Heat-Affected Zone

Heat-affected zone infiltration may be a potential cause of erosion/corrosion failures of carbon steel piping when exposed to a continuous stream of wet steam or feed water. The problem is associated with high-velocity and turbulences created by the presence of the welding penetration as a flow obstacle.

The deterioration is developed for two reasons:
1) The mechanical wear due to the increase of velocity (red color area) would abrade the welding;
2) The turbulences generated in the region downstream the welding (blue area) would affect the formation of the protective metal-oxide layer and erode the surface.
3) Localized turbulences may also contribute to release CO2 or other acid-forming anions contained in the water chemistry and develop weak acid attack (see microscope image)

As a result, carbon steel piping becomes vulnerable to general deterioration and wall thickness dissolution. Make sure to have a good field supervisor on board… and better welders!