Semi-conductive properties of melanization of chromate zinc plates

The anodic polarization curves of chromate zinc plates were studied under the condition of with and without illumination. It is found that the chromate film of zinc plate is characterized by an n-type semi-conductive property. The illumination on the surface and the addition of Pb in the surface raises the anodic photocurrent and plays a part of the n-type property in the film. Contrarily, Ag in the film plays a part of the p-type dopant. The effects of quantitative Pb²⁺ and Ag⁺ in bath on the melanization(black patina) of zinc plate after chromate treatment also has been studied by comparing the lightness differences AL by means of a whiteness meter. It is found that the lightness decreasing process ( the melanization rate) is consistent with the Wagner equation ΔL = kt^t/2 + C, and that Pb in the film increases k, on the other hand, Ag in the film decreases k of the melanizing rate.