If the parts are truly black anodised, there will be no problem with any
cleaning method or process, except those that use a highly alkaline
aqueous cleaner, unbuffered for amphoteric metals. Unfortunately, many
parts, including most heat sinks, are not anodised at all, but
conversion coated and dyed. These should be cleaned only with
circumspection.
To explain the difference: true anodisation oxidises the surface of the
aluminium, forming open pits of oxide which are receptive to the dye.
The parts are then "sealed", usually by placing them in boiling water
for half an hour. This hydrolyses the dyed oxide pits by converting the
oxide to hydroxide, which assumes a greater volume and closing them off,
with the dye actually sealed within the structure. Conversion coating is
done by immersing the parts in, say, a phosphoric acid (there are
several types of conversion used) based mixture. This forms a
microscopically thin layer of aluminium phosphate which is receptive to
some dyes. However, the dye is not sealed in and will leach out again at
the drop of a hat or with a drop of solvent of almost any nature.
If, after cleaning, the part looks discoloured or the solvent takes on a
colour, then the chances are that it is a conversion coating, although
unsealed anodisation will also suffer somewhat (sealing is very
energy-intensive and long and is sometimes skipped for low-cost items by
unscrupulous anodisers).
Brian
Morse, Carrie wrote:
> Just came across an in-house spec that calls for some metal components that have a
> black anodized coating to be cleaned in an ultrasonic cleaner prior to assembly. We are currently
> NOT doing that, but, I suspect if we do, the bond between the component and the coating will
> break down.
>
> Before I test one and scrap it, are there any opinions (professional or otherwise?)
>
> -Carrie
>
> P.S. I don't have the specifics on the coating or the cleaning (ex: energy, time, etc.), but, just
> looking for a general opinion.
>
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