Monday, 18 May 2015

CONCEPT CATHODIC PROTECTION SYSTEM



The basic concept of cathodic protection is that the electrical potential of the subject metal is reduced below its corrosion potential, and that it will then be incapable of going into solution, or corroding.

This mechanism has been defined by many scientists and has become established beyond dispute. Indeed the principles of corrosion reactions are used in the design and construction of expendable and re-chargeable batteries and accumulators which play such a major part in modern life.

A battery that is 'dead' has no energy left and does not corrode any further. Likewise a car battery on charge does not corrode, in fact in this case the reaction is reversible, and energy is 'pumped back in'.

However, a battery has a very carefully composed electrolyte which has qualities to ensure a predictable reaction with the other components of the battery. We know that the corrosion within a battery can be controlled very accurately, by external electrical input, as this technique is in common use with rechargeable batteries which are nowadays controlled by computers which balance the reaction equilibrium to suit their own power demands.

Unfortunately a cathodic protection system is not composed of simple elements in the way that batteries are, because the electrolyte is the ground itself. This electrolyte is uncontrollable and has an almost infinite variety of qualities. The chemical composition and electrical conductivity can span a vast range, as can the temperatures and pressures to which the reaction is subjected.

Cathodic protection of such subjects as ships hulls and storage tank bases is relatively simple as the electrolyte is likely to be almost homogeneous, but as the size of the structure increases, it extends through different electrolytes and the reaction at each interface varies.

Offshore oil rigs, for example have different temperatures and pressures at the sea bed to those at the surface, and a study of this situation has shown that it has a substantial influence on corrosion.

Pipelines can be regarded as many interface reactions connected together in parallel. The metal element can be well defined, as this is specified to a high degree by the designers, as is the coating material.

However it is accepted that no coating can be perfect, and the faults, or 'Holidays' introduce the first indefinable variable to the system.

During the construction of a pipeline all possible measures are taken to detect and repair coating faults, so it follows that those remaining are undefined. It is possible to calculate the theoretical resistance of a perfectly coated pipeline, given the specification of the coating and dimensions of the pipeline, but it is impossible to calculate the actual resistance of the total pipeline.

The electrical current measurements, taken during routine cathodic protection monitoring, show that there is little resistance in the total coating (with faults) of a pipeline and this can be explained by the difficulty in quality control, during the construction period.

Undetected coating faults are the path of cathodic protection current and a perfect coating would prevent any output from the CP system. We therefore, know that there are many unspecified 'metal to electrolyte' interfaces present on an average pipeline.

The electrical resistance of the pipeline metal itself can be calculated, and is found to be very low. In fact the effect that the pipeline resistance has on the complex current paths and variation in potentials, is so small that it can almost be ignored.





The complication is due to each interface being capable of a different reaction, electro- motive-force (EMF) which cannot be measured as it is in parallel with all other EMF’s on the same section of pipeline. The magnitude of the current from each of these reactions is dependent on the earth resistance immediately adjacent to the interface, and the direction of all the resulting currents is the result of the combined effects of all the resistances and electrical pressures caused by all the EMF's.



Although it is simple to understand each corrosion cell and the mechanism of corrosion itself, the reality of applying the science, to the field, becomes immensely complex. This becomes more obvious when the circuit has been subject to computer modelling as discussed later.
To be effective, cathodic protection must reduce the metal at each single interface, to below it's corrosion potential. This is not too difficult to achieve, as each interface is part of the same metal structure, which has a very low electrical resistance. The difficulty is knowing when all the interfaces have been reduced to below their corrosion potential in relation to the electrolyte in their reaction vicinity. ( Don't forget, if we knew where each interface was we would repair them all!!!!)


OVER PROTECTION

There are several other problems, however, as too much current passing onto a steel surface can cause embrittlement, which under certain circumstances can be as detrimental as corrosion itself. This is manifest in such applications as the protection of the external surfaces of drill pipe casings, where a considerable amount of cathodic protection current is used.


