ASTM G4-01(2008)
(Guide)Standard Guide for Conducting Corrosion Tests in Field Applications
Standard Guide for Conducting Corrosion Tests in Field Applications
SIGNIFICANCE AND USE
Note 1—This guide is consistent with NACE Standard RP0497.
Observations and data derived from corrosion testing are used to determine the average rate of corrosion or other types of attack, or both (see Terminology G 15), that occur during the exposure interval. The data may be used as part of an evaluation of candidate materials of construction for use in similar service or for replacement materials in existing facilities.
The data developed from in-plant tests may also be used as guide lines to the behavior of existing plant materials for the purpose of scheduling maintenance and repairs.
Corrosion rate data derived from a single exposure generally do not provide information on corrosion rate change versus time. Corrosion rates may increase, decrease, or remain constant, depending on the nature of the corrosion products and the effects of incubation time required at the onset of pitting or crevice corrosion.
SCOPE
1.1 This guide covers procedures for conducting corrosion tests in plant equipment or systems under operating conditions to evaluate the corrosion resistance of engineering materials. It does not cover electrochemical methods for determining corrosion rates.
1.1.1 While intended primarily for immersion tests, general guidelines provided can be applicable for exposure of test specimens in plant atmospheres, provided that placement and orientation of the test specimens is non-restrictive to air circulation.
1.2 The values stated in SI units are to be regarded as the standard. The values given in parentheses are for information only.
1.3 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. See also 10.4.2.
General Information
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Designation: G4 − 01(Reapproved 2008)
Standard Guide for
Conducting Corrosion Tests in Field Applications
This standard is issued under the fixed designation G4; 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.Asuperscript
epsilon (´) indicates an editorial change since the last revision or reapproval.
1. Scope G30 Practice for Making and Using U-Bend Stress-
Corrosion Test Specimens
1.1 This guide covers procedures for conducting corrosion
G36 Practice for Evaluating Stress-Corrosion-Cracking Re-
tests in plant equipment or systems under operating conditions
sistance of Metals and Alloys in a Boiling Magnesium
to evaluate the corrosion resistance of engineering materials. It
Chloride Solution
does not cover electrochemical methods for determining cor-
G37 Practice for Use of Mattsson’s Solution of pH 7.2 to
rosion rates.
Evaluate the Stress-Corrosion Cracking Susceptibility of
1.1.1 While intended primarily for immersion tests, general
Copper-Zinc Alloys
guidelines provided can be applicable for exposure of test
G41 Practice for Determining Cracking Susceptibility of
specimens in plant atmospheres, provided that placement and
Metals Exposed Under Stress to a Hot Salt Environment
orientation of the test specimens is non-restrictive to air
G44 PracticeforExposureofMetalsandAlloysbyAlternate
circulation.
Immersion in Neutral 3.5 % Sodium Chloride Solution
1.2 The values stated in SI units are to be regarded as the
G46 Guide for Examination and Evaluation of Pitting Cor-
standard. The values given in parentheses are for information
rosion
only.
G47 Test Method for Determining Susceptibility to Stress-
1.3 This standard does not purport to address all of the Corrosion Cracking of 2XXX and 7XXX Aluminum
safety concerns, if any, associated with its use. It is the
Alloy Products
responsibility of the user of this standard to establish appro- G58 Practice for Preparation of Stress-CorrosionTest Speci-
priate safety and health practices and determine the applica-
mens for Weldments
bility of regulatory limitations prior to use. See also 10.4.2. G78 Guide for Crevice Corrosion Testing of Iron-Base and
Nickel-Base Stainless Alloys in Seawater and Other
2. Referenced Documents Chloride-Containing Aqueous Environments
2.2 NACE Standard:
2.1 ASTM Standards:
RP0497 Field Corrosion Evaluation Using Metallic Test
A262 Practices for Detecting Susceptibility to Intergranular
Specimens
Attack in Austenitic Stainless Steels
E3 Guide for Preparation of Metallographic Specimens
3. Significance and Use
G1 Practice for Preparing, Cleaning, and Evaluating Corro-
NOTE 1—This guide is consistent with NACE Standard RP0497.
