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.
Microstructural and segregation effects affecting the corrosion behavior of a high‐temperature Bi‐Ag solder alloy in dilute chloride solution
In electronic devices the solder joint is exposed not only to the air but also to moistures and other corrosive media such as chlorine and sulfur compounds. Bi–Ag alloys meet the melting temperature requirement to be classified as high-temperature solders, therefore, knowledge of corrosion behavior is important for a long-term reliability of Bi–Ag solder connections. However, corrosion studies of Bi–Ag alloys are quite restricted in the literature. In this study, the role of the representative length scale of the microstructure as well as of the effects of Ag segregation on the resulting corrosion behavior of Bi–4 wt% Ag alloy samples are investigated. Cyclic potentiodynamic polarization and electrochemical impedance spectroscopy measurements were performed, and an equivalent circuit was also proposed to simulate the electrochemical corrosion behavior. All the used techniques indicated a tendency of better corrosion resistance associated with the sample having coarser microstructure and less Ag content.
In situ scanning tunnelling microscopic study of the initial stages of growth and of the structure of the passive film on Ni(111) in 1 mM NaOH(aq)
In situ electrochemical scanning tunnelling microscopic (STM) measurements of the growth mechanism of the passive film on Ni(111) single-crystal surfaces in an alkaline 1 mM NaOH aqueous solution are reported. Stepping the potential from −1,050 mV/SHE, where the terrace topography of the bare Ni(111) surface is observed, to −800 mV generates a continuous step flow resulting from a slow dissolution of the surface localized at step edges of the surface. Islands are observed on the terraces that possibly correspond to the specific adsorption of hydroxide anions, without formation of an ordered structure. The nucleation of the passivating oxide is preferentially located at the step edges at this potential. The ongoing dissolution at the nearby step edges not yet passivated leads to the formation of isolated 2D islands of the passivating oxide. At −780 mV, the formation of extended islands constituted of 2D nanocrystals (~2 nm) is observed on the substrate terraces and blocks the dissolution at the step edges. The nanocrystals have an hexagonal lattice assigned to the formation of a monolayer of Ni(OH)2(0001) in strained epitaxy on Ni(111). At E≥−630 mV, the formation of a facetted topography is observed as a result of the 3D growth of the passive film in tilted epitaxy on the substrate. Its structure is in excellent agreement with that expected for an unstrained 3D β-Ni(OH)2(0001) layer. This 3D layer of β-Ni(OH)2 is not reduced by a cathodic sweep of the potential down to −1,500 mV whereas the 2D islands are reduced under similar conditions. Ageing of the passive film shifts cathodically the potential of non-reversibility.