Transcript Copper

Cathodic Protection of
Pipeline
1
Contents
principle of corrosion
Forms of corrosion
Environment Effects
corrosion protection
preparation of pipeline
Corrosion of pipeline
Cathodic protection of
pipeline
EIGHT : Case Study
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SEVEN :
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CHAPTER ONE
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Ch.1 principles of corrosion
 Introduction
Corrosion is the destructive attack of a material by reaction with its environment.
The serious consequences of the corrosion process have become a problem of
worldwide significance. In addition to our everyday encounters with this form of
degradation, corrosion causes plant shutdowns, waste of valuable resources, loss
or contamination of product, reduction in efficiency, costly maintenance, and
expensive over design; it also jeopardizes safety and inhibits technological
progress.
1.1 corrosion definition
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Corrosion is the deterioration of materials by chemical interaction with
their environment. The term corrosion is sometimes also applied to the
degradation of plastics, concrete and wood, but generally refers to
metals.
 Or Destruction of a metal by chemical or electrochemical reaction with
its' environment.
 Or A process in which a metal is destroyed by a chemical reaction
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1.2 corrosion principles
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The cathode is that portion of the metal surface where reduction takes place and does not
dissolve.
The anode is that portion of the metal surface that is corroded. It is the point at which metal
dissolves, or goes into solution. When metal dissolves, the metal atom loses electrons and is
oxidised
a. Corrosion occurs by an electrochemical process.
The phenomenon is similar to that which takes place when a carbon-zinc “dry” cell
generates a direct current. Basically, an anode (negative electrode), a cathode
(positive electrode), an electrolyte (environment), and a circuit connecting the anode
and the cathode are required for corrosion to occur (see Figure 1-1).
Figure (1-1), dry cell
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the predominant cathodic reaction is
O2 + H2O + 4e- → 4(OH)
The cathodic reaction that usually occurs in deaerated acids is
2H+ + 2e- → H2
In aerated acids, the cathodic reaction could be
O2 + 4H+ + 4e- → 2H2O
eq (1-2 )
eq (1-3)
eq (1- 4)
b. The number of electrons lost at the anode must equal the number of electrons gained at the
cathode.
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For example, if iron (Fe) was exposed to an aerated, corrosive water, the anodic reaction
would be
2Fe → 2Fe ++ + 4e(anodic)
eq (1-5)
O2 + 2H2O + 4e- → 4(OH- )
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(cathodic)
These can be summed to give the overall oxidation reduction reaction
2Fe + O2 + 2H2O → 2Fe ++ +4(OH- )
eq (1-6)
eq (1-7)
c. After dissolution
ferrous ions (Fe++) generally oxidize to ferric ions (Fe+++ ); these will combine with
hydroxide ions (OH- ) formed at the cathode to give a corrosion product called rust (FeOOH
or Fe2O3 x H2O).
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1.3 classification of corrosion
* General / Uniform Corrosion :
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Atmospheric
Galvanic
Stray-current
 General biological
 High-temperature
* Localized Corrosion
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Filiform
Crevice
 Pitting
 Localized microbiological
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Ch:2 Forms of corrosion
2.1 Uniform corrosion or general corrosion
as sometimes called, is defined as a type of corrosion attack (deterioration) that is more or less
uniformly distributed over the entire exposed surface of a metal (see illustration below).
Fig (2-1) , uniform corrosion
2.1.1 Mechanisms
The anodic reaction in the corrosion process is always the oxidation reaction:
M = M+ + eeq (2-1)
In acidic environments, i.e., pH < 7, the cathodic process is mainly the reduction of hydrogen
ions:
2H+ + 2e = H2
eq (2-2)
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With uniform distribution of cathodic reactants over the entire exposed metal surface, reactions (2-2) take
place in a "uniform" manner and there is no preferential site or location for cathodic or anodic reaction. The
cathodes and anodes are located randomly and alternating with time. The end result is a more or less
uniform loss of dimension.
Fig (2-2) Real uniform corrosion
2.1.2 Prevention or Remedial Action
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Uniform corrosion or general corrosion can be prevented through a number of methods:
Use thicker materials for corrosion allowance
Use paints or metallic coatings such as plating, galvanizing or anodizing
Use Corrosion inhibitors or modifying the environment
Cathodic protection (SA/ICCP) and Anodic Protection
selection of a more corrosion resistant alloy (i.e. higher alloy content or more inert alloy)
utilize coatings to act as a barrier between metal and environment.
modify the environment or add chemical inhibitors to reduce corrosion rate
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2.2 Galvanic
Accelerated corrosion which can occur when dissimilar metals are in electrical contact in the presence
of an electrolyte (i.e. conductive solution).
Fig (2-3) Galvanic corrosion
2.2.1 Mechanism
Different metals and alloys have different electrochemical potentials (or corrosion potentials) in the
same electrolyte. (i.e., the voltage) between two dissimilar metals is the driving force for the
