Page 75 - Copper and Bronze in Art: Corrosion, Colorants, Getty Museum Conservation, By David Scott

I ASSESSING THE CORROSIVITY OF THE ENVIRONMENT The
International Organization for Standardization (is o ) 1 8 has adopted a series of criteria for evalu­
ating the corrosivity of the environment based on (1) measurements of the atmospheric gases
(sulfur dioxide, nitrogen oxides, ammonia, ozone, and hydrogen sulfide); (2) measurements of
particulate matter; and (3) the rates of corrosion of a series of control samples of copper, zinc,
iron, and aluminum. An important measurement concerning relative humidity and precipi­
tation is time-of-wetness (TOW), which was first measured with a sensor of platinum foil on
a zinc panel (Sereda, Croll, and Slade 1982). Time-of-wetness is the time period during which
the potential of the galvanic couple exceeds 0.2 V. This serves to express the rate of corrosion
in terms of milligrams per square decimeter per day of wetness (sometimes abbreviated as
MDDw), which can, in fact, be correlated with the rate of corrosion. Other, more sophisticated
sensors have since been developed to take this measurement, TOW can also be determined from
temperature and relative humidity data for a specific set of measurements, which are taken as
part of any detailed assessment of outdoor bronzes, TOW is closely related to corrosive events,
and, therefore, any projections regarding the maintenance of a bronze or the aggressiveness of
the environment to which it is subjected should include a TOW measurement. A measure of the
corrosivity of the environment, based partly on data for sulfur dioxide and sodium chloride pol­
lutants, was proposed by Knotkova-Cermakova and Barton (1982).
The simple model for TOW calculations, which are so critical to understanding metallic cor­
rosion events, has come under scrutiny recently, resulting in a much more sophisticated model
being proposed by Tidblad, Mikhailov, and Kucera (i998). They suggest that dose-response
relationships can be improved by using the original climatic data for temperature and relative
humidity, instead of TOW measurements. To accomplish this, however, it is necessary to account
for the nonlinear temperature effect by, for example, subdividing the data into cold and warm
temperature regions prior to analyzing the information. Another difficulty with the time-of-
wetness concept is that different corrosion processes may be going on in different areas of an
i
outdoor bronze statue —as, for example, n areas exposed to or sheltered from rain—which
results in limited utility for this parameter. More important is the RH, the amount and pH of
precipitation, and other ionic species and gases in the environment.
I ASSESSING THE CORROSIVITY OF THE METAL SPECIES In
addition to environmental data, standard test panels of iron, copper, aluminum, and zinc can be
used to assess the corrosivity of a metal species in ambient conditions. These panels, measuring
10.16 cm by 15.24 cm by 0.5 mm (4 by 6 by 0.02 in.), are used as controls for the particular met­
als to be tested or for coatings placed on them. Duplicate panels are withdrawn from the rack
for evaluation after periods of one, two, four, and eight years of exposure. The testing rack is
usually inclined 30 to the horizontal, facing south, and held in place with insulated plastic
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grommets with stainless-steel screws to avoid any galvanic effects with the superstructure of the

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