A method and a system for monitoring a mechanical device for internal corrosion are provided. An exemplary method includes placing a sampling thermoelectric polymer sheet (TEPS) on an external surface of the mechanical device to be monitored for internal corrosion, and placing a reference TEPS on an external surface of the mechanical device not susceptible to internal corrosion. A current from the sampling TEPS is measured, and a current from the reference TEPS is measured. Potential internal corrosion is identified from changes between the current from the sampling TEPS and the current from the reference TEPS.