ASM Metals HandBook Vol. 8 - Mechanical Testing and Evaluation

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Fig. 12 Loading frame used for rising step-load test

The rising step-load test was used to evaluate high-strength HY ship steels and weldments in an environment

simulating seawater under conditions of cathodic protection commonly used to protect ship hulls (Ref 66).

Samples from the heat-affected zone (HAZ) and other locations in the weld metal were tested. Interlayer gas

tungsten arc heating was evaluated as a means of providing a refined, homogeneous, tempered microstructure

with improved resistance to hydrogen stress cracking. Test results showed that HY-180 is more susceptible to

hydrogen stress cracking than HY-130 and that the resistance to hydrogen embrittlement of specimens taken

from the HAZ and the fusion line is consistently higher than that of weld metal specimens (Ref 66).

Another application example of the rising step-load bend technique is the evaluation of potential-step

polarization and hydrogen over-potential on crack propagation and crack arrest of T-250 maraging steel and

PH-13-8Mo (UNS S13800) steel (Ref 67). In this case, the critical stress-intensity threshold for hydrogen-

assisted cracking was measured as a function of potential. A fractographic examination of specimens tested at

different potentials revealed distinct changes in the fracture mechanism as a function of potential. These

changes are explained in terms of the decohesion model for hydrogen-assisted cracking. The crack arrest and

propagation results reveal that the crack tip responds instantly to changes in the hydrogen activity from step

polarization. The ability to arrest and restart crack propagation by stepping the applied potential at a constant

applied stress intensity is demonstrated, proving that stress-intensity threshold is a direct function of hydrogen.

The disk-pressure test method measures the susceptibility to hydrogen embrittlement of metallic materials

under a high-pressure gaseous environment (Ref 68). In this test, a thin disk of the metallic material to be tested

is placed as a membrane in a test cell and subjected to helium pressure until it bursts. Because helium is inert,

the fracture is caused by mechanical overload; no secondary physical or chemical action is involved. An

identical disk is placed in the same test cell and subjected to hydrogen pressure until it bursts. Metallic

materials that are susceptible to environmental hydrogen embrittlement fracture under a pressure that is lower

than the helium-burst pressure; materials that are not susceptible fracture under the same pressure for both

hydrogen and helium.

The test is used for the selection and quality control of materials, protective coatings, surface finishes, and other

processing variables. The ratio, S

, between the helium-burst pressure, P

, and the hydrogen-burst pressure,

, indicates the susceptibility of the material to environmental hydrogen embrittlement. If S

is equal to or

less than 1, the material is not susceptible to environmental hydrogen embrittlement. When S

is greater than

2, the material is considered to be highly susceptible. At values between 1 and 2, the material is moderately

susceptible, with failure expected after long exposure to hydrogen; therefore, the material must be protected

against exposure.

A compilation of test results (Ref 3 and 68) indicate:

• Alloys having little or no sensitivity: 7075-T6 aluminum; Haynes 188 (cobalt base); beryllium copper

(copper base); types 304, 316, and 310 austenitic stainless steel; type 430 ferritic steel; and age-

hardened austenitic A 286 steel

• Alloys with high sensitivity: Cobalt- and iron-base alloys, including Haynes 25 and medium- and high-

strength steels

Finite element models of the specimen have also been used for interpreting experimental results and to achieve

a more general understanding of the capabilities of the disk pressure test for the characterization of hydrogen

embrittlement effects (Ref 69).

Slow-strain-rate tensile tests can be used to evaluate many product forms, including plate, rod, wire, sheet, and

tubing, as well as welded parts. Smooth, notched, or precracked specimens can be used. The principal

advantage of this standardized test is that the susceptibility to hydrogen stress cracking for a particular

metal/environment combination can be rapidly assessed. A variety of specimen shapes and sizes can be used;

the most common is a smooth bar tensile coupon, as described in ASTM E 8. The specimen is exposed to the

environment and is stressed under displacement control. For stainless steel in chloride solution, the strain rate is

-6

-1

. One or more of the following parameters are applied to the tensile test at the same initial strain rate:

