Hydrogen-Induced+Cracking+Mechanisms,+Inspection,+Repair,+and+Prevention - VVImp.pdf

8/10/2019 Hydrogen-Induced+Cracking+Mechanisms,+Inspection,+Repair,+and+Prevention - VVImp.pdf

1/5SPE Production & Facilities, February 1996 49

Hydrogen-induced cracking (HIC) is a materials- and corrosion-re-lated problem that occurs in surface production systems. Steels usedto construct sour-gas production facilities and flowlines may cor-rode from wet hydrogen sulfide (H2S) gas in the production stream.The corrosion process generates hydrogen that may damage thesteel, resulting in HIC and other forms of damage from hydrogen.HIC control and prevention are an important consideration in oper-ating surface-facility equipment in a safeand efficient manner. Byeffectively controlling or preventing HIC damage in surface-facilityequipment, operating costs are reduced, potential costly downtimefrom equipment failure is avoided, and a safe work environment ismore easily realized.

We developed an overall approach to understand and deal withthe HIC problem in sour-surface production facilities. The approach

dealt with understanding the mechanism in steel materials used insurface-facility equipment; implementing state-of-the-art inspec-tion techniques; fitness-for-service (FFS) evaluation to assess dam-age effect on performance; repair procedures on existing equip-ment; and finally, establishing steel performance and fabricationrequirements to eliminate or reduce HIC damage risk for newly fab-ricated equipment.

This paper presents the results of laboratory examinations of over40 steels covering both new and existing equipment, leading to theapproaches developed for controlling HIC in existing equipmentand for controlling or preventing HIC in new construction. We alsopresent the basis for the approaches developed to deal with HIC insurface production facilities.

Wet H2S- or cyanide- (CN-) cracking in surface production processequipment, such as pressure vessels, is now a major concern in theindustry. Industry attention turned to this potential cracking prob-lem in the mid-1980s, following the catastrophic rupture of anamine contactor at the Union Oil refinery in Lemont, IL.1

Two steps were taken to address this concern in production opera-tions: 1) special requirements were placed on steel, vessel fabrica-tion, and operation to avoid process equipment and vessel cracks af-ter understanding the steel-cracking mechanism, and 2) a sour-gasplant inspection/maintenance program was initiated2in the operat-ing plants with an FFS capability developed to assess cracking sig-nificance on equipment service ability. This overall approach has anindustry-wide impetus.

To address materials performance in hydrogen-charging environ-ments for new construction, we initiated an effort to study the per-

formance of pressure vessel steels in wet H2S/CN environments.From these studies, recommendations about the benefit of spe-

cialized chemistry control, normalization after rolling, special met-allurgical processing techniques, and other controllable variableswere determined. These recommendations were then incorporatedinto a material-purchase specification.

Wet H2S/CN Cracking Mechanisms in Process Equipment. WetH2S- or CN- assisted hydrogen cracking (HC)often known as

Copyright 1996 Society of Petroleum Engineers

Original SPE manuscript received for review June 1, 1993. Revised manuscript received July13, 1995. Paper peer approved Aug. 15, 1995. Paper (SPE 25583) first presented at the 1993SPE Middle East Oil Technical Conference & Exhibition held in Bahrain, April 36.

HIC to cover a multitude of related cracking mechanismsoccursin both refinery and production facilities. Upstream production faci-lities are exposed to H2S, while downstream systems experience hy-drogen charging into the steel from both H2S and CN. The hydro-gen-driven cracking is observed in several modes. The modes ormechanisms are sulfide-stress corrosion cracking (SSC), hydrogenblistering, hydrogen-induced (stepwise) cracking (HIC), and stress-oriented hydrogen-induced (stepwise) cracking (SOHIC).

Other forms of stress-corrosion cracking (SCC), by direct conse-quence of the gas or by gas-treating agents, are also reported in re-finery and production facilities that deal with wet acid gas. One SCCtype is related to amine cracking where the H2S- or CO2-rich gas-process stream is sweetened by a lean amine such as monoethanol(MEA) or diethanol amine (DEA). Cracking is observed in the H2S-rich MEA or DEA stream. The second SCC type is carbonate/bicar-bonate cracking. Both these cracking mechanisms are consideredsimilar, occurring in an alkaline environment and exhibiting inter-granular branched crack growth in low-strength ferritic/pearliticsteels.

