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Corrosion Engineering
N Balasubramanian
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CHE3166: Materials and Corrosion
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Corrosion:Corrosion: ?
Corrosion: Destruction of a material by reactionCorrosion: Destruction of a material by reaction
with its environment.with its environment.
Corrodere (Latin)Corrodere (Latin) -- To Eat AwayTo Eat Away
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The basic corrosion reaction
M M+ + e-
e a ecomes a ca on
Oxidation reaction
Electrochemical reaction
Need to have a corresponding reduction
reaction
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Categories
Infrastructure
Utilities
Cost of Corrosion
Transportation
Production & Manufacturing
Government
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Motor Vehicles 23.4
Ships 2.7
Aircraft 2.2
Oil & Gas Exploration & Production 1.4
Mining 0.1
Petroleum Refining 3.7
Chemical, Petrochemical, & Pharmaceutical 1.7
Cost of Corrosion
Railroad Cars 0.5
Hazardous Materials Transport 0.9
TOTAL: 29.7
Pulp & Paper 6.0
Agricultural Production 1.1
Food Processing 1.1
Electronics -
Home Appliances 1.5
TOTAL 17.6
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Cost of Corrosion
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The eight forms are:
forms of corrosion
5. intergranular corrosion. , ,
2. galvanic, or two-metal corrosion,
3. crevice corrosion,
4.pitting
6. selective leaching, or parting
7. erosion corrosion, and
8. stress corrosion.
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Forms of Corrosion
Uniform CorrosionUniform Corrosion
Galvanic CorrosionGalvanic Corrosion
Crevice CorrosionCrevice Corrosion
ErosionErosion--corrosioncorrosion
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Uniform Corrosion
Uniform attack is the most common form of corrosion. It is normally
characterized by a chemical or electrochemical reaction which
proceeds uniformly over the entire exposed surface or over a large
area. The metal becomes thinner and eventually fails. For example, a
piece of steel or zinc immersed in dilute sulfuric acid will normally
dissolve at a uniform rate over its entire surface. A sheet iron roof willshow essentiall the same de ree of rustin over its entire outside
surface.
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Forms Of Corrosion
Uniform CorrosionUniform Corrosion
Galvanic CorrosionGalvanic Corrosion
PittingPitting
Crevice CorrosionCrevice Corrosion
ErosionErosion--corrosioncorrosion
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Dissimilar Metal Couples
GraphiteTitanium
Alloy C-276
Stainless Steel
OK?
Copper
Cast Iron
SteelAluminum
Zinc
OK
Can BeBad
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Galvanic Corrosion
A potential difference usually exists between two dissimilar metals
when they are immersed in a corrosive or conductive solution. If these
metals are placed in contact (or otherwise electrically connected), this
potential difference produces electron flow between them. Corrosion of
the less corrosion-resistant metal is usually increased and attack of the,
these metals when they are not in contact. The less resistant metal
becomes anodic and the more resistant metal cathodic. Usually the-
cathode or cathodic metal corrodes very little or not at all in this type of
couple. Because of the electric currents and dissimilar metals involved,
this form of corrosion is called galvanic, or two-metal, corrosion. It is
electrochemical corrosion, but we shall restrict the term galvanic to
dissimilar-metal effects for purposes of clarity.
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Galvanic Corrosion
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Forms Of Corrosion
Uniform CorrosionUniform Corrosion Galvanic CorrosionGalvanic Corrosion
PittingPitting
Crevice CorrosionCrevice Corrosion ros onros on--corros oncorros on
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Pitting Corrosion
Pitting is a form of extremely localized attack that results in holes in the
metal. These holes may be small or large in diameter, but in most cases they
are relatively small. Pits are sometimes isolated or so close together that
they look like a rough surface. Generally a pit may be described as a cavity
or hole with the surface diameter about the same as or less than the depth.
Pitting is one of the most destructive and insidious forms of corrosion. It
weight loss of the entire structure. It is often difficult to detect pits because
of their small size and because the pits are often covered with corrosion
products. In addition, it is difficult to measure quantitatively and compare
the extent of pitting because of the varying depths and numbers of pits that
may occur under identical conditions.
