New insights into magnesium alloys corrosion using scanning probe techniques

Fuel consumption in automobiles is significantly dependent on their overall mass. This challenge is highlighted in the next-generation of electric vehicles, which make use of hefty components (batteries and electric motors) that increase energy consumption. In this context, replacing heavier automobile components with lighter materials would decrease fuel consumption and increase vehicle autonomy. However, replacement of materials in automotive applications is not straightforward since mechanical properties and safety remain important prerequisites.The lightest metal commercially available is magnesium (Mg), which possesses highly desirable mechanical properties, and has already shown some interesting capabilities in automobiles. However, the overall proportion of Mg in car bodies still remains surprisingly low (<0.3% of an entire vehicle). Limitations towards the direct commercial application of Mg in automobiles is in part due to its poor corrosion properties. Although corrosion properties have been improved by alloying Mg with more noble elements, Mg alloys still corrode rapidly. Mg alloy corrosion occurs when electrically connected with a more noble material (galvanic corrosion) but also suffers from self-dissolution due the formation of surface heterogeneities (microgalvanic corrosion). This thesis describes the characterization of both galvanic and microgalvanic corrosion in Mg alloys using scanning probe techniques. Scanning probe techniques are particularly advantageous since they allow for in-situ investigation of corrosion processes. In addition, scanning probe techniques have the capability of probing local chemical fluxes and potential gradients involved in corrosion. When probed selectively, these chemical fluxes and potential gradients can be used to determine the overall rate of corrosion, along with the corrosion mechanism of the investigated sample. To obtain a greater comprehension of the corrosion mechanisms in Mg alloys, electrochemical tools in scanning probe techniques were developed.More specifically, we have studied the galvanic corrosion of a Mg alloy using the scanning vibrating electrode technique (SVET), which allowed estimation of their corrosion rates. Due to the high reactivity of Mg alloys, especially when galvanically controlled, the development of electrochemical and imaging tools was necessary. Major advances in the field of scanning electrochemical microscopy (SECM) were made in order to target a precise chemical flux during the corrosion of Mg alloys. For this purpose, the rapid corrosion rate of Mg alloys was evaluated with a micropipette positioned at a constant distance to extract the surface topography. The rate of material dissolution was then investigated in small volumes of water (50 μL) using a fabricated Mg2+ ion-selective microelectrode. This dissolution rate was correlated withMg alloy production methods (sand, graphite and die casting) and their respective microgalvanic properties. Another approach to investigate Mg alloy microgalvanic corrosion properties was probing the evolution of molecular hydrogen (H2). The sources of H2 produced from immersed Mg alloys were identified in the perspective of helping the optimization of alloy fabrication and improving their corrosion resistance. This in-situ investigation of Mg alloys is also necessary to understand their corrosion mechanism prior to the development of corrosion prevention strategies.In the same perspective, an additional electrochemical tool was developed and used for the quantification of local heterogeneous kinetics in Mg alloys. The successful quantification of local kinetics helped determine the corrosion product thickness and porosity through the development of a numerical predictive tool. Numerical predictive tools to describe Mg alloy galvanic and microgalvanic corrosion are described in this thesis and represent an efficient approach to improve their use in a wide range of automotive and other applications.Le poids des véhicules est directement lié à leur consommation de carburant. Cette problématique demeure importante lorsqu’il est question du développement de la prochaine génération de véhicules électriques nécessitant des apports énergétiques élevés dû à l’utilisation de composantes lourdes. Dans ce contexte, remplacer les composantes lourdes par des matériaux plus légers permettrait de diminuer la consommation d’essence. Toutefois, la sélection de matériaux légers demeure complexe en raison des prérequis liés à leurs propriétés mécaniques ainsi que des raisons de sécurité.Le matériau commercial le plus léger est le magnésium (Mg). Ce dernier possède des propriétés mécaniques attrayantes et avantageuses pour l’industrie automobile. Malgré cela, la proportion de Mg présentement utilisée demeure basse (