destructive attack on the active metal (anode). Current flows through the electrolyte to the more noble
metal (cathode) and the less noble (anode) metal will corrode.
Fig (2-4) Real example of Galvanic corrosion
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2.2.3 Prevention or Remedial Action
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selection of alloys which are similar in electrochemical behavior and/or alloy content.
area ratio of more actively corroding material (anode) should be large relative to the
more inert material(cathode).
use coatings to limit cathode area.
insulate dissimilar metals.
use of effective inhibitor.
Select metals/alloys as close together as possible in the galvanic series.
2.3 crevice
Crevice corrosion is a localized form of corrosion usually associated with a stagnant
solution on the micro-environmental level This form of attack is generally associated
with the presence of small volumes of stagnant solution in occluded interstices,
beneath deposits and seals, or in crevices, e.g. at nuts and rivet heads. Deposits of
sand,
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Fig (2-5) Crevice corrosion
2.3.1 MECHANISM
. Autocatalytic process are three stage :
2.3.1.1 Stage one of a crevice formation ( Induction )
Fig (2-6) Stage one of a crevice formation
2.3.1.2 Stage two of a crevice formation (Restricted Convection)
Fig (2-7) Stage two of a crevice formation (Restricted Convection)
2.3.1.3Stage three of a crevice formation(Obstruction and
Electromigration)
Fig (2-8) Stage three of a crevice formation(Obstruction and Electromigration)
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2.3.2 Prevention
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Crevice corrosion can be designed out of the system
Use welded butt joints instead of riveted or bolted joints in new equipment
Eliminate crevices in existing lap joints by continuous welding or soldering
Use solid, non-absorbent gaskets such as Teflon.
Use higher alloys for increased resistance to crevice corrosion
design installations to enable complete draining (no corners or stagnant zones)
2.4 Pitting
Pitting: Pitting Corrosion is the localized corrosion of a metal surface confined to a point or
small area, that takes the form of cavities. Pitting is one of the most damaging forms of corrosion
Fig(2-9) Morphology of pitting
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2.4.2 Mechanisms
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For a homogeneous environment, pitting IS caused by the MATERIAL that may contain inclusions
(MnS to pit initiation )
Fig (2-10) Real pitting corrosion
2.4.3 Prevention or Remedial Action
* Pitting corrosion can be prevented through:
• Proper selection of materials with known resistance to the service environment
• Control pH, chloride concentration and temperature
• Cathodic protection and/or Anodic Protection
increase velocity of media and/or remove deposits of solids from exposed metal
surface
•
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2.5 Stress-corrosion cracking (SCC)
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Stress-corrosion cracking (SCC) is a cracking process that requires the simultaneous action of
a corrodent and sustained tensile stress. This excludes corrosion-reduced sections that fail by fast
fracture. It also excludes intercrystalline or transcrystalline corrosion, which can disintegrate a
alloy without applied or residual stress.
Fig(2-11) stress corrosion cracking
2.5.1 Mechanisms
Stress corrosion cracking results from the conjoint action of three components:
(1) a susceptible material;
(2) a specific chemical species (environment) and
(3) tensile stress.
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2.5.2 Prevention
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Stress corrosion cracking can be prevented through :
Control of stress level (residual or load) and hardness.
Avoid the chemical species that causes SCC.
Use of materials known not to crack in the specified environment.
Control temperature and or potential
2 .6 intergranular corrosion
Intergranular corrosion is sometimes also called "intercrystalline corrosion" or "interdendritic
corrosion".
Fig (2-12) intergranular corrosion
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2.6.1 Mechanisms
This type of attack results from local differences in composition, such as coring commonly
encountered in alloy castings. Grain boundary precipitation, notably chromium carbides in
stainless steels, is a well recognized and accepted mechanism of intergranular corrosion.
The precipitation of chromium carbides consumed the alloying element - chromium from a
narrow band along the grain boundary and this makes the zone anodic to the unaffected grains.
The chromium depleted zone becomes the preferential path for corrosion attack or crack
propagation if under tensile stress.
Fig (2-13) intergranular corrosion
2.6.2 Prevention
•Intergranular corrosion can be prevented through:
•Use low carbon (e.g. 304L, 316L) grade of stainless steels
•Use stabilized grades alloyed with titanium (for example type 321) or niobium (for example type 347).
Titanium and niobium are strong carbide- formers. They react with the carbon to form the corresponding
carbides thereby preventing chromium depletion.
•Use post-weld heat treatment.
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2.7 selective leaching
*Dealloying
is the selective corrosion of one or more components of a solid solution alloy. It is also called parting,
selective leaching or selective attack. Common dealloying examples are decarburization,
decobaltification , denickelification, dezincification, and graphitic corrosion or graphitization

