3 2 Pourbaix Diagram

PPOURBAIXOURBAIX DDIAGRAMIAGRAM} Graphical representation of the domains of stability of metal, metal

ions, oxides, hydroxide, etc. in aqueous solution.ions, oxides, hydroxide, etc. in aqueous solution.

Potential (E)

M+2H+

OH-

pH

MMO

H+H2O

1

pH

Pourbaix Diagram (cont.)Pourbaix Diagram (cont.)

} The Nernst Equation allows us to compute lines on the diagram for equilibrium reactions of interest

g ( )g ( )

equilibrium reactions of interest

Electrochemical reactions of pure charge transfer (horizontal lines -since no H+ or OH- dependence - corresponding to potentials of o o O d p d o po d g o po a oequilibria at given concentrations)

Pure acid-base reactions (vertical lines - since no electron transfer and no dependence on potential - corresponding to equilibrium concentrations of H+ (OH-) for given concentrations of species)

Electrochemical reactions involving charge transfer and H+ (OH-) (sloping lines).

2

Pourbaix Diagram (cont.)Pourbaix Diagram (cont.)(1) Pure Charge Transfer Reaction(1) Pure Charge Transfer Reaction

Consider Ni2+ + 2e- = Ni

(1) Pure Charge Transfer Reaction(1) Pure Charge Transfer Reaction

RT3032So, the Nernst equation becomes,

With e0 = -0.25 V, so

)Nilog(F2RT303.2

ee 20 ++=

)Nilog(03.025.0e 20 ++=We usually compute for 4 concentrations 100, 10-2, 10-4, 10-6 M

Ni2+

Ni

3

Partial Pourbaix Diagram forNi2+ + 2e- = Ni

Ni

Pourbaix Diagram (cont.)Pourbaix Diagram (cont.)(2) Pure Acid(2) Pure Acid Base ReactionBase Reaction

Consider Ni2+ + 2H2O = Ni(OH)2 + 2H+

(2) Pure Acid(2) Pure Acid--Base ReactionBase Reaction

There is no charge transfer, so the Pourbaix diagram is a vertical line for a given Ni2+concentration.

We evaluate the equilibrium constant K form the standard free energy change for theWe evaluate the equilibrium constant, K, form the standard free energy change for the reaction.

KlnRTG0 =or

RT3032G

)Klog(0=RT303.2

4

Pourbaix Diagram (cont.)Pourbaix Diagram (cont.)((22) Pure Acid) Pure Acid Base ReactionBase Reaction

calculated from the standard free energy of the reactants and products

((22) Pure Acid) Pure Acid--Base ReactionBase Reaction0G

+= 000 GGG or

G2GG2GKlog

0H

0)OH(Ni

0OH

0Ni 222 ++ +=

+= rrpp GGG

G0 values are tabulated

RT303.2Klog =

G values are tabulated= -11,100 cal/mol= -56,690 cal/mol= -108,300 cal/mol

0Ni2

G +0

OH2G

0)OH(Ni 2

G

and = 0 (convention)

So, at 250C, log K -12

0H

G +

5


For reaction Ni2+ + 2H2O = Ni(OH)2 + 2H+


]H][)OH(Ni[ 2+

Si th ti h H O d ( lid) Ni(OH) h ti iti f 1

]OH][Ni[]H][)OH(Ni[

K2

2

22

++

=

Since the continuous phase H2O and (solid) Ni(OH)2 have activities of 1,

12]Ni[]H[

log 22

=++

or

][

]Nilog[12]Hlog[ 22 ++ +=

]Nilog[5.06pH 2+=

6


Again, we compute for [Ni2+] = 100, 10-2, 10-4 and 10-6 M


Ni Ni(OH)2

The region to the left of a particular line (i.e., more acid) is the region of stability for Ni2+ So, if we are on the equilibrium line at pH 9 for a Ni2+ concentration of 10-6 M, and we drop the pH to 7 (say), the Ni(OH)2 solid in contact with the solution will dissolve to

7

we drop the pH to 7 (say), the Ni(OH) solid in contact with the solution will dissolve to try and make the concentration to 10-2 M.

