ASTM G94-92(1998)
(Guide)Standard Guide for Evaluating Metals for Oxygen Service
Standard Guide for Evaluating Metals for Oxygen Service
SCOPE
1.1 This guide applies to metallic materials under consideration for oxygen or oxygen-enriched fluid service, direct or indirect, as defined in Section 3. It is concerned primarily with the properties of a material associated with its relative susceptibility to ignition and propagation of combustion. It does not involve mechanical properties, potential toxicity, outgassing, reactions between various materials in the system, functional reliability, or performance characteristics such as aging, shredding, or sloughing of particles, except when these might contribute to an ignition.
1.2 This document applies only to metals; nonmetals are covered in Guide G63. Note 1-The American Society for Testing and Materials takes no position respecting the validity of any evaluation methods asserted in connection with any item mentioned in this guide. Users of this guide are expressly advised that determination of the validity of any such evaluation methods and data and the risk of use of such evaluation methods and data are entirely their own responsibility. Note 2-In evaluating materials, any mixture with oxygen exceeding atmospheric concentration at pressures higher than atmospheric should be evaluated from the hazard point of view for possible significant increase in material combustibility.
1.3 The values stated in SI units are to be regarded as the standard.
1.4 This standard does not purport to address all of the safety problems, 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.
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Designation:G94–92 (Reapproved 1998)
Standard Guide for
Evaluating Metals for Oxygen Service
ThisstandardisissuedunderthefixeddesignationG94;thenumberimmediatelyfollowingthedesignationindicatestheyearoforiginal
adoptionor,inthecaseofrevision,theyearoflastrevision.Anumberinparenthesesindicatestheyearoflastreapproval.Asuperscript
epsilon (e) indicates an editorial change since the last revision or reapproval.
1. Scope D2512 Test Method for Compatibility of Materials with
Liquid Oxygen (Impact Sensitivity Threshold and Pass-
1.1 This guide applies to metallic materials under consider-
Fail Techniques)
ation for oxygen or oxygen-enriched fluid service, direct or
D2863 Test Method for Measuring the Minimum Oxygen
indirect, as defined in Section 3. It is concerned primarily with
Concentration to Support Candle-Like Combustion of
the properties of a material associated with its relative suscep-
Plastics (Oxygen Index)
tibility to ignition and propagation of combustion. It does not
D4809 Test Method for Heat of Combustion of Liquid
involve mechanical properties, potential toxicity, outgassing,
Hydrocarbon Fuels by Bomb Calorimeter (Intermediate
reactions between various materials in the system, functional
Precision Method)
reliability, or performance characteristics such as aging, shred-
G63 Guide for Evaluating Nonmetallic Materials for Oxy-
ding, or sloughing of particles, except when these might
gen Service
contribute to an ignition.
G72 Test Method for Autogenous Ignition Temperature of
1.2 This document applies only to metals; nonmetals are
Liquids and Solids in a High-Pressure Oxygen-Enriched
covered in Guide G63.
Environment
NOTE 1—The American Society for Testing and Materials takes no
G86 Test Method for Determining Ignition Sensitivity of
position respecting the validity of any evaluation methods asserted in
Materials to Mechanical Impact in Ambient Liquid Oxy-
connection with any item mentioned in this guide. Users of this guide are
genandPressurizedLiquidandGaseousOxygenEnviron-
expresslyadvisedthatdeterminationofthevalidityofanysuchevaluation
ments
methods and data and the risk of use of such evaluation methods and data
are entirely their own responsibility. G88 Guide for Designing Systems for Oxygen Service
NOTE 2—In evaluating materials, any mixture with oxygen exceeding
2.2 Compressed Gas Association Document:
atmospheric concentration at pressures higher than atmospheric should be
Pamphlet G-4.4, Industrial Practices for Gaseous Oxygen
evaluated from the hazard point of view for possible significant increase 8
Transmission and Distribution Piping Systems
in material combustibility.
2.3 ASTM Adjuncts:
1.3 The values stated in SI units are to be regarded as the
Test Program Report on the Ignition and Combustion of
standard.
Materials in High-Pressure Oxygen
1.4 This standard does not purport to address all of the
3. Terminology
safety concerns, if any, associated with its use. It is the
responsibility of the user of this standard to establish appro-
3.1 Definitions:
priate safety and health practices and determine the applica-
3.1.1 direct oxygen service—in contact with oxygen during
bility of regulatory limitations prior to use.
normaloperations.Examples:oxygencompressorpistonrings,
control valve seats (see Guide G63).
2. Referenced Documents
3.1.2 impact-ignition resistance—the resistance of a mate-
2.1 ASTM Standards:
rial to ignition when struck by an object in an oxygen
D2015 Test Method for Gross Calorific Value of Coal and
atmosphere under a specific test procedure (see Guide G63).
