SSS Korrosionsschutztechnik

Corrosion and Corrosion Protectionof Underground Steel Pipelines

SSS KorrosionsschutztechnikGmbH & Co. KGMünchener Str. 69D-45145 Essen

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Preface

1 Definitions:Corrosion, corrosion manifestation,corrosion damage, and corrosionprotection

2 Basics of underground corrosion and itsconditions

2.1 Partial reactions in corrosion chemistry2.2 Influencing factors

and consecutive chemical reactions2.3 Interactions between anodic

and cathodic partial reactions -mutual dependence

2.4 Corrosion with uniform metal consumption2.5 Formation of electrochemical cells2.5.1 Anodic region; localized attack2.5.2 Cathodic region;

formation of covering layers2.5.3 Interaction between anodic

and cathodic regions2.5.4 Influence of anodic

and cathodic area ratio

3 Estimation of corrosion danger for pipelinesnot subject to electrical influence;Fundamentals of protection methods

3.1 Aggressiveness of the soil3.1.1 Uniform oxygen corrosion3.1.2 Cell formation in the soil3.1.3 Intensity of corrosion

in different types of soil3.2 Methods of corrosion protection3.2.1 Coatings: „passive“ corrosion protection3.2.1.1 Corrosion chemical interactions

on a coated steel surface3.2.1.2 Required protective properties

of coating materials3.2.2 Cathodic protection:

“active“ corrosion protection3.2.2.1 Potential dependence of corrosion3.2.2.2 Application of cathodic protection;

verification of protection potential3.2.3 Active protection + passive protection

= complete protection

4 Cathodic protection of pipelines4.1 Constructional requirements

put by cathodic protection4.2 Cathodic protection by using galvanic anodes4.3 Cathodic protection by impressed current4.4 Current feed test and adjustment4.5 Side effects of the protection current4.5.1 Interference with extraneous equipment4.5.2 Internal interference

within water pipelines

5 Corrosion danger caused by foreigncathodes; localized cathodic protection

5.1 External currents5.2 Corrosion caused by foreign cathodes5.2.1 Characteristic of a foreign cathode5.2.2 Examples of foreign cathodes5.2.3 Estimation of corrosion danger5.2.4 Coating influence5.3 Protection measures5.3.1 Galvanic separation5.3.2 Local cathodic protection,

hot spot protection6 Corrosion caused by stray currents;

protection methods6.1 Origins of stray currents6.2 Characteristics of stray current action6.3 Protection measures

against stray currents6.3.1 Immediate drainage6.3.2 Rectified drainage6.3.3 Forced drainage6.3.4 Use of controlling rectifiers

for forced drainage

Contents

7 High voltage interference, protectivemeasures and effect to cathodic protection

7.1 Types and causes of high voltage influence7.2 Protective measures against too

high voltages in personal contact7.2.1 Short time interference caused by short

circuit currents7.2.2 Permanent interference caused by operating

currents7.3 Grounding the pipeline

8 Conditions related to stress corrosioncracking; preventive measures

8.1 Critical agents causing SCC8.2 Danger caused by corrosion on the inside

surface of the pipes; protective measures8.3 Danger caused by corrosion on the outside

surface of the pipes; protective measures8.3.1 SCC caused by nitrates8.3.2 SCC caused by sodium hydroxide8.3.3 SCC caused by sodium bicarbonate8.3.4 SCC in hard spots8.3.5 SCC caused by cathodically produced

hydrogen

9 Behaviour of stainless steel in the ground9.1 When to use stainless steel9.2 Stainless steel parts acting as foreign

cathodes vs. unalloyed steel9.3 Overall corrosion properties

and influence of alien currents9.4 Conditions requiring protective measures9.4.1 Sensitization9.4.2 Concentrated chloride

10 Symbols

In this paper corrosion and corrosion protection of undergroundsteel pipelines are treated. Complementary to a number ofexisting technical regulations it describes causes of corrosionprocesses and possible protection measures based upon manyyears practical experience of our customers. It gives hints andexplanations to engineers in charge of planning and operatingsteel pipelines in order to help them realize and assess corrosiondangers that may arise, and provide for suitable protectionmeasures in time.

The possible corrosion processes being manifold and complex,and the service conditions of the pipelines not always beingthoroughly anticipated, completeness cannot be claimed, normay any guarantee be given for security against corrosiondamage. The planning, construction, and operation of activecorrosion protection facilities are the tasks of pertinent expertcontractors but are not the subject of this text.

As a coherent and compact treatise on corrosion-relatedproblems and based upon the general principles of corrosionchemistry, this paper is intended to contribute to a more commonunderstanding of these. This understanding is to promote theapplication of suitable corrosion protection, e.g. high gradepassive protection by polyethylene coating and application ofmodern electrochemical protection methods so that steelpipelines can be put to use with optimal profit and operatedsecurely.

The paper has been subdivided into many paragraphs similarto a handbook so as to facilitate the locating of particularproblems.

The definitions of terms, and the electrochemical principles ofunderground corrosion have been described in great detaildeliberately because for a comprehension of corrosion dangersarising in practice and of appropriate protective measures thisknowledge is necessary.

Preface

Steel pipelines are used for the transportation of gases,water, mineral oil, long-distance heating water andchemical products as well as for the hydraulictransportation of solid materials. In most cases the pipematerial is unalloyed or low alloy steel. There is practicallyno difference in the chemical properties of thesematerials. High alloy steels - so-called stainless steels- used for special applications only, have totally differentchemical properties varying considerably from type totype. The weldability of line pipe steels allows theconstruction of almost homo-geneous and mechanicallydurable pipeline systems.

In this presentation mainly the corrosion behaviour ofunalloyed and low alloy steels is covered. Minordifferences in the chemical compositions of these maybe ignored; so for simplification, in the following chaptersthis group of steels will just be called „steel“.

Corrosion is understood to be reactions of the materialin question with chemical constituents of its environment.The changes resulting from these reactions aremanifestations of corrosion. In the case of steel in waterand humid soil the corrosion manifestation is always thetransformation of iron into corrosion products, mostlysolid, called rust. The question, whether or not this resultsin damage, is answered exclusively by the extent of theprocess in relation to the required performance of theconstruction element.

In case the construction element does no longer performits task or if it may stop functioning within its projectedservice life, then there is damage. In the case of apipeline the word „damage“ means the pipe wall isperforated or does no longer fulfill the servicerequirements. Generally corrosion damage may betaken to have occurred, if the wall thickness falls undulyshort of the specified minimum.

In this way of differentiating the various corrosion relatedconcepts from one another, as put down in DIN 50 900,the fact is taken into account that all the metals incommon use generally corrode more or less rapidlywithout this causing damage in every case.

Essential for the assessment of corrosion resistanceis always the definition of the requirements to be fulfilledby the construction element in question, and, consequently,the maximum tolerable rate of corrosion. With steelpipelines this may be assumed to be a few 0.01 mmper year. In case, however, corrosion rates amount tosome 0.1 mm per year, this may result in later damage,depending on wall thickness, or have no consequencesif, for instance, corrosion rates drop in the course oftime due to the formation of surface layers. In projectingpipelines this consuming corrosion is usually allowedfor by means of extra, compensatory wall thickness.Very dangerous are conditions that lead to corrosionrates of some millimeters per year, because in mostsuch cases perforation results within a few years.

Corrosion protection is understood to be methods that,correctly applied, ensure corrosion to be mitigated to arate not exceeding the tolerable maximum. There isneither a technical necessity nor the possibility incommon practice to achieve zero corrosion rate. Thatthis is not possible by methods applicable in practicecan be proved; as opposed to this, technically negligiblecorrosion rates below 0.01 mm per year may well beattained.

The different methods of corrosion protection for under-ground pipelines work in different ways. Which of themshould be applied mainly depends on the relevantconditions that may stimulate or mitigate corrosion,these not always being obvious and generally difficultto be assessed. There are even borderline cases wherea protection method, unsuitably applied, acceleratescorrosion. This is why pipeline protection, as a rule, is ajob for an expert.

1 Definations:Corrosion, corrosion manifestation,corrosion damage, and corrosion protection

2.1 Partial reactions in corrosion chemistry

Steel is made up predominantly of iron atoms; theirchemical symbol is Fe. Like all other metals steel ischaracterized by an exceptionally high electricconductivity in the order of magnitude of 105 Ω-1 cm-1.Responsible for this is a high concentration of freeelectrons in the metal lattice. These electrons beingconsidered separate particles, for a realistic descriptionit is more appropriate not to use the symbol Fe fordenoting steel, but instead:

Fe <---> (Fe2+ + 2 e-) (1)

This manner of writing is used in chemistry to denotemesomeric conditions of substances. The meaning isthat steel behaves like being in a state between twolimiting ones which are signified by the opposite sidesof formula (1).This means steel can react in both these „modifications“:(Fe), or (Fe2+ + 2 e-). This fact is the reason for the electro-chemical nature of the corrosion of metals in electrolyticsolutions. Both components of the metal may react withthe environment independently of each other:

(Fe2+)steel

---> (Fe2+)electrolyte solution

(2a)

(e-)steel

---> (e-)electrolyte solution

(2b)

Electrons (e-) as such are not soluble in water, but they mayreact directly with oxidizing components of the elec-trolytic solution:

4e- + O2 + 2 H

2O ---> 4OH- (3)

2e- + 2 H2O ---> 2 OH- + H

2(4)

Eqs. (2), (3), and (4) may be described in this way: Eq.(2a) signifies a transfer of electrically positively chargedparticles from the material into the soil; the transfer isaccompanied by a loss of bulk material. This processis an anodic electrochemical reaction immediatelyresulting in a corrosive metal consumption. This typeof corrosion is called electrolytical corrosion, or better:anodic corrosion. According to Faraday’s law its velocityw is equivalent to an electric current I

A or, more

correctly, to the current density JA defined by the

relation of the current to the surface area of the material:

wmm/year

= 11.6 mA cm-2J

A = 1.16 A m-2J

A

Eq. (2b) denotes a transfer of electrons - i.e. negativecharges - from the material into the soil. The electrontransfer is not accompanied by a loss of bulk material.This process is a cathodic electrochemical reaction,and it is possible only if a chemical consecutive reactionlike those in eqs. (3) and (4) can occur. Obviously, forthe reaction in eq. (3) to take place the access of oxygen(O

2) to the steel surface within the soil is necessary.

Consequently corrosion rendered possible this way iscalled oxygen corrosion. The oxygen reacting withelectrons according to eq. (3) may be replaced by otheroxidizing agents. However, these are rarely present inthe soil except in the case of nitrates from fertilizers orhydrogen ions from humic and/or carbonic acid. In theabsence of oxygen, i.e. in anaerobic regions of the soil,the action of sulfate reducing bacteria may lead to thefollowing reaction:

8 e- + SO4

2- + 4 H2O ---> 8 OH- + S2- (6)

Characteristic of this reaction is the occurrence of sul-fides, e.g. iron sulfide FeS, recognized by the well knownodour of hydrogen sulfide upon addition of hydrochloricacid. Compared to the reduction of oxygen accordingto eq. (3) all these alternative reactions are of minorimportance.

