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      Corrosion of Carbon Steel Under Disbonded Coatings in Acidified Leaching Processes

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      TOLOUEI-ASBFOROUSHANI-THESIS.pdf (2.590Mb)
      Date
      2015-06-12
      Author
      Tolouei Asbforoushani, Maryam
      Type
      Thesis
      Degree Level
      Masters
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      Abstract
      In this research, corrosion behaviour of A36 carbon steel under engineered disbonded coating was investigated in sulphuric acid solutions containing sodium chloride and iron (III) sulphate. Scanning electron microscopy (SEM) and x-ray diffraction (XRD) analyses were carried out to study the morphology and phase composition of corrosion products formed on the carbon steel surface. The results of the SEM analysis showed that only general and pitting corrosion occurred on the carbon steel surface with the engineered crevice. The size of the pits increased as the sulphuric acid and sodium chloride concentrations increased. Moreover, the corrosion products had an open, irregular and loose structure at the pits mouth. The loose and open structure of the corrosion products facilitates diffusion of chloride ions, oxygen, water and contaminants into the carbon steel surface. In contrast, the corrosion products had a very compact and continuous structure outside the pits which provided a good protection against further corrosion. The x-ray diffraction analysis showed that the corrosion products layer mainly consisted of lepidocrocite (γ-FeOOH), goethite (α-FeOOH) and iron sulphide (FeS) on the crevice edges. The Pourbaix diagram of iron in sulphuric acid solution at room temperature indicates that iron sulphide is formed on the metal surface at different pH values. The akaganeite (β-FeOOH) diffraction peak was not identified in any spectrum which could be due to the low concentration of chloride ions in the solutions. Furthermore, the number of lepidocrocite peaks decreased as the sulphuric acid concentration increased from 10 g l-1 to 50 g l-1. The lepidocrocite is dissolved in the presence of sulphuric acid, and the dissolved ion acts as an oxidant to the metal and hence lower lepidocrocite peaks are identified. Electrochemical noise measurement (ECN) testing was also performed to investigate the corrosion process occurring on the carbon steel surface with the engineered crevice. The results of the ECN measurements showed that current increased during first few minutes and then decreased slightly. Also, the coupled potential did not change after an initial shift in negative direction. The low current flowing through the carbon steel electrodes and the constant potential showed that the crevice corrosion did not develop. These results imply that the crevice corrosion may not occur on the carbon steel surface in acidic solutions containing chloride ions.
      Degree
      Master of Science (M.Sc.)
      Department
      Chemical and Biological Engineering
      Program
      Chemical Engineering
      Supervisor
      Evitts, Richard; Heppner, Kevin
      Committee
      Odeshi, Akindele; Meda, Venkatesh; Boulfiza, Mohamed
      Copyright Date
      May 2015
      URI
      http://hdl.handle.net/10388/ETD-2015-05-2060
      Subject
      Corrosion, Carbon Steel, Scanning Electron Microscopy, X-ray Diffraction, Electrochemical Noise Measurement
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      • Graduate Theses and Dissertations
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