An aluminum housing fastened with steel bolts, a copper pipe screwed into a galvanized manifold, stainless cladding on a steel frame: on a technical drawing, joints like these look correct, yet in service they can fail much sooner than the design assumed. The material may be flawless and the assembly done by the book. The problem comes from an electric current flowing between two metals in the presence of ordinary moisture. The result is unplanned downtime, disassembly, and component replacement, and yet the phenomenon is still often overlooked at the design stage.

What is galvanic corrosion? Definition and mechanism

Galvanic corrosion, also called contact, bimetallic, or dissimilar-metal corrosion, is the accelerated corrosion of a less noble metal that is in electrical contact with a more noble metal in the presence of an electrolyte. It is a form of electrochemical corrosion: the more active metal corrodes faster than it would on its own, while the more noble metal gains protection in this arrangement.

The key word here is „accelerated.” Carbon steel or aluminum would corrode on their own as well, but once paired with a more noble metal, the rate of metal loss rises, often many times over. Among the various types of metal corrosion, galvanic corrosion stands out because of its source: it is driven by the potential difference between two materials in contact, and an aggressive environment only amplifies the effect.

How does galvanic corrosion form? The galvanic cell in a structure

The mechanism is easiest to understand by analogy to an ordinary battery. A galvanic cell consists of two electrodes with different potentials immersed in an electrolyte, and exactly the same arrangement forms unintentionally in a structure where two different metals exposed to moisture have been bolted together.

Four elements work simultaneously in such a cell:

1. Anode: the less noble metal (e.g., zinc, aluminum, carbon steel) gives up electrons and passes into solution as ions. This is the metal that dissolves, i.e., corrodes.
2. Electron flow: the released electrons travel through the metallic contact to the more noble metal.
3. Cathode: the more noble metal (e.g., copper, stainless steel) accepts the electrons. A reduction reaction takes place on its surface, in water and humid air most often of oxygen dissolved in the electrolyte, while the metal itself remains intact.
4. Electrolyte: closes the circuit by allowing ions to flow. In practice, this role is played by moisture condensed from the air, rain, and especially water containing dissolved road salt, which conducts current very well.

As a result, the structure turns into a working cell in which one element is steadily „eaten away” to power the reaction on the other. As long as current flows, the anode keeps losing material.

3 necessary conditions: dissimilar metals, electrical contact, electrolyte

For a designer, one observation matters most: galvanic corrosion will not occur if even one of the three conditions is missing. Breaking a single link in the chain of risk is enough.

1. Two different metals with different electrochemical potentials: the greater the difference, the stronger the driving force for corrosion.
2. Electrical contact between them: direct contact or a conductive connection (bolt, weld, clamp).
3. An electrolyte connecting both metals: moisture, condensation, salt water, damp soil.

Laid out this way, the conditions form a practical intervention map. Materials cannot always be changed, but it is often possible to cut off electrical contact or keep the electrolyte away from the joint.

Why the cathode-to-anode area ratio matters

The rate of damage depends both on the choice of metals and on the ratio of their surface areas. As a guide from the UK’s National Physical Laboratory explains, the larger the cathode relative to the anode, the more oxygen is reduced on it and the higher the galvanic current. With a small anode and a large cathode, all of that current concentrates on a small area of the active metal, leading to rapid, localized metal loss.

A classic example is an aluminum rivet or a carbon steel bolt in a stainless steel sheet: the large, noble cathode loads the small fastener, which corrodes very quickly. The reverse configuration, a stainless bolt in a large carbon steel sheet, is much less dangerous because the loss is spread over a large area. Hence the practical rule: a fastener should be made of a metal at least as noble as the parts being joined, never less noble.

The same logic explains why steel bolts in an aluminum plate usually do not corrode at an accelerated rate. In this pair, aluminum is the anode, so it is the plate that may lose material around the holes, especially in the presence of salt and moisture.

Galvanic series of metals: which combinations are the riskiest?

Which metal will be the anode and which the cathode is read from the galvanic series, a ranking of materials by the potential they actually take on in a given environment, most often seawater. The method for building and using such series is described in the ASTM G82 standard. The farther apart two metals sit in the series, the greater the risk of intense corrosion once they are joined.

Metal or alloy	Approximate potential in seawater [V vs. calomel electrode]*
Magnesium	−1.63 to −1.60
Zinc	−1.03 to −0.98
Aluminum alloys	−1.00 to −0.76
Carbon steel, cast iron	−0.72 to −0.60
304/316 stainless steel, active (e.g., in crevices)	−0.58 to −0.43
Brass	−0.40 to −0.30
Copper	−0.36 to −0.30
304/316 stainless steel, passive	−0.10 to 0.00
Titanium	−0.05 to +0.06
Graphite	+0.20 to +0.30

* Approximate values for flowing seawater, consistent with the galvanic series published in sources including ASTM G82. In other environments, the position of metals may change.

