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MetriCorr ApS – Produktionsvej 2 – DK 2600 Glostrup - Denmark Tel. +45 72 177 410 – Fax +45 72 177 216 - [email protected] – www.metricorr.dk
MetriCorrMetriCorr
2006.25.04 - 0 DGC.FAUGT.10
AC / DC interference corrosion in pipelines
Summary Report
Distribution: Participants Report: Folke Galsgaard, Lars Vendelbo Nielsen Review: Peter Cohn, Bent Baumgarten Date: 25-4-2006
Prepared by MetriCorr for
The Danish Gas Technological Centre.
1
Summary and Conclusions
This report gives an overall view of activities performed in the period June 2004 to February 2006
within the project AC/DC interference corrosion in pipelines, sponsored by the Danish Gas
Technological Centre – DGC.
Participants in the project have included the Danish natural gas distribution and transmission
companies, Balslev consulting engineers, and MetriCorr. The work has included laboratory as well
as field investigations along pipelines owned by Gastra (now Energinet.dk), Greater Copenhagen
Natural Gas (HNG) and Naturgas Midt/Nord.
An executive group has bee responsible for the progress of the work. This group consisted of:
Mr. Bent Baumgarten (Greater Copenhagen Natural Gas, HNG)
Mr. Peter Cohn, Gastra – Energinet.dk
Mr. Per B. Sørensen (Naturgas Midt-Nord)
Mr. Dan B. Jensen (Naturgas Fyn)
Mr. Henrik Rosenberg (Balslev Consulting Engineers)
Mr. Lars Vendelbo Nielsen (MetriCorr).
The original project description as approved by DGC has been included in annex 1.
The individual activities within the project are all continuations from previous research activities
within this field conducted by the participants – in particular Gastra and HNG along with their
consultants, Balslev and MetriCorr. These activities have continuously been presented at
conferences, primarily CeoCor and NACE.
Rather than going into great details in discussing results from the project, this report seeks to provide
an overview of the problems encountered on AC/DC interference corrosion, to present state-of-the-
art and to pin point the results from the project in relation to this. Furthermore, the report seeks to
point out investigations worth considering in the further efforts on interference corrosion. As
continuations from this present project, a GERG project has been proposed, and a CeoCor working
group on coupon technology has been proposed.
2
A short introduction to relevant corrosion mechanisms is given in section 1. AC influenced corrosion
can be shown to involve the formation of very high pH at a coating defect combined with the
potential vibration caused by superimposed AC. Since cathodic protection (CP) increases the pH at a
coating defect, it is essential not to apply the CP in excessive amounts as the AC corrosion risk will
increase. This is in contradiction to DC interference corrosion, which will require an extra amount of
CP, and a conflict therefore seems to exist between CP requirements for AC corrosion mitigation and
CP requirements for mitigation of corrosion caused by DC stray currents.
A short description of relevant measuring techniques has been given in the report section 2. Section 3
constitutes a line up of the activities included in the work.
An overall discussion on observations (field and laboratory) on AC influenced corrosion is presented
in section 4, and the AC corrosion mechanisms are discussed based on the results achieved. Clear
correlations have been established between CP current density and the spread resistance of the
coating defect. Above a threshold level, the CP (DC) current density and consequent pH increase
results in a significant lowering of the spread resistance. In turn, the lowering of the spread resistance
results in an increase of the AC current density, which combined with a pH increase and potential
vibration results in increased corrosion rate. Under such conditions, corrosion rates of several
millimeters per year can be measured on coupons. In field and in lab, it has been demonstrated that
corrosion can be reduced or even effectively stopped by reducing the CP level. In other words, the
results clearly show an unintended detrimental effect of excessive CP under such circumstances.
