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1、| | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | BRITISH STANDARD BS 7886 : 1997 ICS 77.060
2、 NO COPYING WITHOUT BSI PERMISSION EXCEPT AS PERMITTED BY COPYRIGHT LAW Method of measurement of hydrogen permeation and the determination of hydrogen uptake and transport in metals by an electrochemical technique Licensed Copy: London South Bank University, London South Bank University, Fri Dec 08
3、11:48:23 GMT+00:00 2006, Uncontrolled Copy, (c) BSI BS 7886 : 1997 This British Standard, having been prepared under the direction of the Engineering Sector Board, was published under the authority of the Standards Board and comes into effect on 15 July 1997 BSI 1997 The following BSI references rel
4、ate to the work on this standard: Committee reference ISE/NFE/8 Draft for comment 95/708873 DC ISBN 0 580 27127 7 Amendments issued since publication Amd. No.DateText affected Committees responsible for this British Standard The preparation of this British Standard was entrusted to Technical Committ
5、ee ISE/NFE/8, Corrosion of metals and alloys, upon which the following bodies were represented: AEATechnology Aluminium Federation British Gas plc British Iron and Steel Producers Association British Non-Ferrous Metals Federation Department of Transport (Transport Research Laboratory) Electricity As
6、sociation Institute of Corrosion Ministry of Defence Society of Chemical Industry Welding Institute Licensed Copy: London South Bank University, London South Bank University, Fri Dec 08 11:48:23 GMT+00:00 2006, Uncontrolled Copy, (c) BSI BS 7886 : 1997 BSI 1997i Contents Page Committees responsibleI
7、nside front cover Forewordii Method of measurement 1Scope1 2Informative references1 3Definitions and symbols1 4Principle2 5Test sample3 6Apparatus3 7Test environment considerations5 8Test procedure5 9Control and monitoring of test environment6 10Analysis of results7 11Test report9 Annex A(informativ
8、e) Recommended test environments for specific alloys10 Tables 1Normalized flux as a function of normalized time based on solution of Ficks law for a rising permeation transient7 Figures 1Polytetrafluoroethylene (PTFE) hydrogen permeation cell, with double junction reference electrodes, used for elec
9、trochemical charging4 2Rising permeation transients for BS 970 410S21 stainless steel in acidified NaCl at 77 C8 List of referencesInside back cover Licensed Copy: London South Bank University, London South Bank University, Fri Dec 08 11:48:23 GMT+00:00 2006, Uncontrolled Copy, (c) BSI ii BSI 1997 B
10、S 7886 : 1997 Foreword This British Standard has been prepared by Technical Committee ISE/NFE/8. Compliance with a British Standard does not of itself confer immunity from legal obligations. Summary of pages This document comprises a front cover, an inside front cover, pages i and ii, pages 1 to 10,
11、 an inside back cover and a back cover. Licensed Copy: London South Bank University, London South Bank University, Fri Dec 08 11:48:23 GMT+00:00 2006, Uncontrolled Copy, (c) BSI BSI 19971 BS 7886 : 1997 Method of measurement 1 Scope This British Standard specifies a laboratory method for the measure
12、ment of hydrogen permeation and for the determination of hydrogen atom uptake and transport in metals using an electrochemical technique. The standard gives a method for evaluating hydrogen uptake in metals based on measurement of steady-state hydrogen flux. It also gives a method for determining an
13、 effective diffusivity of hydrogen atoms in a metal and for distinguishing reversible and irreversible trapping. This standard gives requirements for the preparation of specimens, control and monitoring of the environmental variables, test procedures and analysis of results. This standard may be app
14、lied in principle to all metals for which hydrogen permeation is measurable and the method can be used to rank the relative aggressivity of different environments in terms of the hydrogen uptake of the exposed metal. 2 Informative references This British Standard refers to other publications that pr
15、ovide information or guidance. Editions of these publications current at the time of issue of this standard are listed on the inside back cover, but reference should be made to the latest editions. 3 Definitions and symbols 3.1 Definitions For the purposes of this British Standard, the following def
16、initions apply. 3.1.1 charging Method of introducing atomic hydrogen into the metal by exposure to aqueous environment under galvanostatic control (constant charging current), potentiostatic control (constant electrode potential), free corrosion, or by gaseous exposure. 3.1.2 charging cell Compartme
17、nt in which hydrogen atoms are generated on the sample surface. This includes both aqueous and gaseous charging. 3.1.3 decay current Decay of the hydrogen atom oxidation current, after attainment of steady-state, following a decrease in charging current. 3.1.4 Ficks second law Second order different
18、ial equation describing the concentration of atomic hydrogen in the sample as a function of position and time. The equation is of the form C(x, t)/t = D12C(x, t)/x2for lattice diffusion in one dimension where diffusivity is independent of concentration. 3.1.5 hydrogen flux Amount of hydrogen passing
