Chloride (Cl-) is widely used for depassivation in electrocoagulation using metallic iron (Fe-0-EC). However, its optimization is still on a pragmatic approach and thus of limited success. This study proposed a novel approach based on a systematic investigation of Cl--induced pitting dissolution behavior of Fe with the evolution of pH and Cr(VI) (0 <= Cr(VI) (mg/L) <= 520) in Fe-0-EC. The depassivation behavior was characterized by potentiodynamic polarization, cyclic voltammograms (CVs), galvanostatic measurements, and scanning electron microscope (SEM) analysis. The proposed approach to optimize Cl- for depassivation was as follows: (1) Build a database of minimum Cl- concentrations for pitting dissolution (MCPD) as a function of pH and Cr(VI) in passivation region III (MCD =f (pH, Cr(VI))); (2) Record the curve function (g) of pH and Cr(VI) evolution (g ={(pH(i), Cr(VI)(i)), ..., (pH(f), Cr(VI)(f))}) without passivation in the Fe-0-EC process; (3) Obtain the optimal concentration of Cl- for depassivation (OCD) by selecting the maximum MCDs by combining the curve (g) and the database of MCPD (OCD = Max{MCPD1, MCPD2, ... MCPDn}). This method was more accurate and stable compared with RSM. (C) 2015 Elsevier B.V. All rights reserved.