NACE Standard RP0170-2004
Item No. 21002
Standard
Recommended Practice
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Protection of Austenitic Stainless Steels and Other
Austenitic Alloys from Polythionic Acid Stress
Corrosion Cracking During Shutdown of Refinery
Equipment
This NACE International (NACE) standard represents a consensus of those individual members
who have reviewed this document, its scope, and provisions. Its acceptance does not in any
respect preclude anyone, whether he has adopted the standard or not, from manufacturing,
marketing, purchasing, or using products, processes, or procedures not in conformance with this
standard. Nothing contained in this NACE standard is to be construed as granting any right, by
implication or otherwise, to manufacture, sell, or use in connection with any method, apparatus, or
product covered by Letters Patent, or as indemnifying or protecting anyone against liability for
infringement of Letters Patent. This standard represents minimum requirements and should in no
way be interpreted as a restriction on the use of better procedures or materials. Neither is this
standard intended to apply in all cases relating to the subject. Unpredictable circumstances may
negate the usefulness of this standard in specific instances. NACE assumes no responsibility for
the interpretation or use of this standard by other parties and accepts responsibility for only those
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Users of this NACE standard are responsible for reviewing appropriate health, safety,
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standard prior to its use. This NACE standard may not necessarily address all potential health and
safety problems or environmental hazards associated with the use of materials, equipment, and/or
operations detailed or referred to within this standard. Users of this NACE standard are also
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Revised 2004-03-27
Reaffirmed 1997-Mar-10
Revised October 1993
Revised December 1984
Approved October 1970
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RP0170-2004
________________________________________________________________________
Foreword
This standard recommended practice provides methods to protect austenitic stainless steels and
other austenitic alloys from polythionic acid stress corrosion cracking (PTA SCC) occurring during
downtimes and contiguous shutdown and start-up periods. This standard is directed toward
preventing stress corrosion cracking (SCC) by polythionic acids that are formed by the reaction of
sulfide corrosion products with oxygen and water. For practical purposes, it should be assumed
that such acids can be formed by reaction of oxygen and water with oxidizable sulfur species
(sulfur, H2S, metal sulfides).
Primary protection methods to prevent polythionic acid formation include appropriate material
selection, avoidance of oxygen entry, alkaline washing of surfaces, and the prevention of liquid
water formation. Regardless of the protection method selected, appropriate confirmation steps to
validate compliance with the requirements of this standard are required by the user to ensure
protection is provided.
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This standard is intended primarily for petroleum refining industry materials and corrosion
engineers as well as inspection, operations, and maintenance personnel. While the focus of this
standard is on refining industry units such as desulfurizing, hydrocracking, and hydrotreating in
which the incidence of PTA SCC has been comparatively high, it can be applied to other units
using austenitic stainless steels and other austenitic alloys, such as crude distillation units and fluid
catalytic cracking units, when the user may have a concern for PTA SCC. The user must consider
other factors such as the effect of the alkaline chemicals on catalysts, as well as the appropriate
means and protective equipment required for handling these chemicals. For the purposes of this
standard, the term other austenitic alloys refers to those alloys of nickel, iron, and chromium that
may be susceptible to PTA SCC.
The techniques described in this standard are not designed to remove chloride deposits, but should
minimize the possibility of chloride SCC (Cl SCC) by the wash solutions.
This standard was originally prepared in 1970 by NACE Task Group T-8-19, revised in 1984 and
1993, and reaffirmed in 1997 by Group Committee T-8. It was revised in 2004 by Task Group (TG)
173 on Polythionic Acid SCC Prevention. TG 173 is administered by Specific Technology Group
(STG) 34 on Petroleum Refining and Gas Processing. TG 173 is sponsored by STG 39 on
Process Industry—Materials Applications, and STG 60 on Corrosion Mechanisms. This standard is
issued by NACE International under the auspices of STG 34.
In NACE standards, the terms shall, must, should, and may are used in accordance with
the definitions of these terms in the NACE Publications Style Manual, 4th ed., Paragraph
7.4.1.9. Shall and must are used to state mandatory requirements. The term should is used
to state something good and is recommended but is not mandatory. The term may is used
to state something considered optional.
________________________________________________________________________
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RP0170-2004
________________________________________________________________________
NACE International
Standard
Recommended Practice
Protection of Austenitic Stainless Steels and Other Austenitic
Alloys from Polythionic Acid Stress Corrosion Cracking
During Shutdown of Refinery Equipment
Contents
1. General ......................................................................................................................... 1
2. Materials and Fabrication Considerations .................................................................... 3
3. Protection Using Nitrogen Purging................................................................................ 5
4. Protection Using Alkaline Washing ............................................................................... 6
5. Protection Using Dry Air................................................................................................ 7
6. Protection of Reactors .................................................................................................. 8
References.......................................................................................................................... 8
Bibliography ........................................................................................................................ 9
Appendix A: Examples of PTA SCC ................................................................................ 10
Table 1: Reported Sensitization Temperature Ranges for Some Austenitic Materials ..... 3
Figure A1: Dye Penetrant Inspection Showing Extensive Cracking Around Welds ........ 10
Figure A2: Polythionic Acid SCC of Austenitic Stainless Steel (~ 200X) ........................ 10
________________________________________________________________________
ii
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RP0170-2004
________________________________________________________________________
Section 1: General
1.2 As in any SCC mechanism, the PTA SCC mechanism
requires three primary contributing factors as described
below. Addressing at least one of these factors can
eliminate or reduce the probability of SCC.
