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Corrosion Behavior of Electroless Ni-B-TiO2 Composite Coatings

Niksefat, Vahid | 2013

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  1. Type of Document: M.Sc. Thesis
  2. Language: Farsi
  3. Document No: 45045 (07)
  4. University: Sharif University of Technology
  5. Department: Materials Science and Engineering
  6. Advisor(s): Ghorbani, Mohammad
  7. Abstract:
  8. Usage of electroless Ni-B coatings as coatings with super hardness and wear resistance has been increased widely, but these coatings have low resistance to corrosion.In this project, in order to increase resistance to corrosion, titania powder of szie 200 to 300 nanometers is added to Ni-B coatings to make a composite coating of Ni-B-TiO2 that has better corrosion properties compared to Ni-B coatings. Ultrasonic use for creating composite coatings of Ni-B-TiO2 and the optimum ultrasonic and electroless bath conditions for creating optimum coating in terms of corrosion resistance are determined. These optimum conditions are as follows: speed of 300 rpm for substrate circulation in bath, 250 W ultrasonic power with 40khz frequency, pulse time of 1 minutes for turning on and off the ultrasonic and concentration of 2 g/liter of Titania. To analyze the corrosion resistance of Ni-B and Ni-B-TiO2 coatings, electrochemical impedance tests, polarization and the OCP determination are used. Moreover, surface morphology of coatings is determined using SEM. The effect of heat treatment in temperature of 450 centigrade and duration of 1 hour on corrosion, hardness and wear of samples is studied and the phase structure of the samples is analyzed using XRD test. Corrosion, roughness (surface topography) and wear resistance of Ni-B coating and composite coatings are measured before and after heat treatment.The result is that compositing the Ni-B coating using titania powder has led to increase in hardness (from 889 Vickers to 1263 Vickers) because compression of coating increases. Corrosion tendency of Ni-B-TiO2 coatings reduced due to the filling of voids and porosity reduction in the composite coating. Corrosion current reduced 6 times less than Ni-B coating and 36 times less than normal steel substarte, because it creates a protective layer on the surface and filling voids with titania particles. Finally, the heat treatment of Ni-B and Ni-B-TiO2 coatings results in increase in hardness (from 889 Vickers to 1669 Vickers) due to the polycrystalline phases (Ni3B Ni2B) was formed. Wear resistance of Ni-B coatings after heat treatment increased because of the creation of polycrystals phases and corrosion current of Ni-B and Ni-B-TiO2 coatings after heat treatment also increased due to formation of galvanic coupling between coating and substrate
  9. Keywords:
  10. Electroless ; Composite ; Titania ; Nickel-Boron Compsite Coating

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