Galvanic corrosion between dissimilar metals

Key takeaways

Galvanic corrosion needs three things at once: two dissimilar metals, a conductive path between them, and an electrolyte that bridges the junction, so removing any one of the three stops the reaction.
The metal that corrodes is the less noble one, which is why a small stainless or zinc-coated fixing can be consumed while the large sheet it fastens looks untouched.
The risk of a pair is set by the difference in nobility and the ratio of exposed areas, so a small anode against a large cathode in a wet location is the worst arrangement you can build.
Isolation detailing means breaking the conductive path with a non-conductive barrier, a compatible fastener, a gasket or a coating, and recording that decision on the drawings.
Corrosion at a fixing or flashing junction typically surfaces after practical completion, when the builder is arguing about defect liability and the subcontractor has been paid.

Galvanic corrosion is what happens when two dissimilar metals are joined in the presence of an electrolyte and a conductive path, and the less noble of the two gives up metal. On a building site this is rarely a dramatic event. It is a slow loss of section at a fixing, a lap or a penetration, hidden behind a sheet or under a bead of sealant, and it usually becomes visible only after the building is occupied.

The reason it matters commercially is timing. Corrosion at a junction does not show up during defects inspection at practical completion because the moisture and the current have not had time to do their work. It shows up later, when the owner is chasing the builder, the subcontractor has been paid, and the argument turns on whether the detail was ever specified.

This article is about metal incompatibility and isolation detailing: how to recognise a galvanic couple before it is built, how to break it, and how to record that decision so it survives a warranty claim. General corrosion protection advice is a separate subject and is covered elsewhere.

What has to be true before a galvanic cell can form

Three conditions must exist at the same time. Two dissimilar metals must be in contact, or connected by a conductive path. There must be an electrolyte bridging the junction, which in practice means water with dissolved salts or ions. And the two metals must be electrically connected so that current can flow from one to the other.

Remove any one of the three and the reaction stops. This is why isolation detailing works: you are not trying to make the metal corrosion-proof, you are trying to break one of the three legs. A non-conductive barrier between the metals breaks the electrical path. A drained and ventilated detail that keeps the junction dry removes the electrolyte. Selecting a compatible fastener removes the dissimilar-metal pair altogether.

Most failures on site come from the electrolyte leg being underestimated. A junction inside a wall cavity, under a capping, behind a flashing upstand or in a roof space is not dry. Construction moisture, wind-driven rain, condensation and wash-down water all find those spaces, and once water is in, it often stays because the detail has no drainage path. The metal roof and flashing details article covers how those junctions get wet in the first place.

The less noble metal is the one that disappears

When two dissimilar metals are coupled, the less noble one becomes the anode and corrodes. The more noble one is protected. This is the same principle that makes sacrificial anodes work on a boat hull or a hot water service, and it means a small component can be consumed while the large component it fastens looks untouched.

The consequence for detailing is the area ratio. A small anode coupled to a large cathode corrodes faster than a large anode coupled to a small cathode, because the corrosion current is concentrated into a small area. A fixing is a small anode. A roof sheet or a structural member is a large cathode. Put them together in a wet junction and the fixing loses section quickly while the sheet looks fine until the fixing lets go or the lap opens.

This is why the fixing is the component you should be most careful about, and why the answer is not always “use stainless”. A stainless fastener into a different metal family can make the sheet or the bracket the anode, which is a worse outcome because the anode is now the expensive, large, hard-to-replace component. The right answer depends on which metal you are prepared to sacrifice, and that should be decided on the drawings rather than at the fixing gun.

How a galvanic cell forms on a building

1Two metals meetA fixing, flashing, bracket or mesh contacts a metal of different nobility at a lap, penetration or fixing point.
2Water bridges the gapRain, wash-down water, construction moisture or condensation carries dissolved salts and ions into the junction.
3Current flowsThe less noble metal gives up metal ions and becomes the anode while the more noble metal acts as the cathode.
4The anode is consumedLoss of section occurs at the anode, often hidden behind the sheet, inside the lap or under the fixing head.
5Corrosion products appearStaining, rust bleed, loose fixings and, eventually, water entry or loss of structural fixing capacity show up.

Which junctions on a residential job actually carry the risk

Galvanic couples turn up in predictable places on residential work in NSW. The list below is not exhaustive, but these are the junctions that generate callbacks.

Roof and wall cladding fixings. Screws, rivets, washers and clips of a different family from the sheet, particularly where the fixing penetrates the coating at installation.
Flashing to cladding laps. Where lead, copper, zinc, aluminium or coated steel flashings lap or sit against a different metal, especially at a cut edge.
Cut edges and swarf. Metal filings left on a roof or wall sheet after drilling create small dissimilar deposits that stain and pit the coating.
Structural connections. Brackets, base plates, hold-downs and post anchors in a different metal from the member they fix, in an exposed or damp location.
Termite mesh, weep holes and cavity flashings. Fine metal mesh and flashings in contact with a dissimilar wall material or mortar, in a cavity that stays damp.
Plumbing, rainwater and drainage. Metal gutters, downpipes, tank inlets and pipework of different families joined without a dielectric break.
Fencing, balustrades and external screens. Aluminium, galvanised steel and stainless components mixed in a coastal environment.

