Please Note: This Certification page is from my personal copy of the project work, hence the reason why it was signed by only my project supervisor (Engr. Shedrach M.U.). Other copies were submitted to the department for signing and documentation and were not returned back to me.

ABSTRACT

Corrosion has been a problem experienced by sea vessels such as ships, dredgers pipelines and other structures, which have led to loss of billions of dollars globally, pollution and even loss of lives, thus necessitating on-going research on how to efficiently curtail its effects. Therefore, this project is basically to design and install a cathodic protection system on coated steel (as used for parts of a dredger). Sacrificial anode cathodic protection system was used as the protection applied to the steel and a single packaged zinc anode was installed alongside the steel. When the whole system was connected, the zinc being higher in the electrochemical series corroded preferentially in place of the steel since they are in the same conductive electrolyte (i.e. the environment). A wooden test post was installed alongside the steel pipe where all the cables from the steel and the anode were displayed and connected for CP monitoring. The CP system was monitored for days and it was observed that the potential readings obtained were within the standard -850mV to -1150mV. This proved that the submerged mild steel was ‘cathodically protected’ from corrosion. Thus cathodic protection method could be used to protect steel of structures or vessels hence prevent losses.

INTRODUCTION

The first practical use of cathodic protection is

generally credited to Sir Humphrey Davy in the

1820s. Davy’s advice was sought by the Royal

Navy in investigating the corrosion of copper

sheeting used for cladding hulls of naval vessels.

Davy found that he could preserve copper in sea

water by the attachment of small quantities of

iron or zinc; the copper became, as Davy put it,

“cathodically protected” (Francis, 2004).

Davy was assisted by his pupil Michael Faraday,

who continued his research after Davy’s death. In

1834, Faraday discovered the connection

between corrosion, weight loss and electric

current and thus laid foundation for future

application of cathodic protection method.

Heldtberg et al (2004), reported that a case study

of a ship James Matthews, 1841 was conducted

and that a laboratory simulation study of the

impact of pH and chloride content on the

corrosion of cast iron and mild steel was carried

out. It was found that there was a linear

relationship between the corrosion rate of cast

iron and the log of chloride ion concentration in

the pH range 7.8< pH >5.5 with only a small pH

effect noted for the given range of conditions.

Studies on a 19th century mild steel sample

indicated that the corrosion rate was linearly

dependent on the square root of the chloride ion

concentration and the corrosion rate fell in a

linear fashion as the pH was increased to strongly

alkaline solutions of sodium hydroxide. It has

been more than 30 years since the first CP system

was installed on a reinforced concrete bridge

deck in 1973 near Sly Park, California. Today, it is

estimated that over two million of concrete

structures are cathodically protected worldwide.

(Callon et al, 2004)

It has been established that electric current can

generate corrosion, corrosion, in turn can

generate electric current. As indicated by these

phenomena, it is then possible to prevent

corrosion by the use of electrical current. This,

there, is the basis for cathodic protection. When

direct current is applied with a polarity which

opposes the natural corrosion mechanisms, and

with sufficient magnitude to polarize all cathodic

areas up to the open circuit potential of the

anodic area, corrosion is arrested. (Bever, 1986)

The theoretical consideration indicate that the

basis for cathodic protection is relatively simple

not difficult to understand. However, practical

designs for various applications can vary

considerably based on the type of structure to be

protected and the conditions encountered.

It is also obvious that the rate of corrosion could

be reduced if every bit of the exposed metal could

be made to collect current. Direct current is

forced onto all the surface of the metal thereby

shifting the potential of the metal in the active

(negative) direction, when the amount of current

flowing is properly adjusted, it will overpower

the corrosion current discharging from the

anodic areas on the metallic structure and there

will be a net flow onto the surface at these points.

The entire surface then will be cathodic and the

corrosion rate will be reduced.

To begin with; what is Corrosion? Generally

corrosion is the deterioration of material, usually

metals due to its reaction its environment. It is a

natural process that occurs because metal dislike

remaining as metal, but prefers its equilibrium

state i.e. its Ore.

