Galvanic Corrosion: The Dissimilar Metals Reference
When a carbon steel bolt seizes and crumbles inside an aluminium housing while the housing itself looks untouched, you are looking at galvanic corrosion in action. The mechanism is straightforward electrochemistry: two dissimilar metals in electrical contact and bridged by an electrolyte form a galvanic cell. The less noble metal acts as the anode and oxidizes preferentially, sacrificing itself to protect the more noble cathode. Understanding how to predict and interrupt this process dictates how you specify fasteners, enclosures, and electrical connections in any environment where moisture is present.
The Mechanism: How the Cell Forms
Three conditions must exist simultaneously for galvanic corrosion to occur: two metals with different electrochemical potentials, direct electrical contact between them, and a conductive electrolyte bridging their surfaces. Remove any one of these, and the corrosion stops.
When these conditions are met, electrons flow from the anode (the less noble metal) to the cathode (the more noble metal) through their physical contact point. At the anode surface, metal atoms lose electrons and go into solution as ions. The electrolyte—whether it is condensation, rainwater, or process fluid—provides the ionic path that completes the circuit. The cathode undergoes a reduction reaction, typically combining oxygen and water to form hydroxide ions, but the cathode material itself does not corrode. It acts as the site where the depolarization reaction occurs, effectively driving the dissolution of the anode.
This is why a galvanic cell often accelerates over time. As the anode dissolves, it roughens, increasing its effective surface area and exposing fresh metal to the electrolyte, which speeds up the reaction rate.
The Galvanic Series for Common Industrial Metals
The galvanic series ranks metals by their actual electrochemical potential in a given environment. The further apart two metals are on this list, the greater the voltage difference between them, and the more aggressive the corrosion of the anode. The order below represents the general behavior of common industrial materials in a neutral, aerated aqueous environment like brackish water or contaminated condensation.
- Magnesium (Active)
- Zinc
- Aluminium (wrought alloys)
- Carbon Steel
- Lead / Tin
- Brass / Copper
- Stainless Steel (passive)
- Titanium
- Graphite
Position matters, but so does state. Note that stainless steel appears as "passive." Active stainless steel, where the chromium oxide layer has broken down due to oxygen depletion or chemical attack, shifts much further down the list, close to carbon steel. If you are specifying fasteners and hardware for a wet environment, you must verify whether the stainless alloy will maintain its passive layer in your specific application.
As a general rule, if the metals are adjacent in the series—like carbon steel and cast iron—the potential difference is small enough that galvanic effects are negligible. When you jump entire categories, like mating carbon steel directly to passive stainless steel, you create a strong driving force for corrosion.
The Area Ratio: Why Fastener Size Dictates Failure
The potential difference tells you if galvanic corrosion will happen; the area ratio tells you how fast it will destroy the anode. The area ratio is the relationship between the exposed surface area of the cathode and the exposed surface area of the anode.
The fundamental rule: a small anode coupled to a large cathode is a catastrophic combination. The total galvanic current flowing through the cell is determined by the cathode's ability to consume electrons. A large cathode can support a massive reduction reaction, pulling a correspondingly large current from the anode. If the anode is small, that entire current dissipates from a tiny surface area, resulting in an extremely high current density. High current density means rapid, concentrated metal loss.
This is why putting a single stainless steel bolt into a massive carbon steel beam is usually fine in all but the harshest environments—the large steel beam is the anode, but it has a huge surface area to distribute the current. The corrosion spreads out and proceeds slowly.
Reverse the setup, and you have a classic maintenance nightmare. Driving a carbon steel fastener into a large stainless steel plate makes the fastener the tiny anode. The massive stainless plate acts as an efficient cathode, driving a high current through the small steel fastener. The fastener will rust, swell, and shear off rapidly. The same logic applies to aluminium plates joined with stainless fasteners. The aluminium is less noble, and the fastener head provides a concentrated point of attack. Always design so the anode has the larger surface area.
Countermeasures: Breaking the Circuit
If material selection alone cannot solve the problem, you must break one of the three conditions for galvanic corrosion. You can isolate the metals, seal out the electrolyte, or choose a more compatible pairing.
Mechanical Isolation
The most reliable countermeasure is breaking the electrical path. Use dielectric isolation washers and sleeves—typically nylon, PTFE, or glass-reinforced epoxy. You must isolate both the bearing surface under the fastener head and the shank of the fastener passing through the hole. If you use an isolation washer but leave the metal shank touching the inside wall of the hole, the cell remains intact. The sleeve must span the entire contact length.
Be aware that isolation hardware changes your joint design. Non-metallic washers can creep under high clamping loads, leading to torque relaxation. Re-torqueing after thermal cycling is often necessary.
Sealants and Coatings
If you cannot isolate mechanically, you can block the electrolyte. Applying adhesives, sealants and chemicals like thread sealants or anaerobic gasket makers to the joint prevents water from bridging the gap between the two metals. A heavy-duty epoxy or polyurethane coating over the entire assembly achieves the same thing on a larger scale.
Coatings require total coverage to work. If you paint the cathode but scratch the anode, you have effectively created a tiny anode-to-large-cathode area ratio. The exposed scratch will corrode far faster than if the entire assembly had been left bare. When coating dissimilar metal joints, coat both metals, or coat the more noble metal to reduce its effective cathodic area.
Material Substitution
Sometimes the only fix is changing the materials. If you must use a wall-mount enclosure made of aluminium in a humid environment, do not use stainless hardware directly on its mounting flanges. Use aluminium hardware, or plate the hardware with zinc to reduce the potential difference. If you are terminating copper power leads to an aluminium bus bar, you must use specialized bimetallic transition lugs. These lugs friction-weld copper and aluminium together, isolating the dissimilar metals from direct contact at the critical current transfer point.
Even in standard electrical wiring, galvanic principles apply. When terminating stranded wire into mechanical screw terminals, using wire ferrules and lugs made of tin-plated copper prevents the individual wire strands from oxidizing and minimizes galvanic interactions with brass or steel terminal blocks. The tin plating acts as a sacrificial barrier and provides a stable, gas-tight connection that resists moisture ingress.
Environmental Severity and Joint Design
The conductivity of the electrolyte dictates the speed of the galvanic attack. In a dry, climate-controlled indoor environment, the lack of a consistent electrolyte means galvanic corrosion is practically non-existent. You can safely mix metals indoors. In a humid plant environment where condensation forms daily, the thin film of water on the metal is enough to drive the reaction. In marine environments or areas with chloride contamination, the high conductivity of the electrolyte accelerates the process dramatically.
When designing joints for outdoor or high-humidity use, assume water will find its way into the joint. Design weep holes so water does not pool in crevices, and specify gasket materials that do not absorb water. Some carbon-based gasket materials are highly conductive and can act as a graphite cathode, accelerating the corrosion of any metal they touch. Verify that your gasket material is rated for galvanic isolation if it sits between dissimilar metals.
If you are dealing with a seized dissimilar metal joint or need to source isolation hardware, bimetallic lugs, or compatible fastener grades for a repair, our team can help you cross-reference the right components for your specific material pairing.
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