The standard in one paragraph
Australia's outdoor environment is divided into six corrosivity categories (A–F), ranked by the Corrosion Rate of Mild Steel (CRMS, µm/year) measured from one year of exposed bare-steel coupons. The categories tell engineers and farmers how aggressive a site is, so they can choose the right zinc or zinc/aluminium-alloy coating thickness and predict service life. Corrosivity falls off rapidly with distance inland from the sea, so a single town can sit in 2–3 categories at once. Beyond macro-climate, microclimate factors (rain shielding, dew, fertilisers, galvanic contact, fires, soils, animal urine, etc.) can move a site into a worse category. Zinc/5 % Al alloy coatings give 2–3× longer life than plain zinc — biggest gain in harsh marine zones.
Australia — measured corrosion sites (Table F2)
Numbers in brackets are CRMS in µm/year (one-year mild-steel corrosion). Sites overlap on the map where the standard records both city & beach measurements.
Category table (Table F1 + ISO 9223)
| Cat | CRMS | ISO | Outdoor |
|---|---|---|---|
| A | <1.3 | C1 | Alpine |
| B | 1.3–25 | C2 | Arid / urban / rural |
| C | 25–50 | C3 | Coastal |
| D | 50–80 | C4 | Sea-shore (calm) |
| E | 80–200 | C5 | Surf / off-shore / industrial |
| F | ≈ B | — | Non-coastal tropics |
Corrosivity falls off with distance from the sea
Schematic of Figure F1: corrosivity drops sharply within the first ~10 km from a surf coast.
Key concepts to remember
1. The corrosion driver chain
2. Microclimate factors that can bump a site to a worse category
3. Coating life rules of thumb
- Total Zn depletion ≈ 1.5× the time to first steel rust (10 % surface).
- Zn corrosion rate vs mild-steel rate: 1/50 (benign) up to 1/5 (marine).
- Zn / 5 % Al alloy coatings give 2–3× the life of plain Zn for the same mass — biggest gain in Cat D / E.
- In water: soft fresh water (≈ 100 g/m²·mo) is more corrosive than seawater; turbulent / splash zones are worst.
YieldMax coating recommendation by zone
A practical starting-point spec sheet — confirm against your site's microclimate and the specifying engineer's requirements.
| Zone | Where | Recommended YieldMax product | Typical coating mass |
|---|---|---|---|
| A · B · F | Arid, inland, urban, non-coastal tropics | Class 3 heavily galvanised wire / fixed-knot field fence | ≥ 240 g/m² Zn |
| C | Coastal cities & bays | Class 4 galvanised or Zn/5 % Al alloy wire | ≥ 300 g/m² Zn or ≥ 150 g/m² Zn-Al |
| D | Sea-shore, sheltered bays | Zn/5 % Al alloy ("ZnAl·5") fixed-knot or hinge-joint | ≥ 215 g/m² Zn-Al |
| E | Surf coast, off-shore, heavy industry | Zn/5 % Al alloy + PVC sheath where exposed | ≥ 230 g/m² Zn-Al + PVC |
Source: AS/NZS 4534:2006 — Guidance on Corrosion Protection (Appendix F, informative). This summary is provided by YieldMax for planning purposes; the published standard remains the authoritative reference.
