Polythionic Acid SCC Mitigation – Materials Selection and Effective
Protection of Austenitic Stainless Steels and other Austenitic Alloys
Materials and Corrosion Control Standards Committee
Polythionic Acid SCC Mitigation:
Materials Selection and Effective
Protection of Austenitic Stainless Steels
and other Austenitic Alloys
Saudi Aramco DeskTop Standards
Table of Contents
1 Scope……………………………………………………………….. 3
2 Use of Best Practice……………………………………………. 3
3 References………………………………………………………… 4
4 Definitions and Abbreviations……………………………….. 5
5 Polythionic Acid Stress Corrosion
Cracking (PASCC)………………………………………. 6
6 Criteria for Soda Ash Washing
(Decision Tree)………………………………………….. 10
7 Guidelines for Soda Ash Washing………………………… 11
8 Material Selection Guidelines for New
Projects and Upgrades……………………………….. 13
9 Other PASCC Mitigation Measures………………………. 15
10 Health and Safety………………………………………………. 15
Figure 1 – Sensitization Curves for
(a) 304/304L and (b) 321/347………………………………. 16
Figure 2 – Dye Penetrant Inspection Showing
Extensive OD Cracking Around Welds
of Austenitic Stainless Steel
(Courtesy API RP571-2003)………………………………… 17
Figure 3 – 304 Stainless Steel, Showing
Intergranual PASCC Cracking in
Base Metal, MAG. X 100
(Ref. CSD/ME&CCD/L-1445/98)…………………………… 17
Figure 4 – 304 Stainless Steel, Showing
Sensitized Microstructure after
18 Years’ Service, Mag. x 320
(Ref. CSD/ME&CCD/L-1445/98)…………………………… 18
Figure 5 – Stainless Steel, Showing
Intergranular Fracture of Opepend
PASCC Crack, Mag. x 750
(Ref. CSD/ME&CCD/L-1445/98)………………………….. 18
Figure 6 – Soda Ash Wash Decision Tree
for Aged Austenitic Stainless Steel
Equipment and Piping ©……………………………………… 19
Attachment 1 – Chemical Hazard Bulletin No. 197………… 20
This Saudi Aramco Best Practice supplements NACE RP0170-2004 “Protection of
Austenitic Stainless Steels and other Austenitic Alloys from Polythionic Acid Stress
Corrosion Cracking during Shutdown of Refinery Equipment”. It specifically provides
• Criteria to determine the need for soda ash washing of existing austenitic stainless
steel equipment and piping during plant downtime.
• Guidelines on the practice and controls related to soda ash washing.
• Guidelines on material selection for new projects or upgrades.
Its purpose is to prevent polythionic acid stress corrosion cracking (PASCC) caused by
the reaction of sulfide corrosion products, oxygen (air) and water/moisture during
shutdowns. Refineries shall adopt this Best Practice to formulate or revise PASCC
mitigation strategies, which are normally documented as RIMs.
Optimized materials selection for new projects or upgrades is the key to equipment
reliability. Significant potential cost savings may be achieved when protection by soda
ash washing is determined to be unnecessary.
In case of discrepancy between this document and NACE RP0170-2004, resolve
discrepancy through the Coordinator ME&CCD.
2 Use of Best Practice
This Best Practice provides PASCC mitigation guidelines based on current
industry practices. Its primary purpose is to provide Saudi Aramco Refineries
with strategies to minimize risk of PASCC during plant shutdowns. It is
intended for Saudi Aramco internal use only.
Saudi Aramco® is a registered trademark of the Saudi Arabian Oil Company,
Copyright, Saudi Aramco, 2004.
2.2 Conflicts with Mandatory Standards
In the event of a conflict between this Best Practice and other Mandatory Saudi
Aramco Engineering Standards or Specifications, the matter shall be referred to
the Coordinator of ME&CCD/CSD for resolution.