• Time to failure

• Ductility, as assessed, for example, by reduction of area or elongation to fracture

• Maximum load achieved or reduction in ultimate and yield tensile stress

• Area bounded by a nominal stress-elongation curve or a true-stress/true-strain curve

• Presence of secondary cracking on the specimen gage section

• Appearance of the fracture surface

Slow-strain-rate tension testing is used extensively in the evaluation of environmentally assisted crack growth,

and several standards have been published including:

Designation

Title

ISO 7539 (Part

Slow Strain Rate Testing

ASTM G 129 Slow Strain Rate Testing to Evaluate the Susceptibility of Metallic Materials to

Environment

Slow-strain-rate testing is used extensively in materials research, as described in more detail in Ref 70. The

decreasing of fracture ductility from testing at a lowered strain rate in tension has been useful in establishing the

threshold stress-intensity factor K

of high-strength steel (Ref 71) consistent with the model of K

behavior of

high-strength steel suggested by Gerberich (Ref 72). The slow-strain-rate tension test has also confirmed that

initiation of cracks with low concentration of hydrogen is a mainly strain-controlled process in steels (Ref 73).

References cited in this section

3. L. Raymond, Evaluation of Hydrogen Embrittlement, Corrosion, Volume 13, ASM Handbook, ASM

International, 1987, p 283–284

5. W.W. Gerberich and S.-H. Chen, Environment-Induced Cracking of Metals, Fundamental Processes:

Micromechanics, Int. Conf. Environmentally Induced Cracking, Oct 1988, National Association of

Corrosion Engineers, 1990, p 167–187

6. W.W. Gerberich, P. Marsh, J. Hoehn, S. Venkataraman, and H. Huang, Hydrogen/Plasticity Interactions

in Stress Corrosion Cracking, Corrosion-Deformation Interactions, CD1 '92, Les Editions de Physique

les Ulis, T. Magnin, Ed., 1993, p 325–343

8. H.K. Birnbaum, I.M. Robertson, D. Sofronis, and D. Teter, Mechanism of Hydrogen-Related Fracture—

A Review, CDI '96, T. Magnin, Ed., The Institute of Materials, 1997, p 172

9. M.-J. Lii, X.-F. Chen, Y. Katz, and W.W. Gerberich, Dislocation Modeling and Acoustic Emission

Observation of Alternating Ductile/Brittle Events in Fe-3wt%Si Crystals, Acta Metall., Vol 38, 1990, p

2435–2452

10. S.-H. Chen, Y. Katz, and W.W. Gerberich, Crack Tip Strain Fields and Fracture Microplasticity in

Hydrogen Induced Cracking of Fe-3wt%Si Single Crystals, Philos. Mag. A, Vol 63 (No. 1), 1990, p

131–155

15. E.A. Steigewald, F.W. Schaller, and A.R. Troiano, Trans. Metall. Soc. AIME, Vol 218, 1960, p 832

17. H. Huang and W.W. Gerberich, Quasi-Equilibrium Modeling of the Toughness Transition During

Semibrittle Cleavage, Acta Metall. Mater., Vol 42 (No. 3), 1994, p 639–647

18. M.S. Daw and M.I. Baskes, Phys. Rev. B, Vol 29, 1984, p 6443

19. N.R. Moody, J.E. Angelo, S.M. Foiles, M.I. Baskes, and W.W. Gerberich, Atomistic Simulation of the

Hydrogen-Induced Fracture Process in an Ion-Based Superalloy, Sixth Israel Materials and Engineering

Conf., D. Itshak, D. Iliezer, and J. Haddad, E., Ben-Gurion University of the Negev, 1993, p 444

20. C.L. Fu and G.S. Painter, J. Mater. Res., Vol 6, 1991, p 719

21. X. Chen, T. Foecke, M. Lii, Y. Katz, and W.W. Gerberich, The Role of Stress State on Hydrogen-

Induced Cracking in Fe-Si Single Crystals, Eng. Fract. Mech., Vol 35 (No. 6), 1989, p 997–1017

