Studies On Corrosion Electrochemical Behavior Of Cu-Ag Alloy With Different Grain Size

Nanocrystalline powders have been applied in many fields recently, and may be prepared in many ways. Aqueous reducing method is one of the classical methods used to prepare nanocrystalline powders. This method has many merits, such as, convenience of operation, simplicity of the process, controlling grain size and shape easily, mixing in molecular level, etc. Thus, it has attracted much attention. In addition, corrosion resistance of nanocrystalline materials increase or decrease after the grain size is refined, which has attracted widespread attention.Nanocrystalline Cu-Ag alloy powders are obtained by aqueous reducing method. The nanocrystalline and the corresponding coarse grained Cu-Ag bulk alloys are prepared through hot pressing at 650℃in vacuum. The corrosion electrochemical behavior of Cu-Ag alloys with different grain size is compared in some media containing sodium sulfate, chloride ions, hydrogen ions or hydrogen ions plus chloride ions, and meanwhile the effect of grain size on corrosion electrochemical behavior of Cu-Ag alloys are also studied.The grain size of Cu-Ag alloy powders obtained by aqueous reducing method is about 45 nm, and about 60nm after hot pressing. The microstructure is homogeneous.In the media containing sodium sulfate, chloride ions, hydrogen ions or hydrogen ions plus chloride ions, the corrosion current densities of nanocrystalline Cu-Ag alloys are higher than those of the coarse grained materials. It shows that the corrosion resistances of Cu-Ag alloys decrease when the grain size is refined. In the corrosion processes, the passivation ability of nanocrystalline Cu-Ag alloys is better than that of the coarse grained materials.In sodium sulfate media, there are no passivating phenomena for Cu-Ag alloys with the different grain size. The corrosion processes are controlled by the electrochemical reactions.In the media containing chloride ions, the corrosion processes of Cu-Ag alloys with the different grain size are controlled by the electrochemical reactions initially and afterwards diffusion. Only when the chloride ions concentrations are 0.10mol/L and 0.5mol/L, there are stable passivating films over the nanocrystalline Cu-Ag alloys. With the increment of chloride ions concentrations, the corrosion current densities of Cu-Ag alloys with the different grain size increase and therefore corrosion rates become faster.In acidic media, the electrochemical impedance spectroscopy (EIS) plots of Cu-Ag alloys with the different grain size consist of two continuous capacitive loops at high and median frequency portion and a diffusion tail at low frequency portion, thus, corrosion processes are controlled by diffusion finally. When the chloride ions concentration is 0.5mol/L, Cu-Ag alloys with the different grain size produce the second passivating phenomena, and there are stable passivating films over the alloys finally. The passivity maintaining current density of coarse grained Cu-Ag alloys is higher than that of nanocrystalline materials. This shows that the power consumption of maintaining the passivating films of nanocrystalline Cu-Ag alloys is lower than that of the coarse grained materials in the same condition. With the increment of hydrogen ion concentrations, the corrosion current densities of Cu-Ag alloys with different grain size have no obvious change. This shows that Cu-Ag alloys with different grain size have better corrosion resistance in acidic media.In acidic media containing chloride ions, EIS plots of Cu-Ag alloys with different grain size consist of two continuous capacitive loops. Only when the hydrochloric acid concentrations are 0.10mol/L and 0.5mol/L, there are stable passivating films over the nanocrystalline Cu-Ag alloys. With the increment of hydrochloric acid, the corrosion current densities of Cu-Ag alloys with different grain size increase, therefore corrosion rates become faster.