Aluminium has a reputation for not rusting, and that is largely deserved. It builds a thin, tight oxide skin that protects the metal underneath. Near the sea, that skin is tested every day by salt, humidity and long periods of moisture. Corrosion on a coastal building rarely starts on a clean, open face. It starts in a crevice, behind a bracket, at a cut edge or where two metals meet. This article explains where attack begins and what a specifier can do about it.
Why coastal air is harder on aluminium
Sea air carries fine salt particles that settle on surfaces and draw moisture out of the air. A film of salty water is far more aggressive than plain rainwater. It conducts electricity easily, which speeds up the electrochemical reactions behind most corrosion. Humidity also keeps surfaces damp for longer, so the salt stays active instead of drying and blowing away.
Exposure is not uniform. A face that is washed by rain stays comparatively clean, while an underside, a soffit or a sheltered recess collects salt and never gets rinsed. The distance from the shore matters, but so do prevailing winds, nearby harbours and industrial sources. Treat the site as having zones, not one simple category.
The forms of attack to look for
Four forms cover most of what is seen on site. Pitting shows as small craters and white or grey powdery deposits where the oxide skin has broken locally. Crevice corrosion happens where moisture is trapped in a tight gap, such as under a gasket or behind a washer. Galvanic corrosion appears where aluminium touches a more noble metal in a wet joint. Filiform corrosion creeps as fine threads under a painted or coated surface, usually starting from a scratch or a cut edge.
Each form has a different cure, which is why a vague note saying "suitable for coastal use" does not help a fabricator. The specification should say which conditions the design must resist and where. Knowing the mechanism also helps on site, because it tells the inspector where to look first.
What the finish does
A finish is the main protection on the visible face. Anodising thickens the natural oxide layer so that it becomes part of the metal itself. Powder coating bonds a baked polymer layer to a pretreated surface. PVDF coating is a liquid-applied fluoropolymer system commonly specified for long outdoor life and colour stability. Each has strengths, and our guide to anodised, powder coated or PVDF compares them in more detail.
For coastal work the pretreatment before the coating is as important as the coating itself. A coating applied over a poorly cleaned or poorly converted surface may look perfect on delivery and fail early at the edges. Ask what pretreatment is used, who carries it out and how it is checked, and ask for the coating system and its warranty to be named in the specification.
Fixings, brackets and dissimilar metals
A facade holds together with screws, bolts, brackets and anchors, and many of them are not aluminium. When aluminium touches another metal in a wet, salty joint, the more reactive metal gives up material and corrodes faster than it would alone. In practice that is usually the aluminium, though it can be the fixing, depending on the pair. Stainless steel fixings are common, but the grade must suit the environment, and not every stainless steel is equal near the sea.
The usual controls are simple. Choose compatible fixings, separate unlike metals with isolating washers, sleeves or tapes, and keep joints sealed so that water cannot sit in them. Our article on sealants and gaskets in facades covers how those materials help keep water out of small gaps. Fixings are hidden and small, so plan their protection at the drawing stage, not on site.
Detailing so water leaves and dries
Good coastal detailing lets water drain away and lets surfaces dry. Horizontal ledges that hold standing water, blind pockets, and flat tops without a fall will collect salt and moisture. Drip edges, sloped tops and open drainage paths reduce the time a surface stays wet. Where a cladding system has a cavity behind it, check that the cavity can drain and ventilate so that salt-laden damp air does not stay trapped.
Pay attention to places that rain cannot reach. Soffits, undersides of canopies and recessed bands are not rinsed by the weather, so salt builds up. Either design these areas for easy washing, or choose a finish and detail that tolerates deposits. A drawing review that asks "where does this stay wet?" finds most of the problem areas before fabrication begins.
Check also how the facade meets other materials. Where aluminium is set against concrete, masonry or timber, wet alkaline material can sit against the metal for long periods. A separating membrane, a sealed joint or a small air gap prevents that contact. These junctions are normally the architect's detail rather than the facade supplier's, so they are easy to leave unresolved between two parties. Ask who owns each interface and make sure it appears on a drawing.
Cut edges, scratches and site damage
A coating is only as good as its continuity. Every drilled hole, trimmed end and scratch exposes bare metal. On a coastal job, those exposed spots become starting points for creeping corrosion under the film. Fabricators should treat cut and drilled edges according to the coating supplier's instructions, and site teams should have an approved touch-up method that is used instead of improvised paint.
Protection film, careful handling and clear site rules matter more than they seem. Site grinding of steel nearby can embed iron particles in aluminium surfaces, which then rust and stain the finish. Cement and wet plaster splashes are alkaline and can attack both coating and metal if left in place. Record the condition at handover so that later claims can be assessed fairly.
Cleaning, inspection and records
Washing is the cheapest corrosion control there is. Rinsing salt off with clean water on a regular schedule keeps deposits from building up. Use cleaning products and methods that the finish supplier approves, because strong or abrasive cleaners can damage the coating and void a warranty. Our facade maintenance checklist gives a practical routine that suits most buildings.
Inspect at the same time. Look at gaskets, sealant lines, fixings visible at the surface, and the underside of projecting parts. Photograph any bubbling, blistering or white deposits and note the location, so that a pattern becomes visible over several seasons. Early repair of a small coating defect is far cheaper than replacing a corroded component.
Design life, warranty and realistic expectations
No finish lasts forever without care, and a coastal site shortens the margin for neglect. When a supplier quotes a warranty, read what it actually promises. Some cover only peeling or loss of adhesion, while others also cover colour change or chalking, and many carry conditions about washing, damage and the distance of the site from the water. A warranty that the owner cannot satisfy in practice is not much protection.
It also helps to separate cosmetic change from structural concern. A slight loss of gloss or a little colour drift on a coated face is not the same as pitting through a section or a failed fixing. The first is an appearance question that the owner can judge. The second is a safety and durability question that needs an engineer. Make sure the project documents say which is which, so that a complaint is routed to the right person.
Finally, agree what happens at the end of the finish's life. Some coatings can be refreshed in place, while anodised surfaces are harder to renew without removal. Knowing this early lets an owner plan a repainting or replacement budget instead of being surprised by it.
What to put in the specification
A coastal specification should be concrete. It should state the exposure zones on the building, the finish and pretreatment for each, the fixing materials and the isolation method between unlike metals. It should also ask for the coating warranty in writing, along with its conditions, since many warranties depend on a washing routine that the owner must follow.
- Which parts of the facade are in the most exposed or least rinsed zones?
- What finish, pretreatment and coating system are proposed for each?
- What fixing materials are used, and how are unlike metals separated?
- How are cut edges and site damage treated and recorded?
- What washing routine does the warranty require, and who carries it out?
A sample or mock-up exposed on site does not predict decades, but it does show how the detail handles water, and it lets the team inspect the real fixing arrangement before a large quantity is made.
General guidance for orientation, not a specification. Applicable standards, values and details are confirmed per project with the facade consultant.
