Spec library

Environment classes C1 to C5-M, in plain English

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

Every protective coating specification you are handed starts in the same place: a letter and a number. C3. C4. C5-M. It sits on the drawing, it drives the system, and it decides whether a job passes.

Most people in a workshop can tell you roughly what the classes rank. Fewer can tell you what they are actually measuring, or where the number on their drawing came from. That is worth knowing, because the class is not a marketing tier. It is a measured corrosion rate.

The classes are a measured rate, not a description

A corrosivity category describes how fast unprotected metal loses thickness in a given atmosphere, measured in micrometres per year over the first year of exposure. The categories are set out in AS 4312 Atmospheric corrosivity zones in Australia and in ISO 9223, and the AS/NZS 2312 series uses them to select protective systems for structural steel.

Class	Rating	Mild steel loss (µm/yr)	Zinc loss (µm/yr)	Copper loss (µm/yr)
C1	Very low	< 1.3	< 0.1	< 0.1
C2	Low	1.3 – 25	0.1 – 0.7	0.1 – 0.6
C3	Medium	25 – 50	0.7 – 2.1	0.6 – 1.3
C4	High	50 – 80	2.1 – 4.2	1.3 – 2.8
C5	Very high	80 – 200	4.2 – 8.4	2.8 – 5.6
CX	Extreme	200 – 700	8.4 – 25	5.6 – 10
T	Inland tropical	Not rate-determined	—	—

Two things fall straight out of that table.

First, the jump from C3 to C5 is not incremental — the steel loss rate roughly quadruples. Second, zinc corrodes at well under a tenth of the rate of bare steel — in most classes somewhere between a twentieth and a thirtieth — which is the entire reason zinc sits at the bottom of most structural systems.

What each class looks like on the ground

The rate tells you the severity. It does not tell you where you are. These are the typical environments the standards attach to each class.

Class	Outside	Inside
C1	—	Heated buildings with clean air — offices, shops, schools
C2	Low-pollution atmospheres, mostly rural	Unheated buildings where condensation can occur — depots, sports halls
C3	Urban and industrial atmospheres, moderate sulphur dioxide; coastal areas with low salinity	Production areas with high humidity and some air pollution — food processing, laundries, breweries, dairies
C4	Industrial areas and coastal areas with moderate salinity	Chemical plants, swimming pools, coastal, ship and boat yards
C5-I	Industrial areas with high humidity and aggressive atmosphere	Buildings with almost permanent condensation and high pollution
C5-M	Coastal and offshore areas with high salinity	Buildings with almost permanent condensation and high pollution

In an Australian context the shorthand runs: C1 is dry indoors, C2 is arid or urban inland, C3 is coastal or industrial, C4 is marine on calm water, and C5 is marine with ocean surf.

Why C5 splits into C5-I and C5-M

AS 4312 has a single C5. The AS/NZS 2312 series splits it into C5-I (industrial) and C5-M (marine).

The reason is worth understanding, because it is the one place where the rate table stops being the whole story. Steel corrodes at the same rate in both. The coatings do not. A chemically aggressive industrial atmosphere and a chloride-loaded marine atmosphere affect a coating film differently, so the standard keeps them separate even though the substrate loss figure is identical.

The two categories most people have never met

T — inland tropical. Added for the tropical zones of Australia. In corrosion-rate terms it sits around C2 for metals, because of the monsoonal weather pattern and the absence of ocean surf inland. For organic coatings it is far more demanding, because of the intensity and duration of UV. In other words: a low-corrosivity environment that still destroys paint.

F — inland tropical, as classified in AS/NZS 2728. Same idea, applied to prefinished sheet metal products.

NATSPEC puts it plainly: categories F and T are generally not very high in corrosivity, but the aggressiveness of the environment towards organic coatings means special protection is required.

The class on the drawing is a macro number. Your steel lives in a micro-environment.

AS 4312 deals with two different things: the macro-environment, generated by normal weather patterns across an area, and the micro-environment, restricted in scale and created by local features or by the orientation of the structure itself.

That distinction is where jobs get caught. The same beam can sit in two environments at once. Chloride does not normally travel more than about a kilometre inland from the coast — but flat terrain and strong prevailing onshore winds are a known exception. A surface that is sheltered from rain never gets washed, so salt accumulates instead of running off. Prolonged surface wetness, abrasion and erosion all shift the real corrosivity away from the map.

AS 4312 also relates the macro-environment to distance from the shoreline, and lists the micro-climatic factors that modify it: industrial pollutants and chemical attack, shelter from rain and regular washing, screening from salt and pollution deposition, prolonged surface wetness, abrasion and erosion.

What this means for you

The class is the first decision on the job and everything downstream inherits it — the system, the film build, the surface preparation grade, and what the inspector measures. If the drawing does not state one, or states one that does not match where the steel is actually going to sit, that is worth resolving before anything is blasted rather than after.

If you are not certain which class your work falls into, or you are quoting across several at once, the Paint Audit is seven questions about how you coat steel today, and the read comes back with your environment class restated in plain English alongside what that classification actually demands.

References

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