weld-inspecta co. corrosion part 1 dr. eng. hamid a. nagy suez canal university

WELD-INSPECTA CO.WELD-INSPECTA CO.

Corrosion Part 1Corrosion Part 1

Dr. Eng. Hamid A. NagyDr. Eng. Hamid A. Nagy

Suez Canal University

Dr. Eng. Hamid A. Nagy

CorrosionCorrosion


Driving forceDriving force

Every Process to take place, we should Every Process to take place, we should have some driving force.have some driving force.

The driving force depends on the energy The driving force depends on the energy of the first state and that of the final state.of the first state and that of the final state.


Driving forceDriving force

En

erg

y

A

B

Bar

rier

Driv

ing

For

ce


ThermodynamicsThermodynamics

Every material has two sources of its Every material has two sources of its energyenergy Heat content, EnthalpyHeat content, Enthalpy Content depending on its randomness, Content depending on its randomness,

Entropy.Entropy. We can not measure this energy directly.We can not measure this energy directly. So we have a reference zero value which So we have a reference zero value which

is the hydrogen molecule formation.is the hydrogen molecule formation.



Now consider the transfer of a metal from Now consider the transfer of a metal from state A to state B.state A to state B.

This can only be done if the state A has This can only be done if the state A has higher energy than state B.higher energy than state B.

GGAA > > GGBB

G is the free energy of material state.G is the free energy of material state.



Now consider the reaction between two Now consider the reaction between two materials a and B to produce C.materials a and B to produce C.

The same law applies.The same law applies.

GGAA + + GGBB > > GGCC

For the reaction to proceed in the directionFor the reaction to proceed in the direction

A + B → CA + B → C

And vice versa.And vice versa.


Effect of Effect of ConcentrationConcentration

Consider the following ReactionConsider the following Reaction

MM++++ + 2e → M + 2e → M As the M ions concentration increases the As the M ions concentration increases the

reaction tends to go the left.reaction tends to go the left. Oxidation reaction increases.Oxidation reaction increases. More anodic tendency.More anodic tendency. Potential decreases. Potential decreases.


KineticsKinetics

Rate of reaction depends on what is called Rate of reaction depends on what is called mechanism.mechanism.

There should be some energy done to There should be some energy done to activate the first stage.activate the first stage.

This is called the energy barrier.This is called the energy barrier. This energy barrier could be high or low This energy barrier could be high or low

depending on the mechanism.depending on the mechanism.


KineticsKinetics

Overcoming energy barrier may consist of Overcoming energy barrier may consist of several steps.several steps.

The rate of occurrence of this reaction The rate of occurrence of this reaction depends on the interaction of steps to depends on the interaction of steps to overcome energy barrier.overcome energy barrier.

There is usually what is called rate There is usually what is called rate determining step.determining step.

Determining the rate of the reaction is Determining the rate of the reaction is what is called KINETICS.what is called KINETICS.


MetalsMetals

Corrosion is a chemical reaction.Corrosion is a chemical reaction. What is considered in corrosion?What is considered in corrosion?

Feasibility.Feasibility. Rate.Rate.

The answer is both.The answer is both. You can even protect metals if you impair You can even protect metals if you impair

the feasibility or slow down the rate.the feasibility or slow down the rate.


MetalsMetals

Now consider the structure of metal atoms Now consider the structure of metal atoms and their mutual relation.and their mutual relation.

Metal atoms have some free electrons.Metal atoms have some free electrons. In the matrix of metals, free electrons do In the matrix of metals, free electrons do

not relate specifically to a certain atom.not relate specifically to a certain atom. They are just Free ELECTRONS.They are just Free ELECTRONS. This is what provides the metals with their This is what provides the metals with their

Characteristic Properties.Characteristic Properties.


MetalsMetals

If the metals are going to react, What is If the metals are going to react, What is better for the atoms?better for the atoms?

To share these electrons.To share these electrons. To loss these electrons.To loss these electrons. The answer, for sure is the second option.The answer, for sure is the second option. So they are going to exchange ions with So they are going to exchange ions with

other reactants.other reactants. Ionic Bond.Ionic Bond.


Corrosion PotentialCorrosion Potential

Now, reaction of metals depends on Now, reaction of metals depends on G.G. But this reaction involves transfer of But this reaction involves transfer of

electrons.electrons. G is a measure of chemical energy.G is a measure of chemical energy. But this energy is transferred to electric But this energy is transferred to electric

energy for the reaction to take place.energy for the reaction to take place. Is it possible to measure this energy using Is it possible to measure this energy using

the electric parameters.the electric parameters.



Now Recall the definition of volt.Now Recall the definition of volt. The voltage between two points is 1 volt if The voltage between two points is 1 volt if

the amount of energy required to transfer the amount of energy required to transfer 1 unit charge is 1 unit of energy.1 unit charge is 1 unit of energy.

This is why This is why

P = V IP = V I Or Energy/ time = Volt X (Charge/ Time)Or Energy/ time = Volt X (Charge/ Time) Energy = Volt X Charge. Energy = Volt X Charge.


