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MINISTRY OF HIGHER EDUCATION AND SCINTIFIC RESEARCH
BRIGHT STAR UNIVERSITY, EL-BREGA
FACULTY OF TECHNICAL ENGINEERING
DEPARTEMENT OF CHEMICAL ENGINEERING
CORROSION AND CORROSION CONTROL BY CATHODIC
PROTECTION
Submitted in Partial Fulfillment of the Requirement for Bachelor's Degree in
Chemical Engineering
Prepared:
SALEH ESSA SAAD 21161039
IBRAHIM ALI MOHAMED 21161139
NASSER SALEM NASSER 21161120
Supervised Name: Mr. EMHAMED AMAITIQ
CHE 2016 0016 A
FACULTY OF TECHNICAL ENGINEERING
DEPARTEMENT OF CHEMICAL ENGINEERING
CORROSION AND CORROSION CONTROL BY CATHODIC
PROTECTION
Submitted in Partial Fulfillment of the Requirement for Bachelor's Degree in
Chemical Engineering
Prepared:
1) SALEH ESSA SAAD 2) IBRAHIM ALI MOHAMED
3) NASSER SALEM NASSER
EXAMINER 1 ……………………………
EXAMINER 2 ……………………………
EXAMINER 3 ……………………………
Approval…………… Date……………….
CHE 2016 0016 A
3
V
Contents
Objective
Chapter 1 : Corrosion
2 1.1 – Introduction
2 1.2 - Corrosion definition
2 1.3 - Corrosion Costs
3 1.3.1- Direct Costs
3 1.3.2 -Indirect Costs
4 1.4-Corrosion Fundamentals
5 1.5-Corrosion Reactions
5 1.5.1-Anodic Rx
6 1.5.2-Catnodic Rx
6 1.5.3-Typical Rx ( Oxidation Rx )
7 1.6-Corrosion Types
7 1.6.1-Uniform ( general ) Corrosion
8 1.6.2-Pitting Corrosion
9 1.6.3-Crevice Corrosion
10 1.6.4-Galvanic Corrosion
11 1.6.5-Stress Corrosion Cracking ( SCC )
12 1.6.6-Erosion Corrosion
12 1.7-Corrosion Control Methods
13 1.7.1-Design
13 1.7.2-Coatings
14 1.7.3-Cathodic and Anodic Protection
15 1.7.4- Corrosion Inhibitors
Chapter 2: Cathodic Protection
17 2.1 - Introduction
18 2.2-Definition of Cathodic Protection
18 2.3 -The Principle of Cathodic Protection
19 2.4-Types Cathodic Protection Systems
20 2.4.1- Galvanic of Cathodic Protection
22 2.4.2 -Impressed Current Cathodic Protection
24 2.5-D.C Current Sources For C.P Systems
24 2.5.1-Transformer Rectifier
25 2.5.2-Anode Materials for Impressed Current C.P. Systems
26 2.6 -Monitoring
27 2.7 -Criterion of Cathodic Protection
27 2.8-Coatings
28 2.9 - Cathodic Protection Disbonding
28 2.10 -Choice of Cathodic Protection Systems
29 2.11-Comparision of C.P Systems Characteristics
29 2.11.1- Galvanic Anodes
30 2.11.2 - Impressed Current Anodes
Chapter 3: Field Survey Cathodic Protection
33 3.1- Ras Lanuf Cathodic Protection system
33 3.2- Survey Area(Polyethylene Plant)
5
34 3.3- Survey Tools
35 3.4 - Level of Protection
36 3.5 -Survey Procedure
36 3.6- Survey Results
37 3.7-Recommendations
38 3.8 - Survey Area(Harbour)
Chapter 4: Electricity Safety Cathodic Protection
45 4.1- Dangers To Be Avoided
46 4.2 - Electricity Safety (CP) Regulations
47 4.3 - Nature and Extent of the Problems
47 4.4 -Policy Objective
48 5.0 -Future Work
49 Conclusions
51 References
List of Figures
4 Figure :1.4- Basic of Corrosion Cell
7 Figure: 1.6.1- Uniform General Corrosion
8 Figure :1.6.2- Pitting Corrosion
9 Figure :1.6.3-Crevice Corrosion
10 Figure: 1.6.4-Galvanic Corrosion
11 Figure: 1.6.4-Galvanic Corrosion
12 Figure: 1.6.6-Erosion Corrosion
19 Figure: 2.3 -Principle of Cathodic Protection
22 Figure: 2.4.1-Sacrificial Anode in the of a Ship
22 Figure : 2.4.2- Impressed Current for Cathodic Protection
24 Figure : 2.5 .1(a) - Basic circuit of cathodic protection rectifier.
24 Figure: 2.5.1(b)- General view of cathodic protection rectifier.
25 Figure: 2.5.2-Anode Materials for Impressed Current CP System
34 Figure:3.3- Survey Tools
38 Figure: 3.8-General View of the Harbour Jetties
List of Tables
21 Galvanic SeriesSimplified -1 :Table
31 2-Comparison of CP System Characteristics :Table
39 Table :3-Potential Reading of Harbour
40 4 - CP Components for offsite Area of Polyethylene Plant :Table
41 Table : 5 -Transformer Rectifier Measurements(Current & Voltage)
42 Table : 6 -Test point potential measurement
43 :7 - Potential Readings Around Freighting Tanks Table
7
Objective
To improve scientific knowledge and technical background about corrosion science
for chemical engineers as the corrosion phenomena is very important factor affected
the safety of operation process of any type of industry. Any engineer should
have enough information about corrosion in general and corrosion control whereas the
cathodic protection method is one of effective corrosion control methods general
widely used in petroleum and petrochemical industry.
Chapter 1: Corrosion
9
1.1- Introduction.
Materials can be metals, polymers (plastics, rubbers, etc.), ceramics (concrete,
brick, etc.) or composites-mechanical mixtures of two or more materials with
different properties. Metals corrode because we use them in environments
where they are chemically unstable. Only copper and the precious metals (gold,
silver, platinum, etc.) are found in nature in their metallic state. All other metals,
to include iron-the metal most commonly used-are processed from minerals or
ores into metals which are inherently unstable in their environments. Some
metals form protective ceramic films (passive films) on their surfaces and these
prevent, or slow down, their corrosion process. )7)
1.2- Corrosion Definition.