1.2.1 Environment
Polythionic acid normally forms in refinery equipment
by the reaction of oxygen and water with sulfide
corrosion products usually present on the internal
surfaces of equipment. When this combination of
reactants occurs on sensitized austenitic stainless steel
and other austenitic alloy process equipment, PTA
SCC can occur, usually during an outage.
1.2.1.1 The most common source of oxygen for
the formation of polythionic acid is opening
equipment and exposure to the atmosphere.
Other sources could be oxygen-containing
cleaning solutions, impure nitrogen sources used
for equipment purging, and/or blanketing gas that
contains small amounts of O2.
1.2.1.2 Liquid water is a common product of
shutdown operations, typically produced from
condensation of steam used for hydrocarbon
removal within equipment. Within heaters that
require decoking, condensed steam from steamair decoking, or water used to propel decoking
pigs, may promote conditions necessary for PTA
SCC. Other shutdown procedures like water
washing are an obvious source. Less frequently,
local ambient conditions, such as rainfall or
regions with high humidity that may easily reach
dew point conditions, may be the source of liquid
water. Shutdown maintenance and inspection
activities may introduce water into equipment from
routine practices like hydrotesting or hydrojetting.
Less obvious sources could include high-pressure
water jet cutting, often used in major repairs of
refractory-lined components in fluid catalytic
cracking units (FCCU), and fluidized bed coking
units.
1.2.1.3 The likelihood of PTA SCC is much
greater in parts of refinery process units where the
environment is conducive to the formation of hightemperature iron or other metal sulfide scales.
These high-temperature scales may then become
wet in the presence of moist air during a unit
shutdown, leading to the formation of PTA and
1
ultimately PTA SCC if the material is sensitized.
1.2.1.4 A thermodynamic assessment may be
used to determine the likelihood of forming metal
sulfide scales in a system. Assessments have
shown the difference between the likelihood of
PTA SCC in hydroprocessing units and FCCU
2
regenerators.
Such assessments assume equilibrium conditions
for the formation of iron sulfide are achieved. This
is a conservative assumption, because in many
cases it is unlikely that equilibrium will be reached,
and that sufficient oxidizing potential with the
austenitic material exists in such a way that
chromium oxide (Cr2O3), rather than iron sulfide,
scale forms. Provided a predominantly oxide
scale is formed, the likelihood of PTA formation is
low, even if the equipment is exposed to moist air
after shutdown. This observation explains why
PTA SCC on the outside of austenitic stainless
steel and other austenitic alloy heater tubes is not
a major concern, even when firing with a sulfurcontaining fuel occurs, provided the firing
conditions produce an oxidizing flue gas.
The theoretical thermodynamic assessment is
supported by general industry experience as
3
documented in the NACE REFIN•COR database.
Overall, there are very few reported problems of
PTA SCC in parts of refinery units that have
predominantly oxidizing conditions prior to
shutdown.
1.2.1.5 In hydroprocessing applications, the
environments are much more reducing (no oxygen
or CO2 present) due to the presence of H2S and
hydrogen, leading to the formation of
predominantly iron sulfide scales and high PTA
SCC susceptibility if the austenitic material is
sensitized.
1.2.1.6 Experience has shown that austenitic
stainless steel and other austenitic alloy
components in the reactor side of a FCCU are
more susceptible to PTA SCC than in the
regenerator side because the environment is much
more reducing (i.e., more H2S and less oxidizing
species) in the reactor.
However, partialcombustion FCCU regenerator systems may also
be susceptible to PTA SCC because of the higher
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1.1 PTA SCC refers to an intergranular form of cracking
that can occur in sensitized austenitic stainless steels and
1
other sensitized austenitic alloys. Polythionic acid refers to
the family of acids that have the form H2SxOy, where x is
generally considered to range from 1 to 5, and y may range
from <1 to 6. Not all sulfur-containing acids are implicated
in PTA SCC. Acids such as H2SO4 may exacerbate the
mechanism and can result in intergranular corrosion, but by
themselves do not cause it. The figures in Appendix A are
examples of PTA SCC.
RP0170-2004
1.2.1.7 In addition, FCCU regenerator systems
generate SO2 and SO3 that can condense as
H2SO3 and H2SO4 in cold areas within the
regenerator and flue gas systems. This mixture of
condensed acids can contribute to general
corrosion as well as intergranular corrosion and
subsequent cracking of sensitized austenitic
stainless steel and other austenitic alloy
components. In some units, polythionic acid can
form within this condensate and will readily cause
PTA SCC of sensitized austenitic stainless steel
components. Both of these cases require differing
mitigation practices because they occur on-stream
as opposed to during shutdowns (condensate
prevention or the use of more corrosion-resistant
alloys).
1.2.1.8 Few instances of PTA SCC have been
reported in crude distillation units. It has been
postulated that heavy oil films in units such as
crude units, coker and FCCU fractionators, etc.,
provide sufficient protection from PTA SCC.
