Abstract
The development of tools that can probe corrosion related phenomena at the (sub)microscale is recognized to be increasingly important in order to understand the surface structural factors (grain orientation, inclusions etc.) that control the (electro)chemical stability (corrosion susceptibility, pitting, passivity etc.) of metal surfaces. Herein we consider the application of scanning electrochemical cell microscopy (SECCM), a relatively new member of the electrochemical droplet cell (EDC) family, for corrosion research and demonstrate the power of this technique for resolving structure and activity at the (sub)microscale. Hundreds of spatially-resolved (2 μm droplet size) potentiodynamic polarization experiments have been carried out on the several hours timescale and correlated to complementary structural information from electron backscatter diffraction (EBSD) and energy dispersive x-ray spectroscopy (EDS) in order to determine the effect of grain orientation and inclusions on electrochemical processes at low carbon steel in neutral solution (10 mM KNO3). Through this approach, it has been shown unequivocally that for the low index planes, anodic currents in the passive region (an indicator of corrosion susceptibility) are greatest on (101) planes compared to (100) and (111) planes. Furthermore, individual sub-micron MnS inclusions have been probed and shown to undergo active dissolution followed by rapid repassivation. This study demonstrates the high versatility of SECCM and the considerable potential of this technique for addressing structure-activity problems in corrosion and electromaterials science.
Highlights
The corrosion of metals and alloys is often caused by the establishment of local galvanic cells at surface heterogeneities when exposed to a corrosive environment
A bias potential (V1) of þ0.1 V was applied between the quasi reference counter electrodes (QRCEs) in order to generate an ion conductance current across the liquid meniscus formed at the end of the probe, which was used as a feedback signal during positioning of the micropipet probe relative to the substrate surface
Our intention here is to show that these limitations are readily overcome with scanning electrochemical cell microscopy (SECCM), positioning it as a powerful technique for spatially-resolved corrosion-related measurements
Summary
The corrosion of metals and alloys is often caused by the establishment of local galvanic cells at surface heterogeneities when exposed to a corrosive environment (e.g., an electrolyte solution). SECCM enables direct voltammetric-amperometric measurements at a series of targeted positions of a sample surface [22e25] and has previously been applied to resolve the relationship between structure and activity in a wide range of electrochemical processes at a diversity of electrode materials (e.g., sp carbon materials [26], molybdenum disulfide [27], iron nickel sulphide catalysts [28], and metallic nanoparticles [23,24]), but has not yet been employed to study corrosion related phenomena. The structural/ compositional dependence is shown unequivocally in this work, where differences in the corrosion susceptibility of the low-index (100), (101) and (111) grains is elucidated from spatially-resolved voltammetric measurements and the direct electrochemical detection and characterisation of sub-micron sized manganese sulphide inclusions is demonstrated
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