Cathodic Protection Training Course.doc

7/24/2019 Cathodic Protection Training Course.doc

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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 198 cathodic protection data has been stored in computers! but thedifference between the electrochemical theory and most basic field practices has

made it impossible 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.

"t is important that each student understands each module as a basis on which theycan move forward to the ne#t.

$t the end of the course each student will be re%uired to present a paper for

publication on the CP& website. The merits of each paper will be assessed by themembership of the CP&.

'#perienced corrosion engineers and scientists will be able to chec( the validity of

each step and are encouraged to e#press their opinions.

This course has been on line for many years now and has had many thousands of


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visitors.

"t is not functioning as intended as there have been no interactive students but this

page has received over )*! hits during this past year.

+rom discussions on a variety of social and business networ( groups it can be saidthat these pages are being used as a sort of reference boo( but it cannot be effective

in this way.

" am now wor(ing my way through the course to ma(e it clear that it is essentialthat students start at the very beginning and do not move on until they thoroughly

understand each module.

" have only received two reports as re%uired for a certificate of understanding and

ability and so there are only three of us reviewing any other reports that aresubmitted.

" have certificated others in understanding and ability to carry out ,C and C"PS

surveys as " have instructed these people in the field.

" have also issued certificates of understanding and ability to university studentswho " have mentored in specific aspects of corrosion control that re%uire

e#periments to ma(e topic repeatedly observable according to the re%uirements oftrue science.

$nyone wanting to develop diagnostic software for cathodic protection and

corrosion control needs to understand the laws of nature that apply to our science.Scientific notation cannot always be e#pressed digitally as much of it is analogue

reasoning that is not definitive and deterministic.

The first /odule of our course is deliberately basic in order to e#plain the laws of

nature that bind us all and to help students to 0&,'ST$&, the e%uations thatare taught 2parrot fashioned3 and tested by tic( bo# e#aminations.

Module 1

VER IM!"RT#$T.


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$ P4T'&T"$5 can only be measured in a laboratory as the voltage that we

measure must be compared to a standard potential 2that is agreed by convention3and must be measured in closed circuit condition.

Therefore! the term 6pipe-to-soil potential6 is incorrect and should not be used! as

this will confuse those who (now what a potential really is.

Therefore those who use the term 6pipe-to-soil potential6 must either not understandor be deliberately intending to confuse.

$ny document or scientific paper that includes the term 6pipe-to-soil potential6! or

any abreviation that means this! must &4T be given credence until the method ofmeasurement is described in detail and the word voltage used instead of the word

6potential6.

,eeper scientific studies into this reach a point where we cannot ma(e themeasurement needed to resolve the e%uations and cannot apply that (nowledge.

" have gone into great details to e#plain electricity in a manner that ac(nowledges

all real science and allows you to ma(e e#periments to repeatedly observe theconse%uences.

7hat is cathodic protection

%athodic protection is an electrical way of stopping corrosion.

In order to understand cathodic protection it is crucial that an engineer can

visualise &electrical pressures&.


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This is a typical illustration of a corrosion cell with the arrows showing thedirection of the current.

This current is driven by the 6pressure6 2'lectro-/otive +orce3 of the corrosionreaction that is ta(ing place on the surface of the metal.

This pressure drives electrical current through the electrolyte to any location with a

lower electrical pressure.

7e should imagine the 6electrical pressures6 as we use the instruments to measure

electrical values.


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This meter is showing that there is a voltage of .: between it6s negative and

positive poles.


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$n electrical pressure is (nown as a 6potential6 - not be confused with a voltage. $

voltage is the difference in potential between two points! measured in volts.

The relationship between voltage! current 2measured by ammeters3 and resistance2measured in ohms3 is defined by 4hm6s 5aw.

7hen measuring voltages any potential can be regarded as ;ero for the purpose of

graphic display and calculations.

This potential can then be compared to another potential using a voltmeter so thatthe potential difference can be e#pressed in volts. The graph above only shows the

difference between the two potentials at each point of measurement. There is noreason to suppose that any two voltages are related. This graph is simply a number

of voltages


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The above e#periment will confirm that the graph base line is a 6floating ;ero6.

Corrosion produces 6electro-motive-force6! which drives current into the electrolyte!

causing the potential of the electrolyte to increase and the corrosion current to


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radiate out into the surrounding electrolyte.

Corrosion is a chemical reaction that discharges electricity from an anode to a

cathode through the electrolyte. /etal is changed into rust at the anode and themetal at the cathode remains undamaged.

The current generated at a coating defect ta(es the least line of resistance to return

to the pipeline metal

The point where current enters the metal is (nown as the cathode. &o corrosionreaction is possible at this site as the potential of the electrolyte is greater than thatof the metal at this immediate interface.