1.3.1 Selection of materials and fabrication practices
resulting in fabricated product resistant to sensitization,
supported by an assessment of the risk of PTA SCC
associated with such selections.
When the risk
associated with potential PTA SCC is judged to be
acceptable, the user may not require the application of
other mitigation methods.
1.3.2 Exclusion of oxygen and water by using a drynitrogen purge.
1.3.3 Alkaline washing of all surfaces to neutralize any
polythionic acids that may form. Field experience has
demonstrated that austenitic stainless steels and other
austenitic alloys are effectively protected with properly
applied alkaline solutions.
1.3.4 The use of dry (dehumidified) air for protection
against PTA SCC is acceptable if the dew point
temperature of the air entering the vessel is maintained
a minimum of 22°C (40°F) lower than the internal
4
surface metal temperature.
1.4 If process equipment remains unopened and “hot”
(above the water dew point of the gas in the equipment),
additional protection is unnecessary.
1.2.2 Stress
As with all SCC mechanisms, tensile stresses are
required. Tensile stresses in process equipment, both
residual from fabrication and applied by mechanical
loads, are sufficient for cracking to occur.
1.2.3 Material
Susceptible materials that can crack in the presence of
polythionic acid are austenitic stainless steels and other
austenitic alloys that are in a sensitized condition.
1.3 The degree of sensitization and stress levels present in
a material are generally not known. Furthermore, the critical
levels of sensitization and stress required to initiate PTA
SCC are not well understood.
Therefore, austenitic
stainless steel and other austenitic alloy process equipment
on which sulfide corrosion products may be present should
be protected using one or more of the methods summarized
briefly below, except in those cases when the equipment
operates below the sensitizing temperature range and the
material has not been sensitized due to welding. More
details on each mitigation strategy are provided in the later
sections of this standard. Users may select one or more
mitigation strategies depending on their needs and
assessment of exposure risk, both in reducing PTA SCC
risk and in creating additional exposures when
implementing a mitigation strategy.
2
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1.5 The internal surfaces of austenitic stainless steel and
other austenitic alloy heater tubes may be susceptible to
PTA SCC. If not decoked, tubes should be kept dry;
otherwise decoking would be required to allow alkaline
solution to reach the tube surfaces.
1.5.1 Thermal decoking procedures should ensure
that the tubes are not subject to condensation prior to
completion of decoke, and protection should be
provided after decoking.
1.5.2 Pig decoking procedures should use alkaline
solutions during and after decoking.
1.6 The need for protection of the external surfaces of
austenitic stainless steel and other austenitic alloy heater
tubes should be considered when sulfur-containing fuels
have been used for heater firing. In many applications,
however, combustion conditions do not form the iron sulfide
film that is a key to polythionic acid formation.
Consequently, many users do not require protection of the
external surfaces of austenitic stainless steel heater tubes
and other austenitic alloy heater tubes. It is only when poor
combustion practices lead to reducing conditions that it is
possible to generate sulfide scales versus oxide scales
externally on heater tubes.
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levels of H2S present, and the reducing ratios of
CO/CO2 in the flue gas.
RP0170-2004
________________________________________________________________________
2.1 General
PTA SCC can occur, depending on thermal history, on most
sensitized austenitic stainless steels and other sensitized
austenitic alloys in the appropriate environment.
2.1.1 PTA SCC normally occurs with the standard
(0.08% carbon max.) and high-carbon (0.10% max.)
grades of austenitic stainless steels and other
austenitic alloys that have become sensitized either by
weld fabrication, or by operation in the sensitizing
temperature range.
2.1.1.1 Sensitization is the precipitation of
chromium carbides, usually at grain boundaries.
This results in a depletion of chromium locally at
the grain boundaries, making this region less
corrosion resistant.
2.1.1.2 The sensitizing temperature range for
these alloys can vary, but sensitization has been
observed for various materials in the range of
approximately 370 to 815°C (700 to 1,500°F). See
Table 1 for reported sensitization temperature
ranges for some austenitic materials.
The
temperature ranges outlined in Table 1 are the
reported (in accordance with references noted)
normal sensitization range for each alloy. It must
be recognized that the sensitization temperature is
a function of a number of variables including
carbon content, stabilizing element-to-carbon ratio,
exposure time, and prior thermal history. Because
of these variables, operational experience has
shown that many plants operate above the
minimum temperatures given in Table 1 without
significant sensitization.
TABLE 1:
Reported Sensitization Temperature Ranges for Some Austenitic Materials
Austenitic Material
Low-carbon grades of SS
UNS S30400 (304 SS)
UNS S31600 (316 SS)
UNS S30409 (304H SS)
UNS S31609 (316H SS)
UNS S32100 (321 SS)
UNS S34700 (347 SS)
UNS N08825 (Alloy 825)
UNS N06625 (Alloy 625)
2.1.1.3 Sensitization readily
welding of alloys in this class.
occurs
Sensitization Range
5, 6, 7
400°C (750°F) to 815°C (1,500°F)
5, 8, 9
370°C (700°F) to 815°C (1,500°F)
5, 8, 9
370°C (700°F) to 815°C (1,500°F)
5, 8, 10, 11
370°C (700°F) to 815°C (1,500°F)
5, 8, 10, 11
370°C (700°F) to 815°C (1,500°F)
5, 12, 13, 14
400°C (750°F) to 815°C (1,500°F)
5, 6, 12, 13, 15
400°C (750°F) to 815°C (1,500°F)
16, 17
650°C (1,200°F) to 760°C (1,400°F)
16, 17
650°C (1,200°F) to 1,040°C (1,900°F)
during
construction codes have temperature limits on the use
of “L” grade material).
