Corrosion study of Stanum-Bismuth-Copper Lead-free solder in acidic solution

The corrosion analysis of copper towards Sn-58Bi alloy was studied by investigated the changes of morphology, structural and elemental of solder before and after corrosion. The open circuit potential test (OCP) was conducted to study the corrosion potential of SnBi-Cu. In order to mimic the sea water concentration, 3.5 wt% of NaCl solution was used as electrolyte. The copper powder was added into Sn-58Bi solder with different wt% that are 0.25, 0.50, 0.75, 1.00 and 1.25 wt%. The morphology and elemental analyses revealed that there are new phase exist after the addition of copper which is Cu6Sn5 intermetallic compound (IMC). The gray and the light region represent to the Sn-rich and Bi rich phase respectively while the dark region represent to the ternary eutectic phase Cu6Sn5 particles. The ternary eutectic phases Cu6Sn5 are not clearly exist at the 0.25 and 0.50 wt% of copper but it is clearly exist as the wt% increase from 0.75 to 1.25. The OCP result shows that SnBi-Cu has almost same potential with Sn-Bi that is at -0.450 mV. However, the evolution of open circuit potential (Eoc) are different. The Eoc increase from 0.25 until 0.75 wt% of copper but the Eoc started to decrease at 1.00 and 1.25 wt% of Cu. This shows that 0.75 wt% is the optimum composition to increase the time taken for the formation of the corrosion layer to completely cover the materials.