Properties of passive films on Cr were studied in situ using the electrochemical quartz crystal microbalance (EQCM). Potential sweeps were performed from the low to the high passive region. From these measurements, it is shown how to calculate a time-resolved fraction of the film growth current and a thickness change. The film thickness change estimated with EQCM matched well with X-ray photoelectron spectroscopy measurements. During the potential sweep experiments, it was observed that the current reached a steady-state value proportional to the scan rate. It is suggested that this proportionality can be used to determine the electric field within the Rim.