Rust on outdoor steel sculpture

Bonni-Dara Michaels <michaels<-a t->ymail< . >yu< . >edu> writes

>Several years ago, we received a stainless steel sculpture which was
>erected outdoors.  It shortly developed rust spots.  My predecessor
>cleaned it with "Zud."
>...
>The sculpture again has unsightly patches of rust.  Do we try to
>clean it, and if so, with what and how do we proceed?

It is unlikely that your leaving the piece alone will do any good,
nor will cleaning as a single action.

The rust you are seeing is basically an electrolytic process that
occurs in the presence of oxygen. Essentially you have cathodic and
anodic centres within the metal. When a circuit is made between
these centres, electrons flow causing the iron to react with
available oxygen and thus producing the corrosion that you see as
rust. The circuit is generally completed by the presence of moisture
from atmospheric condensation or rain. Although this is by no means
the only method of completing the circuit, it is probably the most
common.

To halt this kind of corrosion, it is necessary to inhibit the flow
of electrons between the centres, as without a completed circuit,
the reaction with oxygen is minimal.

This is usually accomplished by the application of an insoluble
insulator which will create a barrier between the object and the
principle components of corrosion. This insulator is usually a wax
or lacquer (or indeed a combination of both) applied to a perfectly
dry object (any trace of residual moisture on the object will allow
corrosion to continue beneath the lacquer).

I would recommend that after cleaning the object, you swab it with
acetone, or some other water displacing solvent, then apply a
lacquer such as Ercaline over a layer of microcrystalline wax, both
of which are available from Conservation Resources. You might find
that it is necessary to perform such treatment as frequently as
annually, although these lacquers may last up to a decade. It is all
dependant upon your local weather system.

Although inhibiting the main cause of corrosion as suggested above
will slow down the process considerably, it will not entirely halt
it. You may find after a trial period that you will need to
implement an alternative procedure such as removing the object to an
atmospherically controlled interior setting, using a volatile
corrosion inhibition agent, or in an extreme situation, storage in
an oxygen free environment.

All of these however are slowing actions, not halting actions. I
believe that it was reported in a Museums Association Journal
several years ago that an iron meteorite owned by a museum, and
stored in the ideal setting of a sealed enclosure containing an
Argon gas atmosphere with a relative humidity of next to zero is
still corroding, but the rate is so slow as to be unnoticable.

I hope this is of help. Regards,

Jonathan Farley

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                  Conservation DistList Instance 11:18
                  Distributed: Friday, August 22, 1997
                       Message Id: cdl-11-18-002
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