19、 through the metal sample per unit area per unit time. 3.1.6 hydrogen uptake Atomic hydrogen absorbed into the metal as a result of charging. 3.1.7 irreversible trap Microstructural site at which the residence time for a hydrogen atom is infinite or extremely long compared to the time-scale for perm
20、eation testing at the relevant temperature. 3.1.8 mobile hydrogen atoms Hydrogen atoms in interstitial sites in the lattice (lattice sites) and reversible trap sites. 3.1.9 oxidation cell Compartment in which hydrogen atoms exiting from the metal sample are oxidized. 3.1.10 permeation current Curren
21、t measured in oxidation cell associated with oxidation of hydrogen atoms. 3.1.11 permeation flux Hydrogen flux exiting the test sample in the oxidation cell. 3.1.12 permeation transient Variation of the permeation current with time from commencement of charging to the attainment of steady-state, or
22、modification of charging conditions. 3.1.13 recombination poison Chemical within the test environment in the charging cell which enhances hydrogen absorption by retarding the recombination of hydrogen atoms on the metal surface. 3.1.14 reversible trap Microstructural site at which the residence time
23、 for a hydrogen atom is greater than that for the lattice site but is small in relation to the time to attain steady-state permeation. 3.2 Symbols For the purposes of this British Standard the following symbols apply. AExposed area of sample in the oxidation cell C(x, t)Lattice concentration of hydr
24、ogen as a function of position and time C0Sub-surface concentration of atomic hydrogen in interstitial lattice sites on the charging side of the sample C0RSummation of the sub-surface concentration of hydrogen in interstitial lattice sites and reversible trap sites on the charging side of the sample
25、 Licensed Copy: London South Bank University, London South Bank University, Fri Dec 08 11:48:23 GMT+00:00 2006, Uncontrolled Copy, (c) BSI 2 BSI 1997 BS 7886 : 1997 DlLattice diffusion coefficient of atomic hydrogen DeffEffective diffusion coefficient of atomic hydrogen based on elapsed time corresp
26、onding to J(t)/Jss= 0.63 FFaradays constant (9.6485 3 104Cmol-1) J(t)Time-dependent atomic hydrogen permeation flux as measured on the oxidation side of the sample JssAtomic hydrogen permeation flux at steady-state as measured on the oxidation side of the sample J(t)/JssNormalized flux of atomic hyd
27、rogen I(t)Time-dependent atomic hydrogen permeation current IssSteady-state atomic hydrogen permeation current LSample thickness tTime elapsed from commencement of hydrogen charging tbElapsed time measured by extrapolating the linear portion of the rising permeation current transient to J(t) = 0 tla
28、gTime to achieve a value of J(t)/Jss= 0.63 xDistance in sample measured in the thickness direction tNormalized time (D1t/L2) tlagNormalized time to achieve a value of J(t)/Jss= 0.63 4 Principle 4.1 The technique involves locating the metal sample of interest between the charging and oxidation cells,
29、 where the charging cell contains the environment of interest. Hydrogen atoms are generated on the sample surface exposed to this environment. 4.2 In gaseous environments, the hydrogen atoms are generated by adsorption and dissociation of the gaseous species. In aqueous environments, hydrogen atoms
30、are produced by electrochemical reactions. In both cases, some of the hydrogen atoms diffuse through the metal sample and are then oxidized to hydrogen cations on exiting from the other side of the metal in the oxidation cell. NOTE. A palladium coating is sometimes applied to one or both sides of th
31、e membrane following initial removal of oxide films. A palladium coating on the charging face of the membrane affects the sub-surface hydrogen concentration in the substrate and the measured permeation current. It is important to verify that the calculated diffusivity is not influenced by the coatin
32、g. Palladium coating is particularly useful for gaseous charging. 4.3 The environment and the electrode potential on the oxidation side of the membrane are selected so that the metal is either passive or immune to corrosion. The background current established prior to hydrogen transport is steady, a
33、nd small compared to the hydrogen atom oxidation current. 4.4 The electrode potential of the sample in the oxidation cell is controlled at a value sufficiently positive to ensure that the kinetics of oxidation of hydrogen atoms are limited by the flux of hydrogen atoms, i.e. the hydrogen atom oxidat
34、ion current density is transport-limited. NOTE. Palladium coating of the oxidation side of the sample can enhance the rate of oxidation and thereby enable attainment of transport-limited oxidation of hydrogen atoms at less positive potentials than for the uncoated sample. 4.5 The oxidation current i
35、s monitored as a function of time. The total oxidation current comprises the background current and the permeation current. 4.6 The thickness of the sample, L, is selected usually to ensure that the measured flux reflects volume (bulk) controlled hydrogen atom transport. NOTE. Thin specimens may be
36、used for evaluation of the effect of surface processes on hydrogen entry (absorption kinetics or transport in oxide films). 4.7 In reasonably pure metals with a sufficiently low density of microstructural trap sites, atomic hydrogen transport through the material is controlled by lattice diffusion.
