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Electrodeposition of Ag-Pd Alloy for Enhancing Corrosion Resistance

Yari, Mohammad Hossein | 2018

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  1. Type of Document: M.Sc. Thesis
  2. Language: Farsi
  3. Document No: 50626 (07)
  4. University: Sharif University of Technology
  5. Department: Materials Science and Engineering
  6. Advisor(s): Dolati, Abolghasem
  7. Abstract:
  8. Silver is a resistant metal against the oxidation under normal atmospheric conditions, but it reacts readily with sulfur bearing species in the environment and corrodes. Alloying silver with more noble elements, such as Palladium, is a promising approach for increasing corrosion resistance against the sulfur pollutants. The purpose of this research is to develop a plating bath for deposition of Silver-Palladium alloy with acceptable appearance and higher corrosion resistance than pure silver. For this purpose, a bath based on thiosulfate complexing agent was investigated. In this solution, with a bath containing 10 mM of each cation, deposits with palladium content of 6, 11 and 14 % were deposited with a current density of 2, 9, and 16 mA.cm-2 respectively. The obtained coatings are uniform, adherent and, in thickness of 1µm and below, are bright. The coating growth rate in current densities of 2, 9, 6 mA.cm-2 was 0.11, 0.32, 0.44 µm.Min-1 respectively, and coatings with a thickness of up to 5µm, were adherent and without any cracks or delaminations. In thiosulfate solution, silver with a reduction potential of -0.4 V is nobler than Palladium with a reduction potential of -0.6V. The microscopic investigation of the deposit represents a spherical grained morphology, growing in size with increase in deposition over potential, which indicates deposition under diffusion control. XRD analysis of the coatings exhibits the formation of single phase solid solution of Silver and Palladium. Linear polarization of alloys in a 50 mM solution of sodium sulfide, illustrates an increase in corrosion potential from -0.63V for pure silver, to -0.58V for Ag-6Pd, -0.42V for Ag-11Pd and -0.33V for Ag-14Pd, indicating that alloying Silver with Palladium, leads to a decrease in tendency of reaction between Silver and Sulfur in polluted environments. Similar result is obtained for the corrosion current investigation, showing a decreasing trend with increasing Palladium content of the deposit. From 0.006 mA.cm-2 for pure silver, it decreases to 0.004, 0.002 and 0.001 mA.cm-2 for alloys containing 6, 11 and 14% of Palladium respectively. Simulated atmospheric corrosion in a sealed box containing vapors of sodium sulfide solution, shows an increase in time of complete tarnishing of silver, from 2 hours for pure silver to 4 hours for an alloy with 6% Palladium and to 6 hours for an alloy containing 11% of Palladium, showing higher corrosion resistance of Silver-Palladium alloys in sulfur bearing atmospheres
  9. Keywords:
  10. Alloy Plating ; Electrodeposition ; Corrosion Resistance ; Silver-Palladium Alloy Coating ; Thiosulfate Bath

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