This paper presents a degradation (wall-thinning) analysis done on the BWR core-spray 3-inch testing pipeline component. Hot-cell examination performed included visual inspection, X-ray diffraction, chemical compositions analysis, metallographic test, wall-thickness measurement, hardness test, and scanning electron microscopic observation. Results obtained showed that the inside surface of the wall-thinned piping component was covered with a wide range of black tubercle-type deposits. Localized pitting corrosion beneath each of the tubercle deposits, which occurred and grew along the stringer texture of the material matrix, was identified. It was concluded that the microbiologically-induced corrosion was responsible for the wall-thinning (and perforation) degradation of the core-spray testing piping component. Tuberculation together with under-deposit pitting (or MIC-pitting) were considered the degradation mechanism.