cathodic protection module 1-00

8/19/2019 Cathodic Protection Module 1-00

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Cathodic Protection Training Course

Introduction to this course

This course is for everyone involved with the application of cathodic protection.

Cathodic Protection has always been divided between the science of electro-

chemistry and the application of cathodic protection technology in the field.

Since the 1980's cathodic protection data has been stored on computers but the

gap between the electro-chemists and the most basic field practices has made itdifficult to achieve computer analysis.

This course includes practical work that is

designed to enable the student to understand

applied cathodic protection from the very basic

principles.

It is important that each student understands each module as a basis on whichthey can move forward to the next.

At the end of the course each student will be required to present a paper for

publication on the CPN website. The merits of each paper will be assessed by the

membership of the CPN.

Experienced corrosion engineers and scientists will be able to check the validity

of each step and are encouraged to express their opinions.

Module 1

dule 1 http://www.rogeralexander1938.webspace.virginmedia.com/cpn/Corre...

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An introduction to cathodic

protection

What is cathodic protection?

Cathodic protection is an electrical way of stopping

corrosion.

In order to understand cathodic protection it is

crucial that an engineer can visualise 'electrical

pressures'.

This is a typical illustration of a corrosion cell with the

arrows showing the direction of the current.

This current is driven by the 'pressure' (Electro-Motive

Force) of the corrosion reaction that is taking place on

the surface of the metal.

This pressure drives electrical current through the

electrolyte to any location with a lower electrical

pressure.

We should imagine the 'electrical pressures' as we use

the instruments to measure electrical values.


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This meter is showing that there is a voltage of 0.530

between it's negative and positive poles.

An electrical pressure is known as a 'potential' - not be

confused with a voltage. A voltage is the difference in

potential between two points, measured in volts.

The relationship between voltage, current (measured by

ammeters) and resistance (measured in ohms) is defined

by Ohm's Law.

When measuring voltages any potential can be regarded

as zero for the purpose of graphic display and

calculations.

This potential can then be compared to another potentialusing a voltmeter so that the potential difference can be

expressed in volts. The graph above only shows the

difference between the two potentials at each point of


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measurement. There is no reason to suppose that any

two voltages are related. This graph is simply a number

of voltages joined together by a line joining the dots.

This fact is dealt with in depth during this course,

including practical experiments. The voltmeter sets the

zero for each measurment.

The above experiment will confirm that the graph base

line is a 'floating zero'.

Corrosion produces 'electro-motive-force', which drives

current into the electrolyte, causing the potential of the

electrolyte to increase and the corrosion current to

radiate out into the surrounding electrolyte.

Corrosion is a chemical reaction that discharges

electricity from an anode to a cathode through the

electrolyte. Metal is changed into rust at the anode and

the metal at the cathode remains undamaged.

The current generated at a coating defect takes the least

line of resistance to return to the pipeline metal


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The point where current enters the metal is known as

the cathode. No corrosion reaction is possible at this site

as the potential of the electrolyte is greater than that of

the metal at this immediate interface.

The reaction can continue until equilibrium is reached

between the chemicals and the electrical energy. Thechemicals have 'eaten away' all the metal and have run

out of 'food'. No current is produced and so the whole

coating fault is 'at rest'. Corrosion product builds up on

the metal blocking the path of the current.

Batteries work on this principle. When batteries reach

equilibrium we have to re- charge or discard them.


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The public are not generally aware that our gas and oil

comes to us through pipes that are inclined to rust but

are protected by 'Cathodic Protection'. As corrosion

control engineers we must constantly be aware of the

electrical energy around us all the time as that drives the

corrosion process and affects everything we do in our

everyday lives.

Pipelines are 'out of sight and out of mind' so little

attention is given to the fact that metal dissolves in

some solutions and gives off electricity.

It is left to the corrosion engineers to worry about such

things until a pipeline fails, causing loss of life,

environmental damage and massive financial

consequences.


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Consultants are then asked why the pipeline failed and

the debate about the criterion for cathodic protection

receives attention for a little while.

Ship and boat owners are constantly aware of the

damage caused by corrosion and consequently metal

boats are protected by cathodic protection. They have

lumps of metal attached to hulls for this purpose. These

lumps of metal disolve in the water and give off

electricity which prevents the hull from corroding.

Sir Humphrey Davy first introduced this system by

attaching 'pig iron' to the copper clad hulls of ships.

There is a considerable amount of information and

computer modelling advertised on the internet in this

respect. A search will reveal a number of specialised

companies offering services and the CPN is not

competing in this market.

