Boubakar Diawara

Three-dimensional modelling of selective dissolution and passivation of iron–chromium alloys

A three-dimensional model has been developed for modelling the selective dissolution and passivation of alloys. The model has been used to simulate the passivation of iron–chromium alloys. The real structure of the alloy is taken into account (bcc in the present case), as well as the structure of the initial surface. The passivation is modelled in considering the formation of “oxide” nuclei, resulting from the presence of local chromium-rich clusters. During the dynamic evolution of the model, based on the Monte Carlo method, surface diffusion and dissolution of atoms occur according to probabilities dependent on the nature of the atom (Cr or Fe) and on its chemical environment. The conditions of simulation can be changed through a set of parameters defining the rules for surface diffusion, selective dissolution and number of Cr atoms in the Cr clusters required to initiate locally the passivation. The effects of these parameters on the simulation have been tested for an alloy containing 22 at.% Cr and compared with experimental data. The results show that the diffusion of Fe has little influence on the course of passivation while the diffusion of Cr has a marked effect. When the number of surface chromium atoms required to form a nucleus of passive film increases, the passivation becomes less rapid, with a marked effect on the composition of the passivated layer. The extent of the chromium enrichment in the passivated surface obtained in the model for the initial stages of passivation is not as high as the one measured experimentally in the stationary state of passivity. Other simulations have then been performed with various chromium contents in the alloy. The results show the existence of a transition, which is not sharp but progressive, between alloys that cannot be passivated to alloys that are passivated.

NEURAL NETWORKS PREDICTION OF PARTITION COEFFICIENTS

Abstract A comparison has been made of the relative ability of neural networks and regression analysis to estimate the octanol—water partition coefficients (log P ) of organic compounds, with a recently proposed model based on geometric and semi-empirical AM1 parameters. The predictive power of the model is estimated by cross-validation. The regression analysis requires the use of a 17-parameter model including higher powers of a 8-parameter model, giving a standard error of prediction (SEP) of 0.371. The neural network approach gives comparable prediction (SEP = 0.379), even with the reduced 8-parameter model. With a 13-parameter model obtained by the addition of five new independent parameters, neural networks are found to give still better results (SEP = 0.300). The variability of the prediction made by neural networks has been related to the leverage of the compounds in the descriptor space of the multilinear model.

A DFT-D study of hydrogen adsorption on functionalized graphene

In this paper, we use density functional theory with dispersion correction functional (DFT-D) as implemented in the Vienna ab initio simulation package in order to investigate hydrogen adsorption on graphene (GH) and fluorographene (GF). The adsorption sites at different surface coverage rates were studied to determine the most stable configurations. The comparison between the results obtained using standard pure DFT functionals and dispersion corrected ones highlight the role of the dispersion effect in the adsorption energies and the orientation of the molecules relative to the surface. The coverage rate is found to increase up to 75% on the two sides, making these nanoporous materials promising candidates for hydrogen storage. Electronic properties such as density of states and band structures were calculated on both GH and GF systems. It is observed that after H2 adsorption the band gap of GH is only slightly modified, whereas the opposite trend is observed on GF.

Energy ordering of grain boundaries in Cr2O3: insights from theory

The grain boundaries (GBs) of corundum Cr2O3 are known to play an important role in the diffusion of ions within the oxide, which is an important phenomenon for the corrosion of stainless steels. The extent of the growth of oxide layers in stainless steel depends upon which interfaces are preferred within Cr2O3. Therefore, we have constructed four different grain boundary planes (rhombohedral, basal, prismatic and pyramidal) and their various associated interface symmetries known in literature for corundum Al2O3. Their structural, electronic, and energetic properties are investigated theoretically with periodic boundary conditions using the DFT + U approach. We find that the prismatic screw GB with a Cr–O plane interface is the energetically preferred GB with the rhombohedral GB with screw symmetry and Cr vacancy termination being the second energetically preferred GB. The increase of the number of in-plane Cr atoms at the interface of prismatic GB enhances the stability, which is also evident in the electronic density of states.

