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 H2S Existence, Surface Hardness and Applied Stress on Corrosion Groove Formationand SSC Initiation in Low Alloy Steel
Junji Shimamura 1, Itaru Samusawa1, Tatsuya Morikawa 2 , Shigeto Yamasaki2and Masaki Tanaka 2
1

Steel Research Laboratory, JFE Steel Corporation, Fukuyama, Japan2Kyushu University, Fukuoka, Japan

ABSTRACTINTRODUCTION

In designing thermo-mechanical controlled processing (TMCP) sourlinepipe, lowering surface hardness is one of the most important points
to prevent sulfide stress cracking (SSC). In previous studies, it was
found that surface hardness should be controlled to be lower value lessthan 270HV0.1 at 0.25mm from surface in 4-point bend (4PB) SSC test
under 0.15 bar H 2S partial pressure condition. Also, it was found thatthe high applied stress activated the anode reaction in 4 point bend(4PB) SSC test. The results indicated that the anodic dissolution
enhanced by applied stress was dominant factor for corrosion groove
formation. However, an initial corrosion groove formation behavior
and a SSC crack initiation behavior are still not clear regarding theeffect of H 2S existence, surface hardness and applied stress which canaffect a microscopic plastic deformation at surface region. In this paper,in order to clarify the effect of those factors on an initial corrosion
groove formation behavior and a SSC crack initiation behavior, 4PB
SSC tests were conducted under 0.15 bar H 2S partial pressure condition
using Grade X65 steel plate with relatively high surface hardness at
0.25mm from surface. Firstly, the effect of H 2S existence wasinvestigated. Two different gas compositions (0.15bar H 2S +0.85 bar
CO 2, 0.15bar N 2 +0.85bar CO 2) were used. Also, the effect of surface
hardness was evaluated using 4PB test specimen taken from threedifferent position (0.25mm, 1.0mm and 7.15mm from surface) inthickness direction. As a result, localized corrosion pit or groove
formation was observed under the H 2S condition, while no local
corrosion pit groove formation was observed under the N 2 condition. It
was confirmed that local corrosion pit groove formation was aphenomenon peculiar to H 2S sour environment. The harder samples
collected on the surface side, the larger the amount of corrosion and thelarger the depth of corrosion pit groove. The mechanism of corrosion
groove formation and SSC crack initiation is also discussed in thispaper.

Low carbon low alloy linepipe steels for sour service need to have asufficient resistance to sulfide stress cracking (SSC) under sour
environment. SSC behavior in TMCP sour linepipe has recently drawn
much attention because of possibility of the formation of local hard zone in the plate surface (Fairchild et al., 2019; Newbury et al., 2019;
Marchebois et al, 2019).
In the previous paper, the effect of surface hardness on SSC wasinvestigated under different H 2S partial pressure conditions, and it was
found that the surface hardness threshold to prevent SSC in the 4PB
SSC test decreased as the H 2S partial pressure increase from 0.07 to 1bar. On the other hand, in the case of 1 bar or higher H 2S partialpressure condition, the surface hardness threshold was approximately
constant as 250HV0.1 (Shimamura et al., 2020). It is considered that
SSC mechanism is different between H 2S partial pressure less than 1
bar and 1 bar or higher. In the environment of 0.15 bar H 2S, theformation of FeS, which is considered to be corrosion -protective (Sunet al., 2008), was small and the amount of corrosion increased. Local
corrosion pits are often formed, and the SSC mechanism is a composite
type of active path corrosion (APC) and hydrogen embrittlement (HE).
On the other hand, in the 1bar H 2S environment, the formation of FeS
was large and the amount of corrosion was small. Since the H 2S partialpressure is high, the amount of hydrogen permeation amount increases.
The SSC mechanism is considered to be HE type mainly. (Shimamuraet al., 2021).
Also, it was found that the high applied stress activated the anodic
reaction in 4PB SSC test in 0.15 bar H 2S condition and plastic strain
enhanced the anodic reaction in round bar and notched round bar
tensile test. The results indicated that the anodic dissolution enhanced
by applied tensile stress and plastic strain was dominant factor forcorrosion groove formation (Izumi, et al., 2020). However, an initialcorrosion groove formation behavior and a SSC crack initiation
behavior in 4PB SSC test in the environment of 0.15 bar H 2S are still
not clear regarding th e effect of H 2S existence, surface hardness andapplied stress level which can affect a microscopic plastic deformation
at surface region. It is known that H 2S suppresses the molecularizationof H atoms formed in the cathode reaction by the catalytic effect of FeS
film and promotes the penetration of H into steel (Kawashima, et al.,
1976; Berkowitz et al., 1984). Dislocations are known to promote

KEY WORDS: linepipe; sour gas; sulfide stress cracking; Thermomechanical controlled processing; Bainite; hardness; corrosion.

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