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. / Applied Science, Faculty of / Materials Engineering, Department of / Graduate

Crevice corrosion experiments have been conducted on Inconel 600 and Inconel 625 exposed to two principle test solutions of 1 M NaCl and 1 M NaCl + 0.01 M Na₂S₂0₃ (Sodium Thiosulphate) at three temperatures, 22°C, 55 °C and 80°C. The crevice corrosion tests were performed in a corrosion cell which was constructed from PTFE (Polytetrafluoroethylene, Teflon) and Pyrex glass. Features of the cell included the utilization of an artificial Teflon-metal crevice and provisions to monitor crevice corrosion current, active crevice corrosion potential and active crevice pH. Additional experiments included potentiodynamic anodic polarization tests on pure Ni, Alloy 600, and Alloy 625 in bulk solution environments and in simulated crevice solutions. Crevice corrosion morphology and compositional analysis of the corrosion products was studied using a scanning electron microscope equipped with an X-ray energy dispersive spectroscopy (EDS) system. Results show that crevice corrosion rates increase with increasing temperature for Alloy 600 in both principle test solutions. X-ray EDS analysis indicated that an insoluble nickel sulphide corrosion product formed on Alloy 600 in a solution of 1 M NaCl + 0.01 M Na₂S₂0₃. For the Alloy 600, in a solution of 1 M NaCl + 0.01 M Na₂S₂0₃, initiation times were significantly reduced and crevice corrosion propagation rates enhanced, as compared to Alloy 600 in 1 M NaCl. The decrease in initiation times has been attributed to the destabilizing nature of the S₂O₃⁻² species on the passive oxide film. Enhanced propagation rates have been attributed to the presence of H₂S in the crevice solution and the formation of an adsorbed species Ni(H₂S)ads which enhances the anodic dissolution reaction. The H₂S in the active crevice solution originated from the thermodynamically favoured electrochemical reduction of the S₂0₃⁻² species in the active crevice solution. Experiments on Alloy 625, which is alloyed with molybdenum, (Mo), show that it was virtually immune to crevice corrosion as compared to Alloy 600 which is not alloyed with Mo. The resistance of Alloy 625 to crevice corrosion initiation has been attributed to the stabilizing nature of MoO₂ in the passive oxide film. For an actively corroding system, the formation of the molybdate species MoO₄⁻² may act as an anodic inhibitor and effectively enhance the repassivation of the passive film. / Applied Science, Faculty of / Mining Engineering, Keevil Institute of / Graduate

Le comportement électrochimique d'une électrode de nickel en milieux aqueux faiblement alcalins (tampon H₂C0₃/HCO₃CO₃²⁻, pH 7,2-10,2) a été étudié dans le but d' identifier les processus de corrosion généralisée et localisée qu'induit un tel environnement. Dans le milieu étudié, le nickel affiche un comportement électrochimique qui n'est pas influencé par la concentration des diverses espèces en fonction du pH. L'eau et les ions hydroxyle présents en solution sont principalement responsables de la passivation du nickel, contrôlant ainsi la compétition qui survient entre la mise en solution du nickel et la formation d'un film passivant protecteur. Afin de ralentir la mise en solution d'ions métalliques de nickel, l'effet d'un inhibiteur de corrosion organique, le benzotriazole, a été étudié. De faibles concentrations, entre 10-6 et 10-3 M, suffisent pour inhiber la corrosion généralisée du nickel dans une solution tampon de carbonate. Des mesures électrochimiques ont permis de déterminer que l'espèce responsable de cette inhibition de la corrosion généralisée du nickel est le benzotriazole sous forme neutre (BTAH). Le benzotriazole s'avère cependant inefficace contre la corrosion localisée induite par les ions chlorure. Une méthode de protection anodique a été étudiée afin de combattre la corrosion localisée induite par les ions chlorure. La formation d'un film bicouche protecteur a été réalisée dans des conditions d'anodisation bien précises. La couche interne qui agit comme une barrière envers les espèces agressives, identifiée par spectroscopie Raman, est constituée de NiO, tandis que la couche externe, identifiée par la combinaison d'analyses électrochimiques et de la spectroscopie de photoélectrons X, s'avère être β-Ni(OH)₂. Le film peu soluble ainsi formé s'épaissit en fonction du temps d'anodisation.