Pourbaix Diagram (cont.)Pourbaix Diagram (cont.)(3) Reactions Involving Electrons and H(3) Reactions Involving Electrons and H++

Consider Ni + H2O = NiO + 2H+ + 2e-

th N t ti

(3) Reactions Involving Electrons and H(3) Reactions Involving Electrons and H++

]H][NiO[RT3032 20+

the Nernst equation:]OH][Ni[]H][NiO[

logF2RT303.2

EE2

0 +=

pH059.0EE 0 =

Calculate E0 V11.0F2GGG

nFG

E0OH

0Ni

0NiO

00 2 ==

So, E = 0.11-0.059pH

8

Pourbaix Diagram (cont.)Pourbaix Diagram (cont.)(3) Reactions Involving Electrons and H(3) Reactions Involving Electrons and H++

NOTEWe can also depict the Ni H2O reaction as:

(3) Reactions Involving Electrons and H(3) Reactions Involving Electrons and H++

Ni + 2H2O = Ni(OH)2 + 2H+ + 2e-

S th t2

20 ]H][)OH(Ni[lRT303.2

EE+

So that

(as for NiO)

22

20

]OH][Ni[]][)([

logF2

EE +=

pH059.0EE 0 =000

With

E = 0.11 0.059 pH

V11.0F2

G2GG

nFG

E0OH

0Ni

0)OH(Ni

00 22 ==

This is identical with the line for the Ni/NiO equilibrium.i.e., Ni(OH)2 is as likely (thermodynamically) as NiO.

9

Pourbaix Diagram (cont.)Pourbaix Diagram (cont.)g ( )g ( )

10

Pourbaix Diagram (cont.)Pourbaix Diagram (cont.)(4) Stability Lines for Water(4) Stability Lines for Water

} In most environments of interest

(4) Stability Lines for Water(4) Stability Lines for Water

to corrosion, there is not a large concentration of metal ions acting as cathodic reactants.

} The important cathodic reactions involve water.

( ) 2H+ 2 H(a) 2H+ + 2e- = H2

(b) O2 + 4H+ + 4e- = 2H2O

11

Pourbaix Diagram (cont.)Pourbaix Diagram (cont.)((44) Stability Lines for Water) Stability Lines for Water

The Nernst equation gives

((44) Stability Lines for Water) Stability Lines for Water

Line (a) on Pourbaix diagram

For line (b) similarly..

pH059.0e2H/H

=+

22

4O0

OO ]OH[

]H[plog

2059.0

ee 222

++=

With [H2O] = 1

(for diagram)

V23.1e0O2 =

atm1p = (for diagram)

Therefore, line (b) is:

atm1p2O=

pH059.023.1e2O

=

12

Pourbaix Diagram (cont.)Pourbaix Diagram (cont.)(4) Stability Lines for Water(4) Stability Lines for Water(4) Stability Lines for Water(4) Stability Lines for Water

O2 + 4H+ + 4e- = 2H2O

2H+ + 2e- = H2

13

Pourbaix Diagram (cont.)Pourbaix Diagram (cont.)(4) Stability Lines for Water(4) Stability Lines for Water

2H+ + 2e- H2 hydrogen evolution in acids

(4) Stability Lines for Water(4) Stability Lines for Water

2H + 2e H2 hydrogen evolution in acids2H2O + 2e- H2 + 2OH- hydrogen evolution in neutral/bases

(th b t ti i l t ti )(the above two reactions are equivalent reactions)

O2 + 2H2O + 4e- 4OH- oxygen reduction in neutral/basesO2 + 4H+ + 4e- 2H2O oxygen reduction in acidsO2 + 4H + 4e 2H2O oxygen reduction in acids

(the above two reactions are equivalent reactions)

14

Pourbaix Diagram (cont.)Pourbaix Diagram (cont.)g ( )g ( )

15

Pourbaix Diagram for ZincPourbaix Diagram for Zincgg

2 0 Equilibrium forEquilibrium for2.0

1.6

1.2

Equilibrium forZn2+ + 2OH- Zn(OH)2Zn(OH)2 + 2OH

- ZnO22- + 2H2On

t

i

a

l

0.8

0.4Zn(OH)2

stable

E ilib i fZnO22-

P

o

t

e

-0.4

0.0

-0.8

Equilibrium forZn Zn2+ + 2e-

Zn2+ stable

in solution

solid Equilibrium forZn + 2OH- Zn(OH)2 + 2e-

2

stable in

solutionEquilibrium for

7 14-1.6

0.8

-1.2

0

Zn metal stable

qZn + 4OH- ZnO22- + 2H2O + 2e-

16

7 140pH

Pourbaix Diagram for ZincPourbaix Diagram for ZincFor indicating corrosion trends, we can use simplified versions

