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Successful in situ spatiotemporal tracking of corrosion processes occurring at heterogeneous Mg alloy microstructures was achieved through tandem analyses involving electron and electrochemical microscopies. Through cross-correlation of scanning electron microscopy and scanning electrochemical microscopy images and subsequent analytical transmission electron microscopy, the morphology and chemical composition of microstructural components on the surface of a sand-cast AM50 Mg alloy were related to their respective local evolution of H 2 with micron scale resolution prior to, during and post corrosion. The results confirm that the preferential water reduction sites in the initial stages of corrosion are the Al 8 Mn 5 intermetallics while a β-Mg 17 Al 12 precipitate contaminated with Ni becomes cathodically active at a later stage of corrosion. This approach demonstrates the power of correlative approaches to probe and understand local electrochemical phenomena. Owing to their light weight, high strength-to-weight ratio, good castability and high damping capacity, Mg alloys are ideal candidates for automotive structural components.1-5 The compromise of formability, room temperature ductility and price represent major challenges in commercializing these materials. Importantly, Mg alloys also suffer from poor corrosion resistance in aqueous environments 6-8 and due to their positions in the electrochemical activity series, are highly susceptible to accelerated corrosion rates when in galvanic contact with a more noble material.9 At open circuit, the overall galvanic current is null due to a balance between the anodic and cathodic current densities, while on a microscopic scale, a difference in electrochemical potential is obtained. In Mg alloys, galvanic coupling between microstructural components of the alloy (commonly referred to as microgalvanic coupling) stems from an inherent electrochemical potential difference between the Mg matrix and its secondary phases, [10][11][12] and can be tracked using local scanning probe methods.Local scanning probe techniques such as several modes of scanning electrochemical microscopy (SECM), [13][14][15][16][17][18][19] scanning Kelvin probe force microscopy (SKPFM), 20-22 the scanning vibrating electrode technique (SVET) [23][24][25] and local electrochemical impedance spectroscopy (LEIS) 26 have been used to probe the heterogeneous surfaces of Mg alloys and to assess the electrochemical behavior of the different microstructural constituents responsible for microgalvanic corrosion. Among these techniques, SECM has the ability to quantify electrochemical fluxes in situ with high lateral resolution using a microelectrode (ME). 27 This technique has been successfully applied in corrosion research to provide in situ analyses of pitting corrosion at oxide films, [28][29][30] to assess lateral variations in corrosion kinetics, 31 and to detect defects within coated metal samples. 32The feedback mode of SECM has been employed to probe corroding AM60 15 and AZ31 17 Mg alloys. The incre...

The electrochemical behaviour of two Al‐Mn materials (Al‐ 5.5 at % Mn and Al‐ 13.5 at % Mn) has been studied in 0.275 M NaCl and 0.138 M MgCl2 solutions to simulate the cathodic environment of Al‐Mn particles during the corrosion of a Mg alloy. Upon polarization in NaCl solution to a potential in the range expected on a corroding Mg alloy, the Al‐5.5 at % Mn alloy proved unstable undergoing de‐alloying (loss of Al) and delamination of layers of the Al(OH)3 formed. This leads to a steady increase H2O reduction current. When polarized in MgCl2 solution the surface was partially protected from de‐alloying and the current for H2O reduction suppressed by the deposition of Mg(OH)2. The Al‐13.5 at % Mn alloy was considerably more stable when cathodically polarized. This increased stability was attributed to the higher density of Mn‐enriched areas in the alloy surface. This simulation of the microgalvanic cathodic behaviour of Al‐Mn intermetallic particles confirms that the appearance of corrosion product domes on the Al‐Mn intermetallic particles during the corrosion of Mg alloys as an indication of their cathodic behaviour and that Al‐Mn intermetallic particles are efficient, yet unstable cathodes.

The corrosion behavior of Mg alloy ZEK100 was investigated in water and in chloride-containing environments. The alloy is composed of an α-Mg matrix and a widespread dispersion of a Mg-Zn-Nd phase and Zr-rich particles. Potentiodynamic polarization measurements showed the corrosion resistance of the ZEK100 alloy improved with decreasing chloride content. Intermittent immersions in 0.16 wt% sodium chloride (NaCl) showed corrosion occurred preferentially by tracking along the surface, interlinking the secondary phase particles. Some of the particles acted as strong cathodes microgalvanically coupled to the α-Mg matrix. Of the three main types of secondary phase identified, i.e., Mg-Zn-Nd, Zr, and Fe-containing Zr particles, the latter were the dominant active cathodes. This was demonstrated by experiments in deionized water (18.2 MΩ cm) where the tracking of corrosion across the surface was absent and active cathodes could be identified by a small surrounding zone of corroded α-Mg and an accumulated dome of corrosion product over the top of the active particle.

The microstructures of copper (Cu) materials were investigated by electron backscatter diffraction, showing that electrodeposited (ED) Cu has a homogenous polycrystalline microstructure, while cold spray (CS) Cu has a heterogeneous microstructure with varying grain size, pores, and interfacial splat regions. The corrosion rate was examined by corrosion potential (ECORR) and polarization resistance (Rp) measurements on Cu specimens in solutions containing 0.1 M NaCl + 1 × 10−3 M Na2S. Although the as sprayed CS-Cu was the least corrosion resistant, the corrosion rate of the heat-treated CS-Cu was similar to that of the ED-Cu and wrought Cu (SKB-Cu). Electrochemical behaviours of Cu materials were investigated by either a potentiodynamic scan or a potentiostatic polarization at a more positive potential (E > ECORR) for various experiment durations up to 4 h, showing that the heat-treated CS-Cu, SKB-Cu and ED-Cu exhibited very similar behaviour while the as sprayed CS-Cu showed erratic behavior consistent with a variable surface reactivity. Nanoscale scanning transmission electron microscopy analysis has been performed at the cross-section of an anodically-oxidized CS-Cu specimen, revealing a two-layer film structure, mostly composed of Cu sulfide, with a minor diffusion of sulfur in the local area of an interfacial splat boundary tip.