Effect of Alloying Elements on Hydrogen Pickup in Zirconium Alloys

Abstract

Although the optimization of zirconium-based alloys has led to significant improvements in hydrogen pickup and corrosion resistance, the mechanisms by which such alloy improvements occur are still not well understood. In an effort to understand such mechanisms, we conducted a systematic study of the alloy effect on hydrogen pickup, using advanced characterization techniques to rationalize precise measurements of hydrogen pickup. The hydrogen pickup fraction was accurately measured for a specially designed set of commercial and model alloys to investigate the effects of alloying elements, microstructure, and corrosion kinetics on hydrogen uptake. Two different techniques for measuring hydrogen concentrations were used: a destructive technique, vacuum hot extraction, and a non-destructive one, cold neutron prompt gamma activation analysis. The results indicate that hydrogen pickup varies not only from alloy to alloy, but also during the corrosion process for a given alloy. These variations result from the process of charge balance during the corrosion reaction, such that the pickup of hydrogen decreases when the rate of electron transport or Manuscript

Corrosion of Zirconium Alloys Used for Nuclear Fuel Cladding

“…The oxidation of SPPs [e.g., Zr(Cr,Fe) 2 in Zircaloy-4, β-Nb and ZrNbFe precipitates in ZIRLO] is delayed relative to that of the Zr matrix so that the precipitates are initially incorporated in metallic form into the oxide layer. This delayed oxidation of alloying elements incorporated into the oxide layer has been verified by TEM (78)(79)(80), by X-ray absorption near-edge spectroscopy (XANES) (81)(82)(83), and more recently by μ-XANES (29,84). As the oxide advances further, the alloying elements in these precipitates eventually become oxidized; the delay is proportional to the tendency of elements to oxidize as specified by Ellingham's or Pourbaix diagrams.…”

The effect of Nb on the corrosion and hydrogen pick-up of Zr alloys

“…The Zr alloys of choice in most current water-cooled reactor designs con-M A N U S C R I P T A C C E P T E D ACCEPTED MANUSCRIPT tain 0.5-2.5 wt. % Nb due to its beneficial effect on HPUF [5,6,7]. Long term autoclave studies have demonstrated that Zr-2.5Nb exhibits near parabolic corrosion kinetics, and a lower corrosion rate than other Zr-based alloys such as Zircaloy-4 and Sn-containing alloys such as ZIRLO, as shown in Figure 1.…”

“…Corrosion of Zry-4, ZIRLO and Zr-2.5%Nb at 360°C in de-ionised water under saturated pressure conditions at 18.7 MPa, reproduced from[7].…”

The influence of alloying elements on the corrosion of Zr alloys

“…It has been demonstrated that alloys with a more electrically conductive oxide layer exhibit a lower HPUF [14]. This allows electrons to move more freely though the oxide and thus allows hydrogen ion/electron recombination to occur further from the metal/oxide interface, which consequently reduces the probability of the hydrogen reaching the cladding metal [15].…”