Chemical Imaging of Reactive Surfaces Using Microsensors As Tips in Scanning Electrochemical Microscopy: Applications in Corrosion Research and Protection
Authors: Souto Ricardo M. 
Published: February 2020

Chemical imaging of reactive surfaces with high spatial resolution has become available with the introduction of scanning electrochemical microscopy (SECM). In fact, this technique has become a powerful tool in the study of a wide range of corrosion processes. This presentation provides a brief review on the applications of scanning electrochemical microscopy (SECM) in measuring, characterising and evaluating corroding systems as developed in our research group at the University of La Laguna (Spain). Localized corrosion processes and electrochemical activity distributions in surfaces can thus be investigated in real time with high spatial resolution.

The SECM is a unique near-field scanning technique that is electrochemically integrated as to detect chemical and electrochemical activities in electrochemical heterogeneous systems such as those operating in corrosion research. The SECM can be used in a variety of ways, which can be broadly classified into amperometric and potentiometric modes, depending on the type of the sensing probe, namely an ultramicroelectrode (UME) and an ion-selective microelectrode, respectively. Because the corroding metal usually involves local changes in ionic concentrations, as they produce Mz+ at the anodic sites transferred under diffusion control, the system can be studied by amperometric SECM. This feature has been exploited to image the generation of specific metal cations for the visualization of metastable pits on austenitic steel [1], and the detection of metal dissolution either from inclusions in alloys [2], or from defects in polymer-coated metals [3]. Additionally, in a neutral aqueous media, the reaction of dissolved oxygen from the electrolyte at the cathodic areas may be analyzed through the monitoring of the subsequent depletion of oxygen content in the solution volume adjacent to the cathodic sites [4]. However, for the detection of concentration distributions in certain corroding samples, especially for metals with sufficiently negative redox potentials in aqueous environments, the use of Pt microelectrodes is limited by the onset of oxygen reduction and hydrogen evolution reactions [5]. Though less employed, these physicochemical parameters may be studied using ion-selective electrodes (ISEs) as tips. In this case, the SECM is operated in the potentiometric mode, which gives greater chemical selectivity. In this respect, the applicability of Zn2+ [6] and Mg2+ ion-selective microelectrodes [7] with SECM has been established, although the onset of high electric fields in the electrolyte phase associated to galvanic corrosion [8] requires new tip assemblies for accurate monitoring [9].

In this contribution, the operation modes of the instrument are briefly described together with typical experiments selected to illustrate their application in sensing localized corrosion. Selected examples regarding the characterization of corrosion processes using scanning electrochemical microscopy are presented, including passivity breakdown and pit initiation, galvanic coupling, and blistering and delamination processes in coated metals.

[1] Y. González-García, G.T. Burstein, S. González, R.M. Souto. Imaging metastable pits on austenitic stainless steel in situ at the open-circuit corrosion potential. Electrochemistry Communications
6, 637-642 (2004).

[2] J. Izquierdo, S. González, R.M. Souto. Application of AC-SECM in corrosion science: Local visualization of heterogeneous chemical activity in AA2024 surfaces. International Journal of Electrochemical Science
7, 11377-11388 (2012).

[3] R.M. Souto, Y. González-García, S. González. In situ monitoring of electroactive species by using the scanning electrochemical microscope. Application to the Investigation of degradation processes at defective coated metals. Corrosion Science
47, 3312-3323 (2005).

[4] J.J. Santana, J. González-Guzmán, L. Fernández-Mérida, S. González, R.M. Souto. Visualization of local degradation processes in coated metals by means of scanning electrochemical microscopy in the redox competition mode. Electrochimica Acta
55, 4488-4494 (2010).

[5] R.M. Souto, Y. González-García, D. Battistel, S. Daniele. In situ SECM detection of metal dissolution during zinc corrosion by means of mercury sphere-cap microelectrode tips. Chemistry, A European Journal
17, 230-236 (2011).

[6] J. Izquierdo, L. Nagy, Á. Varga, I. Bitter, G. Nagy, R.M. Souto. Scanning electrochemical microscopy for the investigation of corrosion processes: measurement of Zn2+ spatial distribution with ion selective microelectrodes. Electrochimica Acta
59, 398-403 (2011).

[7] J. Izquierdo, A. Kiss, J.J. Santana, L. Nagy, I. Bitter, H.S. Isaacs, G. Nagy, R.M. Souto. Development of Mg2+ ion-selective microelectrodes for potentiometric Scanning Electrochemical Microscopy monitoring of galvanic corrosion processes. Journal of The
Electrochemical Society
160, C451-C459 (2013).

[8] A. Kiss, D. Filotás, R.M. Souto, G. Nagy. The effect of electric field on potentiometric Scanning Electrochemical Microscopic imaging. Electrochemistry Communications
77, 138-141 (2017).

[9] D. Filotás, B.M. Fernández-Pérez, A. Kiss, L. Nagy, G. Nagy, R.M. Souto. Double barrel microelectrode assembly to prevent electrical field effects in potentiometric SECM imaging of galvanic corrosion processes. Journal of The Electrochemical Society
165, C270-C277 (2018).