CATHODIC DISBONDMENT

Another fear of 'over-protection' is that of cathodic disbondment of the coating. This happens when the coating manufacturers specifications are exceeded. Cathodic protection current passing onto the metal causes the release of hydrogen which disbonds the coating. In reality this is rarely a problem, and a careful study reveals why.

The current will only pass onto the metal at a coating fault, and the density of the current will depend on the size of the coating fault and the current locally available. As the current blows the coating from the metal, the volts drop at the interface will decrease, and equilibrium will be reached with a very small increase in additional disbondment.

If there is no coating fault, then no cathodic disbondment will occur as recognised in the British Standard Code of Practice for testing the coating manufacturers specification. This requires a specific size of coating fault on a steel coupon, to be subjected to an increasing voltage over a specified period. The test cannot be carried out on a coupon with perfect coating as the disbondment is observed under the coating at the edge of the fault.

It is logical to deduce that if cathodic disbondment is caused by current and that if all current is prevented by a perfect coating, then no disbondment will take place. This is not common sense, however, as many excavations have been dug in areas where high 'pipe- to-soil potentials' have caused concern about cathodic disbondment. In the event, it has proved the logic (above) and no disbondment has been found.

In one particular example voltages of over 5 volts had been recorded when the electrode was place on the surface above the buried pipeline which was subsequently excavated, at several spots, for examination. A coating fault was found at one location but no disbondment. The current passing onto the metal at this coating fault, caused a drop in the voltage of the electrode as it got nearer to the pipe. Whereas at the surface the reading had been over 5 volts, this reduced to 0.950 volts when the electrode could be placed close to the actual interface between the metal and the earth.

This simple drawing shows that the earth at the surface has a higher potential than the earth close to the pipeline at the coating fault, due to the current passing from 'mass earth' into the pipe metal.



At such site it is easy to plot the 'potential gradient' using a static electrode as a reference and a moving electrode to trace the potential isobars. As soon as the coating fault is fully exposed to the air, the gradient disappears completely, as the current stops. The meter then reads 5 volts, even with the electrode placed in the ground a few mm from the metal.

by : Roger Alexander

Cathodic protection is important

An idiots guide to cathodic protection




What the heck IS cathodic protection in the first place???


Cathodic protection is an electrical way of stopping rust.

Rust is chemical and electrical. Metal dissolves in some solutions and gives off electricity. Metal can be 'plated' onto other metal electrically.
All 'batteries' work on this principle and everyone knows that batteries drive loads of the things we use daily.
Not many people know that our gas and oil comes to us through pipes that are inclined to rust, but are protected by 'cathodic protection'.
Some people know that metal boats are protected by cathodic protection, and have seen lumps of metal attached to hulls for this purpose. These lumps of metal dissolve in the water and give off electricity which prevents the hull from rusting.
When you put two different metals in contact and submerge them in liquid (or wetness) one of the metals dissolves and discharges an electrical current into the liquid. The liquid (or damp material) is the 'electrolyte' and gets 'charged up' with electricity. It's 'electrical potential' is increased.
Electricity works by 'pressure' and anything with a higher 'pressure' gives off electricity to anything with a lower 'pressure'.
The electrolyte is then at a higher electrical 'pressure' than the metal that is not dissolving and so the electricity passes into it.
The metal that is dissolving is the 'anode' from which the electrical current passes into the electrolyte and the other metal is the cathode into which the current passes because the electrical pressure must be balanced out. (everything tries to equalise).
The dissolving metal is sacrificed to prevent the subject metal from corrosion, and this method is known as 'sacrificial cathodic protection'.
There are limits to which sacrificial cathodic protection can be used but the same principle can be used by causing a manufactured electrical pressure which is 'impressed' into the electrolyte. The electricity is then 'drained' out of the subject metal....... boat hull or pipeline.... and this interferes with the natural tendency of the metal to dissolve....or rust!

Impressed current cathodic protection


Electricity is generated by a sort of pumping action which causes it to flow backwards and forwards in 'waves', but this is no use for our purposes so we have to get it going in one direction through a circuit known as a 'rectifier'. At the same time we can control the amount of current by transforming it, so the apparatus is know as a transformer-rectifier.
A transformer-rectifier can be regarded as an electrical pump which is sucking the electricity out of the pipeline (etc) and pumping it into the ground (or sea ... or swamp... or wherever else you want to pump it).
The effect of this is amazing. It stops rust! And it's cheap!
But there are some snags.
Because it's so good, it gets installed .... then ignored...... well most people don't even know it exists... and because it's cheap some people don't think it's important.


