Dissolution of brass alloys naturally aged in neutral solutions – an electrochemical and surface analytical study

Abstract

The ageing process of brass alloys, the dissolution rate versus time, was studied in artificial saliva (pH 7.4) and in a phosphate buffer solution (pH 7). Mechanically polished brass alloys (Zn content 18–38 wt%) were exposed for 1, 3 and 16 hours to the solutions open to air at room temperature. The open circuit potential (OCP) and linear polarization resistance (R_p) increased asymptotically with ageing time, indicating a decrease of the dissolution rate upon time. In the artificial saliva the corrosion rate decreased from initially 60 μm per year to 0.5 μm per year after 16 h of exposure, and alloys with a higher zinc content in the alloy showed higher dissolution rates. In the less aggressive phosphate buffer solution the corrosion rate decreased only by a factor of two. This different reaction mechanism can be interpreted on the basis of the surface film composition determined by X-ray photoelectron spectroscopy (XPS) and X-ray induced Auger electron spectroscopy (XAES): while following the exposure to artificial saliva a thick protective film of CuSCN and zinc phosphate was formed, the film present on the brass alloys was very thin and was made of copper oxide Cu₂O and ZnO.