NormalityNormality

If you dissolve one mole (atomic weight) in If you dissolve one mole (atomic weight) in one liter of water, concentration is called one liter of water, concentration is called morality (1 molar solution).morality (1 molar solution).

If you dissolve one equivalent weight in If you dissolve one equivalent weight in one liter of water, concentration is called one liter of water, concentration is called Normality (1 Normal solution).Normality (1 Normal solution).

So, for one and the same materialSo, for one and the same material 1 N solution = 1M solution if n =1.1 N solution = 1M solution if n =1. 1 N solution = ½ M solution if n = 2.1 N solution = ½ M solution if n = 2.


NormalityNormality

For explanation let us consider sodium chloride For explanation let us consider sodium chloride (NaCl)(NaCl)

At. Wt. for Na = 23At. Wt. for Na = 23 At. Wt. for Cl = 17.At. Wt. for Cl = 17. At. Wt. for NaCl = 40.At. Wt. for NaCl = 40. 1 M solution means 40 gram NaCl in 1 liter 1 M solution means 40 gram NaCl in 1 liter

water.water. 1 N solution is the same.1 N solution is the same. Concentration of NaCl in sea water is about Concentration of NaCl in sea water is about

3.5% (35 grams in 1 liter).3.5% (35 grams in 1 liter).


NormalityNormality

Another example is (FeClAnother example is (FeCl22))

At. Wt. for Fe = 56At. Wt. for Fe = 56 At. Wt. for Cl = 17.At. Wt. for Cl = 17. At. Wt. for FeClAt. Wt. for FeCl22 = 90. = 90.

1 M solution means 90 grams FeCl1 M solution means 90 grams FeCl22 in 1 liter in 1 liter

water.water. 1 N solution means 45 grams FeCl1 N solution means 45 grams FeCl22 in 1 liter in 1 liter

water.water. Normality is more expressive than molality.Normality is more expressive than molality.


Electrolytic CellElectrolytic Cell

Now consider Electro-refining of Cu.Now consider Electro-refining of Cu.

Cu +ve potential

Oxidation prevails

(Anode)

Cu –ve potential

Reduction prevails

)Cathode(


Galvanic CellGalvanic Cell

Now consider a cell having Cu in 1N Now consider a cell having Cu in 1N CuSOCuSO44 solution in half cell and Zn in 1N solution in half cell and Zn in 1N ZnSOZnSO44 solution in the other half cell. solution in the other half cell.

ECu = 0.34 V

Reduction prevails

Positive Electrode

(Cathode)

EZn = -0.76 V

Oxidation prevails

Negative Electrode

)Anode(

V

O.C.P. = 1.1 Volts

Electrons

Current


Anode/ CathodeAnode/ Cathode

Now if you consider ZnNow if you consider Zn

ZnZn++++ + 2e → Zn + 2e → Zn This called reduction.This called reduction.

Or Or Zn → ZnZn → Zn++++ + 2e + 2e This is called oxidation.This is called oxidation. As a convention we use the potential As a convention we use the potential

measurement for the reduction reaction.measurement for the reduction reaction. If V increases, If V increases, G decreases, more reduction G decreases, more reduction

takes place, more protection.takes place, more protection.



Back to Cu + ZnBack to Cu + Zn Cu will not be dissolved (protected).Cu will not be dissolved (protected).

CuCu++++ + 2e → Cu + 2e → Cu Zn will dissolve (Corroded).Zn will dissolve (Corroded).

Zn → ZnZn → Zn++++ + 2e + 2e As the difference in potential increases As the difference in potential increases

more current takes place but not more current takes place but not necessarily.necessarily.


Corrosion RateCorrosion Rate

As we know OC potential is higher than As we know OC potential is higher than short circuit potential.short circuit potential.

How much is the highest current provided How much is the highest current provided by any galvanic cell, let us see.by any galvanic cell, let us see.


Concentration Concentration PolarizationPolarization

3- Concentration Polarization:3- Concentration Polarization: Consider the case of placing steel part in aerated water.Consider the case of placing steel part in aerated water. Anode: Fe dissolution Anode: Fe dissolution Cathode: Oxygen Reduction (Oxygen).Cathode: Oxygen Reduction (Oxygen).1- Transport of oxygen to steel by diffusion.1- Transport of oxygen to steel by diffusion.2- Reduction of Oxygen2- Reduction of Oxygen

OO22 + 2H + 2H22O +4e → 4OHO +4e → 4OH--

Ste

el

Low diffusion


Concentration Concentration PolarizationPolarization

Factors affecting Factors affecting this phenomenon:this phenomenon:

1- Temperature.1- Temperature.

2- Agitation.2- Agitation.

3- Pressure3- Pressure

4- Flow Rate.4- Flow Rate.

5- Concentration.5- Concentration.

O2

Pot

entia

l

Current

Fe

High Oxygen

DiffusionLow Oxygen Diffusion


Control of RateControl of Rate

Decrease the metal conductance.Decrease the metal conductance. Lower the electrolytic conductance.Lower the electrolytic conductance. Control one of the surfaces (Larger Anode Control one of the surfaces (Larger Anode

is Better.is Better. Control one of the two reactions (anodic Control one of the two reactions (anodic

and Cathodic).and Cathodic).


