Potential-pH (Pourbaix) Diagrams · 2021. 1. 26. · Potential/pH (Pourbaix) Diagram Pourbaix diagram for Al Chemical equilibrium between Al 2O 3and Al3+ Dissolved substances µo(J/mol)

Corroding Metals: Potential-pH (Pourbaix) Diagrams

There are there general reactions by which a metal, M, may react anodicallywith water:

1. Oxidation to aqueous cations:

2. Oxidation to metal oxide or hydroxide:

3. Oxidation to aqueous anions:

M = Mn+ + ne−

M + nH2O = M OH( )n + nH + + ne−

M + nH20 = MOn− 2n−l( ) + 2nH + + le−

The above reaction occurs in an acid solution.

An equivalent reaction in neutral or alkaline solutions is:�

2H + + 2e− = H2pHEE

HHHH059.00

// 22-= ++

�

2H2O+ 2e− = H2 + 2OH− pHEEHHHH

059.00// 22

-= ++

Cathode reactions Supporting Corrosion

Hydrogen reduction

2/0 0.059

H HE pH+ = -

E = E0 + 0.059nlog

A⎡⎣ ⎤⎦a

B⎡⎣ ⎤⎦b −mn0.059pH

The above reaction occurs in an acid solution.

An equivalent reaction in neutral or alkaline solutions is:

Cathode reactions Supporting Corrosion

Oxygen reduction (dissolved O2 in the electrolyte)

�

O2 + 4H + + 4e− → 2H2O pHEE OHOOHO 059.00// 2222

-=

2 22 4 4O H O e OH- -+ + ® pHEE OHOOHO 059.00// 2222

-=

2 2/ 1.229 0.059O H OE pH= -

E = E0 + 0.059nlog

A⎡⎣ ⎤⎦a

B⎡⎣ ⎤⎦b −mn0.059pH

Potential/pH (Pourbaix) Diagram

It turns out to be very convenient to represent the results of all these anodic metal oxidation processes and cathodic reduction process on a map-like “phase diagram”.

The map considers the parameters of voltage and solution pH as parameters and shows the thermodynamically possible electrochemical/corrosion reactions that can occur for a particular metal such as Fe, Al, Cd,Zn, … and water.

Potential/pH (Pourbaix) Diagram

2/0 0.059

H HE pH+ = -

oxygen evolutionand acidification

hydrogen evolutionand alkalization

water thermodynamically stable2 2/ 1.229 0.059O H OE pH= -

Water

Potential/pH (Pourbaix) DiagramGeneric diagram for a metal

-2 0 2 4 6 8 10 12 14

1.2

pH

0.8

0.4

-0.4

0.0

-0.8

-1.2

-1.6

-2.0

-2.4

PO

TEN

TIA

L, E

(V)

Corrosion

Corrosion

Passivation

Immunity

Corrosion-soluble ions of the metal are stable

Passivation- oxides are stable

Immunity-reduced form of the metal is stable

Potential/pH (Pourbaix) Diagram

Reaction 1: Metal oxidizes to aqueous cations

Pourbaix diagram for Al

�

M = Mn+ + ne−

Al = Al3+ + 3e−

( )( )

( )

3

33

/

/

0 0.059log 0.059

0.0591.662 log

3

a

bAl Al

Al Al

A me e pHn

e

n

l

B

A

+

++= - +

= + - Independent of pH since no H+ is involved.

Only depends on Al3+

activity

Al3+

Aluminum

Potential/pH (Pourbaix) Diagram

Reaction 2: Metal reacts to metal hydroxide or oxide

Pourbaix diagram for Al

�

M + nH2O = M OH( )n + nH + + ne−

2Al + 3H2O = Al2O3 + 6H + + 6e−

2 3/ 1.55 0.059Al Al OE pH= - -

At higher pH Al2O3 is formed. At lower pH Al2O3 chemicallydissolves to Al3+

Intersection depends depends on Al3+ activity (dashed lines are portions of the reactions with no significance)

E = E0 + 0.059nlog

A⎡⎣ ⎤⎦a

B⎡⎣ ⎤⎦b −mn0.059pH

Al3+

Al2O3

Aluminum

Potential/pH (Pourbaix) DiagramPourbaix diagram for Al

Chemical equilibrium between Al2O3 and Al3+

Dissolvedsubstances

µo (J/mol)

Al3+ -481,160

AlO2- -839,771

H+ 0

Solid substances

Al 0

Al2O3 -1608874

Liquid substances

H2O -237,191

Potential/pH (Pourbaix) Diagram

The chemical reaction rate constant for

Pourbaix diagram for Al

�

2Al3+ + 3H2O = Al2O3 + 6H +

�

K =H +( )6Al3+( )2

=10−11.4

Independent of potential

K = 10−11.4;[Ag3+ ] = 10−6M

pH = 3.9

Al3+ Al2O3

logK = 6log(H + )− 2log(Al3+ ) = −6pH − 2log(Al3+ )pH = −1/ 6logK − 2log(Ag3+ )

Potential/pH (Pourbaix) Diagram

Reaction 3: Metal reacts to form soluble aqueous anions

Pourbaix diagram for Al

-+- ++=+ eHAlOOHAl 342 22

At higher pH, Al2O3 dissolves to AlO2-

2 3 2 22 2Al O H O AlO H- ++ = +

214.6 log[ ]pH AlO-= +

( )2

2/1.262 0.020log 0.079

Al AlOAlO pHE -

-= - + -

( ) ( )2 2 29.22 10K H AlO+ - -= =

E = E0 + 0.059nlog

A⎡⎣ ⎤⎦a

B⎡⎣ ⎤⎦b −mn0.059pH

Al3+

Al2O3

AlO2−