2.1.1.4 Some sensitized austenitic stainless
steels will “heal” themselves after exposure above
649°C (1,200°F) due to diffusion of chromium from
the grain interiors to the grain boundaries. This
diffusion has the effect of raising the chromium
content in solution immediately adjacent to the
grain boundaries back to the level at which
resistance to PTA SCC is restored, even though
chromium carbides remain in the grain boundaries.
Careful consideration to the time/temperature
history and specific metallurgy of a component is
required in determining whether this “healing”
process has occurred or will occur in a particular
component.
2.1.2.1 The minimum sensitizing temperature
range for these materials (low carbon and
stabilized grades) is generally higher than the
temperature range for the standard and highcarbon alloys that are not chemically stabilized.
See Table 1.
2.1.2 Low-carbon (0.03% maximum) and chemically
stabilized grades (e.g., with titanium or niobium alloying
additions) of austenitic stainless steel alloys may also
become sensitized by prolonged exposure in the
sensitizing temperature range. (Please note that
2.1.2.2 The
sensitization
resistance
for
chemically stabilized materials may be dependent
on the ratio of the stabilizing element to carbon as
well as the heat-treatment condition.
2.1.2.3 Sensitization is usually not considered to
occur during welding or typical postweld heat
treatment (PWHT) cycles used for ferritic base
metals when these materials are used for cladding
or weld overlay.
2.1.3 Industry experience suggests that austenitic lowcarbon, chemically stabilized weld overlays, and
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Section 2: Materials and Fabrication Considerations
RP0170-2004
2.1.4 Sensitization is more rapid in the presence of
carbon (coke).
2.2 Austenitic Stainless Steel
Austenitic stainless steels can become sensitized due to
their carbon content, thus becoming susceptible to PTA
SCC.
This paragraph describes alloy selection
considerations and controls to minimize this sensitization.
Many users select materials resistant to sensitization to
avoid more cumbersome environmental controls.
2.2.1 Low-carbon grades of austenitic stainless steels,
(1)
such as UNS S30403 (304L), UNS S31603 (316L),
and UNS S31703 (317L), may be used. These alloys
generally have carbon contents limited to 0.03 to 0.04
wt% maximum, depending on product form, by the
materials specification. The reduced quantity of carbon
limits the amount of chromium that can subsequently
be tied up as chromium carbides after exposure to
elevated temperatures, creating a sensitized structure.
2.2.1.1 No special heat
prescribed for these alloys.
treatments
are
2.2.1.2 Matching low-carbon filler metals should
be used for fabrication of these alloys.
2.2.1.3 Dual-certified grades (i.e., grades
carrying a dual designation such as UNS S30400/
S30403 [304/304L]) of these materials may be
used as if they are low-carbon grades.
2.2.2 Chemically stabilized grades of austenitic
stainless steels such as UNS S32100 (321), UNS
S34700 (347), and UNS S31635 (316Ti) may be used.
These alloys contain stabilizing titanium or niobium
alloying elements, which have a stronger affinity to form
carbides than does chromium. The carbon tied up with
the stabilizing element thus reduces the amount of free
carbon that can react with chromium to form chromium
carbide after exposure to elevated temperatures.
2.2.2.1 While materials specifications often
require minimum titanium-to-carbon ratio of no less
than 5:1, some users have elected to specify
higher titanium-to-carbon ratios to further enhance
resistance to sensitization.
2.2.2.2 Similar specifications may require a
minimum niobium-to-carbon ratio of 8:1. Likewise,
some users have specified higher ratios to
improve the alloy’s resistance to sensitization.
2.2.2.2.1 Niobium is a ferrite stabilizer.
Ferrite may transform to a brittle phase called
sigma during elevated temperature exposure.
Consequently, some users have specified a
maximum Nb:C ratio and/or limited the total
ferrite when the material may be exposed
above 538°C (1,000°F), or exposed to PWHT
conditions.
2.2.2.3 The sensitization resistance of chemically
stabilized austenitic stainless steels may be
enhanced through application of a stabilizing heat
treatment typically performed by the steel
manufacturer.
Some
materials
standard
(2)
(3)
specifications (e.g., ASTM, ASME ) provide the
option for such a heat treatment through
supplementary requirements to many product form
specifications. Such heat treatments are typically
performed in the temperature range of 843 to
900°C (1,550 to 1,650°F) for periods of 2 to 4
hours to preferentially allow the precipitation of
titanium or niobium carbides rather than chromium
carbides.
2.2.2.3.1 The stabilizing heat treatment
should be performed after the material has
been solution annealed.
2.2.2.3.2 Weld and heat-affected zones
(HAZ) of welded stabilized material that have
not received a post-fabrication thermal
stabilization can be subject to PTA SCC due
to dissolution of the carbides. For this reason,
post-fabrication thermal stabilization may be
performed on welded joints to improve the
resistance of the HAZ to sensitization when
the risk associated with PTA SCC warrants it.