sion Test Specimens
3.1 Observations and data derived from corrosion testing
G15 Terminology Relating to Corrosion and Corrosion Test-
are used to determine the average rate of corrosion or other
ing (Withdrawn 2010)
types of attack, or both (see Terminology G15), that occur
G16 Guide for Applying Statistics to Analysis of Corrosion
during the exposure interval. The data may be used as part of
Data
an evaluation of candidate materials of construction for use in
similar service or for replacement materials in existing facili-
ties.
This guide is under the jurisdiction ofASTM Committee G01 on Corrosion of
Metals and is the direct responsibility of Subcommittee G01.14 on Corrosion of 3.2 The data developed from in-plant tests may also be used
Metals in Construction Materials.
as guide lines to the behavior of existing plant materials for the
Current edition approved May 1, 2008. Published May 2008. Originally
purpose of scheduling maintenance and repairs.
approved in 1968. Last previous edition approved in 2001 as G4–01. DOI:
10.1520/G0004-01R08.
3.3 Corrosion rate data derived from a single exposure
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
generally do not provide information on corrosion rate change
contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
Standards volume information, refer to the standard’s Document Summary page on
the ASTM website.
3 4
The last approved version of this historical standard is referenced on Available from NACE International (NACE), 1440 South Creek Dr., Houston,
www.astm.org. TX 77084-4906, http://www.nace.org.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
G4 − 01 (2008)
versus time. Corrosion rates may increase, decrease, or remain 4.7.4 Parting or dealloying is a condition in which one
constant,dependingonthenatureofthecorrosionproductsand constituent is selectively removed from an alloy, as in the
the effects of incubation time required at the onset of pitting or dezincification of brass or the graphitic corrosion of cast iron.
crevice corrosion. Close attention and a more sophisticated evaluation than a
simple mass loss measurement are required to detect this
4. Limitations phenomenon.
4.7.5 Pitting corrosion cannot be evaluated by mass loss. It
4.1 Metal specimens immersed in a specific liquid may not
is possible to miss the phenomenon altogether when using
corrode at the same rate or in the same manner as in equipment
small test specimens since the occurrence of pitting is often a
in which the metal acts as a heat transfer medium in heating or
statistical phenomenon and its incidence can be directly related
cooling the liquid. In certain services, the corrosion of heat-
to the area of metal exposed.
exchanger tubes may be quite different from that of the shell or
4.7.6 Stress-corrosion cracking (SCC) may occur under
heads. This phenomenon also occurs on specimens exposed in
conditions of tensile stress and it may or may not be visible to
gas streams from which water or other corrodents condense on
the naked eye or on casual inspection. A metallographic
cool surfaces. Such factors must be considered in both design
examination (Practice E3) will confirm this mechanism of
and interpretation of plant tests.
attack. SCC usually occurs with no significant loss in mass of
4.2 Effects caused by high velocity, abrasive ingredients,
the test specimen, except in some refractory metals.
etc. (which may be emphasized in pipe elbows, pumps, etc.)
4.7.7 A number of reactive metals, most notably titanium
may not be easily reproduced in simple corrosion tests.
and zirconium, develop strongly adherent corrosion product
films in corrosive environments. In many cases, there is no
4.3 The behavior of certain metals and alloys may be
acceptable method to remove the film without removing
profoundly influenced by the presence of dissolved oxygen. It
significant uncorroded metal. In these cases, the extent of
is essential that the test specimens be placed in locations
corrosioncanbestbemeasuredasamassgainratherthanmass
representative of the degree of aeration normally encountered
loss.
in the process.
4.7.8 Some materials may suffer accelerated corrosion at
4.4 Corrosion products from the test specimens may have
liquid to atmospheric transition zones. The use of small test
undesirable effects on the process stream and should be
specimens may not adequately cover this region.
evaluated before the test.
