Marine-grade aluminium isn't a single alloy. It's a category of aluminium alloys with magnesium content tuned for resistance to saltwater corrosion. The dominant marine alloys are 5052 (general marine, easy to bend), 5083 (high-strength marine, used in boat hulls and structural marine work), and 6061-T6 (heat-treated structural marine).
For gate and balustrade work in coastal Australia, 5052 H32 is the common choice for sheet and extrusion infill. Anodised or powder-coated finishes layer extra protection on top of the base alloy.
Non-marine alloys (3003, 6063 commercial-grade) will pit and white-rust within months in surf-zone exposure. Always check the alloy spec when ordering for any address inside the coastal salt-spray zone.