ASTM G148-97(2011)
(Practice)Standard Practice for Evaluation of Hydrogen Uptake, Permeation, and Transport in Metals by an Electrochemical Technique
Standard Practice for Evaluation of Hydrogen Uptake, Permeation, and Transport in Metals by an Electrochemical Technique
SIGNIFICANCE AND USE
The procedures described, herein, can be used to evaluate the severity of hydrogen charging of a material produced by exposure to corrosive environments or by cathodic polarization. It can also be used to determine fundamental properties of materials in terms of hydrogen diffusion (for example, diffusivity of hydrogen) and the effects of metallurgical, processing, and environmental variables on diffusion of hydrogen in metals.
The data obtained from hydrogen permeation tests can be combined with other tests related to hydrogen embrittlement or hydrogen induced cracking to ascertain critical levels of hydrogen flux or hydrogen content in the material for cracking to occur.
SCOPE
1.1 This practice gives a procedure for the evaluation of hydrogen uptake, permeation, and transport in metals using an electrochemical technique which was developed by Devanathan and Stachurski. While this practice is primarily intended for laboratory use, such measurements have been conducted in field or plant applications. Therefore, with proper adaptations, this practice can also be applied to such situations.
1.2 This practice describes calculation of an effective diffusivity of hydrogen atoms in a metal and for distinguishing reversible and irreversible trapping.
1.3 This practice specifies the method for evaluating hydrogen uptake in metals based on the steady-state hydrogen flux.
1.4 This practice gives guidance on preparation of specimens, control and monitoring of the environmental variables, test procedures, and possible analyses of results.
1.5 This practice can be applied in principle to all metals and alloys which have a high solubility for hydrogen, and for which the hydrogen permeation is measurable. This method can be used to rank the relative aggressivity of different environments in terms of the hydrogen uptake of the exposed metal.
1.6 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.
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Designation: G148 − 97 (Reapproved 2011)
Standard Practice for
Evaluation of Hydrogen Uptake, Permeation, and Transport
in Metals by an Electrochemical Technique
This standard is issued under the fixed designation G148; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision.Anumber in parentheses indicates the year of last reapproval.A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
1. Scope G96Guide for Online Monitoring of Corrosion in Plant
Equipment (Electrical and Electrochemical Methods)
1.1 This practice gives a procedure for the evaluation of
hydrogen uptake, permeation, and transport in metals using an
3. Terminology
electrochemical technique which was developed by Devana-
than and Stachurski. While this practice is primarily intended 3.1 Definitions:
for laboratory use, such measurements have been conducted in
3.1.1 charging, n—method of introducing atomic hydrogen
field or plant applications. Therefore, with proper adaptations,
into the metal by galvanostatic charging (constant charging
this practice can also be applied to such situations.
current), potentiostatic charging (constant electrode potential),
free corrosion, or gaseous exposure.
1.2 This practice describes calculation of an effective diffu-
sivity of hydrogen atoms in a metal and for distinguishing
3.1.2 charging cell, n—compartment in which hydrogen
reversible and irreversible trapping.
atoms are generated on the specimen surface. This includes
both aqueous and gaseous charging.
1.3 This practice specifies the method for evaluating hydro-
gen uptake in metals based on the steady-state hydrogen flux.
3.1.3 decay current, n—decay of the hydrogen atom oxida-
tion current due to a decrease in charging current.
1.4 This practice gives guidance on preparation of
specimens, control and monitoring of the environmental
3.1.4 Fick’s second law, n—second order differential equa-
variables, test procedures, and possible analyses of results.
tion describing the concentration of diffusing specie as a
function of position and time. The equation is of the form
1.5 This practice can be applied in principle to all metals
]C x,t /]t5]/]xD ]/]x C x,t for lattice diffusion in one di-
~ ! @ ~ !#
and alloys which have a high solubility for hydrogen, and for 1
mensionwherediffusivityisindependentofconcentration.See
which the hydrogen permeation is measurable. This method
3.2 for symbols.
can be used to rank the relative aggressivity of different
environments in terms of the hydrogen uptake of the exposed
3.1.5 hydrogen flux, n—the amount of hydrogen passing
metal.
through the metal specimen per unit area as a function of time.
The units are typically concentration per unit area per unit
1.6 This standard does not purport to address all of the
time.
safety concerns, if any, associated with its use. It is the
responsibility of the user of this standard to establish appro-
3.1.6 hydrogen uptake, n—the concentration of hydrogen
3 3
priate safety and health practices and determine the applica-
absorbed into the metal (for example, g/cm or mol/cm ).
bility of regulatory limitations prior to use.
3.1.7 irreversible trap, n—microstructural site at which a
hydrogen atom has a infinite or extremely long residence time
2. Referenced Documents
compared to the time-scale for permeation testing at the
2.1 ASTM Standards:
relevant temperature, as a result of a binding energy which is
large relative to the migration energy for diffusion.
1 3.1.8 mobile hydrogen atoms, n—hydrogen atoms that are
This practice is under the jurisdiction ofASTM Committee G01 on Corrosion
ofMetalsandisthedirectresponsibilityofSubcommitteeG01.11onElectrochemi- associated with sites within the lattice.
cal Measurements in Corrosion Testing.