The reaction according to eq. (4) may be totallyneglected in the case of normal soil corrosion. Thisreaction is very slow; the equivalent corrosion rate stayswell below 0.01 mm per year. But in case this reactionproceeds under electrical constraint it is of specialimportance as will be pointed out in section 2.5.2.

2 Basics of underground corrosionand its conditions

(5)

2.2 Influencing factors and consecutive chemical reactions

All electrochemical reactions in eqs. (2a, b) depend onchemical and electrical influencing factors in a typicalmanner.

• Electrolytic corrosion: Anodic reaction ace. to eq. (2a).Stimulation by: High dissolving power of the electrolyticsolution for Fe2+-ions (low pH, large content of dissolvedsalts, e.g. chlorides and sulfates); clean surface of thesteel (no deposits, reaction products, and/or coatings);shifting of the voltage between the steel and the soiltowards more positive values.

Mitigation by: Immediate precipitation of Fe(II)compounds constituting solid corrosion products on thesteel surface (high pH, small content of dissolved salts);covered steel surface (coatings or deposits of reactionproducts); shifting of the voltage between the steel andthe soil towards more negative values.

• Oxygen reduction: Cathodic reaction following eq. (2b).Stimulation by: Large oxygen content and easy accessto the steel surface (aerated soil); steel surface totallywithout a cover or with a coating of relatively low electricresistivity; shifting of the voltage between the steel andthe soil towards more negative values.

Mitigation by: Low oxygen content, and restricted accessto the steel surface (unaerated soil); coating with arelatively high electrical resistivity; shifting of the voltagebetween the steel and the soil towards more positivevalues.These influencing factors may be summarized asfollows:The voltage between the steel and the soil, commonlycalled the pipe-to-soil potential, determines theelectrochemical reactions in alternative ways. A shiftof the potential towards more positive values stimulatesanodic corrosion. A shift of the potential towards morenegative values stimulates the cathodic reaction.

Reaction products and coatings on the steel surfaceimpede the cathodic reaction much less than the anodicone. As shown by eqs. (3), (4), and (6), the cathodicreactions result in a formation of OH2 -ions, i.e. in anincrease of the pH on the steel surface. Thisphenomenon is sometimes called surface alkalinity.Surface alkalinity promotes the formation of solidreaction products on the steel surface, and thusimpedes the anodic reaction.

2.3 Interactions between anodicand cathodic partial reactions -mutual dependence

It is true that the electrochemical reactions accordingto eqs. (2a, b) may be discussed independently of eachother, but there are interactions that must be kept inmind:

• Mutual influence via chemical consecutive reactions(e.g. impediment of the anodic reaction by products ofcathodic reactions);

• Conservation of electrical charges or balance ofcurrents.

Corrosion being electrochemical in character, currentbalance is of paramount importance for its influencingfactors and its consequences.

To achieve a fundamental comprehension, it is helpfulto first consider a totally homogeneous steel surface inthe ground. In this connection the word „homogeneous“is to signify that the electrochemical reactions on thissurface proceed at uniform velocity independent of theparticular site on this surface. The reaction velocitiesare represented by currents, the equivalence given byeg. (5): Anodic partial current I

A, and cathodic partial

current IK. Both of these are dependent on the pipe-to-

soil potential, but with opposite tendencies. This potentialis understood to be the voltage between the steelpipeline and a reference electrode at the surface of theground. Commonly used as such is a copper/coppersulfate electrode (see figure 1).

Housing

Gasket with filling holeand plug

Copper

Ceramic-Diaphragm

Lid

Cable

Saturated coppersulfate solution

Figure 1 Copper/copper sulfate electrodeRefernce electrode for measuring pipe-to-soil potentials(Potential values obtained with the Cu/CuSO4 electrode may beconverted to the standard hydrogen scale by adding 0.32 V)

V

ACurrentsource

Capillary probe

Referenceelectrode

Electrolyte solution

The dependence of the partial reactions on the potential isschematically shown in fig. 2. Superposition of the twopartial current vs. potential curves results in a total currentvs. potential curve I(U):

IA (U) - IK (U) = I(U) (7)

Figure 2 Partial and total current potential curves valid for ahomogenoues steel-to-soil electrode (schematized)

Anodic partial current - potential curveCathodic potential current - potential curve

IA anodic partial current; IK Cathodic partial current

IA anodic partial current; IK Cathodic partial currentTotal current - potential curve

Figure 3Measuring circuitry for determining total current - potential curvesby means of a current source, a voltage (V) and a current (A) meter(Also suitable for the determination of corrosion rate - potential curves;in this case a potential controlling rectifier serves as the current source)

This total current - potential curve is the electrical charac-teristic of the homogeneous steel-to-soil electrode, i.e. itcan be established by direct measurement. The procedureis shown in fig. 3. In this special case the reference elec-trode works with a capillary probe, which is necessary fortapping the potential immediately in front of the steelsurface. If this probe is omitted, the measured potentialdifference contains a deviation corresponding to ohmicvoltage in the ground, and proportional to the total current.

2.4 Corrosion with uniform metalconsumption

If the steel surface is not electrically influenced fromwithout the total current equals zero. This is the situation offree corrosion; the pertinent potential is the rest potential.Following eq. (7) in the case of free corrosion we have

IA (UR) = IK (UR) (8)

In this case the corrosion rate is directly equivalent to thevelocity of the cathodic reaction, i.e. of oxygen reduction.In unaerated soils free corrosion is negligibly slow. A totalcurrent different from zero may either increase or decreasethe corrosion velocity, depending on its direction. Anegative total current signifies cathodic polarization.Corrosion protection achieved this way is called cathodicprotection. A positive total current establishes anodiccorrosion danger and may be due to various externalorigins. The corrosion rate which is possible in such asituation - in some cases it is very large - is almostindependent of the nature of the surrounding soil.

Risk of anodic corrosion

Cathodicprotection

Potential

Neg

ativ

e c

urre

nt Ik I

K

IA

Ia

UR

Pos

itive

cur

rent

2.5 Formation of electrochemical cells

In the case of a homogeneous electrode eq. (8) is validfor every point of the surface, so this relation also holdsgood for current densities:

JA (UR) = JK (UR) (9)

As a consequence of soil corrosion usually solidcorrosion products are formed, which differently affectdifferent electrochemical reactions. So, in the course ofcorrosion, a homogeneous electrode is transformed intoa heterogeneous electrode composed of different regionsin each of which one of the electrochemical reactionsprevails.Figure 4 schematically shows such a heterogeneouselectrode with a central anodic area and peripheralcathodic regions. The electrochemical reactions in theseareas combined with their chemical consecutivereactions are such that the heterogeneous situation ofthe electrode is stabilized. This will be discussed in thefollowing sections.

Figure 4 Corrosion cell with anode and cathode onheterogeneous steel surface in the soil

Cell circuit (arrows only denoting migration of particle inquestion)Surface layer on the cathode (formation favoured by surfacealkalinity NaOH); due to HCl formed by hydrolysis, the anoderemains free of surface layer within the rust nodule

Fe2+ + 2 Cl- + H2OFe (OH) Cl + HCl

2e- Fe2+

Cathode Cathode

Rust nodule

2 Na

+

2 Cl -

Migrationsof ions

O2 - diffusion

Fe (OH)+O2 FeOOH

Anode

½O2+

H2O+

2e-

Na+

OH-2

Soil

FeSteel

2.5.1 Anodic region; localized attack

At the anodic site the anodic reaction prevails. Thismeans the total current density is positive:

Ja = J

A - J

K > 0 (Anode) (10)

Such a region may develop due to the fact that here thesteel surface is loosely covered or otherwise lessaerated. A positive total current corresponds to ananodic cell current which, according to eq. (2a),causesanodic corrosion. In the ground it is propagated in theform of an ionic current, partly represented by cationsmoving into the soil, partly by anions moving towardsthe steel surface. This „anion migration“ enables thepositively charged corrosion products (Fe2+) to beelectrically neutralized in the soil. Together with aconsecutive hydrolysis, this may be described asfollows:

Fe2+ + 2 Cl- + H2O ---> (Fe(OH)+ + Cl-) + (H+ + Cl-) (11)

On the left hand side of the equation we have a correctbalance of currents (corrosion current minus migrationcurrent), on the right hand side we see two electricallyneutralized ion pairs dissolved in the water. In this thehydrochloric acid (H+Cl-) lowers the pH and thus keepsthe steel surface clear of deposits of corrosion products.At some distance from the steel surface the corrosionproduct may be further oxidized by oxygen to forminsoluble rust:

4 Fe(OH)Cl + O2 + 2 H

2O ---> 4 FeOOH + 4 Hl (12)

This process causes the well known rust nodules todevelop above the anodic spots of the steel surface.They cover the anode and, thus, stabilize it purelymechanically.

2.5.3 Interaction between anodic andcathodic regions

In the formation of electrochemical cells it is importantthat there is an increase in pH at the cathode (surfacealkalinity, eq. (14)).This decreases the solubility ofcorrosion products, as can be seen immediately fromthe solubility product:

c(Fe2+) x c2(OH-) = 5 x 10-16 mole3 1-3 (15)

Because of hydrolysis the pH at the anode is definitelylower than 6, so that c(OH-) < 10-8 mole/1. This allowsc(Fe2+) > 5 mole/1 = 280 g/1. At the cathode pH is defi-nitely above 8, corresponding to c(OH-) > 10-6 mole/1.The result is c(Fe2+) < 5 x 10-4 mole/1. In most casespH is around 9. The possible iron concentration is thenc(Fe2+) = 0.3 mg/1. This explains that there can be nosolid corrosion products at the anode within a corrosionnodule, whereas the cathodic regions must be closelycovered. Within the latter the anodic reaction is moreimpeded than the cathodic one.

This consideration shows how the heterogeneous state ofthe corrosion cell is stabilized by consecutive chemicalreactions in both the anodic and cathodic regions. Saltsdissolved in the moisture of the soil participate in theseconsecutive reactions, sodium and chloride ions beingparticularly effective. Without these salts neither anodesnor cathodes can stabilize themselves. Anotherconsequence of the absence of salts is the limitation ofelectrolytic conductivity; this means that for merelyelectrical reasons no effective cells can develop. Theexistence of electrical space charges not being possible,the production of Fe2+ and OH- ions according to eqs.(2a), (2b) and (3), respectively, must take place withinthe immediate neighbourhood of each other. This, however,does not lead to localized corrosion attack below rustnodules but to the formation of homogeneous rust layers:

Fe2+ + 2 OH ---> Fe(OH)2 ---> FeOOH (16)

The presence of dissolved salts being essential forlocalized corrosion to proceed the symbols of sodium andchloride ions were incorporated in fig. 4. Their omissionfor the sake of simplification would be incorrect. If theywere omitted, a wrong concept of the action of saltfreewater would be signalized, and the essential effect of theions would be missed. To simplify the scheme in fig. 4, JKwas taken to be zero in the anodic region, the same holdsgood for JA in the cathodic one. This simplification isindeed acceptable, because it makes the picture moredistinct without changing its meaning.