Control of Cathodic ReactionsControl of Cathodic Reactions

2H2H++ + 2e → H + 2e → H22

Increase or Control pH.Increase or Control pH. Increase Pressure (not a solution)Increase Pressure (not a solution) Decrease Pubbling rate (not a solution in Decrease Pubbling rate (not a solution in

tanks and pipelines.tanks and pipelines.


Control of Cathodic ReactionsControl of Cathodic Reactions

OO22 + 2 H + 2 H22O + 4 e → 4 OHO + 4 e → 4 OH--

Increase or Control pH.Increase or Control pH. Use Scavengers.Use Scavengers. In open vessels, temperature lowers the In open vessels, temperature lowers the

reaction rate.reaction rate. In closed vessels, temperature increases In closed vessels, temperature increases

rate.rate.


Uniform AttackUniform Attack

If we have one steel plate, corrosion will If we have one steel plate, corrosion will take place.take place.

The anode and cathode will alternate from The anode and cathode will alternate from a point at the surface to another.a point at the surface to another.

As the polarization of hydrogen increases As the polarization of hydrogen increases at a certain point.at a certain point.

The other point will act as a cathode.The other point will act as a cathode. Uniform corrosion is not very severe Uniform corrosion is not very severe

usually.usually.


Measurement of Measurement of CorrosionCorrosion

Conversion of Current to Corrosion Rate:Conversion of Current to Corrosion Rate: If i A/cmIf i A/cm2 2 is the current density, i Coulombs/cmis the current density, i Coulombs/cm22 transfer per second. transfer per second. OR (iX365X60X60X24) = (31,536,000Xi) Coulombs per year. OR (iX365X60X60X24) = (31,536,000Xi) Coulombs per year. OR 31,536,000 (i/96,500) = 326.79 (i) Farads per year per cmOR 31,536,000 (i/96,500) = 326.79 (i) Farads per year per cm22.. If the equivalent weight of the metal is (EW), this means thatIf the equivalent weight of the metal is (EW), this means that )326.79XiXEW()326.79XiXEW( gms/year/cmgms/year/cm22

If the density of the metal is (If the density of the metal is () grams/cm) grams/cm33, this means that , this means that (326.79XiXEW/r) cm(326.79XiXEW/r) cm33 of metal corrode in one year from 1cm of metal corrode in one year from 1cm2 2 OR the OR the metal loses metal loses )326.79XiXEW/)326.79XiXEW/(( cm/yearcm/year..

Metal loses (13.617/2.54)X(iXEW/r) or Metal loses (13.617/2.54)X(iXEW/r) or )128.66XiXEW/)128.66XiXEW/(( in./yearin./year.. This means that the metal lost This means that the metal lost )128,660XiXEW/)128,660XiXEW/(( mils/year {mpy}.mils/year {mpy}.

NOTE THAT THIS IS VALID ONLY FOR

UNIFORM CORROSION


Galvanic corrosionGalvanic corrosion

If you place two dissimilar metals beside each If you place two dissimilar metals beside each other, the more negative potential will corrode.other, the more negative potential will corrode.

Corrosion effect will increase as Corrosion effect will increase as Ratio of anode to cathode decreases.Ratio of anode to cathode decreases. Resistance of electrolyte deceases.Resistance of electrolyte deceases. Criticality of corroded part increases.Criticality of corroded part increases. Some notes about painting.Some notes about painting.


Electrochemical Electrochemical SeriesSeries

Metals are ranked in accordance with their Metals are ranked in accordance with their potential in 1 N solution of their solutions.potential in 1 N solution of their solutions.

Hydrogen is zero reference: Hydrogen is zero reference:

2H2H++ + 2e → H + 2e → H22

If metal has positive value (Au, Ag, Pt), it is If metal has positive value (Au, Ag, Pt), it is called noble metal or semi-noble (Cu).called noble metal or semi-noble (Cu).

If metal has negative value (Fe, Al, Mg, Zn), it is If metal has negative value (Fe, Al, Mg, Zn), it is called active metal.called active metal.

Such Ranking is called Electrochemical Series.Such Ranking is called Electrochemical Series.


Galvanic SeriesGalvanic Series

From all the discussion, it can be noticed From all the discussion, it can be noticed that every metal will have different that every metal will have different potentials in different media.potentials in different media.

The behavior in different media depends The behavior in different media depends on many different correlated factors.on many different correlated factors.

This is why Electorchemical series can be This is why Electorchemical series can be not indicative of the corrosion state.not indicative of the corrosion state.

So, Galvanic Series is more practical.So, Galvanic Series is more practical.


PassivityPassivity

But what about if a product of corrosion is But what about if a product of corrosion is formed.formed.

The rate of generation of product The rate of generation of product increases with current.increases with current.

At a certain amount of product, it could At a certain amount of product, it could hinder ions from dissolution into solution.hinder ions from dissolution into solution.

This makes the rate of corrosion very This makes the rate of corrosion very slow.slow.

This takes place in a few metals only. This takes place in a few metals only.


Pitting and CrevicePitting and Crevice

At a certain value passivity breaks down to At a certain value passivity breaks down to start the transpassivity stage. start the transpassivity stage.

