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Materials Degradation LibraryModule S1

Wet H2S Cracking (Refineries And Gas Plants)

Note: The plain formatted text has been copied from API Publication 581, Base ResourceDocument On Risk-Based Inspection, Appendix V, October 1996 Revision. Imperial units inthat document have been converted to rounded metric units. The italics text is additionalinformation or changes consistent with SIOP practices and experience.

Description of Damage

The term wet H2S cracking covers a range of damage mechanisms that can occur due to theeffects of aqueous hydrogen charging in "Wet H2S" refinery or gas plant processenvironments. Types of material damage that can occur as a result of aqueous hydrogencharging include sulphide stress corrosion (SSC) cracking of hard weldments andmicrostuctures, hydrogen blistering, hydrogen induced cracking (HIC) and stress-orientedhydrogen induced cracking (SOHIC).

In refinery and streams containing free water and H2S, the pH is often around 6 or higher dueto the presence of ammonia. Trace quantities of contaminants such as chloride or fluoridemay also be present, which form ammonium salts (ammonium chloride and ammoniumfluoride). Cyanides can also play an important role in refinery streams as they reduce theiron sulphide scale persistence and impact the amount of hydrogen absorbed into the steel. Inhigh pH streams containing significant levels of sulphide and carbonate ions, carbonatecracking is a concern. This module addresses the impact of pH and cyanides on wet H2Scracking and also covers carbonate cracking.

The following subsections taken from three technical modules in the API RBI Project BaseResource Document (Reference 1) provide a more detailed description of these wet H2Sdamage mechanisms:

Sulfide Stress CrackingSulfide stress cracking is defined as cracking of a metal under the combined action of tensilestress and corrosion in the presence of water and hydrogen sulfide. SSC is a form of hydrogenstress cracking resulting from absorption of atomic hydrogen that is produced by the sulfidecorrosion process on the metal surface. SSC usually occurs more readily in materials having ahigh hardness and in hard weld deposits or hard heat-affected zones.

Susceptibility to SSC is related to the hydrogen permeation flux in the steel, which is primarilyassociated with two environmental parameters - pH and H2S content of the water. Typically,the hydrogen flux in steels has been found to be lowest in near neutral pH solutions, withincreasing flux at both lower and higher pH values. Corrosion at low pH values is caused byH2S, whereas corrosion at high pH values is caused by high concentrations of the bisulfide ion.Presence of cyanides at elevated pH can further aggravate the hydrogen penetration into thesteel. SSC susceptibility is known to increase with H2S content, e.g. H2S partial pressure inthe gas phase or H2S content of the water phase. The presence of as little as 1 ppm of H2S inthe water has been found to be sufficient to cause SSC.

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Susceptibility to SSC is primarily related to two material parameters - hardness and stresslevel. Steels with a high hardness have increased susceptibility to SSC. SSC has not generallybeen a concern for carbon steel base metals typically used for refinery pressure vessels andpiping in wet hydrogen sulfide service because these steels have sufficiently low hardnesslevels. However, weld deposits and HAZs may contain zones of high hardness and highresidual stresses from welding. High residual tensile stresses associated with welds increasesusceptibility to SSC. PWHT significantly reduces residual stresses and also tempers (softens)weld deposits and HAZs. A postweld heat treatment of about 620ºC (1150°F) for one hourper 25 mm (1 inch) of thickness (one hour minimum) is considered effective for carbon steel.Somewhat higher temperatures are required for low alloy steels. Control of hardness andreduction of residual stresses are recognised methods for preventing SSC as outlined in NACEStandard RP0472 (Reference 2) and NACE Standard MR0175 (Reference 3).

HIC and SOHICHIC is defined as stepwise internal cracks that connect adjacent hydrogen blisters on differentplanes in the metal, or to the metal surface. No externally applied stress is needed for theformation of HIC. The driving force for the cracking is high stresses at the circumference ofthe hydrogen blisters caused by build-up of internal pressure in the blisters. Interactionsbetween these high stress fields tend to cause cracks to develop that link blisters on differentplanes in the steel.

