Q: What is the best way to specify powder coating performance on steel substrates? With aluminum, I can use AAMA 2604. Is there something similar for steel?
A: Defining corrosive environments is a critical component in specifying and selecting the appropriate material to meet the rigors expected in the life of a coated product. Two specifications capture the requirements of a corrosion-resistant industrial coating, whether it is a powder coating or some other form of organic finishing.
ISO 12944-6 provides an excellent delineation of typical environments where corrosion occurs. Its categories span from C1 (essentially a dry indoor environment) to C5-M (highly corrosive coastal and marine conditions).
| Category | Example Environments |
| C1 – Very Low | Heated buildings with clean atmospheres such as schools or offices |
| C2 – Low | Atmospheres with low levels of pollution, mostly rural areasUnheated buildings where condensation may occur such as depots or sports halls |
| C3 – Medium | Urban and industrial atmospheres, moderate sulfur dioxide pollution, or coastal areas with low salinityProduction rooms with high humidity and some air pollution such as food processing plants, laundries, breweries, or dairies |
| C4 – High | Industrial areas and coastal areas with moderate salinityChemical plants, swimming pools, coastal ship and boat yards |
| C5-I – Very High (industrial) | Industrial areas with high humidity and aggressive atmospheresBuildings or areas with almost permanent condensation and with high pollution |
| C5-M – Very High (marine) | Coastal and offshore areas with high salinityBuildings or areas with almost permanent condensation and with high pollution |
These categories are commonly specified in European and other overseas locations; however, the classes depicted in ISO-12944-6 are representative throughout the world.
Another useful corrosion-resistance specification was developed by the Society for Protective Coatings (SSPC), now the Association of Materials Performance and Protection (AMPP), although its focus is the performance of exterior-grade liquid paints. The SSPC Paint Specification No. 36 Two-Component Weatherable Aliphatic Polyurethane Topcoats delineates three levels of coating durability:
SSPC Environmental Zones
- 1A (interior, normally dry)
- 1B (exterior, normally dry)
- 2A (frequently wet by fresh water, excluding immersion)
- 2B (frequently wet by salt water, excluding immersion)
- 3B (chemical exposure, neutral)
- 3C (chemical exposure, alkaline)
Both resources are scant on any metal preparation specifications beyond galvanizing, blasting, and the use of primers. Specifying chemical metal pretreatment requires another guide, perhaps directly from a vendor of these products.
In addition, NAAMA (National Association of Architectural Metal Manufacturers) has published the Metal Finishes Manual for Architectural and Metal Products. This manual focuses on a variety of coating technologies, including powders. Powder and other organic coatings are described in Chapter 5. This is less a specification and more a thorough explanation of coating options and processes to finish a wide array of substrates, including various steel grades.