There are three essential elements necessary for

corrosion to occur:

Water

Contaminants in the water (e.g. salts)

Oxygen

Corrosion is primarily electrochemical in nature,

where the chemical reaction is accompanied by a

passage of electrical current due to difference in

electrical potential between different areas of the

substrate. Corrosion can either be atmospheric or

immersed corrosion. The corrosion process

involves:

(1) Removal of electrons – oxidation of the metal

(i.e. anodic reaction). Fe = 2Fe++ + 4e-

(2) Consumption of these electrons by some

other reduction reaction (i.e. cathodic reaction),

O2 + 2H2O + 4e- = 4OH- ; 2H2O + 4e- = H2 + 2OH-.

Both electrochemical reactions are necessary for

corrosion to occur.

True, corrosion has been a threat to investment

as it has caused loss of billions of dollars on a

global scale. This led to researches into methods

of combating or at least reducing the act and

effect of corrosion on metallic structures

submerged or buried.

TECHNICAL OBJECTIVES

This research aims to:

Analyze the necessary parameters in

designing cathodic protection system.

Analyze the possible advantages of

using a cathodic protection system in

Nigerian Waters.

Prove and establish the importance of

cathodic protection in pipelines, ship

hulls and other metallic parts of a

dredger.

WATER RESISTIVITY

In water, resistivity is one of the main factors

influencing corrosion. However, because oxygen

dissolves better in brackish water, steel will

usually corrode more rapidly in brackish water

than in normal seawater. Similarly, corrosion rate

in well-oxygenated water is normally higher than

in warmer water. This can be illustrated in the

“Evans diagram” for neutral oxygenated water.

Higher corrosion rate is also expected in

turbulent water where the diffusion of oxygen is

faster.

CORROSION PREVENTION

The principal methods for preventing or at least

reducing corrosion are:

Application of coating

The use of cathodic prevention method.

Alloying is also another viable method.

COATING OF METALS

Coatings normally are intended to form a

continuous film of an electrically insulating

material over the metallic surface to be

protected. The function of such a coating is to

isolate the metal from direct contact with the

surrounding electrolyte (preventing the

electrolyte from connecting the metal) and to

interpose such a high electrical resistance that

the electrochemical reaction cannot readily occur

(Peabody, 1967).

In reality, all coatings, regardless of overall

quality, contain holes formed during applications,

transportation, and installation or in service as a

result of degradation of the coating, stresses, or

movement of the ship (or Dredger). Coating

degradation in service also can lead to

disbonding from the metal surface, further

exposing metal to the environment. A high

corrosion rate at a holiday or within a disbanded

region can result in a leak, crack or rupture of

vessel, even when the coating effectively protects

a high percentage of the structure surface.

The primary function of coating on a cathodically

protected surface is to reduce the surface area of

exposed metal, thereby reducing the current

necessary to cathodically protect the metal. This

is accomplished by shifting the potential of the

metal in the negative direction by use of an

external power source (referred to as impressed

current cathodic protection) or by utilizing a

sacrificial anode. Thus, coating is often best used

in the presence of cathodic protection.

ALLOYING

Metals can be alloyed to change their corrosion

behavior. Stainless steel is a very good example;

here chromium is added to the steel to enhance

its corrosion resistance properties. This works

because chromium in the metal forms a

passivating layer similar to that of aluminum.

GALVANIC SYSTEM OF CATHODIC PROTECTION

When two metals having different energy level or

potential are coupled together, current will flow

from one metal to other. This is so because

nature has endowed each metallic substance with

a certain natural energy level or potential. The

direction of positive current flow will be from the

metal with more or most negative potential

through the electrolyte to that which is more

positive. Corrosion occurs at the point where

positive current leaves the metal surface,

(Bushman et al, 2005). A dry cell battery is a

typical example of a corrosion cell.

