Flow Effects Upon the Repassivation of Iron and Cobalt in Neutral and Acidic Solutions
Weisbrod, Kirk Ryan
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https://hdl.handle.net/2142/66645
Description
Title
Flow Effects Upon the Repassivation of Iron and Cobalt in Neutral and Acidic Solutions
Author(s)
Weisbrod, Kirk Ryan
Issue Date
1981
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, Chemical
Language
eng
Abstract
In order to determine the mechanism by which metal reforms a protective oxide layer after removal though erosion or other mechanical means, experiments were performed in a specially designed flow cell. Particle impaction and subsequent depassivation were simulated through amperometric reduction of the oxide layer. Repassivation of iron, cold rolled steel, and cobalt in neutral and acidic aqueous sulfate solutions was investigated by observing electrochemical current and potential transients under controlled variation of parameters.
Experiments were performed in a flow channel with a 0.3 cm by 0.4 cm cross section for 4,000 < Re < 60,000. In order to determine the metal ion concentration at the surface, the mass transfer rate for both smooth and rough surfaces was obtained by ferricyanide reduction on a nickel sectioned electrode. For a roughness Reynold's number less than 2, the mass transfer rate to a surface roughened by anodic dissolution was at most 20 percent greater than for a smooth surface.
Following oxide reduction, an anodic amperometric step initiated passivation which was observed through current and potential transients. A mathematical model was developed from visual evidence for supersaturation of solution with metal salt with subsequent precipitation of a salt film at the moment of passivation. It was assumed that the mass transfer resistance could be modelled by a Nernstian diffusion layer to determine nonsteady-state concentration profiles. After the surface concentration of metal salt at the moment of passivation was determined, the time required to achieve maximum salt film thickness, and time for salt film dissolution, were calculated and compared with experimentally observed values.
Results indicated that for acidic solutions, a saturated concentration of metal salt at the metal interface was usually required for passivation under steady-state conditions. For short times, passivation occurred when the metal salt concentration reached a value 2-3 times the saturated concentration. Good experimental agreement for time required to achieve maximum salt film thickness, and the time for complete dissolution, was observed for short passivation times.
The interpretation of results was based upon crystallization phenomena and thermodynamics. Homogeneous nucleation was responsible for the constant level of supersaturation observed at short times to passivation. The role of the salt film was apparently twofold in acidic solutions. First, the salt film must form the barrier required to raise the surface pH to allow formation of (gamma)-Fe(,2)O(,3). Second, the ferrous ion concentration (for iron passivation) must be adequate to stabilize the (gamma)-Fe(,2)O(,3) layer from autocatalytic reduction as it grows across the iron surface.
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