The top of the table lists materials that readily give up electrons and sacrifice themselves in a pair; the bottom lists noble materials, protected at their partner’s expense. Note that stainless steel appears in the list twice. As long as it is protected by a thin passive layer, it behaves like a noble metal. In crevices, under deposits, and in areas with limited oxygen access, however, this layer can break down, and the steel’s potential then moves closer to that of carbon steel.

In industrial practice, the most common and riskiest combinations are:

• carbon steel + copper or brass, e.g., steel pipes and tanks with brass fittings or valves, where the steel corrodes heavily;
• aluminum + stainless steel, typical in mixed-material structures, frames, and enclosures, where the aluminum is the victim;
• carbon or stainless steel + graphite, found with graphite gaskets in flanged joints, where graphite acts as a highly noble cathode;
• zinc (galvanized coating) + copper, where galvanized parts in contact with copper or brass lose their zinc coating at an accelerated rate.

The last pair can be deceptive because contact does not always require direct touching. According to the American Galvanizers Association, even water running off copper or brass surfaces can carry enough dissolved copper to accelerate zinc corrosion. This is one of the more common reasons why galvanized sheet metal rusts faster than its coating thickness would suggest.

The galvanic series also depends on temperature and electrolyte composition. The NPL guide mentioned above notes that in water above about 60°C (140°F), the zinc-steel pair can reverse roles so that iron becomes the anode, which explains problems seen in galvanized hot water systems.

Galvanic series vs. electromotive series of metals

Most of us remember the electrochemical series, also called the electromotive series, from chemistry class. It ranks metals by their standard potentials, determined under strictly defined laboratory conditions. The galvanic series, by contrast, is based on measurements in a real environment and accounts for factors such as passive layers and alloy composition, which is why it better predicts how joints will behave in a structure.

Aluminum illustrates the difference well. In the electromotive series, it sits clearly ahead of zinc as the more active metal, yet in seawater, thanks to its oxide layer, it behaves more nobly than zinc. Assessing risk based solely on the electromotive series can therefore lead to the wrong conclusions.

Galvanic corrosion of aluminum and stainless steel

Joining aluminum to stainless steel calls for particular care because both materials are associated with corrosion resistance. In this pair, aluminum becomes the anode and stainless steel the cathode, so the damage appears on the aluminum, usually around mounting holes and under bolt heads.

In the dry air inside industrial buildings, the risk is low. It rises with humidity and salinity: in coastal areas, near roads treated with deicing salt, or around industrial chemicals. The safest solution is stainless fasteners, i.e., small cathodes, run through insulating washers and sleeves, and in highly corrosive environments, additional sealing of the joint itself.

Effects of galvanic corrosion on structures and industrial equipment

The consequences rarely end with cosmetics. Localized material loss means loss of load-bearing cross-section right at the joint, exactly where the structure carries the highest loads. Corroded bolted joints lose preload and work loose, pitting and delamination appear near welds, and tanks and pipelines develop perforations and leaks at flanges or nozzles made of a different metal.

In the automotive industry, the problem affects chassis and frame fasteners exposed to road salt; in rail, it affects fittings, mounts, and mixed steel-aluminum structures operating in rain and moisture; and in agriculture, it affects machinery and tanks in contact with fertilizers and liquid manure, which act as an aggressive electrolyte. Every such failure means the cost of parts, but downtime, disassembly, and restarting the line or machine usually prove far more costly.

It is worth knowing the typical failure points by heart:

• fasteners: bolts, rivets, and screws made of a metal different from the parts they join;
• fittings and hinges: especially brass or stainless ones on carbon steel or aluminum;
• mixed steel-aluminum structures: frames, enclosures, panels;
• flanged joints in pipelines and tanks with components made of other alloys;
• contact areas under gaskets and washers, where moisture collects.

How to prevent galvanic corrosion: design rules and protective coatings

Good design is the cheapest protection against galvanic corrosion. The Institute of Corrosion, in an article on eliminating corrosion hotspots, points out that besides coatings and cathodic protection, the layout of the structure itself plays an important role, so risky joints are best eliminated while still at the drawing stage. Among the available methods of protecting steel against corrosion, the most effective in this case are those that break one of the three conditions needed for a cell to form:

• choosing metals that sit close together in the galvanic series: avoiding combinations of materials far apart in potential;
• a favorable area ratio: fasteners at least as noble as the parts being joined;
• electrical insulation of the joint: nonmetallic washers, sleeves, and gaskets that interrupt current flow;
• cutting off the electrolyte: sealing crevices, providing drainage, and avoiding areas where water collects;
• sacrificial protection: intentionally added sacrificial anodes, usually zinc, magnesium, or aluminum, that corrode instead of the protected structure (a form of cathodic protection);
• zinc coatings: serving as both a barrier and cathodic protection.

