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About ISO 12944

Selecting protective coating systems using ISO 12944

Protective coatings for steel are seen everywhere, from structural steel in buildings, internal and external roof construction, bridges, storage tanks and pipes for our water and fuel infrastructure, industrial structures in factories and off-shore as parts of renewable energy or oil and gas assets. And then there are the invisible parts: buried, immersed or seldom seen internal parts, tidal and splash zones.

Together these possible scenarios form the environment parameters described in the widely used ISO 12944 standard “Paint and Varnishes: Corrosion Protection for steel structures by protective paint systems”.

The challenge of finding the right protective coatings is reduced understanding this standard and using tested and certified Transocean Coating solutions.

Introducing exposure & durability

In short, the ISO 12944 standard proposes coating systems for carbon steel and galvanized steel depending on exposure conditions. It also includes a list of laboratory tests to confirm suitability of coating systems, more on the that further down on this page.

Where coated steel is used, i.e. what corrosive environment it will be exposed to, has a major influence on how long a coating system will last: the durability. In the standard, durability dictates coating type and the minimum film thickness. In ISO 12944, four ranges of durability are used as period to first major maintenance:

  • low (L)                <7 years
  • medium (M)      7-15 years
  • high (H)              15-25 years
  • very high (VH)  >25 years

green and white pipe - partly buried

On-shore and off-shore

Since the first version in the late 1990s, the main use of ISO 12944 probably has been for on‑shore atmospheric corrosion protection. The environmental or corrosivity categories that go with this exposure scenario are going from light to severe: C1, C2, C3, C4 & C5 as described in the first table below. 

There are a number of non-atmospheric service categories: in fresh water, in salt/brackish water and buried as shown in the second table. Three off-shore environments were added to the ISO 12944 standard in 2018: atmospheric (CX), immersion with cathodic protection (Im4) and splash zone (a combination of CX and Im4  with its own set of thickness specifications).

After coating selection, it is up to for contractors,
applicators and those involved in quality control to get the most out
of the coating system. There are parts to the ISO 12944 standard
focussing on the application phase that can assist with this. The impact
of this part of the process should not be overlooked.

The different parts of the ISO 12944 standard, including the full film thickness tables can be bought from local standardisation organisations or the ISO website.

ISO 12944-2 exposure categories

Atmospheric Corrosion category Steel loss after 1st year of exposure Examples Exterior Examples Interior
C1 (very low) ≤ 1,3 µm Heated buildings in rural areas such as offices, hotels, public buildings
C2 (low) > 1,3 – 25 µm Rural areas with low level of pollution. Unheated buildings where condensation may occur such as warehouses.
C3 (medium) > 25 – 50 µm Urban and industrial areas with moderate sulphur dioxide pollution. Coastal areas with low salinity. Industrial facilities with high humidity such as food processing industries, breweries.
C4 (high) > 50 – 80 µm Industrial and/or coastal areas with moderate salinity. Chemical plants, swimming pools, boat yards, ship interiors.
C5 (very high) > 80 – 200 µm Industrial areas prone to high humidity and chemical attack. Coastal areas with high salinity. Facilities exposed to permanent condensation and chemicals.
CX (extreme) > 200 – 700 µm Offshore areas with high salinity. Industrial areas exposed to extreme humidity, aggressive atmosphere and tropical areas. Facilities exposed to extreme humidity and aggressive atmosphere.
Note: In previous versions of ISO 12944, the C5 category was split in Marine (C5M) and Industrial (C5I), part 6 requiring additional tests for the industrial (chemical) exposure. Therefore, in older documents C5 M may not refer to C5 medium durability and C5I should not be misread as C5 low durability (C5 L, using the capital letter L instead of lower-case l).

