Industrial gas turbine blades operate under severe thermo-mechanical conditions such as high temperature, corrosion, erosion, and cyclic loading, which make them highly prone to stress concentration and fatigue failure. This study investigates the failure mechanism of the interconnection tube connecting the first-stage blades of a gas turbine used in the oil industry. A combined experimental and numerical approach was employed, including hardness testing, chemical and structural analyses, visual inspection, macro- and micro-fractography, and finite element modeling. Fractographic analysis revealed clear fatigue striations on the fracture surface of the interconnection tube, confirming a fatigue-induced failure initiated at corrosion pits and eroded regions. The finite element results showed severe stress concentration at the fir-tree root region of the turbine disk, coinciding with the actual fracture zone observed in the failed component. Failure of the interconnection tube increased the transmitted stress to adjacent blades and led to an approximately 22% reduction in the fatigue safety factor at the fir-tree root, as determined by Goodman diagram analysis. The strong correlation between numerical and experimental results confirms that stress concentration, combined with corrosion, erosion, and vibration effects, plays a dominant role in accelerating fatigue crack initiation and reducing the structural reliability of the rotor assembly.