*Decarburization is the selective loss of carbon from the surface layer of a carbon-containing alloy
due to reaction with one or more chemical substances in a medium that contacts the surface.
*Dezincification is the selective leaching of zinc from zinc-containing alloys. Most commonly found
in copper-zinc alloys containing less than 85% copper after extended service in water containing
dissolved oxygen.
Fig (2-14) forms of dezincification
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2.7.1 Mechanisms
Different metals and alloys have different electrochemical potentials (or corrosion potentials) in the
same electrolyte. Modern alloys contain a number of different alloying elements that exhibit
different corrosion potentials. The potential difference is the driving force for the preferential attack
on the more "active" element in the alloy.
Fig (2-15) dezincification
2.7.2 Prevention
Dealloying, selective leaching and graphitic corrosion can be prevented through the following
methods:
•Select metals/alloys that are more resistant to dealloying. For example, inhibited brass is more
resistant to dezincification that alpha brass, ductile iron is more resistant to graphitic corrosion than
gray cast iron.
•Control the environment to minimize the selective leaching
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2 .8 Erosion Corrosion
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Erosion corrosion is the corrosion of a metal which is caused or accelerated by the relative
motion of the environment and the metal surface.as shown in Fig(2-16)
Fig(2-16) Erosion corrosion
2.8.1 Mechanism
There are several mechanisms described by the conjoint action of flow and corrosion that
result in flow-influenced corrosion:
* Mass transport-control: Mass transport-controlled corrosion implies that the rate of corrosion is
dependent on the convective mass transfer processes at the metal/fluid interface.
Fig (2-17) Real of erosion corrosion
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*Erosion-corrosion: Erosion-corrosion is associated with a flow-induced mechanical removal
of the protective surface film that results in a subsequent corrosion rate increase via either
electrochemical or chemical processes. It is often accepted that a critical fluid velocity
must be exceeded for a given material
2.8.2 Prevention or Remedial Action
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selection of alloys with greater corrosion resistance and/or higher strength.
re-design of the system to reduce the flow velocity, turbulence, cavitation or impingement of the
environment.
reduction in the corrosive severity of the environment.
use of corrosion resistant and/or abrasion resistant coatings.
cathodic protection.
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Ch:3 Environment Effects
3.1 Atmospheric Corrosion
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Atmospheric corrosion can be defined as the corrosion of materials exposed to
air and its pollutants, rather than immersed in a liquid. Atmospheric corrosion
can further be classified into dry, damp, and wet categories.
3.1.1 Types of atmospheres and environments
 Rural. This type of atmosphere is generally the least corrosive and normally
does not contain chemical pollutants, but does contain organic and inorganic
particulates. The principal corrodents are moisture, oxygen, and carbon dioxide.
Arid and tropical types are special extreme cases in the rural category.
 Urban. This type of atmosphere is similar to the rural type in that there is little
industrial activity. Additional contaminants are of the SOx and NOx variety, from
motor vehicle and domestic fuel emissions.
 Industrial. These atmospheres are associated with heavy industrial
processing facilities and can contain concentrations of sulfur dioxide, chlorides,
phosphates, and nitrates.
 Marine. Fine windswept chloride particles that get deposited on surfaces
characterize this type of atmosphere. Marine atmospheres are usually highly
corrosive, and the corrosivity tends to be significantly dependent on wind
direction, wind speed, and distance from the coast. It should be noted that an
equivalently corrosive environment is created by the use of deicing salts on the
roads of many cold regions of the planet.
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3.2 Corrosion By Water
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Nearly all corrosion problems which occur in oilfield production operations are due to
the presence of water.
3.2.1 Effect OF Electrolyte Composition
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There are two aspects to the effects of electrolyte composition on the corrosion
circuit. The first is the conductivity of the electrolyte and the effect of electrolyte on
the base corrosion potential of the system.
3.2.2 PHYSICAL VARIABLES
 The variables of temperature, pressure, and velocity need to be accounted
for when designing and implementing a corrosion control program. Correct
application inhibitors and cathodic protection as corrosion control methods
are very dependent on these variables. Temperature and pressure are
interrelated, and the corrosivity of a system is further influenced by velocity
3.3 Soil in the Corrosion Process
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Soil has been defined in many ways, often depending upon the particular
interests of the person proposing the definition. In discussion of the soil as
an environmental factor in corrosion, no strict definitions or limitations will be
applied; rather, the complex interaction of all earthen materials will come
within the scope of the discussion.
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* The Corrosion Process in Soil

