What Is Galvanic Corrosion? Causes and Prevention
Galvanic corrosion is accelerated metal loss caused by coupling materials with different corrosion potentials. It needs an electrical connection and a shared electrolyte, such as saltwater or a damp, contaminated film. The more active material becomes the anode and corrodes faster than it would alone. Dissimilar metals touching in a truly dry environment do not, by contact alone, create this wet corrosion cell.
How the galvanic corrosion circuit works
A wet, electrically connected pair behaves like a small battery. The anode supplies electrons as metal atoms dissolve. The cathode receives those electrons in a reduction reaction. AMPP’s definition emphasizes the change caused by coupling: faster corrosion of the anodic member and generally slower corrosion of the cathodic member.
The exposed materials behave differently in the actual fluid and surface condition. Alloy name, coating, and passive film all matter.
Direct contact, a bolt, a weld, a wire, or a common frame can carry electrons between the materials.
An ion-conducting liquid or wet film connects the exposed surfaces. Condensation or damp salt deposits can be enough.
Direct contact at the wet surface is not essential. Two parts separated by a gasket may still be electrically connected through a bolt shank or bonding wire elsewhere. If a conductive liquid also links their exposed surfaces, the circuit can remain complete.
Electrons move through the electrical connection; ions carry charge through the electrolyte. At the cathode, a reaction such as oxygen reduction consumes the arriving electrons. In aerated, near-neutral water, the simplified reactions can be written as:
Anode: metal dissolutionM → Mn+ + ne−
Cathode: a common reaction in aerated neutral waterO₂ + 2H₂O + 4e− → 4OH−
M represents the metal and n its ionic charge. Other electrolytes can support different cathodic reactions.
Which metal corrodes first?
The material that is more active in the service environment normally becomes the anode. A galvanic series ranks materials by their measured corrosion potentials in a specified electrolyte. ASTM G82 explains how to use such a series and why its environment matters.
The following are useful tendencies for many aerated wet environments. They do not approve a joint for every alloy, coating, fluid, or temperature.
| Connected materials | Usually more vulnerable | What to check |
|---|---|---|
| Aluminum + passive stainless steel | Aluminum | Exposed areas, salts, trapped water, hole edges, and the full fastener isolation detail. |
| Aluminum + copper or brass | Aluminum | The electrical connection, shared moisture, coatings, and the size of each exposed surface. |
| Carbon steel + passive stainless steel | Carbon steel | A small steel fastener supporting a large wet stainless surface is an unfavorable arrangement. |
| Galvanized steel + copper or passive stainless steel | The zinc coating | Additional coupling can consume zinc faster. Check the exposed zinc area and environment. |
| Aluminum + conductive carbon-fiber composite | Aluminum | Exposed fibers, conductive dust, fasteners, and edge damage. Carbon can act as the cathode even though it is not a metal. |
Swipe sideways on small screens. References: BSSA’s material-pair guidance, American Galvanizers Association, and FAA discussion of graphite/epoxy composites.
Surface condition can change the ranking. Stainless steel with an intact passive film can behave differently from a local area where that film has broken down. A seawater series is also not a reliable ranking for hot acid, alkaline solution, or a particular coolant. Standard electrode potentials describe idealized half-reactions; subtracting two of those values does not establish field compatibility or service life.
For the zinc-protection mechanism and coating choices, see galvanized steel: types, grades, and uses. For the wider fabrication decision, see stainless steel versus aluminum for sheet-metal work.
Why a small anode and a large cathode are a problem
A large wet cathode can support reduction current that a much smaller anode must supply. That concentrates anodic current over little exposed metal. A bolt, rivet, cut edge, or coating defect may therefore lose section faster than the size of the overall assembly suggests.
Count the wetted, participating surface—not just the contact patch. A small bolt contact can electrically connect a large panel. Conversely, only part of that panel may be wetted by the same conducting film. Coatings, electrolyte resistance, drainage, and the geometry of the wet region affect the effective areas.
Coating only the anode can create a weak point. A scratch in that coating exposes a tiny anodic area beside a large bare cathode. The coated-anode problem discussed by the National Bureau of Standards follows this same area-ratio principle. Plan protection for both members where practical, including holes and edges; reducing the exposed cathode is generally the safer direction if only one member can receive a barrier coating.
How to prevent galvanic corrosion at a joint
Start by removing a condition that sustains the cell. Where that is impractical, combine material selection, barriers, water management, and a suitable area ratio. The American Galvanizers Association’s guidance uses this circuit-based approach for galvanized assemblies.
Select materials for the actual exposure
Specify the base alloys and finishes, including the fastener coating. Use an applicable galvanic series, relevant service experience, or testing to assess the pair. A material that is acceptable indoors may need different protection in a marine splash zone or a chloride washdown area.
Keep the part’s other duties in view. Corrosion resistance, mechanical load, temperature, and required electrical conductivity must be satisfied together. Replacing a fastener with a more noble material may protect that fastener while transferring the corrosion concern to the surrounding panel.
