Heidersbach R. Metallurgy and Corrosion Control in Oil and Gas Production

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CORROSION CONTROL 169

− 850 mV CSE Criterion Over the years, many author-

ities came to the conclusion that the − 850 mV CSE

potential advocated by R. Kuhn and his colleagues in

Louisiana was the easiest and most reliable way to

determine if cathodic protection had been achieved.

This idea was incorporated into the ﬁ rst international

standard on cathodic protection, NACE RP0169 (since

changed to SP0169).

102

Mr. Kuhn ’ s arguments were

based on leak records that showed that if cathodically

protected structures were kept at − 850 mv or more com-

pared to CSE, then leaks due to corrosion were substan-

tially eliminated.

−

79,103,104

This approach was reinforced by Peabody, who pub-

lished a practical galvanic series of metals in soil in 1967

(Table 6.26 ).

103

This showed that carbon steel ( “ mild

steel ” in Peabody ’ s terminology) would have a native,

or unprotected, potential of somewhere between − 0.2

and − 0.8 V CSE. Thus, Kuhn ’ s recommended potential

of − 0.85 V ( − 850 mV) is at least a 50 mV shift in the

cathodic direction, and usually much more. Peabody

and Parker were the two standard references on cathodic

protection of pipelines in 1969 when NACE RP169 was

ﬁ rst published, and both books advocated the − 850 mV

criterion, although they do discuss other criteria for

determining if cathodic protection has been achieved.

103,104

Many authorities pointed out, and still do, that it is

unnecessary to have steel at − 850 mV CSE in order to

achieve cathodic protection.

105

While this has always

been the case, most owner operators choose to use the

− 850 mV criterion in NACE SP0169 and similar stan-

dards, because it is easy to measure and to train inspec-

tors on how to perform the necessary measurements.

Electricity is generally cheaper than trained labor, which

is necessary to inspect according to the other, more

complicated, criteria.

In cases where microbially inﬂ uenced corrosion

(MIC) is suspected or at elevated temperatures, the

protection potential is considered to be − 950 mV

CSE.

79,85,102

Little controversy has appeared over the idea of a

similar criterion for locations where MIC is suspected.

The change of potential to − 950 mV due to temperature

is not controversial and can be understood by anyone

who considers the Nernst equation, developed long

before cathodic protection was common, that clearly

explains why electrode potentials for any reaction will

be affected by temperature. This is in contrast to the

continuing controversy over the necessity to use an

“ instant off ’ ” or similar IR - compensation technique to

identify the “ true ” potential of a structure.

All of the above discussion has related to buried

structures, primarily pipelines. Other reference elec-

trodes are used in different applications. The corre-

sponding voltage for silver - silver chloride electrodes,

which are used in seawater, is − 805 mV, although this is

usually rounded to − 800 mV.

100

− 100 mV Shift Criterion Advocates of the − 100 mV

shift criterion point out that − 850 mV CSE is not neces-

sary to achieve cathodic protection (an acceptable

reduction in corrosion activity) in many, perhaps most,

cases.

105,106

They also claim that in some circumstances − 850 mV

CSE, however determined, may not produce protection.

This latter claim is very controversial and, except in the

cases of elevated temperature, parallel zinc anodes, or

microbial activity, it has not been unequivocally

documented.

The − 100 mV shift criterion assumes that unshifted

potentials can be determined. This is impractical for

galvanic anode systems. It also assumes that the

unshifted (or native) potential of the structure does not

change with time. Areas with changing groundwater

levels due to seasonal wet and dry seasons are one

example of where native (unprotected) potentials are

likely to change.

Turning off ICCP systems to determine the native

potential requires up to 48 h for the potential to decay

to the unshifted potential.

The difﬁ culties and limitations discussed above have

led most operators to prefer to use the − 850 criteria for

determining cathodic protection.

E Log i Criterion There are structures where it is

inconvenient or impossible to place reference elec-

trodes along the structure being protected. Well casings

are an excellent example of this situation. While the top

of the well casing is available for electrical connections,

the bottom of the casing is inaccessible. The E log i

criterion (Tafel curve method) is used in these situations

to measure the current necessary to provide cathodic

protection.