The build-up of pressure in the blisters is related to the hydrogen permeation flux in the steel.The source of the hydrogen in the steel is the corrosion reaction with wet hydrogen sulfide.Water must be present for this corrosion reactor to occur, and the resultant hydrogen flux isprimarily associated with two environmental parameters - pH and H2S content of the water.Typically, the hydrogen flux in steels has been found to be lowest in near neutral pH solutions,with increasing flux at both lower and higher pH values. Corrosion at low pH values is causedby H2S, whereas corrosion at high pH values is caused by high concentrations of the bisulfideion. Presence of cyanides at elevated pH can further aggravate the hydrogen penetration intothe steel. Hydrogen permeation is know to increase with H2S content, e.g. H2S partialpressure in the gas phase or H2S content of the water phase. The presence of as little as 50ppm of H2S in the water has been sufficient to cause HIC.

Hydrogen blisters are planar hydrogen-filled cavities formed at discontinuities in the steel (e.g.voids, inclusions, laminations, sulfide inclusions). Blisters most often occur in rolled platesteels, especially those with a banded microstructure resulting from elongated sulfideinclusions. Susceptibility to hydrogen blistering, and therefore HIC is primarily related to thequality of the plate steel, i.e., the number, size and shape of the discontinuities. In this regard,the sulphur content of the steel is a key material parameter. Reducing the sulphur content ofthe steel, combined with a proper normalising heat treatment for grain refinement, reducesthe susceptibility to blistering and HIC. Additions of calcium which controls sulfide inclusionshape control is generally beneficial.

SOHIC is defined as a stacked array of small blisters joined by hydrogen-induced cracking thatis aligned in the through-thickness direction of the steel as a result of high localised tensilestresses. SOHIC is a special form of HIC which usually occurs in the base metal, adjacent tothe heat-affected zone of a weld, where stresses are highest due to the additive effect ofapplied stress (from internal pressure) and the residual stresses from welding. As with HIC,plate steel quality is a key parameter for SOHIC susceptibility. In addition, reduction ofresidual stresses by PWHT can reduce, but may not eliminate, the occurrence and severity ofSOHIC. The level of applied stress also influences the occurrence and severity of SOHIC.

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Although HIC/SOHIC is much more prominent in plate steel fabrications, it has been observedto a limited extent in steel pipe fabrications, usually in the more severe hydrogen chargingenvironments.

Carbonate Cracking (As experienced in alkaline sour water)Carbonate cracking is a common term applied to cracking of a metal under the combinedaction of tensile stress and corrosion in the presence of an alkaline sour water (i.e., high pHwater containing some dissolved H2S and in most cases cyanides) containing moderate to highconcentrations of carbonate. The cracking is predominantly intergranular in nature, andtypically occurs in as-welded carbon steel fabrications as a network of very fine, oxide-filledcracks. Carbonate cracking typically propagates parallel to the weld in adjacent base metal,but can also occur in the weld deposit or heat-affected zones. The pattern of crackingobserved on the steel surface is sometimes described as a spider web of small cracks, whichoften initiate at or interconnect with weld-related flaws that serve as local stress raisers.

Carbonate cracking has been most prevalent in the catalytic cracking unit main fractionatoroverhead condensing and reflux system, the downstream wet gas compression system, and thesour water systems emanating from these areas. Assuming the presence of a sour water phase,three key parameters are used to assess the susceptibility of steel fabrications to carbonatecracking. They are the pH of the sour water, carbonate concentration of the sour water, andlevel of tensile stress.

Studies have concluded that the cracking occurs in a narrow range of electrochemicalpotential, which is very dependent upon the sour water composition. Presence of moderate tohigh levels of carbonates in an alkaline sour water often produces an electrochemical potentialof steel which is in this narrow range where carbonate cracking is likely to occur. Anothercommon contaminant in these sour waters, cyanides, has been shown to influence crackingsusceptibility. Despite this mechanistic understanding, the electrochemical potential of in-service equipment and piping may not be readily available. Therefore, pH and carbonateconcentration of the sour water are judged to be the key environmental parameters influencingthe susceptibility of steel fabrications to carbonate cracking. Based on a survey of many unitsreported in Reference 4, cracking susceptibility increases with increasing pH and carbonateconcentration.

