Surface Corona Aging of Epoxy Resin under Bipolar Square Wave Field with Varied Frequencies

Corona aging characteristics of epoxy resin are investigated under bipolar square wave field with varied frequencies and compared with that of sinusoidal field at 50 Hz and 25 kHz. Surface morphology observed by optical microscope demonstrates that erosion area of bipolar square wave field is severer than that of sinusoidal field with corrosion area of about 1.7 times after 1 min exposure at 4 kV-25 kHz. Corrosion trails break remarkably in center beneath needle tip and turn sparser at edge in shape of “branch” under both bipolar square wave with high frequency and sinusoidal field. Furthermore, surface roughness measured by surface profilometer shows that contour arithmetic mean deviation ( Ra) increases from 0.08 μm at 50 Hz to 2.69 μm at 10 kHz after 1 h exposure under bipolar square wave field. And corrosion depth is decreased under sinusoidal field at same voltage. Corona discharge intensity obtained by ultraviolet imager (UV) is more intense under bipolar square wave with about 2 times of that under sinusoidal field and increases with frequency, corresponding to more severe corrosion degradation and deeper corrosion depth. Drastic collision ionization produces increased electrons and positive ions, leading to more anti-excitation and charge recombination, and thus more photons during corona discharge. Rigorous discharge and higher discharge energy caused by rapid polarity reversal of bipolar square wave can accelerate corrosion rate.