TABLE 6.26 Practical Galvanic Series

Metal

Volts

(CSE)

Noble or

cathodic

Copper, brass, bronze

− 0.2

Mill scale on steel

− 0.2

Mild steel (rusted)

− 0.2 to − 0.5

Mild steel (clean and shiny)

− 0.2 to − 0.8

Active or

Anodic

Zinc

− 1.1

Magnesium

− 1.75

Note : Condensed from Table 2.2 in Peabody, Control of Pipeline

Corrosion .

103

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170 METALLURGY AND CORROSION CONTROL IN OIL AND GAS PRODUCTION

Figure 6.74 Test setup for E - log i testing to determine the

necessary cathodic protection current for a well casing.

112

Electrical isolation

Voltmeter

Rectifier

+–

Test ground

bed

Well casing

Reference electrode

Figure 6.75 Plot of E log i data for a well casing.

Log current →

corr

Potential →

Figures 6.74 and 6.75 show how this is done. A

cathodic current source is connected to the casing and

current is applied. The potential at the casing head is

measured before the current is applied and the change

in potential is monitored as additional cathodic current

is applied to the casing.

At the corrosion potential no current is being sup-

plied from the external power supply, and the corrosion

potential, E

corr

, is due to the natural oxidation and reduc-

tion reactions on the structure. The total oxidation and

reduction currents are unknown. As current is supplied

from an external power source, the potential versus log

of applied current plot begins to curve downward when

the applied current is similar in magnitude to the natural

current. Increased applied current leads to a situation

where virtually all of the cathodic current is coming

from the power supply. The plot then becomes linear or

straight. The applied current where the potential - log

current plot becomes “ straight ” or linear is assumed to

be the current necessary to provide adequate cathodic

protection. At one time, it was suggested that the “ linear ”

portion of the E log i plot should extend over one

decade (or order of magnitude) of current. In recent

years, a two - decade (100 - fold change in current) linear

region has been recommended.

107 – 109

The current requirement from the E log i method is

considered conservative, and leak records seem to

conﬁ rm that idea.

110 – 112

The E log i or Tafel extrapolation method can also

be used to determine the corrosion current. This is dis-

cussed in Chapter 7 , Inspection, Monitoring, and Testing.

Inspection and Monitoring

The continued operation of cathodic protection systems

requires monitoring to insure that the system is working

adequately. Third - party damage, coating degradation

leading to increased current demands, changes in envi-

ronment, and ageing of cathodic protection components

can all cause systems to degrade. There are many inspec-

tion and monitoring methods used in conjunction with

cathodic protection. This section only discusses some of

the more important methods. More complete discus-

sions are available.

78,106 – 118

Inspection and monitoring is

required on at least an annual basis for most pipeline

systems in the United States, and more frequent inter-

vals are sometimes necessary. Rectiﬁ er operations must

be monitored every 2.5 months in the United States.

Potential Surveys The most common means of inspect-

ing a cathodically protected structure is by means of a

potential survey. In any potential survey, it is necessary

to measure the potential of the structure in question

relative to a standard potential. The most commonly

used reference electrode is the saturated copper - copper

sulfate electrode (CSE), which is used onshore and in

freshwater applications. This is shown in Figure 6.76 .

Silver - silver chloride electrodes are used in marine

applications, and the conversion from one standard to

the other is fairly simple. The − 850 mV CSE standard

theoretically becomes − 805 mV with the silver - silver

chloride electrode, but it is usually rounded to − 800 mV.

Zinc is sometimes used as a robust reference anode

for permanently mounted test stations on offshore

structures.

Reference electrodes can degrade and must be main-

tained.

119

It is common for inspectors to carry three

electrodes with them. Two are used and checked against

each other. If they do not produce the same result, they

are then checked against a “ less weathered ” electrode

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CORROSION CONTROL 171

Figure 6.76 Saturated copper - copper sulfate electrode.

Removable

Cap

Transparent

Window

Undissolved

Crystals

Copper Rod

Connection

to Test Lead

Saturated

Copper

Sulfate

Solution

Porous

Ceramic Plug

Figure 6.77 Measurement of pipe - to - soil potential.

