EFFECT OF ELECTROLYTIC TREATMENT OF BALLAST WATER ON CORROSION BEHAVIOR OF TANK STEEL

Abstract

In order to regulate discharges of the ships' ballast water and reduce the risk of non-native species introduced from the ballast water,the international maritime organization(IMO) has developed international legislation,the ships' ballast water should be treated before discharge, and electrolysis has been developed as a promising method for ballast water treatment.However, the oxidative species such as Cl_2,HClO and NaClO generated during electrolytic treatment may not only kill the non-native aquatic species but also accelerate the corrosion of the ballast tank steel. In this paper,the effect of the total residual chlorine(TRC) concentration in 3.5%NaCl solution on the corrosion of Q235 steel was studied using mainly the open-circuit potential-time measurement (E_(OC)-t) and electrochemical impedance spectroscopy(EIS).Four TRC concentrations,0,5,10 and 20 mg/L,which are around the concentration proved to be effective for removal of non-native aquatic species,were tested.Both the E_(OC)-t and EIS results suggested that the corrosion of the Q235 steel in 3.5%NaCl solution with or without TRC can be divided into three stages:rapid oxidation of steel, coverage of metal surface by oxide film,and breakdown of the oxide film and initiation of corrosion. The negative shift of E_(OC) and reduction of charge-transfer resistance(R_(ct)) of the steel immersed in the solutions containing low TRC concentrations such as 5 and 10 mg/L demonstrated that the corrosion of steel is slightly enhanced.The enhancement of corrosion is due to the increase of the depolarization concentration present in the solutions.But for solution with higher TRC concentration such as 20 mg/L,both the E_(OC) and R_(ct) increased,suggesting retardation of the corrosion process.The retardation of corrosion can be attributed to the passivation of the steel surface due to the strong oxidation of the active species and the formation of compact corrosion product that covers the corrosion sites.This means that increasing TRC concentration to 20 mg/L does not accelerate the corrosion of the tank steel. Therefore,higher TRC concentration for on-board electrolytic treatment is allowable,which is beneficial for shortening the treating process and improving the removal efficiency.