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.

Under the conditions described, all stainless steels will rust.
Stainless steels are not "rust-proof", only more corrosion-resistant
if they are taken care of.

Concerning the question on materials, only a chemical/metallographic
analysis will tell you what you have, but it is the question whether
or not you are allowed to take a sample. (Or check the artist's
records.) My experience is that artists choose one of the standard
types of stainless steel, AISI 314 or 316. These are commonly used
for most architectural applications and are "relatively stable" in
outdoor air and not as expensive as other types (since they are the
most commonly used). Most stainless steels are not magnetic, and the
ones that are, are more specialty high-strength types for industrial
applications.

Note that a pure chemical analysis is not sufficient. You will need
to have a metallographic analysis done to see what kind of
microstructure the steel has, and whether it is the correct one for
the application, read further on.

Concerning remedies, if you want to do it right, you should have a
"real" materials scientist do a complete examination of the
sculpture. There are many reasons stainless steels corrode, and
often on one part and not the other. However, the reason can only be
determined by on-site examination by someone with the proper
background and experience. Our experience shows various problems:

    Rain droplets hanging on dirt and grime particles lead to
    so-called "pitting" or "crevice" corrosion, evidenced by a
    spotty appearance of the rust. The wind will play a major role
    here. We have seen statues with rust on the lee side of the
    statue, protected from the wind, thus slower to dry. These kinds
    of conditions could explain why you see rusting on one part and
    not on the other.

    Exhaust gases from the traffic provide all sorts of elements
    (road salt is a real bad guy, sulfur also), which combined with
    rainwater will accelerate corrosion (pitting, crevice,
    intergranular corrosion are the technical terms)

    Poor welding evidence by corrosion along the weld line (not on
    the line but right next to it)

    Poor casting: We had a case hear of a stainless steel statue
    which was cast and exhibited what looked like pitting corrosion.
    Small unobtrusive samples taken and metallographically prepared
    showed however, that the statue was poorly cast. This meant that
    locally, the material was not well mixed and did not have the
    proper composition. For our case, this occurred only in certain
    areas of the sculpture.

Given that examination, here are a few general suggestions. These
will have to be tailored to fit to your situation:

    All work should be conducted under the watchful eye of a metals
    conservator (restorer). There are many conservation and ethics
    issues involved in treating outdoor sculpture (just like there
    are with any indoor objects). A (non-conservation)
    materials/corrosion expert is also to be recommended.

    Regular cleaning is, as Ms. Costa points out, one of the major
    requirements for stainless steel outdoors. The artist's 5 and 10
    schedule is actually far too little for the conditions you
    describe. Annual inspection and the removal of all dirt and
    grime would be a minimum.

    There are acid solutions which can be used to remove corrosion
    products and repassify the stainless steel, but one should talk
    to a corrosion consultant about that. If you do any
    post-cleaning polishing, consult the artist concerning
    appearance.

    Furthermore, contrary to what Ms. Costa writes, there certainly
    are coatings for stainless steels which can be used to protect
    the stainless steel. Again, a corrosion consultant can point you
    in the right direction. However, there are ethical issues
    involved, since coatings will age and discolour with time, and
    would change the appearance of the statue even when knew.
    Assuming the artist is still alive, you would need to consult
    with him/her. Such protective coatings also need to maintained,
    and eventually removed and reapplied.

    Depending on their position and the aesthetics/ethics of the
    situation, it might be useful to put up unobtrusive
    "splash-guards" around the statue to reduce the amount of road
    grime which is thrown up on the object.

    Look into improving the drainage under the object.

Dr. W. (Bill) Wei
Senior Onderzoeker
Instituut Collectie Nederland
PO Box 76709
NL-1070 KA Amsterdam
+31 20 3054 741


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                  Conservation DistList Instance 24:20
                Distributed: Wednesday, October 13, 2010
                       Message Id: cdl-24-20-003
                                  ***