4.5 Corrosion products from the plant equipment may
5. Test Specimen Design
influence the corrosion of one or more of the test metals. For
5.1 Before the size, shape, and finish of test specimens are
example, when aluminum specimens are exposed in copper-
specified, the objectives of the test program should be deter-
containing systems, corroding copper will exert an adverse
mined, taking into consideration any restrictions that might
effect on the corrosion of the aluminum. On the contrary,
dictatefabricationrequirements.Theduration,cost,confidence
stainless steel specimens may have their corrosion resistance
level,andexpectedresultsaffectthechoiceoftheshape,finish,
enhanced by the presence of the oxidizing cupric ions.
and cost of the specimen.
4.6 The accumulation of corrosion products can sometimes
5.1.1 Test specimens are generally fabricated into disks or
have harmful effects. For example, copper corroding in inter-
rectangular shapes. Other shapes such as balls, cylinders, and
mediate strengths of sulfuric acid will have its corrosion rate
tubes are used, but to a much lesser extent.
increased as the cupric ion concentration in the acid increases.
5.1.2 Disks are normally made by one of three methods: (1)
by punching from sheet material, (2) by slicing from a bar, or
4.7 Tests covered by this guide are predominantly designed
(3) by trepanning by a lathe or mill. Punched disks are by far
to investigate general corrosion; however, other forms of
the least expensive and should be considered if material
corrosion may be evaluated.
thickness is not a limitation. Some of the positive characteris-
4.7.1 Galvanic corrosion may be investigated by special
tics of disks are: (1) the surface area can be minimized where
devices that couple one specimen to another in electrical
there is restricted space, such as in pipeline applications, (2)
contact. It should be observed, however, that galvanic corro-
disks can be made inexpensively if a polished or machined
sion can be greatly affected by the area ratios of the respective
surface finish is not required, and (3) edge effects are mini-
metals.
mized for a given total surface area. Some negative character-
4.7.2 Crevice or concentration cell corrosion may occur
isticsare:(1)disksareverycostlytofabricateifagroundfinish
when the metal surface is partially blocked from the bulk
and machined edges are required, (2) disks fabricated from
liquid, as under a spacer. An accumulation of bulky corrosion
sheet material result in a considerable amount of scrap mate-
products between specimens can promote localized corrosion
rial, and (3) disks sliced from a bar present a surface orienta-
of some alloys or affect the general corrosion rates of others.
tion that can result in extensive end-grain attack. Using a bar is
Such accumulation should be reported.
undesirable unless end-grain effects are to be evaluated.
4.7.3 Selective corrosion at the grain boundaries (for ex-
ample, intergranular corrosion of sensitized austenitic stainless 5.2 Rectangular specimens are fabricated by either punch-
steels) will not be readily observable in mass loss measure- ing, shearing, or saw cutting. Punched disk shaped specimens
ments and often requires microscopic examination of the are the most economical if the quantity is sufficiently high to
specimens after exposure. justify the initial die cost. Fabrication is more cost-effective for
G4 − 01 (2008)
rectangular specimens than for disks when ground finished and heat affected zone. For example, gas tungsten arc welding has
machined sides are required, and they can be made using very lower heat input than oxygen fuel welding and causes a
few shop tools. In some cases, rectangular specimens are more narrower heat affected zone, which is also closer to the weld
awkward to mount. bead.