The reaction can continue until e%uilibrium is reached between the chemicals and

the electrical energy. The chemicals have 6eaten away6 all the metal and have runout of 6food6. &o current is produced and so the whole coating fault is 6at rest6.

Corrosion product builds up on the metal bloc(ing the path of the current.


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=atteries wor( on this principle. 7hen batteries reach e%uilibrium 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


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that are inclined to rust but are protected by 6Cathodic Protection6. $s corrosion

control engineers we must constantly be aware of the electrical energy around us allthe time as that drives the corrosion process and affects everything we do in our

everyday lives.

Pipelines are 6out of sight and out of mind6 so little attention is given to the fact that

metal dissolves in some solutions and gives off electricity.

"t is left to the corrosion engineers to worry about such things until a pipeline fails!

causing loss of life! environmental damage and massive financial conse%uences.

Consultants are then as(ed why the pipeline failed and the debate about thecriterion for cathodic protection receives attention for a little while.


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Ship and boat owners are constantly aware of the damage caused by corrosion and

conse%uently metal boats are protected by cathodic protection. They have lumps ofmetal 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 >umphrey ,avy first introduced this system by attaching 6pig iron6 to the copperclad hulls of ships.

There is a considerable amount of information and computer modelling advertised

on the internet in this respect. $ search will reveal a number of specialisedcompanies offering services and the CP& is not competing in this mar(et.

7e are concerned with the analysis of data gathered relating to the cathodic

protection of buried and submerged! coated! steel pipelines that carry most of theworlds 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 ta(en during the construction of the networ( of gaspipelines that have been buried in the 0? for over : 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 bac(-fill.

"t is clear that coating faults were sometimes missed.

The pipe metal at these coating faults is in contact with the ground 2the 6electrolyte63! which gets 6charged up6 with electricity. The electrical potential6 of this bit of

ground is increased to a higher electrical 6pressure6 than the metal surroundingground and so the electricity 6radiates6 into the earth.

The metal that is disolving is the 6anode6 from which the electrical current passes

into the electrolyte.


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The other metal is the 6cathode6 into which the current passes from the 6charged up6eletrolyte! because the electrical pressure must be balanced out. 2everything tries to

e%ualise it6s electrical potential with everything around it3.

The disolving metal is sacrificed to prevent the sub


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$ transformer-rectifier can be regarded as an electrical pump which is suc(ing the

electricity out of the pipeline 2etc3 and pumping it into the ground 2or sea ... orswamp... or wherever else you want to pump it3.

The effect of this is ama;ing. "t stops rust@ $nd it6s cheap@

=ut there are some snags.

=ecause it6s so good! it gets installed .... then ignored...... well most people don6teven (now it e#ists... and because it6s cheap some people don6t thin( it6s important.

=A

T'E (#)I% %"$%E!T "* !I!E+I$E %#T'",I%!R"TE%TI"$

$s stated before! everything has a 6potential6! which has an effect on it6s relationship

with it6s environment. Corrosion is effected by this relationship! as it is an electro-chemical reaction.

The basic concept of cathodic protection is that the electrical potential of thesub


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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 bymany scientists and has become established beyond dispute. /any boo(s and

papers have been published! giving details of the scientific bac(ground of corrosionand corrosion control! as a result of many years of research by respected and

sincere specialists. "t is not intended to dispute any of this wor( or the conclusionsdrawn.

(attery technology can be compared to corrosion control

Technology

The principles of corrosion reactions are used in the design and construction ofe#pendable and re-chargeable batteries and accumulators! which play such a ma


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Corrosion within a battery can be controlled by e#ternal electrical techni%ues whichare in common use. Some batteries have a reversible reaction which enables them

to be recharged by ad


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%#T'",I% !R"TE%TI"$ I) ,I**ERE$T

0nfortunately! cathodic protection is not a unit composed of simple elements in the

way that batteries are! because the electrolyte is the ground itself. This is anuncontrollable feature with an almost infinite variety of %ualities.

The picture above is an e%uivalent circuit diagram of the cathodic protection

systems that were preventing corrosion over an area of tens of thousands of s%uaremiles of pipelines serving a ma


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The chemical composition and electrical conductivity of the ground can span a vastrange and can include environments such as sea water! deserts! freshwater swamps!

arable 2fertilised3 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.


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Cathodic protection of such sub


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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 isgenerally accepted that no coating can be perfect! and the faults 2or 6>olidays63

introduce the first indefinable element to the system.

,uring the construction of a pipeline! all possible measures are ta(en to detect andrepair coating faults! so it follows that the location and si;e of those remaining are

un(nown and not definable. "t is possible to calculate the theoretical resistance of aperfectly 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 anactual pipeline.

