Fundamental interfacial chemical processes, structures, and compositions underlying corrosion reactions

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Many of the metals that we use to construct such diverse items as nuclear reactors, coins, aircraft, orthopaedic implants, and pop cans are so unstable that, without the protection of a 3-5 nanometres-thick layer of oxide (a “passive film”), they would quickly react with their surroundings and revert to the chemical forms in which they existed before we mined and refined them.This project uses advanced surface analysis and electrochemical techniques capable of probing such thin layers to investigate how passive films form on titanium, zirconium, and nickel-based superalloys, their physical structure, their composition (which varies from metal to metal), and how they degrade or react with their surroundings. The goals are to learn how to form passive oxide films that resist degradation and how to make passive films that repair themselves if they become damaged.Another significant challenge for metals is localized corrosion, in which the damage, rather than being distributed across the metal surface, is focused in a small area, resulting in deep penetration. An insidious form of localized corrosion is crevice corrosion, which occurs largely unseen in cracks and hidden areas in joints and under coatings where the narrow geometry restricts movement of chemical species into or out of the crevice. A build-up of corrosive chemical species occurs within the crevice, leading to accelerated attack.This project develops innovative electrochemical methods to analyze and map the distribution of corrosive species and resultant corrosion damage inside crevices, which we will relate to the geometry of the crevice and the composition of the materials from which it is formed. We will also use X-ray imaging to “see” the internal structure inside a crevice as it corrodes. The knowledge gained will be used to help understand, avoid, and mitigate the effects of crevice corrosion and to design more resistant alloys.Finally, we will explore the role of hydrogen in corrosion reactions. Hydrogen is abundant in corrosion, as it is produced by the reaction of a metal with water or acid, but difficult to detect in or on metal surfaces without destroying the metal being analyzed. We will use beams of neutrons or electrons to determine concentration of hydrogen on metal surfaces during or following corrosion reactions to investigate the possibility that surface hydrogen is responsible for two curious phenomena in corrosion called the “negative difference effect” (which accelerates corrosion of magnesium alloys) and “cathodic modification” (which protects titanium alloys from corrosion). We will also measure amounts of hydrogen-driven corrosion occurring hidden within crevices, unaccounted for in damage predictions based only on oxygen-driven corrosion occurring outside the crevice. Preliminary measurements on titanium and nickel-based alloys suggest that hydrogen reactions multiply corrosion damage by 60-500% or more.