Galvanic corrosion and dissimilar metals

At a glance

OK: low risk in that condition. Check: acceptable if the detail is dry, open and drained. Isolate: keep the metals apart. Avoid: change the metal or the way round. These are the studio's readings of the sources on this page, not table values; the last column is our reading throughout. A dry interior means heated, aerated and free of condensation.

What a corrosion couple needs, and why area matters

Euro Inox, the stainless steel industry's European association, lists three things that must all be present: metals with different corrosion potentials, an electrically conductive connection between them, and a conductive film of moisture over both. Take away any one and the effect stops. In a heated, aerated interior there is no film, so Euro Inox says virtually any combination of metals can be used in lighting fixtures and decoration without restriction. Condensation, rain, spray and wet cleaning put the film back.

When the effect does occur, the less noble metal, the anode, is attacked and the more noble metal, the cathode, is protected. The relative areas decide how bad it is. A small cathode on a large anode spreads the attack thinly. A small anode surrounded by a large cathode concentrates it, so the small part corrodes quickly. Euro Inox therefore says fasteners should always be made of the more noble material, so that the cathodic surface is small.

The numbers show how steep the effect is. In Euro Inox's one-year North Sea field test, with the stainless steel as cathode, the partner metal corroded faster as the stainless area grew relative to it:

That test was in sea water, far harsher than any interior. In ordinary air, Euro Inox adds, the effect reaches only a narrow zone along the line of contact, so the area rule applies only in part. Designs with small anodes and large cathodes are still to be avoided, because they can fail even in well-aerated places.

The galvanic series: what it tells you, and what it does not

A galvanic series ranks metals by their corrosion potential in a named electrolyte. The two we read are both for sea water. Euro Inox prints one (its Figure 2, sea water at 10 °C, taken from a NACE guide by R. Francis, 2001) and BRANZ, the New Zealand building research body, prints another (its Table 1, 2020). The drawing shows the order in each. They agree that steel is more noble than aluminium and zinc, that copper alloys are more noble than steel, and that passive stainless steel is more noble than copper. They disagree on which of aluminium and zinc is the less noble.

The ranges above are BRANZ's, as printed: it does not name the reference electrode in the text we read, and Euro Inox plots its figure on a different scale, so the two sets of numbers are not directly comparable. Do not mix them.

Euro Inox is blunt about what a series can do. The difference in potential alone is not a good indicator of the real risk: it only says whether a risk has to be considered. Differences as low as 100 mV can lead to corrosion, while metals with considerably higher differences can be joined without difficulty, because the kinetics of the reactions depend on the metal and on films that form on it. This is why it publishes the empirical tables used in this sheet, for particular environments, rather than a ranking alone. The Galvanizers Association says the same: electrode potential alone cannot be used to decide whether and how severely corrosion will occur.

Reading the compatibility table

The table at the top is built from three published tables and the rule that a joint needs a film of moisture. Euro Inox Table 7 rates pairs in ambient air by which metal has the large area and which the small. The Galvanizers Association table rates zinc against other metals from rural air to sea water. BRANZ names the pairs a building code treats as incompatible. Where a source stops, the table says so, and the last two columns are our reading, one step more cautious as the environment gets harsher. This is how each row was reached:

No published rating was read for pewter, nickel silver, lead or titanium, or for timber and fixings, so they are not in the table. Copper-based wood preservatives corrode mild steel and galvanised fixings in timber, BRANZ reports, which is a different mechanism from the one on this page.

Three cases that come up in bespoke metalwork

Brass inlay in steel

Euro Inox Table 7 rates a small copper part in a large carbon steel part as acceptable for the copper, with a warning: the steel's own rusting is rapid, so the pairing is not recommended without protecting the steel. In practice the question is the finish. Euro Inox says coating only the less noble metal is not a suitable way to prevent galvanic corrosion, because any chip in the coating exposes a small anode beside a large cathode. A blackened or lacquered steel face with a bare brass inlay is that situation. Ask the partner workshop how the joint is sealed and how a chip is repaired, and see our sheets on lacquered and unlacquered brass and patination.

Stainless fixings in aluminium

The safe way round. Euro Inox shows stainless fasteners on much larger aluminium or galvanised components causing virtually no corrosion, even in corrosive conditions, and BRANZ says stainless fasteners in aluminium sheets or frames are normally considered safe if there is no heavy deposit of salt. Near the sea, keep the crevice open and drained, or isolate. The reverse, an aluminium fixing in a large stainless part, is rated uncertain to poor.

Galvanised or zinc-plated screws in brass

The wrong way round. Euro Inox Table 7 rates a small zinc or galvanised part in a large copper part as poor. The Galvanizers Association says copper and copper-containing alloys should ideally not be used with galvanised steel and that isolation is strongly advised even in air. Euro Inox shows what it looks like: galvanised screws holding a stainless cover showed white rust and the start of steel corrosion after about a year in an urban atmosphere. Use brass or stainless screws in brass.

A coating does not cure any of the three. Qualicoat, the powder-coating quality scheme for architectural aluminium, lists contact with other materials among the causes of failure that pre-treatment and the powder coating cannot influence, and its UK and Ireland association says fixings to dissimilar materials need to be electrically insulated. See our powder coating sheet.

