Marine Grade Aluminium for Coastal Projects

If you’re specifying external aluminium building products for a coastal project, the material grade, the pre-treatment process and the powder coat system all need to work together. Get one wrong and the finish fails early, regardless of what the other two are doing.

This article covers what “marine grade” actually means in the context of architectural aluminium, how pre-treatment protects the substrate before powder coating, and what to confirm with your fabricator before you sign off on a specification for a coastal site.

What makes aluminium “marine grade”

Marine grade aluminium refers to alloys in the 5000 series, where magnesium is the primary alloying element. The magnesium content increases the material’s resistance to pitting and stress corrosion cracking in salt-affected environments.

The alloys most commonly used in architectural fabrication are 5005, 5052 and 5083. Each has a different magnesium content and a different strength profile.

5005 is a general purpose alloy with good formability and anodising characteristics. It’s commonly used for architectural sheet and decorative applications where the corrosion exposure is moderate.

5052 contains approximately 2.5% magnesium and offers higher strength and better corrosion resistance than 5005. It performs well in salt spray environments and is a common choice for fabricated building products that will be exposed to coastal conditions long term. It bends, folds and welds cleanly, which makes it well suited to precision fabrication.

5083 is the strongest of the non-heat-treatable marine alloys, with around 4.5% magnesium. It’s typically specified for heavy structural marine applications (hulls, pressure vessels) rather than architectural sheet products.

For external building products such as window shrouds, window hoods, AC condenser covers, planter boxes and louvre screens, 3mm marine grade aluminium in the 5000 series is the standard specification. It gives fabricators the combination of corrosion resistance, formability and weld quality that architectural products require, without the excess weight and cost of heavier structural alloys.

How aluminium resists corrosion

All aluminium alloys form a thin oxide layer on the surface when exposed to air. This layer is self-repairing: if it’s scratched or damaged, it reforms almost immediately on contact with oxygen. That oxide film is the primary reason aluminium performs so much better than steel in exposed environments.

In marine grade alloys, the magnesium content strengthens this natural protection. The oxide layer on a 5052 alloy is more resistant to chloride attack (salt) than the layer on a lower-magnesium alloy like 3003 or 1050. In salt spray testing, 5052 corrodes at roughly one third the rate of standard aluminium alloys.

The oxide layer works well on its own for bare aluminium in mild conditions. But for architectural products on coastal facades, it’s a starting point, not a complete corrosion strategy. The pre-treatment and powder coat system do the rest.

Pre-treatment: the step that determines finish life

Pre-treatment is the process applied to the aluminium surface before powder coating. Its job is to remove contaminants (oils, oxides, fabrication residues), etch the surface for adhesion, and apply a conversion coating that bonds the powder coat to the substrate.

Without proper pre-treatment, powder coat adhesion is compromised from the start. The coating may look fine at handover, but within two to five years in a coastal environment, you’ll see blistering, peeling and filiform corrosion tracking under the film. The powder coat itself hasn’t failed. The bond between the coating and the aluminium has.

There are two broad categories of pre-treatment used in architectural powder coating.

Chrome-based (chromate) systems were the industry standard for decades. They use hexavalent chromium compounds to create a conversion coating with strong corrosion resistance. However, hexavalent chromium is a known carcinogen and environmental pollutant. Regulatory pressure is phasing these systems out across Europe (under REACH regulations), and the Australian industry is following suit.

Chrome-free systems use zirconium, titanium or silane-based chemistry to achieve the same outcome without the health and environmental risks. Modern chrome-free immersion systems are now widely accepted by quality assurance bodies (including Qualicoat and GSB) and deliver equivalent corrosion protection when properly controlled.

The key word is “immersion.” In an immersion pre-treatment system, the aluminium is fully submerged in a series of chemical baths: degrease, rinse, acid etch, rinse, conversion coat, rinse. Full submersion ensures every surface, including internal folds, weld zones and hard-to-reach areas, receives consistent coverage. Spray systems can miss recessed areas, which creates weak points in the coating system exactly where moisture is most likely to sit.

At Alumac, we use a chrome-free immersion pre-treatment system across all of our product lines. Every component is fully submerged before powder coating, which means the conversion coating is applied consistently to the entire surface of the product, including internal faces and weld zones that spray systems can miss.

