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External Atmospheric Corrosion of Malleable Iron and Cast Iron Fittings and Component Parts in Wet Pipe Fire Sprinkler Systems

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Atmospheric Corrosion of Malleable Cast Iron Fittings White Paper (195 kb)

This paper explores external corrosion that occurs on the outside surfaces of wet pipe fire sprinkler piping from contact with ambient air. Three different installation scenarios are addressed:

  • Structures that are temperature controlled during winter months to prevent freezing but not temperature controlled during other times of the year, e.g. warehouses with open garage doors
  • Structures that are temperature controlled year round that experience the intrusion of warm moist air at opened doors during the summer months, e.g. entry vestibules and retail store receiving docks
  • Structures that house freezer/cooler installations and employ dry pendent sprinklers that protrude from the unconditioned air space above the freezer into the freezer itself

In order for corrosion to initiate on the external surfaces of a wet pipe fire sprinkler system, liquid water must be present. As water condenses on the cooled metal surfaces of the fire sprinkler piping, there are two different corrosion mechanisms that can take place:

  1. Acid condensate corrosion is the result of condensate water from the air that is essentially distilled water. Once the water droplet is formed, gases in the air begin to dissolve into the water. The dissolved gases immediately affect the chemistry and physical nature of the water. There are multiple acidic gases in the air that can dissolve into the condensed water droplet.
  2. The second corrosion mechanism is oxygen corrosion. Oxygen from the air will dissolve in the condensed water on the pipe and immediately react with the iron to produce iron oxide. Every time water condenses on the pipe surface, oxygen corrosion will act to liberate iron from the surface of the pipe.


atmospheric corrosion above freezer

atmospheric corrosion above freezer