Galvanic Corrosion: The Galvanic Series and How to Stop It

Galvanic Corrosion: The Galvanic Series and How to Stop It is the accelerated attack on the less-noble of two dissimilar metals when they are electrically connected and share a common electrolyte. The more active metal becomes the anode and corrodes, while the more noble metal becomes the cathode and is protected. Knowing which metal wins that contest, and by how much, is the core of preventing a failure that is entirely predictable once the couple, the electrolyte, and the area ratio are known.

How the Galvanic Cell Forms

Galvanic corrosion needs three ingredients at once. Remove any one and the reaction stops.

• Two dissimilar metals with different electrochemical potentials.
• An electrical connection between them, so electrons can flow from anode to cathode (a bolt, weld, wet contact, or shared structure).
• A continuous electrolyte bridging both metals: seawater, condensate, rain, process fluid, or even a damp salt film.

The couple then behaves like a short-circuited battery: metal ions leave the anode, electrons travel through the metallic path, and a cathodic reaction (usually oxygen reduction or hydrogen evolution) consumes them at the noble surface. The anode thins, pits, or perforates while the cathode stays bright.

The Galvanic Series

The galvanic series ranks metals by their measured corrosion potential in a specific electrolyte, most commonly seawater. It is not the same as the standard EMF series of pure elements: it uses real engineering alloys in a real environment, which is why passive stainless steels appear near the noble end. The metal that sits higher (more anodic, more negative) corrodes; the metal lower (more cathodic, more positive) is protected. The larger the separation between two metals, the stronger the driving force for attack.

Values are representative and shift with temperature, aeration, flow, and alloy grade, so treat the ranking as a guide rather than an absolute number.

The Area Ratio Effect

The single most damaging design mistake is a small anode wired to a large cathode. The cathodic current is spread over the whole noble surface, but all of the corresponding anodic current is forced to leave a tiny active area, so current density on the anode becomes enormous and the small part is eaten quickly.

The classic example is a carbon-steel fastener holding a large stainless or copper-alloy plate: the bolt is sacrificed in weeks. Reverse the geometry, and the attack spreads thin and tolerable. The rule is simple: never let the anode be the small component. If a mixed couple is unavoidable, make the less-noble metal the large member and cut fasteners from the more noble material.

Prevention by Material Choice and Design

Prevention starts on the drawing board. The most reliable defenses, in order of effectiveness, are:

• Avoid the couple. Select metals close together in the galvanic series, ideally within about 0.15 V, or use a single alloy throughout.
• Electrically isolate. Break the metallic path with non-conductive gaskets, sleeves, and washers so no electron flow is possible even when both metals are wet.
• Coat the cathode, not just the anode. Painting only the anode is dangerous: a coating holiday concentrates the whole cathode current on a pinhole. Coat both, and prioritise the noble surface.
• Design out traps. Slope surfaces to drain, avoid crevices, and keep the electrolyte from pooling at the joint. Trapped moisture under lagging is a frequent cause of hidden attack, closely related to corrosion under insulation.

Where galvanic conditions combine with tensile stress and a specific environment, the same couple can also promote stress corrosion cracking, so material selection must consider more than uniform metal loss.

Sacrificial Anodes: Galvanic Corrosion Put to Work

The same physics that destroys a bolt can protect an entire structure when the couple is chosen on purpose. Connect a very active metal such as zinc, aluminium, or magnesium to steel, and the active metal becomes the anode and corrodes preferentially, driving the steel cathodic and protecting it. This is the basis of galvanic (sacrificial) cathodic protection used on ship hulls, tanks, buried pipelines, and heat-exchanger waterboxes.

The sacrificial anode is a consumable and must be inspected and replaced before it is fully depleted. Zinc coatings on galvanised steel work the same way at small scale: the zinc corrodes first and protects the steel at scratches. For anode sizing, reference electrodes, and impressed-current systems, see the guide to cathodic protection.

Inspection and Monitoring in Maintenance

Galvanic attack is predictable, which makes it ideal for planned inspection rather than run-to-failure. Focus on dissimilar-metal joints, fasteners, tube-to-tubesheet interfaces, and any location where two materials meet in wet or humid service. Look for preferential wastage, poultice deposits, and staining on the active side while the noble side stays clean.

A structured maintenance program keeps these findings from being lost. Teams that log anode readings, coating condition, and joint inspections as recurring tasks in a CMMS such as Fabrico can trend wall loss over time and trigger anode replacement before a leak, turning a corrosion surprise into a scheduled job. Book a Fabrico demo to see how dissimilar-metal inspection routes fit into a preventive schedule.

Frequently Asked Questions

Which metal corrodes in a galvanic couple?

The less-noble, more anodic metal always corrodes. Find both metals in the galvanic series: the one higher on the active end thins and pits, while the more noble metal is protected. Magnesium, zinc, aluminium, and carbon steel are common anodes; stainless steel, copper alloys, titanium, and graphite are common cathodes.

Does painting stop galvanic corrosion?

Coatings help but only if applied correctly. Coat both metals, and give priority to the cathode. Painting only the anode is risky, because any pinhole focuses the entire cathode current onto a tiny bare spot and accelerates perforation there.

Are sacrificial anodes just galvanic corrosion on purpose?

Yes. A sacrificial anode is a deliberately chosen active metal, usually zinc, aluminium, or magnesium, connected to the structure so it corrodes first and protects the steel. It is a consumable and must be inspected and replaced before it is fully used up.