A single test post at which low AC voltages levels but high DC interference levels occur has received
special attention, and results from this station are presented and discussed in section 5. In a particular
study, the cathodic protection level of the pipeline (rectifier current output) was gradually lowered in
steps of one week, and the corrosion rate on coupons were measured and compared with the
electrical parameters describing the CP level and DC interference condition. An increased corrosion
was quantified by the coupons a quite accurate correlation with the amount of current escaping the
coupons under anodic interference peaks could be observed. This study clearly shows that under DC
interference conditions, it is wise to maintaining a certain level of CP.
Pipelines experiencing both AC and DC interference constitute a dilemma when it comes to
optimized CP. The survey methods applied for AC and DC interference corrosion respectively have
been used on a pipeline experiencing both AC and DC interference. The interference patterns have
3
been clarified and compared with the corrosion rate of the coupons. The results are discussed in
section 6. It seemed to be impossible to dose a suitable amount of CP in this case. The peaks in
cathodic current density seemed to be severe enough to create a critically high pH, which combined
with the AC caused potential vibration creates a classic AC corrosion case with very high corrosion
rates. For the purpose of protecting against the anodic DC current peaks a certain level of CP should
be applied, but in such severe interference cases, the actual level of CP seemed to be less important,
the corrosion proceeds anyway. These experiences lead to further investigations and measurements
of line currents at several positions along the pipeline to establish the location of the source for the
DC interference and consequent qualified dialog with the local traction system operators in order to
identify and minimize the interference at its very source.
In section 7, the effect and role in AC corrosion of selected parameters have been discussed in brief
terms. The parameters include both electrical parameters like AC voltage and – current density, DC
potentials and –current density, and spread resistance, as well as physico-chemical parameters like
conductivity/soil resistivity, pH, presence of ground water, soil texture and selected chemical
substances. The discussion is made in view of the results obtained in the present project, as well as in
previous projects and from outside the Danish sector. The points are discussed in view of the recently
published CEN/TC 15280 document and AC corrosion likelihood. Among conclusions are that a
coupon DC current density and spread resistance combined with the pipeline AC voltage seems to be
the best indicators of the corrosion risk.
The CEN/TS 15280 document recommends to a large degree the use of coupons for quantification of
the various parameters that are used for assessment of the AC corrosion likelihood. For this reason, it
has been a Danish viewpoint in the ad hoc group that formed the document that techniques that could
also quantify the extent of corrosion on the coupon should be implemented in the document.
The whole idea of using coupons for the risk analysis seems today to be the best suited way (or
perhaps the only option), although one may question the extent to which a coating fault simulated in
a coupon represents an actual coating fault at the pipeline surface. The question could also be turned
upside down, in the sense that one could ask why a coupon should not represent a coating fault, as
long as a careful registration of the environment in which the coupon is placed is made along with
appropriate electrical descriptions of the coupon behavior. One could further ask what is the
difference in behavior of two different coating defects. Today, a NACE recommended practice
(RP0104-2004) on use of coupons for CP monitoring purposes exists, but it is not particularly
focusing on nor necessarily applicable for aspects relating the assessments of AC/DC interference
4
corrosion. In the CEN standards, the use of coupons is widespread, but no official document deals in
details with the possibilities and/or limitations when using coupons.
For the above reasons it has been suggested that a new working group activity is initiated in CeoCor
sector A, dealing with the issue, and a proposal for a GERG project which include activities on
coupon research has been forwarded through the Danish Gas Technological Centre.
Other interesting spin offs from the project has included some premature ideas on optimized cathodic
protection by closer CP control and thoughts on what could be called “dynamic corrosion control”. A
rectifier system, which measures the off-potential frequently throughout the day and adjusts the
rectifier current output accordingly to meet the pre-programmed CP requirement, is now operating in
HNG pipelines. It has been observed here that adjustments within 200 mV in the on-potential are
sometimes necessary within just a 24h period in order to keep a constant off-potential.
