POURBAIX DIAGRAM
Graphical representation representation of the domains of stability of metal, metal ions oxides h droxide etc. in a ueous solution.
Potential (E)
H+
OH-
M+2 M H2O
H+
MO
Po ur baix Dia ra m co n t.
The Nernst Equation allows us to compute lines on the diagram for for e uilibrium rea eact ctio ions ns of in inte tere rest st… … Electrochemical reactions of pure charge transfer (horizontal lines • Electrochemical since no H+ or OH- de endence - corres ondin to otentials of equilibria at given concentrations) concentrations) • Pure acid-base reactions (vertica (verticall lines - since no no electron electron tran transfer sfer and
no dependen dependence ce on potential potential - correspo corresponding nding to equilibr equilibrium ium concentrations of H+ (OH-) for given concentrations concentrations of species) Electrochemical mical reactions involving charge transfer and H+ (OH-) • Electroche (sloping lines).
Pourbaix Diagram Diag ram (cont.) (cont .) Consider Ni2+ + 2e- = Ni So, the Nernst equation becomes, With e0 = -0.25 V, so…
e = e 0 +
. 2F
log( Ni 2 + )
e 0 = −0 .25 + 0 .03 log( Ni 2 + )
We usually compute for 4 concentrations 10 0, 10-2, 10-4, 10-6 M
Ni2+
Partial Pourbaix Diagram for Ni2+ + 2e- = Ni
Pourbaix Diagram (cont.) Consider Ni2+ + 2H2O = Ni(OH)2 + 2H+ There is no charge transfer, so the Pourbaix diagram is a vertical line for a given Ni 2+ concentration. , , reaction. ΔG
0
= −RT ln K
or
log( K ) = −
ΔG
.
0
Pourbaix Diagram (cont.) ΔG
0
calculated from the standard free energy of the reactants and products… 0
=
p
0 p
r
0 r
or 0 0 0 0 + 2GH 2O − GNi( OH )2 − 2GH GNi 2 +
+
2.303 RT 0
… 0 GNi 2 = GH0 2O = 0 GNi ( OH)2 = GH0 = +
and So, at 250C,
log K
+
-12
≈
-11,100 cal/mol -56,690 cal/mol -108,300 cal/mol 0 (convention)
Pourbaix Diagram (cont.) For reaction
Ni2+ + 2H2O = Ni(OH)2 + 2H+
K =
nce
e con nuous p ase
2
an
2
[ Ni 2 + ][ H 2O ] so
2
ave ac v es o
[ H + ] 2 = −12 log 2+ Ni or
log[ H + ] 2 = −12 + log[ Ni 2 + ] pH = 6 − 0 .5 log[ Ni 2 + ]
,
Pourbaix Diagram (cont.) 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 Ni 2+ concentration of 10 -6 M, and 2
try and make the concentration to 10 -2 M.
Pourbaix Diagram (cont.) Consider…
Ni + H2O = NiO + 2H+ + 2e-
e erns equa on:
+
.
0
E = E +
2F
log
2
[ Ni ][ H 2O ]
E = E 0 − 0 .059 pH
Calculate E0 So,
0
E = −
ΔG
0
nF
=
0 0 0 GNiO − GNi − GH 2O
E = 0.11-0.059pH
2F
≅ 0 .11V
Pourbaix Diagram (cont.) NOTE
We can also depict the Ni – H2O reaction as: Ni + 2H2O = Ni(OH)2 + 2H+ + 2eo
a
=
2.303RT Ni OH H + og + 2F [ Ni ][ H 2O ] 2
E = E 0 − 0 .059 pH
With
0
E = −
ΔG
0
nF
=
GNi( OH )2 − GNi − 2GH 2O 2F
E = 0.11 – 0.059 pH This is identical with the line for the Ni/NiO equilibrium. i.e., Ni(OH)2 is as likely (thermodynamically) as NiO.
2
(as for NiO) ≅ 0 .11V
Pourbaix Dia ram cont.
Pourbaix Diagram (cont.)
