# An Electrolyte Life Indicator for Plasma Electrolytic Polishing Optimization
###### Abstract  
This work shows that electrolyte current-density as an indicator can assist in the optimized timing of the addition of the electrolyte to plasma electrolytic polishing (PEP) to keep it active and in operation. In this experiment, 2 wt% ammonium sulfate was used as an electrolyte to polish 1 cm \(\times\) 1 cm stainless steel SUS304. The hot-bath heating method was successfully used to heat it from 60 to 90 \({}^{\circ}\)C, followed by suction filtration. The cathode was fixed at the beaker edge in the electrolyte and the input voltage was 340 volts. Once the gas-phase layer formed stably around the workpiece, the plasma went through the electrolyte to polish the workpiece surface. Then, the anode was slowly immersed into the electrolyte and the current-density measured. It was found that based on the current-density-temperature curve, for the timing of the addition of the electrolyte, the current-density difference could be used to decide whether it needed to be supplemented or not. When the temperature was from 75 to 80 \({}^{\circ}\)C and 85 to 90 \({}^{\circ}\)C, it was found that the 2 wt% ammonium sulfate solution should be supplemented. The result showed that the electrolyte life indicator, using the current-density, is a feasible method of practical technology for PEP. plasma electrolytic polishing; electrolyte life indicators; metal polishing +
Footnote †: journal: Applied Sciences  
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Footnote 1: institutetext: Graduate Institute of Precision Engineering, National Chung Hsing University, Taichung City 40227, Taiwan 1  
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Footnote 2: institutetext: Instituto de Ciencias, Universidad de La Laguna, La  
In the past, traditional polishing was extensively applied to medical instruments [6], and electronic devices and in aerospace. The purpose of traditional polishing is acquiring a low-roughness and high-gloss surface. Traditional polishing mainly includes mechanical polishing, chemical polishing and electrochemical polishing. Mechanical polishing involves making the surface smooth through plastic deformation; the disadvantages of this method are that it needs more workers to operate it, and it cannot be promised that each workpiece surface will be of the same quality. Chemical polishing can remove the workpiece surface through a chemical reaction. Electrochemical polishing is similar to chemical polishing in that the workpiece is immersed in a chemical solution, but it is processed under a current. The metal workpiece is is an anode, while the electric tool is a cathode. Applying a direct current (DC), the workpiece anode is dissolved into metallic ions and removed atom by atom [7]. However, a high concentration of a strong acid or a strong base is used as an electrolyte. During the reaction, harmful gases and liquids will cause damage to workers and their environment [8]. Thus, it will produce waste liquid after the reaction. Unlike the strong acid and caustic alkali solutions used in chemical and electrochemical polishing, the electrolyte used in the plasma electrolytic polishing process is a low-concentration aqueous salt solution. The processing parameters used in the PEP process, such as electrolyte composition, temperature and high voltage applied to the metal alloys, result in the polishing effect of brought about by surface-material removal. The process is environmentally friendly and follows the safety requirements. Plasma electrolytic polishing applications have already been successfully used in steel [9; 10; 11; 12; 13], aluminum [10; 14], copper [15], titanium and their alloys [5]. However, during the PEP process, polishing ability will be decreased because the electrolyte is evaporated at high temperatures and participates in electrochemical action. Thus, polishing ability will be affected by factors such as conductivity, the generation of metal impurities during the polishing process, electrolyte concentrations and pH value changes. Therefore, its status must be monitored and maintained or replaced in the process.