We are concerned with the analysis of data gatheredrelating to the cathodic protection of buried and

submerged, coated, steel pipelines that carry most of the

worlds energy supplies from source to the consumer.

This is a very specialised study that must begin with at

the interface between the pipeline metal and the

electrolyte in which it is submerged or buried.


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This old photograph was taken during the construction

of the network of gas pipelines that have been buried in

the UK for over 30 years. This particular stretch was

coated with coal tar enamel and was handled by heavy

construction machines. The coating was often damaged

and repaired before back-fill.

It is clear that coating faults were sometimes missed.

The pipe metal at these coating faults is in contact with

the ground (the 'electrolyte') , which gets 'charged up'

with electricity. The electrical potential' of this bit of

ground is increased to a higher electrical 'pressure' than

the metal surrounding ground and so the electricity

'radiates' into the earth.

The metal that is disolving is the 'anode' from which the

electrical current passes into the electrolyte.

The other metal is the 'cathode' into which the current

passes from the 'charged up' eletrolyte, because the

electrical pressure must be balanced out. (everythingtries to equalise it's electrical potential with everything

around it).

The disolving metal is sacrificed to prevent the subject

metal from corrosion, and this fact is harnessed by

providing a less noble metal in the corrosion circuit... a

system known as 'sacrificial cathodic protection'.

There are limits to which sacifical cathodic protection

can be used but the same principle can be used bycausing 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 interfers with the natural tendency

of the metal to disolve....or rust!

Students should try to form a mental image of electrical

potential (pressure) and the resulting flow of 'charges'.

Do not get confused by the flow of electrons as we

cannot see this on our meters. It might be important tothe academics but it is irrelevant to field engineers


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Impressed current cathodic protection

Alternating Current 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 acircuit 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. BR>

THE BASIC CONCEPT OF

PIPELINE CATHODIC

PROTECTION

As stated before, everything has a 'potential', which has

an effect on it's relationship with it's environment.

Corrosion is effected by this relationship, as it is an

electro-chemical reaction.

The basic concept of cathodic protection is that the


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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. The reasons for this

are given in thermo-dynamic theory but these will not

be discussed at this stage.

The corrosion reaction and cathodic protection

mechanism has been defined by many scientists and has

become established beyond dispute. Many books and

papers have been published, giving details of the

scientific background of corrosion and corrosion

control, as a result of many years of research by

respected and sincere specialists. It is not intended to

dispute any of this work or the conclusions drawn.

Battery technology can becompared to corrosion control

Technology

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 is designed

to allow a chemical reaction to cause an electrical

current to pass through a desired path, giving energy tothe appliance. The battery has a very carefully

composed electrolyte which has qualities to ensure a

predictable reaction with the other components of the

battery.


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Corrosion within a battery can be controlled by external

electrical techniques which are in common use. Some

batteries have a reversible reaction which enables them

to be recharged by adjusting the electrical 'pressure' at

the terminals. Many appliances are nowadays controlled

by computers to balance the reaction equilibrium to suit

their own power demands.

All this is possible because the battery is a

manufactured unit, designed for the purpose of

receiving and supplying electrical current.

CATHODIC PROTECTION IS DIFFERENT

Unfortunately, cathodic protection is not a unit

composed of simple elements in the way that batteries

are, because the electrolyte is the ground itself. This isan uncontrollable feature with an almost infinite variety

of qualities.

The picture above is an equivalent circuit diagram of

the cathodic protection systems that were preventing

corrosion over an area of tens of thousands of square

miles of pipelines serving a major oil and gas

production company.


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The chemical composition and electrical conductivity of

the ground can span a vast range and can include

environments such as sea water, deserts, freshwater

swamps, arable (fertilised) land, etc. etc. Climatic

effects cause variations in the temperature, and depth of

cover causes pressure variations which effect the

reaction, adding yet more indeterminable factors in the

reaction.

Cathodic protection of such subjects as gathering

stations (shown above) and storage tank bases is

relatively simple but as the size of the structure

increases, it extends through electrolytes of different

nature 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 studies of these conditions have shown that

they have substantial influences on corrosion.

UNDEFINABLE ELEMENTS

Pipelines are more complex, and can be regarded as

many interface reactions connected together, in parallel.


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The metal element, of the reaction, can be well defined,

as this is specified to a high degree by the designers.

The coating material is carefully designed but it is

generally accepted that no coating can be perfect, and

the faults (or 'Holidays') introduce the first indefinable

element to the system.

During the construction of a pipeline, all possible

measures are taken to detect and repair coating faults, so

it follows that the location and size of those remaining

are unknown and not definable. 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 not possible to

calculate the resistance of the coating of an actual

pipeline.

