ASTM G4-95

Standard Guide for Conducting Corrosion Coupon Tests in Field Applications

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

• Status: Historical
• Publication Date: 09-Oct-2001
• ICS: 77.060 - Corrosion of metals
• Technical Committee: G01 - Corrosion of Metals

Relations

• Replaced By: ASTM G4-01 - Standard Guide for Conducting Corrosion Tests in Field Applications
• Effective Date: 10-Oct-2001
• Referred By: ASTM G111-21a - Standard Guide for Corrosion Tests in High Temperature or High Pressure Environment, or Both
• Effective Date: 10-Oct-2001
• Referred By: ASTM G78-20 - Standard Guide for Crevice Corrosion Testing of Iron-Base and Nickel-Base Stainless Alloys in Seawater and Other Chloride-Containing Aqueous Environments
• Effective Date: 10-Oct-2001
• Referred By: ASTM G199-09(2020)e1 - Standard Guide for Electrochemical Noise Measurement
• Effective Date: 10-Oct-2001
• Referred By: ASTM G217-16(2022) - Standard Guide for Corrosion Monitoring in Laboratories and Plants with Coupled Multielectrode Array Sensor Method
• Effective Date: 10-Oct-2001
• Referred By: ASTM G71-81(2019) - Standard Guide for Conducting and Evaluating Galvanic Corrosion Tests in Electrolytes
• Effective Date: 10-Oct-2001
• Referred By: ASTM G162-18 - Standard Practice for Conducting and Evaluating Laboratory Corrosion Tests in Soils
• Effective Date: 10-Oct-2001
• Referred By: ASTM G96-90(2018) - Standard Guide for Online Monitoring of Corrosion in Plant Equipment (Electrical and Electrochemical Methods)
• Effective Date: 10-Oct-2001
• Referred By: ASTM D7665-22 - Standard Guide for Evaluation of Biodegradable Heat Transfer Fluids
• Effective Date: 10-Oct-2001
• Referred By: ASTM C1187-20a - Standard Guide for Establishing Surveillance Test Program for Boron-based Neutron Absorbing Material Systems for Use in Nuclear Fuel Storage Racks in Pool Environment
• Effective Date: 10-Oct-2001
• Referred By: ASTM F3268-18a - Standard Guide for <emph type="bdit">in vitro</emph> Degradation Testing of Absorbable Metals
• Effective Date: 10-Oct-2001
• Referred By: ASTM D5372-20 - Standard Guide for Evaluation of Hydrocarbon Heat Transfer Fluids
• Effective Date: 10-Oct-2001

Standards Content (Sample)