Corrosion can be defined as the degradation of a material due to a reaction with
its environment. Degradation implies deterioration of physical properties of the
material. This can be a weakening of the material due to a loss of cross-
sectional area, it can be the shattering of a metal due to hydrogen embitterment,
)1( .or it can be the cracking of a polymer due to sunlight exposure
1.3- Corrosion Costs.
Studies have been conducted of the cost of corrosion during the past few
decades by a number of countries. These studies indicate, in general, that the
corrosion is very expensive, and have a significant impact on the economies of
industrialized countries. .(2)
The corrosion cost is composed of two types which are .
1.3.1- Direct costs.
The costs of corrosion protection are divided into:
Increase design: an increase in the design process done to prolong
the life of the equipment to protect against corrosion and this in
itself is an increase in costs.
Modify used materials using materials more resistant to corrosion
and higher cost.
The cost of anti-corrosion.
1.3.2- Indirect costs.
These costs included the following :
Lack of production.
Reduce the efficiency.
Pollution outputs corrosion.
11
1.4- Corrosion Fundamentals.
Corrosion is an electrochemical process. It is essentially the tendency of a
refined metal to return to its native state. There are certain conditions which
must exist before a corrosion cell can function. Figure 1 illustrates the four
essential elements of a corrosion cell. Necessary elements include the anode, the
cathode, an electrical path between each and an electrical conductive
electrolyte. Figure 1.4. shows the basic of corrosion cell. .(3)
Figure :1.4 - Basic of corrosion cell.
The driving force behind the corrosion cell is a potential or voltage difference
between the anode and cathode. Once each of the four conditions has been met,
an active corrosion cell is set in place. When functioning properly, there will be
a measurable DC voltage which can be read in the metallic path between the
anode and the cathode. When the two are electrically bonded, the anode is
positively charged and the cathode is negatively charged. Conventional current
flows from positive to negative and thus current discharges from the anode and
is picked up at the cathode through the electrolyte. The current then returns
from the cathode to the anode through the electrical path. This flow has a
detrimental effect on the anode known as corrosion. In order for electrochemical
reactions to occur, four components must be present and active. These
components are the anode, cathode, electron path, and electrolyte.
1.5- Corrosion Reactions.
Corrosion reactions are electrochemical in nature. They involve the transfer of
charged ions
across the surface between a metal and the electrolyte solution in which it is
immersed. There are two types of electrode reaction occurring at the metal
)1(. surface: anodic and cathodic
1.5.1- Anodic Rx.
Dissolution of metal takes place. As result metal ions are formed with the
liberation of free electrons. Anodic reactions involve oxidation: electrons appear
-+ ne +non the right hand side of the equation : M ↔M
In a solution with higher pH, the anodic reaction produces a surface film of
)1( .ferric oxide according to reaction
Examples:
eFeFe 22
eAlAl 33
13
1.5.2- Cathodic Rx.
Cathodic reactions involve electrochemical reduction: electrons appear on the
left hand side of the equation. There are several different cathodic reactions,
which are encountered in metallic corrosion. The most common reactions are:
Oxygen reduction (in acidic solutions)
2O + +4H + -4e O22H
Oxygen reduction (in neutral or alkaline solutions)
2O + O22H + -4e -4OH
Hydrogen evolution (in the absence of oxygen)
H2 2e- + 2H+
1.5.3 - Typical Rx (Oxidation Rx).
The generic chemical formula for this metal loss at anodic sites is:
M ---> M+ + e- where: M = uncharged metal atom at the metal surface
M+ = positively charged metal ion in the electrolyte
e- = electron that remains in the metal
This type of chemical reaction is called oxidation even though it does not
directly involve oxygen but only results in an increase in positive charge on the
atom undergoing oxidation. More than one electron can be lost in the reaction as
in the case for iron where the most common anodic reaction is:
Fe --->Fe2++ 2e
where: Fe = metallic iron, Fe2+= ferrous ion that carries a double positive charge
1.6- Corrosion Types.
1.6.1- Uniform (general) corrosion.
Uniform or general corrosion is considered to be the most conventional
form of corrosion, but in term of safety and cost is not the most important type
of attack. It is characterized by several individual electrochemical processes that
proceed evenly over the metal surface area (general thinning). The
consequences of uniform corrosion are the decrease in metal thickness per unit
)1(.time. Uniform corrosion attack is shown in Figure 1.6.1 (a&b)
Figure:1.6.1 (a&b) - Uniform (general) corrosion.
1.6.1.b 1.6.1.a
The prediction and measurement of uniform corrosion are relatively easy, which
makes the sudden failures rare to occur. As corrosion occurs uniformly over the
metal surface, it can be controlled by coatings , cathodic protection , inhibitors
and proper materials selection
15
1.6.2- Pitting corrosion.
Pitting is a localized form of corrosion and considered a very dangerous form of
corrosion. The attack is in the form of highly localized holes or cavities that
formed in the material and can penetrate inwards extremely rapidly, while the
rest of the surface remains undamaged. The pitting is considered to be more
dangerous than uniform corrosion attack. Even though the overall metal loss is
negligible, a small narrow pit can lead to the failure of the engineering system.
The produced pits could be uncovered (mouth open) or covered with corrosion
)1( .products. The shape of these pits is shown below in Figure1.6.2 (a&b)
Figure:1.6.2 (a & b) - Pitting corrosion.
1.6.2.a 1.6.2.b
The effects of pitting corrosion are reduced by using of more pitting resistant
materials and improve the design of the system.
1.6.3- Crevice Corrosion.
Similar to pitting, crevice corrosion is considered to be a localized form of
corrosion but usually associated with a stagnant micro-environmental solution.
Crevice corrosion occurs at spaces between two metals and non-metal surfaces.
