CATHODIC PROTECTION SYSTEM FOR A LARGE BURIED STRUCTURE
By Maher Ouselati(Graduate Student,Civil Engg) Pavani Chennapragada(Graduate Chennapragada(Graduate Student,Chemical Engg) SUBMITTED ON 23 APRIL 2003
1
CONTENTS
S.NO
Title
Page No
1
Introduction
3
2
Galvanic Anode Protection
4
3
Impressed Current Cathodic Protection
11
4
Ground Bed Design
15
5
Backfill Materials
16
6
Economic merits of Cathodic Protection
17
7
Merits and Demerits
17
8
Comparison of Galvanic Anodes
19
9
Comparison of Impressed Current Anodes
19
10
Comparison of Impressed and Galvanic Current Anodes
20
11
Conclusions
20
12
Bibliography
21
Introduction 2
The science of cathodic protection (CP) was born in 1824, when Sir Humphrey Davy made a presentation to the Royal Society of London: “The rapid decay of the copper sheeting on His Majesty’s ships of war, and the uncertainty of the time of its duration, have long attracted the attention of those persons most concerned in the naval interest of the count. ... I entered into an experimental investigation upon copper. In pursuing this investigation, I have ascertained many facts ... to illustrate some obscure parts of electrochemical science... seem to offer important application.” Davy succeeded in protecting copper against corrosion from seawater by the use of iron anodes. From that beginning, CP has grown to have many uses in marine and underground structures, water storage tanks, gas pipelines, oil platform supports, and many other facilities exposed to a corrosive environment . Recently, it is proving to be an effective method for protecting reinforcing steel from chloride-induced corrosion. The basic principle of CP is simple. A metal dissolution is reduced through the application of a cathodic current. Cathodic protection is often applied to coated structures, with the coating providing the primary form of corrosion protection. The CP current requirements tend to be excessive for uncoated systems. The first application of CP dates back to 1824, long before its theoretical foundation was established. Cathodic protection has probably become the most widely used method for preventing the corrosion deterioration
of
metallic
structures
in
contact
with
any forms
of
electrolytically conducting environments, i.e. environments containing
enough ions to conduct electricity such as soils, seawater and basically all natural waters. Cathodic protection basically reduces the corrosion rate of a metallic structure by reducing its corrosion potential, bringing the metal closer to an immune state. The two main methods of achieving this goal are by either: •
Using sacrificial anodes with a corrosion potential lower than the metal to be protected
3
•
Using an impressed current provided by an external current source
1.Galvanic Anode Protection The earliest experiments on cathodic protection were performed with zinc anodes that were electrically connected to copper plates immersed in seawater. As can be seen on the galvanic series, such an arrangement would produce a cathode (copper) and an anode (zinc). In the large galvanic cell so formed the zinc cylinder corroded away in a manner to protect the copper substrate. This method of cathodic protection can be used with other combination of metals the necessary current to the metal to be protected. When two metals are electrically connected to each other in a electrolyte e.g. seawater, electrons will flow from the more active metal to the other, due to the difference in the electrical potential, the so called “driving force”. When the most active metal (anode) supplies current, it will gradually dissolve into ions in the electrolyte, and at the same time produce electrons, which the least active (cathode) will receive through the metallic connection with the anode. The result is that the cathode will be negatively polarized and hence be protected against corrosion. To calculate the rates at which these processes occur, one has to understand the electrochemical kinetics associated with the complex sets of reactions that can all happen simultaneously on these metals. 1.1 Galvanic anode material The galvanic series shows that magnesium heads the list as the most anodic metal and is widely separated from iron in the galvanic series. Magnesium coupled to iron provides sufficient galvanic potential to provide positive protection. An important feature of a sacrificial anode system is that it is inherently a safer system than impressed cathodic protection systems because the normal potentials generated are insufficient to damage coatings present on the surface to be protected. Because of the low potentials generated, sacrificial systems can be used only in low-resistance soils, i.e., with a resistivity less than 3000
cm.
