Aluminium vs galvanised steel vs mild steel: how to choose by location

Updated September 2026 by Meghan Ristea, Procurement Manager.

Inland and under cover, powder-coated mild steel does the job at the lowest price. In coastal suburbs, hot-dip galvanised steel or aluminium. Within 500 metres of the sea, aluminium: the Alu-Deluxe Trellis Door and Alu-Deluxe Fixture carry a ten-year manufacturing warranty and do not rust. Robo Door rates every site by zone and exposure before recommending the material.

Distance from the sea, and exposure

We sort every site into one of three bands. Inland: Gauteng, Free State, Limpopo, Mpumalanga, North West and the KZN Midlands. Coastal suburb: within the coastal influence but more than 500 metres from the sea, such as most of Cape Town, Durban and inland Gqeberha. Seafront: within 500 metres of the ocean or a tidal estuary. Exposure runs from sheltered, under a roof or carport, to fully exposed to rain, dew, ground splash and sprinklers. Together they set the warranty on the finish.

Mild steel

Mild steel is what most Robo Door gates, trellis doors and bars are made from in Alrode, because it welds, forms and takes a finish well. On its own it rusts, so the finish does the protecting. Powder coat over a standard clean carries a one-year paint warranty. Sandblast, primer and powder coat carries three years sheltered, two fully exposed. Inland and out of the weather, that is a sound and affordable specification. Finishes without zinc carry no rust warranty; the paint warranty is the cover.

Galvanised steel

Hot-dip galvanising coats the steel in zinc before the powder coat, and it earns the longest rust warranty on a steel product: ten years inland and sheltered, seven years semi-exposed, five years fully exposed. In a coastal suburb it drops to five years with powder coat, and within 500 metres of the sea to three. One manufacturing detail is worth knowing: hinges cannot be galvanised because zinc seizes the bearing surfaces, so they are welded on after galvanising and the heat-affected zone is protected with zinc-rich cold-galvanise paint. Inspect those welds once a year. Electroplating is the lighter zinc option, with a five-year rust warranty inland and three years in a coastal suburb.

Aluminium

Aluminium does not rust. It is lighter, it is powder coated, and it costs more per gate. Robo Door's aluminium products are the Alu-Deluxe Trellis Door, Alu-Deluxe Fixture Burglar Bars and Alu-Deluxe Fixture Lite, all with a ten-year manufacturing warranty, plus aluminium fencing, balustrades, railings and staircases. Within 500 metres of the sea it is the default recommendation. Aluminium can still pit where salt is trapped or where it touches dissimilar metal, so fasteners and isolation details are specified, not assumed. The Alu-Deluxe products do not carry the Break-In Repair Guarantee; that stays with the steel Deluxe and Deluxe Fixture.

The decision table

The paint warranty by finish

Weight, automation and handling

Aluminium is the lighter metal, which matters on a wide sliding driveway leaf or a long trellis: less load on rollers, hinges and the motor. Steel is stiffer for the same section, which is why a heavy rigid gate is usually hot-dip galvanised steel rather than aluminium. The frame section and the running gear are sized to the material on every quotation, so a lighter panel is not hung on a frame designed for a heavier one.

Price

Powder-coated mild steel is the least expensive way to secure an inland opening. Hot-dip galvanising adds a process and a cost, and aluminium costs more again. Trellis doors start from R3,500 and burglar bars from R1,000 in steel; the material and finish move the number from there, and every quotation states both.

Mixing materials on one property

A coastal house often ends up with Alu-Deluxe on the sea-facing side, hot-dip galvanised steel on the sheltered street side and aluminium fencing on the boundary. That is a sensible specification, not a compromise, provided each choice is deliberate and written on the quotation with its own warranty.

Stainless steel

Robo Door offers stainless steel on balustrades and railings where the architecture calls for it. It is not a standard material for gates or bars, and it is not immune to tea-staining at the coast. It is specified per project.

What the quotation states

Material, product line, finish, the zone and exposure assumed for your address, and the manufacturing, paint and rust warranties that follow. If your site is within 500 metres of the sea or gets sprinkler overspray, say so at the site visit. An undisclosed coastal site or fully exposed opening is a warranty exclusion.

Next step

Tell us your suburb and how far you are from the sea, and we will name the material and finish on the quote. Request a free quote, WhatsApp +27 82 888 1919 or call 082 888 1919.