Corrosion is among the most detrimental damage to befall a bronze statue. The severity of bronze corrosion depends on the metal's composition, the climate, pollutants in the atmosphere and previous protective measures. While a dark brown copper oxide or light green copper sulfate patina may sometimes be a desirable appearance,
severe forms of chloride corrosion or bronze disease require complex treatments to save the piece from destruction. (Learn more in the article The 5 Factors of Atmospheric Corrosion.)
Five Stages of Bronze Corrosion
The U.S. General Service Administration’s report on bronze protection lists five stages that a corroding outdoor bronze undergoes. Initially, a copper oxide film forms. The composition and rate that this initial film forms factor into how well this layer acts as a protective barrier. Next, the metallic surface converts to copper sulfate over areas with the harshest exposure. Over time, the next stage occurs where copper sulfate and sulfide products pool and streak. The appearance of black “scabs” and discoloration caused by the pooling and streaking of these compounds can make a piece unrecognizable.
Depending on the presence of chloride, the pitting stage spreads around the scabs and can spread below the surface and cause unseen damage. Finally, the exposed surface completely transforms to a green copper sulfate.
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