Isolating a joint

Euro Inox lists three ways to prevent the effect: electrically insulate the parts (insulators, plastic bushes, polyamide washers), put the joint where it is not exposed to moisture, or coat the cathode, or both metals, near the joint. The Galvanizers Association gives practical forms: neoprene or plastic washers on bolted connections, plastic spacers or paint between overlapping surfaces, painting the more cathodic metal. For a stainless part in a galvanised one, Euro Inox shows coating a strip of the stainless along the contact line, a few centimetres wide in ordinary air and wider in salty films, preventing rust in a 48-hour salt-spray test.

• Sleeves, washers and a gasket break every metal path. Polymer washers alone do not: the thread still touches. Euro Inox still recommends them, because they cover the areas most at risk.
• Crevices that stay damp are the problem areas. Elastic seals that fill the crevice are a proven remedy; sealants that embrittle and crack in the crevice can make it worse.
• Drain and ventilate. Euro Inox: occasional short wetting does not normally cause the effect, while permanently damp, dirty or sheltered corners do.
• Run-off. The Galvanizers Association asks that water running off copper or copper alloys does not fall on galvanised articles, and BRANZ notes that water that has passed over a nobler metal can corrode a less noble one below it.

When to ask an engineer

Ask before you fix, not after, for anything that carries load or people: balustrade posts and brackets, suspended items, fixings into stone or concrete, and anything in a pool hall, spa or coastal setting. The consequence of a wrong fixing there is failure, not stain. Our sheets on metal by environment, 304 against 316 and balustrade and handrail rules cover the neighbouring decisions. Put the environment and the fixing material on the drawing: our metalwork brief checklist shows where. For finishes that add a layer, see plating and coating thickness.

Fixings to be [stainless steel, grade A4 / 316] unless stated, and of the same or a more noble metal than the part they fix. No galvanised, zinc-plated or mild steel fixings in [brass, bronze or copper] parts. Dissimilar metals to be isolated by [polyamide sleeves and washers and a gasket] wherever moisture or condensation can reach the joint, at the locations marked [ ]. Fixings and isolation to be shown on the shop drawing.

Inlay of [brass] in [mild steel]: joint to be [sealed against moisture] and the steel finished with [system] on every face including the rebate. The fabricator is to state in writing how the joint is sealed and how damage to the finish is repaired.

Environment for [element]: [dry heated interior / damp interior / external inland / coastal / pool hall]. Run-off from [copper, brass, steel] not to fall on [aluminium, zinc or galvanised steel, stainless steel]. Every contact between different metals to be confirmed by the fabricator in writing, for the environment stated, against [PD 6484, edition [ ], or the fixing maker's guidance], before shop drawings are approved.

Standards and sources

• PD 6484:1979, Commentary on corrosion at bimetallic contacts and its alleviation (BSI): the UK reference; paywalled, not read. Check the ratings against it.
• BS EN ISO 9223:2012, Corrosivity of atmospheres: environment classes; text not read.
• Euro Inox, Stainless Steel in Contact with Other Metallic Materials (Materials and Applications Series, Vol. 10, 2009)
• Galvanizers Association, HDG Datasheet 10: Bimetallic Corrosion (October 2024)
• BRANZ Facts, Metal corrosion in New Zealand buildings #2 (April 2020): a New Zealand source.
• Qualicoat TIS No. 4 (May 2023) and Qualicoat UK & Ireland Guidance Note 7 (April 2025)

Questions we are asked

Can I fix brass to steel?

In a dry heated interior, yes: with no wet film there is no corrosion current, and Euro Inox says virtually any combination can be used. Where the joint can get damp, a small brass part in a large steel part adds little harm to the brass, but the steel is the anode and rusts anyway, so it needs a finish on every face and a sealed joint. A steel screw in a large brass plate is the poor way round.

Are stainless steel screws safe in aluminium?

Usually. Euro Inox shows stainless fasteners on much larger aluminium or galvanised parts causing virtually no corrosion, and BRANZ says they are normally considered safe if there is no heavy deposit of salt. Near the sea keep the crevice drained, or isolate the screw with a sleeve and washers.

Does galvanic corrosion matter indoors?

Not in a heated, aerated interior without condensation. It starts to matter wherever a wet film can form: bars, kitchens, bathrooms, spas, pool halls, and anything cleaned with a lot of water. Use the 'humid' and 'wet' columns for those.

Does paint or powder coating stop it?

No. Qualicoat lists contact corrosion among the things that pre-treatment and powder coating cannot influence, and Euro Inox says coating only the less noble metal is not a reliable fix, because one chip leaves a small anode. Coat the more noble metal near the joint, or isolate the metals.

Can I rely on a galvanic series chart?

Only as a first look. A series ranks metals in one environment, usually sea water, and two published series disagree on the order of neighbours such as aluminium and zinc. Euro Inox says potential difference alone is not a good indicator of risk: area, environment and detail decide.

What isolation do I need?

Break every metal path: sleeves in the holes, washers under head and nut, and a gasket or spacer between plates. Polymer washers alone leave the thread in contact. Keep the joint where it can drain and dry. For anything structural, have an engineer confirm it.