In this article the long term stability of Co-Sm thin films, both with no overcoat and covered with silicon nitride layers of varying thickness, has been investigated by means of accelerated corrosion tests in vacuum, air, and in a humidity chamber. Bare Co-Sm films exhibit an accelerated decrease in magnetization with respect to reference Co-Pt films, dependent in part on the effective surface area of the magnetic film. X-ray photoelectron spectroscopy depth profiling and corrosion tests demonstrate that silicon nitride layers of about 8 nm should provide adequate protection against degradation of the magnetic properties of Co-Sm thin films.