Reference

Electrode

High-Impedance

Voltmeter

Connection to

Structure

Soil

0.907

+–

Pipeline

Figure 6.78 Typical at - grade test station.

114

Removable cover

Terminal

block

Lead

block

Housing

To pipeline

Street paving

or 18″ × 18″ × 6″

concrete block

in the hopes that one or the other electrode will still be

in calibration.

In order to measure the potential of a structure, it

must be connected through a high - impedance voltmeter

to a reference cell in direct electrical contact with the

same electrolyte. This is shown in Figure 6.77 .

At one time, it was common for pipeline surveyors

to make electrical connections with the buried pipeline

by driving a pointed rod into the soil over the pipeline.

This caused unnecessary coating damage. It is now more

common to locate test points along the right of way.

These test points are electrical connections to the pipe-

line and allow the surveyor to make electrical connec-

tions to the pipeline without damaging the coating.

A secondary advantage of using test points is that they

are permanent locations and insure that connections

on subsequent surveys will be made at the same loca-

tion. A typical ﬂ ush - mounted test point is shown in

Figure 6.78 .

Test stations of this type are available from most

cathodic protection equipment suppliers. The at - grade

design has the advantage of being less likely to suffer

vandalism or other third - party damage. It is, however,

hard to ﬁ nd and is subject to being covered over by soil

erosion. Above - ground designs are also available. The

minimum spacing for test stations is at the midpoint

between anode locations, the most likely location for

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172 METALLURGY AND CORROSION CONTROL IN OIL AND GAS PRODUCTION

the pipeline potential to be unprotected. These test sta-

tions are often required by regulatory agencies, and

their locations are recorded on maps of the cathodic

protection system. It is possible to instrument these test

points and relay the readings to remote locations.

Test points on cross - country transmission pipeline

are typically located at intervals up to several kilome-

ters (miles) and at cased road crossings, wherever they

cross another utility, and at buried insulated joints.

Pipelines run for long distances, and the most common

surveys are over - the - line close - interval potential surveys

(CIPS) where the surveyors follow the right of way and

make measurements at predetermined intervals. This is

shown in Figure 6.79 . The intervals between readings

can vary but are typically in the hundreds of meters

(yards) for many cross - country pipelines. These surveys

supplement the information obtained from readings at

the test points, which usually are spaced much farther

apart.

A typical pipe - to - soil potential proﬁ le is shown in

Figure 6.80 . Virtually all cathodic potentials are nega-

tive, and it is common to plot the larger negative volt-

ages higher on the vertical axis. As long as the potential

is more negative than − 850 mV CSE, most authorities

will consider the pipeline to be protected. The problem

areas identiﬁ ed by this survey are shown near the center

of Figure 6.79 where the negative voltages are less than

− 850 mV. The problem could be caused by a discon-

nected magnesium anode or by other factors. Once the

unsatisfactory potential survey results are available, it is

usually necessary to inspect the pipeline in these loca-

tions in greater detail to determine the source of the

problem.

Resistivity Surveys Soil resistivity is commonly mea-

sured when planning ICCP ground - bed locations or

when determining the corrosivity of pipeline rights of

way. The most accurate method is the in situ Wenner

four - pin method, which has been the industry standard

for over 50 years. The Wenner method has the advan-

tage of measuring the average soil resistivity at a depth

determined by the pin spacings. Thus, it can measure,

without disturbing the soil, the resistivity near the

surface, frequently high due to drying between rainfalls,

and at the depth of the proposed structure.

The setup for this measurement is shown in Figure

6.81 . Four electrical contact pins are placed in the soil

surface. An AC electrical current is applied between

the outer pins to produce current ﬂ ow through the soil.

The voltage measured between the inner pins is used

to calculate resistivity. The four pins are arranged in a

straight line, and the distance between pins is adjusted

to reﬂ ect the depth of the soil of interest. Once

the measurements have been made, it is easy to

calculate the average resistivity of the soil at the depth

equal to the pin spacing. Adjusting the pin spacing

allows determination of changes in resistivity with

depth.

113,115,116

Figure 6.79 Over - the - line potential survey setup.

