Abstracts Chemistry

Corrosion Inhibition of Magnesium Alloys and Influence ofAtmospheric Carbon Dioxide

by Daniel Thomas Kaminski

The widespread use of magnesium alloys as an engineering material is yet to be realized due, in part, to poor corrosion resistance in condensing moisture environments. Application of corrosion inhibitor compounds by way of coatings-based inhibitor systems is one corrosion mitigation method that can be implemented to address this concern.This work comprises a fundamental study of corrosion inhibitors for magnesium alloys AZ31 and WE43. A broad array of corrosion inhibitors were examined at 1-100 mM concentration including transition metal oxyanions, group IVA through group VIIA anions, as well as rare earth cations, and selected organic compounds.Due to potent and rapid surface alkalization of magnesium alloys and the potential application of inhibitors as functional pigment additives in organic coatings, it is of concern whether atmospheric carbon dioxide absorbed within alkaline electrolytes could interact with corrosion inhibitors in a favorable, negative, or neutral way. Reactive-transport modeling of carbon dioxide gas absorption in such electrolytes was conducted with respect to experimental conditions. From this, it was understood that equilibrium concentration of dissolved inorganic carbon at experimentally observed pH values can be realized in thin electrolyte layers upon time duration of several hours to days. Slow absorption of carbon dioxide gas into alkaline aqueous solution of experimentally observed pH is primarily due to the rate limiting kinetics of carbon dioxide hydroxylation at a given gas-liquid interface surface area and electrolyte volume.To help develop an understanding of the interaction of corrosion inhibitors for AZ31 and WE43 with ambient carbon dioxide, a novel closed-system test cell technique was developed to decouple CO2 from the system. It was observed that ambient levels of CO2 play an important role in the corrosion of AZ31 and WE43 alloys and furthermore influences corrosion inhibitor effectiveness.In the absence of corrosion inhibitors, experimental results showed that ambient carbon dioxide acts to buffer the pH of the electrolyte in contact with either AZ31 or WE43 during corrosion towards a lower pH than would develop if carbon dioxide was absent. For AZ31, results showed that dissolved carbon dioxide affects the corrosion morphology and surface films favorably and can lead to an approximately five times decrease in corrosion rate. For WE43, dissolved ambient levels of CO2 slightly accelerated the corrosion rate and increased general surface corrosion compared to a CO2-free environment.In the presence of corrosion inhibitors, the critical concentration for inhibition by the tested compounds was much better resolved by use of the novel closed-system test cells compared to conventional methods. This proved to be an unanticipated advantage of the novel test cell’s use. Side-by-side comparison of known, effective, corrosion inhibitors in the presence and absence of ambient levels of CO2 indicated mostly neutral or negative interaction of test inhibitors with dissolved CO2, although… Advisors/Committee Members: Buchheit, Rudolph (Advisor).