ASTM G148-97
(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
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.
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Designation: G 148 – 97
Standard Practice for
Evaluation of Hydrogen Uptake, Permeation, and Transport
in Metals by an Electrochemical Technique
This standard is issued under the fixed designation G 148; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (e) indicates an editorial change since the last revision or reapproval.
1. Scope into the metal by galvanostatic charging (constant charging
current), potentiostatic charging (constant electrode potential),
1.1 This practice gives a procedure for the evaluation of
free corrosion, or gaseous exposure.
hydrogen uptake, permeation, and transport in metals using an
3.1.2 charging cell,, n—compartment in which hydrogen
electrochemical technique which was developed by De-
atoms are generated on the specimen surface. This includes
vanathan and Stachurski. While this practice is primarily
both aqueous and gaseous charging.
intended for laboratory use, such measurements have been
3.1.3 decay current,, n—decay of the hydrogen atom oxi-
conducted in field or plant applications. Therefore, with proper
dation current due to a decrease in charging current.
adaptations, this practice can also be applied to such situations.
3.1.4 Fick’s second law,, n—second order differential equa-
1.2 This practice describes calculation of an effective diffu-
tion describing the concentration of diffusing specie as a
sivity of hydrogen atoms in a metal and for distinguishing
function of position and time. The equation is of the form
reversible and irreversible trapping.
]C x,t!/]t 5]/]xD ]/]x C x,t! for lattice diffusion in one
~ @ ~ #
1.3 This practice specifies the method for evaluating hydro- 1
dimension where diffusivity is independent of concentration.
gen uptake in metals based on the steady-state hydrogen flux.
See 3.2 for symbols.
1.4 This practice gives guidance on preparation of speci-
3.1.5 hydrogen flux,, n—the amount of hydrogen passing
mens, control and monitoring of the environmental variables,
through the metal specimen per unit area as a function of time.
test procedures, and possible analyses of results.
The units are typically concentration per unit area per unit
1.5 This practice can be applied in principle to all metals
time.
and alloys which have a high solubility for hydrogen, and for
3.1.6 hydrogen uptake,, n—the concentration of hydrogen
which the hydrogen permeation is measurable. This method
3 3
absorbed into the metal (for example, g/cm or mol/cm ).
can be used to rank the relative aggressivity of different
3.1.7 irreversible trap,, n—microstructural site at which a
environments in terms of the hydrogen uptake of the exposed
hydrogen atom has a infinite or extremely long residence time
metal.
compared to the time-scale for permeation testing at the
1.6 This standard does not purport to address all of the
relevant temperature, as a result of a binding energy which is
safety concerns, if any, associated with its use. It is the
large relative to the migration energy for diffusion.
responsibility of the user of this standard to establish appro-
3.1.8 reversible trap,, n—microstructural site at which a
priate safety and health practices and determine the applica-
hydrogen atom has a residence time which is greater than that
bility of regulatory limitations prior to use.
for the lattice site but is small in relation to the time to attain
2. Referenced Documents steady-state permeation, as a result of low binding energy.
3.1.9 mobile hydrogen atoms,, n—hydrogen atoms that are
2.1 ASTM Standards:
associated with sites within the lattice.
G 96 Guide for Online Monitoring of Corrosion in Plant
3.1.10 oxidation cell,, n—compartment in which hydrogen
Equipment (Electrical and Electrochemical Methods)
atoms exiting from the metal specimen are oxidized.
3. Terminology
3.1.11 permeation current,, n—current measured in oxida-
tion cell associated with oxidation of hydrogen atoms.
3.1 Definitions:
3.1.12 permeation transient,, n—the increase of the perme-
3.1.1 charging,, n—method of introducing atomic hydrogen
ation current with time from commencement of charging to the
attainment of steady state, or modification of charging condi-
tions (that is, rise transient). The decrease of the permeation
This practice is under the jurisdiction of ASTM Committee G01 on Corrosion
of Metals and is the direct responsibility of Subcommittee G01.11 on Electrochemi-
current with time resulting from a decrease in charging current
cal Measuremnents in Corrosion Testing.
(that is, decay transient).
Current edition approved Apr. 10, 1997. Published January 1998.
