Corona charging of a free-standing polymer film can produce a quasi-permanent potential difference across the film thickness, while the absolute amplitude of the surface voltage may be highly sensitive to the free charges. To precisely control the voltage distribution, we investigated the flow electrification technology by exposing corona-charged polyethylene terephthalate films to sodium salt solutions. The surface voltage and the free-charge density were adjusted by the salt concentration, the anion size, and the flow rate. The dipolar component of electric potential remained unchanged. This result has significant scientific interest and technological importance to surface treatment, filtration, energy harvesting, among others.