As mentioned by Peabody (2001), the corrosion

cell results from contact of dissimilar metals. In

such a cell, one metal is active with respect to the

other and corrode. With galvanic anodes, this

effect is taking advantage by purposely

establishing a dissimilar metal cell strong enough

to counteract corrosion cell.

The table below shows a galvanic energy series

for metals, where Magnesium has the highest

energy and Gold has the least energy.

Table 1: A Galvanic Energy Series Table for Metals Energy Level in Volts Vs Cu-Cus04 Electrode

How Galvanic Anode Works

When steel is electrically connected to a metal

higher in the electrochemical series and both are

Incr

easin

g En

ergy

Le

vel

Aluminum - 1.1Zinc - 1.1Steel - 0.6Steel in concrete with Ct - 0.5Steel in concrete without Ct - 0.1Copper - 0.1Carbon + 0.4Silver + 0.5Platinum + 0.9Gold + 1.2

bare in a common electrolyte such as the earth or

water, the more active metal is corroded and

discharges current in the process. Magnesium,

aluminum and zinc are such metals. If the amount

of current needed for a CP application is known,

anode system can be designed using sufficient

anode material to produce the desired current

output continuously over a desired number of

years. The corrosive nature of the underground

environment may cause self-corrosion of the

anode material. Electrical currents produced by

this corrosion do not result in producing CP

current.

Figure 3: Sacrificial Anode CP System in Seawater (Baxter and Britton, 2006)

Characteristics of Zinc Anodes

They are usually long slender shaped to achieve

low resistance and practical current output at the

usual low driving voltage between anode and

protected structure. Packaged magnesium and

zinc anodes (anode and backfill furnished as a

complete unit ready for installation) are standard

with most suppliers.

CATHODIC PROTECTION WITH IMPRESSED CURRENTTo be free of the limited driving voltage

associated with galvanic anodes, current from

some outside power source may be impressed on

the steel parts by using a ground bed and a

power source. The most common power source is

the rectifier. This is a device which converts

alternating current (A.C.) to electric power to low

voltage direct current (D.C.) power. (Fontana,

1987)

Fig 4: Principle of Cathodic Protection with Impressed Current (Baxter and Britton, 2006)

SELECTION OF THE TYPE OF CATHODIC

PROTECTION SYSTEM

According to Washington’s Department of the

Army Technical Manual (1985), before deciding

which type - galvanic or impressed current

cathodic protection system - will be used and

before the system is designed, certain

preliminary data must be gathered. They include:

Physical dimensions of structure to be protected,

drawing of structure to be protected, electrical

isolation, short circuits, electrolyte resistivity

survey, current requirement, coating resistance,

protective current required, the need for cathodic

protection and structure versus electrolyte

potential survey.

CRITERIA FOR CATHODIC PROTECTION

Various criteria have been developed over the

years that permit a determination of whether

adequate protection is achieved. Those criterions

in more common usage involve measuring the

potential between the pipeline and earth

(Peabody, 2001).

Basically potential criterion is used to evaluate

the changes in structure potential with respect to

the environment that are caused by cathodic

protection current flowing to the structure from

the surrounding soil or water.

The steel-to-water potential can be measured

measuring the voltage between the steel and a

reference electrode placed directly over the steel

surface. The most common electrode used for

this purpose is a copper-copper sulphate

reference electrode, (CSE). The potential is

referred to as “ON” potential if the measurement

is made with the cathodic protection system

energized. The “OFF” or “INSTANT OFF” potential

estimates the polarized potential when the

measurement is made within one second after

simultaneously interrupting the current output

from all cathodic protection current sources and

any other current sources affecting that portion

of the measured steel.

CASE STUDY: A study was done on JEPH

KEBBI INTERNATIONAL LIMITED. Jeph Kebbi

international limited is a private company

incorporated in Nigeria in 1983, they provide

design construction, installation, operation,

maintenance, dredging and marine services to

Nigerian energy, oil and gas industries. Jeph

Kebbi International is located along Chevron

Clinic road Opposite D.B.S off N.P.A Express way,

Delta, Warri. It was observed that the company

uses a cathodic protection system in addition to

coating on its dredgers steel parts (e.g. Abigail 2)

to protect it from corrosion. This article will

show why this method was chosen.