O2

2.5.2 Cathodlc region; formation of coveringlayers

In the cathodlc region the cathodic reaction prevails.So the total current density is negative:

Jk = J

A - J

k < 0 (cathode). (13)

Such a region may develop, e.g., owing to the steelsurface being amply aerated or covered by coatings orreaction products with only minor electrical resistance.A negative total current corresponds to a cathodic cellcurrent, enabling cathodic reactions to take placeaccording to eq. (2b). Within the soil it is propagated inthe form of an ionic current, partly represented by OH- -anions moving off, partly by cations moving towardsthe steel surface. The latter process again is a migrationcurrent that covers the electrical neutralization of thecathodic reaction products according to eqs. (3), (4),and (6), namely the OH- -ions, in thesoil.

Immigration of alkaline earth ions into the cathodic regionleads to the formation of solid cover-layers, immigrationof alkali ions to the formation of caustic alkali:

2 OH- + Ca2+ ---> Ca(OH)2 ------> CaCO

3, (14a)

OH- + Na+ = NaOH (14b)

Eqs. (14a, b) should be understood the same way aseq. (11), showing the current balance on the left handside (cathodic reduction current minus migration current),and the electrically neutralized ion pairs (metalhydroxides) on the right hand side. In addition to this, ineq. (14a) the formation of solid calcium carbonate withcarbonic acid from the soil is indicated. These reactionsare substantially promoted by cathodic protectionbecause the negative total currents enforced by it speedup the reactions according to eqs. (3) and (4) on allbare parts of the steel surface.

CO2

2.5.4 Influence of anodic and cathodic area ratio

In the case of a corrosion cell, i.e. any heterogeneoussteel surface, neither are the current densities of theelectrochemical reactions J

A and J

K locally uniform, nor

does the total current density equal zero with freecorrosion. It is true that taken over the complete steelsurface the integral of current densities equals zero.Then, considering a simplified corrosion cell with oneanode and one cathode like in fig. 4 we find:

Ja x S

a + J

k x S

k = 0 (17)

Sa and S

k stand for the areas of the anodic and cathodic

regions of the steel surface respectively. The total currentdensity J

a of the anodic region increases proportionally

to the ratio Sk/S

a, provided the total cathodic current

density remains constant. This may be assumed to bethe case with corrosion in the soil, JA in eq. (13) beingneglected, and J

k being determined only by a uniform

access of oxygen. Then, what follows from eqs. (13)and (17) is the law of areas:

Ja = J

K x (18)

Ja, and J

k stand for the total current densities

corresponding to eqs. (10), and (13), respectively. Anexterior current may not only influence J

a and J

k but

also via consecutive chemical reactions alter the areasS

a and S

k, provided its duration is sufficient. Sufficiently

strong positive or negative currents totally convert aheterogeneous steel surface into one anode or onecathode respectively. Following eq. (19) a negativecurrent J

k x S

k may make J

a = 0. Then, corresponding to

eq. (10), the anodic region is no longer threatened bycell formation but only by the cathodic reaction proceedingwithin this very region at the velocity J

K. = J

A. Even this

kind of a cathodic protective action may be very helpfulin preventing early failure.

This consideration concerning the interaction of chemicaland electrical parameters is fundamental for a com-prehension of corrosion processes. It enables a semi-quantitative description of steel corrosion in the soil tobe given with reference to the conditions set in practiceby the type of soil and the construction or type ofinstallation. It also gives an insight into how to applyprotective measures.

Sk

Sa

The danger of anodic corrosion caused by this cellformation is obvious from fig. 2, with a positive totalcurrent I

a = J

a x S

a. Only in case I

K in the anodic region

may be neglected compared to IA, the total current

density Ja may be taken to equal the corrosion current

density JA. If exterior electric currents are active,

corresponding to eq. (7), eq. (17) may be extended to:

I = Ja x S

a + J

k x S

k(19)

3 Estimation of corrosion danger for pipelinesnot subject to electrical influence;fundamentals of protection methods

3.1 Aggressiveness of the soil

3.1.1 Uniform oxygen corrosionAs indicated by eq. (9), on a homogeneous steel surfacethe corrosion velocity corresponds to the cathodic currentdensity JK. On a heterogeneous steel surface the localcorrosion rate within the anodic region is higher by afactor Sk(/Sa - the ratio of cathodic and anodic areas -as is indicated by eq. (18). So, in any case for assessingthe corrosion rate, it is essential to know the cathodiccurrent density JK, or, correspondingly, the rate of oxygenaccess. An approximation may be derived from the lawof diffusion. By combining eqs. (5) and (9), we get arelation between corrosion rate and oxygen diffusion:

In the case of air saturation c (O2) may be taken equal to10 mg/l.The quantity lD denotes the effective path of diffu-sion. In the case of a rapidly flowing underground water,which cannot form a covering layer on the steel surface,lD = 0.2 mm may be assumed. Air saturation given, thiscorresponds to a corrosion velocity of 0.8 mm/year. Insoil being at rest, lD is bigger by a factor of 10, at least,corresponding to a corrosion velocity of 0.08 mm/year.Covering layers of corrosion products further increase lD,so corrosion slows down with time. Except in cases offlowing ground water the corrosion velocity on ahomogeneous steel surface in the soil equals a few 0.01mm/year. So, as a rule, it may be neglected.

3.1.2 Cell formation In the soil

But other considerations (in section 2.5) showed also thatdissolved salts which are present in different concen-trations in all types of soil, cause any homogeneous steelsurface to become a heterogeneous one in the course oftime. The velocity of this process grows with the saltcontent, i.e. the electrolytic conductivity of the soil, and,above all, with the extension of the steel surface. Especiallyextended, however, are pipelines; moreover they may tra-verse areas with different types of soil. Then anodes andcathodes develop in a typical manner, as Is described insection 2.2, with respect to influencing factors:

ωmm/year

c(O2)

mg/l

lD

mm = 1.6 x 10-2 : (20)

anodic region: rich in salts, aerated little or not at all,dense, and wet. Example: Clay; especially aggressiveare sour soils (humus) and soils containing hydrogensulfide.

•

Experiments made with steel sheets in different typesof soil verified that corrosion is never totally uniform.Fig. 5 shows samples that freely corroded for 6 years,one in sand and one in clay. The mean rates of metalconsumption were 0.01 mm/year in sand and 0.03 mm/year in clay. Local maxima were ten times that. Extensi-ve investigations made by the National Bureau of Stan-dards tor the U.S. Department of Commerce showedrates of metal con-sumption ranging from 0.01 to 0.09mm/year, depending on the aggressiveness of the soil;in weakly aggressive soils there was a distinct drop inconsumption rates after a few years.

cathodic region: well aerated, moderate moisture andsalt content, light and porous soil. Example: Soilscontaining lime; sandy soils are little effective in thecase of low conductivity, but they too favour thedevelopment of cathodic regions.

•

Sand (ρ = 4 x 106 Ω cm) Clay (ρ = 7 x 106 Ω cm)

Figure 5 Surface of steel samples (100 x 150 x 3 mm) after 6years` corrosion in sand and clay

Humous soil

ClaySandLoamLoam Clay

Peaty soil

Cla

y

Sand

Sand

Sand

ym

arl Sandy

marl

Loess

Clayeymarl

Clayey

marl

Loamy marl

Lime

Calcareousmarl

Calcareous

marl

Calca

reou

sm

arl

Humous soil

Hum

ous

soil

LimeLim

e

Lime 100%Humus 100% Humus 100%

Peaty soilPeaty soil

Sand 100% Clay 100%

Humus 100%

Not aggressive(cathodic regions

Conditionallyaggressive

Aggressive

(anodicregions)

3.1.3 Intensity of corrosion in different types of soil

Fig. 6 gives a systematic survey of the different types ofsoil. The rates of metal consumption in aggressive soilsmay range from 0.05 to 0.10 mm/year. JK is quite large;the reaction of humus acids contributes to this. In soilsconditionally aggressive corrosion velocities are frequentlyfound to be negligibly small. In both these types of soil,though, anodic regions develop on traversing pipelines.Cathodic regions develop only in soils which are not ag-gressive.

The development of extended corrosion cells on a pipelinepassing through various types of soil does not allow anestimation of corrosion velocity. As opposed to this thevery positions of the anodic regions may be located quitewell by referring to the groups in fig. 6. Corrosion velocitywill be all the greater, the smaller the anodic and the greaterthe cathodic regions are, compare eq. (18). This holdstrue provided the electric conductivity of the ground issufficient; if it is not, for instance in dry sandy soils, evenlarge cathodic regions remain inactive with respect tocorrosion cell action. As opposed to this, intensively activecathodic regions afford fast corrosion of the anodicregions, then the properties of the soil near the anodeare less influential. So, evidently, there is no absolutemeasure of corrosiveness of a soil. A rating ofaggressiveness in terms of grades may be gained basedon a soil analysis.

Figure 6 Type system of soils and their corrosion aggressiveness

Approximate assumptions of corrosion velocities withinthe anodic regions of pipelines may range over a few0.1 mm per year. Local metal consumption rates above1 mm/year are definitely due to other reasons to bedetailed in sections 5 and 6. Corrosion danger imposedby soil containing coal (or coke!) belongs to this group.The reason is cell formation between the steel surfaceand the coke, the latter constituting the cathode, theformer being the anode. In this case there is but onesuitable protective measure: replacement of the soil alongthe pipeline.

The presence of coke and foreign electric influencesexcluded, unprotected steel pipelines can definitely beused without the danger of early failure, i.e. for severalyears. This is in accordance with experience. At somelater time though, depending on the wall thickness,damage must be expected. This means corrosionprotection measures are indispensable.

3.2 Methods of corrosion protection

3.2.1 Coatings: „passive“ corrosion protection

The most simple measure of corrosion protection iscoating the steel surface with bitumen or plastics. To besure there is a lot of literature and specifications con-cerning the required properties of the coating materials,but the purpose of each of these requirements withrelation to corrosion protection is not always obvious.

Essential are these two properties:

The coating material must be stable for a long timeunder the relevant service conditions in the ground.

The applied coating must be mechanically as resistantas possible to minimize frequency and extent ofmechanical damage.

