Electrochemical impedance spectroscopy (EIS) has some significant drawbacks with regard to in situ measurements of the corrosion rate beneath a polymer coating. The vibrating sample magnetometer (VSM) can be used to measure the saturation magnetic moment (μ5) of ferromagnetic materials, thus indicating their mass (volume). A novel approach is introduced in this work to monitor the corrosion of bare and acrylic-coated cobalt in situ and in real time by coupling EIS and VSM. This combination is found to be powerful when the corrosion products are not ferromagnetic. For the first time, the corrosion rate beneath a polymer coating is measured accurately in situ. Furthermore, it is suggested that the VSM can be used to enhance the interpretation of EIS data.