The sludge in Zn-Ni electroplating on steel is a major source of various surface defects and low productivity. We investigated the electrolyte and sludge with ICP. EPMA, TEM, and XRD. Generally the sludge was formed of Fe-rich yellow brown phase and Si rich dark gray one and had various colors. We notified that the dissolved impurity ion(Fe, Si and Al) played an important role in the sludge formation and suggested an effective removal process of those impurity ions by controlling the pH of the electrolyte before and after filtering process. Based on the results, it was possible to propose an electrolyte cleaning system for electro-galvanizing line of POSCO.