AS 4312: atmospheric corrosivity zones in Australia
AS 4312:2019 maps Australian atmospheric corrosivity zones C1 to CX, setting steel coating grades, fastener specs, and product warranties.
Ask Chalkline about this →AS 4312:2019, Atmospheric corrosivity zones in Australia is the Australian Standard that maps the country into corrosivity categories, setting the minimum protective coating grade for steel, fasteners, and cladding at any given site (verified 2026-06-11, Standards Australia).
The 2019 edition is the current version, superseding AS 4312:2008.
The categories
Six zones, based on how fast bare mild steel corrodes in the local atmosphere:
| Zone | Severity | Typical location |
|---|---|---|
| C1 | Very low | Dry climate-controlled interiors |
| C2 | Low | Rural inland, low pollution |
| C3 | Medium | Urban areas, light coastal fringe |
| C4 | High | 200 m to 1 km from rough surf; shoreline of sheltered bays |
| C5 | Very high | Within 200 m of rough surf; heavy industrial with high humidity |
| CX | Extreme | Direct surf-break shoreline; very high salt deposition |
A separate T (tropical) modifier covers far-north Queensland and Northern Territory environments where heat and humidity compound salt-zone effects.
How it translates on site
The zone determines what spec you put on every exposed metal component:
- C1/C2: standard zinc-plated or Z275-coated fixings acceptable for dry internal use.
- C3: hot-dip galvanised (HDG) screws and brackets as minimum for external work.
- C4/C5: HDG service life is reduced; use 316 stainless for structural fixings; confirm cladding warranty covers the zone.
- CX: 316 stainless for all fixings; standard Colorbond void, Colorbond Ultra minimum; consult manufacturer before specifying.
Product warranties (roofing, cladding, fasteners) reference AS 4312 zones directly. Using a C3-rated product in a C5 zone voids the warranty and leaves the defect with the builder.
For a builder
- Establish the zone at design stage: distance to breaking surf is the first question. A few hundred metres can shift a project from C3 to C5.
- The fixing is usually the weak link. Upgrading the cladding without upgrading every fastener and bracket is a callback waiting to happen.
- Keep records of the zone determination and the specified products.
Category: Standard / corrosion.
Related
See also
References
- AS 4312:2019, Atmospheric corrosivity zones in Australia, Standards Australia (verified 2026-06-11)
- Simpson Strong-Tie: corrosion protection for metal connectors in Australia (verified 2026-06-11)
Last updated: 2026-06-11. Verified: 2026-06-11. Quarterly review for currency.