Stainless steel in outdoor sculptures

N.J. Bud Goldstone <budgoldstone [at] yahoo__com> writes

>Stainless steel reinforcements within cement mortar sculptural
>members had been used for repairs and conservation in 1979-1985 of
>the complex sculpture "The Watts Towers" by Simon Rodia. After only
>eight years (1995-1998) during failure inspections, we found severe
>damage from cathodic reaction between the stainless and Rodia's
>original mild steel reinforcements. Rodia's work was done between
>1924 and 1955. Has anyone else found this unexpectedly early
>problem? The degradation of the artist's 1924-1945 mild steel was
>severe in 1995 after stainless steel was used nearby, not
>necessarily attached to the original.

Galvanic coupling occurs when two dissimilar metals are connected
electrically. This may not need  more than a thin moisture film to
complete the electrical circuit. The more "noble" (higher on the
galvanic series) will promote the corrosion of metals lower in the
galvanic series.

Even if the stainless steel was not in direct physical contact with
the mild steel (yet connected by the concrete) there is still
opportunity for damage to occur as there is a slight electrical
connection. Complete electronic insulation would be the only way to
prevent the reaction between the stainless and mild steel. This may
be done with polymer, rubber or ceramic insulators at the points of
metallic connection.

Other ways to prevent galvanic coupling are to have the metals as
close together or the same in the galvanic series. Your
recommendation against using stainless reinforcements in a mild
steel sculpture is valid. Are you able to use mild steel
reinforcements of similar composition?

Another option is to put something in that is anodic to both
stainless steel and mild steel.  This is called "cathodic
protection" and is widely used in numerous industries.  If the mild
steel is not in physical contact with the stainless steel, you can
use an 'impressed current' method to maintain the potential of the
mild steel close to that of the stainless steel so neither will
corrode.

Deborah Lau
Analytical and Conservation Scientist
CSIRO MIT
Corrosion Science and Surface Design
PO Box 56, Graham Rd
Highett VIC 3190 Australia
+61 3 9252 6403
Fax: +61 3 9252 6253


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                  Conservation DistList Instance 16:19
               Distributed: Thursday, September 12, 2002
                       Message Id: cdl-16-19-003
                                  ***