The electrical current measurements! ta(en during routine cathodic protectionmonitoring! show that there is little resistance in the total coating of a pipeline andthis can be e#plained by the difficulty in %uality control during coating operations

and preventing damage during the construction period.

Perfect coating would prevent any output from the CP system but undetectedcoating faults provide paths for cathodic protection current. 7e! therefore! (now

that there are many unspecified metal-to-electrolyte interfaces present on anaverage 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 comple# currentpaths and variation in potentials! is found to be so small that it can almost beignored.

*-RT'ER %"M!+EIT

'ach coating fault is a metal-to-electrolyte interface which is capable of a different

reaction! electro-motive-force 2'/+3 which cannot be measured as it is in parallel

with all other '/+s on the same section of pipeline.

The magnitude of the current from each of these is dependent on the earth


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resistance immediately ad


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T>'4B P$CT"C'

7e simply want to stop corrosion but we need to (now when we have succeeded.

Cathodic protection is immensely successful! and cost effective! but every lea( is ademonstration that we have not applied it correctly.

5in( to page on Cathodic Protection /easurements

=efore going any further it is necessary to imagine electricity and this has beenli(ened to water pressure! with containers connected by pipes to allow current to

flow.

>owever! it can be seen that the levels would e%ualise as soon as enough water had

run from one container to the other. &o current would then flow.

"f water was added to the higher container at the same rate that it is passing through

the connecting tube! then the current will continue.
http://www.pipeline-corrosion-control.com/CorrespondenceCourse/Module01/measure.htmhttp://www.pipeline-corrosion-control.com/CorrespondenceCourse/Module01/measure.htm


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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 thatwater flows through the connecting tube at the bottom of the two containers.7hen the reaction energy has run out! the battery is dead and the potential levels

are the same at each pole.

$ corroding nail is similar in that the corrosion current flows from the anode of thenail! into the damp cloth and then goes bac( through the cloth to the cathode of the

nail.


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The corrosion reaction on the nail can be forced in a variety of ways to be defined

in this course.

efering bac( 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 waterwill 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 theimbalance 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 drycell battery connected through a light bulb! but in a cathodic protection circuit! or

when corrosion ta(es place on a pipeline we have no means of measuring each

separate cell in this way.

"f we e#amine the techni%ue that is used in the laboratory then it becomes clear that

provision has been made to eliminate outside influences in this 6open circuitmeasurement6.


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This is not possible in cathodic protection field wor(! 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 tothe same metal and yet each having it6s own corrosion reaction. This can be

imagined li(e this.


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"t can be seen that it is impossible to measure the pressure differences in each cellby ma(ing a single connection to the common reservoir at the bottom. >owever it

would be possible to stop the flow of water in each of the cells by continuouslyma(ing the water level e%ual in each pair of containers.

>owever! 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 wholesystem.


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This is achieved in cathodic protection by flooding all the containers as shown ingreen. The current then stops flowing between each pair. =ecause of the nature of

electricity this re%uires that current is drained from the pipeline and pumped intothe ground in sufficient %uantity to 6fill all the containers6 or overcome the corrosionreaction potential 2'/+3.

lin( to page showing water containers to demonstrate electrical potentials and inrelation to pipeline cathodic protection

The difficultyin ma(ing this voltage measurement is shown in the demonstrationwith water holders buried in sand.
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7e can measure the level of the water against the level of the sand.

7e 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.

7ater can pass between some of the visible containers to others in the same way ascorrosion current.


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7e can never (now if the corrosion current has been stopped when 2whole systemis in e%uilibrium3 as we have no reference to ;ero potential. "t is out of sight and

reach@

"n the same way! we cannot (now the '/+ 2water level3 of each corrosion cell. 7ecan only measure the voltage between the potential of the ground and the potential

of $55 4+ T>' /'T$5. That is the e%uivalent of the level of all of the water inthe containers. 7e do not (now how deep each containers is and we do not (now

which are connected.

The established method of measuring the effectiveness of cathodic protection is byrecording the voltage between two variables. This cannot determine if corrosion has

stopped.

"pen circuit measurements

The term 6open cuircuit measurements6 was ac(nowledged by ,r Peabody of &$C'when recognising the problem that was termed 6the " drop in the soil6

&atural corrosion cells are much different from those set up in a laboratory! as theycan be physically minute or large. 5arge corrosion cells can contain micro-cells

within the same area where anodic areas completely surround cathodes or vice-versa. 7hen studying such cells! we are not able to separate the component parts!


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and the measurements have come to be (nown as 6open circuit measurements6.

This type of measurement involves connections to the electrolyte as well as the

metal and this re%uires the use of an electrode. There is a danger that this willintroduce another '/+ into the circuit! by the reaction between the electrode and

the electrolyte. 7e therefore use an electrode in a solution of its own salts! whichhas a (nown reaction '/+. 7e can then ma(e a connection between the electrolyte

in the cell and the earth electrolyte! in the hopes that there will be no electricaldisturbance to the measuring circuit.

