(Invited) Combined Electrochemical and Surface Analytical Techniques for Probing Complex Multi-Scale Localised Corrosion of Metals and Alloys: An Overview
Authors: Tan Yongjun (Mike) yongjun , Parvizi Reza , Laleh Majid , Varela Bob , Huo Ying , Hughes Anthony ewart 
Published: December 2020

Localized corrosion processes are dynamic, initiating from micro-sized event and propagating into macro-sized corrosion problems. This attribute suggests that in order to fully understand localized corrosion, there is a need of characterization techniques that have appropriate temporal and spatial resolutions for probing and visualizing pre-existing electrode inhomogeneity and the propagating electrochemical heterogeneity at various length and time scales [1]. Over the past decades various electrochemical and analytical techniques have been developed and utilized to understand the influence of complex hetero-structures on corrosion initiation and propagation of metals and alloys. For instance, scanning probe techniques such as scanning Kelvin probe force microscopy (SKPFM) [2], confocal scanning laser microscope [3], nanotomography [4], atom probe tomography (APT) [5], capillary-based micro-electrochemical test [6] have been used to achieve the high temporal and spatial resolution required for probing localized corrosion initiation. However, it should be noted that these techniques and methods often have limitations in terms of either relating the metastable events to complex micro or even nano structures of alloys or correctly assigning the current transients to localized events in longer exposure times due to poor spatial resolutions because the results are generally averaged over the whole electrode surface. A method that was developed to address this issue is an electrochemically integrated multi-electrode array, often referred to as the wire beam electrode (WBE) [1], that has been described as a rapid, quantitative method to measure localized corrosion of metals and especially steels with a capability to monitor quasi in-situ localized corrosion processes on material surfaces [1, 7-9], often used in combination with complementary techniques such as scanning vibrating electrode [7] and galvanic noise technique and optical profilometry (Figure 1)[9].
Several cases of the combined use of electrochemical and surface analytical techniques will be described in order to provide an overview of technical developments. Tan et al. [10] investigated the localized corrosion initiation and inhibition of pure Al and AA2024 WBEs in chloride containing solutions by adding various amount of chlorides. They have found that the galvanic corrosion in these electrodes was suppressed by generation of considerable number of insignificant anodes. They characterized the localized corrosion behavior and inhibition of aluminum in presence and absence of a sodium chromate inhibitor using the WBE in combination with scanning reference electrode technique (SRET) [11]. Parvizi et al. [12] combined a range of interfacial characterization techniques such as the WBE, SVET, SEM, EDS, EBSD and APT in visualizing microstructural features and their effects on the localized corrosion initiation and propagation processes [12]. Corrosion probes based on the WBE has been developed for monitoring and understanding complex forms of localized corrosion on buried pipelines [13,14]. Evidence found on localized corrosion under disbonded coatings and stray currents illustrate some of the unique advantages of the electrode array method for visualizing and understanding localized corrosion of buried steels occurring at different spatial and temporal scales. Recently Laleh et al [15] studied localized corrosion of 316L stainless steel produced by selective laser melting (SLM) at different spatial scales using combined electrochemical and three-dimensional analytical techniques. Specimens containing lack-of-fusion pores were found to be extremely susceptible to localized corrosion, as indicated by their lower breakdown potentials measured in electrochemical polarization tests. Computed tomography analysis, capable of linking the microstructure and corrosion propagation paths in three dimensions, showed the development of localized corrosion at the sites of pores upon exposure to ferric chloride solution.
An overall conclusion is that the combined used of advanced electrochemical and surface analytical techniques is a practical approach to facilitating the visualization and probing of heterogeneous metallurgical, electrochemical and chemical processes occurring on metal and alloy surfaces at different spatial and temporal scales.

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Figure 1