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CHEM-E6185
Applied Electrochemistry and
Corrosion
Lecture 2: Solutions, solids and interfaces
Contents
• Solid – solution interface
• Electrochemical double layer
• Potential difference
• Solution properties
• Solid phase properties
• Passivation
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Electrified interface
• Electrochemical reactions are heterogeneous reactions.
• They happen at the interface between solid electrode and
electrolyte solution.
• Electrochemical reactions include always a charge transfer
step.
• The interface between solid and electrolyte has different
properties than bulk solid or electrolyte.
• The interface between sold and electrolyte consists of
several layers with thicknesses from nanometer to millimeter
scale.
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Electrochemical double layer
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MOLECULE LAYER
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HELMHOLTZ
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FIRST AND SECOND WATER
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SPESIFICALLY ADSORBED ANION
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HYDRATED CATION
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METAL
ELECTRODE
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DIFFUSION LAYER
HYDRODYNAMIC LAYER
LAYERS
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Electrified interface
• On the solution side closest to the electrode surface is the
electrochemical double layer that consists of inner and outer
Helmholtz layer and diffuse double layer.
• In the Helmholtz layer the dissolved ions are fixed.
• In the diffuse double layer dissolved species will move
because of electrostatic forces and thermal movement.
• There is no clear boundar between diffuse double layer and
the diffusion layer in the electrolyte.
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Electrified interface
• From the electrochemical double layer to the bulk electrolyte
is the diffusion layer, where reacting species move by
concentration gradient.
• No solution flow in the diffusion layer.
• From the diffusion layer to the bulk electrolyte is the Prandtl
layer, where the solution flow rate changes from zero to that
of the bulk solution.
• By increasing the bulk electrolyte flow rate the thicknesses
of Prandtl layer and diffusion layer decrease enabling more
rapid mass transfer.
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Electrified interface
Metal
Potential difference
between solid and
electrolyte typically 1 V.
Field strangth over
double layer 108 V/cm.
Solution
Charge
Mesolid « Me2+
solution +2e solid
Potential
F metal - F solution = constant +
RT
ln(Me z+ )
zF
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Electrified interface
• The reacting species not
specifically adsorbed sense the
potential difference fM-fs.
• They approach to the OHP.
• The potential gradient df/dx at
OHP (x=x2) is important for the
driving force.
• The potential difference f2-fs
• is not assisting the reaction.
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Electrochemical double layer
• The double layer has charges of opposite sign separated by
an interface.
• There is a potential difference across the interface.
• The electrochemical double layer is analoguous to a
capacitor.
• Double layer capacitance in pure aqueous solutions is
approximately 10-50 µF/cm2.
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Electrochemical double layer
• The double layer capacitance describes the excess charge
distribution in the solution, C = dQ/dE.
• The double layer capacitance consists of Helmoltz layer and
diffuse layer capacitances in series.
• 1/C = 1/CH + 1/Cd
• Charges in the double layer will affect the electrochemical
reactions by changing potential differences inside the
double layer.
• Adsorption of charges of the same sign that the surface
charge increases the potential gradient, adsorption of
opposite charges decreases.
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Electrochemical double layer
• When the electrode surface has no excess charge it is at the
zero charge potential or zeta potential.
• At higher potentials the surface has positive charge and at
lower potentials a negative charge.
• At potentials higher than zeta potential anion adsorption
becomes easier and at lower potentials cation adsorption.
• The zeta potential depends on material and solution, and it
is possible to modify to material.
• For example alloying to increase zeta potential to prevent
adsorption of chloride ions that damage passive film.
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Electrolyte properties
• Solution properties affect both the probabality and
rate of electrochemical properties:
• acidity or alkalinity,
• redox potential (oxidizing or reducing system),
• temperature,
• dissolved ion concentrations.
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Electrolyte properties
• Solution composition can be given as:
• Molality, moles of dissolved species per kg of solvent [mol/kg].
• Concentration, moles of dissolved species in one litre of solution
[mol/dm3].
• Normality or equivalent concentration, usually moles of H+ or OHions per litre of solution [N].
• 1 N HCl solution contains 1 mol/dm 3 acid
1 N H2SO4 solution contains 0.5 mol/dm3 acid
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Electrolyte properties
• pH describes the acidity or alkalinity of the solution using
H+ ion concentration, pH = -log[H+].
• Grams per litre [g/l] is used sometimes for concentrated
solutions. For example copper electrorefining electrolyte
may contain 180 g/l H2SO4, 60 g/l Cu, 10 g/l Ni,….
• Weight percent [p-%], for example 5 wt-% NaCl solution
contains 50 g NaCl and 950 g water.
