Galvanic Corrosion
When two different metals touch in the presence of an electrolyte, one corrodes faster to protect the other. Here is how to predict it, prevent it and avoid coating it wrong.
Key takeaways
- Galvanic corrosion occurs when two electrically connected metals with different potentials share an electrolyte; the more active metal corrodes faster.
- The galvanic series ranks metals by their potential in a given environment, usually seawater, and predicts which metal will be attacked.
- A small anode connected to a large cathode corrodes rapidly; the reverse arrangement is far less damaging.
- Never coat only the anodic metal — coat the cathode, or both, and isolate the metals where possible.
Every metal has a natural tendency to corrode, and that tendency differs from metal to metal. Join two different metals and expose them to moisture, and they form a battery: current flows, the more active metal dissolves faster than it would alone, and the more noble metal corrodes less. The principles behind this are introduced in corrosion science basics; this article focuses on dissimilar-metal couples.
Galvanic action is not always harmful. Zinc coatings and sacrificial anodes are deliberate applications of it. The problems arise when it occurs by accident, in a joint, fastener or piping connection nobody thought about.
How galvanic corrosion works
Three conditions must all be present:
- Two metals with different potentials. Their difference provides the driving voltage.
- An electrical connection. Direct contact, a bolt, a weld or any conductive path lets electrons flow from the active metal to the noble one.
- A shared electrolyte. Water, condensation, wet soil or a salty film completes the circuit by carrying ions between them.
The active metal becomes the anode and loses metal. The noble metal becomes the cathode, where oxygen reduction or hydrogen evolution consumes the electrons. Remove any one condition — by insulating the metals, keeping the joint dry or choosing compatible metals — and galvanic corrosion stops.
The galvanic series
The galvanic series lists metals and alloys by their measured corrosion potential in a specific environment, most often flowing seawater. It differs from the theoretical electromotive series because real alloys carry oxide films and alloying elements that change their behavior. The order below is simplified and approximate.
| Position | Metal or alloy (seawater, simplified) | Typical role in a couple |
|---|---|---|
| Most active (anodic) | Magnesium and its alloys | Sacrificial anode material |
| Active | Zinc; galvanized coatings | Protects steel sacrificially |
| Active | Aluminum alloys | Attacked when coupled to steel, copper or stainless |
| Intermediate | Carbon steel, cast iron | Anodic to copper and stainless; cathodic to zinc |
| Intermediate | Lead, tin | Varies with conditions |
| Noble | Brasses, bronzes, copper, copper-nickel | Accelerate corrosion of steel and aluminum |
| Noble | Passive stainless steels, nickel alloys | Strong cathode to carbon steel and aluminum |
| Most noble (cathodic) | Titanium, graphite, carbon fiber composites, platinum | Very strong cathodes |
Two caveats matter. First, passive stainless steel sits much nobler than the same alloy in its active state, so behavior can change if the passive film breaks down. Second, the order can shift in fresh water, soils, hot water or chemicals, and in some hot fresh waters zinc can even become cathodic to steel. Distance in the series indicates the driving force, but not the actual corrosion rate.
What controls severity
Area ratio
The cathode’s area largely determines how much current flows, and that current is concentrated on the anode. A large cathode connected to a small anode — carbon steel rivets in a copper sheet, or aluminum fasteners in stainless — produces intense local attack. A small cathode on a large anode, such as stainless fasteners in a large steel plate, causes much less damage because the current is spread out.
Electrolyte
Seawater and salt-contaminated condensation are highly conductive, so current spreads farther from the joint. In low-conductivity water or intermittent damp, attack concentrates close to the junction line.
Polarization and films
Some metals polarize or form protective films that limit current. Others, especially at high flow rates or temperatures, do not. That is why laboratory couple tests and service experience are more reliable than potential difference alone.
Coating a galvanic couple correctly
The intuitive move — coating the metal that is corroding — can make things worse. No coating is perfect, so the anodic metal will always have small holidays. If the cathode is bare and large while the anode is coated, all of the galvanic current focuses on those tiny defects and drives deep, rapid pitting.
Never coat only the anode in a dissimilar-metal joint. Coat the cathodic (noble) metal, or preferably both metals, extending the coating well past the joint. Reducing exposed cathode area is the most effective way to cut galvanic current.
The same logic applies to mill scale, which is cathodic to steel. Scale left under a coating creates many small anodic sites at breaks in the scale, one reason high-performance specifications require it to be removed.
Common problem couples
- Stainless or copper fasteners in aluminum — aluminum around the fastener corrodes; see coating aluminum.
- Copper or brass piping connected to steel — steel near the connection is attacked.
- Stainless steel welded or bolted to carbon steel — carbon steel along the junction corrodes, especially if the stainless area is large.
- Carbon fiber composites joined to aluminum — the composite acts as a strong cathode.
- Galvanized steel in contact with copper or wet treated timber — the zinc layer is consumed faster than expected.
Preventing galvanic corrosion
- Select compatible metals. Choose metals close together in the galvanic series for the service environment, and make fasteners nobler than the parts they join.
- Insulate the joint. Use dielectric unions, insulating flange kits, non-conductive washers, sleeves and gaskets to break the electrical path.
- Exclude the electrolyte. Seal crevices, use jointing compounds and design for drainage so joints do not stay wet.
- Coat strategically. Coat the cathode, or both metals, with a sound barrier system.
- Add sacrificial protection. Zinc-rich primers, hot-dip galvanizing or anodes can protect the more vulnerable metal.
The same principle used deliberately — connecting a more active metal to protect steel — underpins cathodic protection.
Insulating kits only work if nothing else bridges the metals. Check for shared supports, conductive bolts, grounding straps and wet deposits that can short-circuit an otherwise isolated joint.
Frequently asked questions
Is it safe to use stainless steel bolts on galvanized steel?
In many dry or mildly corrosive atmospheres the small bolt area causes little attack on the larger galvanized surface. In wet, marine or immersed service, insulating washers and sleeves or compatible fasteners are a safer choice.
Does galvanic corrosion happen without water?
No. An electrolyte is required, although a thin film of condensation or a humid salt deposit can be enough. Fully dry joints do not corrode galvanically.
How far from the joint does galvanic corrosion reach?
It depends on electrolyte conductivity. In seawater, attack can spread a considerable distance; in thin moisture films it is usually concentrated close to the junction.
Educational reference. Coating performance varies by formulation. Always follow the manufacturer’s product data sheet, safety data sheet and your project specification.
