1、Designation: G 148 97 (Reapproved 2003)Standard Practice forEvaluation of Hydrogen Uptake, Permeation, and Transportin Metals by an Electrochemical Technique1This standard is issued under the fixed designation G 148; the number immediately following the designation indicates the year oforiginal adop
2、tion or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. Asuperscript epsilon (e) indicates an editorial change since the last revision or reapproval.1. Scope1.1 This practice gives a procedure for the evaluation ofhydrogen uptake, p
3、ermeation, and transport in metals using anelectrochemical technique which was developed by De-vanathan and Stachurski.2While this practice is primarilyintended for laboratory use, such measurements have beenconducted in field or plant applications. Therefore, with properadaptations, this practice c
4、an also be applied to such situations.1.2 This practice describes calculation of an effective diffu-sivity of hydrogen atoms in a metal and for distinguishingreversible and irreversible trapping.1.3 This practice specifies the method for evaluating hydro-gen uptake in metals based on the steady-stat
5、e hydrogen flux.1.4 This practice gives guidance on preparation of speci-mens, 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 metalsand alloys which have a high solubility for hydrogen, and
6、forwhich the hydrogen permeation is measurable. This methodcan be used to rank the relative aggressivity of differentenvironments in terms of the hydrogen uptake of the exposedmetal.1.6 This standard does not purport to address all of thesafety concerns, if any, associated with its use. It is theres
7、ponsibility of the user of this standard to establish appro-priate safety and health practices and determine the applica-bility of regulatory limitations prior to use.2. Referenced Documents2.1 ASTM Standards:G 96 Guide for Online Monitoring of Corrosion in PlantEquipment (Electrical and Electrochem
8、ical Methods)33. Terminology3.1 Definitions:3.1.1 charging, nmethod of introducing atomic hydrogeninto the metal by galvanostatic charging (constant chargingcurrent), potentiostatic charging (constant electrode potential),free corrosion, or gaseous exposure.3.1.2 charging cell, ncompartment in which
9、 hydrogenatoms are generated on the specimen surface. This includesboth aqueous and gaseous charging.3.1.3 decay current, ndecay of the hydrogen atom oxi-dation current due to a decrease in charging current.3.1.4 Ficks second law, nsecond order differential equa-tion describing the concentration of
10、diffusing specie as afunction of position and time. The equation is of the formCx,t!/t 5/xD1/xCx,t!# for lattice diffusion in onedimension where diffusivity is independent of concentration.See 3.2 for symbols.3.1.5 hydrogen flux, nthe amount of hydrogen passingthrough the metal specimen per unit are
11、a as a function of time.The units are typically concentration per unit area per unittime.3.1.6 hydrogen uptake, nthe concentration of hydrogenabsorbed into the metal (for example, g/cm3or mol/cm3).3.1.7 irreversible trap, nmicrostructural site at which ahydrogen atom has a infinite or extremely long
12、 residence timecompared to the time-scale for permeation testing at therelevant temperature, as a result of a binding energy which islarge relative to the migration energy for diffusion.3.1.8 reversible trap, nmicrostructural site at which ahydrogen atom has a residence time which is greater than th
13、atfor the lattice site but is small in relation to the time to attainsteady-state permeation, as a result of low binding energy.3.1.9 mobile hydrogen atoms, nhydrogen atoms that areassociated with sites within the lattice.3.1.10 oxidation cell, ncompartment in which hydrogenatoms exiting from the me
14、tal specimen are oxidized.3.1.11 permeation current, ncurrent measured in oxida-tion cell associated with oxidation of hydrogen atoms.3.1.12 permeation transient, nthe increase of the perme-ation current with time from commencement of charging to theattainment of steady state, or modification of cha
15、rging condi-tions (that is, rise transient). The decrease of the permeationcurrent with time resulting from a decrease in charging current(that is, decay transient).1This practice is under the jurisdiction of ASTM Committee G01 on Corrosionof Metals and is the direct responsibility of Subcommittee G
16、01.11 on Electrochemi-cal Measuremnents in Corrosion Testing.Current edition approved Apr. 10, 1997. Published January 1998.2Devanathan, M.A.V. and Stachurski, Z., Proceedings of Royal Society, A270,90102, 1962.3Annual Book of ASTM Standards, Vol 03.02.1Copyright ASTM International, 100 Barr Harbor
17、Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.3.1.13 recombination poison, nchemical specie presentwithin the test environment in the charging cell which en-hances hydrogen absorption by retarding the recombination ofhydrogen atoms adsorbed onto the metal surface into hydrogeng
