At a glance
| Category | Name | Steel thickness loss, first year (µm) | Outdoor examples | Indoor examples |
|---|---|---|---|---|
| C1 | Very low | 1.3 or less | None normally | Heated buildings with clean air: offices, shops, schools, hotels |
| C2 | Low | Over 1.3 to 25 | Rural areas, little pollution | Unheated buildings where condensation can occur: depots, sports halls |
| C3 | Medium | Over 25 to 50 | Urban or industrial air with moderate sulphur dioxide; coast with low salinity | Humid production rooms with some air pollution: food plants, laundries, breweries |
| C4 | High | Over 50 to 80 | Industrial areas; coast with moderate salinity | Chemical plants, swimming pools, coastal boatyards |
| C5 | Very high | Over 80 to 200 | Industrial areas with high humidity and aggressive air; coast with high salinity | Spaces with almost permanent condensation and high pollution |
| CX | Extreme | Over 200 to 700 | Offshore, extreme humidity, sub-tropical or tropical | Industrial areas with extreme humidity and aggressive air |
Loss is for low-carbon steel exposed for one year; the standard notes that losses may reduce in later years, and the examples are informative only. The same loss values appear in BS EN ISO 9223. For C5 a coating maker's summary gives a mass loss of over 650 to 1,500 g/m² and for CX over 1,500 to 5,500 g/m²; mass losses for C1 to C4 were not in the sources read.
The category describes the air, measured on steel
A corrosivity category answers one question: how fast would plain low-carbon steel rust here? The scale is set by how much thickness the steel loses in its first year, from 1.3 micrometres or less in category C1 to over 200 micrometres in CX. Because the scale is made on steel, it describes steel best; other metals do not follow it exactly, and our sheet on metal by environment places hospitality settings on it.
- C1: no coating needed. steelconstruction.info says steel in a dry heated interior, which it classes as C1, needs no corrosion coating at all; its examples include offices, shops, schools, hotels, residential buildings, airport terminals and hospitals. A coating there is for appearance, wear or fire, not corrosion.
- Kitchens, laundries and pools are different. The same page notes that kitchens and laundries inside a hospital may be C3. Swimming pools are listed under C4, and the page does not cover interior C4 and C5 in detail, advising specialist input.
- Outside, the air sets the category. Rural air is C2, urban or industrial air with moderate sulphur dioxide is C3, industrial areas and the coast with moderate salinity are C4, and aggressive industrial air or a highly saline coast is C5. CX is for offshore and extreme sites.
The examples are informative: a particular site can sit in a different category from its description, and the source says so. Where it matters, ask the coating partner for the category they have assumed, and why.
Durability: what Low, Medium, High and Very high mean
Durability is the expected life of the coating to its first major maintenance. steelconstruction.info takes that as the point where 1 per cent of the surface has rusted (degree Ri3 in BS EN ISO 4628-3:2016). It is a considered estimate, and the source says plainly that it does not imply a warranty period.
| Durability band | ISO 12944-1:2017 | Before the 2017 revision | Reading |
|---|---|---|---|
| Low (L) | Up to 7 years | 2 to 5 years | Short-term or easily repainted |
| Medium (M) | 7 to 15 years | 5 to 15 years | A common choice for interior visible steel |
| High (H) | 15 to 25 years | Over 15 years | The usual ask for exterior building steel |
| Very high (VH) | Over 25 years | Not used | New in 2017: hard to reach or costly to repaint |
One trap for older specifications. The 2017 revision of ISO 12944-1 moved the bands: low used to be 2 to 5 years and is now up to 7, medium was 5 to 15 and is now 7 to 15, and high used to be anything over 15 years and is now 15 to 25, with a new very high band above that. A specification written to the old scheme that says only “high durability” means something different. Always state the standard edition or the years.
What the published examples show
The chart plots every example system on that source's interior and exterior tables. Four things stand out, all from the source's own figures.
- Most examples sit between 10 and 25 years. That is the low, medium and high bands. Only the hot-dip galvanised examples go further: 20 years or more outside in category C3, and 40 years or more for visible galvanised steel inside. Zinc is doing the work there.
- A harsher category does not buy a longer life. The listed exterior systems for C4 and C5 are 15 to 20 years, not more, and the systems are heavier: the source's C5 paint system carries 200 µm of micaceous iron oxide in all, against 100 µm in its C3 system.
- Small changes in the system move the life. The source notes that a zinc-rich primer at 80 µm adds roughly five years of durability.
- Visible interior steel is a different job from hidden steel. Hidden steel in a dry interior is described as a structure life of 50 years or more with no coating life given; visible steel is coated for looks, and its coating lives are 10 to 25 years, apart from the galvanised example.
These are structural steelwork systems for buildings, not decorative metalwork. They show what a durability band costs in layers, and they are not a specification for your piece. Our sheet on powder coat or wet paint sets two of these systems beside powder.
