Status : Verified
| Personal Name | Buquiz, Jan Lowell P. |
|---|---|
| Resource Title | Electrochemical corrosion of Pb-Ca-Sn alloy in sulfuric acid solutions |
| Date Issued | 26 July 2019 |
| Abstract | Pb-Ca-Sn alloys are one of the primary materials for the positive grids of automotive batteries. Various studies have been done to improve the mechanical and electrochemical properties of such alloys. Despite these developments, corrosion of the positive grids remains to be one of the main causes of battery failure. The study aims to determine the corrosion behavior of different parts of an as-cast Pb-Ca-Sn alloy used in service to provide insights on improving grid design and casting conditions. The XRF results showed that the elemental composition of each grid location was comparable with each other averaging 97.85 wt% Pb, and 1.4 wt% Sn. The Ca content was determined at 0.07 wt% through ICP-OES. The microstructures of each grid location are characterized by Pb-rich α phase with eutectic mixtures on the intercellular boundaries. Differences in grain size and cellular size exist between the grid locations. Location A has higher grain and cellular sizes than locations B and C. Electrochemical techniques such as Electrochemical Impedance Spectroscopy (EIS), Liner Sweep Voltammetry (LSV), and Cyclic Polarization (CP) were applied on different grid locations at varying sulfuric acid (H2SO4) concentrations and temperatures. A three-electrode set-up was used with the Pb-Ca-Sn alloys as the working electrode, a platinum sheet as the counter electrode, and a mercury/mercurous sulfate reference electrode (Hg/H2SO4) in saturated potassium sulfate (2M K2SO4) solution. An electrical circuit model representing the electrochemical system consisting of electrolyte layer, electrical double layer, porous PbSO4 layer, and metal surface was fitted on the Nyquist plots to determine the charge transfer (R_CT ) and polarization resistance (R_P). Results showed that R_CT decreased with the increase in temperature indicating a decrease in corrosion resistance. The R_P was affected by the change H¬2SO4 concentration and temperature. The R_P increase with acid concentration at elevated temperature seemed to relate with the formation of a thicker corrosion layer indicating faster corrosion rates. On the other hand, LSV results showed that the corrosion potential (E_corr) shifted towards the negative potential as the acid concentration increased. The LSV results also validated the results derived from EIS that at elevated temperature, the (R_P) increased with the acid concentration. The corrosion rates derived from Tafel plots vary from 0.34 to 1.48 mm/year at 28℃, and from 0.21 to 2.73 mm/year at 40℃. The difference between the repassivation potential (E_P) and breakdown potential E_b derived from CP results indicated that the different grid locations had a small tendency for pitting. Photomicrographs showed the presence of such pits. And in general, differences in the corrosion behavior of the three grid locations exist based on the results of R_CT derived from EIS, and R_P derived from LSV. |
| Degree Course | MS Metallurgical Engineering |
| Language | English |
| Keyword | corrosion; electrochemical impedance spectroscopy; pb-ca-sn alloy; polarization resistance |
| Material Type | Thesis/Dissertation |
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