Crevice corrosion is initiated by Oxygen depletion , inhibitor depletion an/or
present of some aggressive ions (Cl-). Figure1.6.3 (a&b). (1)
Figure :1.6.3 (a&b)-Crevice corrosion
1.6.3.b 1.6.3.a
The differential aeration between the shielded areas gives the crevice an anodic
character, which leads to high corrosive condition in the crevice. Crevice
corrosion widely recorded in flanges , washers , gaskets and threaded joints . It
may be reduced by using proper design such as selection of more resistant
materials to crevice corrosion and minimize of aggressiveness of the
environment.
17
1.6.4. Galvanic Corrosion.
Galvanic corrosion occurs when two dissimilar materials are coupled in a
corrosive electrolyte. An example which illustrates galvanic corrosion would be
joining two dissimilar metals in electrical contact in seawater. In that case one
of the metals, which is less noble, becomes the anode and the other metal. (1)
Figure:1.6.4(a&b)- Galvanic Corrosion.
1.6.4.a 1.6..4.b
The anodic metal corrodes faster and known as sacrificial anode. Zinc ,
Aluminum and magnesium sacrificial anodes are commonly used to provide
better protection in some cases.
1.6.5-Stress Corrosion Cracking (SCC).
Environment, exposed materials, and tensile stress cause stress corrosion
cracking. All the mentioned above three conditions must be met simultaneously
for stress corrosion cracking to occur. Temperature plays a big role as a factor
that affects cracking. Stress corrosion cracking is dangerous type of failure as it
can occur without any external load applied. Stainless steel and aluminum are
well known examples of stress corrosion cracking problems. Figure 1.6.5(a&b)
shows the view of stress corrosion cracking. (1)
a&b) -Stress corrosion cracking (SCC). )Figure:1.6.5
1.6. 5.a 1.6.5.b
There are several methods to prevent stress corrosion cracking failures include
proper selection of the appropriate material, removing the chemical species that
promotes cracking and change the manufacturing process or design to reduce
the tensile stresses.
19
1.6.6- Erosion Corrosion.
The corrosion that is associated with or influenced by fluid flow is often known
as flow dependent corrosion and is usually called erosion corrosion. Erosion
corrosion takes place due to mechanical wear and tear; corrosion occurs on the
surface of a metal. Figure 1.6.6(a,b&c) below shows some erosion corrosion
cases. (1)
a,b&c: ) - Erosion corrosion) 1.6.6 : Figure
1.6.6.a 1.6.6.b 1.6.6.c
1.7- Corrosion Control Methods.
There are four basic methods for corrosion control and protection as following:
1. Design.
2. Coatings.
3. Cathodic and Anodic protection.
4. Corrosion Inhibitor.
1 .7.1 – Design.
Engineering design is a complicated process that includes design for purpose,
manufacturability, inspection, and maintenance. One of the considerations
often overlooked in designing manufactured products is drainage.
The corrosion of the automobile side panel above could have been minimized
by providing drainage to allow any water and debris to fall off of the car instead
of collecting and causing corrosion from the far side of the panel . All of the
other methods of corrosion control should be considered in the design process .
The design of a structure is usually as important as the choice of materials of
construction. Design should consider mechanical and strength requirements
together with an allowance of corrosion.(2)
2- Coatings. 1.7.
Protective coatings are the most widely used corrosion control technique.
Essentially, protective coatings are a means for separating the surfaces that are
susceptible to corrosion from the factors in the environment which cause
corrosion to occur. Remember, however, that protective coatings can never
provide 100 percent protection of 100 percent of the surface. If localized
corrosion at a coating defect is likely to cause rapid catastrophic failure,
additional corrosion control measures must be taken. Coatings are particularly
useful when used in combination with other methods of corrosion control such
)2( . protection athodicc as
21
1.7.3- Cathodic Protection and Anodic Protection.
Catholic protection interferes with the natural action of the electrochemical cells
that are responsible for corrosion. Cathodic protection can be effectively applied
to control corrosion of surfaces that are immersed in water or exposed to soil.
Catholic protection in its classical form cannot be used to protect surfaces
exposed to the atmosphere. The use of anodic metallic coatings such as zinc on
steel (galvanizing) is, however, a form of Cathodic protection, which is
effective in the atmosphere There are two basic methods of supplying the
electrical currents required to interfere with the electrochemical cell action.The
first method, cathodic protection with galvanic anodes ,Cathodic protection
converts all anodic areas on a metal surface to cathodes as a result corrosion
stops.
There are two methods of cathodic protection:
By connecting a sacrificial anode to that metal is to be protected.
By applying an electric current, this is called impressed-current cathodic
protection.
The basic principle of cathodic protection (CP) is to minimize anodic
dissolution by the application of a cathodic current onto a protected structure. .(2)
4 - Corrosion Inhibitors. 1.7.
The definition of corrosion inhibitor favored by the National Association of
Corrosion Engineers (NACE) is “a substance which retards corrosion when
added to an environment in small concentrations
Corrosion Inhibitor function by one or more of these mechanisms
1. By adsorption as a thin film on the surface of a corroding material
2 . By inducing formation of a thick corrosion product
3 .By forming a passive film on the metal surface .
4. By changing characteristics of the environment either by protective producing
precipitates or by removing or inactivating an aggressive constituent
If corrosion is viewed as the consequence of an electrochemical cell
composed of anode, cathode, electrolyte, and electronic conductor, inhibitors
retard corrosion by
.Increased polarization of the anode 1.
.Increased polarization of the cathode 2.
3. Increased electrical resistance of the electrolyte circuit resulting from the
formation of a deposit on the surface of the metal . The most important
corrosion inhibitors are passivating , cathodic, organic, precipitate-
inducing, and vapor phase corrosion inhibitors. .(11)
23
Chapter 2 :Cathodic Protection
2.1- Introduction.
In 1824, Sir Humphry Davy, on contract to the royal Navy, discovered the
principle of cathodic protection for the mitigation of natural corrosion
processes. He was searching for a method to prevent corrosion of the copper-
clad wooden hulls of English ships. He attached billets of zinc to the copper and
observed that the zinc would corrode to save the copper. Today, over one and
one-half centuries later, corrosion engineers are still using this same method of
preventing corrosion damage by applying this same zinc anode cathodic
protection to steel ships around the world.