NOTICE: This standard has either been superseded and replaced by a new version or discontinued.
Contact ASTM International (www.astm.org) for the latest information.
Designation:G4–95
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. A number in parentheses indicates the year of last reapproval. A superscript
epsilon (e) indicates an editorial change since the last revision or reapproval.
1. Scope G 41 Practice for Determining Cracking Susceptibility of
Metals Exposed Under Stress to a Hot Salt Environment
1.1 This guide covers procedures for conducting corrosion
G 44 Practice for Evaluating Stress Corrosion Cracking
coupon tests in plant equipment under operating conditions to
Resistance of Metals and Alloys by Alternate Immersion in
evaluate the corrosive attack upon engineering materials. It
3.5 % Sodium Chloride Solution
does not cover electrochemical methods for determining cor-
G 46 Practice for Examination and Evaluation of Pitting
rosion rates.
Corrosion
1.1.1 While intended primarily for immersion tests, general
G 47 Test Method for Determining Susceptibility to Stress-
guidelines provided can be applicable for exposure of test
Corrosion Cracking of High Strength Aluminum Alloy
coupons in plant atmospheres, provided that placement and
Products
orientation of the coupons is non-restrictive to air circulation.
G 58 Practice for Preparation of Stress-Corrosion Test
1.2 The values stated in SI units are to be regarded as the
Specimens for Weldments
standard. The values given in parentheses are for information
G 78 Guide for Crevice Corrosion Testing of Iron-Base and
only.
Nickel-Base Stainless Alloys in Seawater and Other
1.3 This standard does not purport to address all of the
Chloride-Containing Aqueous Environments
safety concerns, if any, associated with its use. It is the
responsibility of the user of this standard to establish appro-
3. Significance and Use
priate safety and health practices and determine the applica-
3.1 Observations and data derived from coupon testing are
bility of regulatory limitations prior to use. See also 10.4.2.
used to determine the average rate of corrosion and the type of
2. Referenced Documents attack (see Terminology G 15) that occur during the exposure
interval. The data may be used as part of an evaluation of
2.1 ASTM Standards:
potential materials of construction for use in similar service or
A 262 Practices for Detecting Susceptibility to Intergranu-
for replacement materials in existing facilities.
lar Attack in Austenitic Stainless Steels
3.2 The data developed from this guide may also be used as
E 3 Practice for Preparation of Metallographic Specimens
guide lines to the behavior of existing plant materials for the
G 1 Practice for Preparing, Cleaning, and Evaluating Cor-
purpose of scheduling maintenance and repairs.
rosion Test Specimens
3.3 Corrosion rate data derived from a single exposure
G 15 Terminology Relating to Corrosion and Corrosion
4 generally do not provide information on corrosion rate change
Testing
versus time. Corrosion rates may increase, decrease, or remain
G 16 Guide for Applying Statistics to Analysis of Corrosion
4 constant, depending on the nature of the corrosion products and
Data
the effects of incubation time required at the onset of pitting or
G 30 Practice for Making and Using U-Bend Stress Corro-
4 crevice corrosion.
sion Test Specimens
G 36 Practice for Performing Stress-Corrosion Cracking
4. Limitations
Tests in a Boiling Magnesium Chloride Solution
4.1 Metal specimens immersed in a specific liquid may not
G 37 Practice for Use of Mattsson’s Solution of pH 7.2 to
corrode at the same rate or in the same manner as in equipment
Evaluate the Stress Corrosion Cracking Susceptibility of
4 in which the metal acts as a heat transfer medium in heating or
Copper-Zinc Alloys
cooling the liquid. In certain services, the corrosion of heat-
exchanger tubes may be quite different from that of the shell or
This guide is under the jurisdiction of ASTM Committee G-1 on Corrosion of
heads. This phenomenon also occurs on specimens exposed in
Metals and is the direct responsibility of Subcommittee G01.12on In-Plant Corro-
gas streams from which water or other corrodents condense on
sion Tests.
Current edition approved Jan. 15, 1995. Published March 1995. Originally issued
cool surfaces. Such factors must be considered in both design
as A224-39. Last previous edition G4-84.
and interpretation of plant tests.
Annual Book of ASTM Standards, Vol 01.03.
4.2 Effects caused by high velocity, abrasive ingredients,
Annual Book of ASTM Standards, Vol 03.01.
Annual Book of ASTM Standards, Vol 03.02.
Copyright © ASTM, 100 Barr Harbor Drive, West Conshohocken, PA 19428-2959, United States.
G4
etc. (which may be emphasized in pipe elbows, pumps, etc.) films in corrosive environments. In many cases, there is no
