Continuum and atomic scale modeling of stress corrosion cracking and hydrogen-accelerated oxidation
Tetsuo Shoji and Nishith Kumar Das
Frontier Research Initiative, NICHe, Tohoku University, Sendai, JAPAN
Abstract
Stress Corrosion Cracking(SCC) is a synergistic phenomena of material/environment/stress, where several model have been proposed.
One of the rather quantitative model is a deformation-oxidation mechanism where deformation such as slip deformation can break a
protective oxide film on a metal surface and generate a bare surface contact with environment, in general aqueous solution. Bare surface is
generally very active and rapid oxidation can take place at the location of oxide film rupture by deformation. This rapid metal oxidation can
be made by two ways, where one is by metal dissolution such as M  M z+ + z e - and the other is solid state oxidation such as M + x O
MOx. In both ways, metal dissolution provides a crack growth or oxide penetrate into metal and provide a crack growth. These oxidation
process can be formulated in a continuum model where metal dissolution rate as metal dissolution current in electrochemistry and solid
state oxidation rate constant plays a key role in evaluation of crack growth. On the other hand, we need to know how these important
parameters can be derived from physical and/or chemical process where atomistic modeling plays a crucial role in understanding of SCC
and suggests a possible mitigation direction.
In this talk, these continuum and atomic scale modeling developed in our group will be described with an special emphasis on the
combination of nuclear structural materials and Light Water Reactor environments. The most important feature of SCC is a self-sustaining
process and the model developed by combination of electrochemistry and elastic-plastic fracture mechanics can handle this process in the
formulation and numerical calculation can numerically reproduce a typical SCC growth behavior of Stage I and II. In order to help the
fundamental mechanisms of SCC, atomic scale simulation was performed to simulate atomic scale oxidation process by water molecule.
Our recent atomic scale analysis and experimental results showed that the hydrogen in metal has a significant effect on the early stage of
oxidation process. The DFT calculations demonstrate that the fcc hollow and octahedral sites are the most favorable for hydrogen
adsorption on the surface and subsurface, respectively [1]. A clean metal surface has a slight inward relaxation in the topmost layer [1,2],
whereas the metal atoms show outward relaxation (2%) due to interstitial hydrogen [1]. The adsorption energies of oxygen and OH have
decreased to 0.26 and 0.13 eV, respectively, and the metal atomic bond further extended in the range of 1~2% in order to hydrogen
remained interstitial site. Hydrogen changes to a negatively charged in the interstitial site by receiving electron. The QCMD results reveal
that the oxygen penetration depth increases when hydrogen occupy into interstitial octahedral site. The deeply diffused or interstitial
hydrogen receives electrons from the metal [1,3,4]. Additionally, interstitial hydrogen initiates the charge transfer and extends the metal
atomic bond. The localized process weakens the metal-metal bonds and it makes the surface chemically active for further interaction. This
process can help oxygen or other species to diffuse into the structure. Furthermore, a transmission electron microscopy (TEM) study has
shown that the hydrogen charged coupons of alloy 600 at 288 oC formed a thicker oxide film than uncharged coupons [5] and oxidation of
316L stainless steel in PWR at 288 oC was accelerated with hydrogen permeation to the oxide/metal interface. Two times thicker oxide was
measured in hydrogen permeation specimen than in no-hydrogen permeation, In both specimens, hydrogen accumulation was confirmed at
oxide/metal interface and more hydrogen accumulation in hydrogen permeation sample measured by DGOES [6]. Those experimental
findings are consistent with our theoretical results. Finally, both of the theoretical and experimental studies suggest that the subsurface
hydrogen accelerates the early stage of oxidation and its localization, and subsequent SCC initiation.
References
[1] N. K. Das and T. Shoji, Int. J. Hydro. Energy 38 (2013) 1644.
[2] N. K. Das and T. Shoji, Appl. Surf. Sci. 258 (2011) 442.
[3] N. K. Das, K. Suzuki, Y. Takeda, K. Ogawa and T. Shoji, Corrosion Science 50 (2008) 1701.
[4] N. K. Das, K. Suzuki, K. Ogawa and T. Shoji, Corrosion Science 51 (2009) 908.
[5] J. Hou, Q. J. Peng, K. Sakaguchi, Y. Takeda, J. Kuniya and T. Shoji, Corrosion Science 52 (2010) 1098.
[6] T.Shoji, France-Japan Symposium on Advanced Materials for Energy Future, Towards Reliable, Durable and Safe Materials for
Energy Production and Storage, June 30 – July 1, 2014, Conference Hall, Science Council of Japan, Tokyo Japan