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 sculpture is intended to appear as silver and shine at the foot
>of the bridge. The artist set up an inspection and maintenance
>schedule of every 5 and 10 years with major cleaning being required
>every 10 years.
>
>The problem with this sculpture is that after just four years of
>being out on display there are already signs of rust appearing on
>the stainless steel. The side facing the more highly traversed
>thoroughfare is highly splattered from road debris. Many of the
>points of contact are now starting to show signs of rust.

Stainless steel is a special class of iron-carbon alloy, containing
passivating elements, as chromium, nickel and molybdenium in its
composition. They will provide the formation of a thin and stable
oxide layer when the surface is exposed to the air, instead of the
voluminous rust often found when iron alloys corrode. The stability
of such thin passive layer is the basis of the corrosion resistance
of stainless steels, because it acts as a barrier, preventing
further reactions with the environment. The type of stainless steel
(nature and content of alloying elements) and the smoothness of the
starting surface are the main factors contributing to the adequate
formation and endurance of the film. Moreover, periodical cleaning
is indispensable, in order to avoid particulate matter deposits
acting as initiating spots for localized corrosion (pitting).

>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) ...

If the above cited conditions (composition, but also smoothness and
cleanness) are not respected corrosion will not be prevented;
anyway, concerning the type of steel used, you can start using a
magnet: the lowest quality steels (but yet stainless!) are magnetic
and maybe further send to analysis;

>... what type of cleaning can be done on
>the sculpture ...

    *   Regular cleaning (without rust) with normal detergent,
        rinsing and drying

    *   Cleaning to remove rust: use an adequate solution to
        dissolve rust and then renew the passive layer by
        application of an oxidizing solution.


>... and is there a coating that can be applied to the
>sculpture to prevent future rusting but still give the sculpture a
>silver look.

Stainless steels should not be coated; the corrosion resistance lies
in the passive layer, which will prevent the adherence of any
coating, conducing to pitting and filiform corrosion.

Virginia Costa
Conservation Scientist


                                  ***
                  Conservation DistList Instance 24:19
                Distributed: Wednesday, October 6, 2010
                       Message Id: cdl-24-19-002
                                  ***