These latter two mechanisms, amine- and alkaline-caused SCC,are briefly summarized here because they are present in surface pro-duction equipment exposed to H2S and CO2 acid gases. Thesecracking mechanisms are typically reported in industry surveys.Such mechanisms are found in recent cracking reports to the NACET8-16 committee on refinery equipment3subjected to H2S/CN- andH2S-production surface-equipment-inspection survey results re-ferred to earlier2in this paper.

HC Mechanisms. The wet H2S/CN HC mechanisms we observedall require the presence of an aqueous phase, temperatures from am-

bient to 300F, and the evolution of hydrogen by a corrosion reac-tion at the environment/steel interface. The presence of hydrogen

ions does not strongly promote absorption into the steel at the inter-face. The presence of the bisulfite (HS) or CN ions does stronglypromote hydrogen absorption into the steel, poisoning the recom-bination of hydrogen into a gas in the corrosion reaction. Similartypes of HC damage are not associated with CO2when the HS or CNions are not present. HS and CN ions promote the transport of hydro-gen across the environment-metal interface and promote blistering,HIC, and SOHIC.

The cracking mechanism for SSC requires extremely low con-centrations of hydrogen in the metal (5 ppm). Low concentrationsof HS or CN can produce this level of hydrogen in the steel. There-fore, SSC is usually controlled by material selection and hardnesscontrol.4-6

The HC mechanisms active in production surface equipment,based on metallurgical reviews of operating vessel wet flourescent/magnetic particle testing (WFMT) indications, are primarily blister-ing and HIC.2Only limited instances of SOHIC and SSC have beenfound.

Blistering. Hydrogen blistering occurs predominantly in low-strength steels (yield strength less than 80 ksi) that are exposed tohydrogen. Blistering occurs from molecular hydrogen that collectsat internal planar nonmetallic inclusions or laminations. Under theinternal hydrogen pressure the inclusion (lamination) disbondsfrom the steel matrix and forms a blister. Blistering is observed inpressure vessels, storage tanks, and piping. This form of hydrogendamage is contrasted with high-temperature (greater than 600F)hydrogen damage in steels where hydrogen reacts with carbon


2/550 SPE Production & Facilities, February 1996

Fig. 1HIC- and SOHIC-cracking morphologies in a steel-platecross section.

found in the pearlite colonies and forms methane that produces in-tergranular fissuring, cracking, and severe loss of ductility.

Hydrogen blistering is not necessarily considered critical to thepressure integrity of a vessel or pipe; often the blisters do not linkfrom one plane to another or break the surface. This influences ves-sel FFS assessments and leads to extended service life. Typically,blisters are observed on the internal surface of the vessel and maybe drilled to relieve pressure and reduce further deformation.

SSC. SSC has been extensively studied in carbon manganese,low-alloy high-strength, and microalloyed steels (Ref. 6). SSC is aspecific form of hydrogen embrittlement that occurs in high-strength steels and hard-weld heat-affected-zones (HAZ). Other re-

search characterizes cracking for steels as dependent on hardness,temperature, H2S concentration, cold work, and alloy content. Al-loys with higher alloying content, called corrosion resistant alloys(CRAs), can still be susceptible to SSC. SSC in CRAs is influencedby the same parameters. SSC is mostly observed in surface produc-tion equipment at internal attachment clip welds where a hard HAZcan be developed.

HIC. HIC is a hydrogen-damage mechanism that occurs in soft,low-strength steels (yield strength less than 80 ksi). Cracking occursby the linking of small inclusions that have blistered. The linkingmechanism proceeds from one blistered inclusion to another in ani-sotropic planes developed from the rolling process in plate manu-facture. The linking is sometimes called stepwise cracking becausethe cracking morphology appears as steps in a metallurgical section(Fig. 1). Another common term that is applied is blister cracking.HIC or blister cracking is differentiated from simple blistering bythe linking or cracking of the steel from one blister to another.

SOHIC.SOHIC is a hydrogen-damage mechanism like HIC. Thedamage mechanism, however, is driven not only by the high pres-sures that are generated by the hydrogen at the inclusion to steel in-terface, but also by the external applied stress.4These stresses causethe crack path between inclusions to move more directly through theplate. The cracking morphology is more reminiscent of a ladder thana stair step; the inclusions link not as stair steps, but more in a verti-cal direction like the rungs on a ladder (Fig. 1). Another commonterm that is used to describe this phenomenon is ladder cracking.

