A local view of corrosion

The effects of corrosion need to be considered in almost every industry. A recent NACE report [1] estimated that corrosion costs $2.5 trillion USD globally, although this could be noticeably reduced with proper implementation of corrosion prevention policies. To effectively combat corrosion, an in-depth understanding of the processes occurring is necessary. Traditionally this would be done using bulk electrochemical techniques, like electrochemical impedance spectroscopy. Bulk electrochemistry provides a global view of the processes occurring, this limits understanding because this is an average view of corrosion. Corrosion often happens at a local level, for example at defect sites. Therefore, to fully understand and prevent corrosion, measurement at the local level is necessary.

What options do we have to measure local corrosion? Starting in the 1980s local electrochemical techniques started to be applied to investigate corrosion locally. This quickly expanded through the next decade to include a wide range of techniques from Scanning Vibrating Electrode Technique (SVET), to measure local current density, to Scanning Kelvin Probe (SKP) to measure the sample work function. Using these techniques it has been possible to investigate the local causes of corrosion In 2010 Intermittent Contact – Scanning ElectroChemical Microscopy (ic-SECM) was introduced by the Warwick Electrochemistry and Interfaces Group. This is a constant distance form of Scanning ElectroChemical Microscopy, allowing the local electrochemical activity of a sample in solution to be measured whilst maintaining a set probe to sample distance throughout. To maintain the probe to sample distance ic-SECM measures the mechanical vibration of the SECM probe perpendicular to the sample. Changes to the probe vibration as it moves in x and y across the sample surface are continually tracked to allow the probe to readjust to maintain the user input vibration. The use of ic-SECM offers a number of distinct benefits for corrosion studies.

• ic-SECM not only tracks the sample topography, it also measures the topography in a single combined measurement with the sample activity. This allows for direct correlation of sample features with corrosion activity.

• Corrosion samples have traditionally been off limits to constant distance SECM because the scan areas of interest are typically too large to reliably measure without the influence of sample tilt. This is not the case for ic-SECM which can track sample topography over large scan areas, which are typically larger than other constant distance techniques. Because the probe to sample distance can be maintained over large ranges, sample tilt is not an issue for ic-SECM making it ideal for corrosion samples.

• Corrosion samples are not just flat sheets. They can be curved, stepped, or otherwise topographically interesting. The control mechanism of ic-SECM means it is well suited to measure samples even with large topography changes.

• When measuring a corrosion sample by SECM it is typically of interest to measure the species released by the sample during corrosion, e.g. Fe²⁺ during steel corrosion. In a constant height SECM experiment, a probe approach curve would be necessary to position the probe for the area scan measurement of interest, which is not trivial when corrosion products are of interest. In ic-SECM, however, a push button approach based on the mechanical signal of the probe vibration is used, making an approach to the surface for corrosion studies more user-friendly.

• ic-SECM can be performed in ac-SECM mode. dc-SECM is by far the most popular form of SECM, however it requires the use of a redox mediator. Often times redox mediators deliberately added for measurement can affect the rate of corrosion, meaning the measurement no longer mimics the real-world scenario. When intermittent contact is used in combination with the ac-SECM technique (ic-ac-SECM), however, the need for a redox mediator is completely removed. Instead the sample can be measured in only the solution of interest. This means ic-ac-SECM can provide a local electrochemical measurement of corrosion samples in an environment matching their real-world use.

ic-SECM is an ideal solution to understanding the processes at work in corrosion at a local level.

Ready to start using ic-SECM in your own corrosion measurements?

SCAN-Lab Application Note #6 features two examples in corrosion to help you get started. This note provides the necessary background to understand how the ic-SECM technique is performed on a Bio-Logic M470 Scanning Electrochemical Workstation. The practical application of ic-SECM to corrosion samples is demonstrated with measurements on a steel weld and a 7075 aluminum alloy.

To learn more about the ic-SECM system available from Bio-Logic please visit the M470 page.

Reference: [1] G. H. Koch et. al., International Measures of Prevention, Application, and Economics of Corrosion Technologies Study. Houston, TX: NACE® International, 2016, iii