"n the laboratory! this disturbance is prevented by the use of a glass capillary filledwith inert gel! which is used as a conductor from the reaction interface to the

reference electrode. The reference electrode is a metal in a saturated solution of itsown salts! as this has a (nown reaction potential. eference electrodes are related to

each other by (nown voltages and are used as international standards.


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7ithout this consistency it would be impossible to evaluate the reaction! develop

theories or design cathodic protection systems etc.

0nfortunately! it became the practice to apply laboratory principles in cathodicprotection field wor(.

This sub


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=oth conductors would carry e%uilibrium current according to the reaction within

each battery and the voltage between the two conductors could be measured byconnecting a meter between the two. 0nless the two cells are separated! it is

impossible to evaluate the voltage of each battery. 'ven this is not as comple# asthe e#pectancies of cathodic protection monitors.

"f we ta(e two batteries and half bury them in an electrolyte with their positive

poles e#posed and connected! we have two corrosion cells in closer condition tothose found on a pipeline.

$ circuit drawing of this arrangement will show that current will pass through the

ground to e%ualise the pressures caused by the interface reactions within eachbattery.

7e must now try to evaluate the reaction within each battery using a high

resistance voltmeter and an electrode. 7e cannot brea( the circuit or separate thebatteries but connections can be made to the metal or the electrolyte or both. "t will


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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 laboratorytechni%ues cannot be applied to these conditions as there are too many variables

which are impossible to evaluate.

"f we increase the number of half buried batteries connected together! we improvethe 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 comple#ity of the situation is now apparent and what seemed to be a simplemeasurement! now seems almost impossible.

$ circuit diagram of the comple# arrangement will show that a different voltage

will be measured with every new position of the electrode! and this is born out incathodic protection field practice. "t is especially obvious on pipelines which are

not connected to cathodic protection systems and which have poor coating.

The different voltages are due to the variety of potentials at each pole of thevoltmeter. 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. "t ispossible 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 comple# when cathodic protection is introduced as thisis an additional voltage which is superimposed over all the others. =eing designed

to drain charges from the whole of the


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pipeline! it has an effect on the e%uilibrium of all the other electrical influences.

>owever! the dynamic effects of an impressed current system can be removed byta(ing voltage measurements immediately after the system has been switched off.


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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 electrodehave 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 partof the corrosion and cathodic protection circuits.


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Cours de formation sur la protection

cathodique

Introduction ce cours

Ce cours est pour tout le monde impliqu dans l'application de la protectioncathodique.

Protection cathodique a toujours t divis entre la science de l'lectro-chimie etde l'application de la technologie de protection cathodique dans le domaine.

Depuis 1980 les donnes de protection cathodique a t stoc! dans lesordinateurs mais la di"rence entre la thorie lectrochimique et la plupart despratiques sur le terrain de #ase n'a pas permis de parvenir $ une anal%se parordinateur.

Ce cours comprend des travau& pratiques qui est conu pour permettre $ l'l(vede comprendre la protection cathodique applique $ partir des principes de #ase.

)l est important que chaque l(ve comprend chaque module comme une #ase surlaquelle ils peuvent aller de l'avant $ l'autre.

* la +n du cours chaque tudiant sera tenu de prsenter un document pourpu#lication sur le site )nternet CP,. es mrites de chaque papier seront valuspar les mem#res du PC,.

)ngnieurs et scienti+ques de corrosion e&priments seront en mesure devri+er la validit de chaque tape et sont encourags $ e&primer leurs opinions.


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Ce cours a t en ligne pendant de nom#reuses annes maintenant et a eu desmilliers de visiteurs.

)l ne onctionne pas comme prvu car il n'% a pas eu d'tudiants interactis maiscette page a reu plus de /.000 visites au cours de cette derni(re anne.

De discussions sur une varit de groupes de rseau& sociau& et d'a"aires onpeut dire que ces pages sont utilises comme une sorte de livre de rrencemais il ne peut pas tre e2cace dans cette aon.

3e travaille maintenant mon chemin $ travers le cours pour qu'il soit clair qu'il estessentiel que les tudiants commencent d(s le d#ut et ne #ougent pas jusqu'$ce qu'ils comprennent #ien chaque module.

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3e l'ai reu que deu& rapports e&igs pour un certi+cat de comprhension et decapacit et donc il n'% a que trois d'entre nous de l'e&amen des autres rapports

qui lui sont soumis.

3e suis dipl5me d'autres dans la comprhension et la capacit de mener desenqutes C)P6 DC7 et comme je l'ai demand ces gens sur le terrain.