Although the soil as a corrosive environment is probably of greater complexity than any other
environment, it is possible to make some generalizations regarding soil types and corrosion.
Fig (3-4) corrosion by soil
3.3.3 Types of Soil Moisture
1. Free ground water. At some depth below the surface, water is constantly
present. This distance to the water table may vary from a few metres to
hundreds of metres, depending upon the geological formations present. Only a
small amount of the metal used in underground service is present in the
ground water zone. Such structures as well casings and under-river pipelines
are surrounded by ground water. The corrosion conditions in such a situation
are essentially those of an aqueous environment.
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2.
3.
Gravitational water. Water entering soil at the surface from rainfall or
some other source moves downward. This gravitational water will flow at
a rate governed largely by the physical structure regulating the pore
space at various zones in the soil profile. An impervious layer of clay, a
‘puddled’ soil, or other layers of material resistant to water passage may
act as an effective barrier to the gravitational water and cause zones of
water accumulation and saturation. This is often the situation in highland
swamp and bog formation. Usually gravitational water percolates rapidly
to the level of the permanent ground water.
Capillary water. Most soils contain considerable amounts of water held in the
capillary spaces of the silt and clay particles. The actual amount present depends
upon the soil type and weather conditions. Capillary moisture represents the
important reservoir of water in soil which supplies the needs of plants and animals
living in or on the soil. Only a portion of capillary water is available to plants.
‘Moisture-holding capacity’ of a soil is a term applied to the ability of a soil to hold
water present in the form of capillary water. It is obvious that the moisture-holding
capacity of a clay is much greater than that of a sandy type soil. Likewise, the
degree of corrosion occurring in soil will be related to its moisture-holding capacity,
although the complexities of the relationships do not allow any quantitative or
predictive applications of the present state of knowledge
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Ch 4 : corrosion protection
4.1 FACTORS THAT CONTROL THE CORROSION RATE
 Certain factors can tend to accelerate the action of a corrosion cell .
These include :
(a) Establishment of well-defined locations on the surface for the anodic and cathodic reactions. This
concentrates the damage on small areas where it may have more serious effects, this being
described as “local cell action”. Such effects can occur when metals of differing electrochemical
properties are placed in contact, giving a “galvanic couple”.
(b) Stimulation of the anodic or cathodic reaction. Aggressive ions such as chloride tend to prevent the
formation of protective oxide films on the metal surface and thus increase corrosion. Sodium
chloride is encountered in marine conditions and is spread on roads in winter for de-icing.
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The rate at which attack is of prime importance is usually expressed in one of two
ways:
(1) Weight loss per unit area per unit time, usually mdd (milligrams per square
decimeter per day)
(2) A rate of penetration, i.e. the thickness of metal lost. This may be expressed in
American units, mpy (mils per year, a mil being a thousandth of an inch) or in metric
units, mmpy (millimetres per year).
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Table (4-1) Corrosion protection techniques
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4.2 Material selection
4.2.1 Alloy steels
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The corrosion resistance of steels can be markedly improved by adding other metals
to produce alloys. The most resistant of the common steel alloys is stainless steel.
4.2.2 Stainless steels
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These steels owe their corrosion resistance to the formation of a passive surface
oxide film, basically Cr2O3
4.3 Corrosion Prevention
By retarding either the anodic or cathodic reactions the rate of
corrosion can be reduced. This can be achieved in several ways :
4.3.1 Conditioning the Metal
This can be sub-divided into two main groups:
(a) Coating the metal, in order to interpose a corrosion resistant
coating between metal and environment. The coating may consist
of:
(i) another metal , e.g. zinc or tin coatings on steel,
(ii) a protective coating derived from the metal itself, e.g. aluminium
oxide on “anodised” aluminium,
(iii) organic coatings, such as resins, plastics, paints, enamel, oils and
greases.
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 *Coating
type
:
1. Internal Lining
Fig (4-1) internal coat
• Description:
Internal coating using a two component liquid epoxy based paint.
• Features:
This coating system has excellent anti-friction properties and good resistance
to chemicals.
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2. Fusion Bonded Epoxy (FBE) Powder Coating
Fig (4-2) Fusion Bonded Epoxy (FBE) Powder Coating
•Description:
Stand alone coating system.
•Features:
This coating system has adequate mechanical properties and effective
anti-corrosion properties with resistance to high temperature operating
service up to 120°C depending on raw materials used
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3. Dual Fusion Bonded Epoxy (D-FBE ) coating
Fig (4-3) Dual Fusion Bonded Epoxy (D-FBE ) coating
• Description:
2-layer coating system composed of FBE primer (first layer), FBE topcoat (top layer).
•Features:
This coating system has good mechanical properties and effective anti-corrosion
properties and resistance to high temperature operating service up to 110°C or
150°C depending on raw materials used
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4. Concrete Weight Coating (CWC)
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Description:
Fig (4-7) Concrete Weight Coating (CWC)
Weight coating system composed of cement, water, aggregates,
heavy or light depending on the required density, and reinforcement.
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Features:
Concrete weight coating is used to provide pipe stability on the sea
bed as well as superior mechanical protection. It can be
manufactured in a range of densities to suit the project specification.
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(b) Alloying the metal
to produce a more corrosion resistant alloy, e.g. stainless steel, in which ordinary steel is alloyed
with chromium and nickel. Stainless steel is protected by an invisibly thin, naturally formed film of
chromium oxide Cr2O3 .
4.3.2 Conditioning the Corrosive Environment
(a) Removal of Oxygen
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By the removal of oxygen from water systems in the pH range 6.5 - 8.5 one of the components
required for corrosion would be absent.
(b) Corrosion Inhibitors
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A corrosion inhibitor is a chemical additive, which, when added to a corrosive aqueous