•

•

PE-coating, fully meets these requirements. In cases ofvery rough service conditions, e.g. laying of the pipes inrocky soil with pointed stones and in soils containing slag,additional rock protection should be given.

3.2.1.1 Corrosion chemical interactions on a coated steel surface

In spite of top quality coatings, careful insulation of theweld joints, repair of gross mechanical coating damage,and in spite of scrupulous supervision of the layingprocedures, the occurrence of damage exposing thesteel surface to the soil cannot be prevented. Withinthese coating defects the electrochemical processes ofcorrosion take place as described in fig. 7. They mustbe considered in connection with the properties of thecoating material.

• Corrosion of the steel surface within a coating damageThe reactions taking place are those according to eqs.(2a, b and 3), the anodic reaction predominantly in themiddle, the cathodic one - producing NaOH -predominantly at the rim of the holiday. The alkali solutiongenerated there may creep some way under the coatingand thus loosen it. This effect may be observed in watersrich in dissolved salts. It is harmless, though, providedthe mechanical properties of the coating ensure thecoating’s unchangingly close fit to the pipe surface, thuspreventing an open cleft from developing.

• Oxygen and Ion permeation through the coatingDue to water absorption, organic coating materials maybecome weak electric conductors. Then they are notinsulators like, for instance, porcelain. Apart from that,organic coating materials are not gastight like, forinstance, a metal. Accordingly, permeation of alkali ions(Na+) and oxygen (O

2) is possible. This results in the

coated surface behaving like a weakly effective cathode.The accompanying anode is the steel surface within theholiday. Thus the formation of cells accelerates the corro-

sion taking place below coating holidays. This corrosiondanger is directly proportional to the electrical con-ductivity of the coating (to be measured in holiday-freeareas!). Another result is the formation of NaOH on thecathodic steel surface below the coating. This maydecrease the coating’s adhesion and, with thin coatings,even lead to blister formation. There are no such celleffects with coating resistivities large enough, say109 Ω m2. As a consequence, one of the requirementsto be put to coating materials is the display of very highresistivity, as previously said, this being measured inholiday-free coatings. The resistivity of PE-coating evenin long time service remains far above this value.

• Permeation of corrosive substances through and rust formation under the coatingBesides oxygen also water and carbon dioxide may per-meate the intact coating, and, below this, give rise tocorrosion of the steel surface. Generally, though, thediffusion velocity is so low as to result only in a minuterate of metal consumption. So, in the beginning, there isno corrosion danger. But as soon as corrosion productsloosen and fracture the coating, i.e. force it open, theprotection is lost. Such processes are possible, forinstance, with thin coating layers used for the protectionof structures in open air. Rust formation below thesecoatings is prevented for several years by usingprotecting pigments in a special primer. Long time orpermanent protection though, as to be specified forunderground pipelines, is not attainable this way. Withthick plastic coatings sitting snugly on the pipe surface- sticking to it or not - such processes are of no concern.Velocities of rust formation below such coatings areequivalent to metal consumption rates less than 0.001mm/year.

Figure 7 Corrosion chemical interactions in connection with coatingsUndercutting by alkaline moisture; cell formation including coated cathode surface; penetration and rust formation underneath the coating

3.2.1.2 Required protective properties of coating materials

Following these lines the requirements to be fulfilled bya good pipeline coating may be summarized as follows:

Passive corrosion protection given to pipelines by suchcoatings is sufficient, provided there are no externalelectrical influences, and the soil is „not aggressive“according to fig. 6, because - within the few spots ofcoating damage - the steel surface corrodes almosthomogeneously, and long line corrosion cells cannotdevelop. This statement is fully corroborated by practicalexperience provided they are embedded in sand andelectrically disconnected from other parts of constructionby insulating joints.

Because of the good electric conductance of welds, withlong pipelines in arbitrary soils cell formation betweendifferent bare parts of the steel surface in coatingholidays may occur, even if these parts are separatedby considerable distances. With a pipeline cover havinga low coating resistivity - measured within an un-damaged area of the coating, as already emphasized -cathodic action of the coated area according to fig. 7,cannot be excluded either. It has to be kept in mind thatthe large surface of a pipeline even with a coatingresistivity of 105 Ω m2 brings about an electricalresistance in the cell current circuit which is quite poor.Anodic corrosion velocity depends on the extent ofdamage in the different soils and on the types of soil. Itmay only be assessed very roughly. Corrosion velocitiesof some 0.1 mm/year are possible. For this reason forall pipelines subject to special safety specifications anadditional active protection, i.e. cathodic protection, isprescribed. In general this kind of protection isrecommended for merely economic reasons, in spite ofthe fact that the probability of damage is much smallerwith passively protected pipelines, compared to thesituation with uncoated lines.

•

•

•

Long time stability of the coating material underservice conditions,

Good durability against mechanical influences and

High electric coating resistivity.

3.2.2 Cathodic protection: „active“ corrosion protection

3.2.2.1 Potential dependence of corrosion

The basis of cathodic protection - „CP“ - was alreadygiven in fig. 2. By means of a cathodic current I

k, the

potential has to be shifted to more negative values, andthat as far as to make the anodic partial current /

Anegligibly small. So a quantitative assessmentnecessitates the knowledge of the anodic partial currentdensity - potential characteristic J

A vs. U. Such relations

may also be obtained by means of a measuring apparatusaccording to fig. 3, provided there is a controlling currentsource maintaining a constant potential for the durationof the test. By measuring the weight loss of the sample,the corrosion rate - potential characteristic ω (U), and,according to eq. (5), the partial current - potentialcharacteristic J

A vs. U, too, may be determined. Fig. 8

shows such results for saline electrolyte solutions. Thereit is obvious that with a potential U

Cu/CuSO4 = -0.85 V in

neutral waters the corrosion rate is less than 0.01 mm/year. This is the cathodic protection potential, well knownfrom practice. In waters containing much carbonic acid,that may flow from mineral springs but are not presentin normal soil, the protection potential is slightly morenegative.

10

3

1

0.3

0.1

0.03

0.01

0.003

-0.90 -0.80 -0.70 -0.60-0.85 -0.75 -0.65

Potential (UCu/CuSO4, V)

Cor

rosi

on r

ate

(mm

/yea

r)

Figure 8 Corrosion rate/potential curve valid for steel in salineelectrolyte solutions

Saturated with oxygenFree of oxygenFree of oxygen, saturated with carbon dioxideRegion of free corrosion (at more negative potentials the current is negative,at more positive potentials it is positive)

Pip

e-to

-soi

l pot

entia

l (U

CU

/SO

4, V

) -1.2

-1.0

-0.8

-0.6

-0.40 20 40 60

Time, starting with current switch-off, h

On-potential

Off-potentialOld pipeline(10 years`cathodicprotection)

Old pipeline(3 years`cathodicprotection)

New pipeline(2 years`cathodicprotection)

Figure 9 Registration of the pipe-to-soil potential after switch-offprotection current(v = Recorder paper travel speed)

Ohmicvoltage drop

IR

“off“

“on“

v = 1 cm/min

v = 1 cm/s

v = 50 cm/sv = 250 cm/s

0 -0.2 -0.4 -0.6 -0.8 -1.0 -1.2 -1.4 V

Potential (UCU/SO4, V)

Pap

er t

rave

l, tim

e (v

)

Figure 10 Decay of pipe-to-soil potentials of different pipelinesafter switch-off protection current

3.2.2.2 Application of cathodlc protection; verification of protection potential

The application of CP in practice is achieved the sameway principally as represented by the measurementarrangement in fig. 3. The proportions, however, are totallydifferent.Generally it is not possible to use capillary probes ofreference electrodes within the ground. The referenceelectrode according to fig. 1, is put onto the surface ofthe ground. The potential determined this way with thecurrent flowing is called on-potential. By the Ohmic po-tential drop (IR) within the ground it is more negativethan the true potential, i.e. the potential free from IR,that would be measured with a capillary probe. In casethe protection current is switched off for a short duration,the potential drop within the ground caused by thiscurrent collapses immediately. The change of potentialdue to chemical changes proceeds much slower, so theoff-potential measured immediately after switching offthe protection current may be taken as a very goodapproximation to the IR-free potential. There might stillbe errors due to Ohmic drops along stray currents fromextraneous direct current systems or due to relaxationor cell currents between differently polarized regions ofthe pipeline including electrically connected foreigninstallations, see sections 5, and 6.

Fig. 9 shows potential - time graphs obtained with diffe-rent paper speeds. A registration effected too slowlymay cause the off-potential to be found more positivethan the true value. In general the potential change afterthe switching off essentially depends on the object beingprotected. As is shown in fig. 10, a high quality coatingof a new pipeline as well as a long duration of CP willslow down the decay of the potential.

PE/PE 30PE/PE 40

PE/PE 270PE/PE 350PE/B 150PE/B 600

B/B 300

B/B 450

1970 1975 1980

3 x 105

3 x 104

105

104r u

(O

hm

m2 )

Coating of pipe/coating of weld joints km

Figure 11 Coating resistivities of differently coated pipelines vs.time in serviceB = Bitumen; PE = Polyethylene

Js

µA m-2x

ru

Ωm2= 3 x 105 (21)

3.2.3 Active protection + passive protection= complete protection

Coating and cathodic protection, when combined, makeup a complete protection of a pipeline. An additional profitafforded by the method is the possibility of continuoussupervision by monitoring the potential. In connectionwith this combined protection method the interactionbetween a coating and CP is sometimes also considered.This interaction may give rise to further requirementsput to the coating. Cathodic currents accelerate theprocesses of blistering and of cathodic disbonding shownin fig. 7. Especially with a thin coating, a gaping cleftbetween the steel surface and the loosened coating maydevelop. An impairment of protection is the consequence,much less critical though with underground pipelinesthan, e.g., with ships and hydraulic structures. Thickpipeline coatings are not subject to cathodic blistering,comp. sec. 3.2.1.1. As opposed to this, cathodicdisbonding starting from coating damage is inevitable.Because of the close fit of thick coatings, the lack ofadhesion within the immediate neighbourhood of a spotof damage does not result in a gaping cleft. This meansthe exposed part of the steel surface does not growthrough cathodic disbonding. The protection currentdemand does not increase. Fig. 11 shows averagecoating resistivities of underground pipelines coated withbitumen and with PE. The slow decrease of r

u with the

bitumen coatings is due to the formation of very smallpores in the bitumen during service, slightly enhancingits electric conductivity. Lines of PE coated pipes thejoints of which were field coated with bitumen show,less distinctly, the same effect, fig. 11.