3e ai galement dlivr des certi+cats de comprhension et capacit d'tudiantsuniversitaires qui je l'ai encadrs $ des aspects spci+ques de contr5le de lacorrosion qui ncessitent des e&priences $ aire le sujet $ plusieurs repriseso#serva#les selon les e&igences de la vraie science.

oute personne dsireuse de dvelopper un logiciel de diagnostic pour laprotection cathodique et de contr5le de la corrosion a #esoin de comprendre leslois de la nature qui se appliquent $ notre science. a notation scienti+que nepeut pas toujours tre e&prime numriquement autant de lui est unraisonnement analogique qui est pas d+nitive et dterministe.


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e premier module de notre cours est dli#rment #ase pour e&pliquer les loisde la nature qui nous lient tous et pour aider les l(ves $ comprendre lesquations qui sont enseignes :perroquet aonna; et test par des e&amens decase $ cocher.

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6 )


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Guelle est la protection cathodiqueH

a protection cathodique est un mo%en lectrique de l'arrt de la corrosion.

@+n de comprendre la protection cathodique il est crucial qu'un ingnieur peutvisualiser Bpressions lectriques.

Ceci est une illustration t%pique d'une cellule de corrosion avec les I(chesindiquant la direction du courant.

Ce courant est entraFn par la Bpression :Jorce Klectro-


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a relation entre la tension le courant :mesur par amp(rem(tres; et larsistance :mesure en ohms; est d+nie par la loi d'?hm.

ors de la mesure des tensions de potentiel peut tre considre comme nulleau& +ns de l'a2chage graphique et calculs.

Ce potentiel peut alors tre compare $ un autre potentiel aide d'un voltm(tre desorte que la di"rence de potentiel peut tre e&prime en volts. e graphique ci-dessus montre que la di"rence entre les deu& potentiels $ chaque point de

mesure. )l n'% a aucune raison de supposer que les deu& tensions sont lies. Cegraphique est tout simplement un certain nom#re de tensions runies par uneligne reliant les points. Ce ait est traite en proondeur lors de ce cours %compris les e&priences pratiques. e voltm(tre d+nit le Nro pour chaque


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e point oE le courant pn(tre dans le mtal est connue comme la cathode.@ucune raction de corrosion est possi#le $ cet endroit que le potentiel del'lectrol%te est suprieure $ celle du mtal $ cette interace immdiate.

a raction peut se poursuivre jusqu'$ ce qu'un quili#re soit atteint entre lesproduits chimiques et l'nergie lectrique. es produits chimiques ont Brongtout le mtal et sont $ court de Bnourriture. Pas de courant est produite et donctoute la aute de revtement est Bau repos. Produit de corrosion se orme sur lemtal qui #loque le chemin du courant.

atteries onctionnent sur ce principe. orsque les #atteries atteignent l'quili#renous devons re- charge ou de les jeter.

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e pu#lic ne sont gnralement pas conscients que notre gaN et de ptrole vient$ nous $ travers les tu%au& qui sont enclins $ rouiller mais sont protgs parBprotection cathodique. Comme les ingnieurs de contr5le de la corrosion nous

devons constamment tre conscients de l'nergie lectrique autour de nous toutle temps que cela entraFne le processus de corrosion et a"ecte tout ce que nousaisons dans notre vie quotidienne.

es pipelines sont Bloin des %eu& et de l'espritB si peu d'attention est accorde au

ait que le mtal se dissout dans des solutions et dgage lectricit.


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)l est laiss au& ingnieurs de corrosion $ vous soucier de ces choses jusqu'$ unpipeline choue causant la perte de la vie les dommages environnementau& etdes consquences +nanci(res massives.

Consultants sont alors demand pourquoi le pipeline a chou et le d#at sur lecrit(re de la protection cathodique reoit l'attention pendant un petit moment.

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Des navires et des propritaires de #ateau& sont constamment au courant desdommages causs par la corrosion et par consquent #ateau& mtalliques sont

protgs par la protection cathodique. )ls ont des morceau& de mtal +&s $ lacoque $ cet e"et. Ces morceau& de mtal dissoudre dans l'eau et dgagentlectricit qui empche la corrosion de la coque.

6ir Qumphre% Dav% introduit ce s%st(me en +&ant Bonte #rute au& recouvert decuivre coques de navires.

)l %a une quantit considra#le d'inormations et la modlisation inormatiqueannoncs sur )nternet $ cet gard. Ane recherche va rvler un certain nom#rede socits spcialises o"rant des services et le PC, est pas en concurrence surce march.

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,ous sommes proccups par l'anal%se des donnes recueillies relatives $ laprotection cathodique de enterres et immerges enro#s des canalisationsd'acier qui transportent la plupart des ournitures mondes de l'nergie de lasource au consommateur.

