Call · (314) 432-1377
Email ·

Weld Seam Corrosion of Fire Sprinkler Piping

Weld Seam Corrosion of Fire Sprinkler Piping (195 kb)

Corrosion related leaks in fire sprinkler system piping have become more common in recent years as buildings and the fire protection systems protecting them continue to age. The most common point of failure in fire sprinkler pipe is caused by weld seam corrosion of Electric-Resistance Welded (ERW) pipe. Post-mortem analysis of failed pipe samples from the field reveals that aggressive corrosion of the weld seam and surrounding area has resulted in the significant removal of the weld metal. Because of its distinctive metal loss characteristics, this type of corrosion is commonly referred to as ‘Knife Cut’ corrosion.
Knife Cut Corrosion of Black Steel Branch Line - Weld Seam Corrosion

Knife Cut Corrosion of Black Steel Branch Line

Preferential Corrosion of the HAZ at a Welded Outlet - Weld Seam Corrosion

Preferential Corrosion of the HAZ at a Welded Outlet

The weld seam and the adjacent pipe metal that experience high temperature and pressure during the manufacturing process are commonly referred to as the Heat Affected Zone (HAZ). As a result of the high temperature and pressure exerted on the HAZ the microstructure of this metal changes and can differ significantly from the structure of the bulk piping. These structural differences in the steel result in the metal of the HAZ becoming anodic with respect to the rest of the pipe wall. The anodic character causes the metal of the HAZ to more readily give up its electrons, or corrode preferentially when the pipe is subject to corrosive conditions.

In an attempt to mitigate the increased risk of developing leaks at the weld seam, it has been suggested that pipe should be rotated during installation to orient the weld seam at or near the top of the pipe. This approach will reduce the likelihood of adherent deposits accumulating over the weld seam and thereby reduce the risk of under-deposit oxygen acceleration of corrosion. Further, this approach will help keep the weld seam out of pockets of trapped water in dry and preaction systems. However, there is the problem of practical application of this approach and the fact that this approach does not address the source of the corrosion. While corrosion rates may be lower with the weld seam removed from the corrosive environment, leaks will still develop given time.