Proceedings of the Thirty-second (2022) International Ocean and Polar Engineering Conference
Shanghai, China, June 5–10, 2022
Copyright © 2022 by the International Society of Offshore and Polar Engineers (ISOPE)
ISBN 978-1-880653-81-4; ISSN 1098-6189

www.isope.org

Effect of Mill Scale, Microstructure and Environment on Microfissures in Carbon Steels
under Sour Service Conditions
Garrett Wadsworth, Fang Cao, Hyun Jo Jun* and Thirumalai Neeraj*
ExxonMobil Research and Engineering Company,Annandale, NJ, USA

typically measured using either a constant load tensile test or by aconstant load four point bend testing (FPB), usually loaded to 80 to 90%
of specified minimum yield strength (SMYS). Conventionally thematerial performance is evaluated by the presence (fail) or absence(pass) of a SSC crack in the coupons after a 720 h (30 day) exposure ina sour environment (NACE_TM-0316, 2016). However, the industry
standard TM0316 does not provide a clear guidance on the acceptancecriteria when there is an absence of a SSC crack, but there is anobservation of surface damage features such as microfissures or stressgrooves on coupon sruface. In the literature there is limited discussion
on the formation of microfissures/stress grooves (Yamane et al., 1986,Pargeter, 2000, Anderson et al., 2020c, Mendibide and Duret-Thual,2021). Further, there is debate about whether the microfissures should betreated as benign stress assisted corrosion features or defects that canpotentially nucleate SSC cracks with additional time (Pargeter, 2000,Anderson et al., 2020c, Yamane et al., 1986, Anderson et al., 2020b,Crolet, 2016a, Crolet, 2016b, Mendibide and Duret-Thual, 2021).
Recently, there has been discussion in the literature about the potential
effect of local hard zones (LHZ) on the ID surface of linepipe steels on
the integrity of pipelines in severe sour service (Fairchild et al., 2019,Baxter et al., 2018, Anderson et al., 2020b). The presence of LHZ onpipe inner diameter surfaces was thought to lower the SSC resistance(Fairchild et al., 2019, Yue et al., 2020, Anderson et al., 2020b),
however, effect of these surface features on SSC initiation andpropagation needs further study.
In this study, microfissure formation (stress grooving behavior) wasexamined using FPB testing in sour solutions with different pH andpH2S. In addition, the effect of various surface conditions such asmachined vs mill scale vs LHZ was also investigated. Conventionally inthe literature cross-sectional metallography is used to analyze thesesurface damage features (Pargeter, 2000, Anderson et al., 2020c,
Mendibide and Duret-Thual, 2021). In this work, a novel approach using
optical surface profilometry of the full constant stress surface area of theFPB coupons was used to study the morphological characteristics of themicrofissures. This allowed the use of image analysis techniques tostatistically analyze the features. The effect of steel microstructure, millscale, and environment conditions on microfissure formation isdiscussed.

ABSTRACT
Sulfide stress cracking (SSC) is a form of hydrogen embrittlement ofsteels involving corrosion and tensile stress in the presence of water andhydrogen sulfide. Despite extensive study of SSC in the literature, themechanisms of SSC initiation and transition to cracking are not fully
understood. The presence of surface damage features, referred to as
microfissures or stress grooving, in NACE Region 3 suggests thatmicrofissures / stress grooves likely act as the initiation nuclei for SSC.
In this study, stress grooving behavior was examined using 4 point bend
specimens in sour solutions with different pH and pH2S. In addition, theeffect of different surface conditions and microstructures wereinvestigated by comparing machined surface with surface preserved withmill scale which was formed during hot rolling as well as surfaces
containing a local hard zone. The morphological characteristics of thestress grooves were studied by optical surface profilometry andquantified using image analysis techniques. The effect of steel
microstructure, surface and environment conditions on stress grooving
will be discussed.
KEY WORDS: sour service; carbon steels; sulfide stress cracking(SSC); microfissure; stress groove; microstructure; mill scale.
INTRODUCTION
Sulfide stress cracking (SSC) is the cracking of carbon steels involving
corrosion and tensile stress (residual and/or applied) in the presence ofwater and H2S in production fluids in Oil and Gas operations. SSC is aform of hydrogen embrittlement (HE) induced by absorption of atomic
hydrogen that is produced by sour corrosion on the metal surface into thesteel (Berkowitz and Horowitz, 1982, Berkowitz and Heubaum, 1984).
Hydrogen uptake is promoted in the presence of H2S as it prevents
atomic hydrogen from recombining into hydrogen molecule as ahydrogen recombination poison (Berkowitz et al., 1976). The atomic
hydrogen can then diffuse into the steel, reduce ductility, and increase
susceptibility to cracking. SSC in high strength low alloy steel linepipe
and tubular goods is of particular concern to the oil and gas industry inenvironments that co-produce hydrogen sulfide and water with oil andgas. It is well known that SSC is strongly dependent on environmental
conditions such as pH2S, pH, temperature, and material parameters suchas hardness and microstructure. However, despite extensive study ofSSC in the literature, the mechanisms of SSC initiation and transition tocracking are not fully understood. In the industry, SSC resistance isEXPERIMENTAL METHODOLOGY
Materials
API-X65 sour linepipe grade steel was utilized for this study (API,3051