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Galvanising

Galvanising applies zinc to steel for corrosion protection. Covers hot-dip vs electro-galvanised, HDG600 vs Z275 grades, and coastal exposure rules.

By Chalkline. Last updated 11 June 2026. Verified 11 June 2026.

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Galvanising applies a zinc coating to steel for corrosion protection. Three processes appear on Australian sites, coating thickness varying roughly 10-fold between them.

Hot-dip galvanising (HDG) immerses fabricated steel in molten zinc at approximately 450 degrees Celsius, bonding metallurgically to produce 45-85 microns of coating (verified 2026-06-11, Aims Industrial). Governed by AS/NZS 4680:2025 for fabricated articles and AS/NZS 1214:2016 for threaded fasteners.

Electro-galvanising deposits zinc by electrical current at room temperature, typically 5-12 microns. Used for pre-galvanised structural sections and purlins under AS 1397:2021.

Zinc plating (electroplating) also produces 5-12 microns. Indoor dry use only; not acceptable for treated-timber applications.

Coating designations

HDG batch system (AS/NZS 4680, fabricated articles): HDG390, HDG500, HDG600, where the number is the minimum coating mass in g/m2. HDG600 (85 microns average) is the standard specification for external structural steel (verified 2026-06-11, GAA).

Z-grade system (AS 1397, continuously coated sheet and strip): Z275, Z350, Z450, where the number is total coating mass in g/m2 across both faces. Applies to pre-galvanised hollow sections, purlins, and framing members (verified 2026-06-11, BlueScope).

Mixing the systems is a category error: “HDG600” does not apply to a pre-galvanised RHS section.

When hot-dip is mandatory

  • H3/H4 treated-timber fixings: HDG minimum. Copper-based preservatives corrode zinc-plated fasteners rapidly. See nails and framing anchors.
  • External structural steel: engineer’s spec calls AS/NZS 4680 HDG. See steel beams.
  • Wall ties and lintels: confirm coating grade matches the exposure zone.

Coastal zones

AS 4312 maps corrosivity zones C2 (mild) to C5 (very severe coastal). Within roughly 1 km of a surf coast (C4-C5), stainless 316 replaces HDG on fixings because chloride ions degrade zinc regardless of thickness (verified 2026-06-11, GAA Design Manual). Structural steel in C4-C5 typically needs a duplex system (galvanise plus paint).

Also known as: Galvanized coating, zinc coating, HDG process.

Category: Materials / Corrosion protection.

See also

Last updated: 2026-06-11. Verified: 2026-06-11. Quarterly review for currency.

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