AZ vs Z — the metallurgy behind aluminium-zinc's coastal survival
Two coating families dominate flat-steel corrosion protection: pure zinc (Z) and aluminium-zinc (AZ or Galvalume). On paper AZ costs a bit more per square metre. In practice, on a coastal roof, AZ150 will still be sound long after Z275 has rusted through. Why?
The zinc mechanism
Zinc protects steel sacrificially — it corrodes first, taking the electrochemical hit for the underlying iron. This is why a scratched galvanised sheet doesn't immediately rust: zinc around the scratch continues to oxidise preferentially. But zinc's oxide film is soluble in chloride environments (i.e. salt air). Coastal zinc loss can hit 6–8 μm per year vs 1–2 μm inland.
The aluminium-zinc mechanism
AZ coatings are 55% aluminium, 43.4% zinc, 1.6% silicon by weight. The aluminium forms a passivating Al₂O₃ layer that's insoluble in chloride and physically blocks further oxygen transport. The zinc component still provides galvanic protection at edges and scratches — but the bulk surface is protected by the aluminium oxide film, which is chemically stable.
Salt-spray test data
- Z275: red rust in ~750 hours ASTM B117 salt-spray
- AZ150: red rust in ~2,000+ hours — 2.6× the life
- AZ50: still beats Z275 on time to red rust in humid conditions
Where AZ wins clearly
- Coastal roofing within 2 km of shoreline
- Industrial atmospheres with SO₂, NO₂
- High-humidity tropical belts (Kerala, Sri Lanka, Indonesia)
Where Z wins clearly
- Cost-driven interior/inland roofing
- Applications requiring welding — Z is easier to weld than AZ's aluminium-rich surface
- Long-edge exposure needing galvanic edge protection (Z's edge-creep is slower)
Ask your paint shop first: PPGI (Z substrate) or PPGL (AZ substrate)? Answer that, then decide top coat.
