← All News YIELD MAX STRONGER FENCES. BIGGER YIELDS.
www.yieldmaxfence.com  ·  sales@yieldmaxfence.com  ·  +86 150 3220 7479
YieldMax · Technical Note · Coating Chemistry

Why ammonia destroys galvanised wire —
and why Zn-Al coating survives.

In dairy sheds, feed pads and animal yards, ammonia attacks galvanised fencing faster than almost anywhere else — and simply adding more zinc doesn't fix it. Here is the chemistry in plain English, and why a Zn-Al (Type ZA) coating is the right specification for high-ammonia country.

1. Where the ammonia comes from

In dairy sheds and feedlots, urease enzymes in manure hydrolyse the urea in cattle urine, releasing ammonia gas continuously. The ammonia dissolves into the moisture film on wire surfaces, creating a persistently alkaline, complexing environment (pH ≈ 9–11).

Urea hydrolysisCO(NH2)2 + H2O → 2NH3↑ + CO2

The root cause is protein. Animal feed protein is roughly 16% nitrogen by mass; that nitrogen is excreted as urea (mammals) or uric acid (poultry), which bacteria break down to ammonia. So the higher the protein intake and stocking density, the more ammonia is released — poultry and pigs are the most aggressive environments, followed by housed dairy cattle, with sheep on open pasture the mildest.

2. How zinc normally protects itself

In ordinary atmospheres, zinc corrodes slowly and its corrosion products build a dense protective patina — this is why galvanising works so well in standard rural conditions.

Anodic / cathodic reactionsZn → Zn2+ + 2e−   |   O2 + 2H2O + 4e− → 4OH−
Protective film formationZn2+ + 2OH− → Zn(OH)2  →  Zn5(CO3)2(OH)6 (patina)

3. What ammonia does to that film

Ammonia is a strong ligand for zinc. It dissolves the protective layer by forming the soluble tetraammine-zinc complex — so the film is stripped as fast as it forms, fresh zinc is re-exposed, and corrosion becomes a continuous linear loss of metal.

Protective film dissolutionZn(OH)2 + 4NH3 → [Zn(NH3)4]2+ + 2OH−
Overall reaction (aerated)2Zn + O2 + 8NH3 + 2H2O → 2[Zn(NH3)4]2+ + 4OH−

This is why a heavier coating doesn't solve the problem. Specifying 240 g/m² of zinc instead of 150 g/m² just means more sacrificial metal to consume — the consumption rate stays the same. The issue is the mechanism, not the mass.

4. Why the Zn-Al coating resists

Pure zinc (HDG)

  • Zn2+ forms soluble ammine complexes — the protective film is constantly dissolved
  • Continuous, linear metal loss in NH3-laden atmospheres
  • Heavier coating = longer countdown, same endpoint

Zn-10%Al (ZA)

  • Al3+ forms no stable ammine complex — ammonia cannot dissolve the alumina (Al2O3) passive film
  • Ammonia is a weak base (pH ≤ ~11) — far too weak to attack Al2O3, which requires pH > 13
  • Corrosion products form dense Zn-Al layered double hydroxides (LDH) — the film becomes more stable over time

The result: at equal coating weight, Zn-Al coatings last roughly 2–4× longer than heavy galvanising in ammonia-laden farm environments — which is why a Zn-Al (Type ZA) build is the right specification for dairy sheds, feed pads and high-stock country.

"Ammonia dissolves zinc's protective layer by forming soluble zinc-ammine complexes — but it can't touch the alumina-rich passive film on a Zn-Al coating. That's chemistry, not coating thickness."

Specifying for high-ammonia country?

YieldMax fixed knot fence is available with Zn-Al (Type ZA) coating to AS/NZS 4534, plus a standard hot-dip galvanised build for lower-corrosion jobs. Coating test reports and mill certificates are available on request.

Email us for a spec quote Compare coatings

This summary is provided by YieldMax for planning purposes. Coating types and classes referenced follow AS/NZS 4534; the published standard remains the authoritative reference.