Suez Canal University
Corrosion Part 1
Dr. Eng. Hamid A. Nagy
WELD-INSPECTA CO.
Corrosion
Dr. Eng. Hamid A. Nagy
Driving force
 Every
Process to take place, we should
have some driving force.
 The driving force depends on the energy
of the first state and that of the final state.
Dr. Eng. Hamid A. Nagy
Barrier
Driving force
Driving
Force
Energy
A
B
Dr. Eng. Hamid A. Nagy
Thermodynamics
 Every
material has two sources of its
energy


Heat content, Enthalpy
Content depending on its randomness,
Entropy.
 We
can not measure this energy directly.
 So we have a reference zero value which
is the hydrogen molecule formation.
Dr. Eng. Hamid A. Nagy
Thermodynamics
 Now
consider the transfer of a metal from
state A to state B.
 This can only be done if the state A has
higher energy than state B.
DGA > DGB
DG is the free energy of material state.
Dr. Eng. Hamid A. Nagy
Thermodynamics
 Now
consider the reaction between two
materials a and B to produce C.
 The same law applies.
DGA + DGB > DGC
For the reaction to proceed in the direction
A+B→C
And vice versa.
Dr. Eng. Hamid A. Nagy
Effect of
Concentration
 Consider
the following Reaction
M++ + 2e → M
 As the M ions concentration increases the
reaction tends to go the left.
 Oxidation reaction increases.
 More anodic tendency.
 Potential decreases.
Dr. Eng. Hamid A. Nagy
Kinetics
 Rate
of reaction depends on what is called
mechanism.
 There should be some energy done to
activate the first stage.
 This is called the energy barrier.
 This energy barrier could be high or low
depending on the mechanism.
Dr. Eng. Hamid A. Nagy
Kinetics
 Overcoming
energy barrier may consist of
several steps.
 The rate of occurrence of this reaction
depends on the interaction of steps to
overcome energy barrier.
 There is usually what is called rate
determining step.
 Determining the rate of the reaction is
what is called KINETICS.
Dr. Eng. Hamid A. Nagy
Metals
 Corrosion
is a chemical reaction.
 What is considered in corrosion?


Feasibility.
Rate.
 The
answer is both.
 You can even protect metals if you impair
the feasibility or slow down the rate.
Dr. Eng. Hamid A. Nagy
Metals
 Now
consider the structure of metal atoms
and their mutual relation.
 Metal atoms have some free electrons.
 In the matrix of metals, free electrons do
not relate specifically to a certain atom.
 They are just Free ELECTRONS.
 This is what provides the metals with their
Characteristic Properties.
Dr. Eng. Hamid A. Nagy
Metals
 If
the metals are going to react, What is
better for the atoms?
 To share these electrons.
 To loss these electrons.
 The answer, for sure is the second option.
 So they are going to exchange ions with
other reactants.
 Ionic Bond.
Dr. Eng. Hamid A. Nagy
Corrosion Potential
reaction of metals depends on DG.
 But this reaction involves transfer of
electrons.
 DG is a measure of chemical energy.
 But this energy is transferred to electric
energy for the reaction to take place.
 Is it possible to measure this energy using
the electric parameters.
 Now,
Dr. Eng. Hamid A. Nagy
Corrosion Potential
 Now
Recall the definition of volt.
 The voltage between two points is 1 volt if
the amount of energy required to transfer
1 unit charge is 1 unit of energy.
 This is why
P=VI
 Or Energy/ time = Volt X (Charge/ Time)
 Energy = Volt X Charge.
Dr. Eng. Hamid A. Nagy
Normality
 If
you dissolve one mole (atomic weight) in
one liter of water, concentration is called
morality (1 molar solution).
 If you dissolve one equivalent weight in
one liter of water, concentration is called
Normality (1 Normal solution).
 So, for one and the same material
 1 N solution = 1M solution if n =1.
 1 N solution = ½ M solution if n = 2.
Dr. Eng. Hamid A. Nagy
Normality