But it 's life and death to some.


The villagers in the picture are gathering water from outside a flowstation in Nigeria. A pipeline in Nigeria leaked petrol and local people collected the petrol in cans and washing up bowls and the site drew hundreds of women and children until the petrol was accidentally ignited.... cooking up to 1000 people.

Cathodic protection IS important.
A couple of years before this incident a pipeline in the USSR exploded and blew a train off it's tracks, killing many and causing ecological devastation. This was thought to be caused by corrosion.

by : Roger Alexander (my great Teacher)























Newsflash 5th December 2000


*** Natural gas spewing in Texas MONT BELVIEU, Texas (AP) - A pipeline ruptured and released a
potentially explosive cloud of natural gas, forcing evacuations of
about 40 homes and the rerouting of airplane flights around the
area. Several minor injuries were reported Monday night when the
pipeline, owned by Channel Industries Gas Co., blew open near
Houston Raceway Park. The blowout was felt and heard as far away as
Baytown, more than 10 miles to the south. There was no fire, said
Baytown police Sgt. Keith Dougherty. However, residents of the
immediate area were told to evacuate and flights east of Houston
were kept at least miles from the site as a precaution, said Texas
Department of Public Safety spokesman Richard Vasser.

Full article at: http://www.infobeat.com/fullArticle?article=405225991

Monday, 26 April 2010

Basic Calculation For Sacrificial System

A. Current requirement ( I ) = SA x CD

Where :
SA = Surface area (taken from Norton Corrosion calculation)
CD = Current density (taken from NACE standard RP 0176-83)
(Standard Recommended Practice Corrosion Control of Steel, fixed Offshore Platform
Associated with Petroleum Production)
Sea water = 55 mA.sq.m
Seabed = 11 mA/sq.m

B. Anode Weight = N

N = (I x Con x L) : U

Where,
I = Current requirement (Amp)
Con = Consumption rate of aluminium anode = 3.35 Kg/AY
Consumption rate of magnesium anode = 7.7 Kg/AY
L = Life time = 10 years
U = Utilization factor = 0.9

C. Example

Data:
Atmospheric Storage Tank:
- Length (L) : 5.54 m
- Diameter (D) : 9.058 m
- Water level Assume : 1 m
Current Density (CD) : 55 mA/m2
Consumption Rate of Aluminum Anode : 3.5 kg/Amp Year
Years design : 10 tahun
Utility Factor : 0.9

Calculation:

1 Surface Area (SA)

- Surface Area of Ground = pi x D2 = 3.14 x 82.047 = 257.62 m2
- Surface Area of wall = pi x D x L = 3.14 x 9.058 x 5.54 = 157.56 m2
Total Surface Area = 415.18 m2

2 Current Requirement (Ip)

Ip = SA x CD
= 415.18 m2 x 55 mA/m2
= 22,834.9 mA = 22.835 Amp

3 Weight of Anode (W)

W.Al = Ip x Y x 3.5 kg/Amp Year
U = 22.835 Amp x 10 years x 3.5 kg/Amp Year : 0.9
= 799.225 kgs

4 Number of Aluminium Anode (N)

Proposed Aluminium Anode type 50 kg - Net weight = 50 kg/ea
N.Al = W.Al : 50 kg
= 799.225 kgs : 50 kg
= 15.985 pcs = 16 pcs

Total = 16 pcs Aluminum Anode type 50 kg


regards
bajakz
email : bajakz@gmail.com

note : if you need more of calculation for cathodic protection, please contact me

Thursday, 22 October 2009

INTERNATIONAL STANDARD ISO 15589-2

Foreword

ISO 15589-2:2004(E)


ISO (the International Organization for Standardization) is a worldwide federation of national standards bodies
(ISO member bodies). The work of preparing International Standards is normally carried out through ISO
technical committees. Each member body interested in a subject for which a technical committee has been
established has the right to be represented on that committee. International organizations, governmental and
non-governmental, in liaison with ISO, also take part in the work. ISO collaborates closely with the
International Electrotechnical Commission (IEC) on all matters of electrotechnical standardization.