environment, reduces the rate of metal wastage.
(i) anodic inhibitors
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Anodic inhibitors are thus classified as “dangerous inhibitors”. Other examples of anodic inhibitors
include orthophosphate, nitrite, ferricyanide and silicates.
(ii) cathodic inhibitors
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Cathodic inhibitors are classed as safe because they do not cause localised corrosion.
(iii) adsorption type corrosion inhibitors
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The main functional groups capable of forming chemisorbed bonds with metal surfaces are amino
(NH2), carboxyl (COOH)
(iv) mixed inhibitors
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4.3.3 Electrochemical Control
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the rate of corrosion reactions may be controlled by passing anodic or cathodic currents into the
metal
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Anodic protection
Fontana and Greene’ state that ‘anodic protection can be classed as one of the most
significant advances in the entire history of corrosion science’, but point out that its
adoption in corrosion engineering practice is likely to be slow. Anodic protection may
be described as a method of reducing the corrosion rate of immersed metals and
alloys by controlled anodic polarisation, which induces passivity. Therefore, it can be
applied only to those metals and alloys that show passivity when in contact with an
appropriate electrolyte. This decrease in corrosion increases the life of
components/plant as well as reducing the contamination of the liquid, so is
particularly beneficial in the manufacture, storage and transport of chemicals such as
acids. Edeleanu first demonstrated the feasibility of anodic protection and also tested
it on small-scale stainless-steel boilers used for sulphuric acid solutions .
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Finally the anodic protection is :
• suitable for active-passive alloys (e.g. stainless steel, nickel alloys, titanium)
• requires a broad potential range for passivity
• need sizable/expensive electrical equipment
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• risky if potential “slips” into the active/pitting region
• used often for very aggressive solutions when other methods fail, e.g. for
protection of tanks storing of strong acids (e.g. sulphuric, phosphoric, nitric)
Fig(4-11) Anodic protection
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Ch :5 preparation of pipeline
5.1 Pipeline Construction
5.1.1 Stringing
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At steel rolling mills where the pipe is fabricated, pipeline representatives will carefully inspect
new pipe to assure that it meets industry and federal government safety standards. For corrosion
control, the outside surface will be treated with a protective coating
Fig(5-1) Stringing Process
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5.1.2 Trenching
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The trenching crew will use a wheel trencher or backhoe to dig the pipe trench. The U.S.
Department of Transportation (DOT) requires the top of the pipe to be buried a minimum of 30
inches below the ground surface in rural areas, so the depth of the trench will be at least five to six
feet deep for pipe 30 to 36 inches in diameter
Fig (5-2) Trenching Process
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5.1.3 Pipe Bending
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The pipe bending crew will use a bending machine to make slight bends in the pipe to account for
changes in the pipeline route and to conform to the topography
The bending machine uses a series of clamps and hydraulic pressure to make a very smooth,
controlled bend in the pipe. All bending is performed in strict accordance with federally prescribed
standards to ensure integrity of the bend.
Fig (5-3) pipe bending Process
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5.1.4 Welding
The pipe gang and a welding crew will be responsible for welding, the process that
joins the various sections of pipe together into one continuous length. The pipe gang
uses special pipeline equipment called side booms to pick up each joint of pipe, align
it with the previous joint and make the first part (pass) of the weld. The pipe gang
then moves down the line to the next section repeating the process. The welding
crew follows the pipe gang to complete each weld.
A second quality-assurance test ensures the quality of the ongoing welding operation.
To do this, qualified technicians take X-rays of the pipe welds to ensure the
completed welds meet federally prescribed quality standards. The X-ray technician
processes the film in a small, portable darkroom at the site. If the technician detects
any flaws, the weld is repaired or cut out, and a new weld is made. Another form of
weld quality inspection employs ultrasonic technology.
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5.1.5 Coating
Line pipe is externally coated to inhibit corrosion by preventing moisture from coming into direct
contact with the steel.
Normally, this is done at the mill where the pipe is manufactured or at another coating facility
location before it is delivered to the construction site.
Fig (5-4) Coating Process
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5.1.6 Lowering In
Lowering the welded pipe into the trench demands close coordination and skilled operators. Using
a series of side-booms, which are tracked construction equipment with a boom on the side,
operators simultaneously lift the pipe and carefully lower the welded sections into the trench. Nonmetallic slings protect the pipe and coating as it is lifted and moved into position.
Fig (5-5) Lowering In Process
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5.1.7 Backfilling
Once the pipe has been placed in the trench, the trench can be backfilled. This is accomplished
with either a backhoe or padding machine depending on the soil makeup. As with previous
construction crews, the backfilling crew takes care to protect the pipe and coating as the soil is
returned to the trench. As the operations begin, the soil is returned to the trench in reverse order,
with the subsoil put back first, followed by the topsoil
Fig (5-6) Backfilling Process
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5.1.8 Hydrostatic Test
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Before the pipeline is put into natural gas service, the entire length of the pipeline is pressure
tested using water. The hydrostatic test is the final construction quality assurance test.
Requirements for this test are also prescribed in DOT’s federal regulations. Depending on the
varying elevation of the terrain along the pipeline and the location of available water sources, the
pipeline may be divided into sections to facilitate the test
5.1.9 Restoration
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The final step in the construction process is restoring the land as closely as possible to its original
condition
Fig (5-7) restoration Process
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Ch :6 Corrosion of pipeline
6.1 TYPES OF CORROSION
6.1.1 General Corrosion