118

Electrodes are

placed on surface

directly above pipe

High resistance

voltmeter

First reading

Second reading

Survey

direction

Rear electrode “leap-frogged”

to forward position for each

subsequent reading

Subsequent

voltmeter

locations

+– +– +–

Pipeline

Etc.

Test point wire or other

metallic contact with

pipeline

CuSO

electrode “A”

position 1

CuSO

electrode “B”

position 2

CuSO

electrode “A”

position 3

Known spacing

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CORROSION CONTROL 173

The soil resistivity is calculated from the following

formula:

118,122

ρπ= 2AR

(Eq. 6.9)

where:

ρ = soil resistivity (ohm - centimeters)

A = distance between pins

R = resistance measured with the ohmmeter

Most commercial instruments for measuring resistivity

automatically calculate the resistivity based on pin

spacing and measured resistance.

Changes in resistivity are often indications of changes

in moisture levels. Low - lying riparian areas, often with

more vegetation, are typical examples of where the

resistivity would be lower and expected corrosion rates

would increase. Some locations have widely varying soil

resistivities depending on the time of the year.

It is sometimes desirable to measure the maximum

conductivity (reciprocal of resistivity) by placing the soil

from the appropriate depth into a soil box with four - pin

connections. The four - pin method is then used to deter-

mine the conductivity of the wetted soil. This method

cannot determine the effects of soil compaction on con-

ductivity and is not as reliable as the in situ four - pin

measurements described above.

Figure 6.80 Protective potential proﬁ le indicating a lack of protection near the center

of the plot.

116

–1.5

–1.4

–1.3

–1.2

–1.1

–1.0

–0.9

–0.8

–0.7

–0.6

–0.5

PIPE-TO-SOIL POTENTIAL-V (CSE)

0.1

CD-TP 4

CD-TP 5

CD-TP6-F

CD-TP7-I

CD-TP 8

CD-TP 9

CD-TP 10

CD-MG 3

CD-R 2

12 13 14 15 16 17 18 19 20 21 22 23

Figure 6.81 Wenner four - pin soil resistivity measurement.

113

aaa

Soil Resistance Meter

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174 METALLURGY AND CORROSION CONTROL IN OIL AND GAS PRODUCTION

TABLE 6.27 Recommended Minimum Cathodic

Protection Design Current Densities for Different Soils

117

Soil resistivity ohm.m

Design Current Density

mA/m

(bare steel)

< 10

10 – 100 10

100 – 1000 5

> 1000

Note : A 2.5% increase for each degree C rise above 30 ° C.

TABLE 6.28 Recommended Minimum Cathodic

Protection Design Current Densities for Different Soils

117

Coating Design Current Density mA/m

None 20

Tape wrap 1.25

Coal tar epoxy 0.75

FBE 0.1

Polyethylene 0.1

Single - probe conductivity measurement is also pos-

sible. This is less work but is less accurate; only the

conductivity at the depth of the local probe is deter-

mined at the precise location where the probe is placed.

Cathodic Protection Design Procedures

Most of the following discussion will emphasize design

of cathodic protection systems for pipelines. The

Handbook of Cathodic Protection contains complete

chapters on cathodic protection of ships (applicable to

designs for spar platforms and ﬂ oating production,

storage, and ofﬂ oading [FPSO] structures or vessels ),

marine structures, well casings, water tanks and boilers,

and process equipment.

117

The ﬁ rst step in any cathodic protection design is to

determine the total electrical current demand. For exist-

ing structures, this can be done by measuring the current

necessary to produce the desired potential shift. This is

done by connecting the structure to a temporary DC

power source and varying the current until the neces-

sary polarization, determined by either the E log i

method or by simple measurement of the potential at

remote locations from the temporary anodes. The choice

of method depends on whether or not the remote loca-

tion is accessible for potential measurement. In either

case, it is necessary to wait until changes in the applied

current have produced steady - state potentials before

increasing the current to the next level. This can take

minutes to hours depending on the size of the structure

involved.