With regard to the level of tensile stress, as-welded or as-bent carbon steel fabrications aresusceptible to carbonate cracking because of the high level of residual stress remaining afterfabrication by these methods. Application of a post-fabrication stress-relieving heat treatment(e.g. postweld heat treatment) is a proven method of preventing carbonate cracking. A heattreatment of about 621ºC (1150°F) for one hour per 25 mm (1 inch) of thickness (one hourminimum) is considered an effective stress-relieving heat treatment to prevent carbonatecracking of carbon steel.

Basic Data

The process and materials data listed in Table S1-1 may be used in conjunction with theTables in the Environmental Severity section of this module to determine the susceptibility ofcarbon and low alloy ferritic steel equipment and piping to the various forms of wet H2Scracking. The process data in this Table is the minimum required for selection of materials ofconstruction.

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If exact process data are not known, contact a knowledgeable technologist to obtain the bestestimates.

Table S1-1 Basic Data Required for Analysis

Basic Data Comments

Presence of Water

(Yes or No)

Determine whether free water is present in theequipment/piping. Consider not only normal operatingconditions, but also start-up, shutdown, process upsets, etc.

H2S Content of Water Determine the H2S content of the water phase. If analyticalresults are not readily available, it can be estimated using theapproach of Petrie & Moore (Reference 5).

Cyanide (HCN) Content ofWater

Determine the HCN (CN-) content of the water phase. Ifanalytical results are not readily available a knowledgeabletechnologist shall be consulted for an assessment basedupon the type of process unit, feed, water wash practices,etc.

pH of Water Determine the pH of the water phase. If analytical resultsare not readily available, it should be estimated by aknowledgeable technologist.

CO3= Conc in Water Determine the carbonate concentration of the water phasepresent in this equipment/piping. If analytical results are notreadily available, it should be estimated by a knowledgeabletechnologist.

Sulphur Content, heattreatment and testing (“Z”quality or “HIC” tested) ofPlate Steel

Determine the sulphur content, heat treatment and testing ofthe steel plate used to fabricate the equipment/piping. Thisinformation should be available on MTR’s in equipmentfiles. If not available, it can be estimated from the materialspecification of the steel listed in the fabrication records inconsultation with a materials engineer. Steel plate that hasnot been given a grain refining heat treatment(normalising) should be considered as equivalent to highsulphur steel (Table S1-4), irrespective of the actualsulphur content. Similarly, plate should be considered asequivalent to high sulfur steel if it has not been tested as “Zquality” or “HIC” tested.

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Steel Product Form

(Plate or Pipe)

Determine what product form of steel was used to fabricatethe equipment/piping. Most equipment is fabricated fromrolled and welded steel plates (e.g. A285, A515, A516,etc.), but some small-diameter equipment is fabricated fromsteel pipe and piping components. Most small-diameterpiping is fabricated from steel pipe (e.g. A106, A53, API 5L,etc.) and piping components (e.g. A105, A234, etc.), butmost large diameter piping (above approximately 16 NPSdiameter) is fabricated from rolled and welded plate steel.

Max Hardness Determine the maximum hardness actually measured at theweldments of the steel equipment/piping. Report readingsactually taken as Vickers. If actual readings are notavailable, use the maximum allowable hardness permitted bythe fabrication specification.

PWHT of Weldments

(Yes or No)

Determine whether all the weldments of theequipment/piping have been properly postweld heat treatedafter welding.

Determination of Environmental Severity

Tables S1-2A and S1-2B may be used to assess the severity of the process environment asapplicable to the type of damage mechanism being considered. Input from these Tables isrequired for the Determination of Susceptibility (Tables S1-3, S1-4, Figures S1-1, S1-2).Environmental severity for Carbonate Cracking is used directly to determine susceptibility tocracking in Table S1-5 and Figure S1-3.

NOTE: When using Tables S1-2A, S1-2B and S1-5 the environment being considered should be presentduring normal operations. Short term upsets should only be considered if the damage mechanism anticipatedor experienced is also likely to occur in the short term. If in doubt, a materials engineer shall be consulted.

Sulfide Stress Cracking

If there is no free water likely to be present then the material is not considered susceptible toSSC. If there is water present, then the basic data from Table S1-1 on the H2S content of thewater and its pH and cyanide content should be used to estimate the environmental severity(potential level of hydrogen flux) using Table S1-2A.

