Role of Iron for Hydrogen Absorption Mechanism in Zirconium Alloys

Abstract

The latest PIE results of Zry-2 and HiFi alloy (0.4 %Fe-Zry2) showed that iron addition reduces the hydrogen pick-up ratio. In order to clarify this lower hydrogen absorption mechanism, three types of experiments were carried out for both alloys: (1) Measurements of the hydrogen pick-up ratio in the pre-transition region using an autoclave. (2) CPD (Contact Potential Difference) and FBP (Flat-band Potential) measurements of oxide film using a High Temperature Kelvin system and Photocurrent system. (3) Investigation of hydrogen absorption properties by Sieverts system and corrosion properties by autoclave for intermetallic compounds simulating SPP's Fe/Cr and Fe/Ni ratio in Zry-2 and HiFi. Autoclave tests showed that the corrosion behaviors of both alloys were similar. However, the hydrogen pick-up ratio depended on not only iron addition but also surface treatment (with and without pickling). Iron addition and non-pickling (as-received) reduced the hydrogen pick-up ratio. ΔCPD and FBP shifted to the positive side as the result of iron addition, and FBP changed with and without pickling. This difference in surface potential might be caused by the existence of anion vacancy concentration in the oxide film and furthermore may affect the electrochemical potential gradient over the oxide film. This potential gradient might control the proton diffusion in the oxide film. In the results of the hydrogen absorption tests on intermetallic compounds, Zr(Fe,Cr)2 with a higher Fe/Cr ratio showed lower hydrogen absorption, and Zr2(Fe,Ni) showed higher hydrogen absorption than Zr(Fe,Cr)2. In the results of corrosion tests on intermetallic compounds, Zr2(Fe,Ni) showed a much larger corrosion rate than Zr(Fe,Cr)2. It is considered that Zr2(Fe,Ni)-type precipitate might be oxidized easily in the oxide film, thus it might not contribute to the window for hydrogen absorption, and Zr(Fe,Cr)2 with a higher Fe/Cr ratio would reduce the amount of hydrogen penetrating through SPP at the oxide/metal interface. Based on these results, a tentative hydrogen absorption mechanism is proposed, explaining both roles of the electrochemical potential gradient over oxide film and the SPP window for the hydrogen absorption.

Change of chemical states of niobium in the oxide layer of zirconium–niobium alloys with oxide growth

“…We also found that the hydrogen diffusivity in the barrier layer increased when the hydrogen pickup fraction increased near the kinetic transition in the oxide layer of the Zry-2 type alloy [19]. The relationship between the barrierness of the oxide layers and the hydrogen diffusivity has been also pointed out in some studies [20][21][22][23], so this kind of hydrogen diffusion measurements on Zr-2.5Nb will discover the relationship between the barrierness to oxidation and that to hydrogen pickup.…”

Reducing Hydrogen Penetration through Corrosion Layer Formed on Zirconium Alloys by Iron Addition

“…According to some papers [2], the precipitate in the oxide film plays an important role as the window of hydrogen absorption. Corrosion test results [7] of intermetallic compounds indicate that, in the role of window of hydrogen absorption, the contribution of Zr(Fe,Cr) 2 seems to be greater than that of Zr 2 (Fe,Ni), because the corrosion rate of Zr(Fe,Cr) 2 is smaller than that of Zr 2 (Fe,Ni), so that Zr(Fe,Cr) 2 possibly survives longer than Zr 2 (Fe,Ni). Based on this result, the hydrogen absorption property of zirconium alloy appears to be controlled by the hydrogen permeability of the precipitate.…”

Effect of Final Annealing Temperature on Corrosion Resistance of SZA-6 Zirconium Alloy Cladding Tubes

“…In addition, the corrosion resistance of Nb-containing zirconium alloys is closely related to the heat treatment process. Studies have shown [10][11] that β quenching make stable elements Nb, Fe, and Cr supersaturate in the βZr, which improved the nodular corrosion performance, inversely, made the uniform corrosion rate get faster [12][13]. The intermediate annealing temperature during the cold processing of Nb-containing zirconium alloys should not exceed a certain temperature, otherwise β-Zr would form, which was unfavorable to corrosion.…”