Electronic Thesis and Dissertation Repository

Degree

Doctor of Philosophy

Program

Chemistry

Supervisor

Professor N. Stewart McIntyre

Abstract

The reaction of polycrystalline Ni metal surfaces with low pressures (~ 130 Pa) of O2 gas or H2O vapour produced thin defective nickel oxide films containing both Ni2+ and Ni3+ species at 25oC and 300oC. Exposure of Ni metal samples to H2O vapour resulted in a much slower reaction rate when compared to similar doses of O2 gas. This difference was attributed to a slower place exchange with a surface hydroxyl (OH (ads)) intermediate compared to that of an adsorbed oxygen atom (O (ads)). This OH (ads) species appears only to be stabilized on metallic Ni and termination of oxide growth is believed to occur once all the available surface metal sites have been covered with oxide.

The Cr 2p3/2 spectra of polycrystalline Cr2O3 contain multiplet structures that bear a strong resemblance to those calculated by Gupta and Sen for the free Cr3+ ion. The Cr 2p3/2 spectrum for Cr(OH)3 was isolated from that produced by Cr2O3 and the multiplet structure was qualitatively reassembled for that of the hydroxide. Reactions of either O2 gas or H2O vapour with metallic Cr both produced thin Cr2O3 films that were deficient in Cr3+ and, depending on the reactant, showed varying concentrations of hypo-stoichiometric Cr moieties. No such species was observed following the oxidation of a NiCr (20%) (NiCr) alloy surface in an acidic (pH = 5) aqueous solution.

The O/Ni ratios derived from the XPS spectra of a number of well-characterized Ni oxides were calculated and found to correspond well to the expected O/Ni ratios for these oxides. These ratios were then applied to the analysis of oxides that were electrochemically grown on Ni and NiCr alloy surfaces.

The exposure of polycrystalline Ni metal and NiCr alloy surfaces to basic aqueous environments under mildly oxidizing potentials (0.0 V vs. Ag/AgCl) and temperatures of 25° and 150°C produced predominately β-Ni(OH)2-rich films. Decreasing the pH of the electrolyte at 150°C resulted in an increase in the Cr2O3 and Cr(OH)3 content of the oxide films formed on the alloy surfaces. Reactions on metallic Ni and NiCr surfaces under highly oxidizing potentials (1.5 V vs. Ag/AgCl) in basic solutions resulted in an increase in the NiO content of these films compared to similar exposures carried out at milder oxidation conditions. This was attributed to accelerated dehydration of the β-Ni(OH)2 phase. In addition, an increase in the Cr(OH)3 contribution on the alloy surface oxidized at a more oxidative potential suggested a more rapid dissolution of Cr under these conditions. The composition of the corrosion product formed after an exposure to a highly oxidizing potential was found to be unchanged following a subsequent reaction of equivalent length a much lower oxidizing potential.


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