Corrosion Process: Hydrogen Generation and Transport.Toproduce HC-blistering, SSC, HIC, or SOHIC, hydrogen must bepresent at the steel surface and then be adsorbed into the metal. Thecorrosion process generates hydrogen and involves the productionof iron at the anode that goes into aqueous solution as

FeFe+22e, (1). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

and at the cathode hydrogen is produced and either enters the steelor forms hydrogen gas and bubbles off. When H2S is present,

2H+2eHH. (2). . . . . . . . . . . . . . . . . . . . . . . . . . . . .

When H2S is not present,

HHH2(gas). (3). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Hydrogen enters the steel first by adsorption onto the steel at thewater to steel interface and then by being absorbed into the steel ashydrogen. The amount of hydrogen absorption depends on the cor-

Fig. 2Steel corrosion and hydrogen entry in an H2S environ-ment.

rosion rate of the steel surface and the concentration of anions, suchas CN or HS, that reduce the tendency to produce hydrogen gas,shown in Eq. 2. CN and HS also promote hydrogen, H, entry intothe steel. Fig. 2shows that in a strictly acid environment withoutH2S present, HIC, SOHIC, and SSC would not occur.

Produced wet sweet-acid gas (CO2) environments are an exampleof the criticality of HS in the HC mechanisms. General weight loss

corrosion occurs, but HIC, SOHIC, SSC, and blistering in steelsdoes not occur when HS and CN are not present. Introduction of asour environment allows this to occur and these damage mecha-nisms become active given a susceptible steel microstructure.

Acid-concentration, typically related to pH, has an influence ondamage. An example is acidizing to clean Fe2S-scaled carbon-steelvessels or piping. Acidizing with a Fe2S scale present produces anacidic environment with HS present. At pH4,

Fe2SacidFe+2H+HS. (4). . . . . . . . . . . . . . . . . . . . . .

At pH4,

Fe2SacidFe+2H+H2S, (5). . . . . . . . . . . . . . . . . . . . . .

which then leads to either hydrogen charging of the steel with hydro-

gen or hydrogen gas evolution.

2H+2eHH , (6). . . . . . . . . . . . . . . . . . . . . . . . . . . . .

and HHH2(gas). (7). . . . . . . . . . . . . . . . . . . . . . . . . . . . .

The presence of the HS promotes the transfer of hydrogen into thesteel (Eq. 6). Normally, acidizing is accomplished with acids witha pH of 4 to 6, producing high fluxes of hydrogen and the risk of blis-tering, HIC, or SOHIC from the acidizing alone.

Conditions for Blistering, HIC, and SOHIC. Blistering, HIC,and SOHIC are interrelated and depend heavily on hydrogen entryinto the steel microstructure. Blistering occurs when the hydrogenconcentration, CH, at a discontinuity exceeds the critical thresholdhydrogen concentration, Cth . The internal discontinuity is usually

an inclusion, but can be produced by SSC of a hardened area wherehigh Mn segregation has occurred. The condition is expressed as

CHCth, (8). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

and Cth is related to the material resistance to the hydrogen gas thathas recombined to produce a pressure on the inclusion nucleatedvoid. The pressure available for causing the inclusion to disbondfrom the metal matrix is related to CHat the point where the inclu-sion is located. Fig. 3 shows a typical profile of CHthrough thick-ness in a pressure vessel steel wall, illustrating the conditions forblistering, HIC, or SOHIC.

Blistering, HIC, or SOHIC damage conditions are determined bythe following critical variables: (1) pressure,pc, (2) steel matrix toinclusion resistance or fracture-toughness, KIC, and (3) the length



Fig. 3Condition for HIC in a steel plate based on CHandCthre-quired for HIC or blister formation.

of the inclusion, a. These variables are related by the condition forinclusion disbonding and crack growth as7

pcKIC/2(/a) (9). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

wherepcF(,CH), (10). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

where is the fraction of hydrogen absorbed into the steel lattice,pcis the pressure at the inclusion location, and CHis the concentra-tion of hydrogen at the inclusion location. The CHdecreases froma maximum at the absorption interface where the concentration ofhydrogen (CH)is maximum to zero at the external surface. This isexpressed in Eq. 11 as (Ref. 8)

CHCH(1x/h), (11). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

where h is the steel-vessel wall thickness and x is the distance infrom the surface exposed to the environment.