Corrosion category Environment Examples of environments and structures
Im1 Fresh water River installations, hydro-electrical power plants
Im2 Sea or brackish water Immersed structures without cathodic protection (e.g. harbour areas with structures like sluice gates, locks or jetties)
Im3 Soil Buried tanks, steel piles, steel pipes
Im4 Sea or brackish water Immersed structures with cathodic protection (e.g. offshore structures)
Note: for corrosivity category Im1 and Im3, cathodic protection can be used with a paint system that has been tested accordingly

stack of Transocean Coatings Drums

Coating Selection​

In the ISO 12944 standard, coating systems are proposed based on expected exposure (corrosivity) and a required durability range. The dry film thickness (DFT) tables are based on different coating technologies. Systems descriptions may not be available for all combinations of durability and corrosion category. For instance for the immersion or Im 1-3 specifications , only H and VH are proposed. More frequent maintenance would not be economically viable in such scenarios.

The specified film thickness goes up with durability requirements and the aggressiveness of the environment. Stepping up a corrosion category and down a durability class generally results in the same DFT requirements, for example: a system for C3-high (C3 H) will be the same as for C4-medium (C4 M).

The tables for CX, Im4 and splash zone protective coating systems specify systems that should give a high (15-25 years) durability (no other durability ranges are included for the offshore scenarios).



Film thickness table for atmospheric systems (see ISO 12944-5)​

DurabilityLow (l)Medium (m)
High (h)Very high (vh)
Type of primer 1Zn(R)MiscZn(R)MiscZn(R)MiscZn(R)Misc
Binder chemistry of
primer 1
Binder chemistry of subsequent coatsEP, PUR², AYEP, PUR², AYAK, AYEP, PUR², AYEP, PUR², AYAK, AYEP, PUR², AYEP, PUR², AYAK, AYEP, PUR², AYEP, PUR², AYAK, AY
C2Number of Coats
Dry Film ThicknessNote 3100µm60µm120µm160µm160µm180µm200µm
C3Number of Coats111122222
Dry Film Thickness100µm60µm120µm160µm160µm180µm200µm200µm240µm
C4Number of Coats11122222232
Dry Film Thickness60µm120µm160µm160µm180µm200µm200µm240µm260µm260µm300µm
C5Number of Coats22223233
Dry Film Thickness160µm180µm200µm240µm260µm300µm320µm360µm

Note 1: Zn(R)= zinc-rich, Misc. = miscellaneous primer, see text, ESI = zinc silicate, EP = epoxy, PUR = polyurethane, AK = alkyd, AY = acrylic (mainly water based)
Note 2: in addition to polyurethane technology, other coating technologies may be suitable, e.g. polysiloxane, polyaspartic and fluoropolymer
Note 3: if a coating is desired (for C2-low), use a system for higher corrosivity category or durability, e.g. C2-high or C3-medium

This is a simplified version of the table in ISO12944 part 5. Number of coats should be read as a minimum. For dry film thickness (DFT) the standard talks about ‘nominal’ DFT. For exact definitions please refer to the standard.

Coating system requirements for Im1, Im2 and Im3 in line with ISO 12944-5

DurabilityHighVery High
Type of primer 4Zn(R)Misc.n/a
(single coat)
(single coat)
Binder chemistry of
primer 4
Binder base of subsequent coatsEP, PUREP, PUREP, PUREP, PUREP, PUREP, PUR
Minimum number of coats221221
Dry Film Thickness360µm380µm400µm500µm540µm600µm

Note 4: Zn(R)= zinc-rich, Misc. = miscellaneous primer, ESI = zinc silicate, EP = epoxy, PUR = polyurethane

Coating system requirements for CX, Splash and Tidal Zones and Im4 in line with ISO 12944-9

Type of EnvironmentCX (offshore)Splash and tidal zones CX (offshore) and Im4Im4
Type of primerZn(R)Other primersZn(R) 5Other primersOther primers
(single coat)
Primer Dry Film Thickness ≥40µm≥60µm≥40µm≥60µm≥200µm≥150µm
Minimum number of coats3333212
Dry Film Thickness of
full paint system

Note 5: This coating system with an organic Zn(R) primer can also be used for IM4 service if a Zn(R) primer is desired. In this case the thickness of the complete system van be reduced to ≥350µm.