4
Schematic of Galvanic Anode System 1.2 Magnesium Anodes Magnesium has an equilibrium potential of -2.61V vs. SCE and therefore theoretically can provide a very large driving voltage. However, practical measurements indicate relatively more noble corrosion potentials probably due to the electrochemical inefficiency of the metal as a sacrificial anode. The low efficiency (50-60%) has been attributed to hydrogen evolution at local cathodes and complex surface chemistry at the anode surface. The theoretical current capacity for a magnesium anode is approximately 2200 Ah kg -1 whereas actual measured values are in the range of 1200 Ah kg -1. Alloyingelements (Al, Zn, Mn) have been added to reduce the rapid activation of magnesium. Magnesium alloy anodes because of their large driving voltage are principally used in soils, water tanks, and similar high resistance media. In high conductivity environments, such as seawater, magnesium anodes are
5
generally not recommended because of risk of overprotection and high consumption rate. 1.3 Anode Efficiency A prospective sacrificial anode must possess a large number of electrons per unit mass and should deliver these electric charges efficiently. Thus the electrical output of an anode is given by current capacity which is expressed in Ah kg-1 or kg A -1 y-1. The value of the current capacity is determined by the electrochemical equivalent, the density and the efficiency of the anodic material. The electrochemical equivalent, which is dependent on the atomic weight and valence, is a characteristic of the anode material. However efficiency is determined by a number of factors including nature of the environment, operating current density and metallurgical microstructure. It is apparent that if the cathode reaction rate on the anode is low then the efficiency will be high, so that there is minimum self corrosion. Similarly large operating currents will yield high anode efficiency. It should be added that the type of corrosion attack experienced by the anode also significantly affects the magnitude of the anode efficiency. For instance, severe pitting and intergranular attack may result in a chunk of the anode to become detached without complete consumption of the electric charge in that piece. 1.4 Designing a sacrificial anode system Several factors enter the determination as to how many sacrificial anodes may be required for a given structure and corrosion problem and the manner of distributing them with respect to the location where corrosion is occurring. The anode requirements for a small installation will normally involve the steps taken in the following examples. For cathodic protection of larger structures involving use of six or more anodes or an impressed current (rectifier) system, additional steps must be taken to assure proper functioning of the system, i.e.,
6
proper distribution of the anodes, prevention of damage to other buried metal work, design of an economic system, and proper operation and maintenance. 1.5 Calculations for the given design problem Given:
A pipe of 25000ft and 3/4ft diameter. The soil resistivity is given as
800ohm-cm.Structure to environmental resistance is 30ohm-ft2.The steel coating on the structure is considered poor and the cost of the power is 4.5cents/kWh. Wanted :
1.The surface area of the component to be protected. 2. Current requirements for protection. 3. Number of anodes required to protect the structure. 4. Lifetime of anodes. 5. Cost requirements for the system.
Solution: 1. Area of the structure,A= 2π Rh = π Dh Therefore A=3.14*0.75*25000 A=58904.86 ft2. 2. since the coating of steel is poor, from the table provided in the Design sheet problem(page 2 in the given handout) We get
ip=0.15mA/ft 2.
Since
I=ip* total structure area (A),
We get
I=0.15*10 -3*58904.86 =8.83A
The current required for protection is 8.83A. 3.The working potential for anode (from Galvanic Anode Specs)EA =-1.45V The protection potential, Eprot=-0.85V (from the National Association of Corrosion Engineers specification for buried utility pipelines). With the help of the above calculated values the maximum tolerable circuit resistance(R) is evaluated.
7
Ra
=
0.1589 L
8 L 2.3 log10 − 1 d
ρ
where Ra = resistance in ohms of the anode ρ
= resistivity of soil
L = length of anode in cm d = diameter of the anode in cm while considering the diameter of the anode the backfill is also considered. 0.1589
Therefore R a =
152 .4
8 *152 .4 −1 15 .24
* 800 * 2.3 log 10
R A = 2.817 ohm Electronic current through the circuit,I =
− E prot R1 + R2 + R3 + R4 E A
R3 is the soil resistance which can be neglected. R4 is the wire and the contact resistance which is very small and hence can be neglected. Hence R3,R4 can be made equal to zero. R2 is the structure to soil resistance. Therefore R 2= R2=
ρ
A
. 30
58904 .86
= 5.09 * 10 −4 ohm.
now since we do not know the number of anodes required let us assume them to be “n”. also assume that the system is equivalent to one anode with resistance R1= RA/n.