Cathodic protection is an electrical method of preventing corrosion on metallic
structures situated in electrolytes. In practical applications, the structures most
commonly provided with protection are constructed of iron or steel (including
stainless steel) and the electrolytes are most often soil and water. Other metals
commonly provided with cathodic protection include, lead sheathed cables,
copper and aluminum piping, galvanized steel, and cast iron. Cathodic
protection has also been used successfully in unusual electrolytes such as
concrete, calcium chloride and caustic soda. However, the vast majority of
cathodic protection systems are used to prevent corrosion on steel structures in
soil and water. Cathodic protection has become a standard procedure for many
structures such as underground storage tanks, pipelines, water storage tanks,
ship hulls and interiors, lock gates and dams, water treatment facilities, well
casings, trash racks and screens, bridge decks, and steel pilings. (4)
25
2.2- Definition of Cathodic Protection.
Reduction or elimination of corrosion by making the metal surface a cathode by
means of a direct current flowing to the metal surface. A basic cathodic
protection installation can protect only those external surfaces of the pipe,
which are in contact with a conducting environment.
Any parts of the pipeline system in air (such as aboveground parts, valves,
aboveground header manifolds, etc.) cannot receive any of the protection
current since air will not carry current to the unburied surfaces. From inspection
of it can be seen that the protective current flows into the environment from a
special ground connection (usually called a ground bed) established for the
purpose.
By definition, the materials used in the ground beds are anodes and material
consumption (corrosion) must occur. It follows that corrosion has not been
eliminated by the application of cathodic protection but has been transferred to
known locations (to ground beds) from the protected structure. Ground beds are
designed to discharge the cathodic protection current for reasonably long time
and when the anode elements are consumed they may be replaced without
interrupting the normal function of the protected structure. (5)
2.3-The Principle of cathodic protection.
Prior to applying cathodic protection the corroding metal structures have both
anodic and cathodic areas. If all cathodic area can be converted to anodic areas
(so the potential of cathodic areas will be the same as the anodic areas) the
entire structure will become a cathode and the corrosion of metal will be
stopped. This conversion is possible if the free steel area is polarized by direct
current flowing from the electrolyte to the metal surface.
Figure: 2.3- Principle of cathodic protection
This is the cathodic polarization. If the rate of polarization is suitable the metal
will be protected..(6)
2.4- Types of cathodic protection System.
The cathodic protection of metals can be:
Galvanic anodes.
Impressed current .
27
2.4.1- Galvanic Cathodic Protection.
The protective current may be provided by a metal which is more
electronegative than the metal to be protected. Usable materials for protection of
steel structures: (4)
In soil environment
Mg (magnesium or magnesium alloy)
Zn (zinc or zinc alloys)
In sea water
Zn (zinc or zinc alloy)
Al (Aluminium or Aluminium alloys)
In order for galvanic cathodic protection to work, the anode must possess a
lower (that is, more negative) electrochemical potential than that of the cathode
(the target structure to be protected).
Table 1 below shows a simplified galvanic series which is used to select the
anode metal. The anode must be chosen from a material that is lower on the list
than the material to be protected.
Table :1 - Simplified galvanic series
Metal
Potential with respect to a Cu:CuSO4
reference electrode in neutral pH environment
(volts)
Carbon, Graphite, Coke +0.3
Platinum 0 to -0.1
Mill scale on Steel -0.2
High Silicon Cast Iron -0.2
Copper, brass, bronze -0.2
Mild steel in concrete -0.2
Lead -0.5
Cast iron (not graphitized) -0.5
Mild steel (rusted) -0.2 to -0.5
Mild steel (clean) -0.5 to -0.8
Commercially pure aluminium -0.8
Aluminium alloy (5% zinc) -1.05
Zinc -1.1
Magnesium Alloy (6% Al, 3% Zn,
0.15% Mn)
-1.6
Commercially Pure Magnesium -1.75
29
Figure: 2.4.1- Sacrificial anode in the hull of a ship.
2.4.2- Impressed Current Cathodic Protection.
The impressed current C.P system has a direct current source to supply the
protective current. Figure 2.4.2 .shows an impressed current CP system with
direct current source .
Figure: 2.4.2 - Impressed Current for Cathodic Protection.
The seven elements of the system:
• Direct current source
• Impressed current anodes (usually in backfill)
• Connecting cables between the structure and d.c. Source and anode
• Protected metal (steel) structure
• Electrolyte (soil or water) around the structure
• Test post
• Insulation from not protected structures.
The positive terminal of the power source must always be connected to the
ground bed, which is then forced to discharge as much cathodic protection
current as desirable. This is important. If a mistake is made and the positive
terminal is connected erroneously to the “protected” structure it will become an
anode instead of a cathode and will corrode actively. Just the opposite of the
desired results. The ground bed anodes forced to discharge current will corrode.
Impressed current system ground bed anodes are consumed at relatively low
rates and thus permit designing ground beds, which can discharge large amounts
of current and still have a long life expectancy. All buried parts of the ground
bed assembly connected to the positive terminal of the power source may
discharge current and corrode at any point where metal contacts the conducting
environment in which the ground bed assembly is placed. This includes the cable
from the rectifier to ground bed anodes and the cable inter-connecting anodes
within the ground bed. To prevent current discharge from cables wire having high
quality electrical insulation suitable for underground use must be provided and all
splices and connections must be insulated perfectly. At any defects in the cable
insulation system there will be current discharge that will corrode the wire. All
buried wires connecting self-power-generating galvanic anodes to a protected
structure are subject to current collection from the environment and are free of
corrosion. Insulation on such wires is used to prevent the picking up of
unnecessary current. (4)
31
2.5 – D.C current Sources For Cathodic Protection Systems.
2.5.1-Transformer Rectifier.
