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Subject: Corrosion


From: Susan Stock <susans>
Date: Friday, October 12, 2001
>I am currently working on space related aluminum alloys (30+ years
>of age) which are combined with copper, magnesium, zinc and other
>white metals in trace amounts. Recent analysis of the corrosion
>forming on the aluminum was identified by SCMRE as cuprous chloride
>with some amounts of aluminum oxide in the mix, but primarily it was
>copper chloride.

>From way out in left field. I wasn't clear whether Lisa was working
experimentally or whether there was an actual 'artifact' composed of
this space age aluminum which was found to have cuprous chloride
corrosion products. Having just re-read Robbiola, Fiaud and Pennec,
'New Model of Outdoor Bronze Corrosion and Its Implications for
Conservation', ICOM, 1993, is it possible that the aluminum is
forming a stable oxide film and the copper then, corroding out
preferentially, through a galvanic mechanism? Important
considerations, including composition would be location of the
material and its environment- indoor or outdoor- and possible
sources of the chloride.

I was always under the assumption that Benzotriazole was slightly
acidic in solution. Therefore, pH may a consideration for acid
sensitive metals. Like Robert Lodge, I also heard a rep speak (many
years ago) on the industrial use of Benzotriazole. He was confused
at the success of our use of benzo on corroded objects and in
ethanol solutions. Industrially, if I remember correctly, they use
it in a water solution. Either he or another colleague mentioned
temperature, the use of it warm, as well. Over the years, I have
come to use benzotriazole as little as possible, partially because
it does not have a permanent result and partially because of its
possible carcinogenicity. I am not sure that its possible in this
situation, but I would suggest lowering the relative humidity to
control or at least reduce the rate of corrosion and hopefully
eliminate the further reaction of CuCl to 'bronze disease'. Also,
consider reducing or preventing chloride contamination.

Susan Stock

                  Conservation DistList Instance 15:32
                 Distributed: Tuesday, October 16, 2001
                       Message Id: cdl-15-32-005
Received on Friday, 12 October, 2001

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