂O + 4e⁻  →  4OH⁻       E° = +0.40 V (vs SHE)

Cell potential drives the current:
    E_cell = E_cathode − E_anode  (a positive value = spontaneous)

The bigger the potential gap between the two metals, the harder the cell is driven and the faster the anode corrodes. That single idea — the gap in potential — is what the galvanic series tabulates, and it is the first thing an engineer checks before bolting two metals together in anything wet.

The galvanic series: who eats whom

The galvanic series ranks metals by their measured corrosion potential in a specific electrolyte (the canonical table is for flowing seawater at about 25 °C). It is not the same as the textbook standard-electrode-potential series — the galvanic series uses real, often passivated, alloys in real seawater, so stainless steels and titanium sit near the noble end thanks to their passive oxide films. The metal lower (more active) in the list is the anode and corrodes; the metal higher (more noble) is the cathode and is protected.

GALVANIC SERIES IN SEAWATER  (noble/cathodic at top)
Potential vs Ag/AgCl reference, approximate

  Graphite / platinum ............ +0.25 V   ← most noble (cathode)
  Titanium ....................... −0.05 V
  316 stainless (passive) ........ −0.05 V
  Nickel (passive) ............... −0.15 V
  Copper / bronze / brass ........ −0.30 V
  Tin / lead ..................... −0.50 V
  Carbon steel / cast iron ....... −0.65 V
  Aluminum alloys ................ −0.80 V
  Zinc ........................... −1.05 V
  Magnesium ...................... −1.60 V   ← most active (anode)

Read it like a food chain. Pair any two and the lower one loses. Copper next to steel: the steel corrodes. Steel next to aluminum: the aluminum corrodes. Aluminum next to zinc: the zinc corrodes — which is exactly why galvanized (zinc-coated) steel works. The voltage gap between the two rows estimates the driving force: aluminum (−0.80 V) bolted to copper (−0.30 V) sees a ~0.5 V driver, a severe couple. Two metals within ~0.15 V of each other (316 stainless next to titanium) form a couple weak enough to ignore in most environments. Designers use 0.15 V as a rough "safe to couple" threshold and 0.25 V as the line above which protection is mandatory in wet service.

Why area ratio decides the damage

The galvanic series tells you which metal corrodes; the cathode-to-anode area ratio tells you how fast. This is the single most misunderstood part of galvanic corrosion, and getting it wrong sinks boats. The total current the cell can pass is governed largely by how much cathode surface is available to consume electrons (via oxygen reduction). But all of that current has to leave through the anode. The thing that destroys the anode is not total current — it is current density, current per unit area:

Corrosion penetration rate ∝ anodic current density

    i_anode = I_total / A_anode

A large cathode pumps a large I_total.
A small anode divides it by a tiny A_anode.
→ i_anode explodes, anode perforates locally.

Faraday's law converts current density to metal loss:
    mass loss rate  ṁ = (i · A · M) / (n · F)
where M = molar mass, n = electrons per atom,
F = 96 485 C/mol (Faraday constant).

For steel (M = 55.8 g/mol, n = 2):
    1 A/m² of anodic current = ~1.16 mm/year penetration

So the rule of thumb is simple and load-bearing: you want a large anode and a small cathode, never the reverse. A steel hull (huge anode) with a bronze propeller (small cathode) spreads the corrosion thinly and survives for years. Flip it — a steel fastener (small anode) clamping a bronze through-hull fitting (large cathode) — and the fastener can be eaten through in a single season. The textbook demonstration: a steel sheet riveted with copper rivets shows almost no rivet damage, but a copper sheet riveted with steel rivets loses its rivets fast and falls apart.

