Hydrogen Sulphide Resistanceof Highly-AlloyedAustenitic Stainless SteelsbyC. Wolfe, P-E. Arnvig, W. Wasielewska,Avesta Sheffield AB, Research & Development,SE-774 80 Avesta, Sweden, andR.F.A. Jargelius-Pettersson,Swedish Institute for Metals Research,Drottning Kristinas väg 48, SE- 1 14 28 Stockholm, SwedenAbstractTwo highly-alloyed austenitic stainlesssteels: 654 SMO (UNS S32654, 24Cr22Ni 7.3Mo 0.5N) and 254 SMO(UNS S31254, 20Cr 18Ni 6.2Mo0.2N) have been evaluated inlaboratory environments simulatingsour service. No evidence of crackingwas observed in specimens coldworked up to 80% in NACE testing at25°C. Data at elevated temperaturesand pressures are compared withpublished application limits for 22Crand 25Cr duplex stainless steels. Inthe most aggressive conditionsinvestigated, 1.4 bar H2S at 150°C,no cracking was observed for654 SMO.BackgroundNumerous factors must be taken intoaccount in laboratory testing ofcorrosion resistant alloys ( CRAs) forsour service in oil and gas production.The primary variables which affectcracking propensity are temperature,H2S partial pressure, pH and chlorideconcentration, while the partialpressure of CO2and the presence ofcation species are of secondary im-portance (1). The most widely-usedstandardized test method is NACETM 0177 (2) for which specimens areimmersed in acidified aqueous en-vironments saturated with H2S atambient temperature and pressure.The test environment recommendedfor CRAs is 5%NaCl + 0.5% glacialacetic acid, standardized loadingmethods include tensile, C-ring anddouble cantilever beam specimens.Materials selection recommendationsfor sour service based on suchlaboratory tests and serviceexperience are included in NACEMR-0 175-96 (3).Other relevant testing standardsinclude ISO-7539 (4) which standard-izes the use of various specimengeometries but does not specify anytest environments. The EuropeanFederation of Corrosion have recentlypublished guidelines for materialstesting for H2S service (5). This makesa clear differentiation between sul-phide stress cracking (SSC), a formof hydrogen embrittlement which isgenerally most severe around roomtemperature, and stress corrosioncracking (SCO which is an anodicprocess and which increases inseverity with increasing temperature.The primary recommended tests areconstant load testing at 90% of theyield stress or constant strain testing(4-point bend, C-ring) at the yieldstress with slow strain rate testing asa supplementary test. Some examplesof characteristic environments aregiven, e.g. 1 g/l NaCl pH 3.5 for gasproduction with traces of formationwater or 165 g/l NaCl (100g/lchloride) pH 4.5 for oil productionwith high dissolved solids, but thegeneral principle is that materials beevaluated under the most severeenvironmental and mechanicalconditions that are realisticallyanticipated for the intended serviceand not under standard "worst case"conditions. The general trend is thustowards standard testing methodo-logies but service-oriented environ-ments.In the present work, testing hasbeen carried out both in the standardNACE environment at 25°C and90°C and in an autoclave environ-ment at 150°C. A combination ofuniaxial tensile specimens (constantload) four point bend and U-bendspecimens (constant total strain) hasbeen used.acom2-1997AVESTA SHEFFIELDCORROSION MANAGEMENTAND APPLICATIONENGINEERINGacom No. 2-972ExperimentalprocedureThe compositions of the superauste-nitic alloys investigated are given inTable 1. All the materials were takenfrom standard production heats andwere initially in the solution annealedcondition. In order to investigate theeffect of cold work (and thus theassociated increase in yield strengthand maximum attainable appliedstress), laboratory-scale cold rolling(for plate) or cold drawing (for wire)was employed.Stress corrosion testing was carriedout under three sets of conditions,which are detailed for 654 SMO and254 SMO in Tables 2 and 3 respect-ively. Testing at ambient temperatureand pressure (Series I) was carriedout on four point bend, U-bend andtensile specimens. In the majority ofcases specimens were taken in boththe longitudinal and transversedirections. The environment was thatspecified in NACE TM 0177 i.e. 5wt %NaCl acidified by the addition of0.5wt % glacial acetic acid to give apH of approximately 3. The solutionwas first deaerated by purging withnitrogen and then saturated with H2Sby bubbling the gas through the testsolution throughout the test period. Atthe end of the 720 hour test period,the specimens were removed from theenvironment and examined in a lightoptical microscope for signs of crack-ing or other corrosive attack.Testing was also carried out ona more limited range of specimensin the same environment at 90°C(Series II).The third series of tests was per-formed in a static autoclave environ-ment at 150°C and elevated pressure(30 bar). The specimens were placedin the autoclave, which was closedand purged with nitr。