Radio-frequency (rf) superconducting devices are verysensitive to corrosion because of the small penetration depthsλ<300 nm of the rfmagnetic fields, Hrf. Qualification of high temperature superconductor (HTS) filters for launchinginto outer space requires resistance against corrosion for three weeks at40 °C and 93% relative humidity. A protective layer is essential to shieldthe HTS against these severe atmospheric conditions and to avoiddegradation of HTS films. Such degradation has been reduced by a thinBaF2/Y F3 protective layerproduced by soaking the YBa2Cu3O7−δ (YBCO) films in HF solution. Results on the surface resistanceRs(77 K,Hrf) of YBCO films of different quality before and after HF soaking, as well as after exposure toa highly humid atmosphere, are presented. They are explained by weak link corrosion bydeoxygenation, easing the permeation of Josephson fluxons. These nonlinear effects, edgeenhanced by strips and holes, ultimately limit the applications of strip line orTE011 modes, being quantitatively related to Josephson fluxon hysteresis losses causing localheating. These results on YBCO films shed new light on HTS material science andcorrosion, into thermal runaway of HTS and nonlinear surface impedances of HTS films,and their dependences on the rf mode.