5.3 Material availability and machinability also affect the
7. Preparation of Test Specimens
cost of producing all types of specimens. Before the shape and
size are specified, the corrosion engineer should determine the
7.1 Controversyexistsastowhetherthetestspecimenedges
characteristics of the proposed materials.
should be machined. The cold-worked area caused by shearing
or punching operations can provide valuable information on
6. Test Specimens
alloy susceptibility to stress corrosion cracking. Also, the
ability to compare information among specimens of different
6.1 The size and shape of test specimens are influenced by
materials can be affected by the amount of cold work per-
several factors and cannot be rigidly defined. Sufficient thick-
formed on the material.Therefore, the decision to machine and
ness should be employed to minimize the possibility of
to test specimens with/without the residual stresses associated
perforation of the specimen during the test exposure. The size
with cold work should be made on a case-to-case basis.
of the specimen should be as large as can be conveniently
7.1.1 The depth of cold work associated with punching and
handled, the limitation being imposed by the capacity of the
shearingoperationstypicallyextendsbackfromthecutedgeto
available analytical balance and by the problem of effecting
a distance equal to the specimen thickness. Removal of the
entry into operating equipment.
cold worked areas can be performed by grinding or careful
6.2 A convenient size for a standard corrosion disk shaped
machining the specimen edges.
specimen is 38 mm (1.5 in.) in diameter and 3 mm (0.125 in.)
7.1.2 Ideally, the surface finish of the specimen should
in thickness with an 11 mm (0.438 in.) hole in the center of the
replicate that of the surface finish of the material to be used for
round specimen. This size was arrived at as being the maxi-
equipment fabrication. However, this is often difficult because
mum size that could easily effect entry through a normal 38
the finish on materials varies between mills, between sheet and
mm nozzle. However, it is also convenient for larger size
plate and even between heat treatments. The mill scale and the
nozzle entries as well as for laboratory corrosion testing. A
amount of oxides on the surface can vary as well.Also, surface
convenient standard specimen for spool-type racks measures
finishes are difficult to apply to edges that have been distorted
25 by 50 by 3 mm (1 by 2 by 0.125 in.) or 50 by 50 by 3 mm
by punching or shearing. Since the primary requirement is
(2 by 2 by 0.125 in.). A round specimen of 53 by 3 mm (2 by
usually to determine the corrosion resistance of the material
0.125 in.) or 55 by 1.5 mm (2 by 0.062 in.) is sometimes
2 itself, a clean metal surface is most often used. The purpose of
employed.Theselastthreemeasureabout0.005dm insurface
the test dictates the required finish of the specimen. For
area.
instance, for water treating applications, relative changes of
6.3 Othersizes,shapes,andthicknessesofspecimenscanbe
weights of specimens are usually compared to optimize inhibi-
used for special purposes or to comply with the design of a
tor additions. The specimens are generally punched or sheared
special type of corrosion rack. Special designs should be
and finished by blasts with glass beads. This is one of the most
reducedtoafewinnumberinpreliminarytests;specialdesigns
economicalwaysofpreparingcorrosiontestspecimens.Manu-
should be employed to consider the effect of such factors of
facturing variables in specimen preparation that can be re-
equipment c
...
This document is not anASTM standard and is intended only to provide the user of anASTM standard an indication of what changes have been made to the previous version. Because
it may not be technically possible to adequately depict all changes accurately, ASTM recommends that users consult prior editions as appropriate. In all cases only the current version
of the standard as published by ASTM is to be considered the official document.
Designation:G4–95 Designation:G4–01 (Reapproved 2008)
Standard Guide for
Conducting Corrosion Coupon Tests in Field Applications
This standard is issued under the fixed designation G 4; 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.Asuperscript
epsilon (e) indicates an editorial change since the last revision or reapproval.
1. Scope
1.1 This guide covers procedures for conducting corrosion coupon tests in plant equipment or systems under operating
conditions to evaluate the corrosive attack uponcorrosion resistance of engineering materials. It does not cover electrochemical
methods for determining corrosion rates.
1.1.1 While intended primarily for immersion tests, general guidelines provided can be applicable for exposure of test
couponsspecimens in plant atmospheres, provided that placement and orientation of the couponstest specimens is non-restrictive
to air circulation.
1.2 The values stated in SI units are to be regarded as the standard. The values given in parentheses are for information only.
1.3 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. See also 10.4.2. See also 10.4.2.