The most common power source is a rectifier. This is a device provided with
power from electric utility system lines. It converts the alternating current to a
lower voltage direct current by means of a step-down transformer and a
rectifying device utilizing commonly selenium or silicon elements. Figure
2.5.1(a) .showed the basic circuit of cathodic protection rectifier while Figure
2.5.1(b) .showed the general view of cathodic protection rectifier. )5)
Figure : 2.5 .1(a) - Basic circuit of cathodic protection rectifier.
Figure: 2.5.1(b)- General view of cathodic protection rectifier.
2.5.2 - Anode Materials for Impressed Current C.P Systems.
The following materials have been used as anodes: magnetite, carbonaceous
materials(graphite), high silicon iron (14-18% Si), lead/lead oxide, lead alloys,
platinised materials .(such as tantalum, niobium, titanium). Platinum, with its
high resistance to corrosion, would be an ideal anode material but has the major
disadvantage of very high cost . Some types of used anodes for impressed
current CP systems are shown in Figure 2.5.2. (6)
Figure :2.5.2- Anode materials for impressed current CP systems.
33
2.6 - Monitoring.
Cathodic protection systems may be monitored effectively by the measurement
of structure-to-electrolyte potentials, using high input impedance voltmeter and
suitable half-cell. The standard practical half-cells are copper/copper sulphate,
silver/silver chloride/seawater, silver/silver chloride/ potassium chloride and
zinc. Adjustments are made to the cathodic-protection current output to ensure
that protective potentials are maintained at a sufficiently negative level as
defined by the project specification. The level of protection in soils and water is
accepted at steel potentials of minus 850 mV (wrt Cu/CuSO4) or minus
800mVw.r.t Ag/AgCl/seawater.
Transformer rectifier outputs may be displayed by telemetry at central control
stations. Many cathodic protection systems are increasingly being controlled
and monitored by remote computers and modem links.
Other communication systems that enable, for example, pipe-to- soil potentials
to be monitored from a helicopter or light aeroplane, are available. Galvanic-
anode outputs may also be monitored, as can currents in electrical bonds
between structures.
Tests to measure interaction are usually conducted annually where areas are at
risk or after adjustments to cathodic-protection current output. Maintenance
includes the mechanical maintenance of power supply equipment and the
maintenance of painted surfaces of equipment.
It is good practice to inform all owners of cathodic protection systems and
infrastructure in the area of influence of any new cathodic protection systems,
or of significant changes to existing systems, so that the effect on these facilities
may be assessed. )6)
Method of Survey
Pipe-to-structure potentials are taken .
Test data is recorded .
Rectifiers are inspected to determine their operating condition .
Whenever feasible, the rectifiers are adjusted for maximum
performance .
The operating volts, amps, and potentials are recorded both before
and after the rectifiers are adjusted .
Whenever feasible, the anodes are inspected to determine their
conditions The quantity and type of anodes are recorded whenever
possible.
2.7 - Criterion of Cathode Protection.
It is almost universally accepted that a steel structure under cathodic protection
is fully protected if the Potential is at least 0.85-volt negative, referred a
standard copper – saturated copper sulphate electrode placed in the electrolyte
immediately adjacent to the metal surface during survey of pipe- to-Soil
Potential. )5 )
2.8 - Coatings.
The provision of an insulating coating to the structure will greatly reduce the
current demand for cathodic protection. When first applied, coatings will often
contain flaws, and in service ,further defects will develop over a period of time.
The conjoint use of coatings and cathodic protection takes advantage of the
35
most attractive features of each method of corrosion control. Thus, the bulk of
the protection is provided by the coating and cathodic protection provides
protection to flaws in the coating. As the coating degrades with time, the
activity of the cathodic protection system develops to protect the deficiencies in
the coating .combination of coating and cathodic protection will normally result
in the most economic protection system.) 6)
2.9 - Cathodic Disbonding.
This is a process of disbandment of protective coatings from the protected
structure (cathode) due to the formation of hydrogen ions over the surface of the
protected material (cathode). Disbanding can be exacerbated by an increase in
alkali ions and an increase in cathodic polarization. The degree of disbonding is
also reliant on the type of coating, with some coatings affected more than
others. Cathodic protection systems should be operated so that the structure
does not become excessively polarized, since this also promotes disbonding due
to excessively negative potentials. Cathodic disbonding occurs rapidly in
pipelines that contain hot fluids because the process is accelerated by heat flow. )6)
2.10- Choice of cathodic Protection System.
In the design of a cathodic-protection scheme, a decision must be made as to
whether the. scheme should be a sacrificial anode or impressed-current system
or a mixture of the two systems . )6)
Sacrificial anode systems have the advantage of being.
simple to install
independent of any source of electric power ,
suitable for localised protection ,
less liable to cause interaction on neighboring structures.
2.11- Comparison of C.P System Characteristics.
2.11.1- Galvanic Anodes .
Protective current generated by galvanic anodes depends upon the inherent
potential difference between the anodes and the structure to be protected. Thus,
if the structure is made of iron or steel, any metal that is more active in the
electromotive force series can theoretically be used as anode material. In
practice, the materials generally used for galvanic anodes are zinc and
magnesium. Although aluminum is also a material which is more active than
iron, it has not yet proved to be an effective galvanic anode material for
underground use because of the polarization films which build up on the
aluminum surface as it corrodes, thereby ceasing the generation of protective
current. In recent years, some alloys of aluminum have been used successfully
in seawater applications and work is progressing on alloys that may prove to be
effective in other applications. It should be noted that galvanic anodes consume
themselves in the process of generating protective currents.
The rate of consumption is dependent upon the magnitude of current generated
as well as the material from which the anode is made.In underground
applications, these anodes are normally surrounded with a special backfill.
37
The backfill is usually a mixture of gypsum, Mennonite and sodium sulfate.
This special backfill serves a number of purposes. First, it provides a uniform
environment for the anode, thereby making the corrosion of the anode uniform;
second, the backfill decreases the anode-to earth resistance; third, it retains
moisture and thereby maintains a lower resistance; and fourth, it acts as a
depolarizing agent.) 4)
2.11.2- Impressed Current Anodes.
