Controlling the color of tin coating on brass by electroless plating

Brass is a metallic alloy that is made of copper and zinc and has a gold-like appearance. Tin coating could endorse the brass with yellowish up to silver-white color and improve its surface durability. In the present work, electroless plating of tin on the brass surface was investigated aiming to obtain a stable and controllable colored surface. In the presence of thiourea, the open circuit potential of brass (-0.652V) is more negative than that of tin (-0.532V), which makes the deposition of tin on brass feasible. The surface morphology and composition of the coatings were characterized by scanning electron microscope and energy dispersive spectrometer and the results indicated that a large number of tin clusters were formed on the brass surface. The amount of tin clusters determined the color of the coated surface, which can be facilely tuned by the plating time. The UV/Vis diffuse reflectance spectra were used for monitoring the changes of color at different plating time. The results show that there is a good linear correlation (R-2=0.9948) between the absorbance and the plating time, which can be used to control the extent of tin plating on brass. Finally, the cross-cut test showed that the Sn particles were firmly coated on the brass substrate.