Effect of Fusion Boundary Microstructure on Flow-Accelerated Corrosion Cracking.

Area of Science:

• Materials Science
• Corrosion Engineering
• Metallurgy

Background:

• Flow-accelerated corrosion (FAC) is a significant degradation mechanism in steam pipe systems.
• FAC commonly targets the heat-affected zone of root-pass welds, compromising structural integrity.

Purpose of the Study:

• To identify the precise initiation sites and underlying mechanisms of FAC in steam pipe girth welds.
• To characterize the microstructural and chemical features at the weld fusion boundary contributing to FAC susceptibility.

Main Methods:

• Detailed microstructural characterization of affected weld regions.
• Analysis of elemental composition and phase transformations along the weld fusion boundary.
• Texture analysis to identify preferred crystallographic orientations.

Main Results:

• The fusion boundary of the root-pass weld was confirmed as the preferential initiation site for FAC.
• Depletion of alloying elements and an increased proportion of Goss {110}<001> textured grains were observed at the fusion boundary.
• These microstructural features suggest a synergistic effect contributing to FAC initiation.

Conclusions:

• The weld fusion boundary represents the weakest link for FAC attack in steam pipe girth welds.
• Chemical segregation and specific crystallographic texture (Schmid factor) at the fusion boundary synergistically promote FAC initiation.
• Understanding these mechanisms is crucial for developing mitigation strategies against FAC in power plants.