Outdoor sculpture

Mark Anderson <manderson [at] wdm__ca> writes

>I was recently asked advice upon an outdoor sculpture, as this is
>not an area I am familiar with I thought I would see what other
>people suggested. The sculpture is a large stainless steel monument
>that is 6.6 m high and 3. 6 m wide. It is located at the base of a
>heavily trafficked bridged and a not as widely used thoroughfare.
>The sculpture is exposed to a lot of road debris including sand and
>salt in the winter and street cleaners in the warmer months. It is
>also splashed frequently in rainstorms.
>...
>The question posed were: was the material used not a high quality
>stainless steel as it is already starting to rust as not every sheet
>of steel used has rust on it (thus resulting in the artist to be
>brought in to do some work); what type of cleaning can be done on
>the sculpture; and is there a coating that can be applied to the
>sculpture to prevent future rusting but still give the sculpture a
>silver look.

There was a good reply by Virginia Costa in Conservation DistList
Instance: 24:19 Wednesday, October 6, 2010 and I would like to
amplify on her response from a materials perspective rather than a
conservation viewpoint.

Stainless steels are iron/carbon alloys with at least 10.5% chromium
although the grades with good corrosion resistance have alloy
additions equivalent to at least about 18% chromium. The chromium
forms a protective, self healing oxide layer a few molecules thick.
The higher the alloy content, the better the resistance to corrosion
but for atmospheric exposures it is unusual to need more alloy
additions than the equivalent of about 25% chromium.  The presence
or absence of magnetism has nothing to do with resistance to
corrosion.

For any given composition, the corrosion resistance can be increased
by chemical treatment to remove impurities (mainly sulphides) in the
stainless steel and, if the treatment is an oxidising acid (such as
nitric acid), it will also strengthen the oxide film.  The same
effects can be achieved by electropolishing which has the added
advantages of slightly smoothing sharp peaks (left after abrasive
grinding) and rounding edges as well as impairing a bright lustre.
It is not usually a mirror reflector.  Electropolishing is usually
done in a bath--not on site.

Corrosion resistance is also strongly affected by surface roughness
with a mirror polish offering the best corrosion resistance if only
a mechanical polishing process is used.  An oxidising acid treatment
on a mirror or near mirror polished surface will provide a
resistance to corrosion approximately equivalent to an
elecropolished surface.

Clean and smooth is the aim for best resistance to corrosion.

To maintain a bright finish, the surface must be kept clean to allow
oxygen and water to reform the passive film after it is affected  by
(usually) chlorides from road salt or the sea. Rain may be
sufficient washing if the structure is freely draining.  If the
sculpture is rain- shadowed, it must be washed by low chloride water
and detergent and rinsed with low chloride non-scaling water. Never
use bleach or abrasive cleaners.  How often depends on the load of
grime. A rule of thumb is to imagine it was a window that you would
clean so you could see through it.

Coatings will dull the surface and if scratched will trap pollutants
and generate a rust line. Temporary oils can be used but they must
be UV stable and must be renewed.  They also tend to collect grime.

Pickling was suggested for rust spots but it uses hydrofluoric and
nitric acids and will locally dull the surface.

Graham Sussex
Australian Stainless Steel Development Association


                                  ***
                  Conservation DistList Instance 24:20
                Distributed: Wednesday, October 13, 2010
                       Message Id: cdl-24-20-004
                                  ***