For explanation let us consider sodium chloride
(NaCl)
At. Wt. for Na = 23
At. Wt. for Cl = 17.
At. Wt. for NaCl = 40.
1 M solution means 40 gram NaCl in 1 liter
water.
1 N solution is the same.
Concentration of NaCl in sea water is about
3.5% (35 grams in 1 liter).
Dr. Eng. Hamid A. Nagy
Normality







Another example is (FeCl2)
At. Wt. for Fe = 56
At. Wt. for Cl = 17.
At. Wt. for FeCl2 = 90.
1 M solution means 90 grams FeCl2 in 1 liter
water.
1 N solution means 45 grams FeCl2 in 1 liter
water.
Normality is more expressive than molality.
Dr. Eng. Hamid A. Nagy
Electrolytic Cell
 Now
consider Electro-refining of Cu.
Cu –ve potential
Cu +ve potential
Reduction
prevails
Oxidation
prevails
(Cathode)
(Anode)
Dr. Eng. Hamid A. Nagy
Galvanic Cell
 Now
consider a cell having Cu in 1N
CuSO4 solution in half cell and Zn in 1N
ZnSO4 solution in the other half cell.
V
EZn = -0.76 V
Electrons
Oxidation
prevails
Current
ECu = 0.34 V
Reduction
prevails
Negative
Electrode
Positive
Electrode
(Anode)
(Cathode)
O.C.P. = 1.1 Volts
Dr. Eng. Hamid A. Nagy
Anode/ Cathode





Now if you consider Zn
Zn++ + 2e → Zn
This called reduction.
Or Zn → Zn++ + 2e
This is called oxidation.
As a convention we use the potential
measurement for the reduction reaction.
If V increases, DG decreases, more reduction
takes place, more protection.
Dr. Eng. Hamid A. Nagy
Corrosion Potential
 Back
to Cu + Zn
 Cu will not be dissolved (protected).
Cu++ + 2e → Cu
 Zn will dissolve (Corroded).
Zn → Zn++ + 2e
 As the difference in potential increases
more current takes place but not
necessarily.
Dr. Eng. Hamid A. Nagy
Corrosion Rate
 As
we know OC potential is higher than
short circuit potential.
 How much is the highest current provided
by any galvanic cell, let us see.
Dr. Eng. Hamid A. Nagy
Concentration
Polarization

Steel
3- Concentration Polarization:
 Consider the case of placing steel part in aerated water.
 Anode: Fe dissolution
 Cathode: Oxygen Reduction (Oxygen).
1- Transport of oxygen to steel by diffusion.
2- Reduction of Oxygen
O2 + 2H2O +4e → 4OH-
Low diffusion
Dr. Eng. Hamid A. Nagy
Concentration
Polarization
O2
Potential
Factors affecting
this phenomenon:
1- Temperature.
2- Agitation.
3- Pressure
4- Flow Rate.
5- Concentration.
High
Oxygen
Diffusion
Low
Oxygen
Diffusion
Fe
Current
Dr. Eng. Hamid A. Nagy
Control of Rate
 Decrease
the metal conductance.
 Lower the electrolytic conductance.
 Control one of the surfaces (Larger Anode
is Better.
 Control one of the two reactions (anodic
and Cathodic).
Dr. Eng. Hamid A. Nagy
Control of Cathodic Reactions
2H+ + 2e → H2
 Increase or Control pH.
 Increase Pressure (not a solution)
 Decrease Pubbling rate (not a solution in
tanks and pipelines.
Dr. Eng. Hamid A. Nagy
Control of Cathodic Reactions
O2 + 2 H2O + 4 e → 4 OH Increase or Control pH.
 Use Scavengers.
 In open vessels, temperature lowers the
reaction rate.
 In closed vessels, temperature increases
rate.
Dr. Eng. Hamid A. Nagy
Uniform Attack
 If
we have one steel plate, corrosion will
take place.
 The anode and cathode will alternate from
a point at the surface to another.
 As the polarization of hydrogen increases
at a certain point.
 The other point will act as a cathode.
 Uniform corrosion is not very severe
usually.
Dr. Eng. Hamid A. Nagy
Measurement of
Corrosion








