A possible mechanism of copper
corrosion in anoxic water
Anatoly B Belonoshko and Anders Rosengren
Theoretical physics, KTH
Background
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Common belief
Thermodynamic databases
Electronic structure theory
Other theoretical studies, other surfaces
(Ren and Meng, Taylor, Feibelman)
Our calculations
• We study (100) surface
• A supercell, six layers of Cu in (001) direction
and a vacuum layer , periodic boundary
conditions. The size 10.905x10.905x21.810 Å3
• Surface energy 1.388 J/m2 , exp 1.83 for (111)
• Adsorption energy of a water molecule 0.22
eV, same as obtained by Tang and Chen 2007
• OH adsorption energy in excellent agreement
with Nørskov et al 2007
• Then inbetween slabs place OH and H
separated laterally
• Calculate energy of adsorbed OH and H, i.e. of
the dissociated water molecule. This energy is
lower than the energy of H2O adsorbed intact.
• Thus we find dissociative adsorption of water
on the surface in agreement with Taylor .
Recently confirmed by another calculation.
Computational cell
Continuous supply of free surface?
• A mechanism that continously provides free
copper surface for water dissociation
• We have earlier suggested one mechanism,
nanoparticles, that would provide this surface
• Another way to increase this surface is to take
grain boundary corrosion into account. If grain
boundaries facilitate the removal of OH from the
surface, the available surface for OH adsorption is
essentially the surface of all grains in the sample
Clusters
• Magic number clusters N=13, 38, 55, 75, …
unusually stable
• Cu clusters have been studied by EAM for 2 to
150 atoms. First principles, up to 13 atoms
• We apply first principles methods from 2 to
55. Put them in cubic box with edge 15 Å.
• Up-method and Down-method
The Cu cluster of 55 atoms
• OH binding to cluster, cluster size + # hydroxyls
• Binding energy of OH to Cu(100) surface is 2.61 eV. This is higher than the OH binding
energy to a reasonably large cluster.
• Question: Can this gain in binding energy
compensate the cost in energy for transferring
Cu atoms from the bulk to the cluster?
• We calculated Cu55(OH)42.
The cluster of 55 Cu and 42 OH
Result
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The energy of Cu55 is -166.63 eV
The energy of Cu55(OH)42 is -620.07 eV
The energy of isolated hydroxyls is -378.78 eV
This gives OH binding energy to cluster -3.21 eV
But transfer of 55 Cu atoms from the bulk and 42
OH from the surface is larger by 9.89 eV
• Conclusion: Formation of nanoparticulates
requires considerable energy and is not relevant.
Diffusion in grain boundaries
• Diffusion of O in bulk Cu is negligible
• Removal of OH adsorbed on the Cu surface is
possible via grain boundaries only
• Grain boundary penetration or intergranular
attack
• At high temperature a grain boundary might
be approximated by a liquid structure due to
premelting
Modeling the grain boundary
• Heat solid Cu to 4000 K
• Anneal the liquid to 300K, 1200 K and 2200K
• At 300 K and 1200 K Cu is solid (no selfdiffusion), however the radial distribution
function remained non-solid. Formation of
quasi-crystalline planes is seen
• At 2200 K the structure is liquid and quasicrystalline planes vanish
Embedding OH in the grain boundary
• Two adjacent Cu atoms were removed from
the center of the computational cell
• One position filled with O the other with H
• O and H were shifted towards each other to
form the OH bond. Initial configuration.
• Run molecular dynamics
• D=2.25x10-8 (2200 K), 1.04x10-8 (1200 K) and
2.08x10-9 (300 K) m2/s
Discussion
• The quantity of emitted hydrogen in the
ongoing experiment was 3x10-6 g/cm2
• A typical grain size in the Cu foil was 10-5 m.
Approximate grains with fcc cubes with edge
10-5 m.
• Assume all surfaces of grains have adsorbed
OH to the same extent as the Cu surface
• Grain boundary thickness 2-10 atomic
distances
Order-of-magnitude estimate
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We obtain 10-6 g/cm2.
The release of hydrogen will continue
Some hydrogen will stay in the copper
Calculations show that OH dissociates
immediately and O and H diffuse
independently
• Strong bond forms between O and Cu, and H
is carried away
• Even more hydrogen is produced
• Copper oxide will be formed inside the crystal,
probably as nanocrystals
• Hydrogen saturation leads to de-cohesion – as
observed in experiments
• Oxidation will lead to a lattice expansion process,
which might give rise to cracks and even more copper
surface will be available
Conclusions
• We have investigated 2 possible mechanisms
for OH removal from Cu surface
• Formation of Cu clusters with OH adsorbed
• Diffusion of OH in grain boundaries
• Possible formation of nanocrystals of copper
oxide. Cracks.