Experimental and Theoretical Investigation of Pitting Corrosion of Nickel in Chloride Solution
Harb, John Naim
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https://hdl.handle.net/2142/69792
Description
Title
Experimental and Theoretical Investigation of Pitting Corrosion of Nickel in Chloride Solution
Author(s)
Harb, John Naim
Issue Date
1988
Doctoral Committee Chair(s)
Alkire, Richard C.
Department of Study
Chemical Engineering
Discipline
Chemical Engineering
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Engineering, Materials Science
Abstract
Pitting corrosion of nickel in 0.5M NaCl (pH 6.3) was investigated with use of single corrosion pits under both stagnant and flow conditions. The pit dissolution current was found to vary as a function of the applied potential. The pit surfaces were crystallographic, although the general shape of the pits observed was approximately hemispherical. A mathematical model was developed to simulate pitting under stagnant conditions in an axisymmetric domain which closely resembled the actual pit geometry. The model included metal ion complexation, surface kinetics, and transport by migration and diffusion. Potential dependent dissolution was simulated with use of the Tafel expression and found to represent adequately the experimental data over the range of potentials studied.
Measurement of pit current as a function of time was made for various hydrodynamic conditions. It was found that, for pits which initiated under stagnant conditions, the sudden imposition of flow caused pit repassivation provided that the pit Peclet number (Pe) exceeded about 1000. At lower Pe, the flow was not sufficient to deactivate the pits, although a decrease in the pit dissolution current was observed. The mathematical model was extended to simulate pit growth in the presence of flow. Theoretical calculations predicted a drop in the pit dissolution current due to flow, in agreement with experimental observations. In addition, theoretical calculations of Pe = 1000 showed rinsing of the pit cavity. Thus, pit repassivation at high Pe occurred owing to disruption of the local pit environment by the flow.
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