The presence of chloride ions was found The presence of chloride ions was found to decrease as the chloride content in the to decrease as the chloride content in the solution increases.solution increases.

Chloride ions were expected to attack the Chloride ions were expected to attack the passive layer leaving unprotected area.passive layer leaving unprotected area.

This case represents high cathode to This case represents high cathode to anode area. anode area.



As resistance of the material increases the EAs resistance of the material increases the Epitpit is expected to increase. is expected to increase.

So it can be taken as a measure of resistance to So it can be taken as a measure of resistance to pitting.pitting.

Chromium is added to iron to increase passivity.Chromium is added to iron to increase passivity. At 12% Cr the surface is expected to be covered At 12% Cr the surface is expected to be covered

with Crwith Cr22OO33.. However further increase in Cr will increase the However further increase in Cr will increase the

passive layer thickness and increase resistance passive layer thickness and increase resistance to damage by chloride ions.to damage by chloride ions.



As a rule of thumb those steels covered with As a rule of thumb those steels covered with 100% passive layer are called stainless steels.100% passive layer are called stainless steels.

Cr and Mo increases both thickness and stability Cr and Mo increases both thickness and stability of passive layer.of passive layer.

However, FeHowever, Fe++++ formed in the pit will hydrolyze formed in the pit will hydrolyze according to the reaction:according to the reaction:

FeFe++++ + H + H22O + 2ClO + 2Cl-- → Fe)OH( → Fe)OH(22 + 2HCl + 2HCl

HCl is a strong acid leading to decrease of the pH.HCl is a strong acid leading to decrease of the pH.



Surface Area

Pro

babi

lity

P/DFe++, H+, Cl-

Fe+++

Transported Cl-



This is why pits more corrosive This is why pits more corrosive environment takes place within pits as environment takes place within pits as they grow leading to autocatalytic action.they grow leading to autocatalytic action.

Nitrogen in steel was found to react with Nitrogen in steel was found to react with HH++ in the pits to form NH in the pits to form NH33 and reduce the and reduce the autocatalytic action.autocatalytic action.

For stainless steels, pitting resistance For stainless steels, pitting resistance equivalent number (PREN) is equal to:equivalent number (PREN) is equal to:

PREN = Cr + 3.3 )Mo + 0.5 W( + 16NPREN = Cr + 3.3 )Mo + 0.5 W( + 16N



How to measure the resistance of material How to measure the resistance of material to pitting:to pitting:1- PREN will identify the grade of stainless steel.1- PREN will identify the grade of stainless steel.

2- Pitting potential.2- Pitting potential.

3- the potential at which the anodic polarization curve 3- the potential at which the anodic polarization curve intersects with the passive zone again (Eintersects with the passive zone again (Eprotprot).).

However, the difference of EHowever, the difference of Epitpit-E-Eprotprot is more is more

indicative of the resistance.indicative of the resistance.



Pitting is expected to grow more downward or at Pitting is expected to grow more downward or at the upstream especially encountering elbows.the upstream especially encountering elbows.

Now how to measure the intensity of pitting:Now how to measure the intensity of pitting: Density.Density. Diameter.Diameter. Depth.Depth.

Pitting factor is a measure of the prevailage of Pitting factor is a measure of the prevailage of pitting against general corrosionpitting against general corrosion

P/d tends to zero for general corrosion.P/d tends to zero for general corrosion.



P-d could be P-d could be measured by:measured by:

1- Metallography.1- Metallography. 2- Machining2- Machining 3- Micrometer.3- Micrometer. 4- Microscopy.4- Microscopy.

Pd



Crevice attack is similar to pitting in a way Crevice attack is similar to pitting in a way or another.or another.

Inside the crevice lack of oxygen and Inside the crevice lack of oxygen and increase in the chloride content take place increase in the chloride content take place leading to break down of passivity.leading to break down of passivity.

Thermal insulation and carbonate deposits Thermal insulation and carbonate deposits may lead to the dame situation.may lead to the dame situation.

Filliform corrosion is an example of crevice Filliform corrosion is an example of crevice attack.attack.



Inert Washer

Stainless Steel



Even in bolts, which after rain contains some Even in bolts, which after rain contains some corrosive media in their crevices (does not dry corrosive media in their crevices (does not dry easily).easily).

Solutions include:Solutions include: 1- Use larger and less number of bolts.1- Use larger and less number of bolts. 2- Tighten the bolts as possible.2- Tighten the bolts as possible. 3- Use ductile caulking.3- Use ductile caulking. 4- Use sealing compounds.4- Use sealing compounds. 5- Use Weathering Steel (A HSLA containing copper, 5- Use Weathering Steel (A HSLA containing copper,

phosphorous and nickel in controlled amounts).phosphorous and nickel in controlled amounts).


Differential AerationDifferential Aeration

can take place even if there is no passivity.can take place even if there is no passivity. Consider immersion of pipe in the earth (soil to Consider immersion of pipe in the earth (soil to

air interface).air interface). There will be difference in the mixed potential There will be difference in the mixed potential

due to different values of oxygen Concentration.due to different values of oxygen Concentration.


