CMT555

THERMODYNAMIC OF

CORROSION:

POURBAIX DIAGRAM

STABILITY OF WATER

Water can be converted to its component

elemental gases, H2 and O2 through the

application of an external voltage.

The reduction reaction of water can be written

either as

or, in neutral or alkaline solution as

)(222g

HeH

OHHeOHg

22)(22

These two reactions are equivalent and follow

the same Nernst equation

Assuming H2 gas is 1 atm; log 1/[H+] = pH

22

//

log2

0592.02

2

H

HEE HH

HH

pHEE 22

0592.0

pHEE 0592.0

As potential becomes more positive or noble, water

can be decomposed into its other constituent (i.e.

oxygen).

The equations representing respectively the acidic

form and neutral or basic form of this equilibrium

are written as:

OHeHO 22 244

OHeOHO 442 22

in acid

in neutral or alkali

And again these equivalent equations can be

used to develop a Nernst expression of the

potential in standard conditions of temperature

and oxygen pressure:

Assuming [H2O] = 1 and [O2] = 1atm;

log 1/[H+] = pH

42

2

2// log

4

0592.022

22

HO

OHEE OHOOHO

pHEE 0592.0

So the E vs. pH plots for both processes have

identical slopes and yield the stability diagram

for water shown below:

water can be oxidized and form oxygen

water is thermodynamically stable

water can be reduced to form hydrogen gas

These diagram is divided into three regions.

1. In the upper one, water can be oxidized

and form oxygen (the stability of oxygen).

2. In the lower one, it can be reduced to form

hydrogen gas (the stability of hydrogen).

3. In the intermediate region, water is

thermodynamically stable (the stability of

water).

It is common practice to superimpose these two

lines (a) and (b) on Pourbaix diagrams to mark

the water stability boundaries.

E-PH OR POURBAIX DIAGRAMS These diagrams are graphical representations of

thermodynamic information appropriate to

electrochemical reactions (available for almost

all metals).

The objectives of the construction of the

diagram:

To predict whether or not corrosion can occur

To estimate the composition of the corrosion

product form

To predict environmental changes which will

prevent or reduce corrosion attack

The parameters in Pourbaix diagrams are:

1. pH is plotted on the x-axis

2. Electrode potential (E) vs SHE is plotted on

the y-axis

There are three kinds of region/domain on the

diagram:

1. Region of immunity – in which the metal itself

is the stable species and is immune to corrosion.

2. Region of corrosion – in which the stable

species is a soluble ion and the metal is expected

to corrode.

3. Region of passivity – in which the stable species

is insoluble (e.g. oxide, hydroxide or salt of the

metal) and the metal is resistant to corrosion.

Any point on the diagram will give the

thermodynamically most stable form of that

element at a given potential and pH condition.

Strong oxidizing agents and oxidizing

conditions are found only at the top of Pourbaix

diagram.

Reducing agents and reducing conditions are

found at the bottom of a diagram.

The are three general reactions by which a metal,

M may react anodically in the presence of water:

a) Oxidation to aqueous cations

b) Oxidation to hydroxide/oxide

c) Oxidation to aqueous anions

(These three reactions are used to construct

the Pourbaix diagram)

neMM n

nenHOHMOnHM n2

nenHMOOnHM

n

n 22

SAMPLE OF POURBAIX DIAGRAMS

Iron

Copper

Gold

APPLICATION AND LIMITATION

Its applications include:

1. Formulation of corrosion control methods

(use of cathodic protection, anodic protection,

corrosion inhibitors etc.)

2. Identification of possible corroding states of

the metal-H2O system (regions of immunity,

passivation, corrosion or cracking)

3. Prediction of most likely corrosion products

(Fe2+ or Fe3+ ) for the metal-H2O system.

Some limitations of such diagrams include:

1. No information on corrosion kinetics is

provided by these thermodynamically derived

diagrams.

2. The diagrams are derived for specific

temperature and pressure conditions.

