The origin of high surface spin-flip rate in metallic nonlocal spin valves has been explored by tracking the evolution of the spin signals during the natural oxidation of the devices in air. The temperature dependence of spin signals evolves from a non-monotonic dependence with a maximum at ∼60 K for as-fabricated devices to a monotonic increase with decreasing temperatures for fully oxidized devices. The experimental results suggest that the high surface spin-flip rate originates from magnetic impurities near the side surfaces of the nonmagnetic channel and can be reduced by oxidation.