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A possible mechanism of copper corrosion in anoxic water

A possible mechanism of copper corrosion in anoxic water. Anatoly B Belonoshko and Anders Rosengren Theoretical physics , KTH. Background. Common belief Thermodynamic databases Electronic structure theory

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A possible mechanism of copper corrosion in anoxic water

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  1. A possiblemechanism of coppercorrosion in anoxic water Anatoly B Belonoshko and Anders Rosengren Theoreticalphysics, KTH

  2. Background • Commonbelief • Thermodynamicdatabases • Electronic structuretheory • Othertheoretical studies, othersurfaces (Ren and Meng, Taylor, Feibelman)

  3. Our calculations • Westudy (100) surface • A supercell, sixlayers of Cu in (001) direction and a vacuumlayer , periodicboundaryconditions. 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

  4. Theninbetweenslabsplace OH and H separatedlaterally • Calculateenergy of adsorbed OH and H, i.e. of the dissociated water molecule. This energy is lowerthan the energy of H2O adsorbedintact. • Thuswefinddissociative adsorption of water on the surface in agreement with Taylor . Recentlyconfirmed by anothercalculation.

  5. Computational cell

  6. Continuous supply of free surface? • A mechanism that continously provides freecoppersurface for water dissociation • Wehaveearliersuggestedonemechanism, nanoparticles, that would provide this surface • Another way to increase this surface is to take grain boundarycorrosionintoaccount. If grain boundariesfacilitate the removal of OH from the surface, the availablesurface for OH adsorption is essentially the surface of all grains in the sample

  7. Clusters • Magic number clusters N=13, 38, 55, 75, … unusuallystable • Cu clusters havebeenstudied by EAM for 2 to 150 atoms. First principles, up to 13 atoms • Weapply first principlesmethods from 2 to 55. Putthem in cubic box with edge 15 Å. • Up-method and Down-method

  8. The Cu cluster of 55 atoms

  9. OH binding to cluster, cluster size + # hydroxyls • Bindingenergy of OH to Cu(100) surface is -2.61 eV. This is higherthan the OH bindingenergy to a reasonablylarge cluster. • Question: Can this gain in binding energy compensate the cost in energy for transferring Cu atoms from the bulk to the cluster? • Wecalculated Cu55(OH)42.

  10. The cluster of 55 Cu and 42 OH

  11. Result • The energy of Cu55 is -166.63 eV • The energy of Cu55(OH)42 is -620.07 eV • The energy of isolatedhydroxyls is -378.78 eV • This gives OH bindingenergy 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 nanoparticulatesrequiresconsiderableenergy and is not relevant.

  12. Diffusion in grain boundaries • Diffusion of O in bulk Cu is negligible • Removal of OH adsorbed on the Cu surface is possible via grain boundariesonly • Grain boundary penetration or intergranular attack • At high temperature a grain boundarymight be approximated by a liquidstructuredue to premelting

  13. 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 self-diffusion), however the radial distribution functionremained non-solid. Formation of quasi-crystalline planes is seen • At 2200 K the structure is liquid and quasi-crystalline planes vanish

  14. Embedding OH in the grain boundary • Twoadjacent Cu atoms wereremoved from the center of the computational cell • One position filled with O the other with H • O and H wereshiftedtowardseachother to form the OH bond. Initial configuration. • Runmoleculardynamics • D=2.25x10-8 (2200 K), 1.04x10-8 (1200 K) and 2.08x10-9 (300 K) m2/s

  15. Discussion • The quantity of emitted hydrogen in the ongoing experiment was 3x10-6 g/cm2 • A typical grain size in the Cu foilwas 10-5 m. Approximate grains with fcccubes with edge 10-5 m. • Assume all surfaces of grains haveadsorbed OH to the same extent as the Cu surface • Grain boundarythickness 2-10 atomicdistances

  16. Order-of-magnitudeestimate • Weobtain 10-6 g/cm2. • The release of hydrogen willcontinue • Some hydrogen willstay in the copper • Calculations show that OH dissociatesimmediately and O and H diffuse independently • Strong bond forms between O and Cu, and H is carriedaway

  17. Evenmore 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 willlead to a lattice expansion process, whichmightgiverise to cracks and evenmorecoppersurfacewill be available

  18. Conclusions • Wehaveinvestigated 2 possiblemechanisms for OH removal from Cu surface • Formation of Cu clusters with OH adsorbed • Diffusion of OH in grain boundaries • Possible formation of nanocrystals of copperoxide. Cracks.

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