3.1.9 oxidation cell, n—compartment in which hydrogen
Current edition approved May 1, 2011. Published May 2011. Last previous
edition approved in 2003 as G149-97(2003). DOI:10.1520/G0148-97R11. atoms exiting from the metal specimen are oxidized.
Devanathan, M.A.V., and Stachurski, Z., Proceedings of Royal Society, A270,
3.1.10 permeation current, n—current measured in oxida-
90–102, 1962.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or tion cell associated with oxidation of hydrogen atoms.
contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
3.1.11 permeation transient, n—the increase of the perme-
Standards volume information, refer to the standard’s Document Summary page on
the ASTM website. ationcurrentwithtimefromcommencementofchargingtothe
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
G148 − 97 (2011)
attainment of steady state, or modification of charging condi- asthemembranethroughwhichmeasurementofhydrogenflux
tions (that is, rise transient). The decrease of the permeation are made. The actual process environment is on the charging
current with time resulting from a decrease in charging current sideofthemembranewhicheliminatestheneedforacharging
(that is, decay transient). cell. See 7.1 for guidance on various specimen configurations.
3.1.12 recombination poison, n—chemical specie present
4.2 In gaseous environments, the hydrogen atoms are gen-
within the test environment in the charging cell which en-
eratedbyadsorptionanddissociationofthegaseousspecies.In
hances hydrogen absorption by retarding the recombination of
aqueous environments, hydrogen atoms are produced by elec-
hydrogenatomsadsorbedontothemetalsurfaceintohydrogen
trochemical reactions. In both cases, some of the hydrogen
gas.
atoms diffuse through the membrane and are then oxidized on
exiting from the other side of the metal in the oxidation cell.
3.1.13 reversible trap, n—microstructural site at which a
hydrogen atom has a residence time which is greater than that
4.3 The conditions (for example, environment and the
for the lattice site but is small in relation to the time to attain
electrode potential) on the oxidation side of the membrane are
steady-state permeation, as a result of low binding energy.
controlledsothatthemetalsurfaceiseitherpassiveorimmune
3.2 Symbols: to corrosion. The background current established under these
3.2.1 For the purposes of this practice the following sym- conditions prior to hydrogen transport should be relatively
bols apply: constant and small compared to that of the hydrogen atom
oxidation current.
A = exposed area of specimen in the oxidation cell
4.4 The electrode potential of the specimen in the oxidation
(cm )
cell is controlled at a value sufficiently positive to ensure that
C(x,t) = lattice concentration of hydrogen as a function of
the kinetics of oxidation of hydrogen atoms are limited by the
position and time (mol/cm )
fluxofhydrogenatoms,thatis,theoxidationcurrentdensityis
C = sub-surface concentration of atomic hydrogen at
diffusion limited.
the charging side of the specimen (mol/cm )
D = effective diffusivity of atomic hydrogen, taking
eff
4.5 Thetotaloxidationcurrentismonitoredasafunctionof
into account the presence of reversible and irre-
time. The total oxidation current comprises the background
versible trapping (cm /s)
current and the current resulting from oxidation of hydrogen
D = lattice diffusion coefficient of atomic hydrogen
l
atoms. The latter is the permeation current.
(cm /s)
4.6 The thickness of the specimen is selected usually to
F = faraday’s constant (9.6485 × 10 coulombs/mol)
I(t) = time dependent atomic hydrogen permeation cur- ensure that the measured flux reflects volume (bulk) controlled
hydrogen atom transport. Thin specimens may be used for
rent (µA)
I = steady-state atomic hydrogen permeation current
evaluation of the effect of surface processes on hydrogen entry
ss
(µA) or exit (absorption kinetics or transport in oxide films).
J(t) = time-dependent atomic hydrogen permeation flux
4.7 In reasonably pure, defect-free metals (for example,
as measured on the oxidation side of the specimen
single crystals) with a sufficiently low density of microstruc-
(mol/s/cm )
tural trap sites, atomic hydrogen transport through the material
J = atomic hydrogen permeation flux at steady-state
ss
2 is controlled by lattice diffusion.
(mol/s/cm )
J(t)/J = normalized flux of atomic hydrogen
4.8 Alloying and microstructural features such as
ss
L = specimen thickness (cm)
dislocations, grain boundaries, inclusions, and precipitate par-
t = time elapsed from commencement of hydrogen
ticlesmayactastrapsitesforhydrogenthusdelayinghydrogen
charging (s)
transport.Thesetrapsmaybereversibleorirreversibledepend-
t = elapsed time measured extrapolating the linear
b
ing on the binding energy associated with the particular trap
portionoftherisingpermeationcurrenttransientto
sites compared to the energy associated with migration for
J(t)=O(s)
hydrogen in the metal lattice.
t = time to achieve a value of J(t)/J = 0.63 (s)
lag ss
4.9 The rate of hydrogen atom transport through the metal
x = distance into specimen from the charging surface
duringthefirstpermeationmaybeaffectedbybothirreversible
measured in the thickness direction (cm ).
