Galvanic Couple Potential
Estimate the driving voltage and corrosion risk when two dissimilar metals touch in a wet environment.
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The engineering
When two dissimilar metals are electrically connected and bridged by an electrolyte, the one lower on the galvanic series (more negative potential) becomes the anode and corrodes preferentially while the nobler metal is protected. The larger the potential gap, the harder the couple drives. This card pulls representative seawater potentials versus a saturated calomel electrode (SCE) and reports the gap plus a compatibility call keyed to MIL-STD-889 practice.
The rule of thumb designers actually use: keep the couple under ~0.15 V for general assemblies and under ~0.25 V for anything that stays wet. Two gotchas dominate real hardware — area ratio (a small anode feeding a large cathode corrodes fast, so never surround a small fastener of the anodic metal with a big noble cathode) and passivity (stainless sits noble when passive but jumps ~0.5 V anodic when the film breaks in crevices). Potentials also shift with alloy, aeration, and temperature, so treat these as ranking numbers, not a corrosion rate.
Where this math comes from
Luigi Galvani's twitching frog legs (1780s) and Alessandro Volta's pile (1800) showed that dissimilar metals in a moist path make current — and, unavoidably, that one of them dissolves. Humphry Davy turned the effect into engineering in 1824 when the Royal Navy asked him to save copper ship hulls: he bolted on iron and zinc blocks, inventing sacrificial anode protection that navies still use.
The ordered galvanic series as engineers know it came from cataloging alloy potentials in seawater, work consolidated by researchers like Robert Wesley of the International Nickel Company and codified for defense hardware in MIL-STD-889, which tabulates couples and the allowable potential gaps this card mirrors.
- 1800Alessandro VoltaVoltaic pile demonstrates dissimilar-metal EMF — and dissolution of the anode.
- 1824Humphry DavyFits sacrificial iron/zinc anodes to Royal Navy copper hulls.
- 1938Wesley & Brown (INCO)Publish measured galvanic series of alloys in seawater.
- 1969US DoDMIL-STD-889 codifies dissimilar-metal compatibility and potential gaps.
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