This Best Practice is based on the latest edition of the references below, unless
3.1 Saudi Aramco References
Saudi Aramco Engineering Standards
SAES-A-206 Positive Material Identification
SAES-L-133 Corrosion Protection Requirements for
Saudi Aramco Engineering Procedures
SAEP-20 Equipment Inspection Schedule
SAEP-325 Inspection Requirements for Pressurized
SAEP-355 Field Metallography and Hardness Testing
Saudi Aramco Materials Specification
01-SAMSS-007 Manufacture of Shell and Tube Heat Exchangers
BI-3789 Project Specification for Material Specification
and Corrosion Control – Rabigh Refinery 1997
NACE 02478-2002 Thick-Wall Stainless Steel Piping in
Hydroprocessing Units – Heat Treatment
NiDI Publication The Role of Stainless Steels in Petroleum Refining
R. L. Piehl Stress Corrosion Cracking by Sulfur Acids –
Proceedings API, Refining Division, 1964
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)
NACE 541-1993 Performance of High Nickel Alloys in Refinery
and Petrochemical Environments
3.2 Industry Codes and Standards
NACE RP0170-2004 Protection of Austenitic Stainless Steels and other
Austenitic Alloys from Polythionic Acid Stress
Corrosion Cracking during Shutdown of
API RP571-2003 Damage Mechanisms Affecting Fixed Equipment
in the Refining Industry
ASTM A262-2002 Standard Practices for Detecting Susceptibility to
Intergranular Attack in Austenitic Stainless
4 Definitions and Abbreviations
Austenitic Stainless Steels are 300 series stainless alloys with an austenitic
microstructure, such as 304, 316, 321 and 347.
Chemical Stabilization involves the addition of titanium (Ti) and niobium (Nb)
to minimize sensitization.
Neutralization is the use of alkali to neutralize acid, or vice versa. In this Best
Practice, it primarily refers to soda ash solution which is used to neutralize
Polythionic Acids are sulfur acids with the formula H2SxO6; where x ranges
from 3 – 6. Such acids can form on plant shutdown in certain refinery
equipment, especially in units operating in H2/H2S service.
Positive Material Identification is a chemical analysis that includes all alloying
elements, including C, Ti and Nb, to determine stabilization ratios.
Sensitization refers to the composition / time / temperature dependent formation
of chromium carbides along the grain boundaries of austenitic stainless alloys.
Sensitization causes a dramatic loss in corrosion resistance along the grain
boundaries and generally occurs in the temperature range 370°C to 815°C
(700°F to 1500°F).
Sigmatization or Sigma Phase Embrittlement refers to the formation of an ironchromium
phase, affecting austenitic stainless steels. It generally forms with
long-time exposure in the range of 565 to 980°C (1050 to 1800°F).
Soda Ash is a sodium carbonate solution, normally used at 1 – 5 %
Stabilization Ratio is the ratio of stabilization element (Ti or Nb) to carbon.
Stress Corrosion Cracking is an environmental cracking phenomenon caused
by a combination of a specific corrosive environment and tensile stress on a
Thermal Stabilization is a heat treatment performed on stabilized stainless
steels at approximately 900ºC for 2 – 4 hours to improve resistance to
sensitization and PASCC.
Cl-SCC = Chloride Stress Corrosion Cracking
HAZ = Heat Affected Zone
PASCC = Polythionic Acid Stress Corrosion Cracking
PMI = Positive Material Identification
PT = Penetrant Testing
RIM = Refinery Instruction Manual
5 Polythionic Acid Stress Corrosion Cracking (PASCC)
5.1 Damage Description
• PASCC is a type of stress corrosion cracking which can occur rapidly under
shutdown or T&I conditions. Cracking is due to sulfur acids forming from
sulfide scale, air and moisture acting on sensitized austenitic stainless steels.
• It usually occurs adjacent to welds or high stress areas.
• Cracking may propagate rapidly (in a matter of minutes or hours) through
the wall thickness of piping and components.
