Real Time Monitoring of Dissolution Processes Using a Microelectrochemical Scanning Flow Cell
The steady progress in material science and the development of new coating materials has an immediate consequence for corrosion science: The complexity rises drastically. With increasing numbers of components in modern alloy systems, the field of electrochemistry thus faces extensive challenges. Identifying the influence of each particular component in order to derive promising pathways for material optimization  becomes more than substantial. Moreover, most surfaces are not uniform in elemental distribution and phase composition, and these microstructural aspects are playing a major role besides the bulk composition itself .
To address these challenges, three key approaches have been combined in one single electrochemical setup of a scanning flow cell (SFC). A high spatial resolution is achieved by a capillary cell which confines the area under investigation to around 200 µm. This typical capillary cell feature was extended by a continuous electrolyte flow that additionally provides a controlled mass transport of reactants and products and the possibility to couple various analytical methods downstream, e.g. in the present case a high sensitivity UV-VIS setup (Figure 1).
The potential of the complementary combination of high throughput electrochemistry and real time analysis is demonstrated by the investigation of zinc coatings, where the parameter space created by environmental conditions is extensive and the dissolution mechanism of zinc highly complex by itself . Figure 2 shows the online Zn detection during a 1000 s open-circuit potential (OCP) measurement in borate buffers of two different pH values with a subsequent anodic sweep (2 mV s-1).
It is obvious that the dissolution profile reaches a plateau value during the OCP scan, indicating a steady removal of zinc by the streaming electrolyte. The magnitude of zinc dissolution is proportional to the proton concentration in solutions of near neutral pH as detected by online analysis and is not reflected by the corrosion potential at all, which rather depends on the integrity of the passive film formed on the surface. The OCP transients recorded over 1000 s as shown in figure 3 demonstrate a clear distinction between the active and passive region or, in other terms, an incomplete (active) or complete (passive) coverage by the passive layer . The nature of this surface film was found to be oxidic as determined XPS measurements with the steady state thickness established in less than 100 s when held at the OCP in borate buffer.
The complementary coupling of classical electrochemistry and dissolution monitoring was therefore shown to provide a very fundamental insight into corrosion processes and bridge electrochemical (e.g. corrosion potential) and applied characteristics (e.g. mass loss).
High throughout capabilities originating from the ease of sample preparation and the full automation of the setup furthermore allow screening of non-uniform surfaces [5,6] or parameter variations on homogeneous substrates covering a wide range with high resolution. As an example, the electrochemical texturing of ZnO:Al for photovoltaic applications was performed utilizing the SFC with a focus on the dissolution mechanism and the influence of electrochemical treatment on the surface morphology . Figure 4 displays an experimental series with a variation of the galvanostatically applied current density.
The reproducibility of the method is evident from the identical geometry of the measurement locations and was reported repeatedly [2,4]. As the SFC enables large numbers of experiments combined with an increased information depth due to complementary couplings to further equipment, it is regarded as a promising method to meet the demand for data which is growing steadily in modern material science.
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