Coke by the Adiabatic Bomb Calorimeter
3.1.3 indirect oxygen service—not normally in contact with
D2382 Test Method for Heat of Combustion of Hydrocar-
oxygen, but which might be as a result of a reasonably
bonFuelsbyBombCalorimeter(High-PrecisionMethod)
foreseeablemalfunction,operatorerror,orprocessdisturbance.
1 4
This guide is under the jurisdiction ofASTM Committee G-4 on Compatibility Annual Book of ASTM Standards, Vol 15.03.
and Sensitivity of Materials in Oxygen Enriched Atmospheres and is the direct Annual Book of ASTM Standards, Vol 08.02.
responsibility of Subcommittee G04.02 on Recommended Practices. Annual Book of ASTM Standards, Vol 05.03.
Current edition approved July 15, 1992. Published September 1992. Originally Annual Book of ASTM Standards, Vol 14.02.
published as G94–87. Last previous edition G94–90. Available from Compressed Gas Association, Inc., 1235 Jefferson Davis
Annual Book of ASTM Standards, Vol 05.05. Highway, Arlington, VA.
3 9
Annual Book of ASTM Standards, Vol 05.02. Available from ASTM Headquarters, Order ADJG0094.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
G94–92 (1998)
Examples: liquid oxygen tank insulation, liquid oxygen pump 5.2.1 Studies of the flammability of gaseous fuels were
motor bearings (see Guide G63). begun more than 150 years ago. To date, an extremely wide
3.1.4 maximum use pressure—the maximum pressure to variety of applications have been studied and documented,
which a material can be subjected due to a reasonably including a wide range of important subtleties such as quench-
foreseeable malfunction, operator error, or process upset (see ing phenomena, turbulence, cool flames, influence of initial
Guide G63). temperature, etc., all of which have been used effectively for
3.1.5 maximum use temperature—the maximum tempera- safety and loss prevention. A smaller, yet still substantial,
ture to which a material can be subjected due to a reasonably backgroundexistsfornonmetallicsolids.Incontrasttothis,the
foreseeable malfunction, operator error, or process upset (see studyoftheflammabilityofmetalsdatesonlytothe1950s,and
Guide G63). even though it has accelerated rapidly, the uncovering and
3.1.6 nonmetallic—any material, other than a metal, or any understanding of subtleties have not yet matured. In addition,
composite in which the metal is not the most easily ignited the heterogeneity of the metal and oxidizer systems and the
componentandforwhichtheindividualconstituentscannotbe heat transfer properties of metals, as well as the known,
evaluated independently (see Guide G63). complex ignition energy and ignition/burning mechanisms,
3.1.7 operating pressure—the pressure expected under nor- clearly dictate that caution is required when applying labora-
mal operating conditions (see Guide G63). tory findings to actual applications. In many cases, laboratory
3.1.8 operating temperature—the temperature expected un- metals burning tests are designed on what is believed to be a
der normal operating conditions (see Guide G63). worst-case basis, but could the particular actual application be
3.1.9 oxygen-enriched—applies to a fluid (gas or liquid) worse?Further,becausesomanysubtletiesexist,accumulation
that contains more than 25 mol % oxygen (see Guide G63). of favorable experience (no metal fires) in some particular
3.1.10 qualified technical personnel—persons such as engi- application may not be as fully relevant to another application
neers and chemists who, by virtue of education, training, or as might be the case for gaseous or nonmetallic solids where
experience, know how to apply physical and chemical prin- the relevance may be more thoroughly understood.
ciples involved in the reactions between oxygen and other 5.3 Relationship of Guide G 94 with Guides G 63 and G 88:
materials (see Guide G63). 5.3.1 This guide addresses the evaluation of metals for use
3.1.11 reaction effect—the personnel injury, facility dam- in oxygen systems and especially in major structural portions
age, product loss, downtime, or mission loss that could occur ofasystem.GuideG63addressestheevaluationofnonmetals.
as the result of an ignition (see Guide G63). Guide G88 presents design and operational maxims for all
3.2 Definitions of Terms Specific to This Standard: systems. In general, however, Guides G63 and G88 focus on
3.2.1 autoignition temperature—the lowest temperature at physically small portions of an oxygen system that represent
which a material will spontaneously ignite in oxygen under the critical sites most likely to encounter ignition.
specific test conditions. 5.3.2 The nonmetals in an oxygen system (valve seats and
packing,pistonrings,gaskets,o-rings)aresmall;therefore,the
4. Significance and Use
use of the most fire-resistant materials is usually a realistic,
4.1 The purpose of this guide is to furnish qualified techni- practical option with regard to cost and availability. In com-
cal personnel with pertinent information for use in selecting
parison, the choice of material for the major structural mem-
metals for oxygen service in order to minimize the probability bers of a system is much more limited, and the use of special
of ignition and the risk of explosion or fire. It is intended for
alloys may have to be avoided to achieve realistic costs and
use in selecting materials for applications in connection with delivery times. Indeed, with the exception of ceramic materi-
the production, storage, transportation, distribution, or use of
als, which have relatively few practical uses, most nonmetals
oxygen. It is not intended as a specification for approving have less fire resistance than virtually all metals. Since non-
materials for oxygen service.