New approaches to effective CP under interference conditions could be born within the above
frames, for instance by quantifying the interference patterns continuously and dosing CP
accordingly. If high DC interference is present during daytime and high AC is present at nighttime
(caused by export of power for instance), the CP requirements in those two situations are quite
different.
5
Content Summary and Conclusions ....................................................................................................................1
Content...................................................................................................................................................5
1. Corrosion mechanisms.......................................................................................................................6
2. Measuring techniques – corrosion rate ..............................................................................................9
3. Activity line-up ................................................................................................................................10
4. Observations on AC corrosion.........................................................................................................12
5. Observations on corrosion caused by DC stray currents .................................................................17
6. Observations on corrosion under combined AC/DC interference ...................................................22
7. Evaluation of selected critical parameters .......................................................................................28
7.1 Electrical parameters..................................................................................................................28 7.1.1 AC voltage, AC current density and Spread Resistance.....................................................28 7.1.2 DC potential and DC current density..................................................................................30
7.2 Fysico-chemical parameters.......................................................................................................30 7.2.1 Conductivity / resistivity.....................................................................................................30 7.2.2 Size of the coating defect ....................................................................................................32 7.2.3 pH........................................................................................................................................32 7.2.4 Humidity – ground water level ...........................................................................................32 7.2.5 Depth of the coupon or pipeline coating defect ..................................................................33 7.2.6 Other chemical factors of the soil .......................................................................................34 7.2.7 Soil texture ..........................................................................................................................34
8. Summary and conclusions ...............................................................................................................36
9. Perspectives......................................................................................................................................37
10. References......................................................................................................................................39
Appendix 1. Description of project and activities ...............................................................................41
Appendix 2. Papers and presentations – now and then........................................................................44
6
1. Corrosion mechanisms Part of the purpose of this project has been clarifying the mechanisms of AC influenced corrosion.
Corrosion caused by DC stray current seems quit straight forward, the corrosion occurs during the
anodic peaks of the interference pattern. Corrosion caused by AC interference may have several
faces, however, it seems now that one mechanism is particularly important; namely the alkalization
mechanism.
When cathodically protected, the pipeline receives a current from the rectifier / anode bed system,
which lowers the potential of the pipeline and ideally keeps it within the immunity region of the
Pourbaix-diagram (figure 1). The cathodic protection can be disturbed by interfering DC stray
currents entering and exiting the pipeline. Where the current enters the pipeline, the potential will be
further lowered (more cathodic), i.e. the interference adds to the CP current. Where the current exits
the pipeline, the potential will be increased (more anodic), i.e. the corrosion risk increases. In case
the pipeline is not sufficiently protected, the potential at the point where DC stray current exits the
pipe may enter the active region in the Pourbaix diagram, and cause corrosion. Accordingly, the
standards (e.g. EN 501621) prescribe that corrosion as a consequence of DC stray current can be
avoided by adding a surplus of CP.
Figure 1. Potential-pH (Pourbaix) diagram for iron (steel) in water.
-1,500
-1,000
-0,500
0,000
0,500
1,000
1,500
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
pH
E (
V v
s S
CE
)
Fe
Fe2+
Fe3+
Fe3O4
Fe2O3
HFeO2-
Domain favoring
corrosion
Hydrogen
equilibrium line
Domain favoring
immunity
Oxygen
equilibrium line
Domain favoring
corrosion
Domain favoring
passivity
Hypothetical line
illustrating critical
pitting potential
7
Intuitively, the same strategy has been applied for protection against corrosion caused by AC
interference, and in case stories on AC corrosion, authors have often stressed out that they made sure
that the CP system was particularly well operating, and that the pH at the coating fault has been high.
Today, it is known and particularly shown in this project, that this strategy has been fatal. The
plausible explanation is that the mechanism of AC influenced corrosion involves an alkalization of
the environment close to the coating defect.
The Pourbaix-diagram (figure 1) is a diagram based on
theoretical calculations, however, its feasibility and
applicability has been proved in numerous practical cases.