2. Referenced Documents
2.1 ASTM Standards:
A 262 Practices for Detecting Susceptibility to Intergranular Attack in Austenitic Stainless Steels
E 3Practice Guide for Preparation of Metallographic Specimens
G 1 Practice for Preparing, Cleaning, and Evaluating Corrosion Test Specimens
G 15 Terminology Relating to Corrosion and Corrosion Testing
G 16 Guide for Applying Statistics to Analysis of Corrosion Data
G 30 Practice for Making and Using U-Bend Stress-Corrosion Test Specimens
G 36Practice for Performing Stress-Corrosion Cracking Tests in a Boiling Magnesium Chloride Solution Practice for
Evaluating Stress-Corrosion-Cracking Resistance of Metals and Alloys in a Boiling Magnesium Chloride Solution
G 37 PracticeforUseofMattsson’sSolutionofpH7.2toEvaluatetheStress-CorrosionCrackingSusceptibilityofCopper-Zinc
Alloys
G 41 Practice for Determining Cracking Susceptibility of Metals Exposed Under Stress to a Hot Salt Environment
G 44PracticeforEvaluatingStressCorrosionCrackingResistanceofMetalsandAlloysbyAlternateImmersionin3.5%Sodium
Chloride Solution Practice for Exposure of Metals and Alloys by Alternate Immersion in Neutral 3.5 % Sodium Chloride
Solution
G 46Practice Guide for Examination and Evaluation of Pitting Corrosion
G 47 Test Method for Determining Susceptibility to Stress-Corrosion Cracking of High Strength 2XXX and 7XXXAluminum
Alloy Products
G 58 Practice for Preparation of Stress-Corrosion Test Specimens for Weldments
G 78Guide for Crevice Corrosion Testing of Iron-Base and Nickel-Base Stainless Alloys in Seawater and Other Chloride-
Containing Aqueous Environments Guide for Crevice Corrosion Testing of Iron-Base and Nickel-Base Stainless Alloys in
Seawater and Other Chloride-Containing Aqueous Environments
2.2 NACE Standard:
RP0497 Field Corrosion Evaluation Using Metallic Test Specimens
ThisguideisunderthejurisdictionofASTMCommitteeG-1onCorrosionofMetalsandisthedirectresponsibilityofSubcommitteeG01.12onIn-PlantCorrosionTests.
Current edition approved Jan. 15, 1995. Published March 1995. Originally issued as A224-39. Last previous edition G4-84.
This guide is under the jurisdiction of ASTM Committee G01 on Corrosion of Metals and is the direct responsibility of Subcommittee G01.14 on Corrosion of Metals
in Construction Materials.
Current edition approved May 1, 2008. Published May 2008. Originally approved in 1968. Last previous edition approved in 2001 as G 4–01.
For referencedASTM standards, visit theASTM website, www.astm.org, or contactASTM Customer Service at service@astm.org. For Annual Book of ASTM Standards
, Vol 01.03.volume information, refer to the standard’s Document Summary page on the ASTM website.
Annual Book of ASTM Standards, Vol 03.01.
Available from NACE International (NACE), 1440 South Creek Dr., Houston, TX 77084-4906, http://www.nace.org.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
G4–01 (2008)
3. Significance and Use
NOTE 1—This guide is consistent with NACE Standard RP0497.
3.1 Observations and data derived from couponcorrosion testing are used to determine the average rate of corrosion and the
typeor other types of attack, or both (seeTerminology G 15), that occur during the exposure interval.The data may be used as part
of an evaluation of potentialcandidate materials of construction for use in similar service or for replacement materials in existing
facilities.
3.2 The data developed from this guidein-plant tests may also be used as guide lines to the behavior of existing plant materials
for the purpose of scheduling maintenance and repairs.
3.3 Corrosion rate data derived from a single exposure generally do not provide information on corrosion rate change versus
time. Corrosion rates may increase, decrease, or remain constant, depending on the nature of the corrosion products and the effects
of incubation time required at the onset of pitting or crevice corrosion.
