Investigation of the Electrode Erosion in Pin-to-Liquid Discharges and Its Influence on Reactive Oxygen and Nitrogen Species in Plasma-Activated Water

Although the erosion of high-voltage electrodes was extensively studied in in-liquid electrical discharges, to the best of our knowledge, the erosion produced by discharges generated above water has been barely explored. This work studies the erosion of three pin electrode materials (hafnium, copper, stainless steel) in two gas atmospheres (oxygen, air). They are powered by repetitive high-voltage nanosecond pulses, producing pulsed streamer discharges above water. The electrode material does not affect the energy deposited per pulse. The surfaces of all three electrodes erode, releasing metal particles into the water. Stainless steel is the material with least erosion, in both gas atmospheres. Overall, copper in air shows the highest erosion. We also examine how the metals released into the water affect three long-lived reactive oxygen and nitrogen species (RONS), H₂O₂, NO₂ ⁻ and NO₃ ⁻, during four weeks post-discharge. After treatment with air plasma, NO₂ ⁻ and NO₃ ⁻ are measured in the treated water, but H₂O₂ is not detected. NO₂ ⁻ is almost completely converted into NO₃ ⁻ after two weeks. H₂O₂ is measured for samples prepared with an oxygen plasma. Neither the RONS nor the conductivity of plasma-treated water are significantly affected by the use of different electrode materials.