316 vs 304: what "marine-grade" stainless steel mesh actually means

Stainless steel is not rust-proof. What makes it "stainless" is a microscopically thin passive layer of chromium oxide that forms on the surface as soon as the metal meets oxygen. Scratch it and it reforms. That self-healing film, not the bulk metal, is what stands between a wire and corrosion.

Chlorides are the one thing that reliably beats it. A chloride ion can break through the passive layer at a single point, and because the surrounding surface stays passive, the attack concentrates instead of spreading — a pinhole that keeps going down. That is pitting corrosion, and salt air is a chloride delivery system.

Grade 304 is the workhorse austenitic stainless: roughly 18% chromium and 8% nickel. Grade 316 keeps that basic recipe and adds about 2% molybdenum.

Molybdenum is specifically good at the failure mode above. It stabilises the passive layer against chloride attack, which is why 316 is the grade specified for marine and coastal service across every industry that deals with salt — not just security screens. The Australian Stainless Steel Development Association puts it plainly: in marine environments, use grade 316 or something of equivalent corrosion resistance.

That is the whole of the difference. It is not a brand, a coating or a weave — it is an alloy specification, and a supplier can tell you which one they use in a single word.

Coastal Australia has a name for what under-specified stainless does. Tea staining is a brown surface discolouration that appears on stainless exposed to salt air — most visibly on sheltered surfaces that rain never rinses, like the underside of a door frame or mesh behind an eave.

It is cosmetic rather than structural: a tea-stained screen has not lost its strength. But it looks like rust on a product sold on the promise that it will not rust, it is difficult to reverse once established, and it is the single most common complaint about 304 mesh within a few kilometres of the surf.

Distance from the coast is the variable that matters most, followed by how often the screen gets rinsed — by rain or by you.

Corrosion resistance and security performance are separate questions, and it is worth being clear about that rather than blurring the two. A 304 screen can pass the Australian Standard tests. Grade does not decide whether a screen resists a knife or a kick — construction, mesh tension, the frame and the locking system do.

So treat the mesh grade as the durability question and the test certificate as the security question. You want both answered, and one does not substitute for the other.

Two things worth asking any supplier, whichever brand you are looking at: what grade is the mesh, in the same sentence as the number; and how is the mesh isolated from the aluminium frame? Dissimilar metals in contact in a salt environment is the other corrosion mechanism, and the mesh-to-frame joint is where it happens.

The everyday range — hinged, sliding, French and multi-fold doors, window screens and patio enclosures — is built with 316 marine-grade woven stainless steel mesh, UV-coated, in a T6 extruded aluminium frame powder-coated in Dulux and Interpon finishes.

ScreenGuard Extreme is the exception, and this guide would be worth nothing if it glossed over that. Extreme runs high-tensile 1.2mm 304 stainless steel with WedgeMAX™ retention, because it is engineered for a different threat: windborne debris at up to 44 m/sec, tested to AS 1170.2:2021 as well as AS 5039.1:2023. The thicker, high-tensile wire and the way the wedge holds it in the frame are what buy that impact performance, and 304 is the grade that comes in that build.

So Extreme is not under-specified, it is optimised for cyclone survivability rather than for chlorides — but by the logic above, that does trade corrosion margin. Within a few kilometres of the surf the care and maintenance schedule matters more on an Extreme screen, not less. If corrosion resistance is your first question and cyclone rating is not, ask for a 316 system, and say the number out loud.

That is also why the range carries a 10-year warranty from installation on materials and manufacturing, provided the care and maintenance schedule has been kept. A ten-year term on a screen is, in practice, a statement about the mesh.

• How to clean a security screen door without wrecking it
• How to read a security screen certificate: AS 5039, AS 5040 and AS 5041

ScreenGuard screens are custom-made and fitted by licensed suppliers around Australia. Tell us about your openings and your local supplier will measure, quote and install.