DESIGN PARAMETERS AND

METHODOLOGY

Cathodic protection systems are designed to

maintain the metallic structure (e.g. pipe) within

an electrochemical potential range (-850mV to

1150mV) that prevents surface corrosion at the

expense of a less noble metal (anode). The

system must be monitored weekly to ensure the

efficiency of the system.

MATERIALS: A single packed zinc anode, low-

carbon steel pipe (steel pipe chosen since steel

forms a major part of the dredger), Wooden test

post, 2.5mm PVC cable, Voltmeter, Bolts and nuts,

Drafting tape, Copper-copper sulphate electrode

(CSE), Grades 320 and 640 of Emery paper/cloth.

Coating material: A two-layer-cold-applied

polyethylene tape with trade name Polyken tape

was used on the pipe surface, in other to achieve

a homogenous pipe surface and to separate the

steel pipe from the environment. The first layer is

for corrosion prevention while the second is for

mechanical protection.

Steel pipe

TABLE 2: Pipe Characteristics

Type Low-carbon steel pipeLength 2.5metresDiameter 0.102meters (4 inches)Thickness 0.002mCoating material Polyethylene (Polyken

tape black & white)

A single packaged Zinc anode: The zinc anode

was completely packaged in a porous sack bag

with a coke breeze as the chemical backfill

surrounding the entire length.

Characteristics: Weight 17kg, length 102cm

(1.02m) and its diameter with the backfill being

10cm (0.1m). Zinc having an electrode potential

of -0.76 volts which is greater than iron

(0.44volts) will corrode preferentially and faster

than iron. Other anode systems that could be

used are magnesium anode and aluminum anode.

PVC cable: A 2.5mm PVC cable of thickness was

also used. The cable has a length of 30 yards and

was divided into three equal parts for the anode

and the pipe connections.

Test post: A treated wood with 4 x 4 inch

dimensions was used in the design of the test

post. After the fabrication, it was painted to

prevent it from degradation. The test post has a

box where the cables from the pipe and anode

were displayed and connected for taking of

readings and monitoring.

Bolt and Nut: Bolts and nuts of the same low

carbon steel material with the steel pipe were

also used for the attachment of cables to the pipe.

Voltmeter: A digital voltmeter was used for

taking or measuring potential differences in volts.

Copper-copper sulphate electrode (CSE): This is

an instrument that helps in the measurement of

the potential between the pipeline and earth (i.e.

pipe to soil or water potential).

DESIGN CALCULATION

Pipe data:

Length of pipe, L or h = 2.5metres

Diameter of pipe, D = 0.102metres

Radius of pipe, r = D/2 = 0.102/2 = 0.051metres

Thickness of the pipe, t = 0.002metres

Total surface area of cylindrical pipe (TSA)TSA = Area of the cylindrical pipe + Area of 2

circular surfaces present.

=2 .π .r .h + 2. π . r2 or 2 . π . r (h + r ) (3.1)

Thus TSA = 2 x 3.142 x 0.051 x (2.5 + 0.051)

= 0.78486m2

Cross-sectional area of pipe (A)

A = (R–t)π 2 (3.2)

A = 3.142 (0.051 – 0.002)2

A = 7.5439 X 10-3m2

Linear (Ohmic) resistance of pipe, Rs

Rs = (Ls x L)/A (3.3)

Ls = resistivity of steel pipe = 1.8 x 10-7 m)Ώ

L = length of pipe = 2.5metres

A = Cross sectional area = 7.5439 x 10-3 m2

Rs = (1.8 x 10-7 x 2.5) 7.5439 x 10-3 = 6.0 x 10-5 Ώ

Coating leakage resistance of pipe, Rc

Leakage resistance, Rc, refers to the total

resistance of the structure-electrolyte interface,

including the Ohmic resistance of any applied

surface coating(s).