Conversion of Current to Corrosion Rate:
If i A/cm2 is the current density, i Coulombs/cm2 transfer per second.
OR (iX365X60X60X24) = (31,536,000Xi) Coulombs per year.
OR 31,536,000 (i/96,500) = 326.79 (i) Farads per year per cm2.
If the equivalent weight of the metal is (EW), this means that
(326.79XiXEW) gms/year/cm2
If the density of the metal is (r) grams/cm3, this means that
(326.79XiXEW/r) cm3 of metal corrode in one year from 1cm2 OR
the metal loses (326.79XiXEW/r) cm/year.
Metal loses (13.617/2.54)X(iXEW/r) or (128.66XiXEW/r) in./year.
This means that the metal lost (128,660XiXEW/r) mils/year {mpy}.
NOTE THAT THIS IS VALID ONLY FOR
UNIFORM CORROSION
Dr. Eng. Hamid A. Nagy
Galvanic corrosion






If you place two dissimilar metals beside each
other, the more negative potential will corrode.
Corrosion effect will increase as
Ratio of anode to cathode decreases.
Resistance of electrolyte deceases.
Criticality of corroded part increases.
Some notes about painting.
Dr. Eng. Hamid A. Nagy
Electrochemical
Series





Metals are ranked in accordance with their
potential in 1 N solution of their solutions.
Hydrogen is zero reference:
2H+ + 2e → H2
If metal has positive value (Au, Ag, Pt), it is
called noble metal or semi-noble (Cu).
If metal has negative value (Fe, Al, Mg, Zn), it is
called active metal.
Such Ranking is called Electrochemical Series.
Dr. Eng. Hamid A. Nagy
Galvanic Series
 From
all the discussion, it can be noticed
that every metal will have different
potentials in different media.
 The behavior in different media depends
on many different correlated factors.
 This is why Electorchemical series can be
not indicative of the corrosion state.
 So, Galvanic Series is more practical.
Dr. Eng. Hamid A. Nagy
Passivity
 But
what about if a product of corrosion is
formed.
 The rate of generation of product
increases with current.
 At a certain amount of product, it could
hinder ions from dissolution into solution.
 This makes the rate of corrosion very slow.
 This takes place in a few metals only.
Dr. Eng. Hamid A. Nagy
Pitting and Crevice
 At
a certain value passivity breaks down to
start the transpassivity stage.
 The presence of chloride ions was found
to decrease as the chloride content in the
solution increases.
 Chloride ions were expected to attack the
passive layer leaving unprotected area.
 This case represents high cathode to
anode area.
Dr. Eng. Hamid A. Nagy
Pitting and Crevice





As resistance of the material increases the Epit
is expected to increase.
So it can be taken as a measure of resistance to
pitting.
Chromium is added to iron to increase passivity.
At 12% Cr the surface is expected to be covered
with Cr2O3.
However further increase in Cr will increase the
passive layer thickness and increase resistance
to damage by chloride ions.
Dr. Eng. Hamid A. Nagy
Pitting and Crevice

As a rule of thumb those steels covered with
100% passive layer are called stainless steels.
 Cr and Mo increases both thickness and stability
of passive layer.
 However, Fe++ formed in the pit will hydrolyze
according to the reaction:
Fe++ + H2O + 2Cl- → Fe(OH)2 + 2HCl
HCl is a strong acid leading to decrease of the pH.
Dr. Eng. Hamid A. Nagy
Pitting and Crevice
Fe++, H+, Cl-
Transported Cl-
Probability
Fe+++
P/D
Surface Area
Dr. Eng. Hamid A. Nagy
Pitting and Crevice
 This
is why pits more corrosive
environment takes place within pits as
they grow leading to autocatalytic action.
 Nitrogen in steel was found to react with
H+ in the pits to form NH3 and reduce the
autocatalytic action.
 For stainless steels, pitting resistance
equivalent number (PREN) is equal to:
PREN = Cr + 3.3 (Mo + 0.5 W) + 16N
Dr. Eng. Hamid A. Nagy
Pitting and Crevice
 How
to measure the resistance of material
to pitting:
1- PREN will identify the grade of stainless steel.
2- Pitting potential.
3- the potential at which the anodic polarization curve
intersects with the passive zone again (Eprot).
 However,
the difference of Epit-Eprot is more
indicative of the resistance.
Dr. Eng. Hamid A. Nagy
Pitting and Crevice

Pitting is expected to grow more downward or at
the upstream especially encountering elbows.
 Now how to measure the intensity of pitting:



Density.
Diameter.
Depth.

