Electrochemical impedance spectra are presented for anodic oxide films grown on Zr-2.5Nb. The spectra are interpreted in terms of a model of parallel generalized Debye responses. The model parameters are linear analogue electric circuit elements ͑resistors and generalized constant-phase-element capacitors͒ that can be associated with macro ͑oxide thickness͒ and microscopic properties ͑surface fractal dimension, cracks, and fissures͒ of the oxide.Zirconium is an important metal for structural components used in the nuclear industry because of its low neutron capture cross section and its resistance to high-temperature oxidation in aqueous environments. CANDU® reactors use Zr-2.5 wt % Nb alloy pressure tubes to contain the heavy water employed to remove fission heat from the fuel ͑the water flows at 10 m/s at 10 MPa and 250-310°C͒. The integrity of the alloy is generally assured by a protective oxide that acts as a barrier to further oxidation. Corrosion is not life-limiting, but ingress into the metal of deuterium released during the corrosion reaction can be. Impedance spectroscopy ͑IS͒ is being used to characterize oxides on pressure tubes removed from service. 1 This convenient and nondestructive technique can provide information about the barrier properties and integrity of oxides on metal substrates. [2][3][4] The hope is that analysis of the spectra will result in new information on deuterium ingress. 5 The current work was undertaken to refine the analysis of impedance spectra in terms of a model whose parameters can be associated with macro and microscopic properties of the oxide. For this purpose, anodic films were chosen for this study, rather than thermal films which are found on pressure tubes, because there is a large body of previous work for comparison, there is no orientation dependence on grain structure, and because reproducible anodic films can be formed quickly. ExperimentalThe corrosion cell was a 1 L borosilicate flask fitted with two carbon counter electrodes, a zirconium wire reference electrode, and the sample working electrode. The sample was a 1.6 cm diam disk punched from a 1 mm thick sheet of cold-rolled commercial grade Zr-2.5% Nb that was subjected for 1 h to a 600°C final stress-relief anneal. The sample disk was ground with 600 grit SiC sandpaper and pickled in a solution of 45% H 2 O, 25% H 2 SO 4 , 25% HNO 3 , and 5% HF ͑by volume͒. The specimen was swirled in the pickling solution until a smooth surface was obtained ͑approximately 2 min͒, and then placed in a beaker filled with distilled water and boiled for approximately 1 h. Finally, the sample was rinsed with methanol and air dried. The specimen was then placed in a flat holder made of Tefzel and a Teflon washer was placed over the sample face so that an area of 1 Ϯ 0.04 cm 2 was exposed to a solution of 1% potassium hydroxide (pH 12.9 Ϯ 0.2).The specimen microstructure shown in Fig. 1 is comprised of elongated ␣-zirconium grains surrounded intermittently by varying amounts of metastable Nb enriched -zirconium and/or -niobi...
Oxides on removed pressure tubes from Pickering Unit 3 after 13.4 effective full power years (EFPY) have been examined to investigate the cause of variability in bulk alloy deuterium contents in outlet regions in order to improve predictions and minimize deuterium uptake in operating CANDU reactors. Secondary ion mass spectroscopy (SIMS) and electrochemical impedance spectrometry (EIS) were used for characterization with minimal sample preparation and modification. Two SIMS techniques were used for quantification: (1) the relative sensitivity factor (RSF) method, which requires a reference material and is subject to matrix effects as a result of variation in the secondary ion intensities of a species when different materials are sputtered; and (2) the SIMS infinite velocity (IV) method, which circumvents matrix effects by extrapolating all secondary ion intensity data to infinite velocity. A novel 13C oxide dating technique was used to determine oxide growth kinetics and ensure that oxide spalling had not occurred in the regions examined. Pressure tubes with high bulk alloy deuterium contents showed characteristics near the metaloxide interface in inside surface oxides that were not present in oxides on tubes with low deuterium contents. In samples with high bulk alloy deuterium content, the inside surface corrosion rate, determined by the 13C dating method, may have increased from ∼0.3 to 1 μm/EFPY about five years before tube removal. A constant rate of corrosion was inferred in samples with low deuterium contents. The inner regions of inside surface oxides in tubes with high deuterium contents, corresponding to the faster growing oxide (up to ∼5 μm from the interface) showed relatively higher porosity (inferred from the 2H profile) and almost constant levels of lithium. These oxides also showed a low value of the electrical resistance term in one of the EIS responses, which has been interpreted as being due to the presence of a larger number of water penetration routes. In order to investigate possible matrix effects in relatively thick inside surface oxides, through-oxide thickness concentration profiles for 2H and 12C, obtained by SIMS RSF and IV methods, were compared. Reasonable agreement was obtained between these methods for 2H concentration profiles. However, evidence of a significant matrix effect for 12C quantification, up to 5 μm from the inside surface metal-oxide interface, was found in tubes with high bulk alloy deuterium uptake. Further work is required to understand the reason for this matrix effect and its implications with respect to SIMS RSF quantification for other elements and analysis of excess 13C profiles. Outside surface oxides generally showed similar characteristics for all tubes. Very low constant rates of corrosion of 0.1 μm/EFPY were inferred from excess 13C profiles. Apparent substoichiometry (O:Zr ∼ 1) was found by SIMS IV analysis in outside surface oxides on a tube with high deuterium content that may be related to breakdown of the efficacy of the oxide as a deuterium permeation barrier. Thus, although present results correlate deuterium uptake with inside surface corrosion effects, a contribution from the gas annulus cannot be ruled out.
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