27. D.G. Ulmer and C.J. Alstetter, Hydrogen Induced Strain Localization and Failure of Austenitic Stainless

Steels at High Hydrogen Concentrations, Acta Metall. Mater., Vol 39, 1991, p 1237

28. P.D. Hicks and C.J. Alstetter, Metall. Trans. A, Vol 23A, 1992, p 237

29. H. Vehoff and P. Neumann, Acta Metall., Vol 28, 1980, p 265

30. W.W. Gerberich, H. Huang, and P.G. Marsh, Macroscopic and Microscopic Modeling of Hydrogen

Embrittlement Thresholds, Second Workshop on Hydrogen Effects on Materials in Propulsion Systems,

NASA Conf. 3182, Marshall Flight Center, 1992, p 196–203

31. J.J. Lewandowski and A.W. Thompson, Hydrogen Effects on Material Behavior, N.R. Moody and A.W.

Thompson, Ed., TMS, 1990, p 817

32. D.S. Shih, I.M. Robertson, and H.K. Birnbaum, Hydrogen Embrittlement of α Titanium in situ TEM

Studies, Acta Metall., Vol 36, 1988, p 111

33. I.M. Robertson and H.K. Birnbaum, An HVEM Study of Hydrogen Effects on Deformation and

Fracture of Nickel, Acta Metall., Vol 34, 1986, p 353

34. H. Mathias, Y. Katz, and S. Nadiv, Hydrogenation/Gas Release Effects in Austenitic Steels:

Quantitative Study, Metal-Hydrogen Systems, T.N. Vezirogen, Ed., Pergamon Press, 1982, p 225

35. Y. Katz, X. Chan, M.J. Lii, M. Lanxner, and W.W. Gerberich, The Anisotropic Nature of Local Crack

Stability in BCC Crystals, Eng. Fract. Mech., Vol 41 (No. 4), 1991, p 541–567

36. N.R. Moody, S.K. Venkataraman, R.Q. Hwang, J.E. Angelo, and W.W. Gerberich, Hydrogen Effects on

the Fracture of Thin Tantalum Nitride Films, Symp. Corrosion/Deformation Interactions, 2, Vol 21, T.

Magnin, Ed., Eur. Fed. Corrosion, 1997, p 227–237

37. W.W. Gerberich, R.A. Oriani, M.-J. Lii, X. Chen, and T. Foecke, The Necessity of Both Plasticity and

Brittleness in the Fracture Thresholds of Iron, Philos. Mag. A, Vol 63 (No. 2), 1991, p 363–376

38. R.N. Parkins, Stress Corrosion Cracking, EICM Proc., R.P. Gangloff and M.B. Ives, Ed., National

Association of Corrosion Engineers, 1990, p 1

39. Y. Katz, A. Bussiba, and H. Mathias, The Influence of Austenitic Stability on Fatigue Crack Growth

Retardation, Material Experimentation and Design in Fatigue, F. Sheratt, J.B. Sturgeon, and R.A.F.

Farnborough, Ed., IPC Science and Tech. Press, Guildford, Surry, 1981, p 147

40. C.W. Tien and C.J. Alstetter, Hydrogen-Enhanced Plasticity of 310S Stainless Steel, CDI '92, Les

Editions de Physique les Ulis, T. Magnin, Ed., 1993, p 355

41. V.G. Gavriljuk, H. Häanninen, S.Yu. Smouk, A.V. Tarasenko, A.S. Tereschenko, and K. Ullakko,

Phase Transformation and Relaxation Phenomena in Hydrogen Charged CrNiMn and CrNi Stable

Austenitic Stainless Steel, Hydrogen Effects in Materials, A.W. Thompson and N.R. Moody, Ed., The

Minerals, Metals, Materials Society, 1996, p 893

42. V.G. Gavriljuk, H. Hanninen, A.S. Tereschenko, and K. Ullakko, Effects of Nitrogen on Hydrogen-

Induced Phase Transformation in Stable Austenitic Stainless Steel, Scr. Metall. Mater., Vol 28, 1993, p

247

43. P. Sofronis and H.K. Birnbaum, Mechanism of Hydrogen-Dislocation-Impurity Interactions—I.

Increasing Shear Modulus, J. Mech. Phys. Solids, Vol 43, 1995, p 49

44. H. Mathias, Y. Katz, and S. Nadiv, High Resolution Micro-Autoradiography as Complementary

Research Technique, Proc. Sixth Congress on Electron Microscopy, D.C. Brandon, Ed., Tal.

