Research On Corrosion And Leaching Behavior Of Sn-based Solder In Soil Environment

Most of electronic information products are wasted and landfilled finally, which can be corroded by acid industrial waste water and acid rain, leading to the toxic heavy metals leached out, resulting in a potential threat to the environment. Corrosion is the intrinsic reason for the leaching of heavy metals from the metallic components for the electronic-waste. However, the environmental researchers focus on the migration characteristics of heavy metals in soil and water, and material researchers pay attention to the corrosion resistance and electrochemical behavior of metals, the literature is scarce on intersection between corrosion and leaching for metals in soil. Therefore, a reasonable experimental design for corrosion and leaching of heavy metals should be performed to illustrate the process of corrosion and leaching of heavy metals in soil, and reveal the relationship including corrosion electrochemical characteristic of metals, corrsoin products of iners surface and interface, and leaching and migration characteristics of heavy metals.Base on the investigation of corrosion and environmental pollution of waste electronic information products, the Sn-based solders were buried in the soil and leached by acidic solutions, to analyze the effect on solution components, concentration and solder components on the leaching behavior of heavy metals, and illustrate the corrosion products and characteristics of solder surface in the leaching process, combining the analysis of corrosion electrochemical characterisitics of solders in solutions to expound the corrosion electrochemical mechanism solders leached with solutions, and correlation mechanism between corrosion and migration of heavy metals. The main conclusions are as follows:(1) The leaching dynamics of heavy metals from Sn-based solders leached by the solutions with different components were clarified. The leaching amounts of Sn were significant for solders leached with H2SO4-HNO3, H2SO4-HCl and HNO3-HCl solutions, and the Sn leaching amounts of SnCu solders were higher than those of SnPb solders. The Pb leaching amounts related to the presence of SO42- ion or not, the presence of SO42- ion in solution contributed to an insignificant leaching amount of Pb in the leachate, but the Pb leaching amounts significantly reduced with increment of the concentration of SO42- ion; however, the absence of SO42-contributed to the greater leaching amount of Pb in the case of HNO3-HCl solution.(2) The effect mechanism of SO42-, NO3- and Cl- ions on the leaching behavior of Sn and Pb was clarified. The presence of SO42- and NO3- contributed to significant leaching amount of Sn, due to the formation of soluble Sn2+ after the active reaction of Sn; the presence of SO42-contributed to insignificant leaching amount of Pb, due to the preferential reaction and deposition of PbSO4, the presence of NO3- and Cl- ions contributed to the formation of soluble Pb2+ which could enter into leachate, resulting in significant leaching amount of Pb.(3) The effect of corrosion on leaching of Sn-based solders were revealed in different solutions. The loose tin oxide was deposited on the SnCu surface with microcracks, resulting in accelerating the corrosion rate, therefore the Sn leaching amounts of SnCu solders were higher than those of SnPb solders. Pb preferentially dissolved and PbSO4 had aggregated on the surface of SnPb solder in the presence of SO42- ion solution, inhibiting the leaching of Pb; the corrosion product was soluble PbCl2 in HNO3-HCl solution without SO42- ion, resulting in significant leaching amount of Pb.(4) The effect of galvanic corrosion characteristics of SnPb joint on leaching behavior was clarified. The effect of SO42-, NO3- and Cl- ions on the leaching behavior of Sn and Pb was still followed by galvanic corrosion of joint. With the presence of SO42- ion, the galvanic corrosion accelerated the significant leaching of Sn but insignificant leaching of Pb, due to the active dissolvble of Sn and the formation of PbSO4 on the surface of solder. However, the galvanic corrosion accelerated the significant leaching of Pb but insignificant leaching of Sn in the solution without SO42- ion, due to the active dissolvble of Pb and the deposition of Sn compounds which decreased the leaching amount of Sn.(5) By comparison of corrosion products and leaching mechanism of heavy metals of Sn-based solders leached with acidic solutions, the potential pollution of heavy metals was revealed. Sn could contaminate the underground water under the condition of the presence of SO42- and Cl- ions and mixed solutions (SO42--NO3-, SO42--Cl- and NO3--Cl-); however, Pb could contaminate soil with the presence of NO3- and Cl- ions, and contaminate the soil with the presence of SO42- ion, and pollute the underground water without SO42- ion or with lower concentration of SO42- ion.