Ceci est une tude tr(s spcialiss qui doit commencer $ l'interace entre lemtal et l'lectrol%te pipeline dans lequel il est enterr ou immerg.


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Cette ancienne photographie a t prise lors de la construction du rseau degaNoducs qui ont t enterrs dans le =o%aume-Ani depuis plus de M0 ans. Cetronon particulier a t revtu de goudron de houille mail et a t traite par

des machines de construction lourde. e revtement a t souvent endommaget rpar avant de rem#la%age.

)l est clair que les dauts de revtement ont t parois manqu.

e mtal de la conduite de ces dauts de revtement est en contact avec le sol:la Blectrol%te; qui o#tient Bcreuser avec l'lectricit. e potentiel lectriquede ce peu de terrain est augmente $ un lectrique plus leveB pression quele mtal sol environnant et ainsi de l'lectricitB ra%onne dans la terre.

e mtal qui est disolving est le Banode $ partir de laquelle le courant lectriquepasse dans l'lectrol%te.

'autre mtal est le Bcathode dans lequel le courant passe de la eletrol%teOrechargO parce que la pression lectrique doit tre compense. :tout tented'galiser son potentiel lectrique avec tout autour d'elle;.

e mtal disolving est sacri+ pour empcher l'o#jet mtal contre la corrosion etce ait est e&ploite en ournissant un mtal moins no#le dans le circuit de lacorrosion ... un s%st(me connu sous le nom de Bprotection cathodiquesacri+cielle.

)l %a des limites $ laquelle la protection cathodique saci+cal peut tre utilis maisle mme principe peut tre utilis en provoquant une pression lectriqueconstruit qui est Bimpressionn dans l'lectrol%te. 'lectricit est ensuiteBvid sur le sujet ....... mtal de la coque de #ateau ou pipeline .... et celainter(re avec la tendance naturelle du mtal $ dissoudre .... ou la rouilleR

es l(ves doivent tenter de ormer une image mentale de potentiel lectrique

:de pression; et le Iu& rsultant des Baccusations. ,e vous trompeN pas par le


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Iu& d'lectrons que nous ne pouvons pas voir cela sur nos m(tres. )l pourrait treimportant pour les universitaires mais il est indi"rent au& ingnieurs de terrain

Protection cathodique $ courant impos

Du courant alternati est gnr par une sorte d'action de pompage qui provoqueson coulement vers l'arri(re et vers l'avant en Bvagues mais cela ne sert $rien pour nos +ns et nous devons donc qu'il avance dans une direction $ traversun circuit connu comme un Bredresseur . Kn mme temps on peut contr5ler laquantit de courant en le transormant si l'appareil est connu comme untransormateur redresseur.

An transormateur-redresseur peut tre considr comme une pompe lectriquequi suce l'lectricit $ partir de la canalisation :etc; et de pompage dans le sol:ou la mer ... ou un marcage ... ou partout ailleurs oE vous vouleN pomper;.

'e"et de cette est incro%a#le. )l arrte la rouilleR Kt elle est pas chereR


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e concept de #ase est de la protection cathodique que le potentiel lectrique del'o#jet mtallique est rduite en dessous de son potentiel de corrosion et qu'il estalors incapa#le d'entrer dans la solution ou la corrosion. es raisons en sont

donns dans la thorie de la thermo-d%namique mais ceu&-ci ne seront pase&amins lors de cette tape.

a raction de corrosion et d'un mcanisme de protection cathodique a td+nie par de nom#reu& scienti+ques et est devenu ta#li au-del$ di"rend. Denom#reu& livres et articles ont t pu#lis donnant des dtails sur le conte&tescienti+que de la corrosion et de contr5le de la corrosion $ la suite denom#reuses annes de recherche par des spcialistes respects et sinc(res. )l nevise pas $ contester toute de ce travail ou les conclusions tires.

echnologie de la #atterie peut tre compar $ la technologie de contr5le de lacorrosion

es principes de ractions de corrosion sont utiliss dans la conception et laconstruction des piles et accumulateurs consomma#les et rechargea#les qui

jouent un r5le majeur dans la vie moderne. Ane #atterie est conu pourpermettre une raction chimique pour provoquer un courant lectrique de passer$ travers une trajectoire dsire donnant de l'nergie $ l'appareil. a #atterie aune tr(s soigneusement lectrol%te compos qui poss(de des qualits d'assurerune raction prvisi#le avec les autres composants de la #atterie.

a corrosion au sein d'une #atterie peut tre contr5le par des techniqueslectriques e&ternes qui sont d'usage courant. Certaines #atteries ont uneraction rversi#le qui leur permet d'tre recharges en ajustant la Bpressionlectrique au& #ornes. De nom#reu& appareils sont actuellement contr5ls parordinateur pour quili#rer l'quili#re de la raction en onction de leurs demandesde puissance.