The calculations are based on thermodynamical data for the
electrochemical reactions presented in the box (here to the
left). The reaction (1) states that iron in the solid state
oxidizes and dissolves – which is equal to a corrosion
process. This is also the case for the reaction (2), while the
process (5) indicates transition from one dissolved stage to
another. The equilibrium potential for these three processes -
calculated as a function of pH – dictates the positions of the
two domains in the Pourbaix-diagram where corrosion is
facilitated. As can be observed from the diagram, the steel
can be protected from corrosion – kept in the immunity area
– if the potential is drawn in the negative (cathodic) Under
the influence of AC interference, the potential will vibrate or
oscillate, with the same frequency (50 Hz) as the frequency of the AC interference. This means that
during the positive half wave of this oscillation, the potential might cross the borderline between
immunity and corrosion. The question is then if the electrochemical reactions are fast enough to
occur within those 10 milliseconds, or if 50 Hz potential vibration is so fast that the corrosion risk
from the AC interference is practically absent. Exactly based on this, the traditional way of thinking
– add a surplus of CP to prevent corrosion when AC is present – is fairly plausible. Experience and
research has shown, though, that a dangerous element shall be found in the small area at high pH
where corrosion also occurs due to reaction (2). The very high pH required here does not exist in any
natural environment, and must be created locally in the coating fault in order to constitute a risk.
Under cathodic protection, the pH will be raised locally in the coating defect by reaction (10) in the
above box.
Fe = Fe2+ + 2e- (1)
Fe + 2H2O = HFeO2- + 3H+ + 2e- (2)
2Fe + 3H2O = Fe2O3 + 6H+ + 6e- (3)
3Fe + 4H2O = Fe3O4 + 8H+ + 8e- (4)
Fe2+ = Fe3+ + 2e- (5)
2Fe2+ + 3H2O = Fe2O3 + 6H+ + 2e- (6)
3Fe2+ + 4H2O = Fe3O4 + 8H+ + 2e- (7)
2Fe3O4 + H2O = 3Fe2O3 + 2H+ + 2e- (8)
2H+ + 2e- = H2 (9)
2H2O + 2e- = H2 + 2OH- (10)
O2 + 2H2O + 4e- = 4OH- (11)
O2 + 4H+ + 4e- = 2H2O (12)
8
H2O
OH-
Pipe surface
OH-
Influx
(IDC)
Out-flux
(Diffusion – texture)Accumulation
(pH increase)
OH- neutralisation
(BNE)
Time to reach
critically high pH value
=
INCUBATION PERIOD
Figure 2. Illustration of a pH increase at a cathodically protected surface. Figure 2 accordingly illustrates a mass balance at a coating defect. The CP current produces OH- ions
(alkalinity), which increase pH. If the production rate is modest (small influx) or if nothing is
blocking for the transport of OH- away from the surface (out-flux) then the accumulation of OH- is
small and the pH increase will be low. Additionally, the situation can be harmless if the soil
chemistry can react with the produced OH- then neutralizing the effect (base neutralizing effect
BNE).
In case the rate of production of OH- increases due to increased CP, then accumulation may or will
occur locally at the coating defect, and pH increases. When sufficiently high, the combination with a
vibrating potential, may lead to periodic entering the high-pH corrosion domain in the Pournax
diagram. In case this mechanism is active, there seems to be some characteristic features to be
expected:
• It is difficult or impossible to provoke AC corrosion in pure aqueous environment without
solid particles acting as diffusion barriers.
• One can expect a certain incubation period due to the time needed to achieve the critically
high pH.
• The corrosion will increase when CP current is increased.
9
These features have been proved in practice, and summarized, the net consequence is that the
precautions taken to protect against corrosion resulting from interference from DC stray current
(increased CP) is directly conflicting with one of the precautions that can be taken to protect against
corrosion resulting from AC interference. In this context, the question is where the limits are for AC
and DC interference before corrosion occurs, and how can one monitor in practice that the corrosion
risk is kept under proper control.
