Dealloying of As-Welded Microstructures in Aluminum Bronzes in Essential Cooling Water Service

Dealloying of aluminum-bronze (Al-Brz) is a potential environmentally-induced degradation mechanism in piping systems and components exposed to marine, brackish, or raw water service. Examples of this form of degradation have been observed in ASME Code Class 3 Essential Cooling Water systems. The degradation mechanism is loss of material due to selective leaching (i.e., dealuminification) due to localized corrosion. The dealloying occurs in the eutectoid phase in binary alloys; the eutectoid consists of α + β or α + β/γ2, and is dependent on aluminum content of the alloy, cooling rate during fabrication/welding, and the presence of a local crevice for the corrosion process to concentrate. Cast components are susceptible to general dealloying but unlike them, weldments of wrought pipe have not exhibited the same extent of degradation. The dealloying process leaves micro-voids in the microstructure within the eutectoid. Leakage (weepage) will occur once dealloying has extended through the wall thickness via a network of porous transformed phase. The dealloying process also causes a reduction in mechanical strength and fracture toughness properties. A detailed metallurgical evaluation has been completed to establish a fundamental understanding of the resistance of the pipe welds to dealloying that explains the better performance of the welds versus the castings. Factors affecting the dealloying susceptibility, such as aluminum and iron content, cooling rates during the solidification process, and the resulting microstructural differences were studied and evaluated. It was found that dealloying will not penetrate into the material unless a continuous network of susceptible microstructure is present. The difference in susceptibility between castings and weldments to dealloying was also determined. The findings of this comparative evaluation were significant and are being used in the aging management program to justify continued use of aluminum bronze piping welds for extended service for both above ground and below ground (buried) piping systems.