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Corrosion behaviour of nickel and monel in aqueous fluoride media.

University of British Columbia

The corrosion behaviour of nickel and monel in aqueous fluoride solutions was studied by potentiostatic polarization techniques and surface examination of the corroded specimens. Nickel does not exhibit the usual active-passive transition for 0 < pH < 4.0 but corrodes rapidly especially at the grain boundaries. In the range 4.0 < pH < 6.5 the nickel-polarization curve contains two active regions. Nickel is passive in contact with a fluoride solution with 6.5 < pH < 12.0. Polarization curves of nickel in fluoride solutions of varied pH's and fluoride ion concentrations in the range 4.0 < pH < 7.0 revealed that the current as a function of potential in the first active region is independent of fluoride ion concentration but dependent on pH. The currents in the first passive and second active regions are a function of pH and fluoride ion concentration. Surface examinations showed that nickel corrodes at the grain boundaries in the second active region. A mechanism has been proposed which accounts for corrosion in the second active region by F⁻ adsorption and passivation by either H₂O or OH⁻ adsorption on the anodically polarized metal surface. A mathematical analysis based on competitive adsorption of these species as a function of electrode potential is shown to be consistent with the experimental data. Monel corrodes at less than half the rate of nickel at the mixed potential in fluoride solutions with 0< pH < 6.5 due to its larger hydrogen overvoltage. Monel exhibits active-passive behaviour similar to nickel but with the passive current up to 6 times as large.

Text

2011-10-04

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http://hdl.handle.net/2429/37769

Metals and Materials Engineering

University of British Columbia

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