Electrical Resistance Welded (ERW) carbon steel piping is typically used for fire sprinkler system service. In the process of manufacturing welded metal piping, the weld metal and the base metal just adjacent to the weld seam end up physically and chemically different in granular structure, composition and properties from the base metal. The rapid cooling of the weld seam and the heat affected zone (HAZ) around the weld seam causes the metal to be more “stressed” than the rest of the pipe. As a result, this area of the pipe becomes more anodic in character and can corrode more aggressively.
In many industrial applications where welded metal piping will be used in applications that expose the piping to corrosive fluids it is common to require post weld treatment of the piping to reduce the weld-induced stresses in the weld seam area. This process is called heat annealing or heat treatment of the metal piping. In effect the finished welded piping is re-heated and slowly cooled (quenched) to impart identical compositional and functional properties to all of the piping metal.
The current ASTM standards for fire sprinkler piping DO NOT REQUIRE heat annealing of fire sprinkler piping to prevent corrosion. As a result it is very common for highly localized weld seam corrosion to occur when the fire sprinkler tubing is exposed to moist, oxygen rich conditions. It is also common to see accelerated corrosion on the internal pipe surfaces where weld-o-lets have been affixed to the fire sprinkler mains for branch line connections. The heat affected zone around the weld-o-let can become anodic to the rest of the metal.
Fire sprinkler piping weld seam corrosion:
- Is very common
- Occurs in both horizontal and vertical piping
- Accelerates under deposits of iron oxide
In the case of galvanized steel fire sprinkler piping weld seam corrosion is very aggressive when the weld seam is trapped under standing pools of water within dry and preaction fire sprinkler systems.