Pitting factor is a measure of the prevailage of
pitting against general corrosion
 P/d tends to zero for general corrosion.
Dr. Eng. Hamid A. Nagy
Pitting and Crevice
 P-d
d
P
could be
measured by:
 1- Metallography.
 2- Machining
 3- Micrometer.
 4- Microscopy.
Dr. Eng. Hamid A. Nagy
Pitting and Crevice
Dr. Eng. Hamid A. Nagy
Pitting and Crevice
Dr. Eng. Hamid A. Nagy
Pitting and Crevice
 Crevice
attack is similar to pitting in a way
or another.
 Inside the crevice lack of oxygen and
increase in the chloride content take place
leading to break down of passivity.
 Thermal insulation and carbonate deposits
may lead to the dame situation.
 Filliform corrosion is an example of crevice
attack.
Dr. Eng. Hamid A. Nagy
Pitting and Crevice
Inert Washer
Stainless Steel
Dr. Eng. Hamid A. Nagy
Pitting and Crevice

Even in bolts, which after rain contains some
corrosive media in their crevices (does not dry
easily).
 Solutions include:





1- Use larger and less number of bolts.
2- Tighten the bolts as possible.
3- Use ductile caulking.
4- Use sealing compounds.
5- Use Weathering Steel (A HSLA containing copper,
phosphorous and nickel in controlled amounts).
Dr. Eng. Hamid A. Nagy
Differential Aeration

can take place even if there is no passivity.
 Consider immersion of pipe in the earth (soil to
air interface).
 There will be difference in the mixed potential
due to different values of oxygen Concentration.
Dr. Eng. Hamid A. Nagy
Pourbaix Diagram
Dr. Eng. Hamid A. Nagy
Reference
Electrodes

Our zero arbitrary
reference electrode.
 Potential =0 at STP.
H+ + 2e → H2
Platinized
Platinum
H2
Hg
H2SO4
H2
Standard Hydrogen Electrode
(SHE) Dr. Eng. Hamid A. Nagy
Reference
Electrodes
Pt wire

Potential =0.241 V Vs.
SHE.
Calomel +
Mercury
Hg2Cl2 + 2e → 2Hg + 2ClSaturated
KCl
Saturated Calomel Electrode
(SCE)
Dr. Eng. Hamid A. Nagy
Reference
Electrodes

Potential = 0.318 V
Vs. SHE
Cu Rod
Saturated
CuSO4
Cu++ + 2e → Cu
Porous Plug
Cu/ CuSO4 Electrode
Dr. Eng. Hamid A. Nagy
Measurement of
Corrosion
V
A
Reference Electrode
Working Electrode
Auxiliary Electrode
Dr. Eng. Hamid A. Nagy
Corrosion
Dr. Eng. Hamid A. Nagy
Corrosion
Dr. Eng. Hamid A. Nagy
Corrosion
Dr. Eng. Hamid A. Nagy
Corrosion
Dr. Eng. Hamid A. Nagy
Corrosion
Dr. Eng. Hamid A. Nagy
Corrosion
Dr. Eng. Hamid A. Nagy
Corrosion
Dr. Eng. Hamid A. Nagy
Corrosion
Dr. Eng. Hamid A. Nagy
Stress Corrosion
Cracking
Chloride SCC




Chloride stress corrosion is a type of intergranular
corrosion
occurs in austenitic stainless steel under tensile stress in
the presence of oxygen, chloride ions, and high
temperature.
It is thought to start with chromium carbide deposits
along grain boundaries.
This form of corrosion is controlled by maintaining low
chloride ion and oxygen content in the environment and
use of low carbon and stabilized grades of stainless
steels.
Dr. Eng. Hamid A. Nagy
Stress Corrosion
Cracking
Caustic SCC
 Carbon
and low alloy steels in Sodium
hydroxide which is added to increase the
pH in boiler waters (for corrosion control).
 Interstitials at the grain boundaries of
weldments, also residual stress increase
the situation.
 Use of NH4OH instead of NaOH and use
of phosphate buffer may be solutions.
Dr. Eng. Hamid A. Nagy
EAC