Pourbaix DiagramPourbaix Diagram


Reference Reference ElectrodesElectrodes

Our zero arbitrary Our zero arbitrary reference electrode.reference electrode.

Potential =0 at STP.Potential =0 at STP.

HH++ + 2e → H + 2e → H22

H2

H2

H2SO4

Platinized Platinum

Hg

Standard Hydrogen Electrode )SHE(



Potential =0.241 V Vs. Potential =0.241 V Vs. SHE.SHE.

HgHg22ClCl22 + 2e → 2Hg + 2Cl + 2e → 2Hg + 2Cl--

Saturated Calomel Electrode )SCE(

Saturated KCl

Pt wire

Calomel + Mercury



Potential = 0.318 V Potential = 0.318 V Vs. SHEVs. SHE

CuCu++++ + 2e → Cu + 2e → Cu

Cu/ CuSO4 Electrode

Saturated CuSO4

Cu Rod

Porous Plug


Measurement of Measurement of CorrosionCorrosion

A

Auxiliary E

lectrode

Working E

lectrode

Reference E

lectrode

V


CorrosionCorrosion


Stress Corrosion Stress Corrosion CrackingCracking

Chloride SCCChloride SCC Chloride stress corrosion is a type of intergranular Chloride stress corrosion is a type of intergranular

corrosioncorrosion occurs in austenitic stainless steel under tensile stress in occurs in austenitic stainless steel under tensile stress in

the presence of oxygen, chloride ions, and high the presence of oxygen, chloride ions, and high temperature. temperature.

It is thought to start with chromium carbide deposits It is thought to start with chromium carbide deposits along grain boundaries.along grain boundaries.

This form of corrosion is controlled by maintaining low This form of corrosion is controlled by maintaining low chloride ion and oxygen content in the environment and chloride ion and oxygen content in the environment and use of low carbon and stabilized grades of stainless use of low carbon and stabilized grades of stainless steels.steels.



Caustic SCCCaustic SCC Carbon and low alloy steels in Sodium Carbon and low alloy steels in Sodium

hydroxide which is added to increase the hydroxide which is added to increase the pH in boiler waters (for corrosion control).pH in boiler waters (for corrosion control).

Interstitials at the grain boundaries of Interstitials at the grain boundaries of weldments, also residual stress increase weldments, also residual stress increase the situation.the situation.

Use of NHUse of NH44OH instead of NaOH and use OH instead of NaOH and use of phosphate buffer may be solutions.of phosphate buffer may be solutions.


EACEAC

EAC includes two mechanisms that should be EAC includes two mechanisms that should be distinguished: Corrosion fatigue and SCC. distinguished: Corrosion fatigue and SCC. “Corrosion fatigue” occurs when chemically “Corrosion fatigue” occurs when chemically reactive agents penetrate fatigue cracks.reactive agents penetrate fatigue cracks.

SCC involves corrosive mechanisms and SCC involves corrosive mechanisms and depends on both an depends on both an aggressive environment aggressive environment and tensile stressand tensile stress..

SCC in pipelines is further characterized as SCC in pipelines is further characterized as “high-pH SCC” or “near neutral-pH SCC,” with “high-pH SCC” or “near neutral-pH SCC,” with the “pH” referring to the environment at the crack the “pH” referring to the environment at the crack location and not the soil pH.location and not the soil pH.


EACEAC

SCC cracking is usually oriented longitudinally in SCC cracking is usually oriented longitudinally in response to the hoop stress of the pipe, which is response to the hoop stress of the pipe, which is usually the dominant stress component resulting usually the dominant stress component resulting from the internal pressure. However, in some from the internal pressure. However, in some cases SCC also occurs in the circumferential cases SCC also occurs in the circumferential direction (C-SCC)direction (C-SCC)

when the predominant stress is an axial stress, when the predominant stress is an axial stress, such as stresses developed in response to pipe such as stresses developed in response to pipe resistance of soil movement, at a field bend, or resistance of soil movement, at a field bend, or due to the residual welding stresses at a girth due to the residual welding stresses at a girth weldweld


SCC PipelinesSCC Pipelines A concentrated carbonate-bicarbonate (CO3-HCO3) solution has A concentrated carbonate-bicarbonate (CO3-HCO3) solution has

been identified as the most probable environment responsible for been identified as the most probable environment responsible for high-pH SCC. This environment develops as a result of the high-pH SCC. This environment develops as a result of the interaction between hydroxyl ions produced by the cathode reaction interaction between hydroxyl ions produced by the cathode reaction and COand CO22 in the soil generated by the decay of organic matter. in the soil generated by the decay of organic matter.

In the case of near neutral-pH SCC, the cracking environment In the case of near neutral-pH SCC, the cracking environment appears to be a diluted groundwater containing dissolved CO2. The appears to be a diluted groundwater containing dissolved CO2. The source of the CO2 is typically the decay of organic matter and source of the CO2 is typically the decay of organic matter and geochemical reactions in the soil.geochemical reactions in the soil.


SCC PipelinesSCC Pipelines The mechanical properties of highest interest for most The mechanical properties of highest interest for most

transmission piping are the yield strength and the transmission piping are the yield strength and the toughness. toughness.