When a rectifier type system is used, the current is derived from an outside
source and is not generated by the corrosion of a particular metal as is the case
with galvanic anodes. However, materials used as energized anodes do corrode.
Thus, junk pipe and steel rails that were at one time used extensively as anode
materials in rectifier type systems corrode at the rate of 20 lbs. per ampere year.
Even a relatively small rectifier system, with a capacity of only 10 amperes,
would consume 2000 lbs. of steel in 10 years. Therefore, longer life anode
materials were sought. The materials that are used almost universally today are
graphite, high silicon cast iron and precious metal oxide coated titanium. In
underground work, special coke breeze backfills are usually used for the
purpose of providing a uniform environment around the anode and for lowering
the anode-to-earth resistance. Table2 Indicates the comparison between the
galvanic cathodic system and impressed current cathodic protection.) 4)
Table: 2 - COMPARISON OF CP SYSTEM CHARACTERISTICS.
Galvanic
Impressed current
NO External Power required
External Power Required
Fixed driving voltage
Adjustable Voltage
Fixed Current
Adjustable Current
Limited Current (10 to 50 Milli-amperes
Typical)
Unlimited Current (10 to 100 Amperes
Typical)
Usually used in lower resistivity
electrolytes
Can be Used in almost Any Resistivity
Environment
Usually used with small or very well
coated structures
Can be Used on Any Size Structure
Low $/Unit Cost
High $/Unit Cost
High $/Sq. Ft. of Metal Protected
Low $/Sq. Ft. of Metal Protected
Low Maintenance
Higher Maintenance
NO cause Stray Current Corrosion
Stray DC Currents Can be Generated
39
Chapter 3: Field Survey
Cathodic Protection
3.1 - Ras Lanuf Cathodic Protection System.
Two cathodic protection systems are available in Ras lanuf complex. Both of
them were found working properly. The sacrificial anodes are widely used in
harbor jetties and boats where the magnesium and aluminum anodes are fixed to
the structure. Some tanks were internally protected by sacrificial anodes. The
impressed current used for tank farm areas to protect the piping system and
bottom of tanks. (12)
.
3.2 - Survey Area (Polyethylene Plant).
The existing cathodic protection system protects the underground pipes, tank
bottom the earthing network and other buried steel structure inside the
polyethylene plant Tank Farm. The cathodic protection equipment consists of one
CP. Station each having a transformer unites with two replaceable magnetite
anode chains deep well ground bed, junction boxes and test stations as follows :
• Five nos. oil-cooled transformer rectifier unites 2840 CP 1,2- 2844 CP 1,2 &
2811. CP1. Each rated at 50 V, 50 A D.C.
• One Nos. oil-cooled transformer rectifier unites 2840 CP1 rated at 120A, 80 V
D.C.
• Nine nos. replaceable deep wall ground beds with PVC casing, each two
ground beds operated by one transformer rectifier with expect one grounded
operated by one transformer rectifier no.2811, each ground-bed having 2
magnetite anode chains with 5 anodes per chains.
41
• Two nos. replaceable deep wall ground beds with PVC casing operated by one
transformer rectifier no.2840 each ground bed having 4 magnetite anode
chains with 5 anodes per chain.
• Thirty three nos. test points distributed inside the plant its offsite, each
having zinc perm anent reference electrode. (12)
3.3 - Survey Tools.
Tools used for potential survey are shown here.
Cu/Cu SO4 reference electrode
Ag/AgCl reference electrode
Avometer
Figure:3.3- Survey Tools.
3.4 - Level of Protection.
The only possibility of removing or consider reducing danger is to balance the
different metal potential at the metal-ground interface is the most important
criterion in the protection effect.
With systems, which are separated by insulating joints, the real potential can be
determined with sufficient accuracy by means of the switch-off method.
For the pipelines and structures in the plant, which are connected to the concrete
foundations and earthing network without insulating joints, the real potentials
cannot be determined because of the differently polarized surfaces of steel-
copper and steel-reinforced concrete, which cause considerable voltage drops in
the ground when using the switch-off method .
However, the criteria used in this survey, {negative (cathodic) potential of at
least 0. 85V with the cathodic protection applied. This potential is measured
with respect to a saturated copper/copper sulphate reference electrode (CSE)
contacting the electrolyte}. (12)
43
3.5 - Survey Procedure.
During the survey for the polyethylene plant, the following procedure was
adopted: (12)
3.5.1- Measurement of the pipe/structure to soil potentials at the critical test
stations where lowest potentials have been recorded.
3.5.2 - Adjustment of the transformer rectifier units to bring the potentials up to
the level of the protection
3.5.3 - Measurement of the pipe/structure to soil potentials at all available test
stations and other selected location
3.5.4 – Inspection of the various cathodic protection equipment such as
transformer .the rectifier units, junction boxe test stations, reference
electrodes .
3.6 -Survey Results.
3.6.1- Complete cathodic protection potential survey was carried out using all
available test stations, which are equipped with permanent zinc reference
electrode and measurement once again with portable copper/copper sulphite
reference electrode. Measurements Transformer rectifiers of polyethylene tank
farm are shown in Table 5.The potential readings indicate that the
underground structures are under adequate cathodic protection. The potential
readings were accordingly recorded Table 6.
3.6.2 - Potential survey was carried out on some fire hydrant pipeline using
portable copper/copper sulphite reference electrode. The potential
measurements indicate that the underground fire hydrant pipelines are under
adequate cathodic protection (max.-1415mv , min -922 mv) and potential
measurements are accordingly recoded as attached Table 7.
3.6.3 - Potential survey was carried out in four equal direction around the tanks
by portable copper / copper sulphate reference electrode and underneath tank
bottom zinc permanent reference electrode and potential measurement indicate
that the tank bottoms under adequate cathodic protection according zinc
permanent reference electrode (max.-397mv ,min -228 mv) and one direction
for two tanks was below the acceptable level of cathodic protection according
portable copper / copper sulphate reference electrode due to copper earthing rod
all measurements were recorded in attached table .
3.6.4- General visual inspection to the condition of transformer rectifier units,
junction boxes, test stations and found in good conditions. Some of them
need touch up paint and replacement of silica gel and replacement of
indication lamps. (12)
3.7 - Recommendations.