This basic condition for blistering, HIC, or SOHIC given in Eqs.

8 and 9 shows that the inclusion size, the amount of hydrogen enter-ing the steel to produce the internal pressure, the location of the in-clusion, and the steel matrix to inclusion interface toughness is criti-cal in determining whether HIC damage occurs. Treating practicesthat reduce the hydrogen entry or reduce the size of inclusionswould reduce or preclude damage to the steel-pressure vessel wall.

Techniques to Control or Prevent Damage. Techniques to controlor reduce the hydrogen entry are (1) water washing to reduce con-centrations of HS, (2) treating with inhibitors to reduce corrosionand available hydrogen, and (3) lining the steel areas that are subjectto corrosion with corrosion-resistant claddings like austenitic stain-less steels.

Steel Resistance.Steel resistance to damage is improved by steelproduction techniques that reduce the inclusion or internal disconti-

nuity size. Techniques used to control inclusion size are (1) reducingsulfur content to low (0.012) or ultra-low (0.002) levels, (2) re-ducing phosphorus and other elements like Mn, Cr, Ni, Mo, and Cu,(3) using specialized continuous casting, slabbing, hot-topping, in-got-casting, bottom-pouring, deoxidization, and calcium-treatingprocesses during steel making, (4) specialized cross-rolling practic-es for thinner plate thicknesses in the - to 1-in. range, and (5) in-spection techniques to examine plate steel internally for cleanlinessbefore finishing rolling or after finishing rolling and shipping accep-tance.

Steel to inclusion toughness or resistance to void extension (KIC)is indirectly controlled by the toughness of the steel matrix and theinteraction of the inclusion with the steel grain size for ductile-tear-ing fracture instabilities. Stronger steel will require more pressure

for an internal void or blister based on Eq. 9 to be formed. Steel grainrefinement should improve the material resistance. Techniques torefine the grain size during steel production are (1) heat treating bynormalizing, (2) using an accelerated thermomechanical treatment,and (3) using aluminum or silicon to semikill or fully kill the steels.These approaches are effective in increasing the blistering, HIC, orSOHIC resistance of a material.9,10

H2S Stress-Cracking Control.HSand CNcaused stress crack-

ing in carbon and low-alloy steels is controlled by limiting the mate-rial strength. Prevention of hydrogen entry into the steel is not con-sidered practical because bulk steel content of as little as 5 ppm ofhydrogen can result in cracking in a hard susceptible microstructure.

Water washing or inhibition would not be considered viable meth-ods of controlling SSC.

Limiting strength is accomplished by placing hardness limits onthe carbon and low-alloy steels. The hardness is proportional to thematerial strength. Hardness limits are used for weld metal. A 200Brinell hardness (BHN) limit is typically used for refinery applica-tions.4This limit is based on NACE RP 0472-87. Hardness limitsplaced on carbon steels, welds, and HAZs, as defined by NACEMR-0175-92, were set at 22 Rockwell C-Scale (RC), which isequivalent to 248 Vickers and 237 BHN. These limits are recom-mended, but not required. When low-alloy steels, such as AISI 4130or 4140, are properly heat treated to produce uniform microstruc-tures virtually free of retained austenite and upper bainites, hardnesslimits are extended to RC 26.

Some approaches to controlling strength or maximum hardness

in fabricated steel vessels and equipment are (1) using lower-strength steels without a tendency to develop high hardnesses; (2)controlling cooling rates below a critical rate during welding byusing higher heat inputs and/or preheat; (3) controlling material car-bon equivalent or being aware of the carbon equivalent when weld-ing; (4) controlling residual elements, such as Ni, Cu, Cr, Mo, andV, to a maximum; (5) controlling cooling rates during mill produc-tion; (6) using normalized carbon-manganese steels; and (7) usinga postweld heat treatment to act as a tempering heat treatment thatreduces hardness.

Production-Facility Service Environments. HC occurs in specificproduction gas plant and refinery locations. While the focus of thispaper is on production equipment, locations in refineries are beingmentioned to provide a complementary view of downstream prob-

lem similarity.11The conditions for blistering, SSC, HIC, and SOHIC have been

discussed. These conditions typically occur in the following refin-ery units: fluid catalytic-cracking units (FCCU), hydrotreating/hy-drocracking units, amine absorbers, sour-water strippers, and liq-uid-propane-gas (LPG) storage vessels. These units, however, willnot be discussed in detail here. In production gas plants, the condi-tions are also present in amine absorbers and sour-gas/water strip-pers.

