THE FORMATION OF BLACK PATINA ON COPPER IN HUMID AIR CONTAINING TRACES OF SO2

The formation of a black patina on copper in humid air containing trac es of SO2 was investigated using on-line analysis of SO2 and corrosion product characterization (x-ray photoelectron spectroscopy, Fourier t ransform infrared absorption spectroscopy, x-ray diffraction, and quan titative analysis of sulfite). In humid air (>75% RH) with 4 to 69 ppb SO2, a dull black cuprite patina (200 to 300 nm thick) formed after 2 0 h exposure. However, when concentrations bf SO2 were higher, copper remained shiny, as did samples exposed at low humidity. Sulfate was th e dominant sulfur species on shiny as well as on black samples. At hig h SO2 concentrations copper is suggested to be passivated by a layer o f chemisorbed sulfite on the thin air-formed oxide film, while at low concentrations the film breaks down. Corrosion mechanisms are suggeste d for this active/passive transition on the copper surface. The format ion of cuprite on the black samples resulted in a high corrosion rate, and an inverse correlation between SO2 concentration and corrosion wa s found. These results are in agreement with field studies on outdoor copper and bronze where the formation of a black cuprite patina and a high corrosion rate is observed in environments with low levels of SO2 .