Local Hydrogen Fluxes Correlated to Microstructural Features of a Corroding Sand Cast AM50 Magnesium Alloy

Abstract

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 stability of aluminum‐manganese intermetallic phases under the microgalvanic coupling conditions anticipated in magnesium alloys

“…On corroded Mg alloys, some Al‐Mn intermetallic particles have been observed to collect a dome of deposited corrosion product while others remain exposed, a trait also observed on supposed cathodic sites on other Mg alloys . Based on scanning electrochemical microscopy (SECM) measurements, it was suggested that deposition of corrosion products occurred on cathodically active sites with microscopic evidence of corrosion product accumulation at these sites. Recently, we suggested that these corrosion product deposits on Al‐Mn intermetallic particles (Al 8 Mn 5 (Al 1.6 Mn)) in the AM50 alloy are a result of their cathodic behaviour when microgalvanically‐coupled to the α‐Mg matrix 20 , with the cathodic reaction being the reduction of H 2 O to H 2 .…”

Role of Zinc in Enhancing the Corrosion Resistance of Mg-5Ca Alloys

“…10 Therefore, improving the corrosion resistance of magnesium and its alloys is an interesting subject of studies with a goal of attaining the required corrosion resistance and a reduced hydrogenevolution rate, which are key for biomedical applications. 5 Many investigations have focused on improving the corrosion resistance of magnesium alloys via using surface treatments, [11][12] controlling orientation, 13,14 controlling microstructure, [15][16][17] and using alloying elements. [18][19][20][21][22][23][24][25] Among these methods, the use of alloying elements in a solid solution strongly influences the corrosive and mechanical properties of magnesium alloys.…”

The local electrochemical behavior of the AA2098‐T351 and surface preparation effects investigated by scanning electrochemical microscopy

“…(reaction 1). The local flux of H 2 gas from the surfaces of metal alloys has been investigated by SECM in the SG/TC mode …”