The electrical current measurements, taken during

routine cathodic protection monitoring, show that there

is little resistance in the total coating of a pipeline and

this can be explained by the difficulty in quality control

during coating operations and preventing damage

during the construction period.

Perfect coating would prevent any output from the CP

system but undetected coating faults provide paths for

cathodic protection current. 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. The effect

that the pipeline resistance has on the complex current

paths and variation in potentials, is found to be so small

that it can almost be ignored.

FURTHER COMPLEXITY

Each coating fault is a metal-to-electrolyte interfacewhich is capable of a different reaction, electro-

motive-force (EMF) which cannot be measured as it is

in parallel with all other EMFs on the same section of

pipeline.


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The magnitude of the current from each of these is

dependent on the earth resistance immediately adjacentto the interface, and the current direction is the result of

the combined effects of all the resistances and electrical

pressures caused by all the other 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 beensubject to computer modeling 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 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.

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 considerableamount of cathodic protection current is used.

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.

The current will only pass onto the metal at a coatingfault, 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


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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 specificsize 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.

These matters will be covered in detail later in the

course

THEORY V PRACTICE

We simply want to stop corrosion but we need to know

when we have succeeded. Cathodic protection is

immensely successful, and cost effective, but every leak

is a demonstration that we have not applied it correctly.

Link to page on Cathodic Protection Measurements

Before going any further it is necessary to imagine

electricity and this has been likened to water pressure,

with containers connected by pipes to allow current to

flow.

However, it can be seen that the levels would equalise

as soon as enough water had run from one container to

the other. No current would then flow.

If water was added to the higher container at the same

rate that it is passing through the connecting tube, then


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the current will continue.

This is similar to a dry cell battery which is, infact a

corrosion cell. The current will flow through a

conductor when the two poles are connected in the same

way that water flows through the connecting tube at the bottom of the two containers.

When the reaction energy has run out, the battery is

dead and the potential levels are the same at each pole.

A corroding nail is similar in that the corrosion current

flows from the anode of the nail, into the damp cloth

and then goes back through the cloth to the cathode of

the nail.

The corrosion reaction on the nail can be forced in a

variety of ways to be defined in this course.

Refering back to the water analogy, it is important to

understand that the pressure is caused by the height of

the water in each container and not the weight. The

water will fill any connecting tube and then the pressure

downwards will be greater in the vessel which has the

highest level. The reason for this is obviously due to the

imbalance between the pressures in the two containers

and electrical potentials have the same tendency when

connected by conductors.

This is fine when visualising a simple circuit such as a

single corrosion cell or a dry cell battery connected

through a light bulb, but in a cathodic protection circuit,


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or when corrosion takes place on a pipeline we have no

means of measuring each separate cell in this way.

If we examine the technique that is used in the

laboratory then it becomes clear that provision has been

made to eliminate outside influences in this 'open circuit

measurement'.

This is not possible in cathodic protection field work,

and yet laboratory derived theories are applied to

readings obtained in the field.

The situation on pipelines is that there are many

corrosion cells, all connected to the same metal and yet

each having it's own corrosion reaction. This can be

imagined like this.

It can be seen that it is impossible to measure the pressure differences in each cell by making a single

connection to the common reservoir at the bottom.

However it would be possible to stop the flow of water


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in each of the cells by continuously making the water

level equal in each pair of containers.

However, it can be seen that the pressure measurement

in such a system would need to be between the lowest

water level and the highest water level in the whole

system.

This is achieved in cathodic protection by flooding all

the containers as shown in green. The current then stops

flowing between each pair. Because of the nature of electricity this requires that current is drained from the

pipeline and pumped into the ground in sufficient

quantity to 'fill all the containers' or overcome the

corrosion reaction potential (EMF).

link to page showing water containers to demonstrate

electrical potentials and in relation to pipeline cathodic

protection

The difficulty in making this voltage measurement is

shown in the demonstration with water holders buried

in sand.


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We can measure the level of the water against the level

of the sand.

We cannot see the bottom of the containers but in this

case some are connected to others by a glass tube

through which the water can pass.

Water can pass between some of the visible containers

to others in the same way as corrosion current.

We can never know if the corrosion current has been

stopped when (whole system is in equilibrium) as we

have no reference to zero potential. It is out of sight and

reach!

In the same way, we cannot know the EMF (water

level) of each corrosion cell. We can only measure the

voltage between the potential of the ground and the

potential of ALL OF THE METAL. That is theequivalent of the level of all of the water in the

containers. We do not know how deep each containers

is and we do not know which are connected.