Generally, the best economics result from selecting the Generally, the best economics result from selecting the highest strength pipe material available for the design of highest strength pipe material available for the design of a new pipeline system. As improved manufacturing a new pipeline system. As improved manufacturing proceduresprocedures

are being developed, higher grades of pipe is being are being developed, higher grades of pipe is being purchased. There is no strong evidence that increasing purchased. There is no strong evidence that increasing strengths up to and through grade X70 increases strengths up to and through grade X70 increases susceptibility to SCC initiation or growth.susceptibility to SCC initiation or growth.

Increases in toughness, which have occurred in parallel Increases in toughness, which have occurred in parallel with strength, have significantly increased the critical size with strength, have significantly increased the critical size of the crack necessary to produce ruptures.of the crack necessary to produce ruptures.


SCC PipelinesSCC Pipelines Below some value of tensile stress, referred to as the threshold Below some value of tensile stress, referred to as the threshold

stress, crack initiation does not occur.stress, crack initiation does not occur. The threshold stress is difficult to accurately define but, depending The threshold stress is difficult to accurately define but, depending

on the range of stress fluctuation, is on the order of 40 to 100 on the range of stress fluctuation, is on the order of 40 to 100 percent of the yield strength for classical SCC.percent of the yield strength for classical SCC.



EfEffective means of preventing fective means of preventing SCC:SCC:

1) Design properly with the right materials; 1) Design properly with the right materials; 2) Reduce residual stresses; 2) Reduce residual stresses; 3) remove critical environmental species such as 3) remove critical environmental species such as

hydroxides, chlorides, and oxygen; hydroxides, chlorides, and oxygen; 4) and avoid stagnant areas and crevices in heat 4) and avoid stagnant areas and crevices in heat

exchangers exchangers Low alloy steels are less susceptible than high Low alloy steels are less susceptible than high

alloy steels, but they may be subjected to SCC alloy steels, but they may be subjected to SCC in water containing chloride ions. in water containing chloride ions.


Cathodic ProtectionCathodic Protection



It is to depress potential towards negative It is to depress potential towards negative value.value.

Pot

entia

l

Current

Fe ++ + 2e → Fe

Fe → F

e++ +

2e

H + + 2e → H2




Pot

entia

l

Current

Current Required

Impressed Current

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.




Pot

entia

l

Current

Zn ++ + 2e → Zn

Zn → Zn++ + 2e

2H + + 2e → H2

Fe++ + 2e → FeFe → F

e++ +

2e

Sacrificial Anode

1. More active anodes.

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.



How to design?How to design?

Pot

entia

l

Current

2H + + 2e → H2

X Y

Remember

1. You may improve the situation by increasing anode surface area or reduce anodic polarization.




Pot

entia

l

Current

Zn ++ + 2e → Zn

2H + + 2e → H2

Estimate

1. Hypothetical life of either structure or CP system.

2. Then estimate the allowable mpy.

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.

X Y




Current

Do not forget

1. You may improve situation by increasing surface area of Zinc.

2. This allows more IR drop.

3. Coating of steel is similar.

4. Estimate life of anode and duration of replacement (Be conservative).

Longest Paths




Current

Refine your results

1. Sum up the two passes together.

2. You will find longer passes.

3. Repeat that for many L.

4. There is always an optimum L to reduce no. of anodes (Increase S).

5. You may use integration for estimating potential of steel at every point assuming parallel connections.

L

S



Variation of potentialVariation of potential

Current

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.AnodeCathode



Variation of potentialVariation of potential

Current

Note that

1. Note that polarization and resistance of both soil and cathode vicinity are beneficial.

2. But polarization at the anode surface and its resistance to soil is hamful.

AnodeCathode

Pot

entia

l



Current

Attenuation:

Attenuation decreases the throwing power of cp system.

For Infinite length: Ex = Eo exp )-X(

Ix = Io exp )-X(

= 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(



Current

Attenuation:

For two drainage: Ex = [Eo cosh )d-X(]/ )cosh ad(

Ix = [Io sinh )a-X(]/ )sinh ad(

= 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(.


Cathodic ProtectionCathodic Protection There are always nodes and attenuation.There are always nodes and attenuation. Coating lowers the current demanded but what about deterioration Coating lowers the current demanded but what about deterioration

and damage with time.and damage with time. Coating lowers attenuation also.Coating lowers attenuation also. Attenuation is lower for less conductive soils (In winter and Attenuation is lower for less conductive soils (In winter and

summer).summer). Nodes may have hydrogen embrittement so rlation to welding.Nodes may have hydrogen embrittement so rlation to welding.

Protective potential

Drain

NodeConservative design


Cathodic ProtectionCathodic Protection Anodic reactions in impressed currents include:Anodic reactions in impressed currents include:

M Mn+ + n e2 H2O O2 + 4H+ + 4e2 Cl- Cl2 + 2e

First reaction is self corrosion.First reaction is self corrosion. Second reaction prevails in soil (Humidity at the vicinity of anode is Second reaction prevails in soil (Humidity at the vicinity of anode is

beneficial).beneficial). Third prevails in seawater and sometimes in salty earth.Third prevails in seawater and sometimes in salty earth. Lowering polarization of the last two reactions include:Lowering polarization of the last two reactions include: 1- Backfill.1- Backfill. 2- Use of vents.2- Use of vents. 3- Humidity (Poured water) [For both sacrificial and impressed].3- Humidity (Poured water) [For both sacrificial and impressed].


