What galvanic corrosion is
Put two different metals in contact, add moisture that can carry a current — salt-laden coastal air does this brilliantly — and you've built a battery. Current flows between the metals, and the less noble of the pair sacrifices itself: it corrodes faster than it ever would on its own, while the more noble metal is protected. That's galvanic corrosion, and it's not a slow background process on the coast. The wrong pairing can eat through a coating or a thin section in a fraction of the fixing's expected life.
Three things have to be present: dissimilar metals, direct contact, and an electrolyte. Remove any one of the three and the circuit is broken. That's the whole prevention strategy in one sentence — everything below is just the practical ways to do it.
Why it bites on the Sunshine Coast
Inland, surfaces dry out and the circuit spends most of its life switched off. Here, salt spray and humidity keep surfaces conductive for long stretches — especially within a few kilometres of the surf, around canals and estuaries, and in sheltered details that never get rain-washed. The classic site pairing is a stainless steel screw driven into a galvanised frame or bracket: the zinc coating becomes the sacrificial metal, corrodes away around the fixing, and then the unprotected steel underneath rusts. Aluminium against copper, and aluminium against untreated steel, are the other pairings to treat with respect.
Bi-metal screws — the decking problem, solved
Coastal decking is where the conflict shows up most often. The deck boards want stainless — it's the only finish that lasts in the salt. But stainless is comparatively soft to drive, and putting a stainless screw into galvanised joists or steel subframe sets up exactly the couple described above.
A bi-metal screw resolves both problems in one fastener: a stainless steel head and upper section where the screw is exposed to the weather, joined to a hardened carbon steel drilling section that does the work of getting through hardwood or steel without snapping or stripping. The exposed portion has marine-grade corrosion resistance; the buried portion is matched to the material it's driven into. It's a specialist product, and it's one of the things we know deepest — bi-metal fastening for coastal decks is core Runway territory.
How to prevent it
- Match the metals where you can. Galvanised fixings into galvanised steel, stainless into stainless or timber. The fewer dissimilar contacts, the fewer circuits.
- Keep the fastener the noble one. A small fastener corroding to protect a large sheet is a fast failure; the reverse is slow. As a general rule the fastener should be no less corrosion-resistant than the material it's fixing.
- Isolate the contact. Isolation washers, nylon grommets, and barrier tapes break the metal-to-metal contact so the circuit can't form. Cheap insurance on any mixed-metal detail.
- Use bi-metal screws where stainless exposure meets a galvanised or steel substrate — decking being the textbook case.
- Design the water out. Details that drain and dry break the electrolyte side of the triangle. Trapped water plus mixed metals is the worst combination on the coast.
Common mistakes
- "Stainless is better, so it's always safe." A stainless screw through aluminium flashing or into a galvanised bracket is a live galvanic pairing. Better metal, wrong context.
- Copper run-off onto aluminium or galv. Water that has passed over copper — pipework, treated timber containing copper compounds — deposits it downstream and sets up corrosion on aluminium and zinc surfaces below. Watch what drains onto what.
- Scratched coatings at the contact point. Overdriving a fixing through a coated sheet damages the coating exactly where the dissimilar contact is. Drive to seat.
- Ignoring it because it's hidden. The couple inside a wall or under a deck corrodes just as happily as one you can see — it's just found later, and dearer.
The counter check
We check galvanic compatibility proactively at the counter — if the fastener you're buying is going into a metal that's going to fight it, we'll flag it before it leaves the shop and give you the right option, whether that's bi-metal, isolation washers, or a different grade. Spec notes are available in writing on request. As always, final selection for a specific application rests with the builder, engineer or certifier.