Research On Electrochemical Corrosion Behaviour Of The Rusted Carbon Steel In Seawater

Rust/metal structure is one of the multiphase and multiple interface complex systems. The corrosion under rust is the uppermost and longest form of metallic corrosion evolution. It is difficult to accurately determine the electrochemical parameters because the existence of rust complicates the electrochemical corrosion process. Corrosion rate measured by traditional electrochemical methods deviates from the weight-loss measurement result and the interface capacitance also deviates from reasonable value. Reliable measurement and analysis for rusted steel become one of the important tasks in metallic corrosion research.In this paper, weight-loss measurement and several kinds of electrochemical methods (including polarization curves (PC), linear polarization resistance measurement (LPR), electrochemical impedance spectra technique (EIS) and potential step (PS)) were employed to study the long-term corrosion behaviour of carbon steel in immobile and flowing seawater. The results show that the electrochemical and weight-loss measurement estimations have the same degressive trends and values during the first 8 weeks'immersion period, while the electrochemical measurement result turns to increase after 8 weeks'immersion in immobile seawater. The longer the immersion time, the bigger the deviation. In flowing seawater, electrochemical measurement result is opposite to that determined by weight-loss measurement from the beginning and the deviation becomes larger with increasing of immersion time.In order to explore the reasons for the deviations between electrochemical and weight-loss measurement results, X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), environmental scanning electron microscope (ESEM), nitrogen adsorption (BET), etc were used to study the morphologies, structures, compositions and physicochemical properties of rust layers. Analysis indicates that a dense and black inner rust layer, which containsβ-FeOOH with high electrochemical activity, will form when the potential in the metal/rust interface achieve a noble value. In electrochemical polarization tests,β-FeOOH can participate in cathodic reduction reaction, which leads to increasing the cathodic reaction rate and overestimating the corrosion rate.In order to further confirm that the formation ofβ-FeOOH was related to the potential, scanning micro-electrode technique (SMET) and wire beam electrode (WBE) were used to study the distribution characteristics of cathode and anode regions for rusted carbon steel in seawater. The relationship between the compositions of corrosion products and potential distribution was illustrated by the analysis of corrosion products. At the same time, electrochemical polarization method was applied to control the formation conditions of rust layer. The results confirm thatβ-FeOOH is indeed produced in the noble potential.In order to study the corrosion behaviour of rusted steel by electrochemical measurement, cathodic reaction rate, that is rust reduction rate, was measured in deaerated conditions. Then deduct it from total cathodic reaction rate, the rest is oxygen reduction rate. The results show that the corrosion rate modified by this method is consistent with the weight-loss measurement result and confirm that it is a feasible and effective compensatory method to determine corrosion rate of rusted steel by electrochemical measurement.