Which metal corrodes in a galvanic couple?

The more active (less noble) metal — the one lower in the galvanic series for that electrolyte — becomes the anode and corrodes faster, while the nobler metal is cathodically protected. Aluminium coupled to stainless steel, for example, sacrifices the aluminium.

Why does the area ratio matter so much?

The anode must supply all the current consumed at the cathode. A large cathode driving a small anode concentrates that current into a tiny area, so the anodic current density — and the penetration rate — is severe. A small cathode on a large anode is comparatively harmless. Never put a small fastener of the active metal into a large noble structure.

Should I coat the anode or the cathode?

Counter-intuitively, coat the cathode (the noble metal). A defect in a coating on the anode just creates a tiny anode under a huge cathode — the worst area ratio. Coating the cathode reduces its effective area and the galvanic driving current.

Corrosion · guide

Galvanic corrosion and the area ratio

Put two different metals in electrical contact in an electrolyte and you have built a battery — one that eats the more active metal. Galvanic corrosion is predictable from the galvanic series and, crucially, from the geometry of the couple.

The driving force: the galvanic series

Each metal sits at a characteristic corrosion potential in a given electrolyte. The galvanic series ranks them from active to noble (the seawater series is the classic one). When two are coupled, the potential difference drives a current: the active metal dissolves as the anode, the noble one is protected as the cathode. The further apart they are in the series, the larger the driving force.

The geometry: cathode-to-anode area

The driving potential sets whether corrosion happens; the area ratio sets how badly. Because the total anodic and cathodic currents must balance, the anodic current density scales with the area ratio:

i_anode ∝ (A_cathode / A_anode) · i_total

A large cathode on a small anode is the dangerous configuration — all the cathodic current funnels into a small anodic area, giving deep, fast attack. The same metals with the areas reversed may corrode negligibly.

Mitigation that actually works

In rough order of effectiveness: avoid the dissimilar couple altogether; electrically isolate the two metals (insulating gaskets, sleeves, washers); keep the electrolyte away (sealing, drainage); coat the cathode; choose a favourable area ratio; or add a sacrificial anode that is more active than both, so it corrodes preferentially and protects the structure.

Open the calculatorGalvanic compatibility tool →Check a metal couple against the galvanic series, see the driving potential and the area-ratio risk, and get mitigation guidance.

A note on environment

The series is electrolyte-specific — positions can shift between seawater, fresh water and acids, and stainless steels in particular can flip from noble (passive) to active if the film breaks down. Use the series for the actual service environment, and remember that conductivity sets how far the galvanic effect reaches from the joint.

Frequently asked

Which metal corrodes in a galvanic couple?: The more active (less noble) metal — the one lower in the galvanic series for that electrolyte — becomes the anode and corrodes faster, while the nobler metal is cathodically protected. Aluminium coupled to stainless steel, for example, sacrifices the aluminium.
Why does the area ratio matter so much?: The anode must supply all the current consumed at the cathode. A large cathode driving a small anode concentrates that current into a tiny area, so the anodic current density — and the penetration rate — is severe. A small cathode on a large anode is comparatively harmless. Never put a small fastener of the active metal into a large noble structure.
Should I coat the anode or the cathode?: Counter-intuitively, coat the cathode (the noble metal). A defect in a coating on the anode just creates a tiny anode under a huge cathode — the worst area ratio. Coating the cathode reduces its effective area and the galvanic driving current.

References

ASTM G82, "Standard Guide for Development and Use of a Galvanic Series for Predicting Galvanic Corrosion Performance."
MIL-STD-889, "Dissimilar Metals."
D.A. Jones, "Principles and Prevention of Corrosion," Prentice Hall.
ASM Handbook Volume 13A, "Corrosion: Fundamentals, Testing, and Protection."

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