Initiation and Study of Localized Corrosion by Scanning Electrochemical Microscopy

This paper describes methods to study the initiation and formation of localized corrosion pits on stainless–steel and aluminum samples. These methods are based on the use of a scanned probe microscope, the scanning electrochemical microscope (SECM). The SECM is specifically designed for operation in electrolyte solution and so is uniquely suited for examination of corrosion processes. SECM imaging of a corrosion pit on stainless steel is presented. In addition, the initiation of single pits on aluminum and stainless steel by using the SECM tip to electrogenerate a local source of Cl– is described

Ohmic Drop Compensation in Voltammetry: Iterative Correction of the Applied Potential

A new method of ohmic potential drop correction for use with potential step and sweep voltammetric methods is described. The method, iterative correction of the applied potential (ICAP), essentially replaces an electronic positive feedback correction with a digital positive feedback correction. This paper will present one form of the ICAP procedure, in which voltammetric current data acquired by a high-speed digital oscilloscope are used to iteratively generate a compensating potential waveform that is then synthesized by an arbitrary waveform generator. The ICAP method is advantageous for high-speed measurements, since many of the difficulties caused by electronic positive feedback compensation are eliminated

Deposition of Conducting Polyaniline Patterns with the Scanning Electrochemical Microscope

Conducting polyaniline patterns were deposited on gold, platinum, and carbon surfaces with the use of the scanning electrochemical microscope (SECM). The patterns were deposited in the “micro-reagent” mode in which a local pH change caused by proton reduction at the SECM tip allowed deposition to occur at the substrate. The effect of tip and substrate potential, tip-substrate separation, and deposition time were studied in order to produce well-resolved patterns of the desired thickness. Lateral resolution of the deposited polymer was as low as 3 mm. Conductivity of the film was verified by SECM imaging

Tip position modulation and lock-in detection in scanning electrochemical microscopy

Small amplitude tip-position modulation (TPM) in combination with lock-in detection of the modulated current signal is applied to a scanning electrochemical microscope (SECM) to improve signal-to-noise ratio and to enhance image resolution. Phase shift information from the alternating current TPM signal and the bipolar response of TPM over insulators and conductors make TPM-SECM superior to conventional SECM imaging methods as well as useful for measuring substrate conductivity.Board of Regents, University of Texas Syste

Patterning and Imaging of Oxides on Glassy Carbon Electrode Surfaces by Scanning Electrochemical Microscopy

The scanning electrochemical microscope is used to form and characterize patterns of oxides on glassy carbon surfaces. Chemically specific imaging of oxides present on these surfaces was demonstrated by taking advantage of differential heterogeneous electrontransfer rates for the Fe(II/III) reaction occurring at unoxidized and oxidized glassy carbon electrodes. Localized generation of surface oxides was demonstrated using both the microreagent and direct modification modes of SECM. The micro-reagent mode was used to perform a chemical oxidation of the surface by generating the strong oxidant Ag(II) at the UME tip while positioned close to the carbon surface, however, this technique was found to have poor reproducibility. Direct mode oxidation was found to be a much more versatile route toward the generation of complex patterns of oxides on carbon surfaces. The reproducibility of the direct mode technique was found to depend heavily on solution resistance. “Charge dose” studies, followed by reaction-rate imaging, qualitatively show that the electron-transfer rate for the Fe(II/III) system scales with the amount of charge “injected” in each oxidation experiment, indicating a correlation between surface oxide density and electron-transfer rate

Impedance Feedback Control for Scanning Electrochemical Microscopy

A new constant-distance imaging method based on the relationship between tip impedance and tip-substrate separation has been developed for the scanning electrochemical microscope (SECM). The tip impedance is monitored by application of a high frequency ac voltage bias between the tip and auxiliary electrode. The high frequency ac current is easily separated from the dc level faradaic electrochemistry with a simple RC filter, which allows impedance measurements during feedback or generation/collection experiments. By employing a piezo-based feedback controller we are able to maintain the impedance at a constant value and, thus, maintain a constant tip-substrate separation. Application of the method to feedback and generation/collection experiments with tip electrodes as small as 2 mm is presented

An in situ Electrochemical Study of Electrodeposited Nickel and Nickel-Yttrium Oxide Composite Using Scanning Electrochemical Microscopy

Electrodeposited nickel and nickel-yttrium oxide composite samples were studied in situ using scanning electrochemical microscopy (SECM). The monitored probe currents in phosphate-citrate buffer (pH 4.2) in the presence or absence of Ru(NH3)63+ as an oxidizing mediator near the Ni surface show that the SECM is a useful tool for study of the electrochemical activity of heterogeneous metal surface at micrometer scales. The SECM ultramicroelectrode probe tip provides information about the shape, activity and location of particles, such as Y2O3 introduced (co-deposited) in the Ni-matrix of the composite. Experiments show that the Ni-matrix in the composite coating is more active than the pure Ni-coating. This fact is expected, because of texture changes in the Ni structure upon introduction (by co-deposition) of Y2O3 particles. In the absence of mediator in the solution, the electrochemical activity of heterogeneous metal surface at a micro-level is investigated by using O2 concentration changes. The rate of reaction for O2 reduction was found to locally vary at electrodes floating at the open-circuit potential (o.c.p) when compared to an electrode potentiostatically polarized at a more positive potential than the o.c.p. This behavior suggests that local anode and cathode regions are being observed at the o.c.p. sample

Fast-Scan Cyclic Voltammetry - Scanning Electrochemical Microscopy

The use of fast scan cyclic voltammetry (FSCV) to generate images in scanning electrochemical microscopy is demonstrated. In the FSCV-SECM method a cyclic voltammogram is acquired at each grid point of the scanned area. Thus, each point in the SECM image is a data array rather than a single value. This provides a significant increase in the amount of data collected per scan and provides the ability to make images of multiple analytes per scan. In addition, the use of cyclic voltammetry allows images based on adsorptive as well as diffusional processes. In this paper, we describe the FSCV-SECM experiment and equipment, we show that FSCV-SECM can resolve multiple chemical species in a single scan, and we demonstrate that cathodic-stripping voltammetry can be used to image the concentration profiles of halide ions