Amine Absorbers and Strippers.Amine absorbers, common toboth refineries and production-acid-gas CO2and H2S sweeteningplants, are prone to HC when HS or CN are present. Wet sour-gasfeed is sweetened by intermixing with H2S/CO2 and lean aminethrough a series of bubble trays. The process occurs between tem-peratures of approximately 100to 250F. These temperatures are

associated with the lower portion of the absorber column. HIC dam-age occurs primarily near wet sour-gas-inlet areas in absorbers asthe gas feed contacts the steel. SOHIC damage occurs near internal-attachment welds associated with the bubble-cap trays. Strippersforce the highly condensed H2S and CO2 out of the rich amine.Strippers could potentially experience SSC, HIC, or SOHIC furtherdownstream in the outlet area for the H2S where aqueous condensa-tion has occurred. These stripper overheads are treated to avoid cor-rosion, discussed previously.

Sour-Water Strippers. Sour-water stripping of single or multi-phase sour-gas and oil feeds leads to acidic aqueous environmentsin the bottom of sour-gas/water strippers or at outlets in multiphaseseparators. These areas are prone to blistering, HIC, and SSC. Tem-peratures associated with separators may vary greatly and depend


4/552 SPE Production & Facilities, February 1996

TABLE 1WFMT INDICATIONS

Cracking-CauseMechanism/Source

Number ofVessel

Instances

Total VesselsInspected

(%)

Weld Fabrication

185

17

Blistering or HIC

64

10

Sulfide Stress

29

5

Alkaline Stress Cracking

15

3

Total

293

on feed-stream input temperature and extreme climactic conditions.Water strippers or knock-out drums typically operate at tempera-tures from ambient to 200F. This temperature range is within therange of susceptibility for HC mechanisms to operate.

After the Lemont failure1that drew a significant amount of industryattention to wet H2S/CN cracking in pressure vessels, refinery oper-ations formed a task force in early 1988 to address specific questionsabout HC in sour-service vessels. The refinery-task-force activitiesled to recommended guidelines for constructing, operating, inspect-ing, and repairing vessels in sour-service process streams. Produc-tion operators initiated inspection and repair programs to identifyand use wet H2S/CN cracking in pressure vessels and update vessel-materials specifications.

The inspection programs are showing that many WFMT indica-tions are caused by fabrication and suggest that a lesser percentageof actual indications are a direct result of HC mechanisms in pres-sure/vessel steels subjected to wet H2S/CN environments.

Inspection Techniques. Vessel inspection for HIC, SOHIC, blister-ing, and SSC is preferably done with a WFMT technique. This tech-nique requires the opening up of equipment and sand blasting orflapper-wheel cleaning of the internal surface to detect HIC, SOH-IC, or SSC.

Ultrasonic P-scanning techniques are also used when vessels arenot opened. This technique is capable of generating large quantitiesof detailed information, but does not clearly identify whether or notindications are caused by HIC or SOHIC damage, or by inclusions(discontinuities) normally associated with steels from which the

equipment is fabricated.Ultrasonic shearwave techniques, such as crack tip diffractiontechniques, are most helpful in determining the extent of crackdepth and morphologya determination critical in FFS assess-ments to be discussed later in this paper.

Industry Inspection Programs. NACE committee T8-16 hasconducted a survey of the refinery-industry process vessels that aresubject to wet H2S/CN cracking and found that 1,285, or 26%, of the4,987 inspected vessels contained cracks.3

Internal Inspection Programs.Inspecting thousands of internalrefinery and production vessels, which costs tens of millions of dol-lars, is nearing completion, and monitoring continues as required.The refinery results were provided to the NACE T8-16 Committeeas part of an inspection survey. The industry survey suggests thatmany of the cracks detected and originally thought to be related toHIC, SOHIC, or SSC are related to fabrication discontinuities.

Of all the production equipment and vessels inspected, 618 (57%)of over 1,084 showed WFMT indications. The indications aregrouped according to the mechanisms or causes described in Table1.

