In Situ Visualization of Corrosion Processes of Metal Thin Films in Aqueous Solutions
Authors: Park Jeung Hun , Steingart Daniel A , Ross Frances M 
Published: February 2020

Chemical reactions with the environment result in corrosion or degradation of the properties of materials. Metals in contact with aqueous solutions, depending on their surface morphology, undergo different modes of corrosion including general, pitting, crevice, inter-granular, galvanic, environmentally induced fracture, and dealloying [1,2]. Many aspects of localized corrosion of metals are not well understood, in part because it is difficult to monitor corrosion processes at high spatial resolution under a liquid layer. Liquid cell transmission electron microscopy (LC-TEM), coupled with electrochemical control and analytic functions, enables the observation of structural changes and chemical processes at the solid/liquid interface [3,4]. Its combination of temporal and spatial resolution, difficult to achieve using other characterization techniques, has already yielded information on corrosion kinetics in aqueous solutions [5].
Here we describe LC-TEM observations of general, pitting, and galvanic corrosion in several metals (Au, Ni, Cu, Pt and Zn) under different aqueous solutions. The experiments were carried out in a FEI CM30 TEM, operated at 300 kV, using a continuous flow Hummingbird Scientific LC-TEM holder with capabilities for three-electrode electrochemical measurements. Metal films were deposited to thicknesses of 20 - 50 nm by electron beam evaporation, to partially cover the 50 nm thick silicon nitride membrane that makes up one window of the liquid cell. After assembling the liquid cell, deionized water was introduced to remove air bubbles inside the cell. Then the electrolyte, either deionized water or a solution containing an acid (HCl or H2SO4) and/or metal ions (for example, ZnCl2 or ZnSO4) was introduced. We used low dose imaging protocols and verified that each of the solutions used can be examined for long periods (hours) without visible beam effects. However, the metal films show dissolution over timescales of minutes to hours. We find that both the solution pH and the intensity of the imaging beam affect the rate and morphology of dissolution. The beam effect is caused by the reactive species created by radiolysis of water by the energetic electron beam [6]. We first show beam-induced dissolution of Ni and Pt in water, showing that the process initiates from grain boundaries. We then show dissolution of Au, Ni, Cu and Pt at low pH and in the absence of the electron beam, including the locations of nucleation sites (grain boundaries vs. inter-granular) and we discuss the etching rate of the substrate as a function of solution composition, pH and thickness. Finally, we describe the dissolution of Au under a potential scan, correlating the film morphology with voltage and current. The ability to observe the initiation of dissolution is a useful step in quantifying the mechanisms that control the processes.

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[7] We gratefully acknowledge funding supports from the BP Climate Mitigate Initiative and the National Science Foundation (NSF-GOALI: DMR-1310639), and the technical assistance of Dr. M. C. Reuter and Mr. A. W. Ellis at IBM.