A Theoretical Study Of Environment Influence
A Theoretical Study of Environment Influence
on a Cluster of the Iron-Chromium Alloy
V. Pokhmurskii, V. Kopylets, S. Kornii
Karpenko Physico-Mechanical Institute
of the National Academy of Sciences of Ukraine,
5, Naukova St., 79601, Lviv, MSP, Ukraine
We have developed a cluster model of iron-chromium alloy (simulation of 20X13 stainless steel). Quantum-chemical calculations allowed to explain the effect of its selective dissolution in acid water solution of sodium chloride.
Elastic deformation rangind from 0.5% to 2.0% leads to essential decrease of activation adsorption energy, ionization potentials and pair interatomic bonds for all particles adsorbed, chloride ions being most active. Iron cations are observed to leave cluster surface with lower activation energies than chromium ions do in an agreement with experimental results. However, surface bond energies for Cr atoms are found to be lower than for Fe atoms. To explain this discrepancy an analysis of the contribution bonding and antibonding orbitals to the formation of summarized strengths has been performed. In the case of strong chemisorption bond formation over the metal atom departing, it loses an essential part of its bonds directed to the neighbours (over 50%). This departing atom is weakened and may leave the surface in very easy way.
Most interesting parameters, obtained by semi-empirical calculations for the cluster are collected in the Table.
We assumed that the process of ion and molecules adsorption is in the following sequence: water ( the highest concentration), hydroxonium ions (the lowest activation energy), chloride ions. Next the removal iron (preferably) and chromiun ions meet two chloride ions and leave the surface as molecules, which can dissociate into cations and anions with negligible activation energies.
The compressive deformations activates selective dissolution of the cluster as well as stretching deformations within 0.5 - 3 % accuracy.
Table. Parameters of Fe-Cr cluster interaction with H3O+ , Cl- ions and water molecules.
Adsorbat
Fe2+
Cr3+
H2O
H3O+
Cl-
Adsorption activation energy,eV
0.365
0.119
0.252
Heat of adsorption, eV
0.486
0.937
1.733
Desorption activation energy,eV
3.214
4.525
0.704
0.261
0.481
Ionization potential, eV
7.375
8.647
4.622
Surface charges, e
-1.187
0.224
-4.276
Bonding energy, eV:
Fe-Fe
6.315
6.512
6.834
Fe*-Fe*
6.200
6.321
6.564
Fe-Fe*
5.712
5.632
5.450
Fe-Cr
5.612
5.522
5.325
Fe-Cr*
5.672
5.477
5.241
Fe*-Cr*
4.964
4.823
4.413
Cr*-Cr*
4.365
4.362
4.131
The reverse adsorption of the FeCl2 and CrCl3 molecules has been shown to be possible if their distance from the surface becomes 2.5 A in a ground state. Note that the departing molecules are in the excited state.
The results obtained may serve as a first step in the explanation of the alloy selective dissolution mechanism at the atomic level during the studies of metal and alloy susceptibility towards corrosive mechanical destruction (Pokhmurskii et al. 1998).
Reference
1. Pokhmurskii V.I., Kopylets V.I., Kornii S.A. (1998) Fiz.-Khim. Mekh. Mater: 34.
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