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 intometal 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 sitesare 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], whereasthe metal atoms show outward relaxation (2%) due to interstitial hydrogen [1]. The adsorptionenergies of oxygen and OH have decreased to 0.26 and 0.13 eV, respectively, and the metalatomic bond further extended in the range of 1~2% in order to hydrogen remained interstitialsite. Hydrogen changes to a negatively charged in the interstitial site by receivingelectron. The QCMD results reveal that the oxygen penetration depth increases whenhydrogen occupy into interstitial octahedral site. The deeply diffused or interstitialhydrogen receives electrons from the metal [1,3,4]. Additionally, interstitial hydrogen initiates thecharge transfer and extends the metal atomic bond. The localized process weakens themetal-metal bonds and it makes the surface chemically active for further interaction. Thisprocess can help oxygen or other species to diffuse into the structure. Furthermore, a transmission electron microscopy (TEM) study has shownthat the hydrogen charged coupons of alloy 600 at 288 oCformed 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 thesubsurface 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