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Pitting Corrosion
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Forms of Corrosion
Uniform CorrosionUniform Corrosion Galvanic CorrosionGalvanic Corrosion
PittingPitting
Crevice CorrosionCrevice Corrosion
ErosionErosion--corrosioncorrosion
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Crevice Corrosion
Intense localized corrosion frequently occurs within crevices and other shielded
areas on metal surfaces exposed to corrosives. This type of attack is usually
associated with small volumes of stagnant solution caused by holes, gasket
surfaces, lap joints, surface deposits, and crevices under bolt and rivet heads. As a
result, this form of corrosion is called crevice corrosion or, sometimes, deposit or
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Crevice Corrosion
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Forms Of Corrosion
Erosion corrosion is the acceleration or increase in rate of deterioration or
attack on a metal because of relative movement between a corrosive fluid and
the metal surface. Generally, this movement is quite rapid, and mechanical
wear effects or abrasion are involved. Metal is removed from the surface as
dissolved ions, or it forms solid corrosion products which are mechanically
swept from the metal surface. Sometimes, movement of the environment
Uniform CorrosionUniform Corrosion Galvanic CorrosionGalvanic Corrosion PittingPitting
Crevice CorrosionCrevice Corrosion
ErosionErosion--corrosioncorrosion
,
conditions, but this is not erosion corrosion because deterioration is not
increased.
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Erosion-corrosion
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Forms of Corrosion
Selective leaching is the removal of one element from a solid alloy by corrosion
processes. The most common example is the selective removal of zinc in brass
alloys (dezincification). Similar processes occur in other alloy systems in which
aluminum; iron, cobalt, chromium, and other elements are removed. Selective
leaching is the general term to describe these processes, and its use precludes
the creation of terms such as dealuminumification, decobaltification, etc. Parting is
Selective LeachingSelective Leaching Intergranular Attack
Corrosion Fatigue
Environmental
Cracking
, .
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Selective Leaching
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Forms Of Corrosion
Grain boundary effects are of little or no consequence in most applications
or uses of metals. If a metal corrodes, uniform attack results since grain
boundaries are usually only slightly more reactive than the matrix.
However, under certain conditions, grain interfaces are very reactive and
intergranular corrosion results. Localized attack at and adjacent to grain
boundaries, with relatively little corrosion of the grains, is intergranular
corrosion. The alloy disintegrates (grains fall out) and/or loses its strength.
Selective Leaching
Intergranular Attack (IGA)Intergranular Attack (IGA)
Corrosion Fatigue
Environmental Cracking26
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Intergranular Corrosion
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Intergranular Corrosion
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Corrosion Fatigue
60
80
100
120
ksi
)
ksi
)
0
20
40
10,000 100,000 1,000,000 10,000,000 100,000,000
In AirIn Air In SeawaterIn Seawater
Stress
Stress
Cycles to FailureCycles to Failure
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Corrosion Fatigue Fracture
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Stress Corrosion Cracking
Alloy Susceptible
Tensile Stress
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Stress Corrosion Cracking
Vertical Heat Exchanger
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Stress Corrosion Cracking
Caustic Carryover In Steam
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Forms of Corrosion
Erosion corrosion is the acceleration or increase in rate of deterioration or
attack on a metal because of relative movement between a corrosive fluid
and the metal surface. Generally, this movement is quite rapid, and
mechanical wear effects or abrasion are involved. Metal is removed from the
surface as dissolved ions, or it forms solid corrosion products which are
mechanically swept from the metal surface. Sometimes, movement of the
Selective Leaching Intergranular Attack
Corrosion Fatigue
Environmental CrackingEnvironmental Cracking
,
under stagnant conditions, but this is not erosion corrosion because
deterioration is not increased.
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Factors Favoring
SCC Stainless Steels
Temperature > 55 C
Chlorides
Evaporative Conditions
Pit - Stress Raiser
Residual Or Applied Stress
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Mechanical And Physical Factors
Strength, Ductility
Formability
,
Thermal Conductivity
Thermal Expansion
Unique Properties
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Other Factors
Safety
Compatibility
Cost
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Economics
Managements Language Is EconomicsManagements Language Is Economics
Use Life Cycle Costs To Compare MaterialsUse Life Cycle Costs To Compare Materials
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Materials Selection
Define Environment
Define Required Equipment Define Test And Inspection
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Define Environment
Major Reactants
Minor Ingredients
pH, Contaminants, Aeration
Process Conditions
Temperature, Pressure, Excursions
Catalytic/inhibitive Effects
e.g., Metallic Ions
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Define Equipment
Pressure Vessels
Valves And Piping
Tankage es ys ems
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Test & Inspection
Fabrication Verify Materials, Workmanship, Dimensions,
Storage Conditions
Testing & Inspection
Develop Program
Consider Corrosion Test Locations
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Dont Expect Inspect
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CHE3166Corrosion: Thermodynamics
Net reaction is oxidation of zinc by copper(II) ions:
Zn(s) + Cu2+ Zn2+ + Cu(s)
Half reactions take place in separate locations:
Left electrode: Zn(s) Zn2+ + 2e oxidation
Right electrode: Cu2++ 2e Cu(s) reduction
Electrochemical cells allow measurement and control of a redox (reduction/oxidation)
reaction. Also, we can force the reaction to proceed in its non-spontaneous, or reversedirection by connecting a source of current to the two electrodes.
