Thermodynamic Evaluation of Hydrogen Absorption
in Nb During SRF Fabrication
Richard E. Ricker
1Materials
Science and Engineering Laboratory
National Institute of Standards and Technology
Gaithersburg, MD 20899-8553
2Thomas Jefferson National Accelerator Facility
Newport News, VA 23606
Background
Richard E. Ricker
• BS, MS, N. C. State University
• PhD, Rensselaer Polytechnic Institute (RPI)
• Areas of expertise
- Effects of environments on the properties and performance of materials
in service (corrosion, mechanical properties, fracture)
- Hydrogen embrittlement
- Springback, dynamic modulus analysis (DMA) and internal friction
Much of this presentation reviews the information published in the
following paper:
R. E. Ricker and G. R. Myneni, “Evaluation of the Propensity of Niobium to
Absorb Hydrogen During Fabrication of SRF Cavities for Particle Accelerators,”
J. Res. NIST, 115 (5) 19 pgs. Sept.-Oct. 2010.
Which will be available later this month at the journals website
http://nvl.nist.gov/nvl3.cfm?doc_id=89&s_id=117
Motivation
Highly variable behavior with processing conditions
Tensile flow curves for UHP Nb
The RF performance of a cavity
following different treatments at 1.7 K
Q0 vs. Bp measured at 1.7 K
Classic indicators of interstitials
• baseline test (empty squares)
• Yield points
• air 4 mos, 1 Atm H2 for 11 h (red),
• Serrations or PLCs
• 2xs H2 at 1 atm at 120 °C for 12 h (blue),
(Portevin-Le Chatelier Effect)
• 600 °C for 10 h in UHV (green)
• Variations in strength and ductility for
• 120 °C for 12 h in UHV (empty
same grain size and composition
diamonds).
Tensile Flow Cures
The Influence of Processing Conditions on Deformation and Fracture
Classic hardening by interstitial solutes is observed:
(1) yield points,
(2) serrations,
(3) shortened easy glide (stage 2) region, and
(4) Reduced ductility at lower strain rates.
Hypothesis
Variations in plastic flow and performance are due to hydrogen absorption
Hydrogen is absorbed into niobium
during SRF processing and influences
performance during fabrication and
service.
Evaluation of the Hypothesis
+/- H testing
H measurement as a function of
proc.
Q as a function of [H] content
Missing Piece
Scientifically sound explanation
for why the hypothesized absorption of
hydrogen should occur in SRF
processing environments
Evaluation of relative tendency
to absorb hydrogen from the different
processing environments
Chemical and Electrochemical Thermodynamics
This analysis will use the standard principles of chemical thermodynamics
[P roducts]

K
[Reactants]
Reaction Constant (K)
Electrode potential (E) and work (∆G)
E  G/nF

Thermodynamic reference reaction and reference electrodes

The fact that the surface coverage created
by 1 barr of hydrogen creates a reference
electrode potential of 0 by definition in a
pH=0 solution relates chemical potentials
(∆G/mol) of reactions to hydrogen activities.
Fugacity is the hydrogen activity expressed
in terms of pressure and is not a real
pressure.
This creates a direct numeric link between chemical potentials and hydrogen surface
activity or “fugacity.”
The equilibrium hydrogen fugacity will be used as a measure of the relative propensity
of different processing environments to cause hydrogen absorption.
Scenario
Postulated sequence of events
Nb metal covered with a passivating film of Nb2O5 is placed into the processing environment.
The passivating film is reduced or breached either locally or generally by mechanical
(abrasion), physical (permeation), or chemical means.
This brings the processing environment at the bulk composition into direct physical contact
with Nb metal allowing reactions of the type below to proceed
xNb(s)  yH2O  NbxOy (s)  2yH   2ye
This results in the build up of negative charge on the surface of the metal that stimulates the
reduction of the adsorbed H ions