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out cela est possi#le parce que la #atterie est une unit manuactur conudans le #ut de recevoir et de ournir du courant lectrique.

a protection cathodique est D)JJK=K,


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Kncore $ la comple&it

Chaque daut de revtement est une interace mtal-lectrol%te qui est capa#led'une raction di"rente Klectro-


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d'enveloppes de tu#es de orage oE une quantit considra#le de courant deprotection cathodique est utilis.

Ane autre crainte de 'protection-dessus' est celui de dcollement cathodique durevtement. Cela se produit lorsque les a#ricants de revtements spci+cationssont dpasses. Courant de protection cathodique passant sur le mtal provoquela li#ration d'h%drog(ne qui dcollements du revtement. Kn ralit cela estrarement un pro#l(me.

e courant ne passe sur le mtal $ un daut de revtement et la densit ducourant dpend de la taille du daut de revtement et le courant disponi#lelocalement. Comme le courant souTe le revtement du mtal la chute volts $

l'interace va diminuer et l'quili#re sera atteint avec une tr(s lg(reaugmentation de dcollement supplmentaire.

6i il n'% a pas aute de revtement alors aucun dcollement cathodique aura lieucomme l'a reconnu dans le Code ritish 6tandard de pratique pour tester laspci+cation des a#ricants de revtements. Cela ncessite une taille spci+quede la aute de revtement sur un coupon d'acier tre soumis $ une tensioncroissante sur une priode dtermine. e test ne peut tre e"ectu sur uncoupon avec un revtement parait que le dcollement est o#serv sous le

revtement sur le #ord de la aille.

Ces questions seront traites en dtail plus tard dans le cours

Q?=)K 7 P=@)GAK

,ous voulons tout simplement d'arrter la corrosion mais nous devons savoirquand nous avons russi. a protection cathodique est un immense succ(s etrenta#le mais chaque uite est une dmonstration que nous ne l'avons pasapplique correctement.

ien $ la page sur les mesures de protection cathodique


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@vant d'aller plus loin il est ncessaire d'imaginer l'lectricit ce qui a tcompar $ la pression de l'eau avec des rcipients relis par des tu%au& pourpermettre le passage du courant.

Cependant on peut constater que les niveau& ne galiser d(s que su2sammentd'eau a courir d'un rcipient $ l'autre. @ucun courant ne serait alors couler.

6i de l'eau a t ajoute au rcipient suprieur $ la mme vitesse que sonpassage dans le tu#e de raccordement le courant va se poursuivre.

Ceci est similaire $ une pile s(che qui est en ait une cellule de corrosion. ecourant circule $ travers un conducteur lorsque les deu& p5les sont connects dela mme mani(re que l'eau circule $ travers le tu#e de raccordement en #as desdeu& rcipients.

orsque l'nergie de raction est puise la #atterie est morte et les niveau&potentiels sont les mmes $ chaque p5le.

An clou corrosion est similaire en ce que le Iu& de courant de corrosion del'anode de l'ongle dans le tissu humide puis remonte $ travers le tissu $ la

cathode de l'ongle.

a raction de corrosion sur l'ongle peut tre orc dans une varit de aonsd'tre d+nis dans ce cours.

6e rrant de nouveau $ l'analogie de l'eau il est important de comprendre que

la pression est provoque par la hauteur de l'eau dans chaque conteneur et nonle poids. 'eau va remplir toute tu#ulure et alors la pression $ la #aisse sera plus


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grande dans le navire qui a le plus haut niveau. a raison de ceci est videmmentdU au dsquili#re entre les pressions dans les deu& rcipients et les potentielslectriques ont la mme tendance lorsqu'il est connect par des conducteurs.

Cela est #ien lors de la visualisation d'un circuit simple comme une pile decorrosion unique ou une pile s(che reli par une ampoule mais dans un circuit deprotection cathodique ou lorsque la corrosion a lieu sur un pipeline nous avonsaucun mo%en de mesurer chaque cellule spare dans Par ici.

6i nous e&aminons la technique qui est utilise dans le la#oratoire puis il devientclair que cette disposition a t aite pour liminer les inIuences e&trieuresdans cette Bmesure de circuit ouvert.

Cela ne veut pas possi#le dans cathodique travail sur le terrain de la protection

et les thories encore de la#oratoire drivs sont appliqus au& lectureso#tenues dans le domaine.

a situation des pipelines est qu'il e&iste de nom#reuses cellules de corrosiontous connects $ la mme mtal mais a%ant chacun sa propre raction $ lacorrosion. Ce peut tre imagin comme a.

