Corrosivity category
A classification of how aggressive an environment is towards steel, from C1 (heated interiors) to CX (offshore and extreme industrial), defined in ISO 12944-2 from measurable parameters rather than from location. It is the input a coating system is specified against, and it is fixed before any product is chosen.
A corrosivity category describes the environment, not the asset and not the coating. It is established from what is actually present — time of wetness, chlorides, sulphur dioxide — and it does not change because a better product was bought.
The most common error is applying one category to a whole site. Categories are microclimates: a chloride source, a stack plume or a cooling tower can put one area two categories above the structure two hundred metres away. Splitting a site into zones is usually more accurate, and cheaper, than assigning it a single number.
Sources and references
- StandardISO 12944-2:2017 — Paints and varnishes — Corrosion protection of steel structures by protective paint systems — Part 2: Classification of environmentsInternational Organization for Standardization · accessed August 29, 2026
- StandardISO 12944-1:2017 — Paints and varnishes — Corrosion protection of steel structures by protective paint systems — Part 1: General introductionInternational Organization for Standardization · accessed August 29, 2026
Where this is covered in depth
- ISO 12944 Corrosivity Categories Explained (C1 to CX)Understand ISO 12944 corrosivity categories from C1 to CX. Learn how to select the right protective coating system for your specific environment.
- Corrosion under insulation: where it startsWhy insulated equipment corrodes fastest where nobody looks, which temperature range does the damage, and what actually stops it.