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Electroplating Nano Crystalline Nickel for Reduction of Corrosion Rate

Hajinejad, Davood | 2012

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  1. Type of Document: M.Sc. Thesis
  2. Language: Farsi
  3. Document No: 43451 (06)
  4. University: Sharif University of Technology
  5. Department: Chemical and Petroleum Engineering
  6. Advisor(s): Baghalha, Morteza
  7. Abstract:
  8. Polycrystalline materials are solids that are composed of many crystallites of varying size and orientation called “Grain”. As the grain size reduces to the values below 100nm, the overall material properties are remarkably changed, and thus the resulting nanocrystalline materials have provided much better engineering properties at the same chemical composition compared to the microcrystalline. The main objective of the present study is to synthesize a nanocrystalline (22nm-25nm) coating based on Nickel over the yellow-brass plates. To do this, an electroplating technique in a modified Watts bath is employed in which the current density and stirring speed varied between 3 to 9.5 Ampere per square Decimeter (A/dm2) and 0 to 600 round per minute (rpm), respectively. In order to determine the grain size of the coating, X-ray diffraction (XRD) technique is used. It is observed that increasing the current density and then stirring speed resulted in reducing the grain size. On the other hand, XRD results are verified with a theoretical model. The comparison shows a good agreement between the experimental data and the theoretical linear relationship between the logarithm of grain size and the logarithm of current density. In addition to this, and according to the slope of the linear equation, a Tafel slope of 0.4140 V is calculated which illustrates a very rational relationship among electrochemical systems. Moreover, the electrochemical behavior of the electroplated samples is investigated using a polarization test in a solution of NaCl 3.5%. The results showed that for those samples that have been prepared in a non-stirring system, it is hard to find a specific correlation between corrosion rate and crystallite size as well as current density. Therefore, the best corrosion resistance as much as 5 A/dm2 is measured. On the contrary, when increasing stirring speed induced turbulent behavior to the solution, the corrosion resistance decreased and therefore the corrosion rate in the polarization test increased. It was not also observed a significant passive behavior through the potentials higher than the corrosion potentials which could be attributed to the weak and defective nature of the passive layer over the surfaces of the electroplated Nickels. At the end, hardness of the specimens is measured by a Vickers Microhardness machine at the loading of 25 gr. Based on the hardness results, the highercurrent density and the finer crystal grains, the higher hardness of the specimens. However, the more turbulent behavior in the solution, the lower hardness observed. Here, the relationship between grain size and microhardness is similar to what was proposed by Hall-Petch, but not when there is turbulence in the system
  9. Keywords:
  10. Electroplating ; Coverage ; Grain Size ; Corrosion Resistance ; Hardness ; Nickel Nanocrystalline ; Nickel Electroplating

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