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Pulsed Electrodeposition of Gold in Sulfite and Mixed Sulfite-thiosulfate Baths with Fine-grained Morphology

Malekpouri, Behzad | 2019

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  1. Type of Document: M.Sc. Thesis
  2. Language: Farsi
  3. Document No: 51739 (07)
  4. University: Sharif University of Technology
  5. Department: Materials Science and Engineering
  6. Advisor(s): Dolati, Abolghasem
  7. Abstract:
  8. One of the most common methods of coating gold on the surface of a metal is electrodeposition which is performed in cyanide and non-cyanide solutions. In recent years, gold cyanide baths have been replaced by Non-cyanide baths because cyanide baths are not suitable due to their very toxic properties despite good coating quality. In this project, the coating was performed through electrodeposition process in two non-cyanide gold baths including sulfite bath and mixed sulfite-thiosulfate bath. The results have shown that the stability of the sulfite bath is about 15 days, but the mixed sulfite-thiosulfate bath has a relatively high stability with 165 days, comparable to the cyanide bath. The main goal of the project was to investigate the effect of pulsed electrodeposition on the morphology of the coating, which was carried out through FESEM and AFM. The electrodeposition process was performed by direct current and pulsed current, and the effect of pulse frequency and duty cycle on the coating was investigated. The results of FESEM revealed that by applying pulse current the grain size generally decreased. Surface roughness has also been reduced by pulsed electrodeposition. The surface roughness of the direct current plating is equal to 9.64 nm, which is reduced by applying the pulse current and the minimum roughness was 10.11 nm at 25% duty cycle and 250 Hz frequency. Generally, the roughness of the surface is reduced by increasing the frequency and reducing the duty cycle. Cathodic efficiency was calculated in different conditions and the maximum efficiency was 98% for the coating in the frequency of 250 and duty cycle of 25%. Due to pulsed electrodeposition, the hardness increased in general due to the finer grain size, and the highest increase in hardness due to the using of pulsed current has been seen at a current density of 3 mA.cm-2 3 of 66.8 Hv for direct current to 92.6 Hv for pulsed current with a duty cycle of 25% and a frequency of 250 Hz. The light reflection rate of the coating was about 10% in the sulfite bath and reached 40% for the mixed sulfite-thiosulfate bath and increased to approximately 50% with the pulsed electrodeposition. It has also been observed that all the coatings from this bath have excellent adhesion and have not been cracked in the bending test. The optimum coating of gold is obtained in terms of pulsed electrodeposition with a current density of 3 mA.cm-2, duty cycle of 25% and frequency of 250 Hz
  9. Keywords:
  10. Gold Plating ; Sulfite Bath ; Mixed Sulfite-Thiosulfate Bath ; Coating Morphology ; Bath Stability

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