2.2.3 Cast austenitic alloys have been used in some
high-pressure hydroprocessing unit applications.
These alloys have higher carbon contents, but
generally a larger proportion of ferrite. The ferrite in
these materials is believed to offer some resistance to
PTA SCC; however, most users apply soda ash
washing or other protective measures to these alloys.
Users should base the need for soda ash washing or
other protective measures to protect this class of
material on a suitable risk assessment and actual plant
experience.
___________________________
(1)
Metals and Alloys in the Unified Numbering System (latest revision), a joint publication of ASTM International and the Society of Automotive
Engineers Inc. (SAE), 400 Commonwealth Dr., Warrendale, PA 15096.
(2)
ASTM International (ASTM), 100 Barr Harbor Drive, West Conshohocken, PA 19428-2959.
(3)
ASME International (ASME), Three Park Avenue, New York, NY 10016-5990.
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chemically stabilized wrought internals in reactors are
very resistant to PTA SCC provided the reactor
operating temperatures are below 455°C (850°F).
RP0170-2004
2.3 Other Austenitic Alloys
2.4 Welding and Fabrication
2.3.1 UNS N08800 (Alloy 800), UNS N08810 (Alloy
800H), and UNS N08811 (Alloy 800HT) have carbon
contents that permit them to sensitize to a degree
comparable to standard and high-carbon grades of
austenitic stainless steels. Nonsolution-annealed or
as-welded materials should be protected from PTA
SCC.
2.3.2 UNS N08825 (Alloy 825) contains titanium as a
stabilizing alloying element. Together with a reduced
carbon content in the alloy, these limit the propensity
for sensitization in service and sensitization as a result
of welding.
2.3.2.1 UNS N08825 (Alloy 825) has been
reported to have a high resistance to sensitization
in the “mill stabilized” condition, consisting of a
final anneal at 940°C (1,725°F).
2.3.3 UNS N06600 (Alloy 600) has a carbon content
that causes it to sensitize in a manner similar to
standard and high-carbon grades of austenitic stainless
steels. Nonsolution-annealed or as-welded materials
should be protected from PTA SCC.
2.3.4 UNS N06625 (Alloy 625) contains niobium (Nb)
as a stabilizing alloying element that limits the
sensitization tendency of this alloy. Note: long-term
exposure of this alloy at elevated temperatures (above
538°C [1000°F]) may result in embrittlement.
Except for the possibility of some heat exchanger tubing or
some vessel internals, welding is often used in the
fabrication of austenitic stainless steels and other austenitic
alloys for components that may see PTA SCC conditions.
2.4.1 It has been reported that austenitic stainless
steel weld overlays, either as-deposited or following
PWHT of a ferritic base material, are very resistant to
18, 19
PTA SCC.
2.4.2 Any special heat treatments to maximize
sensitization resistance should be undertaken after all
hot-forming operations have been completed.
2.4.3 Thermal gradient controls may be required
during postweld thermal stabilization heat treatments of
chemically stabilized grades to avoid high thermal
stresses that can lead to cracking of weldments. This
is a particular concern in heavy-wall sections (greater
than 12 mm [0.5 in.]).
2.4.4 Both the low-carbon grades and the chemically
stabilized grades of austenitic stainless steel and other
austenitic alloys are adequate to resist sensitization
during the welding cycle and short-term PWHT cycles
used for fabrication of ferritic-based materials in clad
construction. However, if these alloys are used at a
sufficiently high temperature for a sufficiently long
period of time, sensitizing occurs.
2.4.5 Stress-relief heat treatments have not generally
been used as a means to control the likelihood of PTA
SCC. However, when a postweld stabilizing heat
treatment is applied, there are also stress-relief
benefits.
________________________________________________________________________
Section 3: Protection Using Nitrogen Purging
3.1 Process equipment may be protected by keeping it
tightly closed and purging it with dry nitrogen to exclude
oxygen. Use of dry nitrogen is an effective means of
lowering the water dew point temperature to less than
ambient. Nitrogen purging provides optimum protection for
catalysts.
3.2 If reactors are to be opened but heaters are not, the
internal heater coils may be purged with nitrogen and
blinded. A small positive nitrogen pressure should be
maintained.
3.2.1 Nitrogen should be dry and free of oxygen. (The
user is cautioned that oxygen levels as high as 1,000
ppm have been found in commercial nitrogen.)
3.3 At the user’s discretion, 5,000 ppm of ammonia may be
added to the nitrogen to prevent PTA SCC.
3.3.1 The addition of ammonia is generally
unnecessary when purging with dry nitrogen, but may
be advantageous when water and/or oxygen may be
present.
3.3.2 Copper-based alloys must be isolated from
ammoniated nitrogen.
3.3.3 It should be determined that ammonia will not
have an adverse effect on catalysts.
