Insight into Oxidation of Ultra-Fine Lead Powder By Electrodeposition

Abstract

Pure lead powder has multiple application such as oil and gas exploration, X-ray shield, golf club manufacture, paints, lubricating grease and anti-friction products. Electrochemical deposition of lead powder can be obtained from both the acid electrolytes based on fluoroborate, acetate, fluorosilicate, nitrate, etc. and an alkaline solution. Morphological characteristics of lead powder with overpotential have been detailed in previous literatures. Nevertheless, oxidation of lead powder during electrodepositing process was out of view, which actually decreased the purity and quality of lead powders compared to atomized ones. In this investigation, ultra-fine lead powders with a minimum median diameter of 4.5 μm were electrodeposited in solution consisting of 2 M CH3COONa, 2 mM Pb(CH3COO)2 and pH values adjusted by acetic acid. The degree of as-deposited lead powder was defined as the content of PbO in wt %, and was determined by complexometric titration with EDTA. The morphology of lead powders shown by SEM was a classical dendrite and branches, which resemble a tree. The oxidation of lead powder reached to about 16% when pH value of electrolyte was 1 and decreased abruptly to around 6 % at pH from 3 to 7. The oxidation of lead powder was stable at ~5.5 % with the cathodic current densities from 23 mA cm-2 to 36 mA cm-2 and increased to 9 % and 13% at current densities of 10 and 45 mA cm-2 respectively. The concentration of gelatin as an additive accelerated the oxidation of lead powder. When ultrasonic was applied, the oxidation degree of lead powder became 3-fold larger than that of the stirred one. More importantly, the average sizes of powder have a directly relationship with the degree of oxidation. The smaller median diameter is, the larger degree of oxidation is. However, the XRD of the lead powder with about 16 % oxidation shows only metallic Pb with strongly preferred orientation in (111) plane, which indicated the amorphous PbO powders. The results of TEM confirmed the amorphous materials around lead particles, which suggested metallic Pb was partly transformed from PbO. Therefore, we suggested a mechanism of lead electrodeposition result in oxidation of lead as follows: Deprotonation: [Pb(H2O)]2+ → [PbOH]+ + H+ Electrochemical reduction: Lead reduction— Directly：[PbOH]+ + H+ + 2e- → Pb + H2O Indirectly: [PbOH]+ → PbO + H+ PbO + 2H+ + 2e- → Pb + H2O Hydrogen evolution— 2[PbOH]+ + 2e- → 2PbO + H2 2H+ + 2e- → H2 The reason of forming [PbOH]+ complex is due to the stability constant, lgβ=13.3, which is about 10 order of magnitude large than that of acetate complexes, lgβ=3.60. Figure 1