18、as.3.2 Symbols:3.2.1 For the purposes of this practice the following sym-bols apply:A = exposed area of specimen in the oxidation cell(cm2)C(x,t) = lattice concentration of hydrogen as a function ofposition and time (mol/cm3)C0= sub-surface concentration of atomic hydrogen atthe charging side of the
19、 specimen (mol/cm3)Deff= effective diffusivity of atomic hydrogen, takinginto account the presence of reversible and irre-versible trapping (cm2/s)Dl= lattice diffusion coefficient of atomic hydrogen(cm2/s)F = faradays constant (9.6485 x 104coulombs/mol)I(t) = time dependent atomic hydrogen permeati
20、on cur-rent (A)Iss= steady-state atomic hydrogen permeation current(A)J(t) = time-dependent atomic hydrogen permeation fluxas measured on the oxidation side of the specimen(mol/s/cm2)Jss= atomic hydrogen permeation flux at steady-state(mol/s/cm2)J(t)/Jss= normalized flux of atomic hydrogenL = specim
21、en thickness (cm)t = time elapsed from commencement of hydrogencharging (s)tb= elapsed time measured extrapolating the linearportion of the rising permeation current transientto J(t) =O(s)tlag= time to achieve a value of J(t)/Jss= 0.63 (s)x = distance into specimen from the charging surfacemeasured
22、in the thickness direction (cm2).t = normalized time (D1t/L2)tlag= Normalized time to achieve a value of j(t)/Jss=0.63 (s)4. Summary of Practice4.1 The technique involves locating the metal membrane(that is, specimen) of interest between the hydrogen chargingand oxidation cells. In the laboratory, t
23、he charging cell containsthe environment of interest. Hydrogen atoms are generated onthe membrane surface exposed to this environment. In field orplant measurements, the wall of the pipe or vessel can be usedas the membrane through which measurement of hydrogen fluxare made. The actual process envir
24、onment is on the chargingside of the membrane which eliminates the need for a chargingcell. See 7.1 for guidance on various specimen configurations.4.2 In gaseous environments, the hydrogen atoms are gen-erated by adsorption and dissociation of the gaseous species. Inaqueous environments, hydrogen a
25、toms are produced by elec-trochemical reactions. In both cases, some of the hydrogenatoms diffuse through the membrane and are then oxidized onexiting from the other side of the metal in the oxidation cell.4.3 The conditions (for example, environment and theelectrode potential) on the oxidation side
26、 of the membrane arecontrolled so that the metal surface is either passive or immuneto corrosion. The background current established under theseconditions prior to hydrogen transport should be relativelyconstant and small compared to that of the hydrogen atomoxidation current.4.4 The electrode poten
27、tial of the specimen in the oxidationcell is controlled at a value sufficiently positive to ensure thatthe kinetics of oxidation of hydrogen atoms are limited by theflux of hydrogen atoms, that is, the oxidation current density isdiffusion limited.4.5 The total oxidation current is monitored as a fu
28、nction oftime. The total oxidation current comprises the backgroundcurrent and the current resulting from oxidation of hydrogenatoms. The latter is the permeation current.4.6 The thickness of the specimen is selected usually toensure that the measured flux reflects volume (bulk) controlledhydrogen a
29、tom transport. Thin specimens may be used forevaluation of the effect of surface processes on hydrogen entryor exit (absorption kinetics or transport in oxide films).4.7 In reasonably pure, defect-free metals (for example,single crystals) with a sufficiently low density of microstruc-tural trap site
30、s, atomic hydrogen transport through the materialis controlled by lattice diffusion.4.8 Alloying and microstructural features such as disloca-tions, grain boundaries, inclusions, and precipitate particlesmay act as trap sites for hydrogen thus delaying hydrogentransport. These traps may be reversibl
31、e or irreversible depend-ing on the binding energy associated with the particular trapsites compared to the energy associated with migration forhydrogen in the metal lattice.4.9 The rate of hydrogen atom transport through the metalduring the first permeation may be affected by both irreversibleand r
32、eversible trapping as well as by the reduction of anyoxides present on the charging surface. At steady state all of theirreversible traps are occupied. If the mobile hydrogen atomsare then removed and a subsequent permeation test conductedon the specimen the difference between the first and secondpe
33、rmeation transients can be used to evaluate the influence ofirreversible trapping on transport, assuming a negligible role ofoxide reduction.4.10 For some environments, the conditions on the chargingside of the specimen may be suitably altered to induce a decayof the oxidation current after attainme