3.4 If steam is being used for purging or steam-air
decoking, steam injection should be stopped before the
metal temperature cools to 72°C (130°F) above the water
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The term other austenitic alloys generally refers to the
family of austenitic iron-nickel alloys and austenitic
nickel alloys. Not all such alloys have been applied in
services in which PTA SCC is considered a concern. A
number of the more commonly used alloys in such
services are discussed below:
RP0170-2004
dew point. When depressurized, but before cooling lower
than 72°C (130°F) above the water dew point, the system
should be purged with dry nitrogen. Some purge flow
should be maintained until blinds are installed. A positive
nitrogen purge pressure should be maintained on the
system after blinding.
3.5 The user is cautioned that nitrogen-purged equipment
requires special precautions in accordance with applicable
safety procedures.
________________________________________________________________________
Section 4: Protection Using Alkaline Washing
4.2 The wash solution should contain about 2 wt% Na2CO3
and have a pH greater than 9. However, while a majority of
users wash with 2 wt% solutions, industry practice varies
from 1 to 5 wt% Na2CO3. A 1.4 to 2 wt% soda ash solution
normally provides a sufficient level of residual alkalinity on
metal surfaces after the solution drains from the equipment.
Additionally, this low concentration facilitates solution
preparation.
4.2.1 Sodium hydroxide (NaOH or caustic soda)
solutions should not be used.
4.2.2 Experience with potassium carbonate (K2CO3)
solutions is limited.
However, those who have
substituted potassium carbonate for soda ash have
reported no cracking.
4.2.3 Sodium sesquicarbonate (Na2CO3•NaHCO3•
2H2O or “trona”) has been used successfully at 5 wt%
strength.
4.3 Chloride control of the alkaline wash solution may vary
with the application. Because of successful past experience
with solutions containing small amounts of chloride, it is not
always necessary to provide chloride-free solution.
4.3.1 For hydroprocessing units in which process-side
chloride salt deposits are expected, the chloride
concentration in the freshly mixed wash solution should
be limited to 250 mg/L (250 ppmw). Also, because
units subject to PTA SCC may contain chloride
deposits, measures should be taken to remove these
deposits.
4.3.2 Chloride pickup due to removal of the salt
deposits is not unusual. A sodium nitrate corrosion
inhibitor should be used in the wash solution (see
Paragraph 4.5) to reduce the likelihood of chloride
SCC. The user should establish a tolerable upper
chloride limit in the circulating soda ash solution that
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may be reached during washing. When that limit is
reached, purging of the high-chloride wash solution and
make-up with fresh wash solution should be done to
reduce the chloride concentration. Every effort should
be made to remove pockets of residual solution that
could concentrate during system heat-up. As the
solution heats up the water evaporates and the chloride
concentration increases, which increases the likelihood
of chloride SCC.
4.3.3 When process-side chloride salts are not
expected and draining of the equipment may be
difficult, a lower initial chloride limit of 25 mg/L (25
ppmw) should be used. Despite this low chloride limit,
every effort should be made to remove pockets of
residual wash solution that could concentrate during
system heat-up. As the solution heats up the water
evaporates and the chloride concentration increases,
which increases the likelihood of chloride SCC. As an
alternative, ammoniated condensate may be used (see
Paragraphs 4.9 and 4.10).
4.4 An alkaline surfactant should be added to the wash
solution at 0.2 wt% concentration to promote penetration of
coke, scale, or oil films. Heating of the wash solution to
49°C (120°F) may accelerate the penetration of oily films
and residues.
4.5 Corrosion inhibitors have been used to decrease the
possibility of chloride SCC by these alkaline solutions.
4.5.1 At the user’s option, 0.4 wt% sodium nitrate
(NaNO3) may be added. In laboratory tests, low
concentrations of sodium nitrate have been found to be
effective in suppressing SCC of austenitic stainless
steel in boiling magnesium chloride solutions. Caution:
Excess NaNO3 can cause SCC of carbon steel.
4.6 The equipment must be alkaline washed before any
exposure to air. All of the equipment’s internal surfaces
must be contacted for the washing to be effective.
4.6.1 The system should be filled with the alkaline
solution under an inert atmosphere to minimize oxygen
contamination.
4.6.2 The equipment should be soaked or the wash
solution circulated for a minimum of two hours. If
deposits or sludge are present, the wash solution
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4.1 Austenitic stainless steel and other austenitic alloy
equipment to be opened to the air is best protected with a
sodium carbonate (Na2CO3 or soda ash) solution. Soda
ash solutions neutralize acids formed at the metal surface
and, after draining, leave a thin alkaline film on the surface
that can neutralize any additional acid formation. These
solutions may also contain an alkaline surfactant and/or a
corrosion inhibitor.
RP0170-2004
should be circulated vigorously (two hours minimum.)
Longer times are not detrimental in either case.
4.8 Hydrojetting of equipment should be conducted using a
soda ash solution.
4.6.3 The circulating wash solution should be
analyzed at appropriate intervals to ensure that pH and
chloride limits are maintained.
4.8.1 After hydrojetting, equipment should be kept dry
and out of the weather. If this is not possible, the soda
ash wash should be repeated as required to maintain a
residual film of soda ash.
Equipment shall be
reinstalled with a soda ash residual film remaining on
surfaces.
4.6.4 To ensure that protection is maintained, the
residual soda ash film remaining on the surface should
not be removed by any subsequent water wash or
precipitation or mechanical work. If the film is removed,
it should be reapplied as quickly as possible using
means suitable to the component. Hand-held sprayers
have been used successfully during equipment
maintenance to replenish films. The film must remain
in place through the downtime to ensure continued
protection.