Transoweld coated plates, grey and redbrown

Primers: Zinc or no Zinc?

In most system descriptions in the standard, there is a choice to use a zinc-rich primer (abbreviated to “Zn(R)” primer). The definition for this type of primer comes down to a minimum of 80% zinc dust pigment by weight in the dry film. Transocean Coatings offers 3 types of zinc rich primers: zinc silicate, zinc epoxy and (moisture cured) zinc polyurethane primers.

All other primer types, including those containing zinc below 80%, are considered “miscellaneous” primers for part 5 or described as “other primers” in part 9 of the standard. A note is made indicating there are local standards that require higher amounts of zinc for a coating to be called a zinc or zinc-rich product and used in certain specifications (for example the zinc-silicate specification in Norsok M-501 (rev.7)).


Primers that have been used in coating systems that have passed ISO 12944-6 and/or 9 testing:

Tested and certified coating systems

The ISO 12944 standard describes the accelerated corrosion performance testing required for the different coating systems. This includes combinations of condensation, UV-light, salt fog and where required immersion in salt water (with cathodic protection). With the test protocols prescribed in the standard, a coating system can be validated giving additional peace of mind with regards to expected performance and durability. Different duration of testing are required for the different environment/durability combinations:
Environment Test Duration
C4VH / C5H 1680h  = 10 weeks
C5VH 2688h   = 16 weeks
CX and splash-zone 4200h   = 25 weeks
illustration of UV/condensation, Salt-spray and freeze cycles

Certified system examples

Transocean Coatings offers a wide portfolio of products that can be combined to the coating systems as specified by the ISO 12944 standard. Third party testing has resulted in a range of coating systems that are certified to meet the performance requirements of the ISO 12944 standard from C3 to CX environments and different durability ranges. Contact your local Transocean Coatings representative for more information on availability.

ExposureDurabilitySubstratePrimer typeProductDFT2nd coatDFT23rd coatDFT3Total DFT
(Sa 2½)(1)
Misc. epoxyTranspoxy Masterbond 467
(TO 4.67)
(Sa 2½)(1)
Misc. epoxyTranspoxy Unicote
(TO 4.32)
150µmTransurethane Shield
(TO 3.45)
(Sa 2½)(1)
Misc. epoxyTranspoxy Masterbond Alu 466
(TO 4.66)
100µmTranspoxy Masterbond
(TO 4.67)
100µmTransurethane Finish
(TO 3.43)
(Sa 2½)(1)
Zn(R), epoxyTransozinc Epoxy Primer 155
(TO 1.55)
60µmTranspoxy Masterbond
(TO 4.67)
160µmTransurethane Finish
(TO 3.43)
(Sa 2½)(1)
Zn(R), silicateTransozinc Silicate 147
(TO 1.47)
90µmTranspoxy Masterbond BT
(TO 4.68)
90µmTranspoxyl PX 370
(TO 3.70)
(Sa 2½)(1)
Zn(R), silicateTransozinc Silicate 147
(TO 1.47)
85µmTranspoxy Masterbond BT
(TO 4.68)
120µmTranspoxyl PX 370
(TO 3.70)

(1) Carbon steel, abrasive blasting to Sa2½ (ISO 8501), see product datasheet of primer for more details on profile requirements
(2) DFT the system was tested at, specification table in ISO 12944 may require higher thickness
(3) systems in ISO 12944-9 for CX and Im4 are designed for high durability only, no very high durability systems are described in this part of the standard

For more information about ISO 12944, download our “Guide to protective coating selection using ISO 12944″.

In this guide we indicate how anyone can select a suitable coating system for steel protection using a the parameters described in the above: environment,  durability, primer and system type selection followed by testing.

For datasheets and information about the mentioned products go to the Products page.