5.2 Susceptible Materials
• 300 series stainless steels, Alloy 600/600H, Alloy 625, Alloy 800/800H and
It is noted that Alloy 625 and 825 are much more sensitization-resistant than the
300 series stainless steels, with a sensitization threshold at about 650°C
5.3 Principal Factors
• A combination of environment, material and stress are required to promote
o Environment – Metals form a surface sulfide scale when exposed to some
sulfur compounds, notably hydrogen sulfide, and organic sulfides. The
scale may react with air (oxygen) and moisture to form sulfur acids
o Material – The material must be in a susceptible, i.e. “sensitized”
o Stress – Residual or applied tensile stress. Residual stresses are often
sufficient to promote cracking.
• Affected alloys become sensitized during exposure to elevated temperatures
during fabrication, welding or high temperature service.
• The carbon content and the thermal history of the alloy have a significant
effect on sensitization susceptibility. Regular and controlled carbon grades
of stainless steels such as types 304/304H and 316/316H are particularly
susceptible to sensitizing in the weld HAZ. Low carbon “L” grades
(< 0.03% C) are less susceptible and usually can be welded without
sensitizing. The L grades will not sensitize provided long term operating
temperatures do not exceed 400°C (750°F). Time – Temperature
sensitization curves have generally been adopted by industry for 304, 304L,
321 and 347 grades (Figure 1).
5.4 Affected Units or Equipment
• All units where alloys that may become sensitized are used in sulfurcontaining
environments. Commonly damaged equipment includes but is
not limited to heat exchanger tubes, furnace tubes and piping.
• FCC units (air rings, plenums, slide valves, cyclone components, expansion
joint bellows and piping).
• Hydroprocessing units (reactors, heater tubes, hot feed/effluent exchanger
• Crude and Coker units (piping).
At the time of developing this Best Practice, no austenitic stainless steel type 300
components exist in Saudi Aramco Crude/Vacuum units.
5.5 PASCC Characteristics
• Typically occurs next to welds, but can also occur in the base metal (Figures
2, 3). It is usually quite localized and may not be evident until a leak
appears during start-up or, in some cases, operation.
• Sensitized microstuctures (Figure 4) are susceptible to cracking that
propagates in an intergranular (along grain boundaries) mode (Figures 3, 5).
• Metal loss is usually negligible.
5.6 PASCC Mitigation
Effective mitigation involves both material selection and control of process
parameters. Material selection is used to specify sensitization-resistant alloys
and heat treatments. Process aspects address shutdown procedures to control
the environment, by preventing polythionic acid formation or including a soda
ash wash neutralization step.
• Low carbon grades such as 304L/316L afford some improvement over
controlled carbon grades (Figure 1). The L grades will sensitize if exposed
more than several hours above about 538°C (1000°F) or long term above
• Improved resistance to PASCC can be achieved with modified versions of
these alloys containing small amounts of Ti and Nb. These chemicallystabilized
grades include types 321 and 347 and also nickel base Alloys 825
• A thermal stabilization heat treatment at 900ºC (1650ºF) may be applied to
chemically stabilized austenitic stainless steels after welding to reduce
sensitization and PASCC susceptibility. This heat treatment may be difficult
to apply in the field.
• Thermal stabilization heat treatment is recommended for heater tubes and
• However, for thicker wall piping (>1 inch) or equipment operating below
427°C, thermal stabilization is not recommended. Experience has shown
that the risks of reheat cracking and sigma phase embrittlement outweigh the
benefits of thermal stabilization of thicker wall material.
• Some ASTM specifications permit mill products to be shipped in a thermally
stabilized rather than solution annealed condition, as a supplementary
• Susceptibility to PASCC can be determined by laboratory corrosion testing
according to ASTM A262 Practice C. A sensitizing heat treatment is
applied to low-carbon (L) and chemically stabilized stainless steel grades
prior to testing.
• If feasible, do not open equipment to air at temperatures where atmospheric
condensation might occur. Use inert gas blankets (see NACE RP0170-2004,
Section 3) or use dry air (see NACE RP0170-2004, Section 5) to exclude
condensation of water. Also, condensation can be prevented if equipment is
heated and maintained at temperatures above the dewpoint, e.g. fired heaters.