EAC includes two mechanisms that should be
distinguished: Corrosion fatigue and SCC.
“Corrosion fatigue” occurs when chemically
reactive agents penetrate fatigue cracks.
 SCC involves corrosive mechanisms and
depends on both an aggressive environment
and tensile stress.
 SCC in pipelines is further characterized as
“high-pH SCC” or “near neutral-pH SCC,” with
the “pH” referring to the environment at the crack
location and not the soil pH.
Dr. Eng. Hamid A. Nagy
EAC

SCC cracking is usually oriented longitudinally in
response to the hoop stress of the pipe, which is
usually the dominant stress component resulting
from the internal pressure. However, in some
cases SCC also occurs in the circumferential
direction (C-SCC)
 when the predominant stress is an axial stress,
such as stresses developed in response to pipe
resistance of soil movement, at a field bend, or
due to the residual welding stresses at a girth
weld
Dr. Eng. Hamid A. Nagy
SCC Pipelines


A concentrated carbonate-bicarbonate (CO3-HCO3) solution has
been identified as the most probable environment responsible for
high-pH SCC. This environment develops as a result of the
interaction between hydroxyl ions produced by the cathode reaction
and CO2 in the soil generated by the decay of organic matter.
In the case of near neutral-pH SCC, the cracking environment
appears to be a diluted groundwater containing dissolved CO2. The
source of the CO2 is typically the decay of organic matter and
geochemical reactions in the soil.
Dr. Eng. Hamid A. Nagy
SCC Pipelines

The mechanical properties of highest interest for most
transmission piping are the yield strength and the
toughness.
 Generally, the best economics result from selecting the
highest strength pipe material available for the design of
a new pipeline system. As improved manufacturing
procedures
 are being developed, higher grades of pipe is being
purchased. There is no strong evidence that increasing
strengths up to and through grade X70 increases
susceptibility to SCC initiation or growth.
 Increases in toughness, which have occurred in parallel
with strength, have significantly increased the critical
size of the crack necessary to produce ruptures.
Dr. Eng. Hamid A. Nagy
SCC Pipelines


Below some value of tensile stress, referred to as the threshold
stress, crack initiation does not occur.
The threshold stress is difficult to accurately define but, depending
on the range of stress fluctuation, is on the order of 40 to 100
percent of the yield strength for classical SCC.
Dr. Eng. Hamid A. Nagy
Stress Corrosion
Cracking
Effective means of preventing
SCC:



