Fundamental Investigation of Magnesium Corrosion Using Experiments and Simulation

• Main Author: Banjade, Dila Ram
• Format: Others
• Published: BYU ScholarsArchive 2018
• Subjects: magnesium; corrosion; galvanic; enhanced HER; NDE; Mg kinetics; HER kinetics; Engineering
• Online Access: https://scholarsarchive.byu.edu/etd/7564; https://scholarsarchive.byu.edu/cgi/viewcontent.cgi?article=8564&context=etd

Magnesium (Mg) is the lightest of all structural metals and has tremendous potential for applications in the lightweight industry. However, the corrosion of Mg is a significant barrier towards its wider use. Despite years of research, the fundamental understanding of Mg corrosion still remains short. The enhanced hydrogen evolution reaction (HER) with anodic polarization, the modeling of galvanic corrosion and the impact of hydrogen bubbles for galvanically coupled Mg are important aspects of Mg corrosion that need to be understood before any mitigation measures can be taken. The results presented in this work provide a key step in that direction. In the first part of the project, we explored how the kinetics of reactions involved in the Mg corrosion was influenced by surface changes. A significant difference in Tafel kinetics between a polished Mg surface and a pre-corroded Mg surface was seen. It was also shown that when the concurrent Mg dissolution was accounted for during the cathodic polarization, the absolute value of Tafel slope decreased by a factor of ~2. The enhanced HER on Mg during high anodic dissolution rate was also investigated. The Mg samples were first pre-corroded at different rates until entire Mg surface was corroded. It was found that the surface roughness decreased with an increased rate of corrosion and therefore did not have any influence in enhancing HER at the Mg surface during the anodic dissolution. In order to observe the catalytic effect of anodic dissolution, the potential was immediately dropped to a constant cathodic potential following the pre-corrosion and the current was observed with the time. A peak current, proportional to the prior dissolution rate, was observed. At longer times, the current decayed and converged to similar values irrespective of the prior pre-corrosion rate providing a strong evidence that the enhanced HER is caused due to the catalytic effect provided by the anodic dissolution. Our results provide new mechanistic insights into the current understanding of enhanced HER.In the second part of the project, a numerical simulation was developed to predict the galvanic corrosion rates of Mg coupled to steel. The simulation showed that the kinetics of HER estimated from the cathodic polarization (where significant anodic dissolution rates were absent) underpredicted the experimental HER rates by two orders of magnitude signifying the catalytic effect of anodic dissolution. The simulation allowed us to independently fit the HER kinetics. The simulation also predicted the galvanic current densities and the corrosion potential within 14% error which is a significant improvement of model precision reported previously for galvanic corrosion of Mg corrosion. For the first time, the influence of HER during galvanic corrosion of Mg was reported. Despite substantial hydrogen evolution observed during the experiment, the influence of hydrogen evolution was found to be only 8-9%.