International Standards are drafted in accordance with the rules given in the ISO/IEC Directives, Part 2.

The main task of technical committees is to prepare International Standards. Draft International Standards
adopted by the technical committees are circulated to the member bodies for voting. Publication as an
International Standard requires approval by at least 75 % of the member bodies casting a vote.

Attention is drawn to the possibility that some of the elements of this document may be the subject of patent
rights. ISO shall not be held responsible for identifying any or all such patent rights.

ISO 15589-2 was prepared by Technical Committee ISO/TC 67, Materials, equipment and offshore structures
for petroleum, petrochemical and natural gas industries, Subcommittee SC 2, Pipeline transportation systems.

ISO 15589 consists of the following parts, under the general title Petroleum and natural gas industries —
Cathodic protection of pipeline transportation systems:

— Part 1: On-land pipelines

— Part 2: Offshore pipelines


Introduction

Pipeline cathodic protection is achieved by the supply of sufficient direct current to the external pipe surface,
so that the steel-to-electrolyte potential is lowered to values at which external corrosion is reduced to an
insignificant rate.

Cathodic protection is normally used in combination with a suitable protective coating system to protect the
external surfaces of steel pipelines from corrosion.

External corrosion control in general is covered by ISO 13623.

Users of this part of ISO 15589 should be aware that further or differing requirements may be needed for
individual applications. This part of ISO 15589 is not intended to inhibit alternative equipment or engineering
solutions to be used for the individual application. This may be particularly applicable where there is innovative
or developing technology. Where an alternative is offered, any variations from this part of ISO 15589 should
be identified.

Deviations from this part of ISO 15589 may be warranted in specific situations, provided it is demonstrated
that the objectives expressed in this part of ISO 15589 have been achieved.

More... you can download the file

http://www.4shared.com/file/142724789/8ca67b26/ISO15589-2-2004forOR.html

thks for all

bajakz

Tuesday, 20 October 2009

Examples of Design for Cathodic Protection Systems

From Estimated Exposed Surface Area

Estimating current requirements from expected exposed surface is always subject to
error. There are many factors, which affect the results.
Consider:
• Total surface area in contact with soil or other electrolyte.
• Dielectric properties of any protective coating.
• Factors which may damage a protective coating during installation.
• Expected protective coating life under service conditions.
• Expected percentage coverage by protective coating.
• Past experience with coating applicators and construction contractors.
• Current density required for cathodic protection of the metal(s) in the
environment.

In the end, the expected current requirement depends on calculating the area of
exposed metal in contact with the electrolyte and multiplying it by the “best estimate”
of current density for the conditions present.
There is an alternate approach for coated electrically isolated structures (pipes, under-
ground storage tanks, etc.) where there is data available on existing cathodic protection
systems.

The approach requires reliable local data on:
• Expected leakage conductance (Siemens/unit area) in 1000 ohm cm. soil for a
class of coating (epoxy, polyethylene tape, etc.) and type of service
(transmission pipeline, gas distribution, fuel tank).
• Soil resistivity in the service area.
• Structure to soil potential shift required to produce polarization needed to meet
cathodic protection criteria. This is the immediate change in potential of an
isolated structure measured to a point at “remote earth” when cathodic
protection is applied.

The value is not a criteria for protection. However, under a given set of operating and exposure conditions, a potential shift will provide a good estimate of current needed to meet accepted criteria.

The approach is best understood by using an example.