Isolate the full electrical path where permitted
A complete detail may need a nonabsorbing gasket between faces, a sleeve around the bolt shank, and insulating washers under the head and nut. A washer alone does not isolate a bolt that still touches the hole wall.
Check for metal burrs, conductive debris, common frames, or bonding connections that bypass the barrier. Where electrical bonding is required, retain that function and choose corrosion controls that work with it. Insulating parts and sealants must also tolerate the joint’s load, movement, and service temperature.
Keep water out, and give it a way to leave
Seal mating surfaces and vulnerable edges using a system suitable for the materials and exposure. Provide drainage and inspection access. Avoid a detail where a failed seal traps water around the fastener.
Specify coating coverage at holes, threads, cut edges, and likely wear points. An intact surface film on the broad panel is little help if installation damage leaves the most vulnerable small area exposed. Inspection and repair provisions are part of the design.
What a published marine-ladder example shows
BSSA reports local pitting in aluminum treads secured with uninsulated stainless bolts in a marine environment. Locations on the same ladder with sound insulating washers did not show that pitting. The observation illustrates the value of interrupting the local connection. It does not prove that one washer will isolate every bolt arrangement or that the same material pair is suitable in every environment.
Use sacrificial protection as a designed system
Galvanic action is also useful. Zinc, aluminum, and magnesium anodes are used in appropriate environments to supply protective current to other structures. AMPP describes this cathodic-protection principle.
The anode must suit the electrolyte, remain electrically connected, and supply enough current for the area being protected. Placement, coating condition, consumption, and replacement need consideration. Attaching an arbitrary piece of active metal is not a complete protection design.
How to check whether the damage is galvanic
Preferential damage near a dissimilar-metal connection is a clue. Establish the circuit and the location of actual metal loss before assigning the cause.
- Identify the connected materials. Record alloys, plating, coatings, and fasteners. Include hidden electrical connections and the internal wetted surfaces of pipework.
- Trace the liquid path. Look for condensation, leaks, washdown, salt deposits, retained water, or runoff. Confirm that the electrolyte can reach the relevant exposed surfaces.
- Preserve evidence and measure the damage. Photograph the pattern and retain needed deposits or samples before cleaning. Then assess pit depth, remaining wall, or fastener section by a suitable inspection method.
- Check competing explanations. Pitting and crevice attack can occur without a second alloy. Broad coating failure, transferred rust, or an external electrical source may need a different correction.
The FAA corrosion guide distinguishes galvanic attack from other mechanisms. Its aircraft repair procedures are specific to that field; use the acceptance and repair requirements applicable to your own component.
Galvanic corrosion and stray-current corrosion have different drivers. A galvanic cell produces current through the coupled material reactions. Stray-current attack is driven by an outside source, such as a DC electrical system. Similar-looking damage does not make the corrective action the same.
At a welded assembly, filler metal, heat-affected regions, surface contamination, and crevice geometry can add other mechanisms. The welded-joint corrosion guide covers that wider diagnosis.
How fast will the metal corrode?
There is no reliable universal rate from two material names or a voltage gap. The rate depends on the actual current, exposed areas, electrolyte resistance, surface films, reaction rates, and transport of substances such as oxygen. Temperature, deposits, and the time the joint stays wet also matter.
A galvanic chart can identify an unfavorable tendency. It cannot turn “aluminum plus stainless steel” into a number of years. Even a low average mass loss can conceal a deep local pit in a critical small area.
For a design decision, test the relevant material stack and surface condition in a representative environment. Useful comparisons include coupled and uncoupled specimens, with a defined wetted area ratio and a clear measure of damage. ASTM G71 guides galvanic-couple tests in liquid electrolytes under conditions that do not introduce erosion-corrosion or cavitation.
Agree on what matters for the part: maximum penetration, remaining section, coating condition, or function. Keep the test duration and exposure conditions with the result. Laboratory exposure time is not, by itself, a service-life prediction.
Can laser cleaning stop galvanic corrosion?
Laser cleaning may remove suitable rust or oxide layers; it does not eliminate the galvanic circuit or restore lost metal. If the exposed material pair, electrical path, and electrolyte remain, the conditions for renewed attack remain too.
TRUMPF’s laser-cleaning applications include removing rust and other layers before joining. Suitability still depends on the substrate, coating, deposits, geometry, and chosen process. A cleaning result on one assembly does not establish the outcome for another.
- Preserve the diagnostic evidence. Record the affected locations and collect samples needed to identify the cause.
- Evaluate the surface and remaining material. Use a suitable cleaning trial and inspection method. Assess any pits or section loss against the component’s acceptance limits.
- Correct the cause and restore protection. Address the connection, materials, wetting, or protective system, then apply and verify the specified repair or finish.
For surface-preparation options, see laser rust removal. A clean appearance alone is not evidence that a load-bearing or pressure-containing part is fit to return to service.
Assess a cleaning application with Oceanplayer Laser. Share the base materials, coating details, clear photographs, affected area, and required final surface. Include any remaining-thickness or repair limits that the cleaning process must respect.
Discuss your cleaning application