A possible mechanism of - PowerPoint Presentation

Slide1

A possible mechanism of copper corrosion in anoxic water

Anatoly B Belonoshko and Anders Rosengren

Theoretical

physics

, KTH

Slide2

BackgroundCommon beliefThermodynamic databases

Electronic

structure

theory

Other

theoretical

studies,

other

surfaces

(Ren and

Meng

, Taylor,

Feibelman

)

Slide3

Our calculationsWe 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 Å

3

Surface

energy

1.388 J/m

2

,

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

Slide4

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 H

2

O

adsorbed

intact

.

Thus

we

find

dissociative

adsorption of water on the

surface

in

agreement

with Taylor .

Recently

confirmed

by

another

calculation

.

Slide5

Computational cell

Slide6

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

Slide7

ClustersMagic 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

Slide8

The Cu cluster of 55 atoms

Slide9

Slide10

OH binding to cluster, cluster size + # hydroxylsBinding 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

Cu

55

(OH)

42

.

Slide11

The cluster of 55 Cu and 42 OH

Slide12

ResultThe energy of Cu55 is -166.63 eV The energy

of Cu

55

(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.

Slide13

Diffusion in grain boundariesDiffusion of O in bulk Cu is negligibleRemoval 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

Slide14

Modeling the grain boundaryHeat solid Cu to 4000 KAnneal the liquid to 300K, 1200 K and 2200K

At 300 K and 1200 K Cu is solid (no

self-diffusion

),

however

the radial distribution

function

remained

non-solid. Formation of

quasi-crystalline

planes is

seen

At 2200 K the

structure

is

liquid

and

quasi-crystalline

planes

vanish

Slide15

Slide16

Embedding OH in the grain boundaryTwo 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) m

2

/s

Slide17

Slide18

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Slide22

DiscussionThe quantity of emitted hydrogen in the ongoing experiment

was

3x10

-6

g/cm

2

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

Slide23

Order-of-magnitude estimateWe obtain 10-6

g/cm

2

.

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

Slide24

Even more hydrogen is producedCopper oxide will be formed inside the crystal, probably as nanocrystalsHydrogen 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

Slide25

ConclusionsWe 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.