ISO 12944 corrosivity category selector
Work out which C1 to CX category matches your environment, and the durability expected per class. The driving factor is shown.
Why the category decides everything downstream
A coating system is selected against an environment. Get the environment wrong and everything downstream is wrong with it — the system will be under-specified for the service, or over-specified for the budget. It is the most expensive error in a specification, and it is made before any technical decision.
How the result is reached
Each factor — humidity, pollution, salinity, chemical exposure — proposes a category, and the most severe is retained. That is the principle of the standard: an environment is classified by what attacks it most, not by an average. A dry workshop on the seafront is still exposed to salt, and salt is what decides.
Where two independent factors point to the same high category, the tool flags it without moving up a step: automatic upgrading would depart from the standard. It is information for your judgement, not a correction applied on your behalf.
Durability ranges
Durability is neither a guarantee nor a service life: it is the expected interval before first major maintenance, used to build a maintenance programme.
| Range | To first major maintenance |
|---|---|
| Low (L) | Up to 7 years |
| Medium (M) | 7 to 15 years |
| High (H) | 15 to 25 years |
| Very high (VH) | Over 25 years |
Method and source
The categories and their descriptions come from ISO 12944-2:2017, and the durability ranges from ISO 12944-1. The decision tree is a reading of those descriptions, not a normative provision.
Limits of use
- ISO 12944-2 defines the categories by environment descriptions, not by a formula. This tool turns those descriptions into a decision tree: it points, it does not determine.
- The standard provides two other routes, more reliable when the stakes justify them: comparison with typical atmospheres, and mass loss measurement on specimens exposed for one year. A questionnaire replaces neither.
- One structure can fall into several categories depending on the zone: a walkway downwind of a stack is not in the same environment as its sheltered face. Assess by zone where the difference justifies it.
- The category says nothing about surface preparation, the acceptable level of soluble salts, or the interval between blasting and coating. Those belong to the specification, and they are the items most often left out.
How to cite this page
Maintenance Blueprint, « ISO 12944 corrosivity category selector », maintenanceblueprint.com/en/tools/corrosivity-category/