− E prot now we have the current I= R A + R2 n E A
E A − E prot − R I
therefore R A/n=
2
8
−1.45 + 0.85 ⇒ RHS : 8.83 ⇒ LHS
:n
=
2.817 0.0674
−4 − 5.09 * 10 = 0.06744
= 41 .76 ≅ 42
The number of anodes required=42
−1.45 + 0.85 The current provided by 42 anodes is I= 2.817 + 5.08 *10 −4
42
Hence, I= 8.878A 4.Now each anode generates a current of I=8.8/42=0.22A But I= nF dm ⇒
=
dt
dm 1 dt Aw
IAW nF
=(0.22*24.805)/(2*96500) =2.77*10 -5g/s dm dm actual=80% over theoretical dt dt
But
= 1.8*2.77*10 -5=4.986*10-5g/s
( M
anode
)
life of the anode would be t= dm
dt
actual
= 4170/(4.986*10 -5) =83618534.66s =2.65 years. The life time of each anode is 2.65 years.
5. Cost of the anode installed=$300 each. Number of anodes installed =42 Hence the cost of one set of anodes is =42*300=$12,600. Assuming that the economy will be stable during the project life
9
of the project , we take the discount rate as 4%. Since the structure is obsolete after 30 years and the lifetime of each anode is 2.65 years we need to replace the anodes 12 times for the lifetime of a structure. The following is a generalized formula between present and future cash flows: PV
=
( F ) (1 + i ) n
n
which states that present value (PV) of a future cash flow (F n) after (n) time periods equals the future amount (C n) discounted to zero date at some interest rate (i). Based on the above formula the cost for each replacement has been calculated and tabulated below. number of sets 1 2 3 4 5 6 7 8 9 10 11 12
year 1 2.65 5.3 7.95 10.6 13.25 15.9 18.55 21.2 23.85 26.5 29.15
future cost($) 12,600
present cost($) 12,600 11,356 10,235 9,221 8,314 7,493 6,753 6,087 5,486 4,944 4,456 4,016
Hence the total cost of the system is $90,961 for a period of 30 years. therefore the cost/year=$3032.03
2.Impressed Current Cathodic Protection Cathodic protection can be also applied if the metal to be protected is coupled to the negative pole of a direct current (DC) source, while the positive pole is coupled to an auxiliary anode. Since the driving voltage is provided by the DC
10
source there is no need for the anode to be more active than the structure to be protected. There are basically three types of anode materials: •
Inert or non consumable anodes
•
Semi-consumable anodes
•
Consumable anodes
All items to be protected shall be electrically connected and should have a welded or brazed connection to an anode.
Schematic of Impressed Current Cathode System
2.1 Calculations for the given design problem Given:
A pipe of 25000ft and 3/4ft diameter. The soil resistivity is given as
800ohm-cm.Structure to environmental resistance is 30ohm-ft2.The steel
11
coating on the structure is considered poor and the cost of the power is 4.5cents/kWh. Wanted :
1.The surface area of the component to be protected. 2. Current requirements for protection. 3. Number of anodes required to protect the structure. 4. Lifetime of anodes. 5. Cost requirements for the system.
