Electrochemical behavior of Sn-xZn lead-free solders in aerated NaCl solution
Electrochemical corrosion behavior of Sn-xZn solder alloys (i.e. hypoeutectic Sn-6.5Zn, eutectic Sn-9Zn and hypereutectic Sn-12Zn, in wt.%) in aerated 0.5 M NaCl solution was investigated by electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization techniques, aiming to assess the effect of Zn content on the corrosion behavior of Sn-Zn alloys. Scanning electron microscopy (SEM) equipped with EDX, X-ray diffraction (XRD) were performed to characterize the surface morphology and to identify the chemical composition of the corrosion products, respectively. It is found that increasing Zn contents lead to an increase in corrosion current density and the corrosion potential shifted towards more negative values, revealing that the corrosion rates increase in the order of: Sn-6.5Zn <; Sn-9Zn <; Sn-12Zn. The acquired electrochemical impedances were successfully fitted with an equivalent circuit model. Fitted results reveal that total impedance decreased with increasing Zn content, indicating that Sn-6.5Zn alloy exhibits the highest corrosion resistance. The results are further confirmed by surface morphology characterization of the corrosion products, which shows a more homogeneous product film for lower Zn content. Analysis of the corrosion products indicates that oxide hydroxychlorides are the dominant corrosion products. Based on the obtained results, it is suggested that Sn-Zn alloy with lower Zn content appears to be more attractive in terms of superior corrosion resistance.
Electrochemical Behavior of Sn-9Zn-xTi Lead-Free Solders in Neutral 0.5M NaCl Solution
Electrochemical techniques were employed to study the electrochemical corrosion behavior of Sn-9Zn-xTi (x = 0, 0.05, 0.1, 0.2 wt.%) lead-free solders in neutral 0.5M NaCl solution, aiming to figure out the effect of Ti content on the corrosion properties of Sn-9Zn, providing information for the composition design of Sn-Zn-based lead-free solders from the perspective of corrosion. EIS results reveal that Ti addition was involved in the corrosion product layer and changed electrochemical interface behavior from charge transfer control process to diffusion control process. The trace amount of Ti addition (0.05 wt.%) can refine the microstructure and improve the corrosion resistance of Sn-9Zn solder, evidenced by much lower corrosion current density (icorr) and much higher total resistance (Rt). Excess Ti addition (over 0.1 wt.%) led to the formation of Ti-containing IMCs, which were confirmed as Sn3Ti2 and Sn5Ti6, deteriorating the corrosion resistance of Sn-9Zn-xTi solders. The main corrosion products were confirmed as Sn3O(OH)2Cl2 mixed with small amount of chlorine/oxide Sn compounds.
In situ electrochemical scanning tunneling microscopy of Co(0001) single crystal electrodes in acidic solution
Abstract The surface of a Co(0001) single crystal electrode was examined by in situ electrochemical scanning tunneling microscopy (STM) in 0.05 M Na 2 SO 4 at pH 3 under potential control. Cyclic voltammetry indicated that the anodic dissolution of Co proceeds without passivation, and the anodic current was almost proportional to applied potential. The Co(0001)-(1 × 1) structure was observed on atomically flat terraces in the potential range between the hydrogen evolution and the onset of the anodic dissolution. At the foot of the rising anodic current-potential curve, a hexagonal surface lattice with (5 × 5) symmetry corresponding closely to that of CoO(111) or Co(OH) 2 (0001) was also observed. In the early stages of anodic dissolution, the process proceeded in the layer-by-layer mode, forming atomically flat terraces extending over large areas. By reapplying a negative potential after the anodic dissolution, well-defined terrace-step structures were restored, suggesting that the electrodeposition also took place preferentially at step edges.