Topic 5 min read

Scanning probes & corrosion research

Latest updated: June 2, 2023

Overview:

Corrosion costs industry greatly. An in-depth understanding of the underlying processes related to these processes is therefore essential. Bulk electrochemistry has been used extensively in the study of corrosion; however, this provides a global view, while corrosion often starts and proceeds locally. Scanning probe electrochemistry offers a broad toolkit of techniques to perform local electrochemical investigations, providing a clearer picture of the progression of corrosion. Scanning probe electrochemistry can answer a wide variety of questions about corrosion as outlined in the following table.

Question	Technique	Information Measured	Example
Does changing the content of different components of an alloy affect its corrosion?	LEIS	The homogeneity of the impedance distribution, as well as the magnitude of the impedance change provides information on the corrosion resistance of a system. Lower corrosion resistance is indicated by lower homogeneity in the impedance and larger changes in the magnitude of the impedance.	T. Liu, et al., Corrosion Science 149 (2019) 153-163
	SKP	The Kelvin potential of an alloy reflects its corrosion potential. The corrosion potential is in turn a reflection of the composition of the alloy.	D. Kong, et al., Applied Surface Science 455 (2018) 543-553
How do the corrosion properties vary across, and away from, a sample feature?	SDC	With SDC direct local electrochemical and corrosion studies can be performed in the area directly under the droplet probe. Electrochemical studies can be performed at the feature of interest, before the droplet is moved to the next site for comparison.	J. M. Thuss, J. R. Kish, J. R. McDermid, Magnesium Technology 2012 (2012) 403-409
The presence of hydrogen in a metal can lead to hydrogen embrittlement. Where is hydrogen locally present in a sample?	ac-SECM	The measured ac-current magnitude reflects hydrogen charging. Higher local current magnitudes are related to the presence of hydrogen within the sample. Local impedance measurements performed by ac-SECM reflect a lower resistance over regions with hydrogen present than those without.	M. C. Lafouresse, et al., Electrochemistry Communications 80 (2017) 29-32
	dc-SECM	dc-SECM is used to measure the Oxygen Reduction Reaction (ORR) at the probe, which competes with ORR at the sample. While ORR can occur naturally at corroding surfaces, it is enhanced when there is hydrogen present. Therefore, in a dc-SECM measurement the lowest current is measured in regions with high hydrogen content.	R. F. Schaller, et al., Electrochemistry Communications 51 (2015) 54-58
Where is corrosion occurring locally?	SKP	SKP measures the work function difference between the probe and the sample, in a non-destructive manner. The work function difference directly correlates to the corrosion potential of a system. Changes in the work function difference reflect local changes in the corrosion potential of a sample. Measurements at different time intervals can show progression of corrosion.	D. Zhang, et al. Journal of Materials Engineering and Performance 24 (2015) 2688-2698
Where is corrosion occurring locally?	dc-SECM	dc-SECM measures the Faradaic current of a redox mediator interacting with the sample of interest. The changing Faradaic current represents changing interaction between the sample and the mediator. This can therefore be used to pinpoint areas where, for example, pitting corrosion is occurring.	D. Kong et al. Applied Surface Science 440 (2018) 245–257
What is the effect of the sample structure on corrosion?	SVET	SVET locally measures dissolution current density. By correlating the local current density to the known sample it is possible to determine if sample structures give rise to local anodic and cathodic sites.	Z. Y. Liu, X. G. Li, Y. F. Cheng, Electrochemistry Communications 12 (2010) 936-938
How does the passive layer form and break down?	dc-SECM	The SECM approach provides information on the rate of electron transfer of a sample. For neat samples positive feedback with the highest rate of electron transfer will be measured. As the passive layer forms the rate of electron transfer will decrease, until eventually negative feedback is measured. When this occurs, the passive layer is fully formed locally. Passive layer breakdown will be apparent from the appearance of positive feedback.	H. Torbati-Sarraf, A. Poursaee, Materialia 2 (2018) 19-22
What is the concentration profile of electroactive species during corrosion?	dc-SECM	dc-SECM locally measures the concentration of electrochemically active corrosion products, with chemical selectivity. When the probe is biased to interact with the corrosion product the resulting current is proportional to the concentration of a given species. dc-SECM area scans can be performed at increasing distance distances to build a picture of the concentration profile in x, y, and z.	M. Zhao et al., Corrosion Engineering, Science and Technology 48 (2013) 270-275
How is corrosion proceeding electrochemically in real time?	SVET	SVET measures the current density in solution above a sample. During corrosion it can be used to visualize anodic and cathodic sites. As corrosion progresses SVET can visualize the movement of anodic and cathodic sites across the sample surface in real time.	C. F. Glover, T. W. Cain, J. R. Scully, Corrosion Science 149 (2019) 195-206

Glossary:

Scanning ElectroChemical Microscopy (SECM): Measurement of local electrochemical activity of a sample with chemical selectivity. The local impedance of a sample can also be measured.
Localized Electrochemical Impedance Spectroscopy (LEIS): Measurement of local impedance differences and point-by-point Electrochemical Impedance Spectroscopy
Scanning Kelvin Probe (SKP): Local non-contact measurement of sample work function. Topography can also be measured.
Scanning Vibrating Electrode Technique (SVET): Measurement of the local current distribution of a sample in electrolyte. Also known as Vibrating Probe.
Scanning Droplet Cell (SDS): Local electrochemical and impedance measurements of a sample within a droplet.
Optical Surface Profiler (OSP): Non-contact measurement of the local sample topography.