Evaluation of cooling rate on electrochemical behavior of Sn-0.3Ag-0.9Zn solder alloy in 3.5 wt% NaCl solution
The corrosion resistance of Sn-0.3Ag-0.9Zn alloy solidified under different cooling rates in 3.5 wt% NaCl solution was evaluated based on potentiodynamic polarization, electrochemical impedance spectroscopy and electrochemical noise. It is found that the corrosion resistance of this alloy improves with the increase of the applied cooling rate, which is attributed to the distinct microstructure. The furnace-cooled and air-cooled alloy with active Zn-rich phase and a microstructure formed by a coarser dendritic array associated with larger size of AgZn3intermetallic compounds exhibit a worse corrosion resistance due to a serious galvanic corrosion. Besides, the Mott-Schottky measurement and the X-ray photoelectron spectroscopy were performed to analyze the semiconductor properties and composition of the passive film formed on the surface of the alloy. It confirms that the stability and protective ability of the passive films formed on furnace-cooled and air-cooled alloy are worse than that of water-cooled alloy due to their higher oxygen vacancy defects and concentration of unstable SnO. Furthermore, the major corrosion product on the surface of the Sn-0.3Ag-0.9Zn alloys is tin oxide chloride hydroxide.