Note that several of these are created by the installation process rather than by the design. Swarf from cutting a sheet on the roof is a classic example: the detail is correct, the workmanship introduces the couple. Tool selection, cutting methods and a clean-down routine are part of isolation detailing, not separate from it.

Junctions that behave differently in service

Lower risk arrangement	Higher risk arrangement
Fastener of the same family and coating as the sheet it fixes	Stainless or coated fastener into a different metal family in a sheltered, wet junction
Small exposed cathode area relative to the anode	Small anode, such as a fixing, against a large cathode, such as a roof or wall sheet
Junction protected by a coating, gasket or sealant and detailed to drain	Junction left open, ponding, or buried behind a cladding line with no drainage
Junction in a dry, ventilated, drained location	Junction in a coastal, industrial or continuously damp location
Isolation recorded on the shop drawings and in the maintenance schedule	Isolation assumed by the installer and never documented

Isolation detailing: breaking the couple in practice

Isolation detailing means designing the junction so that one of the three conditions cannot be met. There are four practical approaches, and a robust detail usually combines two of them.

Separate the metals. A non-conductive barrier between the two metals breaks the electrical path. This can be an isolating tape, a gasket, a nylon or neoprene washer, a plastic saddle, a coated clip or a purpose-made isolation pad. The barrier must be continuous across the contact area, including around the fastener hole and under the washer face, and it must survive the installation process without being cut or pinched. A barrier that is torn during fixing is not a barrier.

Choose a compatible fastener. Where the manufacturer of the sheet or the system specifies a fastener type, coating or grade, that specification governs the junction. If you substitute, you own the outcome. If the specification is not clear, confirm the current requirement in the manufacturer literature and the NCC before ordering, and record the confirmation on the shop drawings.

Keep the junction drained and ventilated. A junction that dries between wetting events corrodes far more slowly than one that stays wet. Design laps to shed water, provide a drainage path out of cavities, avoid upward-facing pockets and do not rely on sealant as the primary defence. Sealant is a secondary barrier; it is not a substitute for drainage.

Control the environment. Coatings, galvanising, anodising and paint systems all act as barriers, provided they are intact and compatible with each other. Coating compatibility matters: a coating applied over a dissimilar metal junction can accelerate corrosion if it traps moisture or if the coatings react. Where cut edges are unavoidable, treat them in accordance with the manufacturer system.

Coastal, bush fire and condensation contexts change the risk

The same detail can be acceptable in one location and a problem in another. Three NSW contexts are worth building into your assessment.

Coastal and industrial environments increase the conductivity and aggressiveness of the electrolyte. Salt spray and airborne pollutants mean the junction is wetted with a more aggressive solution and dries more slowly. In these locations, the margin for error in fixing selection and isolation is smaller, and the case for a continuous barrier plus drainage is stronger.

Bush fire design can push you towards metal components and away from others. Planning for Bush Fire Protection sets out the requirements that shape external materials and detailing in designated areas, and the substitution of one metal component for another to meet those requirements can create a new galvanic couple that was not in the original detail. Check the couple whenever the material changes.

Condensation is the quiet one. The Condensation in buildings handbook produced by the Australian Building Codes Board explains how moisture accumulates in building assemblies, and that moisture is exactly the electrolyte a galvanic cell needs. A junction inside an insulated wall or a roof space can be wet for much of the year without anyone seeing water. If your isolation detail assumes a dry cavity, it is assuming something the condensation performance of the assembly may not deliver.

Energy and water efficiency requirements also shape the assembly. BASIX drives design decisions around glazing, insulation and services that change the temperature and moisture profile of cavities, and those changes affect how long a junction stays wet. None of this makes BASIX a corrosion requirement; it means the corrosion assessment has to be done against the building as designed, not against a generic detail.

The mistake that costs the most is specifying a stainless fixing as a blanket answer. It is often the right choice, but applied without checking which metal becomes the anode, it can turn a large, expensive, hard-to-replace component into the sacrificial one, and by the time the staining or the section loss appears the only remedy is replacement of the component rather than the fixing.

What to check before you order, and what to record

The decisions that prevent galvanic corrosion are made before the materials arrive. Work through the following on every job where dissimilar metals are proposed.

List the couples. Go through the drawings and the specification and identify every point where two different metals touch or are connected, including fixings, flashings, brackets, mesh and pipework.
Identify the anode. For each couple, decide which metal will corrode if the junction gets wet, and confirm that this is acceptable in terms of cost, access and replacement.
Check the area ratio. A small anode against a large cathode is the arrangement to eliminate, not to manage.
Confirm the manufacturer requirement. Where the sheet, system or component manufacturer specifies a fastener, coating or separation layer, use it, and keep the documentation.
Design the drainage. Show how water leaves the junction, and do not rely on sealant as the primary path.
Record the isolation on the drawings. Mark the barrier, the gasket, the washer or the compatible fastener, so the installer does not have to guess and the certifier can see the intent.
Include it in the maintenance schedule. Note the junctions that need to be inspected and the intervals, so the owner has a reason to look before failure rather than after.