37、4.8 The effect of alloying and of microstructural features such as dislocations, grain boundaries, inclusions, and precipitate particles is to introduce traps for hydrogen atoms which delay hydrogen transport. NOTE 1. Reversible and irreversible traps can both be present in a particular metal. NOTE
38、2. The rate of hydrogen atom transport through the metal during a first permeation test can be affected by both irreversible and reversible trapping. At steady-state all of the irreversible traps are occupied. If the mobile hydrogen atoms are then removed and a subsequent permeation test conducted o
39、n the sample, the difference between the first and second permeation transients may be used to evaluate the influence of irreversible trapping on transport. NOTE 3. For some environments the conditions on the charging side of the sample may be suitably altered to induce a decay of the oxidation curr
40、ent after attainment of steady state. The rate of decay is determined by diffusion and reversible trapping only and hence can also be used to evaluate the effect of irreversible trapping on transport during the first transient. NOTE 4. Comparison of repeated permeation transients with those obtained
41、 for the pure metal can be used in principle to evaluate the effect of reversible trapping on atomic hydrogen transport. NOTE 5. The technique is suitable for systems in which hydrogen atoms are generated uniformly over the charging surface of the sample. It is not usually applicable to corroding sy
42、stems in which pitting attack occurs unless the charging cell environment is designed to simulate the localized pit environment and the entire metal surface is active. 4.9 The method may be used for stressed and unstressed samples but testing of stressed samples requires consideration of loading pro
43、cedures. Licensed Copy: London South Bank University, London South Bank University, Fri Dec 08 11:48:23 GMT+00:00 2006, Uncontrolled Copy, (c) BSI BSI 19973 BS 7886 : 1997 5 Test sample 5.1 Dimensions Samples shall be in the form of plate or pipe. The dimensions shall be such as to enable analysis o
44、f the permeation transient based on one-dimensional diffusion, e.g. for plates with a circular exposed area, the radius exposed to the solution should be sufficiently large relative to thickness. NOTE. A ratio of radius to thickness of 10:1 or greater is recommended. This condition may be made less
45、stringent if the exposed area on the oxidation side is smaller than that on the charging side. A ratio of radius to thickness of 5:1 or greater is recommended if the radius of the exposed area on the oxidation side is reduced to 90 % of the area of the charging side. For pipes, the ratio of the oute
46、r radius to the inner radius shall be less than 1.1:1 if the experimental results are to be analysed based on planar one-dimensional diffusion. 5.2 Preparation 5.2.1 As hydrogen atom permeation can be influenced by microstructural orientation, the form of the original material shall be recorded (e.g
47、. bar) as well as the location and orientation of the sample relative to that of the original material (see clause 11). 5.2.2 Samples shall be prepared using one of the following methods: a) electrochemical discharge machining (EDM); b) mechanical cutting. NOTE 1. Careful consideration should be giv
48、en to the method of manufacture of sheet samples. NOTE 2. EDM is particularly useful for preparing thin sheets of material but can introduce hydrogen into the metal. Although hydrogen atoms dissolved in lattice sites or reversible trap sites are gradually lost subsequent to EDM, hydrogen atoms can b
49、e retained in irreversible trap sites. The amount of hydrogen generated and the extent of ingress into the metal depends on the details of the EDM process and the material characteristics but sufficient material should be removed by subsequent machining to ensure that all residual hydrogen atoms are removed. NOTE 3. The preferred method for the preparation of thin sheets of material is fine mechanical cutting. 5.2.3 Sheet samples shall be machined to the required thickness. Care shall be taken in machining to minimize surface damage. 5.2.4 The thickness of the sample in the
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