Powder coating for coastal environments

The Australian Standard for architectural powder coatings on aluminium is AS 3715. The 2025 revision of this standard now aligns powder coating classifications with the atmospheric corrosivity categories defined in AS 4312, which means the coating specification should be matched to the site conditions, not just selected from a generic colour chart.

For coastal projects, the corrosivity category will typically fall into C3 (medium, coastal with low salinity) through to CX (very high, beachfront and surf zone). AS 4312 defines these categories based on expected corrosion rates, taking into account factors like humidity, salt deposition, distance from the water and local microclimate. Distance alone no longer defines “coastal” under the updated standard.

What this means in practice: a project 200 metres from the water in a sheltered bay may sit in a different corrosivity category to a project 200 metres from an exposed surf beach. The powder coat specification should reflect that.

Super-durable polyester powder coats, such as the Dulux Duralloy range, are engineered for exterior architectural applications and tested against UV degradation, chalking and colour shift over extended service life. For Alumac products, Satin Black and Satin White in Duralloy are held as standard colours. Colorbond colour matching is also available for facades specifying a consistent finish across roofing, cladding and aluminium building products.

The pre-treatment and the powder coat work as a system. A premium powder coat over a poor pre-treatment will fail. A proper pre-treatment under a substandard powder coat will also fail. The fabricator needs to control both.

Galvanic corrosion: where aluminium meets other metals

On coastal projects, galvanic corrosion is a risk wherever aluminium is in direct contact with a dissimilar metal in the presence of moisture. The most common scenario on building facades is aluminium products fixed to the structure with stainless steel fasteners.

Galvanic corrosion occurs because different metals have different electrical potentials. When connected by an electrolyte (moisture containing salt), current flows between them and the less noble metal (aluminium, in this case) corrodes at an accelerated rate. The closer the project is to the water, the more conductive the moisture, and the faster the corrosion.

The severity of the corrosion depends on the ratio of surface areas. A small aluminium rivet in a large stainless steel sheet will corrode rapidly, because the small anode (aluminium) is overwhelmed by the large cathode (stainless steel). A stainless steel screw in a large aluminium panel is a more manageable scenario, because the large anode surface distributes the corrosion effect over a wider area.

In practice, stainless steel fixings in aluminium building products are common and generally acceptable, provided the surfaces are isolated. Nylon washers, plastic bushings or isolation tape between the stainless fastener and the aluminium substrate will break the electrical path and prevent galvanic corrosion from occurring.

Architects should note this on the specification and builders should confirm it on site. If isolation is missing at the fixing points, the powder coat around the fastener will blister and the aluminium will pit, even if the rest of the product is in perfect condition.

What to confirm with your fabricator

If you’re specifying aluminium building products for a coastal project, these are the things worth confirming before the order goes to fabrication.

• Alloy grade. Confirm the product is manufactured from a 5000 series marine grade alloy. Ask for the specific alloy designation (5005, 5052, etc.) and confirm it’s appropriate for the corrosivity category of the site.
• Pre-treatment method. Ask whether the fabricator uses an immersion or spray pre-treatment system, and whether it is chrome-free. Immersion provides more consistent coverage, particularly on fabricated components with folds, returns and welded joints.
• Powder coat specification. Confirm the powder coat product is rated for the corrosivity category of the site under AS 3715:2025 and AS 4312. For coastal projects, a super-durable polyester is the minimum. Ask for the specific product name and the manufacturer’s warranty conditions.
• Fixing detail. Confirm how the product is fixed to the building structure and whether dissimilar metal isolation is specified at every fastener location.
• Warranty. Ask what the warranty covers and what it excludes. A 10 year warranty that excludes coastal environments is less useful than a shorter warranty that includes them. Confirm whether the warranty covers the finish only, or the substrate as well.

At Alumac, all external building products are manufactured from marine grade aluminium, pre-treated using our chrome-free immersion system, and powder coated in-house. Our 10 year warranty covers both the finish and the substrate. If you’re working on a coastal project and want to confirm the right specification for your site conditions, call us on (02) 9525 2177 or contact us here.