International, Jerusalem, 1976, p 399

45. P. Lacombe, M. Aucouturier, and J. Chene, Hydrogen Trapping and Hydrogen Embrittlement,

Hydrogen Embrittlement and Stress Corrosion Cracking, R. Gibala and R.F. Hehemann, Ed., American

Society for Metals, 1984

46. A.M. Brass and J. Chene, Hydrogen-Deformation Interactions in Iron and Nickel Base Alloys, CDI '96,

T. Magnin, Ed., The Institute of Metals, 1997, p 197

47. A. Bussiba, H. Alush, and Y. Katz, Corrosion/Deformation Interactions in U/Water Systems, Mater. Sci.

Res. Int., Vol 3, 1997, p 244

48. R.A. Oriani and P.H. Josephic, Acta Metall., Vol 22, 1977, p 979

49. H. Kobayashi and K. Ema, Physics and Technology of Amorphous SiO

, R.A.B. Devine, Ed., 1987,

Plenum Press, p 71

50. E.A. Clark, R. Yeske, and H.K. Birnbaum, The Effect of Hydrogen on the Surface Energy of Nickel,

Metall. Trans. A, Vol 11, 1980, p 1903

51. J.P. Hirth, Philos. Trans. R. Soc. London Ser., Vol 295, 1980, p 139

52. J.P. Hirth and J.R. Rise, Metall. Trans. A, Vol 11A, 1984, p 1501

53. T.Y. Zhang and J.E. Hack Metall. Mater. Trans. A, Vol 30, 1999, p 155

54. W.W. Gerberich, P. Marsh, and H. Huang, The Effect of Local Dislocation Arrangements on Hydrogen-

Induced Cleavage, Parkins Symposium on Fundamental Aspects of Stress Corrosion Cracking, S.

Breummer and W.W. Gerberich, Ed., TMS-AIME, 1992, p 191–204

55. S.T. Rolfe and I.M. Barsom, Fracture and Fatigue Control in Structures, Prentice-Hall, 1977

56. R.P. Wei, S.R. Novak, and D.R. Williams, Some Important Considerations in the Development of Stress

Corrosion Cracking Test Methods, Mater. Res. Stand., Vol 12 (No. 9), Sept 1972, p 25

57. C.A. Zanis, P.W. Holsberg, and E.C. Dunn, Jr., Seawater Subcritical Cracking of HY-Steel Weldments,

Weld. J. Res. Suppl., Vol 59, Dec 1980

58. “Rapid Inexpensive Tests for Determining Fracture Toughness,” NMAB-328, National Academy of

Sciences, 1976

59. D.L. Dull and L. Raymond, Stress History Effect on Incubation Time for Stress Corrosion Crack

Growth in AISI 4340 Steel, Metall. Trans., Vol 3, Nov 1972, p 2943–2947

60. G. Liu-Nash, J.A. Todd, and S. Mostovoy, An Accelerated Test Method for Determining Near-

Threshold Stress Intensity Values in HSLA Steels, Fatigue and Fracture of Engineering Materials and

Structures, Vol 20 (No. 12), 1997, p 1657–1664

61. X.P. Zhang and Y.W. Shi, The Reasonable Design of Charpy-Size Specimen for Fracture Toughness

Test in Nuclear Surveillance, Int. J. Pressure Vessels Piping, Vol 62 (No. 3), 1995, p 219–225