Electrical insulation and material selection

If joining two metals far apart in the series cannot be avoided, the electrical contact must be broken. In practice, this is done with dielectric washers and sleeves (made of polyamide, PTFE, or other engineering plastics) that separate the bolt from the hole, and insulating coatings applied at the contact point.

For example, when an aluminum panel is fastened to a steel frame with a stainless bolt, the fastener is run through a plastic sleeve, and dielectric washers are placed under the bolt head and the nut. The aluminum then has no metallic connection to either the bolt or the frame, so the galvanic current has no path to flow. After assembly, it is worth checking with a meter that the insulation actually breaks the circuit. NPL recommends this check after installation and, for critical equipment, also during periodic inspections.

Protective coatings as a barrier against the electrolyte

A zinc coating, applied by hot-dip galvanizing or electroplating, works in two ways. First, it physically shields the base steel from the electrolyte: as long as the layer is intact, moisture cannot reach the substrate and the cell cannot close. Second, zinc is a typical anodic coating. Even where it is locally damaged, it remains more active than steel, so it acts as a sacrificial anode and corrodes first, protecting the exposed steel underneath. This dual function is why galvanizing performs so well in structures exposed to moisture and salt.

Keep in mind, though, that zinc protects the steel it has been applied to. When a galvanized part is in contact with copper or stainless steel, the zinc becomes the anode of that pair and is consumed faster, so in such joints the coating should be supplemented with insulation at the contact point.

Similar caution applies to painting. Paint the more noble metal (the cathode) or both metals, never the anode alone. Every scratch or pore in the paint on the anode creates a tiny anode connected to a large cathode, the most dangerous arrangement described earlier. For galvanized structures operating in particularly harsh conditions, a good solution is a duplex system, in which a paint coating additionally protects the zinc from being consumed.

Which protection method to choose for multi-component structures: hot-dip galvanizing or electroplating?

For structures made of many components, it is worth choosing the protection method using a simple checklist. Any of the points below can decide the choice of technology.

1. Required coating thickness: hot-dip galvanizing produces thick, highly durable layers (per EN ISO 1461, a mean of at least 85 µm for steel over 6 mm thick), while electroplating produces thin coatings, usually from a few to around a dozen micrometers, but uniform and precise.
2. Part size and weight: large, heavy welded structures are the domain of hot-dip galvanizing, while small fasteners and precision parts are best suited to electroplating.
3. Service environment: the higher the corrosivity category (road salt, moisture, chemicals), the stronger the case for a thicker hot-dip coating.
4. Dimensional accuracy and appearance: when tolerances and aesthetics matter, zinc electroplating has the advantage.
5. Budget and batch size: weigh the unit cost against the required service life.

The two technologies differ in durability, cost, and range of applications. Massive structures and parts operating in harsh conditions are best protected by hot-dip galvanizing, while small parts, hardware, and components requiring precision are best served by coatings applied in an electroplating shop.

If you are planning a structure that combines different metals or want to protect finished parts against galvanic corrosion, consult the Strumet team on coating selection. We will help match the galvanizing technology to the corrosivity category, dimensions, and budget of your project.

FAQ: most common questions about galvanic corrosion

How is galvanic corrosion different from ordinary steel corrosion?

Ordinary steel corrosion results from the reaction of the metal itself with its environment. Galvanic corrosion is further accelerated by electrical contact with another, more noble metal, causing the more active metal to deteriorate much faster than it would on its own.

Can galvanized steel be safely joined to aluminum?

In most atmospheric conditions, yes. Zinc and aluminum sit close together in the galvanic series, so the potential difference is small. In salty or constantly damp environments, corrosion can still accelerate, so it is better to insulate the joint there and avoid small-anode configurations.

Does stainless steel corrode in contact with aluminum?

In this pair, stainless steel is the cathode, so it usually stays intact while the aluminum corrodes. The risk increases in the presence of salt and moisture, which is why these joints are insulated with dielectric washers and sleeves.

How can I tell whether two metals will corrode once joined?

Check their positions in the galvanic series for the given environment: the greater the potential difference, the higher the risk. You also need to assess whether electrolyte reaches the joint and what the area ratio of the two metals is.

Does galvanizing protect against galvanic corrosion?

A zinc coating effectively protects the steel it has been applied to: it shuts out the electrolyte and, if damaged, acts as a sacrificial anode. In contact with a more noble metal, such as copper or stainless steel, however, the zinc itself is consumed faster, so such joints should also be insulated.

What washers or insulators are used to prevent galvanic corrosion in bolted joints?

Dielectric washers and sleeves made of plastics such as polyamide or PTFE are used to separate the fastener from the joined parts. They are supplemented with nonmetallic gaskets and insulating coatings at the contact point to interrupt the flow of galvanic current.