The established method of measuring the effectiveness

of cathodic protection is by recording the voltage

between two variables. This cannot determine if

corrosion has stopped.

Open circuit measurements


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The term 'open cuircuit measurements' was

acknowledged by Dr Peabody of NACE when

recognising the problem that was termed 'the IR drop in

the soil'

Natural corrosion cells are much different from those

set up in a laboratory, as they can be physically minute

or large. Large corrosion cells can contain micro-cellswithin the same area where anodic areas completely

surround cathodes or vice-versa. When studying such

cells, we are not able to separate the component parts,

and the measurements have come to be known as 'open

circuit measurements'.

This type of measurement involves connections to the

electrolyte as well as the metal and this requires the use

of an electrode. There is a danger that this will

introduce another EMF into the circuit, by the reaction between the electrode and the electrolyte. We therefore

use an electrode in a solution of its own salts, which has

a known reaction EMF. We can then make a connection

between the electrolyte in the cell and the earth

electrolyte, in the hopes that there will be no electrical

disturbance to the measuring circuit.

In the laboratory, this disturbance is prevented by the

use of a glass capillary filled with inert gel, which is

used as a conductor from the reaction interface to the

reference electrode. The reference electrode is a metalin a saturated solution of its own salts, as this has a

known reaction potential. Reference electrodes are

related to each other by known voltages and are used as


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international standards.

Without this consistency it would be impossible to

evaluate the reaction, develop theories or design

cathodic protection systems etc.

Unfortunately, it became the practice to apply

laboratory principles in cathodic protection field work.

This subject can now be studied in greater detail by

computer modeling which makes it clear that the fixed

potential is normally that of the pipeline metal, and the

variation in the measured voltage is due to the different

potentials elsewhere in the measuring circuit.

Imagine that we require to know the voltage of two dry

cell batteries which are arranged in parallel. That is to

say that each is in connection with a common conductor

to the positive pole and another common conductor (the

ground)to their negative poles.

Both conductors would carry equilibrium current

according to the reaction within each battery and the

voltage between the two conductors could be measured by connecting a meter between the two. Unless the two

cells are separated, it is impossible to evaluate the

voltage of each battery. Even this is not as complex as

the expectancies of cathodic protection monitors.


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If we take two batteries and half bury them in an

electrolyte with their positive poles exposed and

connected, we have two corrosion cells in closer

condition to those found on a pipeline.

A circuit drawing of this arrangement will show that

current will pass through the ground to equalise the

pressures caused by the interface reactions within each

battery.

We must now try to evaluate the reaction within each battery using a high resistance voltmeter and an

electrode. We cannot break the circuit or separate the

batteries but connections can be made to the metal or

the electrolyte or both. It will be seen that we are only

capable of measuring voltages across various spans of

the circuit, and cannot establish a reference within that

circuit. The laboratory techniques cannot be applied to

these conditions as there are too many variables which

are impossible to evaluate.

If we increase the number of half buried batteries

connected together, we improve the similarity to a

pipeline, but in order to be more realistic, we must

include some which have their positive poles buried.

This has been shown earlier in this page.

The complexity of the situation is now apparent and

what seemed to be a simple measurement, now seems

almost impossible.

A circuit diagram of the complex arrangement will

show that a different voltage will be measured with

every new position of the electrode, and this is born out

in cathodic protection field practice. It is especially

obvious on pipelines which are not connected to

cathodic protection systems and which have poor

coating.


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The different voltages are due to the variety of

potentials at each pole of the voltmeter. These can be

caused in many ways, as described later, but it is

important to realise that they are all components of the

voltage shown on the meter. It is possible to eliminate

them in the laboratory but not in the field, therefore they

must be evaluated and considered in the analysis of

survey results.

The problem is even more complex when cathodic

protection is introduced as this is an additional voltage

which is superimposed over all the others. Being

designed to drain charges from the whole of the

pipeline, it has an effect on the equilibrium of all the

other electrical influences. However, the dynamiceffects of an impressed current system can be removed

by taking voltage measurements immediately after the

system has been switched off.

This cannot be achieved where sacrificial anodes are

used, unless they have a special facility designed for

this purpose at construction stage.

The voltages obtained between the pipeline metal and a

randomly placed electrode have a certain amount of

value when compared to others obtained from

connections to the same pipeline. This is because of the

very low electrical resistance in this part of the


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corrosion and cathodic protection circuits.

link to page about electrical potentials and in relation to

pipeline cathodic protection

Students are now required to read Procedure 1a

Students are required to understand the instruments they

will be using.

Students are required to carry out experiments and

submit a report.

Link to some old report forms dating back to before the

original CIPS survey

Go to field trip

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