• Some equipment or processes can be severely damaged by contact with
water. Some catalyst systems must not contact water or other aqueous
solutions. In selecting a PASCC control methodology, consider both the
positives and negatives of each method on the total system. Split systems
into smaller segments and address accordingly.
• If equipment containing sensitized stainless steels is to be opened or exposed
to air, preventive measures should be taken to minimize or eliminate
PASCC. These include flushing the equipment with soda ash solution to
neutralize sulfur acids immediately after or during shutdown or purging with
dry nitrogen or nitrogen/ammonia during the shutdown to prevent air
5.7 Inspection and Monitoring
• Penetrant Testing (PT) examination can be used to detect PASCC (Figure 2).
However, because the cracks are filled with a tight deposit, flapper disc
sanding may be needed to improve the PT sensitivity.
• PASCC can be an inspection challenge because the cracking may not occur
until well into a turnaround.
• Monitoring for PASCC during operation is not usually practical. Conditions
causing the cracking are not usually present while operating.
6 Criteria for Soda Ash Washing (Decision Tree)
If the proponent wishes to eliminate soda ash neutralization, a careful verification of
installed metallurgy and thermal history is essential. CSD/MEU has developed a
methodology based on a decision tree, addressing this issue.
A decision tree for soda ash washing for existing equipment and piping is presented in
Figure 6. Requirements for new equipment/upgrades are addressed in Section 8.
The key points to consider in applying this decision tree are as follows:
• Alloy verification to confirm austenitic material type and stabilization ratios for 321
and 347 grades. Specifically, stabilization ratios shall be as follows:
o Greater than 8 for Ti:C and
o Within the range 10 to 12 for Nb:C
• Review of thermal history to establish maximum metal temperature.
o For stabilized materials, i.e. 321 and 347 grades, a maximum metal temperature
limit of 455°C is permitted. If the representative thermal history shows that this
metal temperature has been exceeded, then a soda ash wash is required.
For fired equipment, the metal temperature is usually derived from skin
thermocouple records. Accordingly, such equipment must be fitted with reliable
skin thermocouples. For other equipment, e.g. exchangers, the process
temperature is regarded as representative of the metal temperature.
o For non-stabilized grades, i.e. 304/316, a soda ash wash is required at every
shutdown. It is noted that at temperatures below 370°C sensitization is not
anticipated for these grades.
• Perform field sensitization test. Testing is based on field metallography, using
ASTM A262 Practice A, in addition to a complementary electrochemical
(potentiostatic) technique that is under development (Technology Item proposal).
• For stabilized materials operating above 455°C, soda ash washing must be applied
at the first shutdown. This should be immediately followed by a field sensitization
test to check degree of sensitization. If the material is not sensitized, then the soda
ash wash can be skipped at the next shutdown, but re-applied at the subsequent
shutdown. The cycle of monitoring sensitization (repeating sensitization test) and
skipping soda ash wash at subsequent shutdowns can be repeated (Figure 6, Pale
Blue Cycle). This approach shall be followed regardless of the operating interval
7 Guidelines for Soda Ash Washing
The guidelines given below are supplementary to those of NACE RP0170-2004.
7.1 Key Requirements
Plants shall develop a RIM based on process licensor’s guidelines. This RIM
shall include the following key points:
1) Soak or circulate for 2 to 3 hours, reversing flow every 30 minutes if
possible. Circulation is preferred to ensure wetting of vapor pockets in
equipment such as furnace tubes. Never open equipment then spray.
2) Monitor and record solution pH (> 9) and chlorides (< 250 mg/L
[250 ppmw]), on an hourly basis.
3) A protective residual film of soda ash will remain after draining, even after
the equipment has dried. Do not rinse or wash off the residual film before
putting equipment back in service.
4) This equipment should then be kept dry and out of the weather to the
greatest practical extent to avoid washing away of the protective film.
5) When cleaning, high pressure jetting, or repair work is needed, the
protective soda ash film might be destroyed. The time without protection
should be minimized and a soda ash film reapplied as quickly as possible.
For high pressure jetting or other washing operations, it is recommended
that a 1 wt% solution of Na2CO3 be used.
