Flow-Accelerated Corrosion Damage in a Steam Pipe Girth Weld

A circumferential girth weld in high-pressure steam distribution pipeline had experienced an unexpected severe degradation. This investigation used metallographic, chemical, crystallographic, and electrochemical methods to identify the damage mechanism. The root pass girth weld of the steam pipe is found to have been severely damaged by flow-accelerated corrosion. Computational fluid dynamics simulation and laser confocal microscopy confirmed a higher fluid velocity would remove the protective magnetite layer, leading to an interactive process of forming a deeper crater, which would generate even higher fluid velocities. The craters initiated at the pre-existing weld toe (or fusion line location) on the downstream side of the girth weld. In the early stage of erosion-corrosion of the 12 o'clock root weld, material removal caused by water erosion was concentrated at the fluid-facing root weld surface. The weld heat-affected zone the weakest link for galvanic corrosion, because it has a substantial portion of high-angle grain boundaries and greatest precipitation of carbides, leaving the grain boundaries depleted of Cr and Mo.

Automated summary

This investigation used various methods such as metallography, chemical analysis, crystallography, and electrochemistry to identify the damage mechanism of a circumferential girth weld in a high-pressure steam distribution pipeline. It was found that the root pass girth weld was severely damaged by flow-accelerated corrosion. Computational fluid dynamics simulation and laser confocal microscopy confirmed that higher fluid velocities resulted in the removal of a protective magnetite layer and the formation of deeper craters. These craters originated at the pre-existing weld toe on the downstream side of the girth weld. In the early stage of erosion-corrosion, material removal caused by water erosion was concentrated at the fluid-facing root weld surface. The weld heat-affected zone was identified as the weakest link for galvanic corrosion due to its high-angle grain boundaries and significant carbide precipitation, leading to depletion of Cr and Mo along the grain boundaries.