First-principles investigation of methanethiol adsorption and dissociation mechanisms on the high-Miller-index vicinal surface Cu(4 1 0)

In this work, we present detailed investigations of methanethiol adsorption on a Cu(4 1 0) surface within the framework of the self-consistent first-principles calculations as implemented in the Vienna ab initio simulation package (VASP). In particular, the adsorption sites, the surface coverage rate and electronic properties have been determined and compared to experimental values. The results indicate that the favorable adsorption site in the case of low coverage rate is a bridge on the step followed by the hollow site on the terrace. The adsorption significantly affects the outermost layer of the surface mainly for a higher coverage rate in a (2 × 2) supercell. The nature of the chemisorption process on the surface is analyzed by means of the density of states which, combined with charge density difference and atomic charge calculations, confirms the ionic character of the S-Cu bond. The specific effect of the presence of steps is highlighted by comparing the adsorption on the (1 0 0) terrace to the adsorption on the extended Cu(1 0 0) surface. Compared to the flat Cu(1 0 0), it is found here that while the stability is almost the same at p(2 × 2) coverage, the CH3S/Cu(4 1 0) becomes more stable than CH3S/Cu(1 0 0) at c(2 × 2) coverage with 0.30 eV per molecule. The mechanism of methanethiol dissociation is explored by the nudged elastic band method and demonstrates that the most favorable path is dissociation followed by migration of hydrogen from the step to its most stable position (hollow on the terrace) with energy barriers less than 0.5 eV.

Atomistic simulation of the passivation of iron-chromium alloys using calculated local diffusion activation barriers

A 3D model has been developed to model the selective dissolution and passivation of alloys, and apply to simulate the passivation of iron-chromium alloys. The real structure of the alloy is taken into account. The passivation is modelled by considering the formation of “oxide” nuclei, resulting from the presence on the surface of local chromium-rich clusters. The dynamic evolution is based on the Kinetic Monte Carlo (KMC) method allowing us to take into account realistic kinetic evolution of the system, with simulation time related to the real time. Using the Modified Embedded Atom Method (MEAM), we have calculated the activation energies for the dissolution and the surface diffusion steps. The calculated surface diffusion probabilities are found to be in agreement with the empirical values used previously. In particular they confirm that chromium preferentially diffuses toward the chromium clusters on the surface while iron atoms show no preferential diffusion. The simulation leads to kinetics of passivation with a KMC time of the order of a few seconds, corresponding to the initial stage of passivation. The simulation exhibits the qualitative evolution of the corroded/passivated surface (Cr content, surface roughness). For a series of Fe-xCr alloys (x in the range 15-22%), the results of the simulation confirm our previous finding that the transition from incomplete or no passivation to complete passivation is continuous.

Prediction of the extraction energy of an atom from the surface of Fe–Cr alloys using topological descriptors

Abstract A 3D model for simulating the passivation of iron–chromium alloys has been developed previously. This paper describes the attempts to estimate the probabilities of dissolution used in the model from the energy of extraction of Fe or Cr atoms according to their chemical environment. Quantum chemistry calculations have been performed to estimate the energy of extraction of Fe or Cr from the cluster for various topological environments of the atom to be extracted. Using the obtained results, a simple multilinear correlation between the energy of extraction of an atom and its environment has been developed. Such a relation will allow us to do more rapid calculations of the energy of extraction for any topology, which can be included in the simulation. The correlation used topological descriptors describing, in a relevant way, the environment of an atom to be extracted. At first, we tested the descriptors connected to the location of the extracted atom in relation with its neighbors. Then, we took into account the parameters describing only the distribution of the neighbors among them, which leads to the autocorrelation function of the cluster. Finally, we combined these two types of descriptors.

Stress Concentration in the Bulk Cr2O3: Effects of Temperature and Point Defects

Modeling the growth and failure of passive oxide films formed on stainless steels is of general interest for the use of stainless steel as structural material and of special interest in the context of life time extension of light water reactors in nuclear power plants. Using the approach, a theoretical investigation on the resistance to failure of the chromium-rich inner oxide layer formed at the surface of chromium-containing austenitic alloys (stainless steel and nickel based alloys) has been performed. The investigations were done for periodic bulk models. The data at the atomic scale were extrapolated by using the Universal Binding Energy Relationships (UBERs) model in order to estimate the mechanical behavior of a 10 μm thick oxide scale. The calculated stress values are in good agreement with experiments. Tensile stress for the bulk chromia was observed. The effects of temperature and structural defects on cracking were investigated. The possibility of cracking intensifies at high temperature compared to 0 K investigations. Higher susceptibility to cracking was observed in presence of defects compared to nondefective oxide, in agreement with experimental observation.