Scandium (Sc) thin films are promising candidates for advanced electronic applications due to their excellent electrical and thermal properties. However, the reliability is challenged by oxidation-induced instability. This study identifies and decouples two distinct oxidation mechanisms: transient oxidation during the deposition phase, driven by residual oxygen at plasma ignition, and ambient-induced oxidation postdeposition. Sequential wafer analysis reveals a pronounced first-wafer effect, with elevated sheet resistance (Rs) and nonuniformity (NU%) in early wafers. Grazing incidence x-ray diffraction and long-term statistical process control confirm surface oxidation and its temporal evolution. An in situ 5 nm Ru capping layer effectively suppresses ambient oxidation, stabilizing Rs, though NU% remains high due to deposition variability. These findings highlight the importance of both surface passivation and precise process control to ensure the electrical stability of Sc thin films.