The paper deals with the degradation phenomena produced on polypropylene films by corona discharges with streamers in air at different humidity levels. The attention is focused on the formation of crystals on the surface of these films. In the case of the thin films under consideration, these crystals appear in about the same time as the beginning of bulk degradation processes and as a precursor signal to the dielectric breakdown of the films monitored in this study by a succession of recurrent streamers. Correlations between the chemical and morphological processes involved in the surface evolution were obtained from coupled diagnostics performed by means of infrared Fourier transform microspectroscopy and electron microscopy. As a result of these correlations, hypotheses on the degradation mechanisms of polypropylene films submitted to corona discharge streamers are set, and in particular a chemical reaction scheme is proposed to explain the nodule to crystal transformation phenomenon observed in the presence of a sufficient amount of water vapour in the gas.