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Properties of water
2 H2O = O2 + 4 H+ + 4 eE = 0,81 - 0,059pH V
2 H+ + 2 e- = H2
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Acids and bases
• Acid hydrolysis in water
HA + H2O = H3O+ + Aequilibrium constant is acidity constant or acid
dissociation constant Ka
• Base hydrolysis in water
B + H2O = BH+ + OHequilibrium constant is basicity constant or base
dissociation constant Kb.
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Acids and bases
Ka or Kb ³ 1, strong acid or base.
10-7 < Ka , Kb < 1, weak acid or base.
10-7 < Ka , Kb < 10-14, very weak acid or base.
Ka tai Kb < 10-14, ineffective acid or base.
HClO4 Ka~109 strong acid
HCl
Ka~105 strong acid
H2SO4Ka~103 strong acid
H3O+ ja OH-, Ka = Kb = 55, strong acid and base
HF
Ka = 6,7·10-4
H2CO3 Ka = 4,4·10-7
weak acid
weak acid
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Oxidants and reductants
• Oxidants will oxidize other species, they take electrons and
themselves get reduced.
• Reductants will reduce other species, they will release
electrons and become oxidized.
• The capacity of a material to oxidize or reduce is estimated
by its electrochemical potential.
• High standard electrode potential Eo = strong oxidant,
low standard electrode potential = strong reductant.
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Oxidants and reductants
Equilibrium potentials
O2 + 4 H+ + 4 e- = 2 H2O
O2 + 4 H+ + 4 e- = 4 OHO3 + 6 H+ + 6 e- = 3 H2O
H2O2 + 2 H+ + 2 e- = 2 H2O
E0 = 1228 - 59pH + 14.7log(pO2)
E0 = 401 + 59pOH + 14.7log(pO2)
= 1228 - 59pH + 14.7log(pO2)
E0 = 1501 - 59pH + 9.8log(pO3)
E0 = 1776 - 59pH + 29.5log(H2O2)
ClO2 + 4 H+ + 5 e- = Cl- + 2 H2O
E0 = 1511 - 47.3pH + 11.8log(pClO2)/(Cl-)
Cl2 + 2 e- = 2 ClClO- + H2O + 2 e- = Cl- + 2 OHFe3+ + e- = Fe2+
S2O32- + 6 H+ + 4 e- = 2 S + 3 H2O
Cu2+ + e- = Cu+
2 H+ + 2 e- = H2
E0 = 1395 - 29.5log(Cl2)/(Cl-)2
E0 = 890 - 59pH + 29.5log(ClO-)/(Cl-)
E0 = 711 + 59log(Fe2+)/(Fe3+)
E0 = 499 - 88.7pH + 14.7log(S2O32-)
E0 = 153 + 59log(Cu+)/(Cu2+)
E0 = 0 - 59pH - 29.5log(pH2)
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Oxidants and reductants
• Oxidant is the starting material of a cathodic reaction,
typically hydrogen ions, dissolved gas (O2, Cl2), oxidizing
compound (ClO2)or metal ion.
• Especially in corrosion engineering:
• Corrosion under reducing conditions happens when redox
potential is low and the oxidant is hydrogen ion.
• Corrosion under oxidizing conditions happen, when redox
potential is high and the system has other, more powerful
oxidizers than hydrogen.
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Oxidants and reductants
• The capability of a solution to oxidize or reduce other
species can be described using redox potential.
• Redox potential is the potential of an inert material (Pt, Au
etc.) measured against any reference electrode.
• Redox potential value depends on the concentrations of
dissolved oxidants and reductants.
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Oxidants and reductants
The solubility of oxygen in
seawater decreases as
temperature and salinity
increase.
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Oxidants and reductants
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Oxidants and reductants
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Oxidants and reductants
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Solution conductivity
• Compounds will dissociate to anions and cations.
• Because of dissoveld anions, cations, H3O+ and OH- the
electrolyte solutions conduct electricity.
• Electrolyte conductivity depends on:
• Concentration of dissolved species
• Charge of dissolved species
• Mobility of dissolved species
• Conductivity unit mS/m = 10 mS/cm
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Solution conductivity
Sea water
conductivity increases
when salinity and
temperature increase
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Solid conductivity
Conductivity of metals 104-106 S-1·cm-1.
Conductivity of insulators 10-22-10-10 S-1·cm-1.
Semiconductors10-9-10-3 S-1·cm-1.
Free charge carriers in metals 1022 1/cm3, semiconductors
1014 1/cm3.
• Many minerals and passive films on metals behave like
semiconductors.
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Solid conductivity
• n-type semiconductor:
• donor impurities form excess electrons in the solid material
lattice to conduct negative charge.
• in some cases anion vacancies will also conduct negative charge
• Anodic reaction would release electrons, but as the lattice
has already negative charge carriers, it will resist this
reaction.
• n-type semiconductor will resist anodic current but pass
cathodic current.