1) Design properly with the right materials;
2) Reduce residual stresses;
3) remove critical environmental species such as
hydroxides, chlorides, and oxygen;
 4) and avoid stagnant areas and crevices in heat
exchangers
 Low alloy steels are less susceptible than high
alloy steels, but they may be subjected to SCC
in water containing chloride ions.
Dr. Eng. Hamid A. Nagy
Cathodic Protection
Dr. Eng. Hamid A. Nagy
Cathodic Protection
Dr. Eng. Hamid A. Nagy
Cathodic Protection
Dr. Eng. Hamid A. Nagy
Cathodic Protection
Dr. Eng. Hamid A. Nagy
Cathodic Protection
Dr. Eng. Hamid A. Nagy
Cathodic Protection
is to depress potential towards negative
value.
Potential
 It
Dr.Current
Eng. Hamid A. Nagy
Cathodic Protection
 It
is to depress potential towards negative
value.
Impressed Current
Potential
1. Rectifier
2. Structure is to be negative.
3. Potential and Current Demand.
4. All materials could serve as anode
but polarization should be as low
as possible.
Current Required
Dr.Current
Eng. Hamid A. Nagy
Cathodic Protection
 It
is to depress potential towards
negative
Sacrificial
Anode
value.
1. More active anodes.
Potential
2. Good in Seawater and
fluids.
3. Electrodes should have high
surface area and low
polarization.
4. IR drop should be as low as
possible.
5. Needs replacement.
6. Mg, Zn and Al.
7. But reverse polarity and
passivity.
Dr.Current
Eng. Hamid A. Nagy
Cathodic Protection
 How
to design?
Remember
Potential
1. You may improve the
situation by increasing
anode surface area or
reduce anodic polarization.
X
Y
Dr.Current
Eng. Hamid A. Nagy
Cathodic Protection
 How
to design?
Estimate
Potential
1. Hypothetical life of either
structure or CP system.
2. Then estimate the allowable
mpy.
X
Y
3. Transform to A/sec.(Point X)
4. Extend to Y.
5. Calculate IR drop.
6. From resistance of
electrolyte, estimate the
longest protected path.
Dr.Current
Eng. Hamid A. Nagy
Cathodic Protection
 How
to design?
Do not forget
1. You may improve situation
by increasing surface area
of Zinc.
2. This allows more IR drop.
Longest Paths
3. Coating of steel is similar.
4. Estimate life of anode and
duration of replacement (Be
conservative).
Dr.Current
Eng. Hamid A. Nagy
Cathodic Protection
 How
to design?
Refine your results
1. Sum up the two passes
together.
2. You will find longer passes.
L
3. Repeat that for many L.
4. There is always an optimum
L to reduce no. of anodes
(Increase S).
S
5. You may use integration for
estimating potential of steel
at every point assuming
parallel connections.
Dr.Current
Eng. Hamid A. Nagy
Cathodic Protection
 Variation
of potential
Potential difference
between anode and
cathode includes:
1. Polarization at cathodse.
2. Ohmic potential change
around cathode.
3. Ohmic Potential change
through electrolyte.
4. Ohmic potential change
around anode.
5. Polarization at anode.
Cathode
Anode
Dr.Current
Eng. Hamid A. Nagy
Cathodic Protection
of potential
Note that
1. Note that polarization and
resistance of both soil and
cathode vicinity are
beneficial.
Potential
 Variation
Cathode
2.
But polarization at the
anode surface and its
resistance to soil is hamful.
Anode
Dr.Current
Eng. Hamid A. Nagy
Cathodic Protection
Attenuation:
Attenuation decreases the throwing power of cp system.
For Infinite length: Ex = Eo exp (-aX)
Ix = Io exp (-aX)

a = Rs/Rk
Rs is the pipe resistance per unit length.
Rk= Rsoil RL
RK is called Characteristic resistance.
RL is the leakage resistance.
RL = (Ex –Eo)/ (Ix-Io)
Dr.Current
Eng. Hamid A. Nagy
Cathodic Protection
Attenuation:
For two drainage: Ex = [Eo cosh (ad-aX)]/ (cosh ad)
Ix = [Io sinh (a-aX)]/ (sinh ad)

a = Rs/Rk
But note all the above equations assume:
1- Very far anode, current has equal access to all points (often not valid)
2. The electrolyte solution is uniform (Not valid in pipeline)
3. Coating resistance is high, uniform and ohmic.
4. Polarization at cathode is linear (Remember exponential or invariant).
Dr.Current
Eng. Hamid A. Nagy
Cathodic Protection





There are always nodes and attenuation.
Coating lowers the current demanded but what about deterioration
and damage with time.
Coating lowers attenuation also.
Attenuation is lower for less conductive soils (In winter and
summer).
Nodes may have hydrogen embrittement so rlation to welding.
Protective potential
Drain
Node
Conservative
design
Dr. Eng. Hamid A. Nagy
Cathodic Protection

Anodic reactions in impressed currents include:
M
Mn+ + n e
2 H2O
O2 + 4H+ + 4e
2 ClCl2 + 2e







First reaction is self corrosion.
Second reaction prevails in soil (Humidity at the vicinity of anode is
beneficial).
Third prevails in seawater and sometimes in salty earth.
Lowering polarization of the last two reactions include:
1- Backfill.
2- Use of vents.
3- Humidity (Poured water) [For both sacrificial and impressed].
Dr. Eng. Hamid A. Nagy
Cathodic Protection





