Studying Oxidation, Mass Transfer of Metals and Corrosion at the Nanoscale with AFM with an EC Cell

Integrating an atomic force microscope (AFM) with an electrochemistry cell (EC Cell) results in a uniquely appropriate tool for examining oxidation, corrosion, and mass transfer of metals at the nanoscale. In this article, the many capabilities and benefits of the Asylum Research Cypher™ ES AFM and Electrochemistry Cell for understanding corrosion are discussed.

Corrosion is a universal infrastructure issue estimated to cost an enormous $2.5 trillion per annum — not including personal and environmental safety impacts — and it is estimated that 15-35% of that cost could be saved if corrosion control best practices and technology were adopted.1 Comprehending corrosion potentials, the nucleation and propagation behavior of defects, and electrochemical reaction rates helps engineers and researchers enhance both materials and designs ranging from precision parts to huge infrastructure to better endure corrosion and minimize this economic loss.

Researchers use electrochemistry methods to probe the potential dependence of the interconversion of chemical and electrical energy2 and establish the corrosion potential of metals and other materials. These measurements are normally performed at a macroscale electrode, exposing bulk charge transfer features, reaction energies, and kinetic constants. Using an AFM, researchers can examine the nanoscale topography of these bias-dependent reactions in real time directly at the charge transfer interface and construct a more vital understanding of the corrosion mechanisms.

Even though EC-AFM is not a new method3, it still presents a challenge to capture and model the nanoscale dynamics of electrochemical processes.4 However, by coupling a Cypher ES AFM with a fully-enclosed EC Cell and the remarkably stable imaging enabled by blueDrive™ photothermal excitation, electrochemical signals and nanoscale variations can be correlated as a function of time in applicable reaction environments.

Reliable Electrochemistry Measurements in Well-Controlled Corrosion Environments

Corrosion studies rely on the ability to imitate the real-world conditions that stimulate corrosion. By using the completely enclosed, inertly-constructed EC Cell (Figure 1), it is possible to reliably produce these specific liquid electrolyte and gas environments and control their compositions during an experiment. In such controlled conditions, measuring the potential reliance of corrosion current for a range of metallic samples becomes simple. For instance, the cyclic voltammograms (CVs) illustrated in Figure 2 expose the oxidation (anodic current) onset for a range of metal samples with filtered ocean water as the electrolyte. For 303 and 316L stainless, it must be noted that the forward and reverse traces in the CVs display hysteresis: the forward scan needs more applied potential to primarily oxidize the electrode surface than is needed to continue corroding the electrode once it is started. This is consistent with the fast accumulation of a passivation layer, which is one of the known mechanisms adding to the anodic corrosion resistance of stainless steels in an oxygen-rich environment.5

It is also worth considering the unique “double peak” shape of the oxidation current trace for brass that possibly arises from the fact that it is a compound metal alloy. Both the Zn- and Ni-coated samples are probably made of a grade of stainless steel that is less corrosion resistant, concluded from their more negative onset potentials for oxidation. The Zn-coated sample also appears to display oxidation of Zn at a very low potential of   <–1.0 V vs. Ag/AgCl, which likely serves as a sacrificial anode to safeguard the underlying steel.

The Asylum Research Cypher EC-AFM

Figure 1. The Asylum Research Cypher EC-AFM comprises the Cypher ES AFM (Left), an external potentiostat (Middle), and the Cypher EC Cell, shown here as an exploded rendering (Right) of the EC Cell components inside the AFM.

Normalized cyclic voltammograms

Figure 2. Normalized cyclic voltammograms (CVs) of various metallic samples. The forward and reverse scan directions of the traces are indicated by triangular arrows, and the oxidation onset potentials are indicated by upward pointing arrows (calculated by extending the linear portion of the curves to the x intercept). Normalizing the current allows for straightforward comparison of samples without concern for the variations in sample surface geometry.

Accurately Correlating Corrosion to EC Signals as a Function of Time

What traditionally stays cryptic in electrochemical corrosion research is the micro- and nanostructural variations at the electrochemically active electrode surfaces — particularly during corrosion. By using the rapid-scanning Cypher ES AFM coupled with the EC cell, it is possible to get a real-time series of nm-scale images during the corrosion process and associate these to the electrochemical signals. Figure 3 illustrates an example of surface topography variation over time during controlled oxidative corrosion of copper.

Corrosion of a copper surface over time under oxidizing potential.

Figure 3. Corrosion of a copper surface over time under oxidizing potential. During this series of 1 μm scans, pits as deep as ~30 nm developed. Note the height scale bars at the right of the images. A 100 nm scan (lower-right zoomed image) reveals fine features of the corroding surface. The electrolyte was 100 mM CuSO4 adjusted to a pH of ~3. Potential was poised with Cu as the working and counter electrodes using an external potentiostat at 0.2 V vs. Ag/AgCl, which is in the regime corresponding to the oxidative corrosion of copper via the reaction Cu0 → 2e + Cu2+ (see the Normal Pulse Voltammetry plot at lower left). Using the AFM software for image processing, it was determined the Cu electrode lost more than 2 μg per cm2 of electrode sheet area (>2 × 105 μm3/cm2) during this experiment. The increasing rate of corrosion in these data (bottom middle plot) suggests that the corrosion accelerates as pits increase in size, consistent with the electroactive surface area increasing as corrosion progresses.