The effective coating resistivity of a pipeline is deter-mined exclusively by number and sizes of holidays,inevitable in practice. So essentially the mechanicalproperties of a coating are of influence in this respect,not so the electrical properties of the coating material.The coating resistivity and the protection current densityof a buried pipeline are related as follows:

The application of CP to pipelines is amply described inspecific literature

• Handbook of Cathodic Protection• Pocket Book of Cathodic Protection

In this section only the main features of this method ofcorrosion protection can be discussed which are ofinterest to those running pipelines.

4.1 Constructional requirements put bycathodic protection

For CP to be applicable to a pipeline, some conditionsmust be fulfilled. The requirements are:

• Continuous lengthwise electrical conductivity of the pipeline and its branches to be included into CP

This means, pipelines with flange and socket jointshaving poor lengthwise conductivity cannot becathodically protected without each joint being electricallybridged; this is quite costly.

In other words: The ends of the pipeline and of itsbranches included into CP must be electrically separatedfrom continuing lines by means of insulating flanges. Inspecial cases, e.g. compulsory grounding for super-ordinate reasons (see sec. 7), this requirement may begiven up. In case constructional reasons prohibit elec-trical disconnection, a special kind of CP, to be describedin sec. 5, may be considered.

• A coating resistivity exceeding 104 Ω m2

This refers to the apparent coating resistivity, describedin sec. 3.2.3, and determined by size and number ofholidays in the coating. According to eq. (21), theprotection current density is inversely proportional tothe coating resistivity. With PE coated pipelines,protection current densities of 1 µA m-2 may be achieved,see fig. 11. The smallness of protection current densityis advantageous as it

extends the range of protection and improves thedistribution of current,

anddecreases the overall current demand and thusreduces any interference with extraneousinstallations.

Electrical separation of the pipeline from all units withlow resistance groundings, and from extraneousinstallations

•

4 Cathodic protection of pipelines

In cases of too great protection current densities onlyenlarged numbers of anodes - quite expensive - help toachieve good current distributions and to avoid detri-mental influences on foreign installations. With pipelinescovered with PE coating, protection ranges far exceeding50 km are easily achieved, and interference withextraneous equipment is virtually excluded.

The ground containing the anode should have the lowestpossible resistivity so as to decrease the requiredrectifier voltage. Where public power supply is notavailable, the application of other sources of energy (e.g.solar, wind, gas) for the generation of current may beconsidered. Usually in these rare cases galvanic anodesrather than rectifiers will be the protection currentsources.

4.2 Cathodic protection by using galvanic anodes

Galvanic anodes are applied for CP in those cases inwhich protection by current drawn from an external po-wer supply is not possible or less economic. Galvanicanodes consist of light metal alloys. They are buried ina special bedding rich in electrolyte, called backfill, andtheir potential is substantially more negative than thatof steel. This potential, negative compared to that of thepipeline, generates the protection current or, in otherwords, shifts the pipe-to-soil potential to the desired,more negative value. Because with galvanic anodesapplied there is no way of controlling the current - exceptfor decreasing it by Insertion of a resistor - the desiredcurrent distribution may only be achieved by a suitabledistribution and sizing of the anodes. This layout mustbe given special care. For underground protection magne-sium anodes may be used, in soils with very low resis-tivities also zinc or aluminium anodes.

The maximum possible current delivered by a galvanicanode depends on the size of the anode and on soilresistivity. Its order of magnitude is 0.1 A. This issufficient for the CP of not too large pipelines having amoderate protection current demand, for instance,pipelines covered with PE.

• A suitable site for the anode to be installed for feedingin the protection current, and a power supply (publicutility)

4.3 Cathodic protection by impressedcurrent

With large pipelines and also for reasons of easy controlmainly externally powered protection facilities areinstalled. Fig. 12 shows such an installation consistingof embedded anodes and a current supply unit. Gene-rally the anodes consist of ferrosilicon castings ormagnetite embedded in coke. The electronic conductivityof the coke helps to cut down the anodic consumptionof anode material. The main advantage of coke beddingan anode is a substantial reduction of its electrical resis-

tance in the soil. For economic reasons horizontal anodes- shown in fig. 12 - are preferred. Where there is notenough space for this type of anode and where foreigninstallations might be influenced due to an excessivefield strength near the anode, vertical anodes may beburled 10 to 100 m deep (deep seated anodes).

Figure 12 Impressed current protection installation with anodebedding, protection current source, and connections to pipeline

Anode lead Cathode lead

Measuring cable leading to the pipeline and the built-inreference electrode( helps to circumvent great IR-drops nearthe anode in the operation of potential controlling rectifiers)

4.4 Current drain test and adjustment

For adjusting the protection current a current drain testis made. At the ends of the protection length and atother accessible places, but above all, at armatures,branch-offs, in the regions of casings and aggressivesoils, on- and off- potentials are measured. Fig. 13 shows,as an example, the result of a current drain test made ata pipeline having a large protection current demand. Onlythen has the protection current been adjusted correctlyif everywhere the on-potential reaches U

Cu/CuSO4 1 V.

In fig. 13 this has been accomplished only for kilometers5 to 9.In most cases an off-potential of U

Cu/CuSO4 -0.85 V can-

not be achieved during a current drain test. Only thisoff-potential indicates complete cathodic protection.

<=

<=

Fig. 13 however, shows that the potential already didshift substantially from the starting point of freecorrosion, thus stopping any corrosion cell action. Whenchecked after about a year, the potential is supposed tohave reached or gone beyond the protection potential.In case this cannot be achieved, even by increasing theprotection current, the reason for this situation must befound. Aside from large damage of the coating, galvanicconnections to extraneous installations may be mainlyresponsible by drawing off an unexpectedly large partof the protection current. Such connections may bediscovered by electrical measurements and eliminatedby constructional provisions. With pipes going throughcasings, crossing over other utility lines, and enteringbuildings, there is always the danger of galvanic contactsensuing. Obviously this danger is especially imminentin urban areas.

Figure 13 Potentials measured in a current feed test with a bitumencoated pipeline of nominal diameter 200 mm, after 4 h polarization

Pipe-to-soil potential under free corrosion conditions

On-potential Off-potential

Locations of current measurement with results;Us = Protection potential

Location of feed-in and current value

0 1 2 3 4 5 6 8 9 10 11 12 13

1.03 A0.3 A 1.17 A 0.36 A

Pipeline length (km)

9.6 A

Pip

e-to

-soi

l pot

entia

l UC

u/C

uSO

4 -1.5

-1.0

-0.5

07

4.5 Side effects of the protection current

4.5.1 Interference with extraneous equipment

It must always be kept in mind that an undergroundcathodic protection current may jeopardize extraneousinstallations which, from it, may suffer stray currentcorrosion. This is illustrated in fig. 14. Symbolicallydepicted is the potential and current field between ananode and a pipeline to be protected, the latter crossinganother pipeline.Near the anode the extraneous pipeline receivesprotection current, i.e. here it is polarized cathodically.Close to the crossover, however, the current leaves thispipeline for the soil in order to reach the nearby pipelineto be protected. In the region of current exit theunprotected line is anodically polarized, and so,corresponding to fig. 2, there is also the danger of anodic

corrosion. In the situation depicted here, the extraneouspipeline may only be protected from anodic danger bybeing included into cathodic protection across a resistor(potential connection) . The conductance of theconnection must be at least sufficient to cause someprotection current to enter the foreign line in thecrossover area; then for sure no current will leave. Theeasiest way to solve problems of interference is to fullyinclude the foreign line in question into CP; this, though,may raise private law problems with the owners of theline. Whoever runs a cathodic protection unit is, in thelegal sense, a producer of stray current and bound toobserve the national standards and regulations. Inpractice, problems of interference should be kept to aminimum by the anodes being suitably placed and by ahigh coating resistivity of the pipeline.

Today cathodic protection of all kinds of pipelines is aproven technique raising no unusual problems. CP mayalso be given to distribution systems in urban areas,provided the pipes are coated well; however, here itentails the heavy expenditure of numerous insulatingjoints becoming necessary in the house connections.

4.5.2 Internal Interference within water pipelines

Naturally the goods conveyed within the pipeline are ofno concern to external protection, provided they are notelectrolyte solutions. But, if this is the case, specialprecautions have to be taken in the regions of insulatingjoints.Protection current may enter an unprotected part of apipeline situated beyond an insulating joint in the ground.From the internal pipe surface the current passes over

into the electrolyte within which it flows into the protectedpipeline section. So danger arises for the internal surfaceof the unprotected pipeline section beyond the insulatingjoint. With water pipelines an electrically insulating liningabout half the length of a pipe on both sides of theinsulating joint is sufficient for corrosion protection. Inthe case of a cement mortar lining there is no danger.With pipelines carrying salt waters, internal cathodicprotection beside the insulating joint also proved useful.

Since in these cases the danger is that of anodiccorrosion, only the conductivity of the electrolyte butnot its corrosion aggressiveness is of concern. In otherwords, with noncorrosive oxygen-free brines and watersused for long distance heating being transported, thedanger of anodic corrosion behind the insulating joint ismuch more severe than the danger of free corrosion inaggressive waters.

Figure 14 Influence of the protection current of a pipeline on acrossing unprotected pipeline

Anodic region Cathodic region

Current leaving the unprotected line tothe protected line

5.1 External currents

In section 3.2.1 for an uninfluenced pipeline the corrosiondanger was assessed, that may be countered by meansof a coating and cathodic protection. An externalinfluence is given, if current enters the pipeline fromextraneous installations across a metallic contact, or if,caused by electric fields within the ground, stray currentsenter and leave certain parts of the pipeline respectively,as was described in fig. 14. Stray current will be dealtwith in chapter 6. Chapter 5 is concerned with thosecurrents exclusively that enter the pipeline at places ofmetallic contact.Normal cathodic protection is an example of this. Theeffect of the negative current fed in is the prevention ofcorrosion damage. This was pointed out in the beginningwith relation to fig. 2. Conversely, there is a definitecorrosion danger, if the current fed in is positive. Soobviously great care has to be taken to make it justimpossible for the rectifier of an external currentprotection unit to be reversed.

5.2 Corrosion caused by foreign cathodes

Positive current enters the pipeline at a place of metal-lic contact whenever the contacted extraneousinstallation has a rest potential more positive than thatof the pipeline.However, some 10 mV need not yet be consideredcritical, a potential difference of 0.3 V is about the margin.Potential deviations of about 0.1 V are quite normal withpipelines and due to different properties of soil andcoating. They signalize the formation of corrosion cellsalong the pipeline as described in section 3.1.2, comparealso the rest potentials in fig. 13.

5.2.1 Characteristic of a foreign cathode

The rest potential of steel in the ground ranges betweenU

Cu/CuSO4 = -0.7 and -0.5 V. A potential of -0.4 V and

more positive values in the neighbourhood of a foreigncontact raises the suspicion of corrosion danger causedby too positive a potential of the extraneous object. Inthis case we have a corrosion cell, the pipeline as awhole representing the anode and the extraneousobject being the cathode.