Cathodic ProtectionCathodic Protection Polarization of anode should be as low as Polarization of anode should be as low as

possible (Not all reactions).possible (Not all reactions). This is why we use back fills (Granulated This is why we use back fills (Granulated

carbon- Coke breeze).carbon- Coke breeze). Use vents to collect the gases from the anodic Use vents to collect the gases from the anodic

reactions.reactions. Deep electrodes has better current distribution Deep electrodes has better current distribution

but not easy to operate and need survey of the but not easy to operate and need survey of the area.area.

Distributed anodes are more easy to manage.Distributed anodes are more easy to manage.



Impressed current anodes include:Impressed current anodes include: Scrap steel and cast Iron:Scrap steel and cast Iron:

Historical.Historical. Only self corrosion.Only self corrosion. Contamination is high.Contamination is high. But low anode material cost.But low anode material cost. You may increase surface area inexpensively.You may increase surface area inexpensively.



Impressed current anodes include:Impressed current anodes include: Cr bearing high silicon cast iron):Cr bearing high silicon cast iron):

In soil (Resistance to abrasion and rough handling) .In soil (Resistance to abrasion and rough handling) . Self corrosion is low but significant).Self corrosion is low but significant). Contamination is high.Contamination is high. Note that common Stainless steel is unsuitable Note that common Stainless steel is unsuitable

(Breakdown of passivity and pitting).(Breakdown of passivity and pitting).



Impressed current anodes include:Impressed current anodes include: Solid compacted graphite:Solid compacted graphite:

In Seawater and soil (Low cost and inertness) .In Seawater and soil (Low cost and inertness) . Fragile.Fragile. No self corrosion.No self corrosion. Very low overvoltage.Very low overvoltage. But C forms COBut C forms CO2 2 S0 buried graphite is limited in S0 buried graphite is limited in

current density.current density. Contamination is high.Contamination is high. Use of carbon as backfill is preferred.Use of carbon as backfill is preferred.



Impressed current anodes include:Impressed current anodes include: Lead alloys:Lead alloys:

Limited to sea water since overvoltage to chlorine Limited to sea water since overvoltage to chlorine evolution is limited.evolution is limited.

In Seawater forms conductive PbOIn Seawater forms conductive PbO22 (limits self (limits self corrosion). corrosion).

No destruction of passive film by ClNo destruction of passive film by Cl22 (PbCl (PbCl22 is is insoluble).insoluble).

But PbClBut PbCl22 increases polarization for cathodic reaction. increases polarization for cathodic reaction. Ag and antimony stabilizes PbOAg and antimony stabilizes PbO22 more than PbCl more than PbCl22.. Not to be buried in sea floor (access to Cl- is limited).Not to be buried in sea floor (access to Cl- is limited).



Impressed current anodes include:Impressed current anodes include: Conductive Oxides:Conductive Oxides:

Magnetite.Magnetite. DSA (Dimensionally Stable Anodes) mixtures DSA (Dimensionally Stable Anodes) mixtures

of Ruthenium oxide and titanium oxide of Ruthenium oxide and titanium oxide sintered on Titanium substrate.sintered on Titanium substrate.



Impressed current anodes include:Impressed current anodes include: Platinum and platinized platinum:Platinum and platinized platinum:

More noble than any anodic reaction.More noble than any anodic reaction. Very low overvoltage.Very low overvoltage. Cost.Cost. Platinized titanium (1-5 mm coating) is good in Platinized titanium (1-5 mm coating) is good in

weight.weight. But affected by DC ripples.But affected by DC ripples.



Anode Type Anode Current Density, A/m2

Consumption Rate per A-yr

Platinized Titanium 540/1080 6 mg PT wire 1080-5400 10 mg Pb-6Sb-1Ag 160-220 0.45-.09 kg Graphite 10.8-40 0.23-0.45 kg Fe-14Si-4Cr 10.8-40 0.23-0.45 kg



Sacrificial Anodes should have:Sacrificial Anodes should have: Highly negative potential (The more distance Highly negative potential (The more distance

and the higher electrolyte negativity and the higher electrolyte negativity necessitates higher negativity).necessitates higher negativity).

Polarization should be very low.Polarization should be very low. The charge available to maintain current The charge available to maintain current

(output) should be high.(output) should be high. Efficiency should be high.Efficiency should be high.



Sacrificial Anodes include:Sacrificial Anodes include: Mg:Mg:

High negativity and low polarization.High negativity and low polarization. Soil and pure water (hot water tanks). These Soil and pure water (hot water tanks). These

media have high resistance.media have high resistance. Not recommended for seawater because of Not recommended for seawater because of

overprotection and inefficiency.overprotection and inefficiency. Alloying elements may be added to allow use Alloying elements may be added to allow use

in low resistivity.in low resistivity.



