Corrosion testing on HVAC coatings can be conducted according to a number of standards, including ASTM-B117, ISO 9227, ISO 12944-6, and ISO 20340. Of these tests, the one we believe represents actual service environments most closely is ISO 20340.

ISO’s testing standards for protective paint systems were developed through long-term material weight loss and surface degradation testing in different corrosive environments. ISO was able to generally classify environments in corrosivity categories of C1 through C5-M or C5-I based upon this loss. Paint systems are typically tested against higher levels of corrosivity because a coating that can protect against the most severe C5-M or C5-I environments is certainly capable of providing protection in a less severe environment.  The corrosivity categories are:

C1 – Very Low

  • Example environments include: heated buildings with clean atmospheres such as schools or offices

C2 – Low

  • Example exterior environments include: atmospheres with low levels of pollution, mostly rural areas
  • Example interior environments include: unheated buildings where condensation may occur such as depots or sports halls

C3 – Medium

  • Example exterior environments include: urban and industrial atmospheres, moderate sulfur dioxide pollution, or coastal areas with low salinity
  • Example interior environments include: production rooms with high humidity and some air pollution such as food processing plants, laundries, breweries, or dairies

C4 – High

  • Example exterior environments include: industrial areas and coastal areas with moderate salinity
  • Example interior environments include: chemical plants, swimming pools, coastal ship and boat yards

C5-I – Very High (industrial)

  • Example exterior environments include: industrial areas with high humidity and aggressive atmospheres
  • Example interior environments include: buildings are areas with almost permanent condensation and with high pollution

C5-M – Very High (marine)

  • Example exterior environments include: coastal and offshore areas with high salinity
  • Example interior environments include: buildings or areas with almost permanent condensation and with high pollution

See a table of surface/thickness loss by corrosivity category

When the corrosivity category is known, it can be a useful tool for selecting the right coating for your HVAC/R equipment. However, it is important to note that a coating capable of providing protection at a higher level also protects well at every corrosivity level below the level tested – without impacting coil efficiency. Therefore, to provide maximum chance for equipment life extension AND coil efficiency, it is best to choose a coating with a higher corrosivity category.


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