Rc = Rp/πDL (3.4)

Rp = coating resistance = 15,000m2Ώ

Rc = 15,000/(3.142 x 0.051 x 2.5)

Rc = 37443.37 Ώ

Attenuation factor, α

Electrical losses in a conductor are caused by

current flow in the conductor. It is a factor that

influences general current distribution on the

surface of the steel pipe. Assuming 50%

attenuation, = (Rs/Rc)α 0.5 (3.5)

= {(6.0 x 10α -5)/37443.37}0.5 = 4.003 x 10-5

Pipeline characteristics resistance, RkRk = {Rs x Rc}0.5 (3.6)

Rk = (6.0 x 10-5 x 37443.37)0.5

Rk = 1.5Ώ

Surface area, SASA = πDL (3.7)

SA = 3.142 x 0.051 x 2.5 = 0.4006m2

Total current requirement, I

I = SA x Cd x f (3.8)

Cd = current density = 0.015A/m2 = 15mA/m2

f = Safety factor = 1.5

I = 0.4006 x 0.015 x 1.5

I = 0.009 or 9.0x10-3 Amperes

Resistivity of water: The seawater resistivity, ρ

(ohm·m), is a function of the seawater salinity

and temperature. In open sea, salinity does not

vary significantly and temperature is the main

factor. It is recommended that design of cathodic

protection systems in such locations is based on

resistivity measurements reflecting annual mean

value and the variation of resistivity with depth.

Resistivity, = ∑ /N or R/(L/A)ρ ρ (3.9)

Table 3: Resistivity of Some Typical WatersWater ( cm)ρ ΏPure water 20,000,000Distilled water 500,000Rain water 20,000Tap water 1000-5000River water (brackish) 200Sea water coastal 30Sea water open sea 20-25

The current output from an anodeFrom Roberge’s hand book on corrosion (2000),

the current from an anode can be estimated using

the Dwight’s equation:

i = (2 . π . E . L ) / [ ρ . In . (8 . LD − 1)](3.10)

Where, i = the current in Amperes (A)

E = design driving voltage of zinc anode = -0.25v

L = anode length in cm = 50cm

= water resistivity (ohm-cm) = 200 -cmρ Ώ

D = anode diameter in cm = 5cm

π = 22/7 = 3.142

Substituting the above values into equation;

I = (2 x 3.142 x 0.25 x 50) {200x ln [8 x(50/5)-1]} = 0.09amperes

Life Expectancy of an AnodeAccording to Bushman et al (2005), anode life can

be calculated using:

Anode Life = [Faraday Consumption Rate (Ampere Hours/Pound)/No. of Hours per Year] x Anode Weight (lbs) x Anode Efficiency x Utilization Factor/Anode Current in Amperes

The utilization factor is usually assumed to be

0.85. The equation may then be reduced to

simpler form by substituting the constant factors:

Life expectancy for magnesium, Lm = (48.5 W/I)

Life expectancy for zinc, Lz =(32.5W/I)

Where, W = Anode metal weight in pounds

I = Current output in milli-amperes

LM = magnesium anode life, years

LZ = zinc anode life, years

Inputting values into the re-written formula:

Life for zinc = (32.5 x 37.48)/(0.09 x1000)

(where 17kg = 37.48Ib) = 13.5 years

DESIGN PROCEDURESInstallation point survey: During this survey,

care was taken to ensure that no other metallic

structure(s) is around the location to avoid loss

of current to the structure(s) or issues of stray

current; also soil resistivity test was carried out.

Cable attachment: Two points were marked for

cable attachment (each 150millimeters apart

from centre of the low carbon steel pipe). Two

bolts and nuts were attached to each of the points

by manual arc welding. After welding operation,

the welded joint was freed of welding electrode

slag and visually inspected for weld integrity.

Pipe coating: The coating material used is a field-

applied coating called Polyken tape. This is a

cold-applied two layer coating system, though a

primer may sometimes be needed. The adhesive

wrap is softer and bonds intimately with the pipe

surface; this ensures proper corrosion

prevention and the polyethylene wrap used

provides great resistance to mechanical stresses

due to handling, backfill, etc.