3.1.13 recombination poison, n—chemical specie present
Devanathan, M.A.V. and Stachurski, Z., Proceedings of Royal Society, A270,
90–102, 1962.
Annual Book of ASTM Standards, Vol 03.02.
Copyright © ASTM, 100 Barr Harbor Drive, West Conshohocken, PA 19428-2959, United States.
NOTICE: This standard has either been superceded and replaced by a new version or discontinued.
Contact ASTM International (www.astm.org) for the latest information.
G 148
within the test environment in the charging cell which en- exiting from the other side of the metal in the oxidation cell.
hances hydrogen absorption by retarding the recombination of
4.3 The conditions (for example, environment and the
hydrogen atoms adsorbed onto the metal surface into hydrogen
electrode potential) on the oxidation side of the membrane are
gas.
controlled so that the metal surface is either passive or immune
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.
4.4 The electrode potential of the specimen in the oxidation
A = exposed area of specimen in the oxidation cell
cell is controlled at a value sufficiently positive to ensure that
(cm )
the kinetics of oxidation of hydrogen atoms are limited by the
C(x,t) = lattice concentration of hydrogen as a function of
flux of hydrogen atoms, that is, the oxidation current density is
position and time (mol/cm )
diffusion limited.
C = sub-surface concentration of atomic hydrogen at
3 4.5 The total oxidation current is monitored as a function of
the charging side of the specimen (mol/cm )
time. The total oxidation current comprises the background
D = effective diffusivity of atomic hydrogen, taking
eff
current and the current resulting from oxidation of hydrogen
into account the presence of reversible and irre-
atoms. The latter is the permeation current.
versible trapping (cm /s)
4.6 The thickness of the specimen is selected usually to
D = lattice diffusion coefficient of atomic hydrogen
l
ensure that the measured flux reflects volume (bulk) controlled
(cm /s)
F = faraday’s constant (9.6485 x 10 coulombs/mol) hydrogen atom transport. Thin specimens may be used for
I(t) = time dependent atomic hydrogen permeation cur-
evaluation of the effect of surface processes on hydrogen entry
rent (μA) or exit (absorption kinetics or transport in oxide films).
I = steady-state atomic hydrogen permeation current
ss 4.7 In reasonably pure, defect-free metals (for example,
(μA)
single crystals) with a sufficiently low density of microstruc-
J(t) = time-dependent atomic hydrogen permeation flux
tural trap sites, atomic hydrogen transport through the material
as measured on the oxidation side of the specimen
is controlled by lattice diffusion.
(mol/s/cm )
4.8 Alloying and microstructural features such as disloca-
J = atomic hydrogen permeation flux at steady-state
ss
tions, grain boundaries, inclusions, and precipitate particles
(mol/s/cm )
may act as trap sites for hydrogen thus delaying hydrogen
J(t)/J = normalized flux of atomic hydrogen
ss
transport. These traps may be reversible or irreversible depend-
L = specimen thickness (cm)
ing on the binding energy associated with the particular trap
t = time elapsed from commencement of hydrogen
sites compared to the energy associated with migration for
charging (s)
hydrogen in the metal lattice.
t = elapsed time measured extrapolating the linear
b
4.9 The rate of hydrogen atom transport through the metal
portion of the rising permeation current transient
during the first permeation may be affected by both irreversible
to J(t) =O (s)
and reversible trapping as well as by the reduction of any
t = time to achieve a value of J(t)/J = 0.63 (s)
lag ss
oxides present on the charging surface. At steady state all of the
x = distance into specimen from the charging surface
irreversible traps are occupied. If the mobile hydrogen atoms
measured in the thickness direction (cm ).
are then removed and a subsequent permeation test conducted
t = normalized time (D t/L )
t = Normalized time to achieve a value of j(t)/J =
on the specimen the difference between the first and second
lag ss
0.63 (s) permeation transients can be used to evaluate the influence of
irreversible trapping on transport, assuming a negligible role of
4. Summary of Practice oxide reduction.
4.10 For some environments, the conditions on the charging
4.1 The technique involves locating the metal membrane
side of the specimen may be suitably altered to induce a decay
(that is, specimen) of interest between the hydrogen charging
of the oxidation current after attainment of steady state. The
and oxidation cells. In the laboratory, the charging cell contains
rate of decay will be determined by diffusion and reversible
the environment of interest. Hydrogen atoms are generated on
trapping only and, hence, can also be used to evaluate the effect
the membrane surface exposed to this environment. In field or
of irreversible trapping on transport during the first transient.
plant measurements, the wall of the pipe or vessel can be used
4.11 Comparison of repeated permeation transients with
as the membrane through which measurement of hydrogen flux
those obtained for the pure metal can be used in principle to
are made. The actual process environment is on the charging
evaluate the effect of reversible trapping on atomic hydrogen
side of the membrane which eliminates the need for a charging
transport.
cell. See 7.1 for guidance on various specimen configurations.
