Interaction Between Arc Spot Plasma and Steel Surface in Descaling

Cathode spots of low-pressure arc discharge have been used for cleaning oxides on the steel surface. A phenomenon of two-step plasma scanning on the oxidized steel surfaces is observed, where arc spots first move on the entire surface of the sample with strong evaporation and explosion of oxides. Afterward, there is a brief scanning, and a remaining thin oxide layer is removed while a clean surface of the bulk metal appears. It is believed that the two-step plasma scanning is due to the layered structure of oxides, which results in the self-adjusting of cathode spots on the oxidized surface. The corresponding spot plasmas are investigated by a high-speed camera and an oscilloscope. The voltage waveform and plasma spot images exhibit various characteristics during the two-step scanning process. Compared with the case of Ar arc discharge cleaning, the surface hardness of the sample cleaned with N2 arc spot plasma is obviously enhanced, which should be attributed to the formation of Fe4N during descaling.