4. Limitations
4.1 Metal specimens immersed in a specific liquid may not corrode at the same rate or in the same manner as in equipment in
which the metal acts as a heat transfer medium in heating or cooling the liquid. In certain services, the corrosion of heat-exchanger
tubes may be quite different from that of the shell or heads. This phenomenon also occurs on specimens exposed in gas streams
from which water or other corrodents condense on cool surfaces. Such factors must be considered in both design and interpretation
of plant tests.
4.2 Effects caused by high velocity, abrasive ingredients, etc. (which may be emphasized in pipe elbows, pumps, etc.) may not
be easily reproduced in coupon simple corrosion tests.
4.3 The behavior of certain metals and alloys may be profoundly influenced by the presence of dissolved oxygen. It is essential
that the test couponsspecimens be placed in locations representative of the degree of aeration normally encountered in the process.
4.4 Corrosion products from the test specimens may have undesirable effects on the process stream and should be evaluated
before the test.
4.5 Corrosion products from the plant equipment may influence the corrosion of one or more of the test metals. For example,
when aluminum specimens are exposed in copper-containing systems, corroding copper will exert an adverse effect on the
corrosionofthealuminum.Onthecontrary,stainlesssteelspecimensmayhavetheircorrosionresistanceenhancedbythepresence
of the oxidizing cupric ions.
4.6 Theaccumulationofcorrosionproductscansometimeshaveharmfuleffects.Forexample,coppercorrodinginintermediate
strengths of sulfuric acid will have its corrosion rate increased as the cupric ion concentration in the acid increases.
4.7Coupon corrosion testing is 4.7 Tests covered by this guide are predominantly designed to investigate general corrosion;
however, other forms of corrosion may be evaluated with coupons. evaluated.
4.7.1 Galvanic corrosion may be investigated by special devices that couple one couponspecimen to another in electrical
contact. It should be observed, however, that galvanic corrosion can be greatly affected by the area ratios of the respective metals.
4.7.2 Crevice or concentration cell corrosion may occur when the metal surface is partially blocked from the bulk liquid, as
under a spacer.An accumulation of bulky corrosion products between couponsspecimens can promote localized corrosion of some
alloys or affect the general corrosion rates of others. Such accumulation should be reported.
4.7.3 Selective corrosion at the grain boundaries (for example, intergranular corrosion of sensitized austenitic stainless steels)
will not be readily observable in mass loss measurements and often requires microscopic examination of the couponsspecimens
after exposure.
4.7.4 Parting or dealloying is a condition in which one constituent is selectively removed from an alloy, as in the dezincification
of brass or the graphitic corrosion of cast iron. Close attention and a more sophisticated evaluation than a simple mass loss
measurement are required to detect this phenomenon.
4.7.5 Pitting corrosion cannot be evaluated by mass loss. It is possible to miss the phenomenon altogether when using small test
specimens since the occurrence of pitting is often a statistical phenomenon and its incidence can be directly related to the area of
metal exposed.
4.7.6 Stress-corrosioncracking(SCC)mayoccurunderconditionsoftensilestressanditmayormaynotbevisibletothenaked
eye or on casual inspection. A metallographic examination (Practice E 3) will confirm this mechanism of attack. SCC usually
occurs with no significant loss in mass of the test coupon,specimen, except in some refractory metals.
4.7.7 Anumber of reactive metals, most notably titanium and zirconium, develop strongly adherent corrosion product films in
corrosive environments. In many cases, there is no acceptable method to remove the film without removing significant uncorroded
metal. In these cases, the extent of corrosion can best be measured as a mass gain rather than mass loss.
4.7.8 Some materials may suffer accelerated corrosion at liquid to atmospheric transition zones.The use of small test specimens
may not adequately cover this region.
5. Test CouponSpecimen Design
5.1 Before the size, shape, and finish of test couponsspecimens are specified, the objectives of the test program should be
determined, taking into consideration any restrictions that might dictate fabrication requirements. The duration, cost, confidence
level, and expected results affect the choice of the shape, finish, and cost of the coupons.specimen.
