When 10:00 AM - 12:00 PM Aug 23, 2017
Where 1690 Beyster
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Microstructural Aspects of Localized Corrosion Behavior of Mg Alloys


Peng-Wei Chu
Thesis Defense

Combining high specific strength and unique electrochemical properties, magnesium (Mg) alloys are promising lightweight materials for various applications from automotive, consumer electronics, biomedical body implant, to battery electrodes. However, the fundamental mechanisms responsible for the unique localized corrosion behavior of Mg alloys, the associated abnormal hydrogen evolution response, and the relationships between corrosion behavior and alloy microstructure are still unsolved.

In this thesis, multiscale site-specific microstructure characterization techniques, including in situ optical microscopy, scanning electron microscopy with focused ion beam milling, and transmission electron microscopy, combined with electrochemical analysis and hydrogen evolution rate monitoring, were performed on pure Mg and selected Mg alloys under free corrosion and anodic polarization, revealing key new information on the propagation mode of localized corrosion and the role of alloy microstructures.

Uniform surface corrosion film on Mg alloys immersed in NaCl solution consisted a bi-layered structure, with a porous Mg(OH)2 outer layer on top of a MgO inner layer. Presence of fine scale precipitates in Mg alloys interacted with the corrosion reaction front, reducing the corrosion rate and surface corrosion film thickness. Protruding hemispherical dome-like corrosion products, accompanied by growing hydrogen bubbles, formed on top of the impurity particles in Mg alloys by deposition of Mg(OH)2 via a microgalvanic effect. Localized corrosion on Mg alloys was found to propagate laterally a few μm inside the alloy and underneath the surface corrosion film, with finger-like features aligned with (0001) Mg basal planes at the localized corrosion/alloy interface. Rising streams of hydrogen bubbles were found to follow the anodic dissolution of Mg and formation of Mg(OH)2 corrosion products at the propagating localized corrosion fronts. Alloying elements segregation to the grain boundaries showed the ability to stop localized corrosion propagation momentarily.

By revealing the microstructure of corrosion features on Mg alloys, a descriptive model was proposed. Relationships between the corrosion behavior and alloy microstructures were also identified. This microscopic information can serve as a guideline for future development of Mg alloys by tailoring the microstructure to achieve proper corrosion responses for applications under different environments.

 

DISSERTATION COMMITTEE

Chair:
Assoc. Prof. Emmanuelle Marquis, MSE

Members:
Prof. J. Wayne Jones, MSE
Prof. Katsuyo Thornton, MSE


Cognate:
Assoc. Prof. Stephen Maldonado