Galvanic couples in the wild

System	Anode (corrodes)	Cathode (protected)	Outcome / fix
Boat hull + bronze prop	Bolt-on zinc	Steel hull & bronze prop	Sacrificial zinc replaced every season (~−1.05 V)
Galvanized steel roofing screw	Zinc coating	Steel substrate	Zinc sacrifices first, steel stays sound until coating depletes
Aluminum panel + steel fastener	Aluminum panel	Steel fastener	Use nylon/cad-plated fasteners + sealant; common car-body rust
Copper pipe + galvanized steel pipe	Steel pipe	Copper pipe	Dielectric union mandatory by plumbing code
Stainless plate + carbon-steel bolt (seawater)	Carbon-steel bolt	Stainless plate	Worst area ratio; use stainless bolts or isolate
Buried steel pipeline	Magnesium anode bed	Pipeline steel	Sacrificial or impressed-current cathodic protection
Aircraft skin: Al alloy + steel/Ti	2024/7075 aluminum	Steel, titanium, CFRP	Alclad, chromate primer, sealant, cad-plated fasteners
Carbon-fiber (CFRP) + aluminum	Aluminum	Carbon fiber (acts noble)	Severe couple; glass-fiber isolation ply between them

The carbon-fiber case surprises people: graphite is the most noble entry in the series, so a CFRP panel bolted directly to an aluminum frame turns the aluminum into a fast-corroding anode. Boeing and Airbus both specify a fiberglass isolation layer and wet-installed sealant wherever CFRP meets aluminum for exactly this reason.

Stopping it: break one leg of the circuit

Galvanic corrosion needs three things at once — two dissimilar metals, an electrical path, and an electrolyte. Kill any one and the cell dies. The standard toolkit, roughly in order of preference:

• Match the metals. The cheapest fix is to not create the couple. Pick fasteners, fittings, and structure from metals within ~0.15 V of each other in the galvanic series. A stainless bolt in a stainless flange has no galvanic driver at all.
• Insulate the contact (break the electrical path). Nylon or phenolic washers, sleeves, and gaskets, or a dielectric union in plumbing. The metals can still both touch the electrolyte, but with no metal-to-metal wire the electrons cannot flow. This is the textbook fix for a steel-to-aluminum or copper-to-steel joint.
• Exclude the electrolyte (coatings and sealant). Paint, anodize, or wet-seal the joint so water never bridges the two metals. Critical rule: coat the cathode, or coat both — never the anode alone. A coated anode with one pinhole becomes a tiny anode against a huge cathode and perforates at that pinhole.
• Sacrificial anode (cathodic protection). Deliberately attach an even more active metal — zinc, aluminum, or magnesium — so it becomes the anode and is consumed while your structure becomes the protected cathode. This is galvanic corrosion turned into a defense. A 5 kg zinc on a steel hull might last a season; a buried magnesium anode protects a pipeline for years.
• Impressed-current cathodic protection (ICCP). A DC power supply pushes protective current onto the structure through inert anodes, holding the whole structure cathodic without consuming a sacrificial block. Used on ships, large tanks, and pipelines where sacrificial anodes would be impractically huge.
• Favorable area ratio. Where a couple is unavoidable, design so the less noble metal has the larger exposed area. Make fasteners from the more noble metal, not the less noble one.

When galvanic corrosion actually matters

• The environment is wet and conductive. Seawater (conductivity ~5 S/m) is the classic accelerant; brackish water, road salt, industrial condensate, and even persistently humid salty air all qualify. Dry indoor air rarely supports a galvanic cell, which is why mixed-metal joints survive indoors but fail outdoors.
• The potential gap exceeds ~0.15–0.25 V. Below that, the driving force is too weak to matter in most service. Above it, you need a deliberate mitigation.
• The area ratio is unfavorable. A small area of the less noble metal next to a large noble surface is the red flag — a steel bolt in a copper or stainless assembly, a zinc pin in a brass body.
• The couple is load-bearing or pressure-bearing. A corroding decorative trim is cosmetic; a corroding fastener, through-hull, or pipeline is a safety failure. Prioritize protection where loss of section threatens structural or containment integrity.

Conversely, ignore it when the metals are close in the series, the joint stays dry, the couple is fully encapsulated, or the less noble metal has the dominant area and only minor surface loss is acceptable.

