Aramco :: Best Practice SABP-A-001

SABP-A-001

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

1 Scope

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

the following:

• 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.

Commentary Note:

In case of discrepancy between this document and NACE RP0170-2004, resolve

discrepancy through the Coordinator ME&CCD.

2 Use of Best Practice

2.1 Disclaimer

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.

3 References

This Best Practice is based on the latest edition of the references below, unless

otherwise noted.

3.1 Saudi Aramco References

Saudi Aramco Engineering Standards

SAES-A-206 Positive Material Identification

SAES-L-133 Corrosion Protection Requirements for

Pipelines/Piping

Saudi Aramco Engineering Procedures

SAEP-20 Equipment Inspection Schedule

SAEP-325 Inspection Requirements for Pressurized

Equipment

SAEP-355 Field Metallography and Hardness Testing

Saudi Aramco Materials Specification

01-SAMSS-007 Manufacture of Shell and Tube Heat Exchangers

Other Documents

BI-3789 Project Specification for Material Specification

and Corrosion Control – Rabigh Refinery 1997

Upgrade Project

NACE 02478-2002 Thick-Wall Stainless Steel Piping in

Hydroprocessing Units – Heat Treatment

Issues

NiDI Publication The Role of Stainless Steels in Petroleum Refining

9021 (1977)

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

Refinery Equipment

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

Steels

4 Definitions and Abbreviations

4.1 Definitions

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.

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 %

concentration.

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

susceptible material.

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.

4.2 Abbreviations

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

Alloy 825.

Commentary Note

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

(1200°F).

5.3 Principal Factors

• A combination of environment, material and stress are required to promote

PASCC:

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

(polythionic acids).

o Material - The material must be in a susceptible, i.e. "sensitized"

condition.

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

tubes, bellows).

• Crude and Coker units (piping).

Commentary Note

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.

Material Factors

• 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

400°C (750°F).

• 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

and 625.

• 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

welds.

• 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

requirement.

• 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.

Process factors:

• 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

exposure.

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.

Commentary Note

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

between shutdowns.

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

solution.

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

7.2.3).

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

necessary.

• 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

gas.

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.

7.2.4 Reactors

• 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

condensate.

• 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

steels components.

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

resistance:

1) Use chemically stabilized grades such as 321/321H or 347/347H or low-carbon

grades, and

Commentary Note:

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

8.

4) For 347 stainless steels, the Nb:C stabilization ratio shall be within the range 10 to

12.

Commentary Note

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

condition (2a).

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.

Revision Summary

31 July 2005 New Saudi Aramco Best Practice.

Figure 1 - Sensitization Curves for (a) 304/304L and (b) 321/347

(a)

(b)

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)