Anodic Oxide Growth and Hydrogen Absorption on Zr in Neutral Aqueous Solution: A Comparison to Ti

Abstract

Anodization and subsequent cathodic polarization of a thin-film sample of Zr were studied with in situ neutron reflectometry and electrochemical impedance spectroscopy ͑EIS͒. The results show the originally 485 Å thick sputter-deposited film generally behaved similar to a bulk electrode in neutral Na 2 SO 4 solution. The anodic oxide, grown by applying 0.5 V potential steps, contained a significant amount of hydrogen in one form or another. The observed anodization ratio was somewhat higher than the literature value determined by coulometry, while the Pilling Bedworth ratio was in good agreement with published data. Oxide layer thickening, accelerated immediately after an anodic polarization was applied, persisted for many hours, suggesting the migration of ions continues for an extended time. The oxide cracked when the applied potential reached 1.5 V and its thickness reached ϳ100 Å, causing loss of passivation. Surprisingly, the EIS behavior of the sample with cracked oxide could still be well represented by a single time-constant equivalent circuit consisting of a capacitor and a leakage resistor, albeit with much lower resistance. This indicates that the cracked and intact regions of the electrode behave essentially independent as parallel electrodes. The proposed modified equivalent circuit can explain a number of other unusual observations, including those seen under cathodic polarization.All the group 4B metals, zirconium, titanium, and hafnium, are known for forming strongly adherent, highly corrosion-resistant passive oxides. These metals owe this property to their s 2 d 2 electronic configuration which gives metal cations in the +4 oxidation state ͑inert gas configuration s 0 d 0 ͒ when protective oxides are formed on the surface of the metals. As originally emphasized by Uhlig since the 1940s, electronic configuration ultimately determines the type and structure of surface oxide films. 1 All three metals, largely due to their corrosion resistance, find unique applications in the nuclear industry: Ti as a potential canister or drip shield material for underground spent nuclear fuel disposal, 2 Zr, with very low absorption of neutrons, 3 as the metal of choice for the calandria and pressure tubes of the CANDU and several other types of reactors, and Hf, with a neutron absorption cross section nearly 600 times that of Zr, 3 in reactor control rods. Interestingly, Zr and Hf are nearly always found together in nature and are two of the most difficult elements to separate, as their chemistry is nearly identical, yet Hf must be thoroughly removed from Zr that is to be used in reactor pressure tubes; because of hafnium's very large neutron absorption cross section, a CANDU reactor made of pressure tubes contaminated even by a small amount of Hf would not start.Both ZrO 2 and TiO 2 are semiconductors with large gaps in the electronic band structure, 5.7 eV 4 and 3-3.4 eV, 5-8 respectively. Although not clean as to semiconductor industry standards, these oxide layers that form on their metals are good insulators...

Hydrogen Absorption into Titanium under Cathodic Polarization: An In-Situ Neutron Reflectometry and EIS Study

“…Neutron reflectometry (NR), amongst other techniques for detecting hydrogen, is the most powerful and perhaps the best method for studying hydrogen ingress into metals. [19][20][21][22][23][24][25][26][27][28][29][30] This ability is empowered by the high contrast between hydrogen isotopes (hydrogen and deuterium) which can be used to precisely probe, non-destructively, the amount of hydrogen penetrating into the metal and any surface films. The fact that NR can be carried out in situ by applying a controlled electrochemical condition adds a new dimension to the study of hydrogen transport through the oxide and metal films, and of their corrosion resistance.…”

Pathways for Hydrogen Absorption into Zircaloy-2 under Cathodic Polarization Assessed by Scanning Electrochemical Microscopy, Scanning Electron Microscopy, and Electrochemical Impedance Spectroscopy

“…It is possible that this poreopening process is enhanced by slight dissolution of the oxide film over secondary phases as the OH − concentration at the alloy surface increases with increasing cathodic polarization, although such dissolution was not observed for an oxide film on pure Zr. 38 The very large magnitude of the Tafel slope for E <−1.3 V SCE indicates that water reduction is still highly polarized, despite the apparent open grain boundaries. A possible explanation for this is that, as the cathodic current increases, the SPPs and exposed β-phase at the grain boundaries are rapidly converted to a hydride,…”

“…where Q is the CPE coefficient, α the exponent, and ω the angular frequency of the applied potential perturbation. Fitting generally yielded α ≥ 0.9, allowing the method of Brug et al 37,38 to be used to convert a) the CPE parameter to an equivalent capacitance value:…”

“…Fitting generally yielded α ≥ 0.9, allowing the method of Brug et al 37,38 to be used to convert a) the CPE parameter to an equivalent capacitance value:…”

In-Situ Polarized Neutron Reflectometry Study of the Passive Film Growth on Fe-20Cr Alloy

“…The scattering length density (SLD) profile obtained from the measured reflectivity curves provides information about the thickness of layers in the sample, interfacial roughness, material density, and composition depth profile. [13][14][15][16][17][18][19][20][21] In this study, we use PNR to study the passive film growth on an Fe-Cr alloy thin film in an alkaline environment.…”