ggCorrosion possible

with oxygen reduction

2.0

1.6

1 2

reduction

Corrosion ibl i h

Corrosion is possible, but likely to be stifled by solid

i

a

l

0.8

1.2

0.4Zn(OH)2

stable

possible with hydrogen evolution

likely to be stifled by solid corrosion product

s

s

i

v

i

t

y

o

r

r

o

s

i

o

n

Corrosion

P

o

t

e

n

t

i

-0.4

0.0 Zn2+ stable

in solution

stable

solidZnO22-

stable in

solution

Corrosion requires strong oxidising agent

Corrosion is thermodynamically

impossible

P

a

s

C

o

1 6

-0.8

-1.2Zn metal stable

Immunity

17

7 14-1.6

0

Pourbaix Diagram for GoldPourbaix Diagram for Goldgg

2 02.0

1.6

1.2

C

CPassivity

n

t

i

a

l

0.8

0.4Gold metal stable Gold cant corrode with

P

o

t

e

-0.4

0.0

-0.8

Immunityoxygen reduction or hydrogen evolution

7 14-1.6

0.8

-1.2

0

18

7 140

Use of Pourbaix Diagram (Cu)Use of Pourbaix Diagram (Cu)g ( )g ( )Corrosion requires

strong oxidizing agentCorrosion requires

strong oxidizing agent

Corrosion possible with O2 reduction Corrosion possible with O2 reduction with O2 reduction with O2 reduction

Corrosion is possible Corrosion is possible Corrosion is possible but likely to be stifled by solid

corrosion product

Corrosion is possible but likely to be stifled by solid

corrosion product


impossible


impossible

20

Pourbaix Diagram for Ti (I)Pourbaix Diagram for Ti (I)

Ti is thermodynamically reactive, but is nevertheless

g ( )g ( )

reactive, but is nevertheless very corrosion resistant because of a highly resistant passive film that is stable at ll H i idi i t ti lall pHs in oxidizing potentials.

21

Use of Pourbaix DiagramUse of Pourbaix Diagramgg

22

Validity of Pourbaix Diagram Validity of Pourbaix Diagram -- ExampleExampley gy g pp

23

Validity of Pourbaix Diagram Validity of Pourbaix Diagram -- ExampleExampley gy g pp

24

Use of Pourbaix DiagramUse of Pourbaix Diagram(Hot(Hot dipped Aldipped Al Zn coated steel)Zn coated steel)(Hot(Hot--dipped Aldipped Al--Zn coated steel)Zn coated steel)

Widely used for fences and roofs after paintingWidely used for fences and roofs after painting.Act as a sacrificial anodeExpected service life: 20 to 30 yrs.

In some locations, although the metalized coatings were still present, the underlying steel had corroded at defects.

25


} To function as a sacrificial anode Al & Zn must remain

26

} To function as a sacrificial anode, Al & Zn must remain active/unfilmed state.


27

Pourbaix Diagram for Tertiary SystemsPourbaix Diagram for Tertiary Systemsg y yg y y

28

Limitations of Pourbaix DiagramsLimitations of Pourbaix Diagrams

} Tell us what can happen, not necessarily what will happen.

gg

} No information on rate of reaction

C l b l d f l d i l l i f} Can only be plotted for pure metals and simple solutions, not for alloys.

29

ReferencesReferences

} M. Pourbaix, Lectures on Electrochemical Corrosion, Plenum Press, New York, 1973.New York, 1973.

} M. Pourbaix, Atlas of Electrochemical Equilibria in Aqueous Solutions, NACE International, Houston, TX.

30

Homework ProblemsHomework Problems

1) Using the Pourbaix diagram for nickel, Fig. 2.11 (d), give the anodic and cathodic reactions that occur on Ni in water for theanodic and cathodic reactions that occur on Ni in water for the following conditions, assuming activity of 10-6 for all soluble species:a) Deaerated pH 2) pb) Deaerated pH 10c) Aerated pH 2d) Aerated pH 10) p

2) Problems #9 of Chapter 2 in textbook.

31

3 2 Pourbaix Diagram

Documents

Transcript of 3 2 Pourbaix Diagram