Phase constitution, microstructure and corrosion performance of binary Sn–Bi alloys

In the present paper, the phase constitution, microstructure, and corrosion behavior of a series of binary Sn–xBi alloys (x = 40, 57 and 80 wt.%) have been studied. The materials were prepared by induction melting of high purity Bi and Sn lumps in argon. Subsequently, the alloys were rapidly solidified to form cast ingots. The as-cast Sn-57Bi alloy had a near-eutectic microstructure. The as-cast Sn–40Bi alloy consisted of dendritic Sn(ss) and eutectic grains, while the as-cast Sn–80Bi alloy consisted of dendritic Bi(ss) and eutectic. The corrosion behavior of the alloys was studied by open circuit potential measurements (OCP) and linear sweep voltammetry in an aqueous NaCl solution (1 wt.%). The experiments were carried out at room temperature in a three-electrode cell controlled by a potentiostat. The results were compared with those for pure Sn and Bi. The open circuit potentials of the Sn-Bi alloys increased in the following order: Sn<Sn-80Bi<Sn-40Bi<Sn-57Bi<Bi. The corrosion currents of the hypoeutectic Sn-40Bi and hypereutectic Sn-80Bi alloys were found to be higher compared to Sn-57Bi alloy. These results indicate that the near-eutectic Sn-57Bi alloy has a higher corrosion resistance compared to the Sn-80Bi and Sn-40Bi alloys. The corrosion behavior of the binary Sn-Bi alloys was found to take place by a galvanic mechanism. The eutectic regions in the alloys were preferentially attacked. Corrosion products on the alloy surfaces were identified using a combination of SEM/EDS and room-temperature XRD. The results are compared with those of previously studied Sn-Bi alloys. The corrosion mechanism is discussed.