Rust on meteorite

Elizabeth M. Scott <emscott [at] ismhs__org> writes

>Our Natural history collection contains a few meteorite specimens.
>These have a high iron content and rust is forming.  Are there any
>storage tips that could help to deter or stop the rust formation
>(e.g. isolation--how, humidity control--recommended level)?  Are
>there any treatments that could remove the rust?

Firstly is this a pallasite or mesosiderite or is it an iron-nickel
meteorite alone?  It used to be thought that lawrencite, ferrous
chloride, was responsible for the dramatic collapse of iron
meteorites which could be reduced to a heap of fragments in a matter
of months following excavation. Well, it turned out that there was
no lawrencite at all and the deterioration is mostly due to the
akaganeite, beta-FeOOH, slowly releasing chlorides which encourages
further corrosion.

First of all you need to reduce the RH to very low levels, I cannot
quite remember the exact figure, perhaps someone else does, but
Turgoose showed that an RH of less than about 25% was necessary. I
think some people had trouble with this and a few iron objects would
collapse anyway as they dehydrated. Anyway, very low RH is a must,
oxygen scavengers would be good too in the enclosure.

Regarding removal of the rust, this depends on what you want to
achieve and the extent of the problem. Since the chloride in
akaganeite can be reduced but not eliminated by washing, this is not
an option for your meteorite I would have thought. Since the object
is of high scientific value in its own right, you do not want to
chemically treat the object, since changes to magnetite or other
iron oxides, or the use of rust converters should generally be
avoided.

David Scott




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                  Conservation DistList Instance 15:45
                Distributed: Thursday, December 20, 2001
                       Message Id: cdl-15-45-002
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