Investigating electrochemical corrosion at Mg alloy-steel joint interface using scanning electrochemical cell impedance microscopy (SECCIM)

Abstract

Developing strategies to prevent corrosion at the interface of dissimilar metal alloys is challenging because of the presence of heterogenous distribution of galvanic couples and microstructural features that significantly change the corrosion rate. Devising strategies to mitigate this interfacial corrosion requires quantitative and correlative understanding of its surface electrochemical reaction. In this work, scanning electrochemical cell impedance microscopy (SECCIM) was employed to study location-specific corrosion in the interfacial region of dissimilar alloys, such as AZ31 (magnesium alloy) and DP590 (steel) welded using the Friction-stir Assisted Scribe Technique (FAST) processes. Herein, SECCM and SECCIM were used to perform correlative mapping of the local electrochemical impedance spectroscopic and potentiodynamic polarization to measure the effect of electronic and microstructural changes in the welded interfacial region on corrosion kinetics. Microstructural characterization including scanning electron microscopy and electron backscatter diffraction was performed to correlate changes in microstructural features and chemistry with the corresponding electronic properties that affect corrosion behavior. The variations in corrosion potential, corrosion current density, and electrochemical impedance spectroscopy behavior across the interface provide deeper insights on the interfacial region-which is chemically and microstructurally distinct from both bare AZ31 and DP590 that can help prevent corrosion in dissimilar metal structures.

Figure 1

(a) SEM microstructure of FSW welded AZ31-DP590 steel. (b) EBSD map in interfacial region and (c) EDS maps of region used in area scan measurement.

Figure 2

(a) Cross-sectional image of Mg-steel joint sample before SECCM/SECCIM experiment (dotted line indicates interface) and (b) image after SECCIM analysis; yellow circled areas are measurement “footprints” to indicate where the SECCM/SECCIM measurements were performed. (c) Schematic of SECCM/SECCIM setup used in this study. (d) Tafel plots for the corresponding points.

Figure 3

(a) Optical microscope image showing the positioning of the SECCIM electrode over the DP590 substrate, interface, and AZ31 regions as marked; (b) E_corr and (c) OCP x–y images of a 70 µm × 500 µm area are shown for the interfacial region. Dotted lines represent the interface between DP590 and AZ31.

Figure 4

AC impedance (Z) measurements for Points 1, 2, 3, 6, and 7. Bode plots representing (a) real Z vs. frequency, (b) imaginary Z vs. frequency, (c) absolute Z vs. frequency, and (d) phase Z vs. frequency are shown to corroborate the Nyquist plots shown in (e) with the equivalent circuit. The frequencies used for EIS imaging are shown with dotted lines. Panel (f) shows the DRT results that can be correlated to specific electron transfer surface events in the Nyquist plots in (e).

Figure 5

Area scan of EIS at four selected frequencies (10 kHz, 1 kHz, 10 Hz, and 1 Hz) measured along with the OCP and Tafel scans presented in Fig. 3. Dotted lines represent the interface between DP590 and AZ31.