34、nt of steady state. Therate of decay will be determined by diffusion and reversibletrapping only and, hence, can also be used to evaluate the effectof irreversible trapping on transport during the first transient.4.11 Comparison of repeated permeation transients withthose obtained for the pure metal
35、 can be used in principle toevaluate the effect of reversible trapping on atomic hydrogentransport.4.12 This practice is suitable for systems in which hydrogenatoms are generated uniformly over the charging surface of themembrane. It is not usually applicable for evaluation ofcorroding systems in wh
36、ich pitting attack occurs unless theG 148 97 (2003)2charging cell environment is designed to simulate the localizedpit environment and the entire metal charging surface is active.4.13 This practice can be used for stressed and unstressedspecimens but testing of stressed specimens requires consider-a
37、tion of loading procedures.5. Significance and Use5.1 The procedures described, herein, can be used to evalu-ate the severity of hydrogen charging of a material produced byexposure to corrosive environments or by cathodic polariza-tion. It can also be used to determine fundamental properties ofmater
38、ials in terms of hydrogen diffusion (for example, diffu-sivity of hydrogen) and the effects of metallurgical, processing,and environmental variables on diffusion of hydrogen inmetals.5.2 The data obtained from hydrogen permeation tests canbe combined with other tests related to hydrogen embrittlemen
39、tor hydrogen induced cracking to ascertain critical levels ofhydrogen flux or hydrogen content in the material for crackingto occur.6. Apparatus6.1 The experimental set-up shall consist of a separatecharging and oxidation cell of a form similar to Fig. 1. Sealedoxidation cells, in which an additiona
40、l material (usually palla-dium), either plated or sputter deposited onto or clampedagainst the specimen and the flux exiting this additionalmaterial is measured may be used provided that it is demon-strated that the introduction of this additional interface has noeffect on the calculated diffusivity
41、. The clamping of thisadditional material may provide inaccurate permeation currentsin some systems due to the barrier effect at the interface (thatis, oxides, air gaps and so forth will act as a diffusion barrier).6.2 Non-metallic materials which are inert to the test envi-ronment should be used fo
42、r cell construction.6.2.1 At temperatures above 50C, leaching from the cellmaterial (for example, silica dissolution from glass in someenvironments) can modify the solution chemistry and mayinfluence hydrogen permeation.6.2.2 Polytetrafluoroethylene (PTFE) is an example of amaterial suitable for ele
43、vated temperatures up to about 90C.6.2.3 Where metallic chambers are necessary (for contain-ment of high pressure environments), the materials chosenshall have a very low passive current to ensure minimal effecton the solution composition and shall be electrically isolatedfrom the membrane.6.3 The O
44、-ring seal material should be selected to minimizepossible degradation products from the seals and contamina-tion of the solution. This problem is particularly of concernwith highly aggressive environments and at high test tempera-tures.6.4 Double junction reference electrodes may be used whereneces
45、sary to avoid contamination of test solutions. At elevatedtemperatures, the use of a solution conductivity bridge arrange-ment with suitable inert materials is recommended.6.5 The location of the reference electrode in each compart-ment shall ensure minimal potential drop between the speci-men and t
46、he reference electrode. A Luggin capillary may beuseful in cases where the solution resistivity is high, small cellvolumes are used and long tests are conducted. See Guide G 96for further guidance.6.6 Recording of oxidation (and, as appropriate, charging)current shall be made using a standard resist
47、or and a highinternal impedance digital voltmeter or by direct measurementusing a current monitoring device.NOTE 1A Luggin capillary should be used for more accurate measurement of potential when the current is large.FIG. 1 PTFE Hydrogen Permeation Cell (with double junction reference electrodes, us
48、ed for electrochemical charging)G 148 97 (2003)36.7 The measurement devices should be traceable to na-tional standards and calibrated prior to testing.6.8 In some cases, stirring of the solution in the chargingcell may be required. This should be performed using suitablestirring motor and apparatus7
49、. Specimen7.1 DesignSpecimens may be in the form of plate or pipe.The dimensions shall enable analysis of the permeation tran-sient based on one-dimensional diffusion. For example, forplates with a circular exposed area, the radius exposed to thesolution should be sufficiently large relative to thickness. Aratio of radius to thickness of 10:1, or greater, is recommended.This condition may be made less stringent if the exposed areain the oxidation side is smaller than that on the charging side.A ratio of radius to thickness of 5:1 is acceptable
