Characterizing Dealloying Using a Bi-modal Electromagnetic Acoustic Method

As part of a successful license renewal application, a nuclear power plant must demonstrate to the NRC that is has identified aging risks through analysis and that it will be able to effectively manage or mitigate these risks during the renewal period. Among the many possible aging management challenges that the U.S. nuclear fleet faces, graphitic corrosion of gray cast iron and dealuminification of Aluminum Bronze have been specifically identified as integrity threats and many plants have committed to inspecting susceptible components in their License Renewal applications. While Aluminum Bronze has been used on a very limited basis in U.S. nuclear power plants, the use of gray cast iron is widespread with uses in fire protection systems and drain line piping, among other things. The application of traditional Nondestructive Evaluation (NDE) techniques to these materials is complicated as the thickness of the affected material does not necessarily change as de-alloying progresses, though significant changes in the material properties, strength, and leak-tightness may be occurring. For these reasons, new innovative, yet practical, NDE approaches are needed that can quantitatively characterize de- alloying without a priori knowledge of material properties and thickness. Structural Integrity Associates, Inc. (SI) has previously demonstrated the feasibility of a bi-modal ultrasonic technique for the characterization of dealuminification of Aluminum Bronze. The developed technique was able to successfully characterize the magnitude of localized dealloying in the material without knowledge of the material wave velocities or thickness by examining the ratio of time-of-flight measurements obtained with both shear- and longitudinal-polarized ultrasonic waves. SI is presently proposing to extend this work through the development of a Bi-Modal Electromagnetic Acoustic Method (BEAM™) that will use an electromagnetic acoustic transducer (EMAT) combined with a piezoelectric transducer to make the bi-modal ultrasonic measurements simultaneously and that will subsequently process the data to obtain a quantitative characterization of dealloying in gray cast iron and copper alloys. Although this technology is being developed for nuclear power plant piping, it has broad ranging applications in other industries utilizing alloyed metals which are subject to dealloying and in structures other than piping.