These results show that production equipment contains a relative-ly high percentage of as-fabricated discontinuities that have existedin the equipment for an extended time period. The percentage ofHIC-related cracking is under 10%.

Production surface equipment with discontinuities or cracks arequite common, according to the inspection survey data. These ves-sels have operated for a number of years with original fabrication

discontinuities. The question is how to determine whether theequipment or vessel is fit for service and can continue to be operatedsafely and profitably. To address this question, a FFS is being ap-plied to process vessels.12The FFS technology is not new, and hashad success in application in the aerospace, nuclear, and offshore-structures industries, among others.

Application to process vessels after inspection has shown thatcracks and other types of discontinuities present allow for repair orreplacement strategies based on sound performance criteria. Threelevels of analysis are available, each one being more involved thanthe one before. Through this analysis, acceptance criteria are set thatallow safe and productive operation of a process vessel with cracks

or discontinuities.Blistering.Blisters in the center of the plate are treated simply and

directly by FFS technology. Blisters that remain in the plane of theplate parallel to the stressed direction are benign and simply ig-nored. Blisters with ends that extend towards the surface of the platerequire a more detailed analysis.

Internal Surface Cracks and Weld Repair.Surface cracks foundby WFMT inspection are typically ground or excavated by arcgouging. Typically, when the grinding or excavation depth exceedsthe corrosion allowance, weld repair is required. By using FFS tech-nology, many of these ground areas do not require weld repair.

Field and Supplier Steels. Steel pressure/vessel materials ASTM

A285C, A515, A516 Grades 60 and 70, A537 Class II and A841were obtained from both field-process service locations and frominternational and domestic steel suppliers where steels are producedwith various processes, representing current steel-production tech-nology.

Steels From Operating Vessels. Field sample sources were devel-oped as available with the support of the operating locations to in-clude pressure/vessel steel grades such as ASTM A285 Class C,A212, A515, and A516 Grade 60 or 70 in the as-welded or postweldheat treatment (PWHT) condition.

Steel-Producer Sources. Materials were obtained from vesselsthat had been in wet H2S/CN service with evidence of HC found byWFMT inspections. Supplier steel pressure/vessel samples wereobtained from both domestic and international sources to providematerials fabricated with different state-of-the-art steel-making

technologies. These steels included ASTM A516 and A841 withsupplemental requirements in both the as-rolled, normalized, ther-momechanically processed (TMCP), and quenched and temperedfinished conditions.

Steel Chemistries and Mechanical Properties. Steel chemistryand mechanical properties were measured for both the field and thesteel-supplier samples evaluated. Heat analysis results were notused in an effort to characterize the actual plates tested and did notconsider variations in properties within a heat. Weldability parame-ters were also calculated from the individual chemistries accordingto the following formulas from the IIW.

HIC-Testing. Steel pressure/vessel materials ASTM A285C,A515, A516 Grades 60 and 70, A537 Class II and A841 were testedin both pH 3.5 and 4.5 solutions. Tests were run in both the moreaggressive pH 3.5 solution (TM 01 77) and the pH 4.5 solution (TM

02 84) to see the relative differences in the corrosion and hydrogen-charging effects of the two solutions.

Test results generally showed that the lower sulfur steels have atendency to produce lower values of crack/sensitivity ratio (CSR),crack/length ratio (CLR), and crack/thickness ratio (CTR). Test re-sults showed that materials can achieve a CLR below 30 when thesulfur is less than 0.006 wt%, the material is normalized, and someinclusion shape control practice is used. Finally, steels that are as-rolled with higher sulfur content show a greater tendency to havehigher values of CLR and lower values of CTR relative to thenormalized steels with lower sulfur contents. Steels that are specifi-cally produced for HIC resistance by the manufacturer for newconstruction have a goal performance-acceptance criterion thatranges from 10 to 15 CLR in the industry.



SOHIC-Testing. The NACE TM 01 77 test method was selectedto simulate HIC cracking under stress, a reported prime influence onproperty degradation and material performance in wet H2S/CN en-vironments. The acidic-pH 3.5 aqueous NACE test solution withH2S, 5% NaCl, and 0.5 wt% acetic acid was selected as the most se-vere and therefore most conservative measure of a steels resistanceto HIC or SOHIC.