62. M. Wall, C.E. Lane, and C.A. Hippsley, Fracture Criteria for Hydrogen and Temper Embrittlement in

9Cr1Mo Steel, Acta Metall. Mater., Vol 42 (No 4), April 1994, p 1295–1309

63. J.J. Lewandowski and A.W. Thompson, Hydrogen Effects on Cleavage Fracture in Fully Pearlitic

Eutectoid Steel, Hydrogen Effects on Material Behavior, TMS, 1990, p 861–870

64. R.D. Kane, M.S. Cayard, and M. Prager, “Test Procedures for Evaluation of Resistance of Steels to

Cracking in Wet H

S Environments,” Paper No. 519, NACE International, 1994, p 22

65. D.L. Dull and L. Raymond, Electrochemical Techniques, Hydrogen Embrittlement and Testing, STP

543, ASTM, 1974, p 20–33

66. P.J. Fast, C.S. Susskind, and L. Raymond, “Charpy V-Notched Specimens for Indexing Stress

Corrosion Cracking in HY Ship Steels and Weld Metals,” Final report, Contract F04701-78-C-0079,

The Aerospace Corporation, Aug 1979

67. P.S. Tyler, M. Levy, and L. Raymond, Investigation of the Conditions for Crack Propagation and Arrest

under Cathodic Polarization by Rising Step Load Bend Testing, Corrosion, Vol 47 (No. 2), 1991, p 82–

68. J.P. Fiddle, R. Bernardi, R. Broudeur, C. Roux, and M. Rapin, Disk Pressure Testing of Hydrogen

Environment Embrittlement, Hydrogen Embrittlement Testing, STP 543, ASTM, 1974, p 221–253

69. M. Beghini, G. Benamati, and L. Bertini, Hydrogen Embrittlement Characterization by Disk Pressure

Tests: Test Analysis and Application to High Chromium Martensitic Steels, J. Eng. Mater. Technol.

(Trans. ASME), Vol 118 (No. 2), 1996, p 179–185

70. Slow Strain Rate Testing for the Evaluation of Environmentally Induced Cracking: Research, STP 1210,

ASTM, 1993

71. B. Ule, F. Vodopivec, and L. Vehovar, The Application of Slow Strain Rate Tension Test for

Quantitative Evaluation of Hydrogen-Induced Cracking Susceptibility of High Strength Steels, ECF 8—

Fracture Behaviour and Design of Materials and Structures, Vol I, Engineering Materials Advisory

Services Ltd., 1990, p 461–468

72. W.W. Gerberich, Effect of Hydrogen in High-Strength and Martensitic Steels, Hydrogen in Metals,

American Society for Metals, 1976, p 115

73. B. Ule, F. Vodopivec, and L. Vehovar, The Application of Critical Strain Fracture Criteria for the

Quantitative Evaluation of Hydrogen Embrittlement of High-Strength Steels, Conference: Use of

Special Steels, Alloys and New Materials in Chemical Process Industries (Lyon, France, 22–23 May

1991), Bull. Cercle D'Etudes Met., Vol 16 (No. 1), May 1991, p 28.1–28.11

Evaluation of Environmentally Assisted Crack Growth

Y. Katz, N. Tymiak, and W.W. Gerberich, University of Minnesota

Evaluation of Stress-Corrosion Cracking

Stress-corrosion cracking (SCC) can be influenced by a combination of anodic and/or cathodic reactions that

influence crack growth mechanisms, as briefly summarized in Table 2. Thus, SCC can be a more complex

process than hydrogen embrittlement, although many of the test methods are similar. Nonetheless, this section

briefly describes the key features of SCC testing, as recently published in Ref 74. More detailed information on

SCC testing is also contained in Ref 75 and 76.

Detailed information on SCC mechanisms and models is also beyond the scope of this article. Alloy design

against SCC is an ongoing mission coupled with environmental control efforts. Generalized statements

concerning interactive problems are not recommended, as they can be misleading. Table 3 summarizes typical

SCC behavior for various alloy systems. In most cases, the scope of a comprehensive analysis must be designed

to fit a given application.

Table 3 Stress-corrosion cracking of selected material systems

Material Environment

(a)

Applications

Remarks

Ferrous alloys

Carbon steels,

ferritic, pearlitic,

or tempered

martensitic

(normally low

carbon)

NaOH and nitrate solution

S, ammonia,

carbonate/bicarbonate,

aqueous solutions

chlorides, seawater

Oil/gas production and

transmission, oil refining, pulp

and paper processing, power

generation, marine and general

application, wire products,

strands, and wire ropes

Alloying has significant

effects on SCC

Crack propagation rate

increases with water

vapor lowering chloride

concentration is

beneficial.