G4–01 (2008)
5.1.1 Testcouponsspecimensaregenerallyfabricatedintodisksorrectangularshapes.Othershapessuchasballs,cylinders,and
tubes are used, but to a much lesser extent.
5.1.2 Disks are normally made by one of three methods: (1) by punching from sheet material, ( 2) by slicing from a bar, or (3)
by trepanning by a lathe or mill. Punched disks are by far the least expensive and should be considered if material thickness is
not a limitation. Some of the positive characteristics of disks are: (1) the surface area can be minimized where there is restricted
space,suchasinpipelineapplications,(2)diskscanbemadeinexpensivelyifapolishedormachinedsurfacefinishisnotrequired,
and (3) edge effects are minimized for a given total surface area. Some negative characteristics are: (1) disks are very costly to
fabricate if a ground finish and machined edges are required, (2) disks fabricated from sheet material result in a considerable
amount of scrap material, and (3) disks sliced from a bar present a surface orientation that can result in extensive end-grain attack.
Using a bar is undesirable unless end-grain effects are to be evaluated.
5.2 Rectangular couponsspecimens are fabricated by either punching, shearing, or saw cutting. Punched coupons disk shaped
specimensarethemosteconomicalifthequantityissufficientlyhightojustifytheinitialdiecost.Fabricationismorecost-effective
for rectangular couponsspecimens than for disks when ground finished and machined sides are required, and they can be made
using very few shop tools. In some cases, rectangular couponsspecimens are more awkward to mount.
5.3 Material availability and machinability also affect the cost of producing all types of coupons.specimens. Before the shape
and size are specified, the corrosion engineer should determine the characteristics of the proposed materials.
6. Test Specimens
6.1 The size and shape of test specimens are influenced by several factors and cannot be rigidly defined. Sufficient thickness
should be employed to minimize the possibility of perforation of the specimen during the test exposure. The size of the specimen
should be as large as can be conveniently handled, the limitation being imposed by the capacity of the available analytical balance
and by the problem of effecting entry into operating equipment.
6.2 A convenient size for a standard corrosion coupon disk shaped specimen is 38 mm (1.5 in.) in diameter and 3 mm (0.125
in.) in thickness with an 11 mm (0.438 in.) hole in the center of the round coupon.specimen. This size was arrived at as being the
maximum size that could easily effect entry through a normal 38 mm nozzle. However, it is also convenient for larger size nozzle
entries as well as for laboratory corrosion testing.Aconvenient standard couponspecimen for spool-type racks measures 25 by 50
by 3 mm (1 by 2 by 0.125 in.) or 50 by 50 by 3 mm (2 by 2 by 0.125 in.). A round couponspecimen of 53 by 3 mm (2 by 0.125
in.) or 55 by 1.5 mm (2 by 0.062 in.) is sometimes employed. These last three measure about 0.005 dm in surface area.
6.3 Other sizes, shapes, and thicknesses of specimens can be used for special purposes or to comply with the design of a special
type of corrosion rack. Special couponsdesigns should be reduced to a few in number in preliminary tests; special couponsdesigns
should be employed to consider the effect of such factors of equipment construction and assembly as heat treatment, welding,
soldering, and cold-working or other mechanical stressing.
6.4 Since welding is a principal method of fabricating equipment, welded couponsspecimens should be included as much as
possible in the test programs.
6.4.1 Aside from the effects of residual stresses, the main items of interest in a welded couponspecimen are the corrosion
resistance of the weld bead and the heat affected zone. Galvanic effects between weld metal and base metal can also be evaluated.
The weld and heat affected zone regions are relatively small; therefore, welded couponsspecimens should be made slightly larger
than the normal size couponnon-welded specimens when possible, for example, 50 mm by 75 mm (2 in. by 3 in.). The optimum
method of welding coupons corrosion test specimens is to join the two halves using a single vee or double vee groove with full
penetration and multiple passes. Double vee joint preparation is use
...
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