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−
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