New pipeline cathodic protection design is often

based on the current expected to be necessary after 20

years of service. The coating degradation is not expected

to worsen after that period of time. Designs provide

more current than is necessary during the early life of

the system and are intended to last indeﬁ nitely. For this

reason, it is common to overdesign the system, because

it is easier, and is supposedly less expensive, to install a

somewhat larger than necessary system at the beginning

than it is to retroﬁ t at some later date. Most authorities

recommend current densities based on expected bare

metal exposed area. Sometimes the assumption is that

1% of the possible surface area will be exposed, but this

is seen as very conservative for some of the newer pipe-

line coating systems. Tables 6.27 and 6.28 show guide-

lines from several different oil companies ’ published

recommendations for the minimum current densities

necessary for buried pipeline cathodic protection.

117

The

effectiveness of high - quality coatings is obvious from

the reduced current demands shown for FBE and

polyethylene.

Once the current requirements have been identiﬁ ed,

the design procedure then must consider a number of

alternatives based on the choice of anode type. Figure

6.82 is one of a number of recommended design proce-

dures that are available.

The procedures for calculating all of the above design

steps have been standardized for many years. Many of

the necessary formulas, for example, for calculating

ground - bed resistance, are based on work done in the

1930s. While the calculations can be done by hand (as

they were originally), it is more common to do them

using computer software. Most of this software is based

on spreadsheets, and many cathodic protection contrac-

tors have developed their own in - house software for this

purpose. In recent years, several websites have become

available that do these calculations. Figures 6.83 and

6.84 are examples of what is available. As the use of

computers increases, this kind of user - friendly software

will become more common.

For many years, the 1960s ’ book, Control of Pipeline

Corrosion by A. W. Peabody, was considered to be one

of the premier reference materials on cathodic protec-

tion.

103

The book was updated in 2001, and the compact

disc that accompanies the updated book has spreadsheet -

based software included. Figure 6.85 is one example of

the screens used by the compact disc that accompanies

this widely used handbook.

The compact disc also contains sample problems

showing the following calculations:

•

Determining protective current requirements

•

Anode resistance to earth

•

Conventional ground - bed design

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CORROSION CONTROL 175

Figure 6.82 Cathodic Protection Design Procedure. Adapted from NACE International CP

Technology Course Slide5/140.

Sample Cathodic Protection Design Procedure

Start

Evaluate Pipeline

and

Environmental Factors

Determine Current

Requirements to Achieve

Desired Criterion

Calculate Resistance

of Anode or

Groundbed

Calculate Number

and Spacing of

Anodes or Groundbeds

Calculate

System Life

Choose Anode Type,

Size, Weight, and

Arrangement

Galvanic

Galvanic or

Impressed Current?

Is Design

Acceptable?

Yes

Design Complete

Impressed

Current

Calculate Resistance

of Anode or

Groundbed

Choose

Power Supply,

Type, and Rating

Calculate

System Life

Estimate Installed

Cost of System

Choose Anode

Size, Weight, and

Arrangement

Estimate Installed

Cost of System

Figure 6.83 Online screen for calculating galvanic anode life. Image courtesy of Mesa

Products, Inc., http://www.mesaproducts.com and http://www.cpdesigncenter.com/public/cp_

calculators/currentrequirement.htm , May 2, 2010.

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176 METALLURGY AND CORROSION CONTROL IN OIL AND GAS PRODUCTION

Figure 6.84 Online screen for calculating total current requirement for cathodic protection

of pipelines or storage tanks. Image courtesy of Mesa Products, Inc., http://www.mesaproducts.

com and http://www.cpdesigncenter.com/public/cp_calculators/currentrequirement.htm , May

2, 2010.

Figure 6.85 Screen from CD accompanying Peabody ’ s Control of Pipeline Corrosion,

2nd edition.

118

NACE

Companion to the Peabody Book

October 26, 2000

Revision 1.1M

Dwight’s Equation for Single Vertical Anode Resistance to Earth - millimeters

Dwight’s Equation for Single Vertical Anode Resistance to Earth—meters

25.6 ohms

10,000 ohm-cm

2.13 m

0.203 m

203 mm

2134 mm

r =

= Soil resistivity in ohm-cm

= Rod length in mm

L =

d =

= Rod diameter in mm

1.59 r

⎧

⎩

⎧

⎩

–1

= Resistance of vertical rod in ohms

r = Resistivity of backfill material (or earth) in ohm-cm

= Length of anode in meters

= Diameter of anode in meters

= Resistance of one vertical anode to earth in ohms

r =

L =

d =

0.00159 r

⎧

⎩

⎧

⎩

–1

•

Deep anode bed design

•

Cathode resistance to earth

•

Total DC circuit resistance

•

Current attenuation

•

System life of galvanic anode systems

They allow the user to input the data in metric or U.S.

conventional units.

