The effect of Nb on the corrosion and hydrogen pick-up of Zr alloys
File(s)
Author(s)
Bell, BDC
Murphy, ST
Grimes, RW
Wenman, MR
Type
Journal Article
Abstract
Zr-Nb alloys are known to perform better in corrosion and hydrogen pick-up than other Zr alloys but the mechanism by which this happens is not well understood. Atomistic simulations using density functional theory of both tetragonal and monoclinic ZrO2 were performed, with intrinsic defects and Nb dopants. The overall defect populations with respect to oxygen partial pressure were calculated and presented in the form of Brouwer diagrams. Nb is found to favour 5 + in monoclinic ZrO2 at all partial pressures, but can exist in oxidation states ranging from 5 + to 3 + in the tetragonal phase. Nb5+ is charge balanced by Zr vacancies in both phases, suggesting that contrary to previous assumptions, Nb does not act as an n-type dopant in the oxide layer. Clusters containing oxygen vacancies were considered, Nb2+ was shown to exist in the tetragonal phase with a binding energy of 2.4 eV. This supports the proposed mechanism whereby low oxidation state Nb ions (2 + or 3+) charge balance the build-up of positive space-charge in the oxide layer, increasing oxygen vacancy and electron mobility, leading to near-parabolic corrosion kinetics and a reduced hydrogen pick-up. Previous experimental work has shown that tetragonal ZrO2 transforms to the monoclinic phase during transition, and that during transition a sharp drop in the instantaneous hydrogen pick-up fraction occurs. The oxidation of lower charge state Nb defects to Nb5+ during this phase change, and the consequent temporary n-doping of the oxide layer, is proposed as an explanation for the drop in hydrogen pick-up during transition.
Date Issued
2017-06-15
Date Acceptance
2017-04-26
Citation
Acta Materialia, 2017, 132 (1), pp.425-431
ISSN
1359-6454
Publisher
Elsevier
Start Page
425
End Page
431
Journal / Book Title
Acta Materialia
Volume
132
Issue
1
Copyright Statement
© 2017 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. This manuscript is licensed under the Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International http://creativecommons.org/licenses/by-nc-nd/4.0/
Sponsor
Rolls-Royce Plc
Identifier
http://gateway.webofknowledge.com/gateway/Gateway.cgi?GWVersion=2&SrcApp=PARTNER_APP&SrcAuth=LinksAMR&KeyUT=WOS:000405881500039&DestLinkType=FullRecord&DestApp=ALL_WOS&UsrCustomerID=1ba7043ffcc86c417c072aa74d649202
Grant Number
5200026668
Subjects
Science & Technology
Technology
Materials Science, Multidisciplinary
Metallurgy & Metallurgical Engineering
Materials Science
Density functional theory
Zirconium
Niobium
Corrosion
Hydrogen pick-up
AB-INITIO CALCULATIONS
ZIRCONIUM ALLOYS
ELECTRONIC-STRUCTURE
OXIDATION
INTERFACES
NIOBIUM
PHASES
SN
Materials
0204 Condensed Matter Physics
0912 Materials Engineering
0913 Mechanical Engineering
Publication Status
Published
Date Publish Online
2017-04-28