Polarization of anode should be as low as
possible (Not all reactions).
This is why we use back fills (Granulated
carbon- Coke breeze).
Use vents to collect the gases from the anodic
reactions.
Deep electrodes has better current distribution
but not easy to operate and need survey of the
area.
Distributed anodes are more easy to manage.
Dr. Eng. Hamid A. Nagy
Cathodic Protection
Impressed current anodes include:
 Scrap steel and cast Iron:





Historical.
Only self corrosion.
Contamination is high.
But low anode material cost.
You may increase surface area inexpensively.
Dr. Eng. Hamid A. Nagy
Cathodic Protection
Impressed current anodes include:
 Cr bearing high silicon cast iron):




In soil (Resistance to abrasion and rough handling) .
Self corrosion is low but significant).
Contamination is high.
Note that common Stainless steel is unsuitable
(Breakdown of passivity and pitting).
Dr. Eng. Hamid A. Nagy
Cathodic Protection
Impressed current anodes include:
 Solid compacted graphite:







In Seawater and soil (Low cost and inertness) .
Fragile.
No self corrosion.
Very low overvoltage.
But C forms CO2 S0 buried graphite is limited in
current density.
Contamination is high.
Use of carbon as backfill is preferred.
Dr. Eng. Hamid A. Nagy
Cathodic Protection
Impressed current anodes include:
 Lead alloys:






Limited to sea water since overvoltage to chlorine
evolution is limited.
In Seawater forms conductive PbO2 (limits self
corrosion).
No destruction of passive film by Cl2 (PbCl2 is
insoluble).
But PbCl2 increases polarization for cathodic reaction.
Ag and antimony stabilizes PbO2 more than PbCl2.
Not to be buried in sea floor (access to Cl- is limited).
Dr. Eng. Hamid A. Nagy
Cathodic Protection
Impressed current anodes include:
 Conductive Oxides:


Magnetite.
DSA (Dimensionally Stable Anodes) mixtures
of Ruthenium oxide and titanium oxide
sintered on Titanium substrate.
Dr. Eng. Hamid A. Nagy
Cathodic Protection
Impressed current anodes include:
 Platinum and platinized platinum:





More noble than any anodic reaction.
Very low overvoltage.
Cost.
Platinized titanium (1-5 mm coating) is good
in weight.
But affected by DC ripples.
Dr. Eng. Hamid A. Nagy
Cathodic Protection
Anode Type
Platinized Titanium
PT wire
Pb-6Sb-1Ag
Graphite
Fe-14Si-4Cr
Anode Current Density,
2
A/m
540/1080
1080-5400
160-220
10.8-40
10.8-40
Consumption Rate
per A-yr
6 mg
10 mg
0.45-.09 kg
0.23-0.45 kg
0.23-0.45 kg
Dr. Eng. Hamid A. Nagy
Cathodic Protection
 Sacrificial Anodes




should have:
Highly negative potential (The more distance
and the higher electrolyte negativity
necessitates higher negativity).
Polarization should be very low.
The charge available to maintain current
(output) should be high.
Efficiency should be high.
Dr. Eng. Hamid A. Nagy
Cathodic Protection
Sacrificial Anodes include:
 Mg:




High negativity and low polarization.
Soil and pure water (hot water tanks). These
media have high resistance.
Not recommended for seawater because of
overprotection and inefficiency.
Alloying elements may be added to allow use
in low resistivity.
Dr. Eng. Hamid A. Nagy
Cathodic Protection
Sacrificial Anodes include:
 Al alloys:





In seawater only.
< 1% Zn, mercury, indium and tin to lower
passivity.
Very high output (High valence and low
density).
Not used in soil or pure water (passivation).
Low cost.
Dr. Eng. Hamid A. Nagy
Cathodic Protection
Sacrificial Anodes include:
 Zn:



In seawater but may be used in others.
Intermediate potential, low polarization and
high efficiency.
Used as pure as possible to reduce
polarization.
Dr. Eng. Hamid A. Nagy
Cathodic Protection
Property
Potential Vs.
SCE
Output, A-h/kg
Efficiency, %
Density,
gm/cm3
Relative cost
Mg
-1.68
Zn
-1.1
Al Alloy
-1.05
2200
50-60
1.7
810
>90
7.1
2000
>90
2.7
3
2
1
Dr. Eng. Hamid A. Nagy
Cathodic Protection
 Stray
current is current flowing from one
conductor to the pipe is but not easy to
manage.
 Solutions include:



Barriers but not easy to manage.
Connecting wires but this increases
attenuation.
Use of Shields.
Dr. Eng. Hamid A. Nagy
Cathodic Protection
Effect of Cathodic Reactions with time.
1- Alkalinity:
 Some offshore are left uncoated.