3.7.1- Weekly follow up to the current and voltage and adjust it when it is
required.
3.7.2- Monthly follow up to existing system to be carried-out.
3.7.3- Repairs to be carried out immediately such as touch up paint.
3.7.4- Annual preventive maintenance program to be performed.
45
3.8 -Survey Area (Harbour).
Rasco harbor and sea water piles are cathodically protected by the use of 2613
Number of various weighs (32 - 95kg) aluminum alloy sacrificial anode
cathodic Protection system. The system has been installed since the harbour
construction (1982) and designed to achieve full protection for 15 years as a
minimum life. The harbor consist of three Finger jetties, three morning dolphins
as shown in figure no. The main track which are supported by 1348 numbers of
coated steel piles for 1.016 meter diameter each 19mm wall thickness.
Structures to sea water potential measurents have been carried out on all
available test points distributed at harbor. All obtained potential found to be
within the protective potential level adequately cathodically protected and the
piling to sea water potential measurements . Readings were tabulated in
Table3. (12)
Figure :3.8 - General view of the harbor jetties.
Table: 3 Potential reading of harbor jetties compared with previous years.
2013
2006
1996
Commissioning
(mv)
908 982 1031 1046
908 974 1049 1052
921 953 1011 1063
912 961 1006 1045
921 971 1018 1021
907 974 1020 1034
911 948 981 1044
903 951 992 1022
905 955 1022 1102
912 948 1007 1104
909 944 1009 1089
912 939 999 1081
938 929 971 1071
907 938 990 1063
894 94 973 1055
907 953 974 1018
47
Table: 4 - cathodic protection components for offsite area of polyethylene plant.
Polyethylene - Tank farm Cathodic Protection System
CATHODIC PROTECTION
OPERATING SURVEY FOR IMPRESSED CURRENT SYSTEM
Data
: sep
/
201
3
Protected Structure: Firewater ,Oily
Watermain,ect
CP Types :
ICCP
Area:2840
Plant : POLYETHLENE
T/R
nos
.2
DC.V
80
3Phase
AC.V :380
Installed
Data .Dec
9495
Types
.ONAN
Manufacture :ROXBY
T/R Data
Anode
nos./GB:4(5per
chain )
Anode Type Magnetite
Ground bed
Nos.2
Ground bed Type :Deep Well(Vertical)
Drawing No. A2/C2911-B
Transformer Rectifier Measurements &Reading
Table: 5 - Transformer rectifier measurements ( Current & Voltage)
Polyethylene Tank farm Cathodic Protection System Transformer
rectifier measurements & readings
Transformer Rectifier DC Output
No.
Area
Remark
s
Oil
Level
S.Gel
Volt
Total 2 1
Amp mV Am
p
mV Amp mV
O.k.
10
65 31.
4
33 31.4 32 31.1 CP-1 2840
Cathode (Structure)measurements(amp)
Anode Chains Measurements
Z/RE
Potenti
al
Total
2
1
C.J.B.
NO
Chain
.4
Chain.
3
Chain.
2
Chain.
1
A.J.B.
No.
Area
-440
66
30.7
36.2
1
40.3
19.2
23.5
0
1
2840
0
11.2
29.4
41.9
2
49
Table: 6 - Test Point Readings ( Potential Readings mV).
Polyethylene . Tank farm Cathodic Protection System Test point
potential measurement
Potential : (-mV)
Reference Electrode
T.P N0.
Remarks
Portable
(CuSo4)
Permanent
(Zn)
2826.S.1A
-881
-391
1
2827.S.1A
-950
-388
2
2829.S.1A
-885
-383
3
2826.S.1B
-912
-340
4
2827.S.1B
-916
-425
5
2829.S.1B
-934
-225
6
-1419
-552
7
-1165
-336
8
-1239
-303
9
Table: 7 - Potential Readings around firefighting tanks.
Polyethylene . Tank farm Cathodic Protection SystemTank bottom potential measurement
Tested Structure : Underneath Tank Bottom & Fire Hydrant Pipeline
Reference Electrode :Portable Cu/CuSO4
Potential (-mV)
Remarks
East West
South
North
Tank No.
1032
881
1006
674
2826.S.1A
912
1145
800
1182
2826.S.1B
1002
950
1077
616
2827.S.1A
916
1150
1055
864
2827.S.1B
1051
855
1042
1116
2829.S.1A
1145
1095
992
934
2829.S.1B
51
Chapter 4 :Electricity Safety
Cathodic Protection
4.1 - DANGERS TO BE AVOIDED.
Consideration must also be given to spark hazards created by the introduction of
electric currents into structures situated in a hazardous atmosphere. Any
secondary structure residing in the same electrolyte may receive and discharge
the cathodic protection direct current by acting as an alternative low-resistance
path. Corrosion will be accelerated on the secondary structure at any point
where current is discharge to the electrolyte. This phenomenon is called
“interaction”.
Interaction may occur, for example, on a ship that is moored alongside a
catholically protected jetty, or on a pipeline or metal-sheathed cable that crosses
a catholically protected pipeline.
Interaction may be minimised by careful design of the cathodic-protection
system, in particular, by design of a scheme to operate at the lowest possible
current density and by maintaining the greater separation between the protected
structure and the secondary structure, and between the ground beds or anodes
and the secondary structure. It is an advantage of sacrificial-anode schemes that
they are not prone to creating severe interaction problems and therefore are
popular for protection in congested and complex locations.
Methods and procedures are available for overcoming interaction, and
testing must be carried out in the presence of all interested parties, so that
the choice of remedial measures maybe agreed if and when the acceptable
limit of interaction is exceeded. (4)
53
4.2- ELECTRICTY SAFETY (C.P) REGULATIONS.