High-strength

steels (low or

high alloy)

normally

tempered

Water and water vapor,

aqueous electrolytes

including phosphate ions,

NaCl solutions, H

S, strong

acids, various organic

Aerospace, construction tools,

dies, fasteners, landing gears,

and general applications

Mutual considerations

and compromises

regarding alloying

elements

Trade-offs in mechanical

martensite compounds

properties like yield

strength, toughness, and

SCC are controlled by

alloying.

Ferritic stainless

steels

Chloride ions aqueous

solutions

Housewares, food industry

(vessels, pipeline transmission,

and general applications)

Highly sensitive to

alloying elements with

synergistic effects

Austenitic

stainless steels

Chloride ion in solutions or

steam, NaOH solutions,

NaCl + H

solutions,

seawater

Food industry (e.g.,

transmission) houseware,

nuclear and chemical industry,

biomedical instruments

Phase-stability aspects

Attention also to carbon

concentration and

elements affecting

sensitization

Duplex structure

(combination of

austenite and

ferrite phases)

stainless steels

Chloride ions (higher

resistance than other

stainless steel in some

situations) aqueous

solutions

Chemical and nuclear industry

or more specific applications

Duplex stainless steel is

picked to preserve a

specific microstructure,

thereby requiring

particular attention to

welding.

Martensitic and

precipitation-

hardening

stainless steels

NaCl + H

, seawater,

NaOH + H

S, NaOH

solutions, other sulfur

compounds, marine

environments

Aerospace, aeronautical,

houseware, surgical tools

Used for applications that

require high strength.

This material group

allows yield strength

values to exceed 1100

MPa, calling for stress-

intensity threshold

evaluation.

Nonferrous alloys

Low- and high-

strength

aluminum alloys

NaCl solutions, seawater,

water vapor, aqueous

solutions, halide ions, Cl,

Br, and I solutions

(synergistic effects)

General building construction

elements, aerospace, aeronautic

transportation, naval vessels

Attention to material

selection in terms of alloy

composition and strength

Commercial aluminum

alloys are well specified in

material groups

(composition and

processing parameters).

Combined environment

and material group

interaction differ

regarding severity.

Nickel alloys Pure steam, NaOH or KOH

solutions, fused caustic

sulfur compounds, various

aqueous halide solutions,

aqueous solutions, and

cupric acid

Heat exchangers, higher-

temperature applications in

nuclear and chemical industry.

Superalloys for jet engine

components

Sensitive to

microstructure and phases

like γ, γ′, and carbides

Copper alloys

(including α or β

brass)

Ammonia solutions,

amines, water, mercury salt

solutions, distilled water

General applications,

munitions, marine components

…

Nickel-copper

alloy (Monel)

HF or H

SiFe

solutions

Mercury salt solutions

Specific corrosion-resistant

applications

…

Titanium alloys Seawater, organic liquids,

, aqueous Cl, B, and I

solutions, HCl + methanol

solutions, HNO

Space, aircraft, biomedical

prostheses, deep sea, and other

specific applications

Microstructure and phase

optimization for specific

environment. Normally

alloy types consider the α

chlorinated hydrocarbons

(hexagonal closed packed)