Danger is evidenced by potentials around -0.2 V andabove being found near the extraneous object. Indicativeare measurements made after breaking the contact, ifthis is possible. The separation having been effected,the electrical potentials of the disconnected parts mustdrift in opposite directions: The pipeline will become morenegative and the foreign cathode more positive. By useof a low resistance ammeter even the cell current maybe measured directly.

By means of such a potential measurement also theoperator of a pipeline may check whether it is anodicallyendangered by foreign cathodes or not. Required is aCu/CuSO

4 reference electrode, easily built by laboratory

personnel referring to fig. 1, and a high resistance voltagemeter, the min. input resistance of which is1 MΩ/V. Fora potential measurement to be made, the referenceelectrode has to be put on moist soil. A potentialmeasurement on an asphalt road surface is impossible!In case there is an indication of corrosion danger, expertsshould assess its extent and plan protective measures.

In connection with this, it must be kept in mind that inthe case of an external cathode raising danger, theelectrical influence on corrosion strongly prevails andthat the properties of the soil are almost of no concern,provided only, a cell current can flow. This means, onlyin sandy soils with very high resistivities is the dangerof corrosion diminished.

5.2.2 Examples of foreign cathodes

Although it is easy to realize that great potentialdifferences mean anodic danger, it is comparativelydifficult for the non-expert, when judging from the mate-rial, to see which parts of construction might act asforeign cathodes.In the case of a pipeline connected to other installations,e.g. groundings, mainly those parts that consist of nob-ler materials like, for instance, copper, copper alloys,and stainless steel may be suspected to act as foreigncathodes. In many instances the surface fraction of thesematerials in the ground is so small that, in accordancewith the law of areas, eq. (18), the corrosion dangermay be neglected. However, the potential is dependentnot only on the kind of metallic material but also on thechemical properties of the environment. In the groundthere may be large areas of steel surface covered byconcrete or cement mortar. Now, the chemical propertiesof cement mortar are vastly different from those of soils.Covered by cement mortar, steel is in a special condition,

5 Corrosion danger caused by foreign cathodes;localized cathodic protection

called passivity. Within cement mortar it behaves likestainless steel surrounded by water. So, foreign cathodesmay be represented by all parts of copper, stainlesssteel and steel covered by cement mortar. Apart fromthis, old rusted steel surfaces of pipes, storage tanks,and other constructional parts may act as foreigncathodes.

Today, steel parts in concrete structures acting as foreigncathodes are the most frequent origins of anodic danger.Possible examples are fixing points and manholes neararmatures, points of entry into buildings, connectionsto building foundations, and to groundings. In this linefig. 15 schematically shows the course of the pipe-to-soil potential near the pipeline contacting a reinforcedconcrete structure. The funnel-shaped potential distri-bution above the coating holiday only exists with respectto the IR-free potential which, theoretically, might bedetermined by using probes. The pipe-to-soil potentialas measured on the surface of the ground is much moreeven due to the ohmic voltage drop of the current leavingthe steel surface through the holiday. Were the pipelinedisconnected from the rebars in the building, the pipe-to-soil potential would assume a value U

Cu/CuSO4 < -0.6 V

belonging to the steel surface below the holiday.

5.2.3 Estimation of corrosion danger

Equation (17), may be used for an assessment of corro-sion danger, the indices a and k referring to the pipelineand the foreign cathode respectively. Steel in concretebeing passive, eq. (18), may be taken to be valid; and,in the case of large corrosion rates, J

a = J

A may be

assumed. So it is the area proportion which is decisive,and with it the magnitude of S

a, the total area of the

steel surface lying bare in an area of coating damage.The better the coating, i.e. the less mechanical damageoccurred along the pipeline, or the higher the coatingresistivity is, the larger will be the anodic current densityaccording to eq. (18), or, according to eq. (5), the fasterwill be the corrosion at places where damage did occur.In such cases corrosion rates may exceed 1 mm/year,and, in saline waters, even reach 1 cm/year. It is truethat the corrosion rate will be lower with a pipeline havinga badly damaged coating with many holidays or evenhaving no coating at all. So lack of passive protectionmight help to avoid early failure, but not damageoccurring later.

Figure 15 Result of a contact between a pipeline and a reinforcedconcrete structure

Pipe-to-soil potential mesured with reference electrode on theground surfacePipe-to-soil potential, free of IR-drops (as if measured withthe probe placed on the pipe surface)

-0.2 V

-0.4 V

-0.6 V

Pot

entia

l UC

u/C

uSo4

(V

)

Length

Reinforced concrete structure

SOIL

5.2.4 Coating influence

Certainly, doing without a coating is not a protectionmeasure to be recommended, just in order to delayfailure. But the user must be aware that, danger by foreigncontacts being given, the choice of a top quality coatingcannot be considered a protective measure as long asthere is no 100% certainty of the coating being free ofany minute holiday remaining that during service.

This seemingly paradoxical effect of the extent of coatingdamage in the case of an external electrical influence isgenerally valid; it is not limited to foreign cathodesgenerating danger. The effect is easily understood inview of these relations:

The electrical resistance in the ground in front of acoating damage (contact resistance, groundingresistance) is inversely proportional to the meanradius of the uncovered steel surface below thedamage:

•

RF = A r-1 (22)

• The size of this uncovered steel surface is proporti-onal to the square of the same mean radius:

SF = B r2 (23)

With a voltage U acting upon the pipeline around thedamages, Ohm`s law gives:

Ia =

UR

F

UrA

= (24)

Ja =

Ia

SF

UA B

= r-1 (25)

So the current density Ja will increase, if the radius r

decreases!

5.3 Protection measures

Protection against danger caused by foreign cathodesmay be accomplished in two ways:

• Galvanic separation• Local cathodic protection

5.3.1 Galvanic separationGalvanic separation is the most effective means becauseby it the very source of corrosion cell danger iseliminated.

The reliability of this depends on easy control of all thesites of possible connections, and on the certainty withwhich accidental contacts may be excluded in practice.

Standards and regulations specify galvanic separationto be effected for house connections in public gas deliverysystems, because for inside the building, intentionalconnections for personal protection are madecompulsory by DIN/VDE-regulations. With reinforcedconcrete construction and interconnection of all domes-tic installations being common today, personal protectionobserving DIN/VDE results in the formation of spaciousforeign cathodes with respect to gas pipelines, that haveto be disconnected from these by inserting insulatingflanges.

5.3.2 Local cathodic protection, hot spot protection

In ranges of industrial plant, galvanic separation isinapplicable because of the presence of too manyinterconnections, in many instances also for construc-tional reasons. Applicable then is local cathodic protec-tion. The techniques of normal CP cannot be followedfor these reasons:• The foreign cathodes will consume much more currentthan the pipeline to be protected. Large ohmic drops inthe ground will result. So IR will become a substantialpart of the on-potential.• The pipeline and the foreign cathodes vastly differwith respect to their electrochemical properties. Freecorrosion endangers the pipeline anodically. Givencathodic polarization, the two areas are polarizeddifferently. After switching off the protective current forthe off-potential to be measured, comparatively largerelaxation currents between the pipeline and the foreigncathode will flow. The corresponding ohmic drop willfalsify the off-potential too. The measured value is morepositive than the IR-free (true) value.

The basis of local cathodic protection may be derivedfrom eq. (19). The protection current is rated and fed inso that the foreign cathodes do not deliver cell currentbut consume protection current. Then eq. (19) gives:

In other words; Aimed at is the polarization of the foreigncathodes so as to render them harmless instead ofbreaking their corrosion cell with the pipeline. Withinindustrial plant many 104 m2 of reinforced concrete surface

I >= J

kS

k and J

a<= 0 (26)

contact the ground and consume a mean current of about5 mA m-2, so the order of magnitude of the currentconsumed for local cathodic protection of a special plantis 102 A. The interference problems raised by suchcurrents can only be solved by all underground in-stallations being incorporated into the protection system.

Fig. 16 shows the application of local cathodic protectionto a power station with large cooling water ducts (nomi-nal diameters 2000 and 2500 mm) and water conduitsfor fire extinguishing. For 19 000 m2 of reinforced concretefoundations and 2000 m2 of copper groundings a currentof 120 A is fed in via eight deep-groundbeds

With a resistivity of 150 to 350 Ωm this soil is a compara-tively poor conductor; so, for a local potential loweringof specific objects to be protected, a „directed“ currentmay be fed in via horizontal groundbeds.

The values given under „A“are pipe-to-soil potentialsmeasured with free corrosion going on before local cath-odic protection was installed. These potentials range fromU

Cu/CUSO4 = -0.5 to -0.1 V. The values given under „B“,

and „C“are on-potentials (off-potentials) measured after4 months and one year, respectively, of the protectionsystem being in operation. As expected, it was notpossible to produce off-potentials more negative thanthe critical protection potential (-0.85 V), this is in spiteof (some) vastly negative on-potentials. However, dueto the IR-bound error still present in the off-potentialmeasurements, this impossibility does not imply thatthe object is not fully cathodically protected. There areways of measuring that work with polarization probesand/or test coupons, and enable off-potentials at criticalpoints of the structure to be measured sufficientlydepleted of IR-errors. This affords a reliableascertainment of protection.

Complicated adjustment steps during the design andconstruction phases, and later on, controlling measure-ments and the handling of very large protection currentsin view of possible interference with foreign objects,necessitate this type of protection to be managed byexpert enterprises having advanced experience. Duringthe last decade progress was manifested in numeroussuccessful applications.

Figure 16 Local cathodic protection in a power station

Deep groundbrdsHorizontal anodesCooling water linesFire extinguishing lines

AB

C

Pipe-to-soil potential under free corrosion conditionsPipe-to-soil potential after 4 months` polarizationWithout brackets: On-potential, in brackets: Off-potentialSame as B, but after one years` polarization

6.1 Origins of stray currents

A stray current is an underground current originatingfrom current-carrying parts of electrical installations. Inmost cases these are the rails of electric railways. ButCP facilities installed for foreign objects to be protectedmay also be responsible. Related interference problemswere discussed already with regard to fig. 14. Dependingon the geometric situation of the pipeline in relation tothe direction of the current, i.e. the electric field generatedby the unit in question, stray current will enter the pipelineover large areas. Within the most positive region thestray current leaves the pipeline for the ground, and hereit causes anodic corrosion.

6.2 Characteristics of stray current action

Corrosion caused by stray currents is in many respectssimilar to that caused by foreign cathodes. However,there are typical differences, too.

• Corrosion danger essentially depends on currentdensity only. Properties of the ground are of no concern,actually, provided a stray current does flow. So only insandy soils of very high resistivity is the corrosiondanger diminished.

• The influence of the pipe coating is varied.