Recording matters more than most builders expect. If a defect claim is made years later, the question is usually whether the builder followed the specified detail. A detail that exists only in the installer’s head is difficult to defend, and a substitution made on site without a variation is difficult to defend at all. This is the same discipline that applies to reinforcement cover and concrete durability, where the specified cover is only as good as the documentation that shows it was achieved.

Worked example: a coastal roof replacement

A builder in a coastal NSW location is replacing a metal roof on a dwelling within a few streets of the water. The existing roof is being replaced with a coated steel sheet system, and the specification calls for the manufacturer’s fasteners and washers. The builder is also replacing the gutters, and the gutter supplier has offered an aluminium gutter at a better price than the coated steel option in the original specification.

The aluminium gutter is installed against the coated steel fascia and the coated steel roof edge, fixed with the same fasteners used on the roof. The roof fixings are correct. The gutter fixings are not, and the gutter is now coupled to a dissimilar metal along its full length in a location where salt-laden water sits in the gutter and wets the junction every time it rains.

The failure appears as staining and paint blistering at the gutter line and at the fixing points, and it appears after practical completion. By the time the owner raises it, the gutter is pitted and the coating on the fascia is damaged. The remedy is replacement of the gutter and repair of the fascia coating, plus an argument about whether the substitution was a variation and who carries the cost.

The prevention was available at three points. The builder could have kept the specified gutter material. The builder could have isolated the gutter from the fascia and the roof edge with a continuous barrier and a compatible fastener. Or the builder could have documented the substitution as a variation with the corrosion assessment attached, so the decision was visible and the cost was allocated. Any one of those would have changed the outcome.

Where this sits in your CPD and your quality system

Corrosion at dissimilar metal junctions is a detailing competency, and it is the kind of thing that is easier to build into a standard detail library than to solve on each job. If your practice has standard details for flashings, cladding fixings, post anchors and gutter junctions, review them once for galvanic couples and record the isolation requirement on each. That is cheaper than reviewing every job individually.

For builders working through their continuing professional development, durability and materials topics sit alongside the compliance and contract content that the scheme requires. The guide to how NSW builder CPD works explains the categories and the record-keeping, and the course catalogue lists the durability and materials units currently available. If you are looking for a starting point on the broader programme, the PointCPD home page links to the current course listings.

The practical takeaway is narrow and worth repeating. Galvanic corrosion is not a product problem to be solved by buying better metal. It is a junction problem, and junctions are drawn, specified, installed and recorded by the builder. Get the isolation detail onto the drawing, use the specified fixing, keep the junction drained, and write down what you did. That is what stands up when the call comes after practical completion.

Corrosion at a junction is a detail you draw, not a product you buy.

Questions builders ask

What actually causes galvanic corrosion between dissimilar metals?

It requires two dissimilar metals in contact or connected by a conductive path, an electrolyte such as water carrying dissolved salts, and an electrical connection between them. When all three are present, current flows and the less noble metal corrodes. Removing any one of the three conditions stops the reaction, which is why isolation detailing works.

Which metal corrodes when two different metals are joined?

The less noble metal becomes the anode and loses section, while the more noble metal is protected. In practice this means a small fixing can be consumed while the large sheet or member it fastens looks unaffected. The area ratio matters: a small anode against a large cathode corrodes faster because the current concentrates into a small area.

Is using stainless steel fixings always the right answer?

No. Stainless fixings are often appropriate, but if they are coupled to a different metal family in a wet junction they can make the larger component the anode. That is a worse outcome because the component is expensive and hard to replace. The right choice depends on which metal you are prepared to sacrifice, and that decision should be made on the drawings.

How do I isolate two dissimilar metals on site?

Use a continuous non-conductive barrier such as an isolating tape, gasket, nylon washer or coated clip across the full contact area, and make sure it is not torn during installation. Alternatively use a compatible fastener specified by the manufacturer, design the junction to drain and ventilate, or protect it with a compatible coating system. A robust detail usually combines two of these.

Why does galvanic corrosion show up after practical completion?

The reaction is driven by wetting and drying cycles, so it takes time for enough section loss or staining to become visible. That timing puts it outside the defects inspection at practical completion and into the period when the owner is chasing the builder and the subcontractor has already been paid. Documentation of the specified detail is what usually decides who carries the cost.

Where should I check the corrosion requirement for a specific junction?

Start with the manufacturer literature for the sheet, system or component, because where a fastener type, coating or separation layer is specified, that specification governs the junction. Confirm the current requirement in the NCC and in the relevant Australian Standard for the material, and record the confirmation on the shop drawings so the installed detail matches the specified one.

Sources

Planning for Bush Fire Protection · NSW Rural Fire Service
Planning for Bush Fire Protection · NSW Rural Fire Service
BASIX · NSW Planning Portal
Condensation in buildings handbook · Australian Building Codes Board

Published 17 September 2026. General information for builders, not legal, financial or engineering advice for a specific job. Check current requirements with the relevant regulator or standard before relying on them.

Galvanic corrosion between dissimilar metals: isolation detailing that survives the warranty period