and the β (body-centered

phase volume fraction). β

at ambient temperatures

is stabilized by alloying

with Cr, Mo, V, Ta, and

Magnesium

alloys

NaCl + K

CrO

solutions,

marine atmosphere,

distilled water, and other

solutions

Specific applications relating to

transportation

Highly reactive material

group with intensive

efforts for development in

alloy design, i.e., ratio of

strength to weight with

acceptable properties

Zr alloys Organic liquids

solution

at elevated temperature

Nuclear applications for

cladding and others based on

beneficial nuclear properties

Attention to SCC

susceptibility to iodine

embrittlement

α-uranium

alloys or

stabilized γ-

uranium

Cl solutions

Water and wafer vapor

Oxygen and hydrogen gas

Nuclear industry application

for heavy metal specific

applications

…

Thin-film

aluminum and

copper

metallization

Hydrogen produced by

thin-film processing or

chemomechanical

polishing; relative

humidity attack of

galvanic couples

Conductors, interconnects for

microelectronics,

optoelectronics,

microelectromechanical

systems, flexible circuits

Small volume effects on

adhesion, strength,

environmental

susceptibility

(a) Environment here is used in the context of those known to produce SCC or hydrogen embrittlement, but

those listed are not meant to be all-inclusive.

Besides common structural materials, there are special applications involving small-volume devices and

multilayer or composite systems. To evaluate such materials, all the microstructural aspects and processing

history must be provided (e.g., specific heat treatments and textures for structural components or sputtering

temperatures and pressures for thin films).

Similar attention to detail applies to the environmental conditions where synergism is one of the major

considerations. For example, in nuclear power stations, neutron or gamma radiation affects materials in terms of

enhanced precipitation kinetics or reduced fracture resistance. Additionally, radiation affects the environment,

for example, through radiolytic decay products. Informative background on such complexities has been

published (Ref 38, 77, and 78). On the conceptual level, it is emphasized to the designer that design criteria can

be established only when combined with experimental SCC evaluations.

Additional considerations include:

• High resistance to general corrosion is frequently a cause of major confusion when considering

localized processes. General corrosion resistance of aluminum alloys, titanium alloys, or even nickel

alloys does not imply resistance to local SCC under specific conditions.

• Attention should be given to processing variables. In many cases, alloy design is aimed at achieving

some optimized conditions. This might necessitate sacrificing mechanical properties to minimize time-

dependent damaging effects.

• Many applications, such as those associated with the power industry and others, follow restricted codes

related to detrimental combinations of alloy concentration and impurities. Familiarity with the

physical/chemical bases of such codes or standards for analogous applications are always beneficial.

• Although in many cases SCC arises from an ill-advised material selection, many also stem from poor

assessment of the local mechanical/chemical loading factors.

• The distinction between long and small crack behavior is also a factor. Observations have confirmed

that higher crack growth rates occur for small cracks compared to long cracks under identical applied

stress-intensity conditions.

Considering the last point, the issue of crack size becomes particularly apparent regarding the initiation stage.

During slow-strain-rate testing on smooth specimens, multicracking phenomena can also occur with different

damage characteristics. At least intuitively, such behavior calls for a statistical approach, which potentially

implies insight as to how one might approach design. Questions that might be included are:

• Is the attention only to the longest crack justified and at what stage of damage evolution?

• What about small crack coalescence that might evolve into a long-crack situation?

The latter is even more understandable in initiation-controlled processes. Critical crack growth does not

necessarily involve a very long crack. Small cracks increasing by a factor of 2 to 3 can consume 80% of the

total life (Ref 79).

Accordingly, other approaches, such as the one addressed by Parkins (Ref 80), have incorporated the

aforementioned aspects. As indicated in Ref 80, the crack propagation rate is related to the crack density. Along

with this, statistical models can be established for crack density and size with particular implication to the

initiation stage and small crack growth. Interrupted tests as a function of time can establish crack-distribution

functions incorporated in engineering models. Activities along such phenomenological lines have been

conducted without using any mechanistic hypothesis (Ref 81). At this stage, a well-established methodology for

design is not yet available, but the practical potential is self-evident. Although such a phenomenological

approach is essential, the combination with mechanistic analysis would be more comprehensive. For such an

analysis to be successful, a crack-distribution assessment should be assisted by high-resolution direct

observation and not only by indirect methods such as acoustic emission.