To sustain the cell reaction, the charge carried by the electrons through the external
circuit must be accompanied by a compensating transport of ions between the two cells.
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To sustain the cell reaction, the charge carried by
the electrons through the external circuit must be
accompanied by a compensating transport of ions
between the two cells.
Relative amounts of charge can be carried by
Galvanic cells and electrodes
negat ve or pos t ve ons epen s on t e r re at ve
mobilities) through the solution.
Salt bridge, consists of an intermediate
compartment filled with saturated salt solution and
fitted with porous barriers at each end, is used forprecise measurements. The purpose of salt bridge is
to minimize the natural potential difference
(junction potential).
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Consider zinc in a solution of hydrochloric acid
Thermodynamics
Zn + 2 ClZnCl2+H2
This reaction can be separated into two half-cell reactions
n n + e
2H+ + 2e H2
potential ea)
Reduction reaction (cathodic reaction withpotential ec)
has an associated free-energy change (G)
G= nFE= nF ea+ ec( )
n=number of electrons exchanged, F=96500 C/e-, E=potential
Nernst Equation
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Standard hydrogen electrode
Pt is inert and acts as
catalyst site
1 atm H2 gas is the standards a e
Solid Zn and 1N Zn2+ is the standard state
Measured potential difference V is 0.762V
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Standard hydrogen electrode
For a pure Cu electrode in a
1 N Cu 2+ solution, the
potential difference in this
cell is 1.104 V
Zn2+ + 2e Zn 0.762V( )
Cu2+
+ 2e
Cu +0.342V( )
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Electromotive Force Series
Zn + 2HClZnCl2+ 2
or
ZnZn2+ + 2e
2H+ + 2e H2
= ea + ec
E= +0.762 + 0
= +0.762V
G= nFE< 0
At standard state, reaction
will occur
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Electromotive Force Series
The half-cell reaction with the
more active (negative) half-
cell potential always proceeds
as an oxidation, and thereaction with the more noble
half-cell potential always
proceeds as a reduction in the
spontaneous reaction
produced by the pairShipboard corrosion
protection
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Electromotive Force Series
Question. Is copper (Cu)
oxidized (dissolved) by ferric
(Fe3+) ions?
Ferric reduction is noble relative
to copper so copper is oxidized
(copper becomes an ion and
gives off electrons)
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Reaction Direction
Written as reduction
reactions from left to right
y rogen re uc on s
chosen (arbitrarily) as the
reference and assigned
zero potential
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Standard conditions
Standard states:
For a solid, the solid is taken as unit activity
For a gas, 1 atm pressure is taken as unit activity
For dilute or strongly dissociated solutes typically found in
most instances of corrosion, activity is reasonably approximated
A( ) = CA
A=activity of atom A= activity coefficient (tabulated)
CA=concentration (g/l) of atom A
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Activities
Gases A=p (partial pressure in atm)
Pure li uids and solids A=1
Solutes A=M (molarity)
Liquids in solutions A=X (mole fraction)
Solvent in dilute solution A=1
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Non-standard conditions
Analyze a general half-cell equation between atom A and atom B in
an aqueous solution
aA +mH+ +ne = bB +dH2O
or reac an s an pro uc s n e s an ar s a e
G0 = bGB0 +dGH2O
0( ) aGA0 +mGH+0( )
For reactants and products in a non-standard state
G= bGB +dGH2O( ) aGA +mGH+( )59
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Non-standard conditions
The departure of free energy in the non-standard state from the
standard state is:
G G0 = b GB GB0( )+d GH2O GH2O
0( )[ ] a GA GA0( )+m GH+ GH+0( )[ ]
The corrected concentration A available for the reaction is known as
the activity of A and is given by:
a GA GA0( )=aRTln(A) =RTlnA( )
a
G G0 =RTln B( )
bH2O( )
d
A( )a
H+( )m
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Nernst Equation
G G0 =RTln B( )
bH2O( )
d
A( )a
H+( )m
= 0 RT B( )b
H2O( )d
G= nFE= nF ea + ec( )Recall
2.3 T = 0.059
At 25C
nF A( )a
H+( )m
e= e0 +2.3RT
nFlog
A( )a
H+( )m
B( )b
H2O( )d
e= e0 +0.059
nlog
A( )a
B( )b
m
n0.059pH
pH= logH+( )
Recall
Activity of water=1 for
aqueous solutions
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Electrochemical potential
e= e0 +0.059
nlog
A( )a
B( )b
m
n0.059pH
Examine specific half-cell reactions
+ e = 2
eH+ /H2
= eH+ /H2
0 0.059pH
,
so activity=1
As the activity of any dissolved oxidizer (e.g., H+) increases,
more noble or positive potentials are routinely measured
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Electrochemical potential
The above reaction occurs in an acid solution. An
equivalent reaction in neutral or alkaline solutions is:
2H+ + 2e =H2 eH+ /H2 = eH+ /H20
0.059pH
0
The electrochemical evolution of hydrogen is the
decomposition of water
2 + e = 2+ eH+ /H2= e
H+ /H2 . p
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Potential/pH (Pourbaix) Diagram
eH+ /H2
= eH+ /H2
0 0.059pH
oxygen evolution
and acidification
For hydrogen evolution at a fixedpotential
2H+
+ 2e
=H2
+ hydrogen evolution
and alkalization
decreases), we are less likely to
form H2 gas
Note that, consistent with the half-cell potential definitions,
the hydrogen line goes through zero potential at zero pH
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Potential/pH (Pourbaix) Diagram
oxygen evolution
and acidification
At higher potential
O2+ 4H+
+ 4e
2H2O
is feasible
eO2 /H2O = eO2 /H2O 0.059pHhydrogen evolutionand alkalization
Note that, consistent with the half-cell potential definitions,the oxygen line goes through 1.229V at zero pH and 0.401V atpH=14.
