Tin corrosion

Jan Willem Pette <jan-willem.pette [at] museum__uu__nl> asked about tin
corrosion.

I work as a pipe organ restorer, and very often I have to face
problems deriving from tin degradation. In fact, antique pipe organs
have facade pipes made with tin sheets, which is in almost all cases
more or less affected by the problems you mention.

My information and experience mainly regard the second of your
problems, as tinned iron is (was) not so frequently used in pipe
organs.

Theoretical literature on the subject affirms that the phenomenon
you have observed derives from a crystalline transformation in the
tin sheet, which converts the metal to a gray-black and powdery
consistence, the "tin pest". Such transformation should happen very
slowly at temperature lower then 13 deg. C.

Actually, several experiments and observations showed that
development of "tin pest" is a very rare occurrence, and what really
happens in most cases is an oxidation of the metal, which converts
it to tin monoxide (SnO) and bioxide (SnO2), something similar to
the common oxidation of iron. If the tin part of the object is not
immersed in water, then the corrosion is classified as atmospheric
corrosion; it is a sort of electrochemical oxidation, which takes
place when some layer of condensed water collects on the object. It
has been showed that the corrosion is greatly accelerated by the
presence of SO2, NO, NO2, and H2S in the air, then the polluted air
of modern cities greatly accelerates the degradation of tin objects.
It seems that the presence of small quantities of other metals in
the alloy also accelerates oxidation. Also, it has been referred
that intergranular corrosion may have some part. As far as I can
say, strain due to bending of the sheet should have no effect on the
corrosion phenomenon.

The process of corrosion is irreversible, then it is impossible to
reconvert the oxide layers to the original metallic state, at least
with the limitations we are subject to when dealing with artistic
and museal objects.

In literature, it has been suggested to remove the oxide layers, and
fill the gaps with tin or soldering alloy; but in many cases, as
perhaps yours, it could be desirable to maintain the oxided layers.
In such cases, the only treatment is to apply some wax on the
surface; it provides the double benefit of acting as a consolidant,
and preventing the water from keep in contact with the metallic
surface, so avoiding further corrosion. I have observed on occasion
that tin pipes, which was treated with wax to fill gaps deriving
from corrosion some 80 years ago, had no further corrosion. Also,
keeping the object in dry environment avoids further oxidation; it
has been showed that in a controlled environment with less then 50%
humidity, the corrosion stops.

It could also be taken into consideration the possibility of using
some sort of resins to provide a mechanical separation of the metal
from the external humidity. For an example, Paraloid B72, which is
currently used as a consolidant in several fields of conservation,
could be a good solution; anyway, it should be experimented if it
would have some negative kind of reaction with the metal. There is a
variant of the paraloid, that is commercially called
"metal-methacrylate", which I currently use to protect iron objects,
with good results.

I would be happy if someone else in the list is aware of recent
issues about this subject. I am especially interested in experiences
related to new materials for gap filling, alternative to the use of
tin or soldering alloys.

Bibliography

    T. Stambolov
    "The corrosion and conservation of metallic antiquities and
    works of art", Central Research Laboratory for Objects of Art
    and Science, Amsterdam.

    M. Leoni
    "Elementi di metallurgia applicata al restauro delle opere
    d'arte" Opus Libri, Firenze 1984.

Leonardo Perretti
Musical Instruments Restorer
71033 Casalnuovo Monterotaro
Italy


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                  Conservation DistList Instance 14:50
                 Distributed: Wednesday, March 28, 2001
                       Message Id: cdl-14-50-003
                                  ***