References cited in this section

38. R.N. Parkins, Stress Corrosion Cracking, EICM Proc., R.P. Gangloff and M.B. Ives, Ed., National

Association of Corrosion Engineers, 1990, p 1

74. G. Koch, Stress Corrosion Cracking and Hydrogen Embrittlement, Fatigue and Fracture, Vol 19, ASM

Handbook, ASM International, 1996, p 481–506

75. D. Sprowls, Evaluation of Stress-Corrosion Cracking, Corrosion, Vol 13, ASM Handbook, 1987, p 245–

282

76. D. Sprowls, Evaluation of Stress Corrosion Cracking, Stress Corrosion Cracking, Materials

Performance and Evaluation, ASM International, 1992, p 363–416

77. R.H. Jones and R.E. Ricker, Stress Corrosion Cracking Mechanisms, Stress Corrosion Cracking,

Materials Performance and Evaluation, ASM International, 1992, p 1–41

78. R.W. Staehle, Understanding “Situation-Dependent Strength,” a Fundamental Objective in Assessing

the History of Stress Corrosion Cracking, EICM Proc.., R.P. Gangloff and M.B. Ives, Ed., National

Association of Corrosion Engineers, 1990, p 561

79. M.A. Gaudett and J. R. Scully, The Effects of Pre-Dissolved Hydrogen on Cleavage and Grain-

Boundary Fracture Initiation in Metastable Beta Ti-3Al-8V-4Cr-4Mo-4Zr, Met. Mater. Trans. A, Vol

30A, 1999, p 65

80. R.N. Parkins, Corros. Sci., Vol 29, 1989, p 1019

81. G. Santarini, A Morphological Model for Quantitative Characterization of Stress Corrosion Cracking,

CDI '96, T. Magnin, Ed., The Institute of Metals, p 55

Evaluation of Environmentally Assisted Crack Growth

Y. Katz, N. Tymiak, and W.W. Gerberich, University of Minnesota

SCC Testing

In order to determine the susceptibility of alloys to SCC, several types of testing are available that are covered

in ASTM standards:

ASTM

No.

Title

G 30

Standard Practice for Making and Using U-Bend Stress-Corrosion Test Specimens

G 35

Standard Practice for Determining the Susceptibility of Stainless Steels and Related Nickel-

Chromium-Iron Alloys to Stress-Corrosion Cracking in Polythionic Acids

G 36

Standard Practice for Evaluating Stress-Corrosion-Cracking Resistance of Metals and Alloys

in a Boiling Magnesium Chloride Solution

G 37

Standard Practice for Use of Mattsson's Solution of pH 7.2 to Evaluate the Stress-Corrosion

Cracking Susceptibility of Copper-Zinc Alloys

G 38

Standard Practice for Making and Using C-Ring Stress-Corrosion Test Specimens

G 39

Standard Practice for Preparation and Use of Bent-Beam Stress-Corrosion Test Specimens

G 41

Standard Practice for Determining Cracking Susceptibility of Metals Exposed under Stress to

a Hot Salt Environment

G 44

Standard Practice for Exposure of Metals and Alloys by Alternate Immersion in Neutral

3.5% Sodium Chloride Solution

G 47

Standard Test Method for Determining Susceptibility to Stress-Corrosion Cracking of 2xxx

and 7xxx Aluminum Alloy Products

G 58

Standard Practice for Preparation of Stress-Corrosion Test Specimens for Weldments

G 64

Standard Classification of Resistance to Stress-Corrosion Cracking of Heat Treatable

Aluminum Alloys

G 103 Standard Test Method for Performing a Stress-Corrosion Cracking Resistance of Low

Copper 7xxx Series Al-Zn-Mg-Cu Alloys in Boiling 6% Sodium Chloride Solution

In addition, other standards include:

Designation

Title

ISO 7539

Metals and Alloys—Stress-Corrosion Testing

DIN 50915

Testing the Resistance of Unalloyed and Low-Alloy Steels to Intergranular Stress-

Corrosion Cracking

NACE

TM0284

Evaluation of Pipeline Steels for Resistance to Stepwise Cracking

NACE

TM0177

Laboratory Testing of Metals for Resistance to Specific Forms of Environmental

Cracking

The general test methods and the type of information derived from testing are illustrated in Fig. 13. These test

concepts apply to both SCC and hydrogen embrittlement. Tests for corrosion fatigue tests are also illustrated.