and reversible trapping as well as by the reduction of any
τ = normalized time (D t/L )
τ = Normalized time to achieve a value of j(t)/J = oxidespresentonthechargingsurface.Atsteadystateallofthe
lag ss
irreversible traps are occupied. If the mobile hydrogen atoms
0.63 (s)
are then removed and a subsequent permeation test conducted
on the specimen the difference between the first and second
4. Summary of Practice
permeation transients can be used to evaluate the influence of
4.1 The technique involves locating the metal membrane
irreversibletrappingontransport,assuminganegligibleroleof
(that is, specimen) of interest between the hydrogen charging
oxide reduction.
andoxidationcells.Inthelaboratory,thechargingcellcontains
the environment of interest. Hydrogen atoms are generated on 4.10 Forsomeenvironments,theconditionsonthecharging
the membrane surface exposed to this environment. In field or side of the specimen may be suitably altered to induce a decay
plant measurements, the wall of the pipe or vessel can be used of the oxidation current after attainment of steady state. The
G148 − 97 (2011)
rate of decay will be determined by diffusion and reversible
trappingonlyand,hence,canalsobeusedtoevaluatetheeffect
of irreversible trapping on transport during the first transient.
4.11 Comparison of repeated permeation transients with
those obtained for the pure metal can be used in principle to
evaluate the effect of reversible trapping on atomic hydrogen
transport.
4.12 Thispracticeissuitableforsystemsinwhichhydrogen
atomsaregenerateduniformlyoverthechargingsurfaceofthe
membrane. It is not usually applicable for evaluation of
corroding systems in which pitting attack occurs unless the
chargingcellenvironmentisdesignedtosimulatethelocalized
pitenvironmentandtheentiremetalchargingsurfaceisactive.
4.13 This practice can be used for stressed and unstressed
specimens but testing of stressed specimens requires consider-
ation of loading procedures.
5. Significance and Use
5.1 The procedures described, herein, can be used to evalu-
atetheseverityofhydrogenchargingofamaterialproducedby
NOTE 1—A Luggin capillary should be used for more accurate
exposure to corrosive environments or by cathodic polariza-
measurement of potential when the current is large.
tion.Itcanalsobeusedtodeterminefundamentalpropertiesof
FIG. 1 PTFE Hydrogen Permeation Cell (with double junction ref-
materials in terms of hydrogen diffusion (for example, diffu-
erence electrodes, used for electrochemical charging)
sivityofhydrogen)andtheeffectsofmetallurgical,processing,
and environmental variables on diffusion of hydrogen in
6.3 TheO-ringsealmaterialshouldbeselectedtominimize
metals.
possible degradation products from the seals and contamina-
5.2 The data obtained from hydrogen permeation tests can
tion of the solution. This problem is particularly of concern
becombinedwithothertestsrelatedtohydrogenembrittlement
with highly aggressive environments and at high test tempera-
or hydrogen induced cracking to ascertain critical levels of
tures.
hydrogen flux or hydrogen content in the material for cracking
6.4 Doublejunctionreferenceelectrodesmaybeusedwhere
to occur.
necessary to avoid contamination of test solutions.At elevated
6. Apparatus
temperatures,theuseofasolutionconductivitybridgearrange-
ment with suitable inert materials is recommended.
6.1 The experimental set-up shall consist of a separate
charging and oxidation cell of a form similar to Fig. 1. Sealed
6.5 Thelocationofthereferenceelectrodeineachcompart-
oxidation cells, in which an additional material (usually
ment shall ensure minimal potential drop between the speci-
palladium), either plated or sputter deposited onto or clamped
men and the reference electrode. A Luggin capillary may be
against the specimen and the flux exiting this additional
useful in cases where the solution resistivity is high, small cell
material is measured may be used provided that it is demon-
volumesareusedandlongtestsareconducted.SeeGuideG96
strated that the introduction of this additional interface has no
for further guidance.
effect on the calculated diffusivity. The clamping of this
6.6 Recording of oxidation (and, as appropriate, charging)
additionalmaterialmayprovideinaccuratepermeationcurrents
current shall be made using a standard resistor and a high
in some systems due to the barrier effect at the interface (that
internal impedance digital voltmeter or by direct measurement
is, oxides, air gaps and so forth will act as a diffusion barrier).
using a current monitoring device.
6.2 Non-metallic materials which are inert to the test envi-
6.7 The measurement devices should be traceable to na-
ronment should be used for cell construction.
tional standards and calibrated prior to testing.
6.2.1 At temperatures above 50°C, leaching from the cell
6.8 In some cases, stirring of the solution in the charging
material (for example, silica dissolution from glass in some
cell may be required. This should be performed using suitable
environments) can modify the solution chemistry and may
stirring motor and apparatus.
influence hydrogen permeation.
6.2.2 Polytetrafluoroethylene (PTFE) is an example of a
7. Specimen
material suitable for elevated temperatures up to about 90°C.
6.2.3 Where metallic chambers are necessary (for contain- 7.1 Design—Specimensmaybeintheformofplateorpipe.
ment of high pressure environments), the materials chosen The dimensions shall enable analysis of the permeation tran-
shall have a v
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