charged interfaces - corrosion
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Copyright @ Dr. I. H. ToorADVANCED CORROSION ENGINEERING
ME 575
(Advanced Corrosion Engineering)
Chapter # 3
Charged Interfaces
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Copyright @ Dr. I. H. ToorADVANCED CORROSION ENGINEERING
Types Interfaces
Electrolytes
The solution/air interface
Metal/solution interface
Metal Ions in Two Different Chemical Environments
The Electrical Double Layer & Models
Significance of the Electrical Double Layer to Corrosion
Measurement of Electrode Potentials
Reference Electrodes
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Copyright @ Dr. I. H. ToorADVANCED CORROSION ENGINEERING
Charged interfaces
Interfaces form at the physical boundary between two phases, such as:solid
and a liquid (S/L), a liquid and its vapor (L/V), or a solid and a vapor (S/V).
There can also be interfaces between two different solids (S1/S2) or between
two immiscible liquids (L1/L2).
Two special interfaces, the solution/air interface and the metal/solution
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Electrolytes: The Interior of an Electrolyte
The interior of an electrolyte mayconsist of a variety of charged and
uncharged species.
(1) H2O molecules
(2) Na+ ions
(3) Cl ions
(4) Organic molecules (which may be
present as impurities, biological entities,
or may be intentionally added as a
corrosion inhibitor).
Fig. 3.1 (a) The water dipole. (b) In the bulk, liquid water
consists of an array of randomly oriented dipoles, so the
net charge is zero
No net electrical field in the interior of liquid water.
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Electrolytes: The Interior of an Electrolyte
Fig. 3.2 A volume element of sodium chloride solution showing
the distribution of ions
no net charge within anyvolume element of
solution due to the
existence of these
dissolved ions of NaCl
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Electrolytes: The Interior of an Electrolyte
Fig. 3.3 Primary waters of hydration for (a) Na+ ion, (b) Clanion. Primary hydration numbers are from Bockris and
Reddy [1]
Central ion is surrounded by watermolecules, which are called primary
waters of hydration.
Secondary region of partially orderedwater molecules (outside the primary
sheath) is called secondary waters
of hydration, which balance the
localized oriented charge which has
developed in the primary water sheath.
No net charge due to ionic hydration.
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Interfaces: Solution/Air Interface
Fig. 3.4 Water molecules at the water/air interface
and the origin of surface tension
An imbalance of forces for molecules located in the surface regionresults in a net force inward into the liquid, and this net inward force
is the origin of the surface tension of the liquid.
What about NaCl solution
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Interfaces: Metal/Solution Interface
Fig. 3.7 The orientation of water molecules at a metal/s
olution interface. Top: the flop-down orientation of the water dipole. Bottom: the flip-up orientation [3]
More complicated than the solution/airb/c of:
First, being a good conductor, metal side of theinterface can be charged negatively or positively
, respectively.
Second, chloride ions are adsorbed at metal/solution interfaces.
Third, the water molecule itself is adsorbed atmetal/solution interfaces
Fourth, the metal/solution interface is notalways a stable one(neither chemically nor
geometrically)
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Metal Ions in Two Different Chemical Environments
Array of positive ions in a Fermi sea of electrons.
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Metal Ions in Two Different Chemical Environments
Corrosion process transfer of a positive ion from the metal lattice into solution Inthe metal lattice, the positive ion is stabilized by the Fermi sea of electrons Insolution, the positive ion is stabilized by its water of hydration
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Metal having a positive charge, ispartially balanced in solution by a diffuse
layer of negative ions.
In diffuse layer, ions are in thermalmotion.
An overall increase in the concentrationof negative ions within diffuse layer (par
tly balance the positive charge on the
metal side of the interface)
Net charge within the diffuse part of theelectrical double layer (no net charge in
bulk/interior of soln)
Electrical Double Layer- GouyChapman Model
Fig. 3.10 The GouyChapman model of the electrical doublelayer at a metal/solution interface
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Electrical Double Layer- The Electrostatic Potential and Potential Difference
The electrostatic potential (at some point)is the work required to move a smallpositive unit charge from infinity to the
point in question. Assumption is
(1) the positive test charge is smallenough not to perturb the existing
electrical field;
(2) the work involved is independent ofthe path taken.
The potential difference (between two points) is the work required to move a small unit
positive charge between the two points [PD
= B A ( joules per coulomb, or volts)
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Electrical Double Layer- Stern Model Model
Fig. 3.12 The Stern model of the electrical double
layer at a metal/solution interface
Adsorption of anions or cations at the metalsurface.
