Corrosion and dissolution in confinement

1. Realtime in-situ studies of actively corroding crevices and inhibition of crevice corrosion

Crevice corrosion (CC) of metals remains a serious concern for structural materials and aerospace engineering. Yet a real-time in situ visualization of corrosion, and its inhibition within a confined geometry, remains challenging. We recently demonstrated how white light interferometry (WLI) could be utilized to directly visualize corrosion processes in real-time, in-situ in a confined (i.e. buried) geometry. We studied in detail how pure aluminum corrodes in crevice geometries and how vanadate ions can effectively inhibit CC of aluminum.

Consecutive 2D FECO images obtained as a function of time after injection of 5 mM NaCl into a confined aluminum/mica contact. The schematics below show progress of the corrosive reaction as deduced from the FECO images. As indicated in the schematic shown in (c) a subtle balance between material transport rates (diffusion and migration) as well as reaction rates may control the locations where rapid Crevice Corrosion initiation is most likely.

Consecutive 2D FECO images obtained as a function of time after injection of 5 mM NaCl into a confined aluminum/mica contact. The schematics below show progress of the corrosive reaction as deduced from the FECO images. As indicated in the schematic shown in (c) a subtle balance between material transport rates (diffusion and migration) as well as reaction rates may control the locations where rapid Crevice Corrosion initiation is most likely.

2. Realtime in-situ studies of electrochemically oxidizing/ reducing noble metal surfaces

Electrochemical solid|liquid interfaces are critically important for technological applications and materials for energy storage, harvesting, and conversion. Yet, a real-time Angstrom-resolved visualization of dynamic processes at electrified solid|liquid interfaces has not been feasible. Using white light interferometry in an SFA we developed a unique real-time atomistic view into dynamic processes at electrochemically active metal interfaces. This method allows simultaneous deciphering of both sides of an electrochemical interface—the solution and the metal side—with microsecond resolution under dynamically evolving reactive conditions that are inherent to technological systems in operando. Quantitative in situ analysis of the potentiodynamic electrochemical oxidation/reduction of noble metal surfaces shows that Angstrom thick oxides formed on Au and Pt are high-ikmaterials; that is, they are metallic or highly defect-rich semiconductors, while Pd forms a low-ik oxide. In contrast, under potentiostatic growth conditions, all noble metal oxides exhibit a low-ik behavior.

3. The Intersection of Interfacial Forces and Electrochemical Reactions

We recently developed a number of experimental techniques that simultaneously combine measurements of the interaction forces or energies between two extended surfaces immersed in electrolyte solutions—primarily aqueous—with simultaneous monitoring of their (electro)chemical reactions and controlling the electrochemical surface potential of at least one of the surfaces. Combination of these complementary techniques allows for simultaneous real time monitoring of angstrom level changes in surface thickness and roughness, surface–surface interaction energies, and charge and mass transferred via electrochemical reactions, dissolution, and adsorption, and/or charging of electric double layers. These techniques employ the surface forces apparatus (SFA) combined with various “electrochemical attachments” for in situ measurements of various physical and (electro)chemical properties (e.g., cyclic voltammetry), optical imaging, and electric potentials and currents generated naturally during an interaction, as well as when electric fields (potential differences) are applied between the surfaces and/or solution—in some cases allowing for the chemical reaction equation to be unambiguously determined. We study how the physical interactions between two different surfaces when brought close to each other can affect their chemistry.

4. Pressure solution – The importance of the electrochemical surface potentials

“Pressure solution”, frequently found in clay-rich sandstone, is characterized by enhanced quartz dissolution at inter-grain contacts. The origin of pressure solution and many other related dissolution processes remains elusive. Using an Electrochemical Surface Forces Apparatus we visualized and measured the dissolution of silica glass surfaces close to an electrode surface. The dissolution rates correlate quantitatively with the electrode potential via the Butler–Volmer equation for corrosion. Our experimental results demonstrate that at low temperature, apparent pressure solution and many other mineral dissolution phenomena can be driven by electrochemical processes rather than a pressure-driven process. This finding highlights the role of electrochemical surface potentials in dissolution phenomena at dissimilar material interfaces, and provides new perspectives on pressure solution in particular and a new theoretical basis for predictive control of dissolution phenomena in general.