6) It is generally not necessary to re-apply the film unless the protective film
was been washed-off or mechanically removed or damaged.
7) Care should be taken during startup and shutdown to avoid washing away
a film and exposing equipment to air and water (e.g., hydrotesting, steam
out. Refer Section 7.2.2).
8) If hydrotesting is required, this shall be carried out using a soda ash
7.2 Additional Requirements for Specific Equipment
The following requirements apply to process side (hydrocarbon) environments
only. External soda ash washing of fire-side surfaces is not required (Refer
7.2.1 Vertical Tube Furnaces – Not Steam-Air Decoked
During neutralization of vertical furnace tubes, it is difficult to achieve
thorough wetting of the top U-bends during filling and removal of soda
ash during flushing and draining. Thus, the solution should be circulated
at a high rate (to provide turbulent flow) to ensure wetting of the upper
U-bends and to remove vapor from the downflow tubes.
When removing solution for maintenance or storage, the tubes should be
blown with dry air or inert gas; this will result in a protective soda ash
film on the tubes.
• If short term (< 2 weeks) work is planned, no further treatment is
• For long term outage (> 2 weeks) the tubes should be blown dry with
air or inert gas and sealed with a slight positive pressure of dry inert
7.2.2 Furnace Tubes – Steam-Air Decoked
During cool down after decoking, condensed steam and air can coexist
and could result in PASCC. To prevent this, 5000 ppm NH3 should be
injected into the steam during the final phase of steaming-out or
decoking. This ammoniated steam can then be displaced with soda ash
as required. Alternatively, nitrogen purging (9.1) can be used.
7.2.3 Furnace Tubes – External Washing
Under normal combustion conditions (non-reducing), fireside scales do
not contain sulfides. Therefore, PASCC is unlikely and external soda
ash washing is unnecessary.
• Reactors with stabilized or low carbon austenitic weld overlays, and
confirmed to be operating at temperatures below 455ºC, do not
require soda ash washing, since they are resistant to PASCC. This
also applies to stabilized wrought internals.
• If soda ash washing of any component in the reactor circuit (reactors,
heaters, exchangers and transfer piping) is judged to be necessary
based on metallurgy/service history, soda ash neutralization of the
reactor circuit must be followed by washing with ammoniated
• Due to their large volume and presence of catalyst, soda ash washing
of reactors is often impractical. Additionally, sodium is a catalyst
poison. Therefore, the most practical means of protecting reactors is
by inerting or keeping dry.
• Note dry air can be used only if the catalyst has been regenerated to
convert the bulk of the pyrophoric iron sulfide which is usually
present. However, soda ash wash can be used for protecting reactors
after the catalyst has been dumped or even for wet dumping catalyst
when replacement is planned.
• Both filling and hosing down the walls with soda ash have been used.
Recirculating the soda ash solution is acceptable for hosing purposes
provided the alkalinity is maintained.
8 Material Selection Guidelines for New Projects and Upgrades
For new projects or upgrades with well-specified sensitization-resistant materials, the
practice of soda ash washing may be unnecessary, leading to potentially significant cost
savings in the early life of the equipment or pipe.
Therefore, when the risk of PASCC is judged to be low, based on optimum material
selection, alloy verification (PMI), fabrication practices and process design parameters,
the Materials Engineering Unit of CSD shall be consulted to determine the need for
soda ash washing. Since PASCC is time – temperature dependent, this approach:
• Is applicable to newly-installed plant, equipment or pipe.
• Is valid for a limited service period, e.g. up to 2nd Turnaround or 7 years maximum.
• Equipment is subject to normal operation with no undue process upset or
temperature excursion. Therefore, it is paramount that Operations maintain
comprehensive metal/process temperature records of the subject austenitic stainless
Subsequent service beyond the 2nd Turnaround requires full re-evaluation based on
criteria given in Section 5.
Accordingly, for new Saudi Aramco projects and upgrades with potential PASCC risk,
optimum material specification and heat treatments (for base materials and welds during
manufacture) shall include the following requirements, to enhance PASCC and Cl-SCC
1) Use chemically stabilized grades such as 321/321H or 347/347H or low-carbon
It is recognized that 347/347H offer better PASCC resistance than 321/321H.
