Electrolytic Solutions

   The electrolyte is the solution in which the anodization process takes
place. For the anodization of Al the electrolyte must be acidic, in addition
to having free ions and the ability to conduct electricity. Because of this,
solutions containing roughly 15% sulfuric acid to water are a commonly
used electrolyte.[7] However chromic acid as well as organic acids may also
be used but under different concentrations and usually higher applied volt-
ages.

    It should be noted that varying the electrolyte pH will have a direct
effect on the resulting thickness of the oxidation layer. This results from
the duel purpose effected by the sulfuric acid; providing a conductive, ion
rich medium while also slowly dissolving the newly formed oxidation layer,
which is a critical characteristic of the anodization process. If the resulting
alumina layer was not partially dissolved while it formed, the stock surface
would be completely sealed by a thin natural layer thereby terminating the
process.

    However the acid serves to dissolve this barrier layer at the molecular
level exposing the Al substrate in small pitts or pores surrounded by the
oxidation layer.[4] The exposed substrate continues to oxidize depositing
new layers of alumina around the pore; thus the acid serves to perpetuate
the oxidation reaction to yield a thicker barrier layer. It is these nanopores,
on the order of tens to hundreds of nanometers in diameter, that receive the
coloring or sealing agents otherwise the pores will themselves oxidize when
exposed to the atmosphere.[7] But this site is concerned chiefly with this
resulting nanoporus array in the context of a template for nano-structured
devices.

P. D. Wallace
UAF Spring 09'
Steve's Templates