In this data, the AFM tip serves as a passive (electrically floating) observer, measuring topography without direct contribution in the electrochemical reaction, which is possible for a range of imaging modes. For tapping mode and other AC modes, the Cypher ES applies blueDrive photothermal excitation for simple, clean tunes and ultra-stable imaging in liquid. Nearly all electrochemical measurements are performed in liquid electrolyte, so this helps make it a lot simpler to obtain high quality data.6

Monitoring Galvanic Processes

The measured oxidation potential data (Figure 2) can be used to predict and trigger galvanic electrochemical processes between two metal samples of interest. To show this, brass (oxidation onset potential –0.10 V vs. Ag/AgCl) was hammered into 303 stainless (oxidation onset potential 0.45 V vs. Ag/AgCl) and then machined and polished flat to yield a sample with an inherent potential difference of 0.55 V. Brass preferentially oxidizes in this configuration in the presence of electrolyte due to its lower oxidation potential, and this process can be seen optically as well as with the AFM. As illustrated in Figure 4, exposure to electrolyte (seawater) causes visible oxidative discoloration of the brass surface. This is due to increased roughness (refer to line sections plot) as well as an increase in the relative height of the brass surface (refer to height histogram plot). These features are readily explained by the fast evolution of brass oxidation products because of the 0.55 V galvanic potential variance between brass and its surrounding 303 stainless steel matrix.

Optical and AFM images of brass in 303 SS

Figure 4. Optical and AFM images of brass in 303 SS. The top row shows optical (Left) and AFM (Middle) images of the brass-in-steel sample before exposure to electrolyte, while the bottom row shows the same sample after 25 minutes of exposure to filtered seawater. The lines marked on the AFM images correspond to the line sections shown on the top-right, which reflect the roughening of the brass due to corrosion. Height histograms (Bottom-Right) calculated from the AFM images show that the peak corresponding to brass has both broadened and increased overall relative to the peak corresponding to stainless steel, which is largely unchanged.

Core Cypher Capabilities and Benefits for Electrochemical Research

Highest Resolution Imaging

The Cypher makes high-resolution imaging straightforward. There is no other AFM that makes true atomic resolution routine.

Cypher EC Cell

Atomic point defect in calcite imaged in the Cypher EC Cell in tapping mode using blueDrive. Scan size 5 nm.

blueDrive Makes Imaging Simpler and More Stable

blueDrive is basically a better way to tap. blueDrive tunes are clean and stable, and nearly match the theoretical response. There is never a “forest of peaks” like one may see when using piezo drive for tapping in liquid, so one can image for hours with no setpoint modifications. blueDrive also enables tapping mode operation in extremely viscous media, such as ionic liquids that are usually at least an order of magnitude more viscous than water.7

Strontium ruthenate

Strontium ruthenate (SrRuO3) sample imaged in a viscous liquid using tapping mode with blueDrive. Step heights are measured as 400 pm, matching the expected 395 pm. Scan size 10μm; scan rate 10 lines/second.

Excellent Top-View Bright-Field Optics

The Cypher features diffraction-limited, top-view optics that realize <1 μm resolution. Variable aperture and field diaphragms help enhance contrast on difficult samples.

Optical images showing surface deposition of copper on gold as it proceeds over time (Left to Right) under oxidizing potential. For scale, the probe is an AC40 cantilever (40 μm long).

An Outlook for Corrosion Studies

It is currently possible to integrate electrochemical measurements with the rapid scanning and cutting-edge nanoscale imaging methods available on the Cypher ES AFM. This paves the way to the possibility of correlating electrochemical and nanostructural variations as they occur.

Whether one is exploring proprietary electrolyte solutions (for example, deep eutectic solvents, ionic liquids, newly discovered electrolytes, etc.) or unique metallic samples and rare alloys (for example, novel Li-battery cathodes or anodes, superalloys, biomedical tools etc.), the future of corrosion studies and imaging is restricted only by the samples one creates.

References

  1. G. Koch, J. Varney, N. Thompson, O. Moghissi, M. Gould, J. Payer, International Measures of Prevention, Application, and Economics of Corrosion Technologies Study, NACE Int. IMPACT. 1–3 (2016).
  2. A.J. Bard, L.R. Faulkner, Electrochemical Methods: Fundamentals and Applications, 2nd ed., 2001.
  3. S. Manne, P.K. Hansma, J. Massie, V.B. Elings, A.A. Gewirth, Atomic- Resolution Electrochemistry with the Atomic Force Microscope: Copper Deposition on Gold, Science. 251, 183–186 (1991).
  4. D.R. Gunasegaram, M.S. Venkatraman, I.S. Cole, Towards Multiscale Modelling of Localised Corrosion, Int. Mater. Rev. 59, 84–114 (2014).
  5. V. Maurice, P. Marcus, Passive Films at the Nanoscale, Electrochim. Acta. 84, 129–138 (2012).
  6. A. Labuda, J. Cleveland, N. Geisse, M. Kocun, B. Ohler, R. Proksch, M. Viani, D. Walters, Photothermal Excitation for Improved Cantilever Drive Performance in Tapping Mode Atomic Force Microscopy, Microsc. Anal. 28, 23–27 (2014).
  7. G. Yu, D. Zhao, L. Wen, S. Yang, X. Chen, Viscosity of Ionic Liquids: Database, Observation, and Quantitative Structure-Property Relationship Analysis, AlChE. 58, 2885–2899. (2012)

This information has been sourced, reviewed and adapted from materials provided by Asylum Research - An Oxford Instruments Company.

For more information on this source, please visit Asylum Research - An Oxford Instruments Company.

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