4.2 In gaseous environments, the hydrogen atoms are gen- 4.12 This practice is suitable for systems in which hydrogen
erated by adsorption and dissociation of the gaseous species. In atoms are generated uniformly over the charging surface of the
aqueous environments, hydrogen atoms are produced by elec- membrane. It is not usually applicable for evaluation of
trochemical reactions. In both cases, some of the hydrogen corroding systems in which pitting attack occurs unless the
atoms diffuse through the membrane and are then oxidized on charging cell environment is designed to simulate the localized
NOTICE: This standard has either been superceded and replaced by a new version or discontinued.
Contact ASTM International (www.astm.org) for the latest information.
G 148
pit environment and the entire metal charging surface is active. 6.2.1 At temperatures above 50°C, leaching from the cell
4.13 This practice can be used for stressed and unstressed material (for example, silica dissolution from glass in some
specimens but testing of stressed specimens requires consider- environments) can modify the solution chemistry and may
ation of loading procedures. influence hydrogen permeation.
6.2.2 Polytetrafluoroethylene (PTFE) is an example of a
5. Significance and Use
material suitable for elevated temperatures up to about 90°C.
5.1 The procedures described, herein, can be used to evalu- 6.2.3 Where metallic chambers are necessary (for contain-
ment of high pressure environments), the materials chosen
ate the severity of hydrogen charging of a material produced by
exposure to corrosive environments or by cathodic polariza- shall have a very low passive current to ensure minimal effect
on the solution composition and shall be electrically isolated
tion. It can also be used to determine fundamental properties of
materials in terms of hydrogen diffusion (for example, diffu- from the membrane.
6.3 The O-ring seal material should be selected to minimize
sivity of hydrogen) and the effects of metallurgical, processing,
and environmental variables on diffusion of hydrogen in possible degradation products from the seals and contamina-
tion of the solution. This problem is particularly of concern
metals.
5.2 The data obtained from hydrogen permeation tests can with highly aggressive environments and at high test tempera-
tures.
be combined with other tests related to hydrogen embrittlement
or hydrogen induced cracking to ascertain critical levels of 6.4 Double junction reference electrodes may be used where
hydrogen flux or hydrogen content in the material for cracking necessary to avoid contamination of test solutions. At elevated
to occur. temperatures, the use of a solution conductivity bridge arrange-
ment with suitable inert materials is recommended.
6. Apparatus
6.5 The location of the reference electrode in each compart-
ment shall ensure minimal potential drop between the speci-
6.1 The experimental set-up shall consist of a separate
men and the reference electrode. A Luggin capillary may be
charging and oxidation cell of a form similar to Fig. 1. Sealed
useful in cases where the solution resistivity is high, small cell
oxidation cells, in which an additional material (usually palla-
volumes are used and long tests are conducted. See Guide G 96
dium), either plated or sputter deposited onto or clamped
for further guidance.
against the specimen and the flux exiting this additional
6.6 Recording of oxidation (and, as appropriate, charging)
material is measured may be used provided that it is demon-
strated that the introduction of this additional interface has no current shall be made using a standard resistor and a high
internal impedance digital voltmeter or by direct measurement
effect on the calculated diffusivity. The clamping of this
additional material may provide inaccurate permeation currents using a current monitoring device.
in some systems due to the barrier effect at the interface (that 6.7 The measurement devices should be traceable to na-
is, oxides, air gaps and so forth will act as a diffusion barrier). tional standards and calibrated prior to testing.
6.2 Non-metallic materials which are inert to the test envi- 6.8 In some cases, stirring of the solution in the charging
ronment should be used for cell construction. c
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