Writing the specification line
A full specification line has five parts, shown below. Parts two and three are decisions for the designer and client: where the piece will be and how long before it is repainted. The standard, preparation and coats can come from the coating partner, who should confirm them in writing before the price.
- Preparation matters as much as the paint. The example systems start from blast cleaning to Sa 2½ in BS EN ISO 8501-1:2007 as cited there. A coating maker's summary of the 2017 revision of ISO 12944-3 adds that preparation grade P3, very thorough preparation, must be used where high or very high durability is wanted in C4, C5 and CX.
- Say what the standard covers. ISO 12944 is for paint systems on steel. Powder coating is outside it, as the Qualisteelcoat scheme notes, so a powder system is described by that scheme's own classes; see our powder coating sheet.
- Detail for drainage. BS EN ISO 12944-3 deals with design for coating: the source notes that good detailing avoids water and dirt traps and keeps later maintenance possible. Ask for the detail before the finish.
Specification wording
Paste into your schedule or specification and replace anything in [square brackets] with your own figures. Wording is a starting point: your specification governs.
Protective paint system to BS EN ISO 12944-5 for corrosivity category [C3], durability [High, 15 to 25 years to first major maintenance, ISO 12944-1:2017]. Surface preparation [blast clean to Sa 2½ to BS EN ISO 8501-1]. Coats and nominal dry film thickness [primer, intermediate, finish, in µm] stated by the applicator in writing before pricing. Durability is not a warranty.
Corrosivity category for [element] taken as [C1 / C2 / C3 / C4] because [heated dry interior / rain-washed inland exterior / coastal, low salinity]. The partner to state the category it has assumed and notify the designer before pricing if the site conditions suggest a higher one.
[Element] is in a heated dry interior, category C1: no corrosion coating is required. [Finish] to be applied for appearance and wear only; the partner to confirm that no part is in a damp or washed zone.
Terms used on this page
- Corrosivity category (C1-C5)
- A classification of how aggressive an environment is for coated steel, from dry heated interiors up to marine and industrial sites. Coating systems and expected life are matched to it.
- Hot-dip galvanising
- Dipping fabricated steel in molten zinc to give a thick protective coating. The standard sets minimum thicknesses by steel thickness, for example 85 um mean on steel over 6 mm.
- Durability (ISO 12944)
- The expected life of a protective paint system to its first major maintenance, in four bands: low (up to 7 years), medium (7 to 15), high (15 to 25) and very high (over 25). It is not a warranty period.
- Micaceous iron oxide (MIO)
- A flaky iron oxide pigment used in an intermediate paint coat to build film thickness and slow moisture. Its colour range is limited, so a finish coat gives the colour.
Standards and sources
- steelconstruction.info (SCI and BCSA): corrosion protection; standard systems for buildings; coating specification: read 7 Nov 2026; cites BS EN ISO 12944-2:2017, -5:2019 and BS EN ISO 9223:2012.
- ISO 12944 series: -1:2017 (durability bands), -2:2017 (environments; current, confirmed 2023 per the ISO catalogue summary), -3:2017, -5:2019: texts not read; BSI listings not checked.
Questions we are asked
What corrosion category is a hotel or office interior?
C1, very low, according to steelconstruction.info: heated buildings with clean air, with offices, shops, schools and hotels as examples. Steel there needs no corrosion coating. Kitchens and laundries inside such a building may be C3, and a swimming pool hall is listed under C4.
What category is an exterior steel balustrade?
It depends on the air. The source puts rural exteriors at C2, urban or industrial exteriors and low-salinity coasts at C3, industrial areas and moderate-salinity coasts at C4, and highly saline coasts at C5. Ask the coating partner which they have assumed.
What does high durability mean in years?
15 to 25 years to first major maintenance, in ISO 12944-1:2017. Before 2017 it meant over 15 years, so an old specification may mean something different. The standard says durability is not a warranty period.
Does ISO 12944 cover powder coating or stainless steel?
No. It is for paint systems on steel. Qualisteelcoat notes that ISO 12944 does not cover powder, and borrows the categories. Stainless steel does not rust like carbon steel, so its choice is by grade and finish: see our metal by environment sheet.
What is category CX?
Extreme: offshore areas, extreme humidity and sub-tropical or tropical sites, with a steel loss over 200 to 700 µm in the first year. A coating maker's summary says CX was added in 2017 and that C5 now covers land-based structures.
Do I need a coating on steel inside a heated building?
Not for corrosion. The source says no corrosion coating is needed in a dry heated interior. You may still want one for appearance, handling or fire protection, which is a different specification.
Download the factsheet
A two-page A4 PDF with the category table, the durability bands, the chart of example systems and paste-in wording, to keep with your project file.