The threshold stress, expressed as a percentage of the actual yieldstrength, typically ranges from 40% to 100%. We should note thatthe results show that steels with relatively high HIC resistance donot necessarily have high thresholds. Steels used for wet sour-gasproduction as oil-country tubular goods (OCTG) must pass an 80%

criterion set by API Specification 5CT. Amoco requires a 90% ac-ceptance criterion for tests run to this method. However, the crack-ing mechanism for the OCTG is not SOHIC, but SSC, suggestingthat this acceptance criterion may or may not be applicable to SOH-IC failure mechanisms that are different in pressure vessels.

Material-Performance Specifications. As a result of laboratorytesting and field-sample evaluation, specifications were establishedfor new construction and fabrication. These requirements are estab-lished to reduce risk of blistering, HIC, SOHIC, and SSC in newconstruction. Key points to steel resistance are control of sulfur, car-bon equivalent, and HIC/SOHIC resistance through acceptancetesting. Fabrication factors in controlling SSC are welding proce-dure, heat input, and PWHT.

Steel Requirements. Steels are required to be made to fine grain

practice (ASTM grain size 5 or smaller), normalized, and have amaximum sulfur content of 0.012 wt%. The steel is also required topass a performance HIC test per NACE TM02 84 requirements,using the more aggressive NACE TM 01 77 test solution. Accep-tance criteria are set at a CLR of 15 % and CTR of 5%. CLR and CTRto some degree measure material resistance to HIC and SOHIC.

Fabrication Requirements. PWHT is required after welding, withWFMT inspection following. In selected situations, weld procedurequalification is also required. As part of the procedure qualificationrecord (PQR), welds are hardness tested in the HAZ and must havea hardness that is less than 248 BHN. Testing is done with a Vickershardness indentor with a 10-kg load. Carbon equivalent is con-trolled to reduce the risk of hardness exceeding 248 BHN. Steelswith plate thicknesses of 1.5 in. or less must have a carbon equiva-lent (CE) of less than 0.42, while steel with plate thicknesses that ex-

ceed 1.5 in. must have a CE of no more than 0.45. CE is computedper the IIW formula.

CECMn/6(CrMoV)/5(NiCu)/15. (12). . . . .

These requirements add to the cost of a surface-process vessel.Estimates of added cost range from 10% to 30% increase in steelcost and 8% to 50% increase in as-fabricated cost of a process ves-sel. However, these costs are easily offset by the cost of periodic in-spection and dismantling of a vessel and making repairs, or, in ex-treme cases, replacing the equipment.

HIC in surface production equipment is an important considerationin operating equipment in a safe and efficient manner. Understand-

ing of the mechanism causing HIC, SOHIC, blistering, and SSC,and the applications of FFS technology have shown ways of dealingwith the phenomenon in a cost-effective manner. Through inspec-tion and repair strategies, process equipment in wet H2S service canbe maintained. Through specifications relating to material perform-ance and fabrication control, new equipment can be placed in ser-vice with more resistance to HIC, SOHIC, blistering, and SSC in wetH2S service.

a characteristic inclusion or crack lengtha fraction of hydrogen absorbed into steel at interface

of steel and corrosive environmentCH hydrogen concentration

CHo hydrogen concentration at the absorbation interface

Ec carbon equivalent (IIW formula)

KIC material fracture toughness at tip of inclusion orcrack

pC critical pressure

h wall thickness of pressurized equipmentx position from surface through wall of vessel

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tant Steels and HIC Test Methods for Hydrogen Sulfide Service, Paper

No. 6, Session 11, 1983 Annual Conference of Metallurgists-Canadian

Institute of Mining and Metallurgy, Edmonton, Alta, March.

10. NACE T8-16, State-of-the-Art Report-Materials and Fabrication Prac-

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ery Service, Draft Report to T8-16 Members, NACE Fall Committee

Week 1992.

11. Kane, R. D., Whilhelm, S.M., and Oldfield, J.W.: Review of Hydrogen-

Induced Cracking of Steels in Wet H2S Refinery Service, paper pres-

ented at the 1989 International Conference on Interaction of Steels with

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ment With Discontinuities, 1992 ASME Pressure Vessel and Piping

Conference-Workshop on Fitness-for-Service in the Petroleum Indus-

try, New Orleans, June 2426.

F (F32)/1.8 Cin.2.54* E00cm

lbm4.535 924 E01kgpsi6.894 757 E00kPa

*Conversion factor is exact. SPEPF

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