Temperature Effects on Cathodic Protection Chemical

reactions associated with corrosion are highly tempera-

ture dependent. Many design guidelines contain advice

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CORROSION CONTROL 177

on increasing current density for above - ambient tem-

peratures. The consumption rate of anodes depends on

temperature, and this must be considered in cathodic

protection design and replacement scheduling. The

increased consumption rates of anodes can be mini-

mized by using remote anode locations in cooler envi-

ronments, but this leaves some designs more prone to

mechanical damage due to soil movement and other

causes.

Computer - Aided Cathodic Protection Design

Computer - aided cathodic protection designs for off-

shore structures have been tried by several organiza-

tions in recent years. These computer - aided designs are

of two types:

82,119,120

•

Personal computers used to make the types of cal-

culations (such as the wetted surface area calcula-

tions discussed above) that have commonly been

used for cathodic protection design. The computer

is a time - saver in these calculations and allows a

greater number of alternatives to be considered,

but the actual design methodology is not changed.

•

The use of numerical techniques, such as ﬁ nite

element, ﬁ nite difference, or boundary integral, to

model the potential current distribution around a

structure. Initial efforts to use these techniques

found limited acceptance because of the time delay

caused by communications difﬁ culties between the

operator, the cathodic protection designer, and the

computer expert.

The increased memory capabilities of personal comput-

ers now allow design engineers to make calculations

once requiring mainframe computers. Figure 6.86 shows

a sample plot of the cathodic protection on an offshore

platform node. The various color arrangements or shad-

ings allow quick assessment of areas that might be inad-

equately protected.

82,119,120

Comparisons of plots for

different anode arrangements allow the designer to

quickly determine which anode locations are the most

effective and where inspection points should be located

to determine if adequate cathodic protection is being

achieved at high - stressed node welds and other critical

locations.

Additional Topics Related to Cathodic Protection

Some of the problems associated with cathodic protec-

tion include stray current corrosion, hydrogen embrit-

tlement and stress corrosion cracking of high - strength

(or high - hardness) steel, and cathodic disbondment of

coatings. In addition to these well - documented prob-

lems, many authorities have questioned the standards

used over recent decades for determining if a structure

is cathodically protected.

“ Instant - Off ” Potentials In recent years, the biggest

controversy in cathodic protection has been over the

idea of measuring “ instant - off ” potentials to determine

if a structure is protected from corrosion. There are

many publications pro and con on this subject, and the

ideas behind “ instant - off ” potentials are the subject of

continuing debates.

The ﬁ rst advocates of “ instant - off ” potentials cited

the need for accounting for IR drops between the struc-

ture and the electrolyte. This was based on the mistaken

assumption that the − 850 mV CSE potential was the

“ equilibrium potential ” for carbon steel in soil. This is

not the case because:

•

T h e − 850 mV criterion came from leak records and

measurements of “ current - on potentials ” on cath-

odically protected pipelines, primarily on the Gulf

Coast of the United States. R. Kuhn from Louisiana

was the most prominent early advocate of this idea,

and he based his reasoning on leak records that

showed that structures held at potentials at least as

negative as − 850 mV CSE had much lower leak

records than unprotected steel. This standard was

considered acceptable for most situations, although

a − 950 mV criterion was generally recommended

when microbial activity was likely.

76,77,79,85

Figure 6.86 Computerized model of the cathodically pro-

tected region around a node on an offshore platform.

126,127

Reprinted with permission of ASM International. All rights

reserved, http://www.asminternational.org .

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178 METALLURGY AND CORROSION CONTROL IN OIL AND GAS PRODUCTION

Figure 6.87 Idealized instant - off potential plot.

