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The oxide chemical composition on metallic Ni and NiCr alloy electrodes has been studied for changes in simulated reactor coolant solution chemistry, through a range of oxidizing potentials and pH settings at 25 °C and 150 °C. Even under strongly reducing conditions, the Ni surface has a 2-nm layer of oxide and hydroxide from which further oxides form, depending on the subsequent oxidative step. For basic (normal operating) pH conditions and under oxidation potentials near 0 volts (vs. SCE), a predominantly Ni(OH)2 layer is formed that appears to remain relatively stable up to at least 48 hours of oxidation at 150 °C. For the NiCr alloy, similar stability is imparted by a thin film of Cr(OH)3/ Cr2O3 and Ni(OH)2./NiO. Under milder oxidizing (but still basic) conditions, the surface is stabilized by a thin film that is mostly Ni(OH)2/NiO. Under neutral solution conditions, the same oxide/ hydroxide films do not seem to be as effective in stabilizing the surface. Copyright © 2017 John Wiley & Sons, Ltd.

The O/Ni ratios derived from the X-ray photoelectron (XP) spectra of a number of well-characterised Ni oxides were calculated and found to correspond well to the expected O/Ni ratios for these oxides. The measured ratios, combined with the characteristic XP spectra, provide a higher level of confidence to the assignment of O 1s chemical species, particularly for surfaces with multiple phases present. As examples of this, we discuss the additional information imparted by O/Ni ratios measured for oxides that were electrochemically grown on Ni and NiCr-20% alloy surfaces.

This paper reports several interrelated XPS investigations of the Cr–O system that provide additional interpretation of the Cr 2p3/2 spectra, along with more detail on the reaction mechanisms between O2 gas and H2O vapour with metallic Cr. 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 O/Cr ratio derived from the corrected Cr3+ and substitutional O2− intensities for all polycrystalline Cr2O3 samples was 1.5. Annealing of freshly cleaved polycrystalline Cr2O3 surfaces above 550 °C resulted in a sharpening of the multiplet structures that may be caused by the increased prevalence of the (0 0 0 1) Cr2O3 orientation on the surface. The slight difference in binding energies between the low energy (0 0 0 1) plane and the other major crystallographic orientations could contribute to the spectral broadening that is observed for the Cr 2p3/2 spectral line shapes. The Cr 2p3/2 spectrum for Cr(OH)3 was isolated from that produced by Cr2O3 and the multiplet structure was qualitatively re-assembled for that of the hydroxide. The Cr 2p3/2 and O 1s spectra of reaction products on polycrystalline metal surfaces were analysed using the above information. The oxidic portion of the Cr 2p3/2 spectra could be cleanly separated from the metallic substrate using spectral subtraction. Reactions of either O2 gas or H2O vapour both produced a thin Cr2O3 film that was deficient in Cr3+ and, depending on the reactant, showed varying concentrations of hypo-stoichiometric Cr moieties. The surface corrected O/Cr ratios calculated for all the oxidised surfaces were 1.7. There was little evidence to support the formation of Cr(OH)3 on these surfaces. For comparison, an aqueous reaction of a Ni–Cr (20%) alloy was studied where both Cr(OH)3 and Cr2O3 components could be analysed. For this example no sub-stoichiometric Cr oxide species were produced.

TM abstract The oxidation of polycrystalline nickel (Ni) metal surfaces with water (H2O) vapour was studied at 25 ◦C and 300 ◦ C using dose rates considerably higher than in previous studies. The reaction rate was followed to a point where the oxide thickness was several nanometers. This was done using X-ray photoelectron spectroscopy (XPS) and modeling of the spectral profiles using QUASES

Quantitative chemical state X-ray photoelectron spectroscopic analysis of mixed nickel metal, oxide, hydroxide and oxyhydroxide systems is challenging due to the complexity of the Ni 2p peak shapes resulting from multiplet splitting, shake-up and plasmon loss structures. Quantification of mixed nickel chemical states and the qualitative determination of low concentrations of Ni(III) species are demonstrated via an approach based on standard spectra from quality reference samples (Ni, NiO, Ni(OH)2, NiOOH), subtraction of these spectra, and data analysis that integrates information from the Ni 2p spectrum and the O 1s spectra. Quantification of a commercial nickel powder and a thin nickel oxide film grown at 1-Torr O2 and 300 °C for 20 min is demonstrated. The effect of uncertain relative sensitivity factors (e.g. Ni 2.67 ± 0.54) is discussed, as is the depth of measurement for thin film analysis based on calculated inelastic mean free paths. Copyright © 2009 John Wiley & Sons, Ltd.

