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Electrodeposition of Ni-P/Zn-Ni Compositionally Modulated Multilayer Coatings using Single bath Technique

Bahadormanesh, Behrouz | 2018

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  1. Type of Document: Ph.D. Dissertation
  2. Language: English
  3. Document No: 50862 (57)
  4. University: Sharif University of Technology, International Campus, Kish Island
  5. Department: Science and Engineering
  6. Advisor(s): Ghorbani, Mohammad
  7. Abstract:
  8. In present study, with the aim of fabrication of a sacrificial coating with superior barrier properties in comparison with Zn-Ni monolayer, a single Zn-Ni-P bath was developed for electrodeposition of Ni-P/Zn-Ni multilayers. The deposits that were prepared in developed bath at 5 mA/cm2 was dominantly Ni-P containing less than 1 percent Zn and the alloy deposited in 60 mA/cm2 was mainly Zn-Ni containing around 3.2 wt.% P content. The effects of bath ingredients on the electrodeposition mechanism was analyzed through voltammetry. Also, the composition, surface morphology and the growth mechanism as well as the phase structure of deposits were studied via SEM, EDS and XRD analysis. Zn-Ni layers in CMMCs exhibited a 3D pattern of growth while that of Ni-P layers was 2D. Moreover, the Ni-P deposits tends to fill the discontinuities in Zn-Ni layers. Structural analysis demonstrated that the Ni-P/Zn-Ni CMMCs comprised of alternate layers of amorphous Ni-P and nanocrystalline Zn11Ni2 phase. The corrosion mechanisms of the deposits were studied via polarization, EIS, salt spray tests as well as chemical and structural analysis of corrosion products. The corrosion current density of CMMCs were around 1/10 of Zn-Ni monolayer and the time to red rust for Ni-P/Zn-Ni CMMC increased in comparison with monolayer Zn-Ni from 600 to around 890 hours. However, the time for white rust formation decreased. The corrosion mechanism of both Zn-Ni and Ni-P (containing small amount of Zn) was dealloying and preferential dissolution of Zn and the corrosion products were comprised of mainly Zn hydroxychloride and hydroxycarbonate. Based on the electrochemical character of the layers and the morphology of the corroded surface, the corrosion mechanism of multilayers was discussed
  9. Keywords:
  10. Electrodeposition ; Corrosion ; Laminates ; Single Bath Method ; Steel Corrosion

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