Lecture 6: Construction of Eh – pH Diagrams (Fe – H2O – O2 Diagram)
NPTEL Web Course
Lecture 6
Construction of Eh – pH Diagrams (Fe- H2O – O2
Diagram)
Keywords: Iron Diagram, Corrosion Regions, Advantages and Limitations.
Construction of an Eh – pH diagram based on thermodynamic data is illustrated
with respect to Fe-H2O – O2 system. Seven major reactions which are
thermodynamically feasible are illustrated along with calculations leading to
simplified Eh – pH relationships which are plotted on the electrochemical
equilibrium diagram. The diagram can be drawn for a given concentration of the
metal ion species. The lines will shift as a function of varying concentrations.
Assume Fe++, Fe+++ ion concentrations (activity), aH2O = 1, pO2 = 1, pH2 = 1.
Room temp 25oC.
Eh – pH diagrams drawn for specific ion activities and partial pressures of
gases at room temperature.
1. Fe = Fe++ + 2e
E0 = - 0.44V
(Reaction dependent only on Eh, independent of pH)
Eh = - 0.44 +
log [Fe++]
For aFe++ = 1, Eh= - 0.44V
2. Fe++ + 2H2O = Fe(OH)2 + 2H+
(Reaction dependent only on pH, independent of Eh)
reaction =
f(FeOH) 2
+2
F0 f(H+) - F0f(Fe++) - 2
-115.57 + 2(0) – (-20.30) – 2(-56.69) = 18.11 (k.cal)
= - RT lnK
= - 1.364 log K
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Course Title: Advances in Corrosion Engineering
Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore
Lecture 6: Construction of Eh – pH Diagrams (Fe – H2O – O2 Diagram)
log K =
H
log
Fe
NPTEL Web Course
= - 13.28
2
= - 13.28
2pH + log [Fe++] = 13.28
Log [Fe++] = 13.28 -2pH
pH = 6.64
3. Fe++ = Fe+++ + e
E0 = + 0.771V
(Eh dependent, but independent of pH)
Eh = 0.771 + 0.059 log
Eh = 0.771 (at unit activity)
4. Fe +++ + 3H2O = Fe (OH)3 + 3H+
(reaction dependent only on pH)
Log [Fe+++] = 4.81 – 3pH
pH = 1.6
5. Fe + 2H2O = Fe(OH)2 + 2H+ + 2e
(dependent both on Eh and pH)
First, estimate E0 for this reaction from reaction free energy values.
E0 = Eh = - 0.05 +
= -0.05V
log [H+]2
Eh = - 0.05 – 0.059 pH
6. Fe(OH)2 + H2O = Fe (OH)3 + H+ + e
Eh = 0.27 – 0.059 pH
7. Fe++ + 3H2O = Fe (OH)3 + 3H+ + e
Eh = 1.057 – 0.177pH – 0.059 log [Fe++]
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Course Title: Advances in Corrosion Engineering
Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore
Lecture 6: Construction of Eh – pH Diagrams (Fe – H2O – O2 Diagram)
NPTEL Web Course
The above Eh – pH relationships as above are plotted on a graph one by one as
indicated below to yield the final Eh – pH diagram depicting all the seven
reactions with respect to stability regions for the various cations and precipitated
hydroxide products (Fig. 6.1).
Fig. 6.1 Reaction – wise plotting of Eh – pH relationships
Domains of immunity, corrosion and passivation
From the Eh – pH diagram, it can be seen that when Fe++ , and Fe+++ are stable,
the metal (Fe) is in the dissolved state, corresponding to regions of metallic
corrosion.
At more active potentials, iron is thermodynamically stable and
immune to corrosion.
In the regions where corrosion products such as Fe (OH)2 and Fe(OH)3 (or Fe3O4
and Fe2O3) are stable, the metal is oxidized with a surface film which can protect
it from further corrosion (passivity). Corrosion behaviour of the metal (Fe) in an
aqueous aerated solution can then be represented as a corrosion diagram.
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Course Title: Advances in Corrosion Engineering
Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore
Lecture 6: Construction of Eh – pH Diagrams (Fe – H2O – O2 Diagram)
NPTEL Web Course
Corrosion diagram for the Fe-H2O – O2 system is given below (Fig. 6.2).
Fig. 6.2 Corrosion diagram for iron
Regions of corrosion for iron (steels) are not only confined to the acidic region,
but also exist in the high alkaline region (beyond a pH of about 12) where
dissolution of iron as HFeO2- species can occur. Regions of immunity (where
metallic iron is stable thermodynamically) and passivation (stability phases for
iron oxides which form a protective passive layer) are shown.
Advantages and limitations of Eh – pH diagrams.
Electrochemical equilibrium diagram shows conditions of solution oxidizing
power (potential) and acidity or alkalinity (pH) at room temperature for a known
activity of dissolved metal species. Stability regions for various solid (elemental
form and precipitated compounds) and ionized forms are demarcated.
This
diagram has applications in several disciplines such as:
Corrosion prediction and protection
Extractive metallurgy
Geology and geochemistry
Geomicrobiology
Fuel cells
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Course Title: Advances in Corrosion Engineering
Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore
Lecture 6: Construction of Eh – pH Diagrams (Fe – H2O – O2 Diagram)
NPTEL Web Course
However, there are some limitations, such as:
a) The diagrams are thermodynamically (theoretically) determined only for
25oC.
Caution should be exercised to predict corrosion behaviour at
higher temperatures using this diagram.
b) Only thermodynamic amenability to corrosion and protection predicted.
Kinetic factors are not considered. Corrosion rates cannot be predicted.
c) No consideration for extraneous ions and effect of complexation.
d) Only pure metals are generally considered. Effects of impurities, alloying
and metallographic phases and heterogeneties are not considered.
Unstable phases not represented.
Modified Eh – pH diagrams for various metals can be prepared taking into
consideration the effect of alloying additions and presence of complexing
agents.
Combined diagrams which take into consideration two or more
metals simultaneously present would be more useful. For example, effect of
chromium addition to steel to increase corrosion resistance can be represented
in a modified Fe – Cr diagram.
Anodic and cathodic protection limits with reference to potentials and pH can
be predicted using the Eh – pH diagrams.
Computer programmes and soft-ware kits are now available to construct EhpH diagrams for various metal systems.
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Course Title: Advances in Corrosion Engineering
Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore