The ISO 12944 standard is intended to assist engineers and corrosion experts in adopting best practice in the corrosion protection of structural steel at new construction locations. ISO 12944 is progressively superseding regional standards to become a truly global benchmark in. ISO 12944 helps engineers select a protective coating system by defining atmospheric corrosivity categories (C1 to C5 and CX) and linking the environment + durability target to coating system performance expectations. Iso 12944. Transformers face corrosion threats from moisture, salt spray, industrial pollutants (SO₂, Cl⁻), and internal insulation oil (80-135°C). The chromium and molybdenum content (Cr≥16.