2. Measuring techniques – corrosion rate
The recently published Technical Specification CEN/TS 152802 on evaluation of AC corrosion
likelihood, the use of coupons is recommended as an evaluation tool. These are buried next to the
pipeline and electrically coupled hereto. The coupon includes an artificial coating defect of a certain
size, e.g. 1 cm2 which is anticipated thereafter to experience the same chemical and electrical
conditions as a similar coating defect in the real pipeline coating. The coupon is used for measuring
current densities (DC and AC), off-potentials etc, which can be compared with the pipeline AC
voltage and DC on-potential. Further, the spread resistance (ohmic resistance between metal and
remote earth) can be measured.
Rectifier Transformer
CSE
Coupon
Counter
Soil box
Datalogger system
• AC voltage• AC current• Spread resistance• Eon• DC current• Corrosion
Corrosion coupons(simulated coating fault)
Test post
Reference
Field application
Lab application
Pipe
Figure 3. Datalogger system with ER coupon used for both field and laboratory investigations.
10
Throughout the last decade, it has increasingly become a common practice in the Danish sector to
make use of coupons, which additionally to the traditional electrical characterizations have had the
feature of being able to tell the corrosion state and corrosion rate using the so-called ER (electrical
resistance) technique. The basic principles of the method has been described in other papers.4-7 All
results described in this report have been conducted with ER coupons with datalogger system picking
up all corrosion- and associated electrical data. Figure 4 is an example of graphical illustration of the
corrosion rate throughout time.
0
100
200
300
400
500
600
700
800
900
1000
08-Jan-05 15-Jan-05 22-Jan-05 29-Jan-05 05-Feb-05 12-Feb-05 19-Feb-05 26-Feb-05 05-Mar-05
Exposure time -
Date
Co
rro
sio
n r
ate
- V
CO
RR
mym
/yr
Corrosion RateCombined AC and DC stray
Figure 4. Graphical presentation of corrosion rate throughout time picked up by the ER coupon
datalogger system.
3. Activity line-up
Laboratory:
1. Establishment of soil boxes with inert pure quartz sand and electrolyte pore solution. Boxes
were equipped with electrode system (large inert counter electrode mesh made from
platinized titanium, CSE reference electrode and an ER coupon as the working electrode), a
BAC rectifier type 1201, and an AC transformer system; figure 3. An attached datalogger
system (MetriCorr interference corrosion logger) measured and stored the AC and DC
parameters and corrosion of the coupon.
2. Performed series of experiments, each of which of three weeks duration. During each
experiment, the corrosion rate and electrical data were picked up. The AC voltage and DC
on-potential was kept constant throughout the entire period. 15 V AC was chosen for all 6
11
experiments, whereas 6 different on-potentials were chosen in order to study exclusively the
effect of DC on corrosion rate and controlling parameters.
3. Performed series of experiments where the DC on-potential has been kept constant at a value
determined under activity 2) to be a critical value. This condition was then superimposed with
15 V AC in order to create corrosion. From this critical corrosion situation, the AC voltage
was decreased to 10 V and later 4 V to study if corrosion could be stopped.
4. Performed preliminary tests in the laboratory on the effects of DC stray currents. These tests
were later released by actual field measurements, since it became clear that further
knowledge and experiences were needed in relation to the DC pattern during DC interference,
and in order to gain some experience on which environment is critical.
Field investigations:
5. Performed investigations of the applicability of a range of different techniques for
measurements on coupons. These included basic multimeter measurements, DONGlog
measurements and Ramlog measurements.
6. Performed investigations of the soil resistivity as a function of depth at selected locations.
7. Based on the above, an electronic template file for measurements on coupons was
established.