Example 5.1

A gas utility is planning to install 3049 meters (10,000 feet) of 5.1 cm (2 inch) coated
steel distribution mains in a new development. The average soil resistivity in the area
is 5,000 ohm cm. The corrosion engineer wishes to estimate the approximate current
required to cathodically protect the pipes.
Experience in the utility has developed the following data on cathodic protection
current requirements:
Average leakage conductance G for distribution type service is 2.14 × 10−3S/m2in
1000 ohm cm soil.
Average potential shift measured to “remote earth” to achieve protection is −0.250
volt.
Calculations:

Total surface area of the proposed pipe.
As=πd L = (5.1 × 3.1416/100) × 3049 = 488 sq. meters

Estimated leakage conductance of new pipe in 1000 ohm cm soil.
g = G × A = 2.14 × 10−3×488 = 1.04 Siemens
Since resistance = 1/conductance
Resistance to remote earth = 1/1.04 = 0.96 ohm

Estimated resistance to remote earth in 5000 ohm cm soil. (Resistance is directly pro-
portional to resistivity).
0.96 × 5 = 4.8 ohms
Estimated current to shift pipe potential to remote earth −0.250 volt. From Ohm’s
Law (I = E/R)
0.250/4.8 = 0.052 A.

for detail you can buy PEABODY or cantact me for getting ebook

thks for all

bajakz

Wednesday, 15 July 2009

CATHODIC PROTECTION BASIC PRINCIPLES

The presence of anodes and cathodes in a structure can be caused by micro or macro influences.
On the micro scale, they may be due to:

Heterogenieties in alloy structure.
Oxide layer.
Difference in stress level.
Micro segregation, etc
On the macro scale, anodes and cathodes may be caused by:
Variation in oxygen availability.
Water composition.
Soil resistivity.
Bi-metallic couples.
Presence or otherwise of protective coatings, etc.
Corrosion results from an electrochemical reaction. It requires an anode, a cathode, a common electrolyte, and an electrical connection between the two zones. The corrosion process results in the flow of a small electric current from the anode to the cathode through the electrolyte. The magnitude of the current which is due to a number of factors is directly proportional to the metal lost due to corrosion. One ampere flowing for one year would result in the loss of 9 kg of steel from a corroding surface.

Freely flowing corrosion current from Anode to Cathode.
In recent years cathodic protection has found a general acceptance amongst engineers and structure owners as being a truly effective method of preventing corrosion under the ground or under the sea. It is now more common than not to find cathodic protection used on marine structures and on buried pipelines.
The concept of cathodic protection is straight forward. Corrosion occurs as the result of electrochemical reactions between zones of differing potential on a metal surface. Oxidation (corrosion) occurs at the anodic zone and reduction (no corrosion) occurs on the cathodic zone. Cathodic protection is achieved when an entire metal surface is converted to a cathodic zone.
The corrosion reactions at each surface may be described as:
Cathodic protection is achieved by supplying a current from an external source so that it reverses the natural corrosion currents and ensures that current is flowing through the electrolyte onto all of the metal surface requiring protection. This current flow causes a change in potential.
Freely corroding mild steel in seawater has a resultant potential between anode and cathode of approximately -0.50 to -0.60 volts compared to a silver/silver chloride reference electrode. When cathodic protection is applied, it will be noted that the surface potential of steel will change to more negative than -0.80 volts when measured relative to a silver/silver chloride reference cell. Thus by using this simple practical measurement, it is possible to determine whether corrosion has been completely eliminated or not.
The external current applied in cathodic protection may be generated from either of two methods, sacrificial anodes or impressed current systems.
GALVANIC ANODES
Sacrificial or galvanic anodes rely on the galvanic corrosion of a more reactive metal to produce current, e.g. aluminium anodes, zinc anodes or magnesium anodes.
Flow of corrosion current suppressed by protective current discharged from sacrificial anode.
Sacrificial anodes are most commonly used to protect metallic structures in electrolytes because of their simplicity of installation and maintenance free operation. Of the alloys available for sacrificial anodes, alloys of aluminium have proven to be the most economical in seawater or very low resistivity muds.
Knowing the total submerged and buried steel areas, the water resistivity and the required system life, a corrosion engineer can determine precisely what energy will be required to protect a structure and can design a galvanic system to suit the environmental requirements.