Plane through the center of these adsorbedions is called the Helmholtz plane.
The excess charge at the metal surface isbalanced in part by ions located in a GouyChapman diffuse double layer, which exists
outside the Helmholtz plane.
A typical potential difference across theHelmholtz plane is of the order of 1 V. The
thickness of the Helmholtz layer is about
10 (1 = 108 cm) and field strength of1 107 V/cm.
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Electrical Double Layer- Stern Model Model
Fig. 3.13 The BockrisDevanathanMller model of theelectrical double layer at a metal/solution interface [4]
Two important consideration:
First: adsorption of water molecules at the M/Sinterface.
Water molecule and ions in solution competefor sites on the metal surface (adsorption of Cl
- ion)
The plane through the center of these adsorbedions is called the inner Helmholtz plane.
Second: charge introduced by the adsorption ofanions at the metal surface is balanced in part
by counter ions of the opposite charge. These
counter ions are not adsorbed on the metal surf
ace, but exist in solution, and have associated
with them their waters of hydration.
The plane through the center of these counterions is called the outer Helmholtz plane.
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Significance of the Electrical Double Layer to Corrosion
Fig. 3.14 Simple equivalent circuit model of the electrical
double layer. Cdl is the double layer capacitance, RP is
the resistance to charge transfer across the edl, and RS
is the ohmic resistance of the solution
EDL is the origin of the PD across an M/Sinterface, which is responsible for electrode
potential.
Changes in the electrode potential can producechanges in the rate of anodic (or cathodic)
processes.
Emerging (corroding) metal cations must passacross the EDL outward into solution, and
solution species (e.g., anions) which participate
in the corrosion process must enter the EDL
from solution in order to attack the metal. So,
the properties of the EDL control the corrosion
process.
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Electrode Potentials: The Potential Difference Across a Metal/Solution Interface
Fig. 3.15 In order to measure the potential difference across the
metal/solution interface of interest (M/S), an additional interface must
be created using a reference metal ref [3]
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Electrode Potentials: The Potential Difference Across a Metal/Solution Interface
The sum total of changes in electrostatic potential must be zero by Kirchhoffs law
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Fig. 3.16 A standard hydrogen reference electrode (SHE)
Relative Electrode Potentials- Reference electrodes
The hydrogen electrode is universally accepted as the primary standard againstwhich all electrode potentials are compared.
Fig. 3.17 Experimental determination of a standard electrode
potential for some metal M using a standard hydrogen
reference electrode
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Table 3.1 Standard electrode potentials at 25C [6]
The Electromotive Force Series
Limitations of EMF series:
The emf series applies to pure metals (notalloys) in their own ions at unit activity.
The relative ranking of metals in the emfseries is not necessarily the same (and is
usually not the same) in other media (such as
seawater, groundwater, sulfuric acid, artificial
perspiration).
The relative ranking of metals in the emfseries gives corrosion tendencies (subject to
the restrictions immediately above) but
provides no information on corrosion rates.
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Fig. 3.18 A saturated calomel reference electrode
1) The saturated calomel electrode(SCE) :
most popular ref. electrode for laboratory use.
Hg, Hg2Cl2/Cl-(aq. saturated KCl),
Half cell rx.: Hg2Cl2 + 2e- = 2Hg + 2Cl-
Relative Electrode Potentials- SCE
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Fig. 3.19 A silver/silver chloride reference electrode
2) The siver-siver chloride electrode :
preferred for use at high temperature.
Ag, AgCl(s)/Cl-(aq. saturated KCl),
Relative Electrode Potentials- Ag/AgCl
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Reference Electrodes for the Laboratory and the Field
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Fig. 3.21 The copper/copper sulfate reference electrode for use
in soils
Relative Electrode Potentials- Cu/CuSo4
3) Cu-saturated CuSO4 electrode : commonly
used in field.
Cu/Cu+2(aq. saturated CuSO4) :
Half cell rx.: Cu+2 + 2e- = Cu and ECu+2/Cu =
0.340 - 0.0295 log (Cu+2)
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Fig. 3.22 Measurement of the electrode potential of a buried pipe using a
copper/copper sulfate reference [15]
Reference Electrodes for the Laboratory and the Field
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Fig. 3.24 A simple cell for measuring electrode potentials in
the laboratory
Measurement of Electrode Potentials
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