Page 172 - Copper and Bronze in Art: Corrosion, Colorants, Getty Museum Conservation, By David Scott

F I G U R E 5.2 Sulfur dioxide deposition rates at 70% RH in the presence and
5
and 00 ppb 0 3 , the deposition rate is substan­
absence of ozone. At 476 ppb S0 2
tially increased over the rate with S0 2 alone (Strandberg and Johansson 1997a) .

of sulfur dioxide. FIGURE 5.2 shows typical deposition rates for sulfur dioxide at 70% relative
humidity (RH); when some ozone is also present, the rate is increased. This increased rate of
deposition helps the sulfur dioxide to form a sulfite on the surface, and this is subsequently
oxidized to sulfate, which tends to attack the passive surface layer, thus encouraging electro­
chemical corrosion. The corrosion of copper with sulfur dioxide is enhanced by the presence of
nitrogen oxides and ozone (see CHAPTER 1, FIGURE 1.7).
Strandberg and Johansson (1997b) also carried out a study that related the appearance of
copper sheets to the corrosion products that had formed on them after four weeks of exposure
to the following conditions:

1. pure air with sodium chloride
2. air with ozone at 500 ppb and sodium chloride
3. air with sulfur dioxide at 476 ppb and sodium chloride
4. air with sulfur dioxide at 476 ppb, ozone at 500 ppb, and sodium chloride

The results of this study are summarized in TABLE 5.2. In three cases, a new basic copper sulfate
corrosion product, temporarily called "phase I," developed; it has not been fully characterized.
I
The existence of a new phase in such relatively simple exposure trials is an indication of how
much work remains to fully characterize the chemistry of these interactions. FIGURE 5.3 shows
the weight gain of copper samples exposed to 70% RH and 90% RH with a variety of pollutant
concentrations (Strandberg and Johansson 1997a). The results again confirm the enhanced cor­
rosion at different RH levels of the mixed sulfur dioxide and ozone gases.

B A S I C S U L F A T E S
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