The ability to vary the temperature of an electrochemical cell provides opportunities to control reaction rates and pathways and to drive processes that are inaccessible at ambient temperature. Here, we...

Different modes of corrosion are possible for metal films in contact with liquid solutions, depending on the metal or alloy present, the composition of the solution and conditions such as temperature and mechanical stress [1]. The difficulty of monitoring corrosion processes with good spatial and temporal resolution under a liquid layer creates a challenge for understanding key aspects of these corrosion processes, such as the nucleation sites and growth rates of individual pits during localized corrosion. However, structural changes and chemical processes at the solid/liquid interface can be probed by means of liquid cell transmission electron microscopy under electrochemical control (EC-TEM) [2]. This technique can provide a combination of electrochemical and temporally and spatially resolved data that has already yielded information on corrosion kinetics in liquid solutions [3]. Here, we use EC-TEM to examine galvanic dissolution processes of metal thin films, such as Cu and Ni, by exposure to ppm levels of chemical additives, typically Pd ions, in acidic solutions. We have examined the kinetics of these processes under conditions of different pH and Pd ion concentration, and will also discuss temperature-dependent corrosion reactions in these materials systems.The experiments were carried out in a JEOL 2010F TEM operated at 200 kV using a continuous flow LC-TEM holder (Hummingbird Scientific Co., Ltd) with capabilities for mixing or exchanging solutions, as well as heating and electrochemical control. Metal films were deposited to thicknesses of 20-100 nm by electron beam evaporation. The films were deposited either over blank window chips or over commercially available heater chips that included an electrically isolated resistance heater built into the window. Patterning using either lithography or a stencil mask ensured that the metal film partially covered the liquid cell window to provide an open reference area for imaging during dissolution. After assembling the liquid cell, deionized water was introduced to remove air bubbles inside the cell. The metal film was imaged in water, then either deionized water continued to flow or a solution containing an acid electrolyte (H2SO4) and/or ppm levels of chemical additives (for example, PdSO4) was introduced. The film morphology was then imaged at regular intervals. The resulting structural changes were followed as a function of both the solution chemistry and the illumination conditions. The electron beam is well known to cause radiolysis of water that changes the solution chemistry, for example typically lowering the pH [4]. Unsurprisingly, both the solution pH and the beam intensity affect the rate and morphology of dissolution [5]. Figure 1 shows examples of galvanic dissolution of Cu in 0.1M H2SO4 + ppm level of PdSO4 and dissolution of Ni at lower electron beam flux. Experiments such as these provide information on the spatial distribution of the corrosion process (for example, distinguishing grain boundary pitting from intragranular pitting, and measurin...

This paper explores the concept of an analog memristive device based on reversible electrochemical deposition and deplating of a submonolayer metal layer on a 108 Ω resistive bar. Initial feasibility experiments demonstrate a continuous resistance change by seven orders of magnitude during physical vapor deposition of Cu on TaNx/SOI, with the most promising range from 5.6 × 107 to 1.1 × 107 Ω/□ during a 0.64 monolayer Cu deposition. Cyclic electrochemical deposition and deplating of Cu on a metal seed on SiO2 in a 0.01M CuSO4/H2SO4 pH 1.4 solution demonstrates a reversible resistance variation with a minimum of 10 ± 1 discrete resistance states. These initial results are promising but also reveal a key materials challenge: the need for controlled and reversible electrochemical deposition/deplating of a submonolayer metal on the surface of a relatively high resistivity (≥10−2 Ω m) material.

While the growth of pits in passive metals exposed to chloride solutions is well understood, the processes associated with the initiation and propagation of stable pits, versus pits that form and apparently re-passivate, are still a matter of conjecture. A major challenge in studying pit initiation using electron microscopy has been alteration of the structure and chemistry of the hydrated corrosion films upon transfer to the vacuum environment of the microscope. A recently developed technique uses a microfluidic liquid cell to maintain the aqueous environment in contact with the sample. This work uses such cells to directly observe pits initiating, and growing before reaching stability, in aluminum thin films under potentiostatic polarization in situ in the electron microscope. Polarization curves developed in the cell show good agreement with those observed under conventional electrochemical experimental conditions. We observed current transients representative of metastable pitting and were able to relate crystalline features found in situ with topographic features using atomic force microscopy (AFM). An accumulation of aluminum surrounding an initiated pit, combined with depth profiling using Auger electron spectroscopy suggests that aluminum metal is deposited during the pit initiation process, and may serve to reduce lateral dissolution of the aluminum film. Work is currently underway to determine if this observation is unique to the geometry of the microfluidics cell or if is a general result that occurs at the very beginning of pit initiation.