Relationship between microstructure and electrochemical characteristics in steel corrosion

Microstructure modification and electrochemical properties were investigated to estimate the effects of internal chlorides on the steel corrosion in the blended cement systems. According to the test results, slag cement system showed low corrosion rate. Through the microstructural investigation, product layer with the thickness of 20 μm was composed of calcium silicate hydrate phases mixed with Fe oxides near the steel-matrix interface of slag cement system. So its product layer-matrix interface was clearly shown. But rough steel-matrix interface of ordinary Portland cement system was only observed and product layer-matrix interface could not be seen. The electrochemical ac impedance spectroscopy data showed three distinct arcs from lowest (mHz) frequency to highest (MHz) frequency due to oxidized product layer, interfacial reaction (double layer capacitance) and bulk cement matrix. For OPC samples without chloride, second arc of impedance Nyquist plot could not be clearly observed. But for other samples with chloride, distinct secondary circle that was considered to an intermediate resistance (R_f) associated with a oxidized product layer at the steel-matrix interface was shown. Studies in a number of samples with different corrosion kinetics showed that, at probable corrosion times, the corrosion rate was highly correlated with the oxidized surface roughness and tortuosity and also with the electrochemical deviation.