Pourbaix10

8/12/2019 Pourbaix10

1/18

*Dr.M Pourbaix(1966)

Elec

trodepotent

ial,V

pH


2/18

E-pHDiagram: Represent states of lowest freeenergy

E.g: Fe-H2O system:

predicting

environment changes

that will prevent or

reduce corrosive

attacks

large regionlabelled Fe in thepotential-pHdiagram for Fe-H2O systemindicates that ironis inert underthese conditions.


3/18

Use of the Pourbaix

Diagrams:

Predicting the

spontaneous direction of

reactions

Estimating the

composition of corrosion

products

Predictingenvironment changes

that will prevent or

reduce corrosive attacks

Limitation:

1. Tell us what canhappen, notnecessarily whatwill happen

2. Cannot predictcorrosion rates.3. Can only be

plotted for pure

metals andsimple solutions,NOT for alloys.


4/18

The diagramme shows conditions of solution oxidizingpower (potential) and acidity of alkalinity (pH) for thevarious possible phases that are stable in aqueous

electrochemical system The most common reduction reactions are the

reduction reactions of dissolved oxygen and thereduction of water ( H2 evolution)


5/18

Hydrogen-Water

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

2H2O + 2e- = H2 + 2OH

- ( neutral/alkaline)

The Nernst equation gives

For 1 atm. hydrogen gas

E= -0.059 pH vs SHE

pH059.0Hlog2059.0

H

Hlog

2

303.2

2

2

2+

E

F

RTE=E

0

0


6/18

Oxygen-WaterAerated Acid O2+ 4 H

++ 4 e-= 2 H2O Acid

O2+ 2 H2O + 4 e-= 4 OH-Neutral /Alkaline

V23.1

SHEvspH059.0Olog4

059.0

0

2

0

E

EE

E = 1.23 - 0.059pH


7/18

Neutral/Basic O2+ 2 H2O + 4 e-= 4 OH-

4

2

220

4

2

220

OH

OHOlog

4

303.2

OH

OHOln

4

F

RTE

F

RTEE

But [H2O]=1, and log([OH-])=-14+pH

56pH4Olog4

303.2therefore 2

0 F

RTEE


8/18

P

otential(Vvs.

SHE)

pH

Upper: water can beoxidized

and form O2:

O2 can be reduced below line b

Lower: water can be

reducedto form H2

Intermediate: wateris

thermodynamically

st le

0

_a

b02+4H

+

+4e=2H2O

2H++2e=H2

0H-+H+=H2O

O2

H2

1.23_

0 1

6*(a) and (b) are commonly being superimposed on Pourbaix

diagrams.


9/18

How to predict corrosion from PourbaixWhen metal surface is at a potential where one or both

of reduction reactions can occur, the possibility ofcorrosion exists provided the metal disssolutionreaction is thermodynamically favourable.


10/18

Reaction of pure charge transfer.

Only electronsinvolved & NO hydrogen ion

e.g: NiNi2++ 2e

]log[03.025.0

ln

2

2/

2

Ni

Ni

Ni

nF

RT

EENi

o

Ni

So potential depends on the activity

of [Ni2+] not pH.

Select 4 activities: 1, 10-2

, 10-4

, 10-6

M:then

E=(-0.25V), (-0.23+0.03log(10-2)=-

0.31V,

E=(-0.37V), (-0.43V)

-0.25-

0.31-

0.37-

0.43

V

pH


11/18

Reaction involving both electrons& hydrogenione.g: NiO + 2H++2eNi + H2O; E

o=0.11V

pH0.0590.11Ethen]log[HpHsince

]log[0295.0

]][[

]][[log2

059.0

2

22

HEE

HNiO

OHNi

EE

o

o

0.11

NiO

0.05

V

pH

Ni

0


12/18

A rod of Ni is immersed in an aqueous deaerated acid solution with a pH of 1 that

contains 10-4 g-ion/L of Ni2+ions. The system is under 1 atm pressure. Will the nickel

corrode? What will happen at pH more than 8? Refer to the Pourbaix Diagram for nickel.

At the metallic Ni/water interface:Ni2++2eNiENi= -0.25 + 0.026log[Ni

2+]= - 0.37VDeaerated acid solution:2H++2e=H2EH= -0.059 pHEH= -0.06V at pH 1Since ENimore active than EH, sothe electron flow from Ni (-ve) to H2(+ve). Nickel not stable at low pH inwater, so the corrosion occurs.

8

0_

-

0.4_

1

Ni2+

Ni

NiO2

Ni(OH)2

pH

E, V

At pH 6 and 8:

Hydrogen more active than Ni

region of immunity of Ni

No corrosion


13/18


14/18

Pourbaix diagram for Copper

Potential

7 14

2.0

1.6

0.8

1.2

-0.4

0.4

0.0

-1.6

-0.8

-1.2

0

Cu metal stable

Cu2+stablein solution

Cu oxides

stable

CuO2

2-stableinsoln.Will copper

corrode in

acid?

No - hydrogen

evolution only

occurs below the

potential for coppercorrosion

Will copper

corrode in

neutral waters?Usually it will just

passivate, but

corrosion can occurin slightly acid

solutions


15/18

Reversible potential for copper metal oxidationreaction is above line a for all pH. Thus in theabsence of O2, metallic copper is thermodymanicallystable in pure water.


16/18

Pourbaix Diagram for Iron

Potential

7 14

2.0

1.6

0.8

1.2

-0.4

0.4

0.0

-1.6

-0.8-1.2

0

Fe metal stable

Fe3+

Fe oxides

stable

Will iron

corrode in

acid?

Fe2+stable

Yes - there is a

reasonably wide

range of potentialswhere hydrogen

can be evolved and

iron dissolved

Will iron

corrode in

neutral waters?Yes - although iron can

form an oxide in neutral

solution, it tends not toform directly on the

metal, as the potential

is too low.

Will iron corrode

in alkaline

solution?

No - iron forms a solid

oxide at all potentials,

and will passivate


17/18

Corrosion Control Strategies

E = -0.2 V-SHE ,pH =4Increase pH : Water

Treatment

Change potential:

Cathodic Protection

Anodic Protection

Extending passive

region: Alloying : SS

Passivating inhibitor

such as chromate ions


18/18

Pourbaix Diagram for Gold

Potential

7 14

2.0

1.6

0.8

1.2

-0.4

0.4

0.0

-1.6

-0.8-1.2

0

Gold metal stable

Immunity

C

CPassivity

Gold cant corrode

with oxygen reduction

or hydrogen evolution

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Pourbaix10