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Table S1-2A Environmental Severity - SSC

H2S CONTENT OF WATER (mg/kg)

pH of water Cyanide content(mg/kg)

< 50 50 to 1000 > 1000

SEVERITY CATEGORY

< 4.0 (Note 1) Moderate High High

4.0 to 5.4 (Note 1) Low Moderate High

5.5 to 7.5 (Note 1) Low Low Moderate

7.6 to 7.9 < 50 Low Moderate High

7.6 to 7.9 50 Moderate High High

8.0 < 20 Low Moderate High

8.0 20 Moderate High High

NOTE 1. HCN level is not significant at pH 7.5 and below.

Blistering, HIC and SOHIC

If there is no free water likely to be present then the material is not considered susceptible toblistering, HIC/SOHIC. If there is water present, then the basic data from Table S1-1 on theH2S content of the water and its pH and cyanide content should be used to estimate theenvironmental severity (potential level of hydrogen flux) using Table S1-2B.

Table S1-2B Environmental Severity - Blistering, HIC and SOHIC

H2S CONTENT OF WATER (mg/kg)

pH of water Cyanide content(mg/kg)

< 50 50 to 1000 > 1000

SEVERITY CATEGORY

< 4.0 (Note 1) Moderate Moderate Moderate

4.0 to 7.5 (Note 1) Low Low Moderate

7.6 to 7.9 < 50 Low Moderate Moderate

7.6 to 7.9 50 Moderate Moderate High

8.0 < 20 Low Moderate Moderate

8.0 20 Moderate High High

NOTE 1. HCN level is not significant at pH 7.5 and below.

General Materials Selection / Mitigation Philosophy

Materials Selection

The general SIOP materials selection philosophy for new equipment and piping to be exposedto wet H2S cracking environments is covered in the DEPs listed below. Mitigation practicesfor specific process plant applications are covered in the unit specific Materials EngineeringPhilosophy Documents (MEPs). The materials selection, testing and PWHT requirements forcarbon steel equipment and piping in wet H2S cracking environments should be based upon

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the potential damage mechanism(s) anticipated or experienced in the service and the severityof the process. The selection criteria for carbon steel pressure vessel and piping materials inthe listed DEPs are based on the environmental severity tables used in this degradationmodule:

DEP 31.22.10.32 Pressure Vessels (Amendments/Supplements To BS 5500), Appendix 4,(Reference 6)

DEP 31.22.20.31 Pressure Vessels (Amendments/Supplements To ASME Section VIII,Division 1 and Division 2), Appendix 4, (Reference 7)

DEP 31.38.01.11 Piping General Requirements, Appendix 10.6 (Reference 8)

Hardness Testing

Hardness testing requirements for equipment and piping to be used in wet H2S crackingenvironments are specified in the following DEPs:

DEP 31.22.10.32 Pressure Vessels (Amendments/Supplements To BS 5500), Appendix 3,(Reference 6)

DEP 31.22.20.31 Pressure Vessels (Amendments/Supplements To ASME Section VIII,Division 1 and Division 2), Appendix 1, (Reference 7)

DEP 31.38.01.31 Shop and Field Fabrication of Steel Piping, (Reference 9)

Determination of Cracking Susceptibility

The cracking susceptibility rating from this section may be used as input to the “Probabilityof Failure- Wet H2S Cracking” Appendix III questionnaire in the SIOP S-RBI Manual(Reference 10).

SCCUsing the environmental severity determined in Table S1-2A and the basic data from Table S1-1 on maximum hardness and postweld heat treatment of weldments, the susceptibility to SSCmay be determined using Table S1-3. A flow chart of the steps required to determine thesusceptibility to SSC is presented in Figure S1-1.

Table S1-3 Susceptibility to SSC

As-welded PWHTEnvironmental Max Vickers Hardness(1) Max Vickers Hardness(1)

Severity < 248 248-290 > 290 < 248 248-290 > 290

High Low Medium High Not Low Medium

Moderate Low Medium High Not Not Low

Low Low Low Medium Not Not Not(1) Actually tested as Vickers or converted from portable techniques, e.g. Equotip, Microdur etc.