2H   2e  H2 (g)
A steady state process of film breakdown, hydrogen ion discharge, and passive film repair
results and continues during processing.
hydrogen absorbed will depend on the kinetics of this steady state
The actual quantity of
process, but the maximum possible thermodynamic driving force for absorption or hydrogen
activity on the surface will be that generated when the unaltered processing environment
initially contacts freshly exposed bare metal. Since this quantity can be calculated from
chemical thermodynamics, it will be calculated as a fugacity and used as a relative measure
of the propensity for hydrogen absorption.
Hydrogen evolution reaction on a metal surface
Principles of Hydrogen Ion Discharge and Absorption
1.
The activity of the absorbed hydrogen is not limited to the external pressure. Since
the hydrogen is generated by tunneling electrons from the metal into the adsorbed
hydrogen ions of the adsorbed molecules, the activity of the adsorbed hydrogen will
be determined by the kinetics of this process compared to the rate that this
population is reduced by mass transport and the recombination of adsorbed
hydrogen ions to form adsorbed molecules of diatomic hydrogen gas. The adsorbed
molecules of hydrogen must then surface diffuse and combine to nucleate the gas
bubbles.
2.
Hydrogen can be absorbed even when H2(g) bubbles are not observed.
3.
In fact, changes to the environment that promote H2(g) bubbling tend to reduce NOT
increase H absorption. This is because they reduce the activity of hydrogen on the
surface required to nucleate the bubbles.
4.
Hydrogen adsorption comes first. Examination of the diagram on the proceeding
page indicates that water molecules tend to adsorb on the metal with the positive
hydrogen ions down due to the negative charge of the metal. Since electron
tunneling into these adsorbed ions do not required mass transport of ions, it will
occur very rapidly enabling the system to approach the thermodynamic limiting
fugacity calculated here using equilibrium thermodynamics.
Processing Environments
Hydrogen Equilibrium Fugacities
1.
Dilute Aqueous Solutions, [H2O]≈1
xNb(s)  yH 2O(l) Nb xOy (s)  yH 2 (g)
logPeq(H 2 )

G
yRT ln(10)
logPeq (H 2 ) 20(barr)
2.

Reactions with Water Vapor, [H2O]≈relative humidity (RH)

logPeq(H 2 )
G
 log(RH )
yRT ln(10)
logPeq (H 2 ) 16(barr)
3.

Solvents and cutting fluids (same as water vapor)

Processing Environments
Hydrogen Equilibrium Fugacities (cont.)
4.
Concentrated Mineral Acids (BCP), [H+]≈?
Approach 1: Assume acid (HF) dictates H
Fugacity
2Nb(s)10HF  2NbF5  5H 2 (g)
The [H] activity is then.
 G 
log[P(H 2 )]  
 2log[HF]

5RT ln(10) 
log[ P(H 2 )]  16(barr )

Approach 2: Assume residual water
concentration determines H Fugacity
logPeq (H 2 ) 19.9(barr)
Processing Environments
Hydrogen Equilibrium Fugacities (cont.)
5.
Electro-polishing
Electro-polishing transfers the hydrogen evolution reaction to the auxiliary electrode. The
extent of this transfer will depend on cell voltages, electrode geometry, etc. Using
representative values for Tafel constants calculations indicates that the hydrogen fugacity may
be as low as 1 barr depending on cell geometry.
Nb-H Phase Diagram
The hydrogen fugacities calculated for bare exposed Nb surface during
processing exceed those required to nucleate the observed hydrides.
Literature
Examples of H uptake found in the literature
Hydrogen content in an Al alloy as a function of
abrasion time in an aqueous slurry. (S. W. Ciraldi,
PhD Univ. of Ill, 1980)
Permeation of water through a passive film to generate H2
and blister an Al alloy observed in a transmission electron
microscope. (Scamans and Tuck, EnvironmentallySensitive Fracture of Engineering Materials, TMS, 1979,
p464)
Silver Bridge Collapse, Ohio River, Dec. 15, 1967 46 dead
Hydrogen absorption in high strength steel fitting.
Conclusions
1.
The calculated H fugacity driving absorption was absurdly high in all
cases except for the electro-polishing solution and potentials.
2.
Therefore, H uptake by Nb during processing is virtually unavoidable if
water is present when processing conditions break the passivating
surface film.
3.
Residual water in an H2(g) environment may have a greater influence on
the surface activity of hydrogen driving absorption than the H2(g)
pressure.
4.
Understanding the tendency of Nb to absorb hydrogen and managing the
passivating surface layers should enable the consistent fabrication of
optimal performing cavities