Preheat: Temperature of pipe surface was

increased to about 50C above normal room

temperature by exposure to midday sunshine.

Inspection: Visual inspection was carried out on

the coated pipe to ensure:

A near perfect bonding/adhesion between

the pipe and the coating

No cracks or holes in the coating

No kinks, gaps or voids noticed.

Test post design and installation: This is the

cathodic protection monitoring point. The test

post is a wooden box where all readings

associated with the cathodic protection is taken.

Design from a treated of wood with 4x4 inch

dimensions, the test post has a box where cables

from the pipe and anode were displayed and

connected for readings. It also has a long bar of

wood which is treated to avoid degradation, then

it was buried close to the installation.

Complete system Installation: After all the

above mentioned processes were completed,

then the complete system was installed. The

complete system comprises of the coated pipe

with the protruding cables attached to it, the

single packaged anode with its own cable and the

test post with points for cable display.

The system was set up where the two cables from

the coated pipe were displayed in the box of the

test post as P1 and P2 respectively. The cable

from the anode was also displayed as A in the

same box. Then another cable of the same

material was used to loop one of the pipes (P1) to

the anode A, thereby setting up a complete

electrical system. The potential was taken

immediately after looping using resistivity meter.

Cathodic Protection (CP) MONITORING

The potential reading was taken using a

resistivity meter. The pipe length was divided

into five points (1 – 5). The potentials at these

different points were then taken from the test

post and recorded using the resistivity meter by

placing the meter on the different points, each

showing its own specific reading. One terminal

was connected to the cable of the meter while the

other was then placed on the connection to

obtain the readings. The cathodic protection

system was monitored for days after installation.

RESULTS AND DISCUSSION

Resistivity Results: The results were computed

using the Microsoft Excel Program to give the

table below:

TABLE 4: Resistivity Readings

S/n

Spacing a(cm) Resistivityρ = 2π aR (ohm−cm)

1 110 199.502 120 200.483 130 201.624 140 199.355 150 201.03

Total 1001.98

Average resistivity, = /Nρ Σ ρ

Where, N = 5

= Total resistivity readings = 1001.98 ohm-Σ ρ

cm

= 1001.98/5 = 200.396 ohm-cmΡ

The resistivity value used for current output from

the anode = 200.396 ohm-cm.

TABLE 5: Corrosivity Ratings

Resistivity ( cm)Ώ Corrosivity rating 0 – 2000 Severe

2000 - 10,000 Moderately Corrosive10,000 – 30,000 Mildly CorrosiveAbove 30,000 Not likelySource: Donald J.D, (1985).

The resistivity values obtained falls within the 0 -

2000 cm range. Therefore, the environmentΏ

that the pipe was submerged is highly corrosive.

As observed, with the resistivity it can distribute

current freely and generally around the pipe.

Measured Voltage & Current of Anode

The measured voltage value = -0.24Volts. This is

the driving voltage of anode obtained after

system installation. The measured current can be

calculated from the measured voltage using the

Dwight’s equation.

i = (2. π . E . L ) / [ρ . In . (8 . LD − 1)]Where, i = the current in Amperes (A),

E = the measured driving voltage = -0.24v,

L = anode length in cm = 50cm,

= soil resistivity (ohm-cm) = 200.396 -cmρ Ώ

D = anode diameter in cm = 5cm,

π = 22/7 = 3.142

Substituting the above values into equation;

I = (2 x3.142 x 0.24 x 50) {200.396 x ln[( 8 x 50/5)- 1]} = 0.0861amperes

The design voltage of the anode was    0.25Volts̶  

while the design anode current output was 0.09A

while the measured voltage of the anode after

installation was 0.24Volts while measured anode

current output was 0.0861Amperes. From the

above, it is obvious that there was a small

reduction in both the measured voltage and

current values. If the design and measured values

are analyzed using statistical measures (F – test),

there will be no significant changes in the values.