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Blistering, HIC/SOHICFor equipment and large-diameter piping fabricated from rolled and welded plate steel, theenvironmental severity determined in Table S1-2B and the basic data from Table S1-1 on thesulphur content of the plate steel and postweld heat treatment, may be used to determine thesusceptibility to blistering, HIC/SOHIC using Table S1-4. Small-diameter equipment andpiping fabricated from steel pipe and piping components should be considered to have a lowsusceptibility to HIC/SOHIC unless it has not been postweld heat treated and is exposed to ahigh severity environment, in which case it should be considered to have a mediumsusceptibility. A flow chart of the steps required to determine the susceptibility to blistering,HIC/SOHIC is presented in Figure S1-2.

Table S1-4 Susceptibility to Blistering, HIC/SOHIC

EnvironmentalSeverity

High Sulphur Steel (1)

> 0.01% S

Low Sulphur Steel (2)

0.002 to 0.01% S

Ultra Low Sulphur (3)

< 0.002% S

As-Welded

PWHT As-Welded PWHT As-Welded PWHT

High High High Medium Low Low Low

Moderate High Medium Low Low Low Not

Low Medium Low Not Not Not Not

(1) Typically includes A70, A201, A212, A285, A515, and most A516 before about 1990. Also includes any plate steel notnormalised and any plate steel not tested as either “Z” quality or tested as “HIC” resistant.

(2) Typically includes early generations of normalised HIC-resistant A516 in 1980’s, with Ca additions. Also includesnormalised “Z” quality steels.

(3) Typically includes later generations of normalised HIC-resistant and HIC- tested A 516 in 1990’s.

Carbonate Cracking (As experienced in alkaline sour water)

If the equipment/piping is properly stress relieved, then it is considered not susceptible tocarbonate cracking. If there is no free water present the equipment/piping is considered notsusceptible. If the equipment/piping contains a water phase with some dissolved H2S orcyanides (Note: the lower threshold levels for H2S/cyanides have not actually beendetermined by either field experience or laboratory experiments, but the prescence of thesecontaminents is believed to be necessary to create the environment for carbonate crackingas experienced in alkaline sour water) at a pH of 7.6 or greater, then the equipment/piping isconsidered susceptible. Using the basic data from Table S1-1 on pH and carbonateconcentration of the water phase and PWHT condition, the environmental severity andsusceptibility to carbonate cracking should be determined using Table S1-5. A flow chart ofthe steps required to determine the susceptibility to carbonate cracking is presented in FigureS1-3.

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Table S1-5 Environmental Severity and Susceptibility to Carbonate Cracking

CO3= Concentration in Water

pH of Water < 100 ppm 100 - 500 ppm 500 - 1000 ppm > 1000 ppm

7.6 to 8.3 Low Low Low Medium

8.4 to 8.9 Low Low Medium High

ò 9.0 Low Medium High High

Inspection Effectiveness

The inspection effectiveness and the right Non-Destructive testing techniques have beendescribed in the Oil Products Non-Destructive Testing (NDT) Handbook, which can be foundat the SIOP Materials Engineering home page, http://sww-ortem.ksla.shell.nl/ (authorisedusers only).

The following cross references to the NDT Handbook may be used as a source of furtherguidance and information on NDT techniques, which can be applied to detect cracking:

Ultrasonic Testing

Liquid Penetrant Testing

Magnetic Testing

Radiology

Acoustic Emission Testing

If the NDT handbook is not available, table S1-6 can be used as guideline for theeffectiveness of intrusive and non intrusive inspection.

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Table S1-6 - the inspection effectiveness table for intrusive and non intrusive inspection.

Inspection EffectivenessCategory

Intrusive Inspection Non-Intrusive Inspection(ambient temperature)

A For the selected weld area:

>95% Wet FluorescentMagnetic particles Testingwith Ultrasonic Testingfollow-up of relevantindications

AND

>20% Wet FluorescentMagnetic particles Testingwith Ultrasonic Testingfollow-up of relevantindications on non selectedwelds

In case of manual ultrasonicscanning:

>95% for the selected weldarea

AND

>30% for the non selectedweld area

OR

In case of automatedultrasonic scanning:

>70% for the selected weldarea

AND

>15% for the non selectedweld area

B For the selected welds:

>60% Wet FluorescentMagnetic particles Testingwith Ultrasonic Testingfollow-up of relevantindications

For the selected welds:

>60% manual ultrasonicscanning

OR

>30% automated ultrasonicscanning

References1. API Publication 581, Base Resource Document for Risk-Based Inspection, Appendix

V, October 1996 revision.

