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Current Topics in Electrochemistry   Volumes    Volume 23 
A novel theory on the mechanisms of generation, transport and release of oxygen gas and electronic current in anodic alumina film during Al anodizing
G. Patermarakis
Pages: 97 - 115
Number of pages: 19
Current Topics in Electrochemistry
Volume 23 

Copyright © 2021 Research Trends. All rights reserved

The generation, transfer and release of oxygen and electronic current during galvanostatic growth of porous anodic alumina films (PAAFs) was studied and novel mechanisms were formulated. Oxygen release is visually detected above a first boundary of current density. Above it, oxygen release starts earlier and is enhanced with increasing current density but remains mildly detectable up to a much higher second boundary. Below but near it, the normal PAAF growth is transformed to abnormal, which is associated with emergence of local burning. Above it intense and massive oxygen release and electropolishing occur rapidly and successively. The high field in the barrier layer of the order 109 V m-1 polarizes the oxide lattice making it more open thereby enabling ionic migrations. It also polarizes the O2- ions so that their electronic structure is destabilized. Field-activated processes generating electrons O2-  O-1 + e- and O-1 O + e- with overall O2- O + 2e- are established and propagated, the first and third being thermochemically prone. Adjacent O atoms form O2 and partly O3, in the form of separate molecules and of nano-bubbles. Molecules and nano-bubbles formed in the oxide|electrolyte interface enter the pore filling solution directly and those formed in the barrier layer are released in this solution as the barrier layer is renewed and the pore walls are chemically dissolved. Then, molecules and nano-bubbles are transferred outwards. Laplace, van der Waals and Henry’s law equations predict compressed nano-bubbles in equilibrium with high-concentration dissolved-oxygen. While remoting from pore bases, nano-bubbles shrink until their depletion. Local accumulation of nano-bubbles in the attached layer at spatial density above a critical one triggers their avalanche-like merge. Their merger is accompanied with pressure fall and volume expansion to larger nano-, micro- and macro-bubbles. The formulated mechanisms explain the results in this work and in the literature on the evolution of electronic current and oxygen and on the growth of peculiar PAAF nanostructures. Development of nano-porous anodes to optimize evolution of nano-bubbles/dissolved oxygen system is also discussed.
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