?n voit qu'il est impossi#le de mesurer les di"rences de pression dans chaquecellule en aisant une seule conne&ion au rservoir commun $ la #ase. outeoisil serait possi#le d'arrter l'coulement de l'eau dans chacune des cellules dea#rication en continu le niveau d'eau gal dans chaque paire de conteneurs.


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Cependant on peut constater que la mesure de la pression dans un tel s%st(medevrait tre entre le niveau d'eau le plus #as et le plus haut niveau d'eau danstout le s%st(me.

Ceci est ralis dans la protection cathodique en inondant tous les conteneurscomme reprsent en vert. e courant arrte alors circulant entre chaque paire.Kn raison de la nature de l'lectricit cela e&ige que le courant est draine $partir de la canalisation et pomp dans le sol en quantit su2sante pour remplirtous les OrcipientsO ou potentiel de surmonter la raction de corrosion :K


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De la mme aon nous ne pouvons pas connaFtre l'K


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@u la#oratoire cette pertur#ation est empche par l'utilisation d'un capillaire enverre rempli de gel inerte qui est utilis en tant que conducteur de l'interace deraction $ l'lectrode de rrence. 'lectrode de rrence est un mtal dansune solution sature de ses sels comme cela a un potentiel de raction connue.es lectrodes de rrence sont relies les unes au& autres par des tensionsconnus et sont utiliss comme normes internationales.

6ans cette cohrence il serait impossi#le d'valuer la raction dvelopper desthories ou concevoir des s%st(mes de protection cathodique etc.


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6i nous prenons deu& #atteries et la moiti les enterrer dans un lectrol%te avecleurs p5les positis e&poss et connects nous avons deu& cellules de corrosiondans un tat proche de ceu& qu'on trouve sur un pipeline.

An dessin de circuit de cet arrangement sera montrer que le courant passe $travers le terrain pour galiser les pressions causes par les ractions d'interaceau sein de chaque #atterie.

,ous devons maintenant essa%er d'valuer la raction au sein de chaque #atterie$ l'aide d'un voltm(tre $ haute rsistance et une lectrode. ,ous ne pouvons pas#riser le circuit ou sparer les #atteries mais les conne&ions peuvent tre aitespour le mtal ou l'lectrol%te ou les deu&. ?n verra que nous sommes seulementcapa#le de mesurer des tensions $ travers diverses traves du circuit et ne peutpas ta#lir une rrence au sein de ce circuit. es techniques de la#oratoire nepeuvent pas tre appliqus $ ces conditions car il %a trop de varia#les qui sontimpossi#les $ valuer.

6i nous augmentons le nom#re de demi enterr #atteries connectes ensem#lenous amliorons la similitude $ un pipeline mais pour tre plus raliste nousdevons inclure certains qui ont leurs p5les positis enterrs. Ceci a t dmontrplus haut dans cette page.

a comple&it de la situation est maintenant apparent et ce qui sem#lait tre unemesure simple sem#le dsormais presque impossi#le.

An schma de circuit de l'agencement comple&e montrer que une tensiondi"rente sera mesure $ chaque nouvelle position de l'lectrode ce qui est ndans la pratique sur le terrain de la protection cathodique. )l est particuli(rementvident sur les pipelines qui ne sont pas connects $ des s%st(mes de protectioncathodique et qui ont un mauvais revtement.


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es tensions sont di"rentes en raison de la varit des potentiels au niveau dechaque p5le du voltm(tre. Ceu&-ci peuvent tre provoqus de plusieurs aonscomme dcrit plus loin mais il est important de raliser que ce sont tous descomposantes de la tension indique sur le compteur. )l est possi#le de lesliminer dans le la#oratoire mais pas dans le domaine par consquent ils

doivent tre valus et pris en compte dans l'anal%se des rsultats de l'enqute.

e pro#l(me est encore plus comple&e lorsque la protection cathodique estintroduit comme cela est une tension supplmentaire qui est superpos au-dessus de tous les autres. tant destin $ drainer des charges de l'ensem#le dela

pipeline il a un e"et sur l'quili#re de toutes les autres inIuences lectriques.outeois les e"ets d%namiques d'un s%st(me $ courant impos peut tre enleven prenant des mesures de tension immdiatement apr(s que le s%st(me ait tcoupe.

Cela ne peut tre ralis lorsque des anodes sont utiliss sau si elles ont unta#lissement spcialement conu $ cet e"et au stade de la construction.

es tensions o#tenues entre le mtal de pipeline et une lectrode place de aonalatoire ont une certaine quantit de valeur par rapport au& autres o#tenues $partir de conne&ions $ la mme canalisation. Ceci est dU $ la rsistance

lectrique tr(s ai#le dans cette partie de la corrosion et des circuits deprotection cathodique.

Documents

Cathodic Protection Training Course.doc