The Electricity Safety (Stray Current Corrosion) Regulations 2009 were made
on 31 March 2009 and revoked the Electricity Safety (Stray Current Corrosion)
Regulations 1999 A Regulatory Impact Statement (‘RIS’) for those regulations
was released for public consultation on 2 January 2009. The Electricity Safety
(Stray Current Corrosion) Regulations 1999 (‘the existing regulations’), manage
the risks of stray current by requiring the registration and testing of CPSs
together with prescribing operating standards, to ensure that CPSs are operated
effectively and safely
Cathodic protection is a technique used by industry and public authorities to
protect metallic structures in contact with the ground from corrosion. It
deliberately introduces electrical currents flowing between the metallic
structures and anodes which are placed underground. Stray currents travel along
paths (in the earth) other than their intended path. They are attracted to metallic
structures, such as gas, oil and water pipelines, and electricity and
telecommunications cables; and cause electrical interference and corrosion
damage to those structures. To remedy this, the current can be returned to its
intended path by a drainage bond (mitigation) or, if generated by a cathodic
protection system, by modifying the operation of the system that is causing the
currents. The owners of the metallic structures install the cathodic protection
systems to protect their own assets. However, problems can arise when
electrical currents, created by these cathodic protection systems cause the
corrosion of other. nearby metallic structures in the ground that are, not
connected to the system and which belong to other owners. This additional risk
can be created if the cathodic protection system is not installed or operated
correctly and the owner of the nearby metallic structure is unaware of the
installation of the cathodic protection systems (9).
4.3- Nature and Extent of Problems.
The nature and extent of the problems are best identified by considering the
likely consequences if there were no relevant regulations or effective
alternatives in place by time the existing regulations expire on 28 April 2009.
Several significant problems would be likely to arise, including
• Risks to public safety, the environment and property of third parties; and
• Inadequate cost recovery, necessitating cross-subsidisation of owners of
cathodic protection systems by taxpayers or others. (9)
4.4- Policy Objective.
In relation to the proposed regulations and possible alternatives, the following
overarching policy objective is identified: To efficiently minimise risks to
public safety, the environment and third party property from stray currents and
where appropriate, to equitably recover the costs of efficiently providing
services under the Act. The main test for assessing the proposed regulations
against the practicable alternatives is their relative net benefit in achieving this
policy objective. Risks to public safety, the environment and property of third
parties If there were no regulations, the following consequences to public
safety, the environment and property of third parties would arise. (9)
55
• No cathodic protection systems or mitigation systems would be prescribed for
the purposes of the Act;
• No cathodic protection standards would be prescribed.
• There would be no power to require testing, modifications or annual audits, or
to withdraw registration of cathodic protection systems
• There would be no requirement to give notice of impending operation of
impressed systems; and
• There would be no requirement for ESV to keep a register of cathodic
protection systems.
5.0-Future Work.
We recommend further work to increase the knowledge about cathodic
protection system used in the transport pipelines used to carry out different
media such as:
Potable water
Man-made river
Natural gas
Sirte - Al-Khams pipe
Crude oil
Sirtco or waha pipes
The goal of the work to check the level of protection at different types of soil
and define the most important problems they suffering if any such as stray
current and ways of solving these problems .
Conclusions:
Corrosion is the action of a metal that has been extracted from ore reverting to
its primary state when exposed to oxygen and water. The most common
example is the rusting of steel. Corrosion is an electrochemical process,
normally occurring at the anode but not the cathode.
Cathodic protection (CP) is a technique to control the corrosion of a metal
surface by making that surface the cathode of an electrochemical cell. It is used
on pipelines, other immersed or buried metal structures and in reinforced
concrete to enhance resistance to corrosion. It enables thinner metal sheets or
pipes to be used, thereby reducing costs. The principle of cathodic protection is
to connect an external anode to the metal to be protected and to pass a DC
current between them so that the metal becomes cathodic and does not corrode .
There are two ways of doing this :
• Using an external galvanic anode, where the DC current arises from the
natural difference in potential between the metals of the anode (eg Zn, Al
or Mg) and the pipe (eg carbon steel). The anode is electrically connected
to the pipeline, causing a positive current to flow from the anode to the
pipe so that the whole surface of the steel becomes more negatively
charged, i.e. the cathode .
57
• Using an external DC power source (rectified AC) to impress a current
through an external anode (usually inert) onto the surface of the pipe,
which becomes the cathode Typical applications are to the external and
internal surfaces of:
• Steel pipelines containing pressurised petrochemicals, gas or water
• Storage tanks
• Marine jetties
• Hulls of ships
• Steel pilings for bridges and buildings
• Subsea structures such as offshore platforms.
Galvanic systems are easy to install, have low operating costs and minimal
maintenance requirements, do not need an external power supply and rarely
interfere with foreign structures. However, they offer limited protection of large
structures and are therefore used for quite localised CP applications.
Impressed current systems are more frequently used to protect pipelines and
underground storage tanks. Their high current output is capable of protecting
large underground metal structures economically, is flexible to deal with
varying conditions and less susceptible to soil resistivity. However, they rely on
the continuity of their AC power source and can interfere with other nearby
buried structures.
The level of CP current that is applied from impressed current systems is
important. Too little current will lead to corrosion damage; excessive current
can lead to disbanding of the coating and hydrogen embrittlement. For these
reasons impressed current systems require regular monitoring.
References
1. Uhlig H.H, Corrosion and Control, Introduction to Corrosion Science and
Engineering ,2nd Edition,USA,1971
2. K R Trethewey and J Chamberlain, Corrosion for Science and Engineering,
2nd edition, UK,1995
3. Herbert H, Uhlig & R. Winston Revie , Corrosion & Corrosion Control, 3rd
edition,USA,1985
4 . V.Ashowrth & C.J.L Booker, Corrosion and Cathodic Protection: Theory
and Practice , 1st edition,UK,1986
5. Baeckmann, Schwenk , Prinz & Editors, Hand Book of Cathodic Corrosion
Protection , 3rd edition, USA,1977
6. Corrosion and Cathodic Protection : Theory by James B. Bushman, USA
7. http://www.corrosionist.com
8. http://www.corrosion-doctors.org
9. www.corrosiontriplece.com
11 . (NACE International) : http//www.nace.org
12. Ras lanuf oil and gas processing co.lnc POLYETHYLENE (C. P ) System