Therefore, it can be deduced that the installation

was in good working condition. However, these

reductions could be as a result of the measuring

instruments and method of measuring.

The voltage value was obtained using the

resistivity meter. In the course of measuring, an

error may have occurred leading to loss due to

error in the measuring instrument.

Resistivity Variations in the Electrolyte Between the Anode and CathodeFrom the pipe-to-water potential readings

obtained, it is observed that the readings are

highest in the middle of the pipe section; this is

due to anode positioning. Areas of low resistivity

attracted a higher current density, with current

flowing preferentially along the path of least

resistance. After installation, the potential

reading was poor; this was partly because the

PVC coated copper cable was damaged. To

correct this current loss to the environment, the

cable was replaced.

Pipe Length and Number of Anode

The pipe length used was 2.5 metres and number

of anode is one (1). It was a single packaged zinc

anode with the following dimensions;

weight17kg, length 50cm and its diameter with

the backfill being 5cm. The pipe length used was

small. From the design calculations therefore, a

single anode of standard dimensions was enough

to protect the buried pipe length. Additional

anodes can be used to achieve a homogenous

ionic current flow in cases of long pipes.

LIFE OF THE ANODEThe calculated life of anode was 13.5yrs using a

utilization factor of 85%.

Then 85% of 13.5 = 11.5years. This means that

before the 12th year of the anode life, the anode

must be changed because it cannot effectively

protect the steel after the 11.5th year.

Pipe PotentialThe pipe potential reading was taken and

recorded as:

TABLE 6: Pipe Potential Reading

Distances  along the pipe length at different points (metres)

Potential readings at each of the five points (milliVolts)Day 1

Day 2

Day 3

Day 4

0.000 -794 -834 -856 -8600.600 -838 -868 -868 -9101.100 -860 -886 -886 -9361.650 -870 -892 -880 -9422.200 -880 -890 -890 -940

Apart from the first two points on the pipe length,

the other points have values within the stipulated

-850mV to 1150mV for the first week; this means

that the pipe was well coated and that the

Polyken tape coating was working efficiently.

After some days more reading were taken and

there was an improvement in the values

observed. The point zero which had -794mV

value in the first day increased to -838mV. Also

along the pipe length there was an obvious

improvement in the potential readings, this

means that polarization of the entire system has

taken place and the zinc anode cathodic

protection system was protecting the submerged

pipe fully.

From the above measurements, it was observed

that the steel pipe readings met the standard -

850 to -1150mV. From the above observation, it

can be deduced that the submerged low-carbon

steel pipe was “fail safe”.

CONCLUSION

As can be seen from the chapters discussed, to

design an effective corrosion control system for

buried/submerged metallic structure, it is

important to have a good understanding of the

soil and water conditions (acidity/salinity,

resistivity, etc.) that the structure will be

subjected to, then the best design and installation

of the of corrosion control measures suitable for

those conditions can be achieved.

The corrosion protection provided by the

polyethylene tape coating on the steel pipe was

complemented by the zinc anode galvanic

cathodic protection system ensuring greater and

effective protection of steel parts. The zinc anode

used was able to protect the submerged low-

carbon steel pipe despite the low driving voltage

of zinc anode. Thus the low carbon steel pipe

coated with polyethylene tape was “fail safe”

indicating that cathodic protection system for

corrosion prevention is a necessary use for

structures experiencing corrosion.

RECOMMENDATIONSFrom the observations made during the project

work, the following are the recommendations;

That the zinc-anode type should be used

for soil low resistivity (less than 2000

Ohm-cm) because of zinc anode’s low

driving voltage.

That the best coating system with its

application method must be selected in

order to achieve maximum protection of

the buried and submerged metallic

structure.

That the coated pipe and the galvanic

anode must be carefully installed to avoid

any form of damage on any of the system.

Constant monitoring of the system should

be done to take weekly readings of the

potential to ensure that the recommended

standard (850mV to -1150mV) is always

achieved.

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