Material Compatibility — Galvanic Series

The galvanic series with anodic index values for twenty-one metals and coatings, and the voltage-difference rules that decide whether two dissimilar metals can be joined directly.

Data verified 2026-09-29 · based on ASTM G82-98(2023)

Quick Answer

Galvanic Series with Anodic Index

The anodic index is a ranking scale, not a measured potential. It is normalised so that gold sits at 0.00 V and the numbers rise as the metal becomes more anodic. Design decisions are made on the difference between two values, never on the absolute number: the difference is proportional to the driving voltage of the corrosion cell. Values are approximate and shift with alloy, surface condition, temperature and electrolyte — passivated stainless in particular moves several tenths of a volt between its passive and active states.

The Voltage-Difference Rule

Galvanic corrosion happens when three things are present at once: two metals with different potentials, an electrical connection between them, and an electrolyte bridging the joint. Remove any one of the three — insulate the metals, or keep the joint dry — and the corrosion stops. This is why the fix for a galvanic problem is usually a washer or a gasket rather than a different material.

Where the electrolyte cannot be excluded, the accepted design rule is based on the difference in anodic index between the two metals:

Difference ≤ 0.25 V. Generally acceptable in any environment, including salt spray and high humidity. Copper against bronze is 0.05 V, and steel against zinc (galvanising) is 0.15 V.

Difference 0.25–0.50 V. Acceptable only in a controlled environment — indoors, low humidity, no salt. Outdoors or in a marine or chemical atmosphere this couple corrodes. Steel against passive 316 stainless is 0.35 V and falls in this band: it is why stainless fasteners in a steel structure rust the surrounding steel in wet weather.

Difference > 0.50 V. Significant risk in any environment where moisture is present. Aluminium against stainless steel is 0.40 V in the table but behaves worse in practice because aluminium's passive film is easily broken; aluminium against copper is 0.55 V and is a well-known failure case. Insulate the joint, use a coated or plated fastener, or accept a sacrificial component and design it to be replaced.

Two practical modifiers. Area ratio matters enormously — a small anodic area coupled to a large cathodic one produces very rapid attack, because all the corrosion current concentrates at the small anode. A stainless bolt in a large aluminium plate is far worse than the voltage difference alone suggests. And the more anodic metal is the one that corrodes, so a zinc coating on steel is not a problem to be solved but a deliberate sacrifice.

Common Couples, and What to Do About Them

Stainless fasteners in aluminium. 0.40 V, and the aluminium loses. Aluminium's passive oxide film normally protects it, but a scratched or crevice area becomes a small anode against a large stainless cathode and pits quickly, especially in salt air. Use stainless fasteners with an insulating barrier — nylon washers, a chromate conversion coating, or an anti-seize compound that keeps the electrolyte out — or use aluminium or zinc-plated fasteners instead.

Steel in contact with stainless. 0.35 V, with steel as the anode. This couple is everywhere in architecture and it is the reason weathering steel structures show rust streaks below stainless handrails. It is tolerable indoors and not tolerable in a coastal or industrial atmosphere. Paint or galvanise the steel, or isolate the contact.

Galvanised steel bolted to bare steel. 0.15 V with zinc as the anode — this is intentional and benign. The zinc sacrifices itself to protect the steel, including at cut edges and scratches, which is the whole point of galvanising. The coating thickness, not the potential difference, sets the service life.

Copper pipework with steel or galvanised fittings. 0.50 V and 0.65 V respectively, both in the danger band. This is why plumbing codes require dielectric unions between copper and galvanised pipe — a short insulating section that breaks the electrical path while the water still flows.

Titanium and stainless in seawater. 0.05 V apart, effectively compatible. Both are used together in marine and desalination plant without isolation, though neither is compatible with the carbon steel structure around them.

Frequently Asked Questions

Can I put stainless steel bolts in an aluminium part?

It works, but it is a galvanic couple with a 0.40 V difference and the aluminium is the anode. In a dry indoor application it is fine. In salt air, or where moisture sits in the joint, the aluminium pits around the fastener — and the small anode against a large cathode makes the attack fast. Use an insulating barrier such as a nylon washer or a chromate conversion coating, and keep sealant in the joint.

What is the maximum safe difference in anodic index?

The usual guidance is 0.25 V for any environment, including marine and high humidity, and 0.50 V for a controlled indoor environment with low humidity. Above 0.50 V the couple needs a deliberate mitigation: electrical isolation, a barrier coating, a sacrificial anode, or a design that accepts the corrosion of the anodic component. The figures are a design guide, not a code limit.

Why does galvanising work if zinc is so anodic?

Because the zinc is meant to corrode. With a 0.15 V difference from steel, zinc is the anode and sacrifices itself to protect the steel — including at cut edges and scratches, where a paint film would fail. This is cathodic protection, not a compatibility problem, and it is why galvanised steel is bolted to bare steel routinely. Service life is set by coating thickness and the corrosivity of the environment.

Does the size of the parts matter?

Enormously, and it is the effect most often missed. Corrosion current concentrates at the anodic area, so a small anode paired with a large cathode corrodes very rapidly. A stainless bolt in a large aluminium plate is far worse than the same pair with comparable areas. The practical rule is to keep the anodic area large relative to the cathodic area, or to insulate the joint if that is not possible.

When is a dielectric union required?

Whenever copper and galvanised or steel pipe are joined in a water system. The difference is 0.50–0.65 V — in the danger band — and the water is a continuous electrolyte, so the couple corrodes steadily. Plumbing codes generally require an insulating union at the transition. The same principle applies to any dissimilar-metal joint that will be permanently wet.

Related

• Plastic Chemical Compatibility
• Stainless Steel 316 Properties
• Aluminium 6061-T6 Properties
• Copper C11000 Properties
• Steel Plate Sizes
• Surface Finish Chart

Value Sources

Each data column on this page is tied to the source it came from. The numbers in square brackets correspond to the table headers above.

Data Sources

Cross-checked against:

• Anodic index values cross-checked between two independent corrosion-control references
• Ordering of the series checked against the published galvanic series for flowing seawater
• The design rules verified against the worked couples in the second section — steel/stainless, aluminium/stainless and copper/galvanised all give the expected verdict

Anodic index values are an approximate ranking scale, not measured potentials. They shift with alloy composition, surface condition, temperature and electrolyte, and passivated stainless moves several tenths of a volt between its active and passive states. Treat the voltage-difference thresholds as design guidance: where a joint is critical or a failure is expensive, confirm with a corrosion test in the actual environment.

Cite This Page

Every value on this page is traceable to the sources listed above. If you use the data in a document, paper or report, cite it as:

Each row in the tables above also has a permanent link — hover a row and use the # link to cite a single value rather than the whole page.