Anode to Cathode

   So far we have discussed the commercial advantages of anodizing Al,
the recommended sample preparation, and the key role effected by the
electrolyte; we now discuss the anodization process itself including proper
circuit connections and the electrochemical surface oxidation of the stock
sample.

    When speaking about batteries or power supplies, one refers to the
negative side as the cathode and the positive side as the anode. The an-
odization of Al then involves connecting the sample to the positive side of
the battery such that the sample itself becomes the anode. The cathode
is connected to the negative side of the power supply and can be lead, ti-
tanium or 6000 series Al alloy, but not iron or steel as the sulfuric acid in
the electrolyte will quickly destroy iron based metals.[6] Using Al cathodes
is common as it is cheaper than titanium, and lead can sometimes contain
unknown metals or alloys picked up during the smelting process. The cath-
ode should be comparable in surface area to the anode or in this case the
sample itself.[6]

    Samples are generally suspended from Al wire or Al racking so they hang
freely in the electrolyte bath; this can pose a difficulty as the connection
needs to be firm enough to support the sample and also to allow sufficient
current to flow, but no anodization will occur at the connection site. In
the formation of nanoporus alumina templates this is not usually an issue
as a predesignated connection point is allowed for in the sample.

    After connecting the anode and cathode to the power supply and plac-
ing both in the electrolyte bath a direct current is passed through the
circuit initiating the oxidation process.[7] Notice that the anode and cath-
ode are not in physical contact but rather the the circuit is completed by
the conductivity of the electrolyte. The positive charge on the anode repels
oxygen molecules from the surface which combine with hydrogen atoms in
the electrolyte to form aluminum oxide on the surface. As we mentioned
previously, the sulfuric acid dissolves a small depression in the alumina
layer exposing the substrate in the form of a nanopore there by perpetuat-
ing the oxidation process.

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