This type of corrosion is chemical or electrochemical in nature. However, there are no discrete
anode or cathode areas. This form of corrosion is uniform over the surface of the metal exposed
to the environment. The metal gradually becomes thinner and eventually fails.
6.1.2 Concentration Cell Corrosion.

This type of corrosion is caused by an electrochemical corrosion cell. The potential
difference (electromotive force) is caused by a difference in concentration of some
component in the electrolyte

Liquids tend to be more uniform, but can vary in the concentration of some components such as
oxygen varies by depth and flow rates
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Fig (6-1). Concentration Cell Caused by Different Environments
6.1.2.1 Dissimilar Environment.
Pipelines tend to pass through many different types of soils. The metal exhibits different electrical
potentials in different soils
The electrical potential in those soils determines which areas become anodic and which areas
become cathodic.
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6.1.2.2 Oxygen Concentration.
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Pipelines or tanks that are exposed to an electrolyte with a low oxygen concentration are
generally anodic to the same material exposed to an electrolyte with a high oxygen conten
Fig (6-2) Concentration Cell Caused by Different Concentrations of Oxygen
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6.1.2.3 Moist/Dry Electrolyte.

Pipelines or tanks that are exposed to areas of low and high water content in the electrolyte also
exhibit different potentials in these different areas. Generally, the area with more water content
becomes the anode in this electrochemical corrosion cell.
Fig(6-3) Concentration Cell Caused by Different Concentrations of Water
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6.1.2.4 Non-Homogeneous Soil.

Pipelines or tanks that are exposed to an electrolyte that is not homogeneous exhibit different
electrical potentials in the different components of the soil. This can occur in any soil that is a
mixture of materials from microscopic to substantially sized components. The area(s) with the
higher potential becomes the anode in this electrochemical corrosion cell
Fig(6-4) Concentration Cell Caused by Non-Homogeneous Soil
48
6.1.2.5 Concrete / Soil Interface.

Pipelines or tanks that are in contact with cement and exposed to another electrolyte exhibit
different potentials in each area. The area not in contact with cement becomes the anode in this
electrochemical corrosion cell.
Fig(6-5) Concentration Cell Caused by Concrete and Soil Electrolytes
49
6.1.3 Galvanic Corrosion.