IMPRESSED CURRENT ANODES

Impressed current systems provide the same electric current as galvanic anodes by the discharge of D.C. current from a relative inert anode energised from an external D.C. power source such as a transformer rectifier or thermo electric generator. Impressed current system anodes include materials such as graphite, silicon iron, platinised precious metals and lead alloys.
Flow of corrosion current suppressed by protective current discharged from Impressed Current System.
Effective cathodic protection guarantees corrosion free existence. Providing the structure is maintained at a potential of -0.8 volts (or more negative) no loss of metal will occur at all during the life of the structure. As cathodic protection can be renewed or added to during the life of the structure, the maintenance of the desired potential is readily achievable. The efficacy of the system can be monitored by simple electrical measurements.
Cathodic protection apart from overcoming the more "normal" causes of corrosion, may be used to counter accelerated corrosion resulting from contact between different metals, from impingement by high velocity water, from the effects of sulphate reducing bacteria and from the effects of stray D.C. currents.
In fact, any metal such as scrap iron may be used as an impressed current anode. In cathodic protection practice, we choose to use either semi-permanent or permanent anode and very seldom non-permanent anode (such as scrap iron).
Examples of semi-permanent anodes are silicon/chromium/iron anode, lead/silver/antimony anode, graphite anode etc.
Examples of permanent anodes are mixed metal oxide anode, platinised titanium anode etc.

Monday, 13 July 2009

Consideration for Design of Galvanic Anode Cathodic Protection System

by Ernest Klechka, P.E, NACE International CP Instructor

N
ormally cathodic protection (CP) can be applied by a sacrificial anode or impressed current system. CP can be applied by galvanic or sacrificial system when limited amounts of current are needed, soil resistivity is low (normally less than 5,000 ohm-cm), and electric power is limited or not available. Sacrificial anode system have the added advantage of requiring minimum maintenance.
To design a sacrificial anode system, the following information is needed:
1. Current requirement (I current required)
2. Anode resistance ( R anode), calculated or based on the manufacturer's data
3. Design life

Design Current Requirements
CP current requirements can be determined by calculation based on bare or exposed surface area, assumptions about existing coatings, the coating damage factor, and the current density (CD) required for CO in environment are needed.

To calculate the current required, the total surface area (Atotal) in sq m of the structure is defined. Base on experience, the coating damage factor (fdamage) or percent bare area is determined. The total surface area to be protected by CP (Acp) in sqm is then :
ACP = Atotal fdamage

Based on the environment to which the structure is exposed, the applicable CD for CP (Icd) is estimated in mA/sqm. Using the ACP and Icd, the calculated current required (Icurrent required) is then :
Icurrent required = ACP Icd

Current Requirements Based on Field Tests

Current requirement test are the most reliable way to estimate the current requirements for existing structure and take info account the condition of the coatings and variations in soil resistivity. If the structure is coated and isolated, it is possible to directly determine the current requirements.

A temporary anode (groundbed) is established and a portable power supply (usually a battery, generator, or portable rectifier) is installed. A current is applied (I) and the change in polarized potential ( V) is determined. The ratio of the current applied dicided by the change in polarized potential (I/V) is used to calculated the current requirement.

For example, if 5A are applied to the structure and a polarized potential shift of 50 mA occur, and a 100 mV shift is needed for CP, then :

Icurrent required = shift needed x (I/V)
= 100 mV x (5A / 50mV)
= 10 A

Determining the Output of a Sacrificial Anode

Often the manufacture of an anode will provide the estimated current output in the form of table or graphs based on the shape and size of their anode and soil resistivity. The average soil (electrolyte) resistivity is needed to use these table or graphs or for calculation.

if this data is not available, the output of an anode can be estimated based on Ohm's Law :
I anode = E driving potential / R anode
Anode Resistance-to-Earth

Typically, since sacrificial anodes are buried vertically. For a single certical anode, such as a package magnesium anode, the resistance-to-earth can be calculated using Dwight's equation for a single vertical rod or pipe :
R = (0.00159 / L) [ln (8L/d)-1]
where :
R = groundbed resistance ()
= resistivity ( -m)
d = diameter of anode (m)

The resistivity value used must be representative of the volume resistivity affecting the anode.

