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Re: Anodizing Etch

Neil Kolwey wrote:

In my earlier question about etching prior to anodizing, I presented the
question as caustic etching vs. acid etching, which is incorrect. The
anodizing shop asking the question actually presently uses a cleaning
step followed by de-smutting, but no etch. So the question is, why would
he want to add a caustic etch in addition to the cleaning step, and why
not. sorry for the misinformation.


If the shop is doing de-smutting after the cleaning step, then some etching
is already occurring in the cleaning solution, although perhaps under less
than ideal conditions. Some cleaning is always required, and the general
recommendation is to try to get by with a non-etch cleaner.

After that point, recommendations will diverge depending on the alloy of
the substrate, the surface condition, both current and desired, and the
type of anodizing. Usually a caustic etch is used to modify the surface and
make it look more even or actually be more even (i.e., remove
concentrations of aluminum soaps or even weird Al-Cu intermetallics that
can cause problems later).  Sometimes an acid etch is used for this
purpose, but fluorides are often required, so it is in general avoided.
Non-fluoride acid etches, when they fit the alloy and end purpose, are a
good choice.

So, if they have quality problems, adding a caustic etch and switching to a
non-etch cleaner would be a good first step.  However, because of the
tendency of the caustic etch to build in aluminum content more quickly than
would be true in an acid solution, a non-fluoride acid etch should be
checked out.  In all cases, because of the tendency to say "A little is
good, so more is better" in terms of dwell times, they should think through
why they are etching and how much they really need.  Usually all that needs
to be removed is a layer a few molecules thick.

Terry Foecke
Managing Partner
Materials Productivity LLC (fka Waste Reduction Institute, Inc.)
1821 University Avenue
Suite S-219
St. Paul, MN   55104
(p)  651-603-8282
(f)  651-603-8286