Scale forms as follows



O2 + 2H2O + 4e
4OHH2O + 2e
H2 + OHCa2+ + HCO3- + OHH2O + CaCO3
Mg2+ + 2OHMg (OH)2
As a result IL decreases with time and more
economic CP is available.
Dr. Eng. Hamid A. Nagy
Cathodic Protection
Effect of Cathodic Reactions with time.
1- Alkalinity on IL:
Potential
Log Current Density
Dr. Eng. Hamid A. Nagy
Cathodic Protection
Effect of Cathodic Reactions with time.
2- Alkalinity on coating:
 Alkalinity degrades organic coatings.
 Resin coatings resistant to alkalinity are
available.
3- Blistering of coating by hydrogen:
 Supplemental coating at nodes are necessary.
 This is called anode shield.
4- Hydrogen embrittlement.
Dr. Eng. Hamid A. Nagy
Cathodic Protection
Effect of Movement.



Seagoing Ship Hulls.
IL depends on flow rate.
Thyristor Rectifier should be used.
Fe
(IL) - High speed or waves
Dr. Eng. Hamid A. Nagy
Cathodic Protection
 Monitoring:



Two electrodes in a line parallel to pipeline
and separated by exact distance.
E = I RX
RX can by estimated from tabulated resistively
of seawater.
X
Dr. Eng. Hamid A. Nagy
Cathodic Protection

Monitoring (How to avoid effect of IR drop in soil)
1. Measure at different distances from pipe (Walking stick
reference electrode).
With Defected
Coating
Cathode
Dr. Eng. Hamid A. Nagy
Cathodic Protection
Monitoring (How to avoid effect of IR drop in soil)
2. Instant off method

switch cathodic current instantaneously.

IR drop instantly diminishes.

Local variation of polarization passes current between spots
leading to another IR drop.

All CP system should be switched off together.

-1200
IR
-1100
Depolarization
Time
Dr. Eng. Hamid A. Nagy
Cathodic Protection

Monitoring (How to avoid effect of IR drop in soil)
3. Bare Coupon Instant off Method:

Use coupon very near to pipe.

Simulates a defect shortened to the structure.

Most of disadvantages by Instant off method are eliminated.
Dr. Eng. Hamid A. Nagy
Corrosion
Dr. Eng. Hamid A. Nagy
Corrosion
Dr. Eng. Hamid A. Nagy
Corrosion
Dr. Eng. Hamid A. Nagy
Corrosion
Dr. Eng. Hamid A. Nagy
Corrosion
Dr. Eng. Hamid A. Nagy
Corrosion
Dr. Eng. Hamid A. Nagy
Corrosion
Dr. Eng. Hamid A. Nagy
Corrosion
Dr. Eng. Hamid A. Nagy
Corrosion
Dr. Eng. Hamid A. Nagy
Corrosion
Dr. Eng. Hamid A. Nagy
Corrosion
Dr. Eng. Hamid A. Nagy
Corrosion
Dr. Eng. Hamid A. Nagy
Design Considerations

For piping and heat
exchanger tubing to drain
completely it is necessary
to slope the piping or heat
exchanger just enough so
that water will drain and
not be trapped where the
pipe or tubing sags
slightly between support
points.
Horizontal - poor design
Horizontal sloped - very
good design
Dr. Eng. Hamid A. Nagy
Design Considerations
Dr. Eng. Hamid A. Nagy
Design Considerations
Dr. Eng. Hamid A. Nagy
Design Considerations
Dr. Eng. Hamid A. Nagy