Analysis of Tarnished Copper Foil

Tarnished copper foil was investigated before surface treatment. Its surface color changed to band strips (streaks) of soft reddish-brown, dark blue, purple, dark blue, and soft reddish-brown. Its surface morphology was related to the color change. Actually, this surface oxide film formation caused the color change. Surfaces of all samples were composed of CuO and Cu2O. The fractions of these oxides differed. The soft reddish-brown out-end sample has the lowest CuO content ; all other samples show almost identical CuO contents. The oxygen contents of the out-end sample show the lowest value. Its degree of oxidation was the lowest of all samples. The out-end and center samples show an almost constant CuO content region. In contrast, the CuO content of the middle of the samples decreased gradually with increasing depth from the surface. The color samples, out-ends, and center samples have similar depth profiles of CuO contents, but they have different surface contents of CuO. However, the other color samples, the middle two samples, show almost identical CuO depth profiles and contents. Therefore, it is concluded that the main factor of color change is the CuO depth profile.