In most environments of interest to corrosion, there is not a large concentration of metal ions acting as cathodic reactants. The important cathodic reactions involve water. a
+ e- =
2
(b) O2 + 4H+ + 4e- = 2H2O
Pourbaix Diagram (cont.) The Nernst equation gives… Line (a) on Pourbaix diagram…
+
0 O2
eO2 = e
For line (b) similarly.. With
eH
/ H 2
= −0 .059 pH
pO2 [ H + ] 4 0 .059 + log 2 [ H 2O ] 2
eO0 2 = 1.23V
[H2O] = 1 O2
Therefore, line (b) is:
=
eO2 = 1.23 − 0 .059 H
Pourbaix Diagram (cont.)
O2 + 4H+ + 4e- = 2H2O
2H+ + 2e- = H2
Pourbaix Diagram (cont.)
+
-
2
2H2O + 2e- → H2 + 2OH-
hydrogen evolution in neutral/bases
e a ove wo reac ons are equ va en reac ons
O2 + 2H2O + 4e- → 4OH+ + +
oxygen reduction in neutral/bases
(the above two reactions are equivalent reactions)
Pourbaix Dia ram cont.
Pourbaix Dia ram for Zinc for . Equilibrium Zn(OH)2 + 2OH- ⇔ ZnO22- + 2H2O 1.6
Zn2+ + 2OH- ⇔ Zn(OH)2
1.2 0.8 l a i t n e t o P
Zn(OH)2
0.4
stable
ZnO
2-
qu r um or solid Zn2+ stable - ⇔ stable in Zn(OH) + 2eEquilibrium for Zn + 2OH 2 2+ + 2e-0.4 ⇔ Zn Zn in solution solution E- uilibrium for . Zn + 4OH- ⇔ ZnO22- + 2H2O + 2e-1.2 Zn metal stable -1.6 0.0
pH
Pourbaix Dia ram for Zinc Corrosion possible with oxygen
For indicating corrosion trends, we can use simplified versions…
2.0 1.6
Corrosion is possible, but Corrosion likel to be stifled b solid possi e wit corrosion product n o hydrogen y i t s i o evolution Zn(OH) v 2 r i Corrosion r s
. 0.8 l a i t n e t o P
0.4 0.0
Zn2+
-0.4
s a P
o 2ZnOC 2
Corrosion requires strong Corrosion is solid stable oxidising agent stable in thermodynamically impossible solution
in solution
-0.8 -1.2
Immunity Zn metal stable
- . 0
7
14
Pourbaix Dia ram for Gold . 1.6
C
Passivity C
1.2 0.8 l a i t n e t o P
0.4 0.0 -0.4 - . -1.2 -1.6
Gold metal stable Immunity
Gold can’t corrode with oxygen reduction or hydrogen evolution
• A
Use of Pourbaix Dia ram Cu Corrosion requires strong oxidizing agent
Corrosion possible
Corrosion is ossible but likely to be stifled by solid corrosion product
Corrosion is thermodynamically impossible
Pourbaix Dia ram for Ti I Ti is thermodynamically very corrosion resistant because of a highly resistant passive film that is stable at a p s n ox z ng po en a s.
Use of Pourbaix Dia ram
Validit of Pourbaix Dia ram - Exam le
Validit of Pourbaix Dia ram - Exam le
Use of Pourbaix Diagram -
.
Act as a sacrificial anode Expected service life: 20 to 30 yrs. In some locations, although the metalized coatings were still present, the underlying steel had corroded at defects.
Use of Pourbaix Diagram -
-
, active/unfilmed state.
Use of Pourbaix Diagram -
-
Pourbaix Dia ram for Tertiar S stems
Limitations of Pourbaix Dia rams
Tell us what can happen, not necessarily what will happen.
No information on rate of reaction
an on y e p otte alloys.
or pure meta s an simp e so utions, not or
References
M. Pourbaix, Lectures on Electrochemical Corrosion, Plenum Press, New York 1973. M. Pourbaix, Atlas of Electrochemical Equilibria in Aqueous Solutions, NACE International, Houston, TX.
