Instantaneous impedance as a tool for in-situ monitoring of surface modification processes

Many systems involving corrosion protection or corrosion mechanisms, such as anodising, conversion coatings, and AC electrograining, involve fast changing surfaces. Classical monitoring of these processes is mostly performed ex-situ, e.g. by SEM or XPS analysis. Due to the nature of these vacuum-based techniques the studied dehydrated surface will already be modified from the surface in solution. Classical electrochemical impedance spectroscopy cannot be used to measure the changing surface properties in-situ, as this goes against the principle of stationarity during such measurement. In contrast, applying an ORP-EIS signal to a non-stationary system, due to each frequency being applied constantly over the course of the measurement, the instantaneous impedance can be calculated for the duration of the experiment [1]. This technique provides information on the system as its properties are changing. It can therefore be used for monitoring the changes in surface morphology during the actual process.

In this work, the AC electrograining of aluminium is taken as a case study for surface monitoring. This process is used for offset printing plate production. High current densities are used to induce controlled corrosion on aluminium surfaces. During the process, three different stages can be identified, namely dissolution of second-phase particles, crystallographic attack of the aluminium, and development of the smut layer [2]. While many studies have studied the composition of the smut layer ex-situ, the exact composition of the smut layer during the process is still unknown. Due to this uncertainty, it is difficult to compare the electrochemistry of different electrograining solutions and additives. As the industrial process uses multiple electrodes in series [3], the smut layer composition can also change during the interval time between the electrodes. The trapped gas inside of the smut layer is expected to be partially released during this period.

For this study, a six-electrode electrograining process is simulated using a galvanostat. The galvanostatic ORP-EIS signal is applied immediately after the simulated electrodes. The instantaneous impedance was then calculated to identify the non-stationarities. Fitting the obtained spectra, the changes in smut layer capitance and smut layer resistance over time can be quantified. It is found that the smut layer capacitance increases over time, which can be due to the gas release in the smut layer, or due to a change in smut layer thickness. The smut layer resistance is found to decrease after 1.5 seconds, which can relate to the dissolution of the smut layer. Interestingly, the electrolyte resistance is also changing over time, due to the change in gas fraction in the solution. As the smut layer properties are monitored in-situ, this technique could be used to identify differences in smut layer composition when using different additives.

[1] T. Breugelmans et al., Electrochimica Acta 76 (2012) 375–382
[2] T. Dimogerontakis, H. Terryn, Corrosion Science 49 (2007) 3428-3441
[3] M. P. Amor, Electrochemical graining method, US Patent 5,755,949, 1998