may not be easily reproduced in coupon tests. acceptable method to remove the film without removing
significant uncorroded metal. In these cases, the extent of
4.3 The behavior of certain metals and alloys may be
corrosion can best be measured as a mass gain rather than mass
profoundly influenced by the presence of dissolved oxygen. It
loss.
is essential that the test coupons be placed in locations
representative of the degree of aeration normally encountered
5. Test Coupon Design
in the process.
5.1 Before the size, shape, and finish of test coupons are
4.4 Corrosion products from the test specimens may have
specified, the objectives of the test program should be deter-
undesirable effects on the process stream and should be
mined, taking into consideration any restrictions that might
evaluated before the test.
dictate fabrication requirements. The duration, cost, confidence
4.5 Corrosion products from the plant equipment may
level, and expected results affect the choice of the shape, finish,
influence the corrosion of one or more of the test metals. For
and cost of the coupons.
example, when aluminum specimens are exposed in copper-
5.1.1 Test coupons are generally fabricated into disks or
containing systems, corroding copper will exert an adverse
rectangular shapes. Other shapes such as balls, cylinders, and
effect on the corrosion of the aluminum. On the contrary,
tubes are used, but to a much lesser extent.
stainless steel specimens may have their corrosion resistance
5.1.2 Disks are normally made by one of three methods: (1)
enhanced by the presence of the oxidizing cupric ions.
by punching from sheet material, (2) by slicing from a bar, or
4.6 The accumulation of corrosion products can sometimes
(3) by trepanning by a lathe or mill. Punched disks are by far
have harmful effects. For example, copper corroding in inter-
the least expensive and should be considered if material
mediate strengths of sulfuric acid will have its corrosion rate
thickness is not a limitation. Some of the positive characteris-
increased as the cupric ion concentration in the acid increases.
tics of disks are: (1) the surface area can be minimized where
4.7 Coupon corrosion testing is predominantly designed to
there is restricted space, such as in pipeline applications, (2)
investigate general corrosion; however, other forms of corro-
disks can be made inexpensively if a polished or machined
sion may be evaluated with coupons.
surface finish is not required, and (3) edge effects are mini-
4.7.1 Galvanic corrosion may be investigated by special
mized for a given total surface area. Some negative character-
devices that couple one coupon to another in electrical contact.
istics are: (1) disks are very costly to fabricate if a ground finish
It should be observed, however, that galvanic corrosion can be
and machined edges are required, (2) disks fabricated from
greatly affected by the area ratios of the respective metals.
sheet material result in a considerable amount of scrap mate-
4.7.2 Crevice or concentration cell corrosion may occur
rial, and (3) disks sliced from a bar present a surface orienta-
when the metal surface is partially blocked from the bulk
tion that can result in extensive end-grain attack. Using a bar is
liquid, as under a spacer. An accumulation of bulky corrosion
undesirable unless end-grain effects are to be evaluated.
products between coupons can promote localized corrosion of
5.2 Rectangular coupons are fabricated by either punching,
some alloys or affect the general corrosion rates of others. Such
shearing, or saw cutting. Punched coupons are the most
accumulation should be reported.
economical if the quantity is sufficiently high to justify the
4.7.3 Selective corrosion at the grain boundaries (for ex-
initial die cost. Fabrication is more cost-effective for rectangu-
ample, intergranular corrosion of sensitized austenitic stainless
lar coupons than for disks when ground finished and machined
steels) will not be readily observable in mass loss measure-
sides are required, and they can be made using very few shop
ments and often requires microscopic examination of the
tools. In some cases, rectangular coupons are more awkward to
coupons after exposure.
mount.
4.7.4 Parting or dealloying is a condition in which one
5.3 Material availability and machinability also affect the
constituent is selectively removed from an alloy, as in the
cost of producing all types of coupons. Before the shape and
dezincification of brass or the graphitic corrosion of cast iron.
size are specified, the corrosion engineer should determine the
Close attention and a more sophisticated evaluation than a
characteristics of the proposed materials.
simple mass loss measurement are required to detect this
6. Test Specimens
phenomenon.
4.7.5 Pitting corrosion cannot be evaluated by mass loss. It
6.1 The size and shape of test specimens are influenced by
