Local electrochemical characteristics of pure iron under a saline droplet II: Local corrosion kinetics

Abstract The local electrochemical corrosion kinetics of pure iron under a 3.5% NaCl droplet was investigated by newly developed concentric three-electrode array (CTEA) measurements. Electrochemical impedance spectra and polarization curves at different locations under a droplet were measured on the CTEA. A mathematical model for the impedance responses was developed to analyse the electrochemical corrosion processes for different areas under saline droplets with different volumes. The local corrosion kinetics parameters, including anodic faradaic impedance, cathodic faradaic impedance, diffusion resistance, effective capacitance, and local interfacial oxygen concentration, were obtained by the regression and subsequent calculation on EIS data for selected electrode units. There are significant differences in the control mechanisms of local corrosion kinetics between different places under the droplets, resulting from the uneven reaction distribution and the divergent evolution of the corrosion process caused by droplet geometry. Under a droplet with a volume of 25 μL, both the anodic and the cathodic electrode units were determined by oxygen diffusion. Under a droplet with a volume of 110 μL, the central anode units was mainly controlled by the mass transfer of oxygen, and the peripheral cathode unit was determined by both anodic faradaic impedance and oxygen diffusion. The local corrosion process was mass-transfer determined under the central part of 340 μL droplet; whereas, the peripheral part was controlled by both anodic charge transfer and the mass transfer of oxygen.