metals are typically introduced into a system to provide a
physical property not achievable from metals, and since
5. Factors Affecting Selection of Material
nonmetals may serve as “links” in a kindling chain (see 5.6.5),
5.1 General:
and since the locations of use are typically mechanically
5.1.1 The selection of a material for use with oxygen or
severe, the primary thrust in achieving compatible oxygen
oxygen-enriched atmospheres is primarily a matter of under-
systems rests with the minor components as addressed by
standing the circumstances that cause oxygen to react with the
Guides G63 and G88 that explain the emphasis on using the
material. Most materials in contact with oxygen will not ignite
most fire-resistant materials.
without a source of ignition energy. When an energy-input
exceeds the configuration-dependent threshold, then ignition
TABLE 1 Comparison of Metals and Nonmetals Flammability
and combustion may occur. Thus, the materials’ flammability
Metals Nonmetals
propertiesandtheignitionenergysourceswithinasystemmust
Combustion products molten metal oxide hot gases
be considered. These should be viewed in the context of the
Autoignition temperatures 900–2000°C 150–500°C
entire system design so that the specific factors listed in this
Thermal conductivities higher lower
guide will assume the proper relative significance. To summa-
Flame temperature higher lower
rize: it depends on the application. Heat release higher due to density lower
Surface oxide can be protective negligible
5.2 Relative Amount of Data Available for Metals and
Nonmetals:
G94–92 (1998)
5.3.3 Since metals are typically more fire-resistant and are high melting point. Designers have very limited control over
used in typically less fire-prone functions, they represent a the integrity of an oxide layer; however, since oxide can have
second tier of interest. However, because metal components significant influence on metal’s test data, an understanding of
are relatively so large, a fire of a metal component is a very its influence is useful.
important event, and should a nonmetal ignite, any consequen-
5.5.2 A protective oxide provides a barrier between the
tial reaction of the metal can aggravate the severity of an
metal and the oxygen. Hence, ignition and combustion can be
ignition many times over. Hence, while the selection of
inhibited in those cases where the oxide barrier is preserved.
nonmetals by Guide G63 and the careful design of compo-
For example, in some cases, an oxide will prevent autogenous
nents by Guide G88 are the first line of defense, optimum
ignition of a metal up to the temperature at which the metal
metal selection is an important second-line of defense.
melts and produces geometry changes that breach the film. In
5.4 Differences in Oxygen Compatibility of Metals and
other cases (such as anodized aluminum wires), the oxide may
Nonmetals:
be sufficiently sturdy as either a structure or a flexible skin to
5.4.1 Thereareseveralfundamentaldifferencesbetweenthe
contain and support the molten base metal at temperatures up
oxygen compatibility of metals and nonceramic nonmetals. to the melting point of the oxide itself. In either of these cases,
These principal differences are summarized in Table 1.
however, autogenous ignition may result at much lower tem-
5.4.2 Common-use metals are harder to ignite. They have
peraturesifthemetalexperiencesmechanismsthatdamagethe
high autoignition temperatures in the range 900 to 2000°C
oxide coating. Such oxide damaging mechanisms may be due
(1650 to 3600°F). In comparison, most combustible nonmetals
to mechanical stresses, frictional rubs and abrasion, or chemi-
have autoignition temperatures in the range 150 to 500°C (300
cal oxide attack (amalgamation, etc.). Depending upon the
to 1000°F). Metals have high thermal conductivities that help
application, a high metal autoignition temperature, therefore,
dissipate local heat inputs that might easily ignite nonmetals.
may be misleading relative to the metal’s flammability.
Many metals also grow protective oxide coatings (see 5.5) that
5.5.3 One criterion for estimating whether an oxide is
interfere with ignition and propagation.
protective is based upon whether the oxide that grows on a
5.4.3 Once ignited, however, metal combustion can be
metal occupies a volume greater or less than the volume of the
highlydestructive.Adiabaticflametemperaturesformetalsare
metal it replaces. Pilling and Bedworth (2) formulated an
much higher than for most polymers (Table X1.7).The greater
equation for predicting the transition between protective and
density of most metals provides greater heat release potential
nonprotective oxides in 1923. Two forms of the Pilling and
fromcomponentsofcomparablesize.Sincemanymetaloxides
Bedworth(P&B)equationappearintheliteratureandcanyield
do not exist as oxide vapors (they largely dissociate upon
different results.ASTM Committee G-4 has concluded that the
vaporization), combustion of these metals inherently yields
most meaningful formulation for the P&B ratio in oxide
coalescingliquidmetaloxideofhighheatcapacityintheflame
evaluations for flammability situations is:
zoneattheoxideboilingpoint(theremaybeverylittlegaseous
P&BRatio 5Wd/awD (1)
metal oxide). In comparison, combustion of polymers yields
where the metal, M, forms the oxide MaO , a and b are the
gaseous combustion products (typically carbon dioxide a
...
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