Hydrogen Absorption Mechanism of Zirconium Alloys Based on Characterization of Oxide Layer

Zirconium in the Nuclear Industry: 17th Volume

Motta

Comstock

2014

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

Section: Oxidation Model Of Precipitatessupporting

confidence: 86%

Effect of Alloying Elements on Hydrogen Pickup in Zirconium Alloys

Zirconium in the Nuclear Industry: 17th Volume

Motta

Comstock

2014

“…The addition of alloying elements generally decreases hydrogen pickup, with the exception of Ni, which increases pickup, and Sn, whose effect is not well determined (102,105). Additionally, f t H increases from the initial pretransition regime through subsequent transitions, at least up to the third transition (50,56,106,107).…”

Section: F Imentioning

confidence: 97%

Corrosion of Zirconium Alloys Used for Nuclear Fuel Cladding

Motta

Comstock

2015

Annu. Rev. Mater. Res.

367

149

During operation, nuclear fuel rods are immersed in the primary water, causing waterside corrosion and consequent hydrogen ingress. In this review, the mechanisms of corrosion and hydrogen pickup and the role of alloy selection in minimizing both phenomena are considered on the basis of two principal characteristics: the pretransition kinetics and the loss of oxide protectiveness at transition. In zirconium alloys, very small changes in composition or microstructure can cause significant corrosion differences so that corrosion performance is strongly alloy dependent. The alloys show different, but reproducible, subparabolic pretransition kinetics and transition thicknesses. A mechanism for oxide growth and breakup based on a detailed study of the oxide structure can explain these results. Through the use of the recently developed coupled current charge compensation model of corrosion kinetics and hydrogen pickup, the subparabolic kinetics and the hydrogen fraction can be rationalized: Hydrogen pickup increases when electron transport decreases, requiring hydrogen ingress to close the reaction.

“…It is also observed that f t H increased significantly before transition and appears to have been stable at transition. These trends have been observed before [28,29] and are best characterized by computing the instantaneous hydrogen pickup fraction f i H . The instantaneous hydrogen pickup fraction f i H is defined as the ratio of the hydrogen absorbed between time t and time t þ Dt to the total amount of hydrogen generated by the corrosion reaction during the same time interval.…”

Section: Instantaneous Hydrogen Pickup Fractionmentioning

confidence: 89%

Hydrogen Pickup Mechanism in Zirconium Alloys

Zirconium in the Nuclear Industry: 18th International Symposium

2018

Because hydrogen ingress into zirconium cladding can cause embrittlement and limit cladding lifetime, hydrogen pickup during corrosion is a critical lifelimiting degradation mechanism for nuclear fuel. However, mechanistic knowledge of the oxidation and hydrogen pickup mechanisms is still lacking. In an effort to develop such knowledge, we conducted a comprehensive study that included detailed experiments combined with oxidation modeling. We review this set of results conducted on zirconium alloys herein and articulate them into a unified corrosion theoretical framework. First, the hydrogen pickup fraction (f H) was accurately measured for a specific set of alloys specially designed to determine the effects of alloying elements, microstructure, and corrosion kinetics on f H. We observed that f H was not constant and increased until the kinetic transition and decreased at the transition. f H depended on the alloy and was lower for niobium-containing alloys. These results led us to hypothesize that hydrogen pickup during corrosion results from the need to balance the charge during the corrosion reaction such that f H decreases when the rate of electron transport through the protective oxide increases. To assess this hypothesis, two experiments were performed: (1) micro-X-ray absorption