Solution :
The impressed current calculations are basically the same as the
galvanic anode calculations except for the potentials that have to be considered since this involves an external power supply. Therefore we get RA=2.81 ohm, I=8.8A and R 2=0.00051 ohm. 1.The surface area of the component to be protected, A=58904.86 ft2 2. The current required for protection, I=8.8A 3. Now assume back voltage as 2V and also that the rectifier voltage output is not more than 5V over back-voltage. we get I=(7-2)/(R A/n+R2) therefore the number of anodes=5 4. given that the consumption rate is 1.5lb/A-year. Mass of each anode =29.414kg=64.71lb Total mass of anodes=323.55lb For a current of I=8.8A, dm/dt=13.332 Therefore the lifetime of anodes=323.55/13.332=24.26years Lifetime of anode=25 years 5. Cost of each anode=$500 Backfill cost=$100
12
Total cost of anode=5*600=$3000 Rectifier cost=$1000. Power=voltage(V)*Current(I) =7*8.88=62.16W Total power required for 24 years=24*62.16*365 =544521.6Wh =544.521kWh at the cost of 4.5cents/kWh, the cost of power =4.5*544.5216 =$24.5 but the actual cost for 25 years =24.5[1+(1.04) -1+…………+(1.04)-24] =24.5{[1-(1.04) -25]/[1-(1.04)-1]} =$398.05. hence for the first installation the total cost =($3000+$1000+$398.05) =$4398.05 A second installation is needed, assuming that the rectifier is maintained in good condition and will not be replaced, the cost for the second installation will only include the cost of anodes and the cost of power required for 5 years. The cost of the power for the remaining 5 years would be =28.5*(1.04)-25*{[1-(1.04) -5]/[1-(1.04) -1]} =$49.49
13
the total cost for the second installation=$3000+$49.49 =$3049.49 the total cost of the system is =$4398.05+$3049.49 =$7447 cost/year=$7447/30=$248.3 The possibility of reducing the cost has been explored by lowering the potential of the rectifier. If the potential of the rectifier is taken as 6V then the cost has been found to be comparatively less. The number of anodes in this case are found to be 6.18. Practically 7 anodes are considered. The lifetime of the anodes now is 34 years and hence we will need only one installation. The cost of power will be $369.92 The cost of anodes will be $4200 The cost of rectifier is $ 1000. The total present cost is $5569.92 and hence the cost/year=$185
3.Groundbed Design For underground structures requiring cathodic protection, the location and nature of the site where the anode is placed needs careful consideration. A low soil resistivity, which would otherwise be classified as a highly corrosive soil , is not the only factor which determines the location of the anode. Other factors 14
to be considered include the presence of foreign metallic structures, accessibility and availability of a power source. The location which is specifically prepared to house a single or a combination of anodes is called a groundbed. •
Impressed anode groundbeds: Once a location is selected and the soil resistivity is determined, the engineer needs to design the type of groundbed and choose anode material and combination. Types of groundbeds are classified as: shallow vertical, shallow horizontal or deep well. Anode materials used for underground impressed current systems are generally graphite or high silicon cast iron. In the groundbed, it is preferred for the anode to be surrounded by a carbonaceous backfill. The backfill particles help to reduce anode resistance to earth, extend anode life by allowing anodic reactions to occur on their surface and provide a porous structure so the gases produced can escape.
•
A basic design incorporates the use of a steel casing to prevent the collapse of the drilled hole. Several anodes attached together with a rope are placed inside the casing. The remaining space is then filled with carbonaceous material. Once the groundbed becomes operative the steel casing will be consumed. After the pipe corrodes away the anode and backfill become active. Deep wells are generally fitted with a vent to allow gases to escape. Gas entrapment tends to increase the groundbed resistance. It should be added that in certain rock formations anodes have been installed satisfactorily without a steel casing. Although deep well groundbeds provide good current distribution they are expensive to construct because of the cost of the drilling. Careful design is also necessary because anode failures cannot be easily rectified.
•
Sacrificial Anode Groundbeds: In certain situations, for example in reducing stray current effects, a sacrificial system may be specified to protect underground structures. The backfill used with these anodes is
15
different from that described for impressed anodes. A typical backfill contains a mixture of clay and gypsum. The function of this chemical backfill to provide conditions favorable to anode dissolution. It also helps to reduce the groundbed resistance. Groundbed resistances can be calculated using the same procedure adopted for impressed current anodes. Individual galvanic anodes in a horizontal groundbed are generally not used. For this type of groundbed a continuous galvanic anode strip is found to be practical.
