Abstract
In oxidizing environments, most tin-based lead (Pb)-free alloys form a tin oxide that is easily eroded or mechanically damaged, affecting corrosion resistance and thus reliability of the soldered joints. In this study, the effect of microstructure heterogeneity on corrosion behavior of Pb-free solder candidate systems has been investigated on the example of as-cast and heat-treated alloys. The research was focused on a comparison between the corrosion resistance of binary Sn-Zn and ternary Sn-Zn-Cu alloys. Accelerated corrosion tests were performed by means of electrochemical methods in the sodium sulfate solution (VI), Na2SO4, of about 0.5 M concentration, pH adjusted to 2 by means of concentrated H2SO4 acid. In these tests, the corrosion potentials as well as polarization curves were determined for the selected alloys in as-cast state and after their heat treatment using different combinations of processing parameters. The measurements of basic electrochemical characteristics were made, i.e., the corrosion current (i corr μA/cm2) and Tafel coefficients, both cathodic (b c V/dec) and anodic (b a V/dec) ones. Detailed structural characterization of as-cast and heat-treated alloys before and after accelerated corrosion tests has been made under a wide range of magnifications using light microscopy and scanning electron microscopy observations. The results showed that structural heterogeneity of the examined alloys, attributed to the presence of secondary phases, and affected by their size and distribution, significantly influences the behavior of the examined Pb-free Sn-Zn-based alloys in the corrosive environment.
Highlights
Due to the considerable toxicity of lead (Pb), health concerns, and environmental as well as legislation reasons, efforts have been made to replace the conventionally used Sn-Pb solders by new Pb-free alloys
The results of accelerated corrosion tests made for reference Sn-Zn alloys (Fig. 2) and summarized results of the measurements of selected electrochemical parameters for both Sn-Zn and Sn-Zn-Cu alloys (Table 1) show a significant influence of the alloy composition on the corrosion resistance of as-cast solder candidates
The results indicate the crucial role of the alloy microstructure, the presence of secondary phases, on corrosion behavior of Pb-free solder candidates from both binary Sn-Zn and ternary Sn-Zn-Cu systems as examined by accelerated corrosion tests in the sodium sulfate solution (VI), Na2SO4, of about 0.5 M concentration, pH adjusted to 2 by means of concentrated H2SO4 acid
Summary
Due to the considerable toxicity of lead (Pb), health concerns, and environmental as well as legislation reasons, efforts have been made to replace the conventionally used Sn-Pb solders by new Pb-free alloys. Rare earth elements, Cu, Bi, In can effectively improve the wettability, mechanical properties and oxidation resistance of eutectic Sn-Zn alloys (Ref 6, 7). The effect of Al and Cr additions on the high-temperature oxidation resistance of Pb-free alloys from the Sn-Zn system was discussed in Ref 8. In spite of numerous literature data on corrosion behavior, the influence of microstructure and processing parameters on the corrosion resistance of some Pb-free solder alloys and their joints has not been univocally defined. The effect of microstructure heterogeneity on corrosion behavior of Pb-free solder candidate systems has been investigated based on systematic studies of the correlation between structure and corrosion resistance of as-cast and heattreated binary Sn-Zn and ternary Sn-Zn-Cu alloys
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