Electropolishing of 304 stainless steel: Surface roughness control using experimental design strategies and a summarized electropolishing model

The electropolishing (EP) settings for obtaining 304 stainless steel (304SS) with variable surface roughness factors (Ra) in a mixture containing phosphoric acid, sulfuric acid, and glycerol were achieved using experimental design strategies, including the fractional factorial design (FFD) coupled with the response surface methodology (RSM) and the path of the steepest ascent. The bath temperature and polishing time were found to be the strongest factors affecting Ra of 304SS in the FFD study. The glycerol content and polishing current density involved strong interactions with the bath temperature although both factors only showed marginal significant effects. The effects of bath temperature and polishing time on Ra of 304SS were examined in the study of the steepest ascent path for controlling the surface roughness. The results showed that Ra of 304SS is decreased with decreasing the bath temperature and polishing time but increased when the temperature was lower than 20 °C. A summarized model with a new idea on the molecular interactions in the polishing bath is proposed to elucidate the phenomena found in this work. The morphologies and Ra of 304SS electropolished under various conditions were examined by means of the SEM photographs and AFM analyses.