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Solid conductivity
• p-type semiconductor:
• acceptor impurities remove electrons in the solid material lattice
and holes will conduct positive charge.
• in some cases cation vacancies will also conduct positivecharge
• Cathodic reaction would consume electrons, but as the
lattice has nonegative charge carriers, it will resist this
reaction.
• p-type semiconductor will resist cathodiccurrent but pass
anodic current.
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PASSIVATION
• The ability of metals to passivate is the most important
reason that allows using metals in corrosive environments.
• Passivation is a series of events that leads to formation of
a protective surface film and causes the metal dissolution
rate to decrease to a tolerable level.
• Generally, the non-passivating metals corrode more or less
uniformly.
• For the passivating metals the general or uniform
corrosion rate is low because of the passive film.
• In the passive state the metal is protected by the passive
film, but local damages in the film can cause localized
corrosion.
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PASSIVATION
• Examples of metals that show passivity are iron,
chromium, nickel, titanium, and alloys containing
these metals:
• Steel, highly oxidizing solutions (strong HNO3, strong
H2SO4), strong alkalis.
• Stainless steel, air, natural water, dilute oxidizing acids.
• Chromium containing nickel alloys, same environments
as stainless steel.
• Titanium, almost all oxidizing environments.
• Aluminum, air, natural waters in neutral pH range.
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PASSIVATION
O2
O2
O2
O2
Anode
icrit
Fe
Cathode
O2
Fe
4 e-
Anode
ipass
Fe = Fe2+ + 2 e-
Epass
Etrans
OH-
OHFe
Adsorption of oxidant
from solution
Charge transfer reactions
Cathode
O2 + H2O + 4e- = 4 OH-
OHFe
Anode
OH-
Cathode
The reaction products
react forming a
protective layer
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PASSIVATION
• Passivation is determined by thermodynamic and kinetic
factors.
• The formation of passive film is possible only if the initial
corrosion rate is sufficiently high to supply metal ions and
the solution conditions are such that film formation is
thermodynamically possible and kinetically sufficiently
rapid.
• Passivation potential describes the thermodynamic
possibility for passivation.
• Critical current density describes the kinetic conditions for
passivation.
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PASSIVATION
• A solution with high redox potential has higher probability
to polarize the metal surface to anodic reaction, above the
passivation potential.
• When the dissolution rate exceeds critical current density,
passivation process can start.
• Dissolution is caused by the cathodic reaction, so the rate
of cathodic reaction must be high enough.
• When the metal dissolution rate is high enough AND when
the solution E-pH range is on suitable range, the metal ions
can react to a compound and possible form a passve layer.
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PASSIVATION
Adsorbed molecular layer
Stainless steel
Oxide-hydroxide passive film
Patina layer
Oxide film
Copper
Porous hydroxide layer
Oxide barrier film
Aluminum
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PASSIVITY
g -Fe2O3
OyO
2-x2H
3 3
g -Fe
g -Fe
Fe3O4
Fe3O4
Fe
Fe
Fe
FeOOH
g -Fe2O3
[Fe(OH)2]x
Fe
2-2x 2O
xO
3 3
g -Fe
Fe2O3
Fe3O4
Fe
Fe
g -Fe2O3
Fe
g -Fe2O3
g -Fe2O3
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PASSIVATION
• A passive film is seldom stoichiometric.
• An oxide film can be metal or oxygen deficient.
• The structure has vacancies and electron
concentration that is not in equilibrium.
• The film has some concentration of free charge
carriers, either electrones, holes or both.
• The passive film behaves as a semiconductor:
• Metal vacancies, p-type conductivity, resistant to
electron-consuming
• Anion vacancies, n-type conductivity, resistant to
electron-releasing anodic current.
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PASSIVITY
• The passive film models for stainless steels are often
developed from the models of iron.
• The development has led to multilayer structures, with thin
Cr2O3 and different non-stoichiometric layers.
• In the inner layer Cr(OH)3 is anion selective and in the outer
layer anion complexes change it to cation selective.
• Cation selective outer layer prevents anion transfer and
anion selective inner layer prevents cation transfer.
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PASSIVITY
Cl-, OH-
x
CrO4nOH-
x
Cation selective
layer
H+
H+ + O2-
Cr(OH)3
Anion selective
layer
O2-
Crn+
Cr
Cr2O3
Inner oxide layer
Metal
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Summary
• The driving force of the charge transfer reaction is the
potential difference across the electrochemical double
layer.
• This potential difference can be changed by adsorption of
species on the surface or by external current source.
• The solution properties that affect electrochemical
reactions are pH, redox potential, temperature and
concentrations of dissolved species.
• The solid phase properties that affect the reactions are
effects on type and strength of chemical bonds and electric
conductivity.
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