This type of corrosion is caused by an electrochemical corrosion cell developed by a potential
difference in the metal that makes one part of the cell an anode, and the other part of the cell the
cathode

Different metals have different potentials in the same electrolyte. This potential
difference is the driving force, or the voltage, of the cell. As with any electrochemical
corrosion cell, if the electrolyte is continuous from the anode to the cathode and there
is a metallic path present for the electron, the circuit is completed and current will flow
and electrochemical corrosion will occur.
50
6.1.3.1 Dissimilar Metals.

The most obvious form of this type of corrosion is when two different kinds of metal are in the
electrolyte and metallically bonded or shorted in some manner. All metals exhibit an electrical
potential; each metal has its distinctive potential or voltage (paragraph 2-4). When two different
metals are connected, the metal with the most negative potential is the anode; the less negative
metal is the cathode
Fig(6-6) Galvanic Corrosion Cell Caused by Different Metals
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6.1.3.2 Old-to-New Syndrome.

This type of corrosion can also be rather severe. Steel is unique among metals because of the
high energy put into the process of producing the steel . New steel is more active, than corroded
steel.
Fig(6-7) Galvanic Corrosion Cell Caused by Old and New Steel
52
6.1.3.3 Marred or Scratched Surface.

A marred or scratched surface becomes anodic to the surrounding metallic surface. This is similar
to the old-to-new syndrome, where new steel is anodic to the old steel. This electrochemical
corrosion cell is set up by the difference in the electrical potential of the scratched surface
compared to the remaining surface of the structure.
Fig(6-8) Galvanic Corrosion Cell Caused by Marred and Scratched Surfaces
53
6.1.3.4 Simultaneous Sources of Corrosion.

Each of these previously discussed types of electrochemical corrosion cells may cause significant
corrosion, but in many cases there are a combination of many different types of corrosion
simultaneously at work to make corrosive situations even worse on the metal surface.
Fig(6-9) Combination of Many Different Corrosion Cells at Work
54
6.1.4 Stray Current Corrosion.

This type of electrochemical corrosion cell is caused by an electromotive force from an external
source affecting the structure by developing a potential gradient in the electrolyte or by inducing a
current in the metal, which forces part of the structure to become an anode and another part a
cathode
Fig (6-10) Stray Current Corrosion Cell Caused by External Anode and Cathode
55
6.1.4.1 DC Transit Systems.

Electrified railroads, subway systems, street railway systems, mining systems, and trolleys that
operate on DC are major sources of stray current corrosion. These systems may operate load
currents of thousands of amperes at a common operating potential of 600 volts
Figure 2-13. Stray Current Corrosion Cell Caused by a DC Transit System
56
6.1.4.2 High Voltage Direct Current (HVDC) Electric Transmission

Lines. Power distribution systems are another source of stray currents. Most power systems are
AC, although sometimes DC systems with grounded neutral may be used. These transmission
lines, under fault conditions, may use the earth as the return path for the DC current.
Fig(6-11) Stray Current Corrosion Cell Caused by an HVDC Transmission System
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Ch7 Cathodic protection of pipeline
*DEFINITIONS

Cathodic Protection : Reduction of corrosion rate by shifting the corrosion
potential of the
electrode toward a less oxidizing potential by applying an external
electromotive force.
 Groundbed : One or more anodes installed below the earth's surface for the
purpose of
supplying cathodic protection.
 Rectifier : A device which converts alternating current to direct current.
7.1 CATHODIC PROTECTION

Cathodic protection is the most widely applied electrochemical corrosion
control technique. This is accomplished by applying a direct current to the
structure which causes the structure potential to change from the corrosion
potential (Ecorr) to a protective potential in the immunity region.
58
7.2 Applications :
1. Petroleum & Petrochemical:
underground piping and storage tanks, above ground storage tank
bottoms, internal surfaces of water storage tanks, heat exchangers
and storage well casings
2. Marine:
ships, barges, buoys, steel or reinforced concrete dock structures,
offshore pipelines, offshore drilling and production platforms
3. Reinforced concrete structures:
bridges, parking garages and foundations
4. Electrical Power Industry:
cooling water pipelines & intakes, grounding systems, tower
footings, penstocks, condensers
59
7.3 Type of cathodic protection
7.3.1 GALVANIC CATHODIC PROTECTION SYSTEM

Galvanic anodes are most efficiently used on electrically isolated coated structures.
The current output of a galvanic anode installation is typically much less than that
which is obtained from an impressed current cathodic protection system.
Fig(7-1) Sacrificial Anode CP System in Seawater
60
*Anodes Materials

Zinc anodes are also available in many shapes and sizes. They are appropriate in
soils with very low resistivities (750 ohm-cm to 1500 ohm-cm). Favorable
environments are sea water and salt marshes. Short chunky shapes are suitable for
low resistivity areas, but long slender shapes should be employed in higher resistivity
areas.