3. The diagrams are derived for selected

concentrations of ionic species (10-6 M).

4. Most diagrams consider pure substances only

– for example the diagram applies to pure

water and pure metal only. Additional

computations must be made if other species

are involved.

5. In area where a Pourbaix diagram shows

oxides to be thermodynamically stable, these

oxides are not necessarily of a protective

(passivating) nature.

Identify all possible chemical and

electrochemical reactions in the given system

Apply Nernst equation to each possible reaction

Determine the relationship between the

potential (E) and the pH of the system

Draw the potential as a function of pH in a chart

[Mn+] = 10-6 M is assumed as borderline for

corrosion and non-corrosion region

The chart is divided into different region

representing different corroding conditions.

Calculation and Construction

of E-pH diagram

POURBAIX DIAGRAM FOR

ALUMINIUM

Al3+

Al2O3

AlO2-

Al

Line 1 Line 2

Line 4

Line 3

Line 5

3.9 8.6

CONSTRUCTION FOR ALUMINIUM

(WATER – ALUMINIUM SYSTEM)

a) Oxidation to Aluminium cation

(dependent on [Al3+], independent of

pH – produce line 1)

VE 66.1

3,10][ 63 nAl

eAlAl 33

Al

Al

nEE

AlAl

3

/log

0592.03

3

/log

3

0592.066.13 AlE

AlAl

b) Oxidation to Aluminium oxide

VE 55.1

0592.0Slope

(dependent of pH – produce line 2)

eHOAlOHAl 6632 322

2

6

32/ log

0592.032 Al

HOAl

nEE OAlAl

HE OAlAl log66

0592.055.1

32/

pHE OAlAl 0592.055.132/

Intersection between line 1 and line 2 → straight line (line 3)

(No electron transfer is required, independent of potential but dependent on concentrations of product & reactant)

The equilibrium pH where line 1 and line 2 intersect may be calculated from the known value of the equilibrium constant of K;

HOAlOHAl 632 322

3

4.11

23

6

10][

Al

HK

63 10 Al

33 log26log2log6log AlpHAlHK

9.3

3

6

6

4.11

3

log

6

log 3

AlK

pH

At above 3.9 : Al2O3 is stable

below 3.9 : Al3+ is stable when potential is sufficiently high

c) Oxidation to Aluminium anion

VE 26.1

pHAlO 079.0log020.026.1 2

eHAlOOHAl 342 22

Al

HAlO

nEE

AlOAl

4

2

/log

0592.0

2

]4[log3

0592.026.1 2/ 2

pHAlOEAlOAl

pHAlOEAlOAl

0592.03

4log

3

0592.026.1 2/ 2

(Dependent on both pH and [AlO2-] – produce

line 4) Slope = -0.079

At some high pH the oxide Al2O3 dissolves as

the aluminate anion (AlO2-).

There is no charge transfer, and the reaction is

independent of potential – produce line 5.

(calculation as similar as shown for line 3)

HAlOOHOAl 22 2232

o At above 8.6 ; AlO2- is stable

below 8.6 ; Al2O3 is stable

When potential is sufficiently high Al2O3 is

stable below pH 8.6 and AlO2- is stable above

pH 8.6.

]log[6.14 2

AlOpH

6

2 10][ AlO 6.8pH

The cathodic reduction reaction during

corrosion in the absence of dissolved oxygen

will be the evolution of hydrogen.

The anodic oxidation reaction:

In acid solutions:

In nearly neutral solutions:

In alkaline solutions:

Conclusion of Water-Aluminium

diagram

eAlAl 33

HOAlOHAl 632 322

eHAlOOHAl 342 22

Corrosion/oxidation is favoured in most

natural conditions.

Corrosion can be made thermodynamically

impossible with cathodic protection by moving

the potential into the range of immunity.

In the intermediate pH range between 4 and 8,

Al2O3 provides a protective film with that the

corrosion rate is low in all electrolytes.

At high and low pH, where the oxide is soluble,

the corrosion rate may be high depending

other ions present.