2. Methods and Controls to Prevent In-Service Environmental Cracking of Carbon SteelWeldments in Corrosive Petroleum Refining Environments, NACE Standard RP0472-95.

3. Standard Material Requirements - Sulfide Stress Cracking Resistant MetallicMaterials for Oilfield Equipment, NACE Standards MR0175-97 (Revised Annually)

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4. Kmetz and D.J. Truax, Carbonate Stress Corrosion Cracking of Carbon Steel inRefinery FCC Main Fractionator Overhead Systems, NACE paper #206,CORROSION/90.

5. R.R. Petrie and E. M. Moore, Jr., Determining the Suitability of Existing Pipelines andProducing Facilities for Wet Sour Service, Materials Performance 28, 6 (June 1989),pp. 59-65.

6. DEP 31.22.10.32 Pressure Vessels (Amendments/Supplements To BS 5500).

7. DEP 31.22.20.31 Pressure Vessels (Amendments/Supplements To ASME Section VIII,Division 1 and Division 2).

8. DEP 31.38.01.11 Piping General Requirements.

9. DEP 31.38.01.31 Shop and Field Fabrication of Steel Piping.

10. S-RBI Manual, OP 97-30007

Bibliography

Review of Published Literature on Wet H2S Cracking of Steels Through 1989, NACEPublication 8X294

Stress Corrosion Cracking and Hydrogen Embrittlement of Iron Base Alloys, NACE-5, Editedby R. W. Staehle, et. al., NACE International, Houston, TX, 1977, pp. 541-559.

C.M. Hudgins, et. al., Hydrogen Sulfide Cracking of Carbon and Alloy Steels, Corrosion, Vol.22, pp. 238-251.

Guidelines for Detection, Repair, and Mitigation of Existing Petroleum Refinery PressureVessels in Wet H2S Environments, NACE Standard RP0296-96.

R.D. Merrick, Refinery Experiences with Cracking in Wet H2S Environments, MaterialsPerformance 27, 1 (January 1988), pp. 30.

R.D. Merrick and M.L. Bullen, Prevention of Cracking in Wet H2S Environments, NACECorrosion/89, paper no. 269.

Materials and Fabrication Practices for New Pressure Vessels Used in Wet H2S RefineryService, NACE Publication 8X194

Research Report on Characterization and Monitoring of Cracking in Wet H2S Service, APIPublication 939, October 1994.

M.S. Cayard and R.D. Kane, Characterization and Monitoring of Cracking of Steel Equipmentin Wet H2S Service, NACE Corrosion/95, paper no. 329.

H.U. Schutt, Intergranular Wet Hydrogen Sulfide Cracking, NACE paper #454,CORROSION/92 (see also Stress Corrosion Cracking of Carbon Steel in Amine Systems,NACE paper #187, CORROSION/87) (see also Materials Performance 32, 11 (1993), pp. 55-60).

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Start

DetermineSusceptibility Using

Table S1-3

NoWaterPresent?

Yes

PWHT?VickersHardness

Susceptibility

EnvironmentalSeverity

DetermineEnvironmental SeverityUsing Table S1-2A

Not Susceptible

pH and HCNof Water

H2S Contentof Water

Figure S1-1 Determination of Susceptibility of Sulfide Stress Cracking

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Start

NoWater Present?

Yes

EnvironmentalSeverity

Determine SusceptibilityUsing Table S1-4

Not Susceptible

PWHT?Plate sulfur level,normalising &testing

Fabricated fromrolled and welded

plate steel?

Yes

NoYes

As-welded?No

HighEnvironmental

Severity?

Yes

No

Susceptibility MediumSusceptibility

DetermineEnvironmental SeverityUsing Table S1-2B

pH and HCNof Water

H2S Content ofWater

LowSusceptibility

Figure S1-2 Determination of Susceptibility to Blistering, HIC/SOHIC

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DetermineSusceptibility Using

Table S1-5

Carbonateconc inWater

pH of Water

Susceptibility

Start

NoH2S and/or

HCN inWater

Not Susceptible

Yes

YesStress

Relieved? Not Susceptible

No

NoWater

Present?Not Susceptible

Yes

Figure S1-3 Determination of Susceptibility to Carbonate Cracking (as Experienced inAlkaline Sour Water)