is possible to miss the phenomenon altogether when using
several factors and cannot be rigidly defined. Sufficient thick-
small test specimens since the occurrence of pitting is often a
ness should be employed to minimize the possibility of
statistical phenomenon and its incidence can be directly related
perforation of the specimen during the test exposure. The size
to the area of metal exposed.
of the specimen should be as large as can be conveniently
4.7.6 Stress-corrosion cracking (SCC) may occur under
handled, the limitation being imposed by the capacity of the
conditions of tensile stress and it may or may not be visible to available analytical balance and by the problem of effecting
the naked eye or on casual inspection. A metallographic
entry into operating equipment.
examination (Practice E 3) will confirm this mechanism of
6.2 A convenient size for a standard corrosion coupon is 38
attack. SCC usually occurs with no significant loss in mass of
mm (1.5 in.) in diameter and 3 mm (0.125 in.) in thickness with
the test coupon, except in some refractory metals.
an 11 mm (0.438 in.) hole in the center of the round coupon.
4.7.7 A number of reactive metals, most notably titanium This size was arrived at as being the maximum size that could
and zirconium, develop strongly adherent corrosion product easily effect entry through a normal 38 mm nozzle. However,
G4
it is also convenient for larger size nozzle entries as well as for amount of oxides on the surface can vary as well. Also, surface
laboratory corrosion testing. A convenient standard coupon for finishes are difficult to apply to edges that have been distorted
spool-type racks measures 25 by 50 by 3 mm (1 by 2 by 0.125 by punching or shearing. Since the primary requirement is
in.) or 50 by 50 by 3 mm (2 by 2 by 0.125 in.). A round coupon usually to determine the corrosion resistance of the material
of 53 by 3 mm (2 by 0.125 in.) or 55 by 1.5 mm (2 by 0.062 itself, a clean metal surface is most often used. The purpose of
in.) is sometimes employed. These last three measure about the test dictates the required finish of the coupon. For instance,
0.005 dm in surface area. for water treating applications, relative changes of weights of
6.3 Other sizes, shapes, and thicknesses of specimens can be coupons are usually compared to optimize inhibitor additions.
used for special purposes or to comply with the design of a The coupon are generally punched or sheared and finished by
special type of corrosion rack. Special coupons should be blasts with glass beads. This is one of the most economical
reduced to a few in number in preliminary tests; special ways of preparing coupons. Manufacturing variables in coupon
coupons should be employed to consider the effect of such preparation that can be removed reasonably should be elimi-
factors of equipment construction and assembly as heat treat- nated. A standard surface finish facilitates the comparison of
ment, welding, soldering, and cold-working or other mechani- results among test samples.
cal stressing. 7.2 Some of the available finishes are:
6.4 Since welding is a principal method of fabricating 7.2.1 Mill finish (pickled, bright annealed, or shot blasted),
equipment, welded coupons should be included as much as 7.2.2 Electrolytic polished, (Note that electrolytic polishing
possible in the test programs. can produce a surface layer enriched in some alloying elements
6.4.1 Aside from the effects of residual stresses, the main while depleted in others. For example, chromium is enriched
items of interest in a welded coupon are the corrosion on stainless surfaces and sulfur is depleted.)
resistance of the weld bead and the heat affected zone. 7.2.3 Blasted with sand or steel shot, (Note that blasting
Galvanic effects between weld metal and base metal can also many metals with sand can cause embedded sand particles and
be evaluated. The weld and heat affected zone regions are steel shot can cause surface contamination with iron or iron
relatively small; therefore, welded coupons should be made oxide. Glass beads are better, but not if broken pieces are
slightly larger than the normal size coupon when possible, for allowed to be used in the blasting.)
example, 50 mm by 75 mm (2 in. by 3 in.). The optimum 7.2.4 Sanded with abrasive cloth or paper,
method of welding coupons is to join the two halves using a 7.2.5 Machine finished, and
single vee or double vee groove with full penetration and 7.2.6 Passivation with nitric acid to remove surface iron
multiple passes. Double vee joint preparation is used for very contamination and other chemical cleaning methods used, for
thick samples. Machining the weld flush is optional, depending example, after welding.
on how closely the sample will be examined afterward (see 7.3 The surface finish most widely used is produced by
practice G58). sanding with an abrasive cloth
...