If there is no direct connection leading in stray currentfrom foreign objects to the pipeline, a high coatingresistivity could be useful, because stray current entrywould be cut down. On the other hand in the region ofcurrent exit the same conditions are given, as werediscussed in section 5.2.4 with respect to foreigncathodes, in view of eqs. (22) through (25), page 24.Hence in this area a high coating resistivity is un-favourable. Useful would be only a coating 100% free ofholidays, and that after the laying of the line and duringservice.

• As opposed to the situation with foreign cathodes, theorigin of danger cannot be eliminated. Stray currentsare present in the ground, and they do not need connec-tions to enter the pipeline. A suitable protection measureis not impediment of stray current entry but that of straycurrent exit.

• As in the case of danger caused by foreign cathodes,the corrosion rate is hardly assessable. Values ex-ceeding 1 mm/year may be expected. The endangeredregions are characterized by very positive pipe-to-soilpotentials. Contrary to the action of foreign cathodesand foreign protection facilities the potential accom-panying railway currents is intensely variable with time.Depending on construction and usage of the railway trackthe regions of entry and exit of stray current are subjectto considerable alterations. For these reasons simplemeasurements of potentials are not sufficient; necessaryis recording the potentials for several hours’ duration.

The same way as with foreign cathodes creating danger,the user of the pipeline may, by potential measurements,recognize stray currents to be present. These measure-ments must by all means be continued for several hourswhile the neighbouring railway tracks are being used.Assessment of the danger can be achieved by an ex-pert only. The intense danger of early damage necessi-tates protection measures as soon as possible.

6 Corrosion caused by stray currents;protection methods

Figure 17a Stray currents originating from railways

6.3 Protection measures against straycurrents

Protection may be achieved by leading the stray currentthrough electrical connections back directly to theinstallation that produces it. Fig. 17 shows schematicallystray currents being created by a D.C. railway and flowingthrough a neighbouring pipeline. Near the rectifier railwayfeeding station of the railway the stray current leavesthe pipeline and causes severe anodic corrosion in thisarea. This is the very location for protection measures tobe taken.

6.3.1 Immediate drainageVia a heavy duty low ohmic resistor the pipeline isdirectly connected to the stray current source, i.e. therailway track.Even this measure alone is very effective,provided the connection is made in the immediateneighbourhood of the rectifier railway feeding station ofthe railway, and the overhead conductor is the positivepole. Thus the negative potential of the rails is transfer-red to the pipeline, so that even a moderate cathodicprotection may be the result.

Figure 17 Protection against stray currents originating fro railwaystray currents in the soil, pipeline and current drain

Anodic region

Cathodic region

of influenced unprotected pipeline

6.3.2 Rectified drainageThis is a special case of immediate stray current drai-nage. It is characterized by heavy duty diodes incor-porated in the draining connections to the rails. Thediodes are meant to ensure that current can only flowfrom the pipeline to the rails and not in the oppositedirection. This type of draining connection must beapplied, if a direct drainage cannot be installed in theimmediate neighbourhood of the rectifier railway feedingstation. Increasing with the distance from the railwayrectifier is the chance of pole changings taking placebetween pipeline and rails according to varying situationsin railway traffic, so that there is the danger of straycurrent entering through the very drainage. This is to beprevented by diodes. Rectified drainage must be installedin several locations along the pipeline, where stray currentmay often leave.

6.3.3 Forced drainageWith this type of drainage a protective rectifier unit isincorporated into the connection. By means of its voltagethe rectifier extracts the stray current from the pipelineand directs it to the rails. Actually this is a normalarrangement for cathodic protection, the rails repre-senting the anode. Since due to varying situations inrailway traffic the voltage pipeline/rail may be subject tosubstantial alterations, the protective rectifier unit shouldbe adjustable to voltage variations with time. There areseveral possibilities to lower the resulting currentvariations, e.g. installation of inductivities.

6.3.4 Use of controlling rectifiers for forced drainageIn the case of very large current variations it is ad-vantageous to use rectifiers controlling the potential. Bymeans of the fixed reference electrode RE the rectifiedcurrent is controlled so as to keep the pipe-to-soil po-tential in the place of drainage at a fixed value U

nom. At

larger distances from the reference electrode the straycurrent influence is different, so there are increases inthe temporal variations of the pipe-to-soil potential again,but this is of no concern to the protection effectiveness.If in some region the pipe-to-soil potential is often foundto be too far positive, another protection unit must beinstalled there.

A

B

7.1 Types and causes of high voltage influenceIn densely populated areas there is not always a wayaround constructing pipelines running in close proximityto open high voltage lines, crossing them or evenparallelling them for some distance. Due to alternatingvoltages induced in these pipelines high contact voltagesmay develop, which necessitates protective measures.

During construction and operation of pipelines specialmeasures must be taken according to relevant standardsand regulations, also with regard to the cathodic pro-tection system. There are two main types of interferencewhich must be assessed in different ways:

Short time interference caused by short circuitcurrents of the high voltage power line

Permanent interference caused by operating currentsof the high voltage power line

In both cases by a suitable grounding of the pipeline theinduced contact voltages must be safeguarded againstexceeding the admissible extremes. In section 4,however, it was pointed out that disconnection fromgroundings is a prerequisite for cathodic protection.Groundings consume protection current, and, becauseof possible relaxation currents flowing after the protectioncurrent switch-off, they may cause measuring problemsin the determination of true potentials by application ofthe off-potential technique.

7.2 Protective measures against too highvoltages in personal contact

According to to relevant standards and regulations thevoltages permissible for bodily contact depend on theduration of their action, this enables different protectionmeasures to be taken against the two above mentionedtypes of interference.

7.2.1 Short time interference caused by short circuitcurrents of the high voltage power line

In the case of a short circuit of this kind, power is cutoff after about 0.2 s. Permissible for bodily contact arevoltages exceeding 1000 V. This means the requiredgroundings may be connected to the pipeline acrossexcess voltage drains, working by way of gas dischargewith starting voltages (threshold voltages) about 250 V.So during normal operation the groundings are electri-cally separated from the pipeline, and neither do theyinterfere with cathodic protection nor with its control bypotential measurement. With a short circuit given, securegrounding is provided for the short duration required.

•

•

7.2.2 Permanent interference caused by operatingcurrents of the high voltage power line

The maximum permissible voltage for long time bodilycontact is < 65 V. This means the required groundingsmay be connected to the pipeline across AC-voltagelimiter IVL-10. The Limiter IVL-10 effectively blocks theprotective DC-current required for cathodic protectionwhile providing a low ohmic ground connection forinduced AC-currents caused by operating or short circuitcurrents of high voltage overhead line systems.Measuring of true potentials by application of the off-potential technique are guaranteed.

Electromagnetic field

Soil resistivity

Length ofparallelism

Coating qualityCoating resistance

Distance OH-line - Pipeline

Important parameters regarding the causes ofinduced AC on pipelines in utility corridors

Dimensions

Coating resistance

Distance OH-line-Pipeline

Buried depth

Soil resistivity

Operating current

Short circuit current

Phase imbalance

Operating frequeny

Length of parallelism

Pipeline Overhead line

Figure 18 Parameters regarding the causes of induced AC onpipelines

7 High voltage interference, protective measuresand effect to cathodic protection

7.3 Grounding the pipeline

With the pipeline having a coating resistivity ru to be

lowered to ruG

because of high voltage interference andwith n groundings of resistances R

E each to be installed

for this purpose

Sr

uG

Sr

u

=n

RE

+ (28)

S = π d l RE is the surface of the pipeline section in

question with the diameter d and the length l. So therequired number of groundings is

n = π d l RE x

ru - r

uG

ru x r

uG

(29)

In the case of very good coating resistance eq. (29) issimplified to

π d RE

ruG

n = l x (30)

Some experience-based practices are generally followedto “earth“ and “isolate“ the pipeline and/or “divert the in-duced high voltage/current surges to ground“. Thesepractices are generally not backed-up by analytical fieldinvestigations.For optimization of grounding systems computer soft-ware is used to determine the necessary locations andthe required earth resistances of single groundings alonga influenced pipeline section. Results of these computercalculations confirm the predictions of experts but maysurprise those who “earth“ and/or “isolate“ based onpractice.

The groundings may be made of galvanized steel, thezinc layer should at least 70 µm. Strip iron with a dimen-sion 30 x 3.5 mm and rods with about 20 mm diameterare suitable for application as horizontal and deep seatedgroundings respectively.

0,00

0,50

1,00

1,50

2,00

2,50

3,00

3,50

4,00

4,50

5,00

5,50

6,00

25 50 75 100 125 150 175 200 250 300

Requiredearthing resistance

[ Ω ]

3.0 Ω

66 m Lengt of earthing strip [ m ]

rho = 150 Ohm x mrho = 120 Ohm x m

rho = 100 Ohm x m

rho = 80 Ohm

x mrho = 60 Ohm

x m

rho = 40 Ohm x mrho = 20 Ohm x m

Figure 19 Determination of earthing strip length depending onrequired earthing resistance and specific soil resistivity

Example of application:Required earthing resistance : 3.0 ΩSpecific soil resistivity : 94.0 Ωm

Diagram: selected ρ =100 Ωm and 3.0 ΩResult: Lengt of earthing strip = 66 m, taken: 75 m

In the previous sections those types of corrosion weredescribed exclusively which result in a more or lessuniform metal consumption, according to eq. (2a.Corrosion damage is given by the diminishing of wallthickness and the development of trough-shaped cavitiesand pittings.Common to all of these processes are the anodic parti-al reaction of eq. (2), and the electrochemical protectionmeasure, to shift the potential to < U

Cu/CuSO4 = 0.85 V,

the protection potential. But there are still other types ofcorrosion, that may lead to pipeline damage. They aregoverned by mechanisms different from those of theabove discussed types and their dependence on poten-tial is quite varied. Among these are the different kindsof stress corrosion cracking, SCC, characterized by thedevelopment of cracks under tensile stresses and with-out any noticeable material consumption.

8.1 Critical agents causing SCC

SCC may occur provided there is a simultaneous actionof sufficiently high tensile stresses and critical agents.Strong influences are exerted by the concentrations ofthese substances, by temperature and potential. In manycases SCC is even restricted to fairly narrow potentialregions, so that the question about susceptibility to SCCcannot be answered without reference to the relevantpotential.According to the knowledge available so far - mainlychemical in nature - the following substances may triggerSCC in steels:

• Intergranular cracksCaustic alkali solutions, alkali carbonate, and bicarbo-nate; nitrates, fuming nitric acid and condensates con-taining nitrous oxides (e.g. from flue gases), ammoniumsalts of weak acids.

• Transgranular cracksAqueous condensates of hydrogen sulphide, prussicacid, carbon dioxide + carbon monoxide (gases commonin refineries), liquefied ammonia and pressurized hy-drogen.