Aluminum anodes are not commonly used in earth burial applications. Some
proprietary aluminum alloy anodes work well in a sea water environment

Magnesium anodes are available in a variety of shapes and sizes, bare or
prepackaged with the most popular being the 17 lb. prepackaged anode. As a
general guideline, one may assume magnesium anodes to be acceptable where soil
resistivities are between 1,000 ohm-cm and 5,000 ohm-cm. Short chunky shapes are
suitable for low resistivity areas, but long slender shapes should be employed in
Higher resistivity areas
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7.3.1.2 Advantages :





Self-powered so no external power source is required.
Easy field installation.
Low maintenance requirement.
Less likely to cause stray current interference problems on other structures.
When the current requirement is small, a galvanic system is more economical than
an impressed current system.
7.3.1.3 Disadvantages :





Low driving voltage.
Limited to use in low resistivity soils.
Low maintenance requirement.
Not an economical source of large amounts of CP current.
Very Little capacity to control stray current effects on the protected structure
62
7.3.2 IMPRESSED CURRENT CATHODIC PROTECTION SYSTEM

An impressed current system is used to protect large bare and coated structures and structures in high resistivity
electrolytes. Design of an impressed current system must consider the potential for causing coating damage and
the possibility of creating stray currents, which adversely affect other structures See fig (7-2) .
video
Fig(7-2) Impressed Current Cathodic Protection System
63
*An impressed current system consists of the following components:
 Rectifier (current supply)
 Counter electrode
 Reference electrode
7.3.2.1 Advantages





Flexibility
Applicable to a variety of applications
Current output may be controlled
Not constrained by low driving voltage
Effective in high resistivity soil
7.3.2.1 Disadvantages

Increased maintenance
 Higher operating costs
 May cause interference on other structures
64
Table (7-1) Comparison of CP System Characteristic
65
7.4 CATHODIC PROTECTION - THEORY

Carbon steel and stainless steel (depending on the temperature) exposed to seawater will suffer
from corrosion. The following reactions will occur on the surface
Anodic reaction:
Fe → Fe2+ + 2eeq(7-1)
Cathodic reactions:
O2 + 2H2O + 4e- → 4OHeq(7-2)
2H+ + 2e- → H2(g)
eq(7-3)
These reactions can be shown schematically in a over voltage diagram (E - logi) according to Figure
Fig(8-3) Over voltage diagram (E-log I) for steel in seawater
66



The protection current can be supplied in two different ways, as schematically shown in Figure
(7-5):
Impressed current from an external power source
Sacrificial anodes
Fig(7-5) A schematic picture of the cathodic protection principle with
a)
sacrificial anodes
b) impressed current.
67
 Anode Shape
Fig(7-6) Anode Shape
a) Stand off,
b) Flush mounted,
c) Bracelet (Jotun Cathodic Protection – today Skarpenord Corrosion)
68
Ch.8 Case Study

SUMED Mission
contribution to world economy growth, development and prosperity through transporting crude
oil efficiently and at competitive cost in addition to offering relevant services that complement and
augment the main activity.
* The System consists of :
 Off-shore facilities at Ain Sukhna.
 On -shore facilities at Ain Sukhna.
 Main pumping stations at Ain Sukhna.
 2 pipelines 42” O.D., 319.349 km long.
 Nile crossing relief station.
 Intermediate future pumping stations piping at Sidi Kerir arrangement.
 Off-shore facilities at Sidi Kerir.
 On -shore facilities at Sidi Kerir
69
*River crossing
The crossing of water courses encountered along the pipeline route has been
achieved according to the design drawing of each individual crossing.
For minor crossing standard drawings and procedures have been adapted such
as shown on drawing 3000-GC-D-66605, from which results a minimum cover
of 2m an minimum distance between pipelines of 10 m.
The crossing pipes have been over-weighted by a continuous concrete coating
of a thickness variable from 11 to 15 cm.
If rocky soil is encountered minimum coverage of 1 m has been adopted.
The line pipe coating is of the reinforced type for a length equal to the overweighted section plus 5 m at both ends.
70
Suez Canal University
Faculty of Petroleum & Mining Engineering
Metallurgy & Materials Engineering Dept.
Supervised by
Prof . Dr. Mohamed abd El Fattah El Zeky
•
Prepared by :
Khaled Mohamed yousif
Emad Mohamed Mahmoud
Taha Abd El - razq Ramadan
71