Galvanic Corrosion: Joining Dissimilar Metals
Galvanic corrosion, also called bimetallic corrosion, can accelerate the deterioration of one metal when it is electrically connected to another and both are wetted by a shared electrolyte. Selecting fasteners therefore takes more than knowing the metal names: moisture exposure, coatings, and exposed surface areas matter. Protection involves choosing compatible materials, breaking the electrical circuit, or preventing prolonged wetting.
When a galvanic couple forms
Three conditions must exist together: different corrosion potentials in the service environment, an electrically conductive connection between the metals, and an electrolyte linking their surfaces. A film of water containing dissolved salts can act as the electrolyte; the entire assembly does not have to be submerged. Condensation and wet cleaning also need consideration indoors.
The less noble metal in the couple becomes the anode and is preferentially attacked; the other acts as the cathode. These roles depend on the environment and surface condition. A galvanic series for seawater is not a universal compatibility chart, and the difference between standard electrode potentials does not establish the corrosion rate of an actual joint.
How joint design changes the risk
What matters is the exposed surface area wetted by the electrolyte, rather than component mass or just the area where the parts touch. A small anode coupled to a large cathode is particularly unfavorable because attack is concentrated on a small surface. The table provides assessment principles, not combinations approved for every application.
| Situation | What determines the risk | Practical response |
|---|---|---|
| Dry joint between dissimilar metals | This mechanism cannot operate without a shared electrolyte | Check for possible condensation and water entry |
| Aluminum fastener in a large stainless steel component when wet | A small aluminum anode is coupled to a large cathode | Reconsider the fastener material and joint protection |
| Stainless steel fastener in a large aluminum component | The area ratio is more favorable, but salt and trapped water still pose a risk | Assess the environment, insulation, and drainage |
| Only the less noble metal is painted | A scratch exposes a small anode beside a large cathode | Select a protection system that considers both sides of the couple |
Protecting a mechanical joint
- Identify the alloys, coatings, and service conditions: rain, condensation, salt contamination, and immersion. Check the combination against the system manufacturer’s documentation.
- Provide drainage and allow the joint to dry. Crevices and dirt deposits can retain electrolyte; sealing must suit the design and service conditions.
- For electrical isolation, check the entire contact path: the gasket between components, the sleeve around the bolt, washers, and threads. One plastic washer does not guarantee an open circuit. Insulating parts must withstand the load and environment.
- When using barrier coatings, protect the cathodic surface or both sides, rather than the anode alone. Check for damage during assembly and inspect the coating during maintenance.
When a separate assessment is needed
The absence of a galvanic couple does not rule out other forms of corrosion. Load-bearing assemblies, piping, and continuously wet joints need engineered details that account for the specific environment; this table cannot establish service life or allowable load.
Electrical isolation of mechanical parts should not automatically be applied to current-carrying contacts, grounding, or equipotential bonding: electrical continuity may be essential there. Use the compatible connectors and protection specified in the design for those connections.