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

Effects of Ammonium Thiocyanate on Hydrogen Entry Efficiency into Low Alloy Steel in Acidic Solution
Hiroyuki Fuji1, Taishi Fujishiro2, and Takuya Hara3

Nippon Steel Corporation Kansai R&D Lab., Fuso-Cho Amagasaki, Hyogo, Japan1
Nippon Steel Corporation Steel Research Laboratories, Shintomi Futtsu, Chiba, Japan2
Tohoku University School of Engineering, Aoba Aramaki, Aoba-ku, Sendai, Miyagi, Japan3and adsorbed hydrogen atoms; therefore, they acceleratehydrogen entry into steel. Further, they discussed that when
poison lowers the binding energy of metal and adsorbed hydrogenatoms, the rate of the hydrogen permeation current density J in
poison-added and poison-free environment is expressed as Eq. 1ABSTRACT
Hydrogen embrittlement is one of the biggest issues facing thedevelopment of high-strength steel. In this study, the effects of NH4SCNon hydrogen entry behavior into low alloy steel were investigated fromthe viewpoints of the efficiency of hydrogen entry, the coverage ofadsorbed hydrogen atoms on the steel surface and the activation energy
of hydrogen entry by the electrochemical permeation technique withthermal dynamic analysis. NH4SCN increased hydrogen permeation
current density and hydrogen entry efficiency. From thermal dynamicanalysis, it is supposed that NH4SCN decreased the activation energy ofhydrogen absorption, however it increased that of hydrogen adsorption.

𝐽𝐽′ 𝑘𝑘′−3 𝑘𝑘3 𝑘𝑘2 1/2=
� �𝐽𝐽𝑘𝑘3 𝑘𝑘−3 𝑘𝑘′2

(1)

where k2 and k3 are the rate constants of hydrogen recombination(Tafel reaction) and absorption. Prime and non-prime indicate
poison-added and poison-free environments, respectively. Minusindicates a reverse reaction. On the basis of this model,

KEY WORDS: Hydrogen; Hydrogen embrittlement; Hydrogen
entry efficiency; Hydrogen permeation; Ammonium thiocyanate;poison.

𝛾𝛾𝛥𝛥𝛥𝛥𝑘𝑘′3 = 𝑘𝑘3 exp ��𝑅𝑅𝑅𝑅
𝛽𝛽𝛥𝛥𝛥𝛥𝑘𝑘′2 = 𝑘𝑘2 exp ��
𝑅𝑅𝑅𝑅(1 − 𝛾𝛾)𝛥𝛥𝛥𝛥
𝑘𝑘′−3 = 𝑘𝑘−3 exp ��
𝑅𝑅𝑅𝑅

INTRODUCTION
For developing high-strength steel, hydrogen embrittlement is
one of the biggest issues. In particular, hydrogen entry catalysts
called poison such as thiocyanide ion (SCN−), cyanide ion (CN−),and hydrogen sulfide (H2S) are known to remarkably acceleratehydrogen entry into steel. Especially, ammonium thiocyanate,
NH4SCN, is widely used for the evaluation tests of hydrogen
embrittlement susceptibility, for example, FIP test (Fèdèration
Internationale de la Prècontrainte) for PC (Prestressed Concrete)steel (FIP, 1980), because of its ease to treat and large
acceleration effect of hydrogen entry into steel. The mechanismsthat poisons accelerate hydrogen entry into steel have been
discussed. For example, Ando and Yamakawa (1991) reported
that hydrogen entry acceleration by sulfide, cyanide and arsenic
was caused by the increase of the coverage of adsorbed hydrogenatoms on a metal surface. Berkowitz and Horowitz (1982)
reported that when poison is present, the hydrogen discharge
reaction remains high but the recombination reaction wassuppressed; therefore, the percentage of hydrogen permeation
became very high. Furthermore, Bockris, McBreen and Nanis(1965) reported that poison lowers the binding energy of metal

(2)
(3)
(4)

where ⊿W is the amount of change of the binding energy ofmetal and adsorbed hydrogen atom, β and γ are the transfer
coefficients, R is the gas constant, and T is the temperature. From
Eqs. 1~4, it follows that𝐽𝐽′(1 − 𝛽𝛽)𝛥𝛥𝛥𝛥= exp ��𝐽𝐽𝑅𝑅𝑅𝑅(5)

Thus, because β < 1, the lowering of the binding energy of metaland adsorbed hydrogen atoms results in an increase of thehydrogen permeation current density.
This study focused on the effects of SCN- on the elementary stepsof the hydrogen entry. Effects of NH4SCN on the hydrogen entry
efficiency were investigated using the electrochemical hydrogen
permeation technique.
EXPERIMENTAL PROCEDURES

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