Galvanic vs other corrosion types

	Galvanic	Crevice	Pitting	Uniform	Stress-corrosion
Trigger	Two dissimilar metals coupled	Stagnant electrolyte in a gap	Local breakdown of passive film	General exposure	Tensile stress + corrosive media
Driving force	Potential gap between metals	Oxygen-concentration cell	Chloride attack on film	Thermodynamic dissolution	Stress + electrochemistry combined
Needs two metals?	Yes (defining feature)	No (one metal)	No	No	No
Location	At/near the metal junction	Inside crevices, under gaskets	Isolated pits on open surface	Whole exposed surface	Along stressed crack paths
Appearance	Wasted anode, spared cathode	Hidden gap attack	Small deep holes	Even thinning, rust scale	Branching cracks
Key control	Insulate / match / anode area	Eliminate gaps, drain	Higher-Cr/Mo alloy, lower Cl⁻	Coatings, alloy choice	Lower stress, alloy, inhibitors
Worst environment	Seawater, road salt	Stagnant seawater	Chloride solutions	Any aggressive media	Chloride + stress (stainless), ammonia (brass)

Putting numbers on it

The voltage drives the cell, but the achievable current — and therefore the corrosion rate — is throttled by the electrolyte's resistance and by how fast oxygen can reach the cathode. A worked feel for the magnitudes:

Aluminum (−0.80 V) coupled to copper (−0.30 V) in seawater
  Driving voltage:        ΔE ≈ 0.50 V
  Typical galvanic current density on Al: 1–10 A/m²
  Faraday conversion for Al (M = 27 g/mol, n = 3):
      1 A/m² ≈ 1.1 mm/year of aluminum loss
  → 5 A/m² ≈ 5–6 mm/year at the worst spot

Standalone aluminum (no couple) in the same seawater:
  Passive film holds rate to ~0.01–0.05 mm/year
  → the couple accelerates loss by 100× or more

Sacrificial-anode sizing (Faraday again):
  Zinc capacity ≈ 780 A·h/kg
  Protect a 10 m² steel hull needing ~0.1 A:
      charge/year = 0.1 A × 8760 h = 876 A·h
      zinc mass   = 876 / 780 ≈ 1.1 kg/year
  → a 5 kg zinc lasts roughly 4 seasons before replacement

Those numbers explain the rituals of boat ownership: divers check and swap zincs on a schedule, and a hull whose zincs have wasted away starts eating its own running gear within weeks. The same Faraday arithmetic sizes the magnesium anode beds protecting thousands of kilometres of buried steel pipeline.

Common misconceptions and pitfalls

• "Stainless steel can't corrode." Stainless is noble only while its passive oxide film survives. Couple a small carbon-steel bolt to a large stainless plate in seawater and the bolt — the small anode against a large noble cathode — is consumed quickly. And if the stainless loses passivity in a starved-oxygen crevice, it can turn active and corrode itself.
• "Just paint the part that's rusting." Painting only the anode is the classic backfire. A scratch or pinhole in the anode coating becomes a tiny exposed anode facing the full cathode area — the worst possible ratio — and perforates at that defect. Coat the cathode, or coat both.
• "More noble means more durable." Nobility is relative, not absolute. The same copper that protects steel will itself be sacrificed when bolted to titanium or graphite. Always ask "noble relative to what?" before choosing a metal for a joint.
• "A tiny dissimilar fastener can't matter." The opposite is true — small dissimilar fasteners are the highest-risk parts precisely because of the unfavorable area ratio. A small anode concentrates damage; a small cathode does almost none. Make fasteners the more noble metal where a couple is unavoidable.
• "It's the same as a battery, so it'll stop when 'discharged.'" Unlike a sealed battery, a galvanic corrosion cell has a near-limitless supply of fresh electrolyte and oxygen in service. It runs until the anode is gone, the joint is isolated, the water is removed, or a sacrificial anode is added.
• "Aluminum and stainless are both shiny metals, so they're compatible." Appearance is irrelevant; position in the galvanic series is everything. Aluminum (−0.80 V) against passive stainless (−0.05 V) is a ~0.75 V couple — one of the more aggressive pairings in marine hardware, and a frequent cause of seized, corroded fastener heads.