Abstract
In this study, we report on the use of optical emission spectrometry (OES) for the online detection of changes in the Al concentration ejected in a 1.0 mol dm −3 sulphuric acid electrolyte during galvanostatic anodising of Al thin film substrates. The technique relies on the coupling of an Inductively Coupled Plasma (ICP) spectrometer to a specially designed electrochemical flow cell. This has allowed to correlate, for the first time, the kinetics of Al dissolution to well-established morphological changes related to porous anodic oxide formation and growth. A deconvolution algorithm was first developed in order to decompose the experimental ICP/OES signal into elementary distributions, each one characteristic for a specific kinetic regime. The highest dissolution rate systematically occurred during the first step, associated with barrier oxide formation. This is followed by a systematic decrease in the rate of Al dissolution during pore formation. During steady-state porous oxide growth, the Al dissolution rate increases again, but still remains below the level established during barrier oxide growth. In each of these three kinetic regimes, a linear variation of the Al dissolution rate with current density was observed in the range 0.5–5.0 mA cm −2, with slope values of, respectively, 35 ± 2, 24 ± 2 and 28 ± 1 μg C −1. Regarding the temporal transitions between the different regimes, a desynchronisation was observed between the kinetic (dissolution) and morphological transitions, the time offset going in opposite directions for barrier and steady-state porous oxide growth. Finally, using the measured Al dissolution rates, the current density dependence of the film formation efficiency for both porous and barrier oxide growth has been established.
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