This reaction is the basis for water electrolysis65
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Potential/pH (Pourbaix) Diagram
Corrosion-soluble ions of the
metal are stable
Passivation- oxides are stable
Immunity-reduced form of the
metal is stable
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Potential/pH (Pourbaix) Diagram
Reaction 1: Metal oxidizes to
aqueous cations
Pourbaix diagram for Al
M=Mn +
+ne
l= l3+ + 3e
eAl/Al3+
= e0 +0.059
nlog
A( )a
B( )b
m
n0.059pH
eAl/Al3+
= 1.662 +0.059
3logAl3+( )
Independent of pH since
no H+ is involved.
Only depends on Al3+
activity
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Potential/pH (Pourbaix) Diagram
Reaction 2: Metal reacts to metal
hydroxide or oxide
Pourbaix diagram for Al
M+nH2O =M OH( )n+nH+ +ne
+
2 = 2 3 e
eAl/Al2O3 = 1.55 0.059pH
At higher pH Al2O3 is formed. At lower pH Al2O3 dissolves to Al3+
Intersection depends depends on Al3+ activity (dashed lines areportions of the reactions with no significance)
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Potential/pH (Pourbaix) Diagram
The reaction rate constant for
Pourbaix diagram for Al
2Al3+
+ 3H2O =Al2O3+ 6H+
K=H+( )
6
Al3+( )2
= 1011.4
For (Al3+)=10-6,
logK= 6logH+( ) 2log Al3+( )= 6pH 2log Al3+( )
pH= 3.9 Independent of potential
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Potential/pH (Pourbaix) Diagram
Reaction 3: Metal reacts to form
soluble aqueous anions
Pourbaix diagram for Al
= n2m + m
Al+ 2H2O =AlO2 + 4H+ + 3e
eAl/AlO2
= eAl/AlO2
0 +0.059
3logAlO2
( )
4
30.059pH
eAl/AlO2
= 1.262 + 0.020log AlO2( ) 0.079pH
At higher pH, Al2O3 dissolves to AlO2-
70
P b i Di F
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Pourbaix Diagram Fe
Example 2: stability diagram for iron (little
bit complicated).
Horizontal lines: reactions areinvolved with electrons (potential),
but independent of pH (H(+a) or OH(-a)
ions):
Fe2+=Fe3++e
Ee=0.771+0.0591lg(aFe3+/aFe
2+)
Fe=Fe2++2e
Ee=0.771+0.0591lg(aFe3+/aFe
2+)
Vertical lines: reactions are
involved with pH, but independent of
electrons:
Fe3++H2O=FeOH2++H+
lg(aFeOH2+/aFe
3+)=-2.22+pH
Fe2++2H2O=Fe(OH)2+2H+
lg(aFe2+)=13.37-2pH
Diagonal lines: reactions are
involved both pH and electrons:
Fe2++H2O=FeOH2++H++eEe=0.877-0.0591pH+0.0591lg(aFe(OH)
2+/aFe2+)
Fe2++3H2O=Fe(OH)3+3H++e
Ee=0.748-0.1773pH-0.0591lgaFe2+
Fe+2H2O=Fe(OH)2+2H++2e
Ee=-0.045-0.0591pH
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Standard Hydrogen Electrode
Platinum Foil suspended in
sulfuric acid with unit activity of
H+. Platinum is noble and does
not participate (acts as catalyst
Connected to another half-cell
through a solution bridge that
allows for charge but not masstransfer
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Ohter Electrodes
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Questions?
74