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Corrosion Protection of Aluminium by Smart Coatings Containing Layered Double Hydroxide (LDH) Nanocontainers

Imanieh, Iman | 2016

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  1. Type of Document: Ph.D. Dissertation
  2. Language: English
  3. Document No: 51069 (57)
  4. University: Sharif University of Technology, International Campus, Kish Island
  5. Department: Science and Engineering
  6. Advisor(s): Afshar, Abdollah
  7. Abstract:
  8. Smart coatings which prevent corrosion by several mechanisms recently have been carefully planned. One of these mechanisms is the use of ion exchangers like Layered Double Hydroxide (LDH) in the coating network. In this study, the LDH containing a kind of green inhibitor (sodium molybdate) was synthesized and utilized in a Poly-Vinyl Alcohol (PVA) base coating applied on the aluminium (5054 series) surface. After evaluating the LDH performance, the inhibitor release from the LDH crystals was examined by changing significant parameters like MII/MIII (the molar ratio of divalent ions into trivalent ions), pH, aging time and temperature. Considering the LDH morphology or the thickness of the LDH flakes is a valuable study because they can control the kinetic release of the inhibitors. It was found that, high ratio of the MII/MIII causes thin layers (40 nm) of LDH and high pHs make spherical grains with the ordered LDH flakes. Morphologies with ordered flakes are better structures due to their enhanced ion exchange performance. In addition, it was found that high levels of aging time and temperature cause loss of intercalated inhibitors (near 50%) during the synthesis process and also quick releasing in times of unnecessary. In order to find the kinetic release of intercalated inhibitor a prediction model was introduced in 3.5% NaCl solution. The model expressed that the pH as the processing parameter has special effect on the kinetic releasing of the inhibitor. It can enhance the amount of released inhibitor up to 6 times larger due to its effect on the LDH morphology and structure. Also MII/MIII has serious effect on the amount of intercalated inhibitor and the release rate. In fact, the ratio alteration can affect the amount of final released inhibitor by a factor of 2 and its interaction with the pH is noticeable. The whole mechanism of releasing was recognized as Fickian diffusion and the released data were well fitted with the Weibull model. Corrosion measurements show the mechanism activation of the synthesized smart coating and improvement of the corrosion properties (the polarization resistance has been enhanced more than 3 times) in compare with the regular coatings. More LDH in the coating network results in better resistance (39250 Ω.cm-1) to corrosion for the produced coating up to approximately 5.7 weight% of the Cl- ions. Then adding more LDH to the coating, result in resistance decrement due to the defect formation in the polymer network
  9. Keywords:
  10. Smart Coating ; Layered Double Hydroxide (LDH) ; Release Mechanism ; Aluminum ; Active Corrosion Protection ; Molybate Sodium

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