Spec library

AS/NZS 2312.1 vs 2312.2 — when each applies

Reference material, not engineering advice. Every technical claim traces to the cited standard or reference. Technically reviewed August 2026.

Published 2 August 2026 · Core Coatings technical desk

If you have been coating structural steel for more than a decade, you learned one standard: AS/NZS 2312. Since December 2014 it has come in parts — Part 1 for paint, Part 2 for hot dip galvanising, and, since 2025, Part 3 for thermal metal spray — and drawings still arrive citing the old 2002 edition alongside all of them. This article covers the two parts a paint shop deals with.

What changed and why

AS/NZS 2312 first appeared in 1967 as a guide for steel designers on protecting structural steel from corrosion. The last combined revision, AS/NZS 2312:2002, covered everything at once — paint, hot dip galvanising, powder coating, thermal spray and wrapping systems.

The problem with a single document was practical. The complexity of specifying protective paint systems meant that most of the useful information on hot dip galvanising was buried inside detail about other systems. During the review it was recognised that designers would be better served by product-specific parts.

The revised standard was released in December 2014 as two parts, both using the same environment definitions but recognising that the design process and the durability behaviour of paint and galvanising are very different things.

Part 1 — paint coatings

AS/NZS 2312.1:2014, Guide to the protection of structural steel against atmospheric corrosion by the use of protective coatings — Part 1: Paint coatings. Since a 2017 Australian-only amendment, its local designation is AS 2312.1:2014.

This is the one you are working to if the steel is being blasted and painted. Its scope covers:

classification of atmospheric and non-atmospheric environments
planning and design for corrosion protection
surface preparation treatments
the factors that influence coating selection
paint coating systems and methods of application
maintenance of protective coating systems
inspection and testing
guidance on preparing a coating specification

It explicitly does not cover paint systems for light structural steel such as purlins, roofing or similar sections.

Part 1 is also where the coating system designations live — the shorthand codes that appear on drawings and in specifications. Government coating specifications reference them directly: inorganic zinc silicate systems such as IZS4, epoxy/polyurethane systems such as PUR5, epoxy/polysiloxane systems such as PSL2, and moisture-cure urethane systems such as MCU2, "all as defined in AS 2312.1".

Part 1 clause 9.4 is the clause a coating contractor is pointed at around project commencement, when the full system, preparation requirements, application method and recoating intervals get stated. The ambient limits an inspection test plan will carry — relative humidity below 85 per cent and steel surface temperature at least 3 °C above dew point — are cited through AS/NZS 5131 alongside it.

Part 2 — hot dip galvanising

AS/NZS 2312.2:2014, same title, Part 2: Hot dip galvanizing.

Part 2 is based on the ISO 14713 series and references ISO 9223 and ISO 9224 for corrosivity. It covers the general principles of design for articles that are going to be hot dip galvanised, and it is where durability estimates for galvanising come from.

The practical change in the 2014 edition was that durability stopped being a single number. Life to first maintenance is now calculated from the minimum average coating thickness specified in AS/NZS 4680 against the corrosivity category, and the estimates went up substantially against the 2002 edition. As an illustration, a specified minimum coating thickness of 85 µm can be estimated to last a minimum of 20 years and a maximum of 40 years in a C4 environment.

Part 2 also carries three things Part 1 does not:

Duplex coatings. A detailed section on painting over hot dip galvanising, with performance options for both aesthetic and corrosion durability. It also carries seven decorative and industrial paint systems suitable for most corrosivity environments, plus Table 7.1 for anti-graffiti systems over galvanising.
Steel chemistry. Guidance on how silicon and phosphorus content affects the galvanised coating — appearance, resistance to mechanical damage and coating mass. This is why two identical-looking sections can come back from the galvaniser looking completely different.
Design detail. Appendices covering venting and draining, bimetallic corrosion, and the interaction of galvanised steel with soil, concrete, water, chemicals and wood.

So which one applies?

They are not alternatives you choose between. They answer different questions.

Your situation	The part that governs
Steel is blasted and painted	Part 1
Steel is hot dip galvanised and left as is	Part 2
Steel is hot dip galvanised and then painted (duplex)	Part 2 governs the duplex system, including the paint systems applied over the zinc — the paint products themselves are specified in the AS/NZS 3750 series
Light sections — purlins, roofing	Neither for paint — Part 1 explicitly excludes them; light sheet products have their own standards

Two related standards sit next to both parts and get confused with them. AS/NZS 4680 is the manufacturing standard for hot dip galvanised coatings on fabricated articles — it tells the galvaniser what to deliver; AS/NZS 2312.2 tells the designer how long it will last. AS/NZS 3750 is the series that specifies the paint materials themselves, including Part 9 for organic zinc-rich primer and Part 15 for inorganic zinc silicate.

The older drawings

AS/NZS 2312:2002 is superseded but still cited on drawings, and it used a different classification shorthand — an A to E rating rather than C1 to C5. Standards Australia's policy is to harmonise to ISO, so newly published standards use the ISO C1 to C5 system with modifications for environments ISO does not cover.

If a drawing cites AS/NZS 2312 with no part number, or uses letter classes, it is worth establishing which edition the specifier meant before anyone orders material.

What this means for you

Most of the confusion we see is not about which system to use. It is about which document the drawing is actually pointing at, and whether the person who wrote the spec knew there were two parts. That is a five-minute conversation, and it is cheaper before the steel is in the blast bay.

If you would rather have it written down: the Paint Audit asks what spec you are working to, and the read comes back with your spec position echoed back alongside the questions your answers raise.

References

Technical answers shouldn't sit behind a phone call.

Most of what's in this library already exists — in the standard itself, on an inspection sheet, in a rep's head. Getting it out has usually meant becoming a customer first, or waiting on someone else to have time for you. We think reference material this trade needs shouldn't be gated behind either.

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