8 Conditions related to stress corrosion cracking;preventive measures

8.2 Danger caused by corrosionon the inside surface of the pipes;protective measures

Examples of critical substances, that may cause stresscorrosion cracking at sufficiently high pressures, andpertinent protection measures are combined in thissurvey:

Critical substances

H2S, moist

HCN, moist

CO + CO2, moist

liquid NH3

H2 (gaseous)

Protection measure

Drying, inhibition

Drying

Drying

Inhibition (by adding H2O);

O2-removal

Inhibition (by adding O2),

avoidance of sharp edged notches

SCC of pipelines so far mainly started from the insideof the pipes. As critical substances H

2S and CO + CO

2had to be given attention. Corrosion and corrosionprotection of the outside surface in the ground are of noconcern to these processes. Protection measures areexclusively directed to conditioning the goods to betransported and to the working conditions of the pipelines.

8.3 Danger caused by corrosionon the outside surface of the pipes;protective measures

Damage caused by SCC starting from the outside sur-faces of pipelines is very rare, and restricted to linesworking above ambient temperature, especially thoseunder high pressure. The table below gives a surveyconcerning this matter. The features of these types ofcorrosion will be described in the following paragraphs.

8.3.1 SCC caused by nitrates

The critical potentials lie in the region of normal restpotentials. Ordinary cathodic protection totally eliminatesthe danger of SCC. Nitrates from artificial fertilizers beingubiquitous in farming areas, cathodic protection is alwaysrecommendable.

8.3.2 SCC caused by sodium hydroxide

The critical potentials are distinctly more negative thanthe protection potential. But SCC only occurs with tem-peratures permanently exceeding 50 °C (which does nothappen in normal pipeline operation), and with very highconcentrations of sodium hydroxide. Causative for itsformation is, following eqs. (2b), (3), (4), cathodicpolarization in combination with ion migration, eq. (14b).So, danger will only arise if there are very high protectioncurrent densities, if high concentrations of alkali ionsare present, and if the sodium hydroxide is impededfrom diffusing off by deposits or solid materials on thepipe surface.

Lessening the protection current density is doublyeffective. In the first place the probability of the criticalpotentials being reached and in the second place theconcentration of sodium hydroxide is decreased. If, dueto a lower current density, the protection potential is notfully attained in some areas, this situation has to be putup with. Avoiding SCC that may lead to early failure takespriority of definitely preventing the material consumingtype of corrosion, that might - with a decreasedprobability! - lead to late occurring damage.

Substance; origin

Nitrates; fertilizers

(NH4)

2CO

3; frtilizers

NaOH; cathodessee eq.(4)!

NaHCO3; NaOH at cathodes

CO2 from surroundings

Na2CO

2; (like with NaHCO

3)

not specific(soil and salts)

Critical potentialsV VS. Cu/CuSO

4

> -0.45

about -0.65

about -1.1

about -0.7

about -0.9

predominantly withcathodic protection

Damage becameknown in

Long-distance heatingpipeline

Long-distance heatingpipeline

High pressure gaspipeline

High pressure pipeline

Protection measures

Cathodic protection

Cathodic protection

Reduction of temperature(<50 °C)

Cooling, blasting of steel surface,use of CO

2-free coatings,

sufficient cathodic protection

(like with NaOH)

Avoidance of hard spotformation (about >400 HV)

Only in hard spots on the pipe surface!

Critical substances at the pipe surface that may lead to stress corrosion cracking in soil if critical potentialsand sufficiently strong mechanical stress are present

8.3.3 SCC caused by sodium bicarbonate

The critical potentials are a bit more positive than theprotection potential. Sodium bicarbonate is formed outof sodium hydroxide, which is cathodically generated,and carbon dioxide from the environment:

NaOH + CO2 ---> NaHCO

3. (31)

The carbon dioxide may come from the soil or from thecoating material. Because of the potential range requiredfor this type of corrosion to occur, the bare steel surfacebelow coating holidays is not in danger, providedsufficient cathodic protection is given. Critical potentialsmay occur below deposits and loose parts of thecoatings, especially if there is mill scale still on the steelsurface. For this reason, giving adequate cathodicprotection, and the removal of mill scale from the steelsurface are important protection measures.

Another protective arrangement is directed against theformation of sodium bicarbonate after eq. (31). Carbondioxide may come from the soil and permeate the coating,or, at elevated temperatures, be emitted by the coatingmaterial.For this reason only such coating materials should beused, that do not emit carbon dioxide at elevatedtemperatures, and have a comparatively smallpermeation coefficient for carbon dioxide. Anotherprotection measure is lowering the temperature.

In all these cases by diminishing mechanical stress thedanger of SCC may be cut down decisively. In thisrespect not only stress created by internal pressure,but also additional loads, e.g. by thermal elongation,should be considered.

8.3.4 SCC in hard spots

At the end of the table still another possible type ofdamage is included that may occur in hard spots of thesteel surface, the type of material, and the potential beingof no concern whatever. Cathodic protection may slightlyfurther the progress of damage but is not actuallycausative. This type of SCC is caused by the hydrogensensitivity of hard spots, the hydrogen being producedby a cathodic partial reaction according to eq. (4).

These hard spots exceeding 400 HV are not commonphenomena on pipe surfaces. In case they do occur,danger is created only then if the coating is damaged inthe very same place. The chance of such a coincidenceis very narrow - and virtually eliminated by applicationof coatings with high mechanical strength.

8.3.5 SCC caused by cathodlcally producedhydrogen

In theory, hydrogen produced cathodically according toeq. (4), may cause hydrogen-induced SCC of pressur-ized pipelines, especially in the case of sulfides beingpresent on the steel surface, a situation which cannotbe excluded in the soil. This type of corrosion, though,is sharply dependent on the pH of the electrolyte inaction. Damage can occur in condensates from sourgas (H

2S), see section 8.2, not so, however, in soils

with much higher pH values. Moreover by cathodicpolarization the pH value at the steel surface is increasedaccording to eqs. (3) and (4), so that this type of damageoccuring in soils is definitely excluded even with higherstrength pipeline steels. This is in conformance with allpractical experience.

9.1 When to use stainless steel

Stainless steel pipelines are very rarely used for under-ground service. One reason for using them is the speci-fication of particular purity of the transported goods,like for instance fuel. Generally a normal austeniticchromium-nickel steel (German Standard material No.1.4301; AISI 304), containing about 18% Cr and 9% Niis used, that, due to its passivity, is totally stable in thesoil.For particularly aggressive substances to be transportedhigher alloy steels with specific chemical properties mayalso be used. For instance in mineral oil and natural gasproduction the ferritic-austenitic duplex steel AF 22(German Standard material No. 1.4462) is used formains. This type of steel is highly resistant againstchloride induced pitting and SCC, and apart from this itexhibits a comparatively high strength.

9.2 Stainless steel parts acting as foreigncathodes vs. unalloyed steel

Mixed installation of stainless and unalloyed steelscauses problems since stainless steel parts will act asforeign cathodes, see section 5.2.2. Corrosion dangermay be relieved by electrically separating the parts ofdifferent materials and by coating the stainless steelparts. The parts may be wrapped with the simplestinsulating tapes; these are only needed to insulate thegreatest part of the surface but not to protect the stain-less steel from corrosion.

9.3 Overall corrosion properties andinfluence of alien currents

Stainless steel cannot be jeopardized by foreigncathodes in the ground, because, with respect to it -they just do not exist. Anodic danger may only be causedby stray currents. This may happen, e.g., in case stain-less steel parts are, by means of insulating flanges,kept separate from cathodically protected pipelines.Protection current may enter the stainless steel line and- similar to the situation in Fig. 14, - leave it near theinsulating flange. Then, in this region an anodicpolarization being present, chloride ions in the soil maycause pitting. For corrosion prevention the stainless steelline should be included into CP by means of a potential-connection. The protection of stainless steel, though,does not require the potential that is recommended forthe protection of unalloyed steel. Pitting is prevented bya protection potential U

Cu/CuSO4 = - 0.1 V; this is definitely

more positive than the protection potential for unalloyedsteel. Stainless steel in the soil may be exposed to anypotential more negative than the protection potential.There is no hazard that this might result in a loss ofpassivity.

9.4 Conditions requiring protectivemeasures

Stainless steels in the soil may be subject to corrosiondanger on these conditions:

9.4.1 Sensitizatlon

Stability might be impaired at welds. Improper heat treat-ment may sensitize stainless steels to intergranularcorrosion. Linked to this is an increased sensitivity topitting corrosion. Sensitization may be avoided by wayof proper selection of materials and welding procedures.Danger of corrosion only exists with severely sensitizedmaterials exposed to aggressive soils. Usually a weakcathodic polarization, which is effected simply bygalvanic contact with pieces of unalloyed steel, givesfull corrosion protection.

9.4.2 Concentrated chloride

With pipelines operated at higher temperatures there isalways the danger of chlorides being concentrated belowdeposits on the steel surface. In the case of stainlesssteel this causes pitting, and if the steel is austenitic,the additional result is heavily branched transgranularSCC. Application of a cathodic protection potentialU

Cu/CuSO4 = - 0.1 V may be considered.

In any case even a galvanic contact with unalloyed steelis sufficient to eliminate a possible corrosion danger.So, for the protection of stainless steel a piece of un-alloyed steel may be used as a galvanic anode.

The material is sensitive to intergranular corrosionaccording to DIN 50 914.

The pipeline is operated at temperatures exceeding50 °C (approximately)

•

•

9 Behaviour of stainless steel in the ground

10 Symbols

c(X)

dIIB

IA

IK

Ia

Ik

J

Js

llD

nRR

E

RF

Concentration of substance (X)

Pipe diameter

Electric current

Operational current in high power lines

Anodic partial current

(for anodic electrochemical reaction)

Cathodic partial current

(for cathodic electrochemical reaction)

Positive (anodic) total current

Negative (cathodic) total current

Current density

(indices A, K, a, k apply with current)

Protection current density

Length of pipeline or section thereof

Effective distance of diffusion

Number of groundings

Electrical resistance

Electrical resistance of a grounding

Contact resistance of the steel surface

under a coating holiday

rr

u

ruG

SS

a, k

SF

UU

R

UB

UB max

Us

UCu/CuSO4

wργ

Radius of coating holiday

Coating resistivity

Maximum admissible coating resistivity in

the case of high voltage influence

Area

Area of an anodic or a cathodic region,

respectively, on a heterogeneous steel surface

Steel surface area under a coating holiday

Voltage, electrical potential

Rest potential, potential in the case of free

potential

Contact voltage

Maximum admissible contact voltage

Protection potential

(in the case of cathodic protection)

Potential with relation to copper/copper

sulfate electrode

Corrosion rate; rate of metal consumption

Soil resistivity

Transmission measure