4.6.5 Each system must be evaluated individually and
precautions taken to ensure that unvented gas pockets
or cascading through down-flow sections do not
prevent surface contact.
4.6.6 If washing the outside of heater tubes is
necessary to remove deposits, use of a soda ash
solution should be considered because these surfaces
may be subject to PTA SCC. See Paragraph 1.6.
4.7 In special cases, flushing with ammoniated condensate
may be necessary (Paragraphs 4.9 and 4.10). The solution
should have a pH above 9 and a chloride content of less
than 5 mg/L (5 ppmw).
4.9 Hydrostatic testing of equipment should be conducted
using a soda ash solution. Ammoniated condensate may
be used if equipment is not reopened or exposed to oxygen.
4.10 If sodium or chloride ions cannot be tolerated in the
process system, the equipment should be washed with
ammoniated condensate after being closed. If the unit is
not started up immediately, the solution may remain in place
or be displaced with nitrogen or dry hydrocarbon. The unit
must not be exposed to oxygen after this procedure.
Ammonia solutions do not leave a residual alkaline film after
being drained.
4.11 Upon completion of alkaline washing, all of the
remaining alkaline wash solution must be drained from each
low point in the system prior to returning the equipment to
service. Failure to do so can result in concentration of
carbonate and chloride salts by evaporation, leading to SCC
in austenitic stainless steels.
4.11.1 Some users have elected to upgrade low-point
drains in austenitic stainless steel piping circuits to a
material that resists chloride SCC resulting from
residual soda ash solution, flushing, or hydrotest water.
________________________________________________________________________
Section 5: Protection Using Dry Air
5.1 The use of dry (dehumidified) air may be an
economical approach to prevent the formation of free water
and thereby reduce the likelihood of PTA SCC.
5.1.1 Because
nonregenerable
catalysts
are
frequently pyrophoric, such catalysts should either be
kept wet or out of contact with oxygen. After removal of
the catalyst, dry air can be used to protect the material
from PTA SCC.
5.2 The use of dry air for protection against PTA SCC is
acceptable if the dew point temperature of the air entering
the vessel is maintained a minimum of 22°C (40°F) lower
than the internal surface metal temperature.
Examples:
internal metal temperature = 30°C
incoming air dew point temperature 30°C - 22°C = 8°C
or
internal metal temperature = 85°F
incoming air dew point temperature 85°F - 40°F = 45°F.
Air with dew point temperatures from -15 to -46°C (5 to
-50°F) have been used. The dry air purge must be
maintained at all times.
NACE International
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RP0170-2004
________________________________________________________________________
Section 6: Protection of Reactors
6.1 Reactors containing catalysts require special
consideration. Personnel safety and protection of the
catalyst may dictate the use of procedures that are less
than optimum in terms of protection from PTA SCC.
increased to 5 wt% soda ash to compensate for the
acidity of deposits held by the catalyst. Unloading may
then be conducted in air while the catalyst is kept wet
with soda ash solution to prevent pyrophoric ignition.
The reactor should then be washed down with soda
ash solution and dried prior to repairs or catalyst
loading.
6.1.1 Nonregenerated
catalysts
frequently
are
pyrophoric. Such catalysts should either be kept wet or
out of contact with oxygen.
6.3.3 If the user wishes to eliminate the use of soda
ash solutions and fresh-air breathing equipment while
unloading the catalyst, the catalyst may be dumped
following wetting with good-quality fresh water (less
than 50 ppm chloride), without nitrogen purging. This
should be preceded by a careful investigation to
determine that:
6.2 Industry experience suggests that austenitic lowcarbon or chemically stabilized stainless steel weld overlays
and chemically stabilized wrought internals in reactors are
resistant to PTA SCC for reactor operating temperatures
below 455°C (850°F).
6.3 Procedures for the protection of reactors opened for
entry and having a history of successful use in the field are
as follows:
(1)
Only low-carbon or chemically stabilized grades
have been used when austenitic stainless steel or other
austenitic materials have been specified.
6.3.1 Trained personnel using appropriate fresh-air
breathing equipment may conduct catalyst unloading
and loading under nitrogen-blanketing conditions.
Following unloading, the reactor should be purged with
dry air as described in Section 5, and this purge should
be maintained while the reactor is open.
(2)
These alloy materials have not become
sensitized as a result of either vessel fabrication
procedures or the reactor’s thermal history during
operation.
This procedure involves some risk of PTA SCC through
either accidental use of nonstabilized or higher-carbon
alloy grades, or misinterpretations of the thermal history
of the reactor.
6.3.2 If the catalyst is to be discarded, the reactor may
be filled with soda ash solution to wet both the catalyst
and reactor parts. The solution strength should be
________________________________________________________________________
References
1. NACE Publication 5B356 (withdrawn), “Effect of Sulfide
Scales on Catalytic Reforming and Cracking Units”
(Houston, TX: NACE).
2. E. Nagashima, K. Matsumoto, K. Shibata, “Effects of
Sensitization and Service Fluid Chemistry on Polythionic
Acid Stress Corrosion Cracking of 18-8 Stainless Steels,”
CORROSION/98, paper no. 592 (Houston, TX: NACE,
1998).
