Chromic acid

Cihat Uzun <objectconservation [at] yahoo__com> writes

>I am searching for information about using "chromium trioxide anhydrous"
>(chromic acids) in conservation, especially in cleaning application.
>Can it be used for cleaning?

I have used proprietary cleaning products containing chromic acid
for chemical cleaning and etching of aluminum in preparation for
application of epoxy, primers and outdoor paint systems.  One
product is West System Aluminum Etch part A and part B:  part A is a
phosphoric acid based cleaner/etchant and part B contains the
chromic acid (<1%).  I also used DuPont Two-Step Aluminum Prep Kit
(also a two part product that contained the chromic acid in part B)
but I'm not sure that this product is still available.   It may have
been replaced by DuPont 225S Aluminum Refinishing System Step A
Metal Cleaner and Step B Aluminum Conversion Coating and DuPont
13205S Aluminum Cleaner and 13206S Aluminum Conversion Coating, both
of which contain chromic acid in Step B and are part of their
aviation and automotive product lines.

The phosphate based cleaner/etchant cleans and removes corrosion
resulting from weathering in preparation for a subsequent conversion
coating.  The conversion coating creates a chromate layer on
aluminum and aluminum alloys that is gold or tan in color and
provides corrosion protection as well as for adhesion of primer
layers and other organic coatings.

As Dr. Knight and Mr. Moore note, chromic acid is extremely toxic
and a suspected carcinogen.  I always used the appropriate gear such
as respirators, goggles, safety suits and gloves when handling it.
Non-chromate conversion coatings and non-hexavalent chromium
conversion coatings are available and I would research whether they
would be suitable for a future project involving cleaning and
painting of aluminum.

Michael Belman
Objects Conservator
Carnegie Museum of Art
4400 Forbes Ave
Pittsburgh, PA  15213-4080


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                  Conservation DistList Instance 23:38
                   Distributed: Sunday, April 4, 2010
                       Message Id: cdl-23-38-009
                                  ***