Parallel Anode

Sunde's equation can be used to estimate the resistance of distributted parallel anodes:

RN = (0.00159 / L) [ln (8L/d)-1] - 1 + (2L/S) ln(0.656 N)
where :
RN = groundbed resistance ()
= resistivity ( -m)
d = diameter of anode (m)
L = length of anode (m)
S = spacing of anode in the groundbed (m)

If the anode are separate by 6 m or more, the parallel effect of anode is negligible.

Anode Current Output

For a single highh-[ptential (-1.75 Vcse) 7.7 kg (17 lb) magnesium anode, the current output can be calculated or determined based on the manufacturer's data.

A typical 7.7 kg anode is 1,295 mm long by roughly 51 mm square. Because Dwight's equation deals with rods, an equivalent rod with the same circumference as a 51mm square is needed; the rod will be 65 mm in diameter. The equivalent diameter d for square with sides S is given as :

3.14 d = 4 S
d = 4 S /3.14

If the structure is to be polarized to -0.850 Vcse, driving potential is then:

Enet = -1.75 -(-0.85) = -0.90 V

Anode resistance can be calculated using Dwight's equation for 5,000 -cm soil :

Ranode = (0.00159 * 5,000 / 3.14*1,295) [ln (8*1,295/65-1]
= 25

Assuming the structure and cable resistance are negligible, the expected current is then :

Ianode = Enet / Ranode
= 0.90 V / 25
= 0.036 A (36mA)

The anticipated current output is then 36mA from single 7.7 kg anode.
Anode supplier literature indicated that in 5,000 -cm soil, a high-potential magnesium anode will have an anticipated output of 0.040 A (40 mA) to a structure polarized to -0.85Vcse. The data infers that the structure has negligible resistance to earth and, therefore, no IR drop. The resistance to remote earth of single high-potential (-1.75 Vcse) magnesium anode can be calculated:

Ranode = Enet / Ianode
=(1.75 - 0.85) / 0.04
= 22.5

The two value of current output and resistance are very similar.

Number of Anode Needed Bases on Current Requirements

Once the total current required and the current output from a single anode are determined, the number of anode needed to protect the structure can be calculated. Based on current requirement :

Nanode = Icurrent required / Ianode

After the number of anode is calculated, the number of anode must be rounded up ti the next integer (no partial anode allowed).

Number of Anode Needed Based on Design Life

The number of anode needed can also be calculated based on the current requires an the design life. Total weight need for design life can be based on Faraday's Law :

Wtotal =K I T

where:
K = Consumption (kg/A-h x 24 h/day x 365 days) ( see the table I )
I = current in A (Icurrent required)
T = time in years (design life)
Then the number of anode needed is equal to the total weight needed divided by weight of single anode:

Nanode = W total / W anode

Table I


How Many Anode Are Really Needed?

The number of anode needed, therefore, is the larger of the number needed based on current requirements and the number needed based on design life.

Because the current provided can be greater than the current required, control resistor may be needed to reduce the initial output of anodes. After a period of time, the current required may change due to changes in the condition of coating or the environment. Control resistor may needed to be changed with time.

Galvanic Anode Life

Once the number of anode needed is determined, the life expectancy for the system should be checked. The following equations can be used to determine the anticipated life of the sacrificial anode system.

Magnesium years of life = 0.256 x anode weght in kg x efficiency x utilization factor Current in A
Zinc years of life = 0.0935 x anode weght in kg x efficiency x utilization factor Current in A

Post-Installation Measurements

After the systems installed, the structur-to-soil potentials sould be measured as we;; as the current output from the anode system. IF the current output needed adjustment, more anodes can be added to increase current or control resisters can be added to reduce the current output.

Anode life can be calculated based on the actual measured current of the sacrificial anode system.

Conclusions

The design of a sacrificial anode CP system can be accomplished as long as the current demand and the anode current output can be determined. Anode separated by more tham 6 m can be assumed to have minimum parallel interference.

Sacrificial of galvanic anode CP can be very effective as long as current demands are low and soil resistivities are moderate to low. Added benefits of these system are the reduce maintenance cost, no electrical power is required, and ease of installation. Low-power sacrificialsytem also cause a minimum of interference and can be used to discharge CP interference