3. NACE REFIN•COR Software (latest revision) (Houston,
TX: NACE).
4. NACE
6A192/SSPC-TR
3
(latest
revision),
“Dehumidification and Temperature Control During Surface
Preparation, Application, and Curing for Coating/Linings of
Steel Tanks, Vessels, and Other Enclosed Spaces”
(Houston, TX: NACE).
5. D.V. Beggs, R.W. Howe, “Effects of Welding and
Thermal Stabilization on the Sensitization and Polythionic
Acid Stress Corrosion Cracking of Heat and CorrosionResistant Alloys,” CORROSION/93, paper no. 541
(4)
(Houston, TX: NACE, 1993).
6. H.F. Erling, M.A. Scheil, Advances in the Technology of
Stainless Steels (West Conshohocken, PA: ASTM), pp.
275–284.
7. G. Vander Voort, ed., Atlas of Time-Temperature
Diagrams for Irons and Steels (Materials Park, OH: ASM
(5)
International, 1991), p. 681.
8. J.F. Grubb, J.D.Fritz, “Stabilization and Sensitization of
Stainless Steels,” CORROSION/97, paper no. 185
(Houston, TX: NACE).
___________________________
(4)
(5)
Corrected version of the Beggs/Howe data is published in REFIN•COR.
ASM International (ASM), 9639 Kinsman Road, Materials Park, OH 44073-0002.
8
NACE International
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RP0170-2004
9. A.J. Brophy, “Stress Corrosion Cracking of Austenitic
Stainless Steels in Refinery Environments,” Materials
Performance 13, 5 (1974), p. 9.
10. D. Peckner, ed., Handbook of Stainless Steels (New
York, NY: McGraw-Hill, 1977), pp. 4-45.
11. Lacombe, et al eds., Stainless Steels (Les Ulis, France:
Les Editions de Physique Les Ulis, 1993), p. 422.
12. C.H. Samans, K. Kinashita, I. Matsushima, “Further
Observations on Sensitization of Chemically-Stabilized
Stainless Steels,” CORROSION/76, paper no. 159
(Houston, TX: NACE, 1976).
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13. A.C. Hotaling, L.R. Scharfstein, “The Effect of Heat
Treatments in the Prevention of Intergranular Corrosion of
AISI 321 Stainless Steel,” Materials Performance 22, 9
(1983), p. 22.
15. G. Vander Voort, ed., ibid., p. 679.
16. J. R. Crum, M. E. Adkins, W. G. Lipscomb,
“Performance of High Nickel Alloys in Refinery and
Petrochemical Environments,” Materials Performance 25, 7
(1986): p. 27.
17. C.H.
Samans,
“Stress
Corrosion
Cracking
Susceptibility of Stainless Steels and Nickel-Base Alloys in
Polythionic Acids and Copper Sulfate Solution,” Corrosion
20, 8 (1964): p.256t.
18. Emery Lendvai-Lintner, “Stainless Steel Weld Overlay
Resistance to Polythionic Acid Attack,” Materials
Performance 18, 3 (1979): p. 9.
19. K. Tamaki, S. Nakano, M. Kimura, “Application of CrNi
Stainless Steel Weld Metals to Polythionic Acid
Environments,” Materials Performance 26, 8 (1987): p. 9.
14. G. Vander Voort, ed., ibid., p. 691.
________________________________________________________________________
Bibliography
Alessandria, A.V., and N. Jaggard. Stainless Steel in
Petroleum Refining and Processes, Proceedings API
40. Washington, DC: API, 1960, p. 111.
Dravnieks, A., and C.H. Samans. Corrosion Control in
Ultra-Forming, Proceedings API 37. Washington, DC:
API, 1957, p. 111.
Backensto, E.B., and A.N. Yurick. “Stress Corrosion
Cracking Studies of Austenitic Stainless Steels in
Aqueous Ammonium Chloride Solutions.” Corrosion
18, 5 (1962): p. 169t.
Heller, J.J., and G.R. Prescott. “Cracking of Stainless
Steels in Wet Sulfidic Environments in Refinery Units.”
Materials Protection 4, 9 (1965): p. 14.
Couper, A.S. “Testing Austenitic Stainless Steels for
Modern Refinery Applications.” Materials Protection 8,
10 (1968): p. 17.
Couper, A.S., and H.F. McConomy. Stress Corrosion
Cracking of Austenitic Stainless Steels in Refineries,
Proceedings API 46. Washington, DC: API, 1966, p.
321.
Piehl, R.L. Stress Corrosion Cracking by Sulfur Acids,
Proceedings API 44. Washington, DC: API, 1964, p.
111.
Stephens, C.D., and R.C. Scarberry. “The Relation of
Sensitization to Polythionic Acid Cracking of Incoloy
Alloys 800 and 801.” CORROSION/88, paper no. 10.
Houston, TX: NACE, 1988.
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RP0170-2004
________________________________________________________________________
Appendix A: Examples of PTA SCC
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FIGURE A1:
Dye Penetrant Inspection Showing Extensive Cracking Around Welds
FIGURE A2:
Polythionic Acid SCC of Austenitic Stainless Steel (~ 200X)
10
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