Sacrificial Anodes include:Sacrificial Anodes include: Al alloys:Al alloys:

In seawater only.In seawater only. < 1% Zn, mercury, indium and tin to lower < 1% Zn, mercury, indium and tin to lower

passivity.passivity. Very high output (High valence and low Very high output (High valence and low

density).density). Not used in soil or pure water (passivation).Not used in soil or pure water (passivation). Low cost.Low cost.



Sacrificial Anodes include:Sacrificial Anodes include: Zn:Zn:

In seawater but may be used in others.In seawater but may be used in others. Intermediate potential, low polarization and Intermediate potential, low polarization and

high efficiency.high efficiency. Used as pure as possible to reduce Used as pure as possible to reduce

polarization.polarization.



Property Mg Zn Al Alloy Potential Vs. SCE

-1.68 -1.1 -1.05

Output, A-h/kg 2200 810 2000 Efficiency, % 50-60 >90 >90 Density, gm/cm3

1.7 7.1 2.7

Relative cost 3 2 1



Stray current is current flowing from one Stray current is current flowing from one conductor to the pipe is but not easy to conductor to the pipe is but not easy to manage.manage.

Solutions include:Solutions include: Barriers but not easy to manage.Barriers but not easy to manage. Connecting wires but this increases Connecting wires but this increases

attenuation.attenuation. Use of Shields.Use of Shields.



Effect of Cathodic Reactions with timeEffect of Cathodic Reactions with time..1- Alkalinity:1- Alkalinity: Some offshore are left uncoated.Some offshore are left uncoated.

OO22 + 2H + 2H22O + 4e 4OHO + 4e 4OH--

HH22O + 2e HO + 2e H22 + OH + OH--

Scale forms as followsScale forms as follows CaCa2+2+ + HCO + HCO3-3- + OH + OH-- H H22O + CaCOO + CaCO33 MgMg2+2+ + 2OH + 2OH-- Mg (OH) Mg (OH)22

As a result IAs a result ILL decreases with time and more decreases with time and more economic CP is available.economic CP is available.



Effect of Cathodic Reactions with timeEffect of Cathodic Reactions with time..

1- Alkalinity on I1- Alkalinity on ILL::

Potential

Log Current Density

Time



Effect of Cathodic Reactions with timeEffect of Cathodic Reactions with time..2- Alkalinity on coating:2- Alkalinity on coating: Alkalinity degrades organic coatings.Alkalinity degrades organic coatings. Resin coatings resistant to alkalinity are Resin coatings resistant to alkalinity are

available.available.3- Blistering of coating by hydrogen:3- Blistering of coating by hydrogen: Supplemental coating at nodes are necessary.Supplemental coating at nodes are necessary. This is called anode shield.This is called anode shield.4- Hydrogen embrittlement.4- Hydrogen embrittlement.



Effect of MovementEffect of Movement.. Seagoing Ship Hulls.Seagoing Ship Hulls. IL depends on flow rate.IL depends on flow rate. Thyristor Rectifier should be used.Thyristor Rectifier should be used.

Fe

(IL) - High speed or waves



Monitoring:Monitoring: Two electrodes in a line parallel to pipeline Two electrodes in a line parallel to pipeline

and separated by exact distance.and separated by exact distance. E = I RE = I RXX

RRXX can by estimated from tabulated resistively can by estimated from tabulated resistively of seawater.of seawater.

X



Monitoring (How to avoid effect of IR drop in soil)Monitoring (How to avoid effect of IR drop in soil)

1. Measure at different distances from pipe (Walking stick 1. Measure at different distances from pipe (Walking stick reference electrode).reference electrode).

Cathode

With Defected Coating


Cathodic ProtectionCathodic Protection Monitoring (How to avoid effect of IR drop in soil)Monitoring (How to avoid effect of IR drop in soil)2. Instant off method 2. Instant off method

switch cathodic current instantaneously.switch cathodic current instantaneously. IR drop instantly diminishes.IR drop instantly diminishes. Local variation of polarization passes current between spots Local variation of polarization passes current between spots

leading to another IR drop.leading to another IR drop. All CP system should be switched off together.All CP system should be switched off together.

IR

Depolarization

-1200

Time

-1100


Cathodic ProtectionCathodic Protection Monitoring (How to avoid effect of IR drop in soil)Monitoring (How to avoid effect of IR drop in soil)

3. Bare Coupon Instant off Method:3. Bare Coupon Instant off Method: Use coupon very near to pipe.Use coupon very near to pipe. Simulates a defect shortened to the structure.Simulates a defect shortened to the structure. Most of disadvantages by Instant off method are eliminated.Most of disadvantages by Instant off method are eliminated.


CorrosionCorrosion


Design ConsiderationsDesign Considerations

For piping and heat For piping and heat exchanger tubing to drain exchanger tubing to drain completely it is necessary completely it is necessary to slope the piping or heat to slope the piping or heat exchanger just enough so exchanger just enough so that water will drain and that water will drain and not be trapped where the not be trapped where the pipe or tubing sags pipe or tubing sags slightly between support slightly between support points. points.

Horizontal - poor designHorizontal sloped - very

good design


Design ConsiderationsDesign Considerations

weld-inspecta co. corrosion part 1 dr. eng. hamid a. nagy suez canal university

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