IR Error

–1200

–1000

–800

CP On

CP Off

Switch Off

Time (milliseconds)

0 100 500 1000 1500 2000

–600

“Instant Off” Potential

Depolarization

Pipe / Soil Potential (V SCE)

•

T h e − 850 mV criterion is at a lower (smaller) nega-

tive number than the equilibrium potential. This

was shown in Figure 6.64 where the equilibrium

potential is shown at − 950 mV in an acid environ-

ment. The equilibrium potential will vary with pH

in accordance with the Nernst equation discussed

in earlier chapters, but the equilibrium potential

will always be at a greater negative potential than

the − 850 CSE protection potential used for cathodic

protection. As stated previously, cathodic protec-

tion does not eliminate oxidation or corrosion on

a protected structure, but it has been shown to sig-

niﬁ cantly reduce corrosion.

In the decades after the NACE RP 0169 standard was

adopted, many authors discussed errors in potential

measurement, but these errors were considered to be

insigniﬁ cant in most cases.

The IR drops that were originally considered to be

insigniﬁ cant for bitumastically coated pipelines pro-

tected with galvanic anodes were questioned for

impressed current systems and for measurements

directly over galvanic anodes.

108

This led to the develop-

ment of NACE RP 0169 (92) which mandated that IR

drops must be compensated for using an “ instant - off ”

criterion. Difﬁ culties in deﬁ ning how this “ instant off ”

should be done, and questions on whether or not this

“ instant - off ” potential is necessary, continue as of this

writing. Since the requirement to determine an “ instant -

off ” negative potential means that the negative “ current -

on ” potential will be greater than the negative

“ instant - off ” potential, the requirement is conservative.

It is, nonetheless, questioned by many and is unpopular

with ﬁ eld personnel.

Figure 6.87 illustrates the concept of the “ instant - off ”

method of determining IR errors in cathodic protection.

When the cathodic protection current is applied, the

structure assumes a potential which is intended to

reduce corrosion. In order to measure the potential, a

voltmeter is connected to the system and the current - on

potential is measured. The measured potential includes

the potential of the structure and any current - resistance

(IR) drops in the circuit, for example, the reference

electrode - electrolyte IR drop and the structure -

electrolyte IR drop . By turning off the current, the

voltage supposedly “ instantaneously ” drops to the

potential of the structure in the electrolyte. The poten-

tial then decays to the unprotected potential, a process

that can take anywhere from minutes to days. Proponents

of the “ instant - off ” method suggest that the “ instant -

off ” potential must satisfy whatever criterion, either

− 850 mV CSE or − 100 mV potential shift, is used for the

system. The IR drop can be as large as 1 V in some

instances.

Proponents cite electrochemistry textbooks which

identify several IR drops in an electrochemical circuit

and the supposed failures of the “ current - on ” criterion

that had been in use for several decades. Opponents of

this concept argue that measuring the potential with the

current applied has worked for decades. The original

“ current - on ” criterion was developed for onshore pipe-

lines along the Gulf Coast of the United States, a region

where soils generally have low resistance. The IR drop

of concern is more likely to be of concern with high -

resistivity environments. The length of time (from

microseconds to several seconds) of the “ instant decay ”

depends, at least in part, on the environment and must

be empirically determined.

108,121,122

Stray Current Corrosion Caused by Cathodic Protection

Systems The potential ﬁ eld around a cathodically pro-

tected structure can cause stray current corrosion on

nearby structures. This is a signiﬁ cant problem with

ICCP systems that share rights of way with nearby

structures/utility systems. Galvanic anodes, with their

much lower driving potentials, are unlikely to cause this

problem.

Figure 6.88 shows two pipelines crossing each other.

The cathodic protection system is causing corrosion

where current from the cathodic protection system

leaves the unprotected line.

Stray or interference current is detected by turning

the cathodic protection system on and off and monitor-

ing the potential of the unprotected line. If the potential

of the unprotected line varies with the cycle of turning

the cathodic protection system on and off, then stray

current corrosion will occur.

Stray current corrosion can usually be handled by

bonding a short section of the unprotected line to the

protected line and using current from the cathodic pro-

tection system to protect both structures. “ Hot spot ”

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