ISO 12944 is one of the most important international standards that can be used as a reference point for the protection of steel against corrosion by a protective paint or coating which was last revised 2018.

ISO 12944-1: Durability

Part 1 of the standard provides for an overall scope and gives some basic terms and conditions, including durability:

Durability Statement 2007 Version 2018 Version
Low Durability (L) 2 – 5 years ≤  7 years
Medium Durability (M) 5 – 15 years 7 – 15 years
High Durability (H) ≥ 15 years 15 – 25 years
Very High Durability (VH)   ≥ 25 years


ISO 12944-2: Classification of Environments

Part 2 provides guidance on classifications of principle environments which the steel parts and fabrications are going to be exposed to in service. The environments are based on the mass (thickness) loss by standard specimens and describes typical natural environments.

Corrosion Category Internal Exterior
C1 Heated Buildings:

Offices, Shops, Schools, Hotels

C2 Unheated Buildings:

Depot’s, Sports Halls

Low Pollution Levels:

Rural Land

C3 High Humidity:

Factories, Laundries, Dairies

Urban and Industrial:

Moderate SO2, Low Salinity

C4 High Pollution:

Coastal Ship, Chemical Plants

Industrial & Coastal:

Moderate Salinity


Formerly C5I & C5M

High Humidity, High Salinity, Permanent Condensation Industrial Areas, High Humidity, High Salinity, Coastal

New Category Introduced 2018

Industrial Areas:

Extreme Humidity and Aggressive Atmosphere

Offshore Areas:

High Salinity, Aggressive Atmospheres, Sub-Tropical


Categories for Water and Soil Immersion Environment Examples
Im1 Fresh Water Rivers, Hydro-Plants

No Cathodic Protection

Sea, Brackish Water Immersed Structures without Cathodic Protection, Harbour Areas, Locks, Jetties
Im3 Soil Buried Structures

With Cathodic Protection

Sea, Brackish Water Immersed Structures with Cathodic Protection, Off-Shore Structures


The new CX category is for extreme environments off-shore and there’s a significant difference between C5 and CX in terms of the mass loss per unit surface/thickness loss:

Mass Loss per Unit Surface / Thickness Loss after one year of exposure
Corrosion Category Low-Carbon Steel Zinc
Mass Loss g/m2 Thickness Loss µm Mass Loss g/m2 Thickness Loss µm
C5 ≥ 650 – 1500 ≥ 80 – 200 ≥ 30 – 60 ≥ 4.2 – 8.4
CX ≥1500 – 5500 ≥ 200 – 700 ≥ 60 – 180 ≥ 8.4 – 25


Summary of Considerations

Decide on the Durability you or your client is looking for and specify this when asking your coatings application partner company to assist in choosing an appropriate corrosion protection coating/paint system.

When considering an acceptable level of thickness loss you may then choose to galvanise the parts for added protection, this will affect coating system choices.

Next describe the environment the steel fabrication is to be placed in.

If parts or all of the fabrication is to be immersed and protected with cathodic protection your coatings application partner company will need to know if they’re being asked to contribute in choosing a solution.

From this information you may be in a position to specify a coating/painting system or you might choose to provide the information to your corrosion protection coatings partner who’ll then assist in choosing an appropriate system.

Equipment and Facilities Owners and Operators both know that all structures represent a valuable asset, both as a capital investment and as a source of ongoing revenues.

Northpoint has been providing high-quality coating and paint application services for industrial steel structures for almost half a century. Northpoint has grown during this period to a corrosion protection coatings partner that offers truly high-performance coatings solutions for both on-shore and off-shore industries with a proven track record.

Get in touch at:

Tel:         0161 330 4551   (+44 161 330 4551)


Blog Author – Philip Dawson, Technical Advisor, for technical and specification enquiries