A disk drive slider supports a microwave-assisted magnetic recording (MAMR) write head with a spin-torque oscillator (STO) and a separate corrosion monitor (CM). The CM includes a corrosion detection layer formed of the same material as the STO's spacer layer. The CM is coplanar with the STO but laterally spaced from the STO. The corrosion detection layer has an edge at the ABS so as to be exposed to the same atmospheric conditions as the STO's spacer layer. Electrical leads are located at the ends of the CM and are electrically connected to pads on the upper surface of the slider. Electrical resistance of the CM is measured by detection of current in the plane of the corrosion detection layer.