The oxidation of polycrystalline nickel (Ni) metal surfaces after exposure to oxygen gas (O 2 ) at 25 and 300 °C and pressures near 130 Pa, was studied using X-ray photoelectron spectroscopy (XPS). Oxide structures involving both divalent (Ni 2+ ) and trivalent (N1 3+ ) species could be distinguished using Ni 2p spectra, while surface adsorbed O 2 and atomic oxygen (O) species could be differentiated from bulk oxide (O 2 -) using O 1s spectra. Oxide thicknesses and distributions were determined using QUASES™, and the average oxide thickness was verified using the Strohmeier formula. The reaction kinetics for oxide films grown at 300 °C followed a parabolic mechanism, with an oxide thickness of greater than 4 nm having formed after 60 min. Exposure at 25 °C followed a direct logarithmic mechanism with an oxide growth rate about four to five times slower than at 300 °C. Reaction of a Ni (100) single crystal under comparable conditions showed much slower reaction rates compared to polycrystalline specimens. The higher reaction rate of the polycrystalline materials is attributed to grain boundary transport of Ni cations. Oxide thickness was measured on a microscopic scale for polycrystalline Ni exposed to large doses of O 2 at 25 and 300 °C. The thickness of oxide was not strongly localized on this scale. However, the QUASES™ analysis suggests that there is localized growth on a nanometric scale - the result of island formation.

The corrosion products produced on polycrystalline Ni metal and Ni-Cr (20%) (NiCr) alloy surfaces exposed to aqueous environments chosen to emulate possible solution conditions in the steam generator (SG) tubing of pressurized water reactors (PWR) were studied using XPS. Additional measurements modelling the distribution of oxidized Ni and Cr species on select alloy specimens were carried out using ToF SIMS. Exposure of Ni metal and NiCr alloy samples to mildly oxidizing potentials in basic solutions resulted in the preferential growth of a β-Ni(OH)2 phase; driven by the dissolution of metallic Ni at both 25°C and 150°C. The presence of β-Ni(OH)2, Cr(OH)3 and small amounts of a Cr6+-containing oxide on NiCr specimens oxidized under mildly oxidizing conditions at 150°C in neutral solutions suggested that the dissolution of both metallic Ni and Cr followed by the back deposition of the corresponding corrosion products was responsible for oxide growth under these conditions. In acidic media oxide nucleation at 150°C under mildly oxidizing potentials was determined to occur via the dissolution of both Ni and Cr species on NiCr specimens as well. The increased stability of Ni2+ in acidic solution led to a limited precipitation of β-Ni(OH)2 resulting in the formation of very thin oxides containing higher levels of Cr(OH)3. Reactions on metallic Ni and NiCr surfaces under highly oxidizing potentials resulted in an increase in the NiO content of these films compared to similar exposures carried out at milder oxidation conditions 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; overall, uneven films were formed from 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 at a much lower oxidizing potential in basic solution.

Practical quantitative chemical state X-ray photoelectron spectroscopy (XPS) analysis of first row transition metals, oxides and hydroxides is challenging due to the complexity of their M 2p spectra. Complex multiplet splitting, shake-up and plasmon loss structure can play a role in the interpretation of the chemical states present. This paper will show practical curve fitting procedures for the quantitative measurement of different chemical states for metal oxides and hydroxides from a survey of transition metals. It will also discuss some of the limitations and pitfalls present as well as give practical examples of their successful use. These curve-fitting procedures are based on 1) standard spectra from quality reference samples, 2) a survey of appropriate literature databases and/or a compilation of literature references, 3) fitting of multiplet split spectra based on spectra of numerous reference materials and theoretical modelling, 4) spectral subtractions routines, again using reference spectra, and 5) specific literature references where fitting procedures are available.