2) Apply the following heat treatments:
a) Solution annealing at 1040 – 1070°C for ¼ – ½ hour per inch followed by
water quenching for 321/321H and 347/347H, and
b) Stabilization 850 – 900°C for 4 hours followed by rapid air cooling.
c) The adequacy of the above heat treatments must be verified by an
intergranular corrosion/sensitization test, i.e. ASTM A262 Practice C.
3) For 321 stainless steels, the Ti:C stabilization ratio shall be greater than or equal to
4) For 347 stainless steels, the Nb:C stabilization ratio shall be within the range 10 to
Sigmatization is promoted by the presence of Nb. Accordingly, an upper limit of 12
is imposed on Nb:C ratio.
5) Chemically-stabilized 321/347 grades are preferred materials for best PASCC
resistance. However, low-carbon grades 304L or 316L can be used for certain
applications, e.g. heat exchanger tubes, provided they are in the solution-annealed
6) Use chemically-stabilized austenitic weld or low-carbon weld metals for overlays.
Industry experience indicates that such overlays are resistant to PASCC.
7) For fired heater tubing or other components operating above 455°C (metal
temperature), items 1 to 4 above are required. However, for thicker wall piping
(>1 inch) or equipment operating below 427°C, thermal stabilization is not
recommended due to the risks of reheat cracking and sigma phase embrittlement.
Therefore, thermal stabilization is not recommended for thick wall components.
8) For heat exchanger U-bends made of unstabilized grades, e.g. 304L, solution
anneal the entire tube at 1070°C after bending (item 2a). Use stabilized grades
321 or 347 if only the U-bends can be heat treated.
9) For drains, where there is a risk of Cl-SCC due to water entrapment during
shutdowns, a stress relief heat treatment at 870 – 900°C for 1 hour per inch
followed by rapid air cooling is required. An alternative is to specify low-point
drains in Cl-SCC resistant alloys, such as Alloy 825 or 625.
10) PASCC failures of Alloy 800 exchanger tubing in Hydroprocessing Units have
been reported; these were attributed to sensitization caused by stress relief heat
treatment or welding. Therefore, where Alloy 800 is specified for such tubing,
this shall be ordered to a maximum carbon content of 0.03%; alternatively, the
stabilized Alloy 801 or Alloy 825 can be used.
9 Other PASCC Mitigation Measures
9.1 Nitrogen Purging
Nitrogen purging is an alternative to neutralization. The nitrogen should be
pure, dry and oxygen-free.
A positive nitrogen pressure should be maintained. For steam-air decoking
(7.2.2), steam injection should be stopped before the metal temperature cools to
72ºC above the water dew point. When depressurized, but before further
cooling, the system should be purged with dry nitrogen.
9.2 Dry Air Purging
The use of dry air is another approach reduce risk of PASCC. The incoming air
dew point must be maintained at a temperature well below internal metal
temperature to prevent water condensation. The recommended temperature
differential (metal temperature minus dew point temperature) is at least 22ºC.
10 Health and Safety
Health and Safety guidelines concerning handling and disposal of soda ash solution are
given in the Chemical Hazard Bulletin (Attachment 1). This states that sodium
carbonate is a severe irritant to the eyes, skin and respiratory tract and hence Personal
Protective Equipment shall include wearing goggles, neoprene gloves and dust mask if
handling solid powder. In addition, operators shall contact Environmental Protection
Department, Dhahran, for safe disposal of the solution.
31 July 2005 New Saudi Aramco Best Practice.
Figure 1 – Sensitization Curves for (a) 304/304L and (b) 321/347
Figure 2 – Dye penetrant inspection showing extensive OD cracking around
welds of austenitic stainless steel (Courtesy API RP571-2003)
Figure 3 – 304 stainless steel, showing intergranular PASCC cracking
in base metal, mag. x100 (Ref. CSD/ME&CCD/L-1445/98)