4.Backfill Materials The type of backfill used in a groundbed depends on whether the cathodic protection system is sacrificial or impressed. •
Chemical backfills: The chemical backfill used with galvanic anodes provides an environment which is conducive for anode dissolution. A typical mixture is 75% powdered gypsum (calcium sufate), 20% granular bentonite and 5% sodium sulfate. This mixture has a resistivity of 50
cm and is suitable for use in high resistivity soils. The function of the bentonite is to absorb water and expand, thus ensuring good contact between anode and soil by lowering groundbed resistance. A 75% bentonite 25 % gypsum mixture (250
cm) is recommended for low
moisture soils. •
Carbonaceous
backfills:
Impressed
current
anodes
are
usually
surrounded by a carbonaceous backfill. Types of materials use include coke breeze, calcined petroleum coke and natural graphite. The dual purpose of the carbonaceous backfill is to reduce the groundbed resistance by increasing the effective size of the anode and to provide a surface on which oxidation reactions could occur. The latter function prolongs anode life. To ensure good electrical contact, the backfill must be tamped around the anode. Resistivity of carbonaceous backfills are in the order of 50
cm.
16
Particle size and shape are also important when specifying a backfill. Both parameters determine the contact area between anode and earth whilst influencing the porosity of the column which is important for gas ventilation. A general purpose coke breeze is for use in shallow horizontal and vertical groundbeds. It has a resistivity of 35
cm. For deep well applications a
special calcined petroleum coke breeze is available. It has a resistivity of 15
cm and can be pumped.
5.Economic Merits of Cathodic protection •
Savings in materials due to reduction in corrosion margin
•
Elimination of production losses
•
Lower depreciation rates through extension of durable life
•
Fewer equipment shutdowns
•
Lower maintenance cost
•
Guarantee against personal and property damage
6.Merits and Demerits 6.1 Galvanic Anode •
Independent of electrical power source. Less maintenance.
•
Usefulness is generally restricted to protection of well-coated
structures or local protection. •
Impracticable except with soils or waters with low resistivity.
•
Simple to install with easy additions.
•
Inspection involves testing at each anode or between adjacent
anodes. •
Requires at large number of positions with different life.
•
Less likely to affect any nearby structures.
•
Self regulating capabilities of protective current against
environmental changes.
17
•
Bulkiness may impair smooth flow.
•
Can be bolted or welded directly to protecting surface avoiding any
perforation. Connecting members are also protected.
•
•
No misconnections.
6.2 Impressed Current Cathode •
Requires electric power source.
•
can be applied to wide range of structures.
•
Less restricted by resistivity of soil or water.
•
Needs careful design against unforeseen or change of conditions.
•
Needs inspection at relatively few positions of easy access.
•
Requires small number of anodes.
•
Affects other structures near groundbed.
•
Damage hazards to coatings are greater due to incorrect
adjustment in spite of simple control. •
Compactness of anodes gives negligible drag.
•
Requires perforation in all cases on ships hulls, plants etc..
•
Requires high integrity of insulation on connections to positive side
of rectifier. •
Requires careful polarity checking. Misconnection can accelerate corrosion.
7.Comparison of Galvanic Anodes
18
8.Comparison of Impressed Current Anodes
9.Comparison of Galvanic and Impressed current anodes 19
Galvanic anode
Impressed current anode
Material
Mg alloy
cast silicon iron
Length(ft)
5
5
Diameter(in) surface area to be protected(ft2) current required for protection(A)
2
2.2
58904.86
58904.86
8.8
8.8
number of anodes
42
7
weight of each anode(kg)
4.17
29.414
lifetime of each anode(years)
2.65
30
total cost of the system($)
90961
5569.92
cost/year($)
3032.03
185
10.Conclusions After the design calculations we have come to the conclusion that the impressed current cathodic system is much cheaper than the galvanic anode protection system. Also it has been found that by considering a rectifier of lower potential the lifetime of the anodes considerably increased and hence the cost/year is also reduced in the case of impressed current cathode system.The number of anodes involved in the case of impressed current is also very less in comparison to the number of anodes involved in the galvanic anode system for the same structure.
11.Bibliography
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1.R.Heidersbach,”cathodic Protection”,p.467 in Metals Handbook,9th. Ed.,Vol.13,ASM International, Metals Park,OH,1987. 2. www.sam-gong.co.kr / 3. www.corrosion-doctors.org/ 4.Steven F.Daily,”Understanding Corrosion and Cathodic protection of Reinforced Concrete Structures”,Corrpro Companies,Inc. 5.http://nace.org/ 6.H.Uhlig and R.W.Revie,”Corrosion and Corrosion Control”,John Wiley,New York,1985.
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