Effect of micro-phase electrochemical activity on the initial corrosion dynamics of weathering steel

The electrochemical behaviors of micro-phases in three types of weathering steel (Q355NHD, Q450NQR1, and Q460q) were characterized by capillary microelectrode techniques (micro-phase potentiodynamic polarization, micro-phase electrochemical impedance spectroscopy, and micro-phase linear polarization) and the scanning vibrating electrode technique. The initial corrosion morphologies of these steels were observed and wet/dry cyclic corrosion tests were conducted. The effects of micro-phase electrochemical activity on the initial corrosion dynamics are analyzed. The results show that the ferrite and pearlite phases can couple as a micro-galvanic corrosion cell, acting as a cathode and anode, respectively, to facilitate preferential corrosion. In contrast, the corrosion behavior of the bainite phase is mainly uniform. At the initial stage of corrosion, the corrosion rates of Q355NHD and Q450NQR1 steels (which are dominated by ferrite and pearlite multiphase microstructures) were significantly higher than that of Q460q (containing granular bainite). Moreover, the initial corrosion rate increases with pearlite content.