8. A number of approximately 20 locations along the Danish gas grid were picked out and sets
of two ER coupons established at each location. The individual gas companies then used the
template file and associated guidelines to follow corrosion conditions etc. throughout an
extended period of time (+1 year) by spot wise measurements.
9. Based on the spot wise measurements, locations showing particularly problematic or
interesting character were picked out for establishments of loggers and modems for
continuous monitoring and special investigations.
12
4. Observations on AC corrosion
The AC current density through a coating fault probably has a large influence on the corrosion
condition and therefore seems to be an obvious fingerprint. This AC current density is simply
approximated as the AC voltage divided by the spread resistance of the coating defect, which is why
one may reduce the AC corrosion risk by lowering the pipeline AC voltage.
Spread resistance is an equally important parameter, since for a given AC voltage this will be
determining the AC current density. The spread resistance depends on the geometry and size of the
coating fault, as well as on the chemistry existing directly in the close proximity of the coating fault.
The soil chemistry is therefore of great importance as is the humidity around the coating defect.
Since the cathodic protection current causes electrochemical reactions to take place in the coating
defect, and according that ions may be produced directly there, the nature and velocity of these
electrochemical reactions are also of very great importance. The close chemistry may totally be
controlled from this electrolysis, and therefore the spread resistance is very likely to be entirely
controlled by this. As already mentioned in section 1, OH- ions (alkalinity) will be produced at the
surface by the CP current, thereby increasing the pH and lowering the spread resistance. This effect
has been experienced in all of the cases (in this study) where AC influenced corrosion has been
detected on a coupon. Figure 5 illustrates how laboratory experiments have shown that above a
certain limiting value for the DC current density (in this case around 8 A/m2) the spread resistance
will decrease since OH- starts accumulating at the surface. Figure 6 illustrates the consequence
hereof on the AC current density, which increases accordingly despite the AC voltage is constantly
15 V.
Figure 5. Correlation between cathodic protection current density and spread resistance in a laboratory experiment – soil box with inert sand and a dilute sodium chloride solution. Note that increased CP corresponds to a more negative current.
Figure 6. Correlation between AC and DC current density in the same experiment.
0.00
0.02
0.04
0.06
0.08
0.10
0.12
0.14
-20 -15 -10 -5 0
Idc - A/m2
Rs -
ohm
.m2
0
50
100
150
200
250
300
-20 -15 -10 -5 0
Idc - A/m2
Iac -
A/m
2
13
Figure 7 shows the corrosion rate in the same experiment as a function of the AC current density. It
can be described as a function of the AC current density (as expected), but due to the influence of
DC current on spread resistance hence on AC current density, the corrosion rate can just as well be
described equally well as a function of the DC current density (figure 8).
Figure 7. Correlation between AC current density and corrosion rate.
Figure 8. Correlation between DC current density and corrosion rate.
The exact same correlations can be established for results obtained in another environment – figures
9-14. This environment is inert sand + dilute (0.005M) NaOH solution. The AC voltage is
maintained in all experiments at 15 V, and 6 different DC on-potentials has been applied for periods
of htree weeks. The development in all parameters were logged continuously and shown in the
graphs. Figure 9 shows the influence of DC current density on the spread resistance, figure 10
shows accordingly the correlation between AC and DC current density, 11 and 12 show correlations
between corrosion rate and AC quantities, whereas figure 13 and 14 show the correlation between
corrosion rate and DC quantities. Note particularly from figure 13, that the more negative potential,
the worse the development in corrosion rate.
0.00
0.02
0.04
0.06
0.080.10
0.12
0.14
0.16
0.18
0.20
-14 -12 -10 -8 -6 -4 -2 0 2
Idc - A/m2
Rs -
oh
m.m
2
-850 mV DC -950 mV DC -1100 mV DC
-1200 mV DC -1250 mV DC -1300 mV DC
0
100
200
300
400
500
600
-14 -12 -10 -8 -6 -4 -2 0 2
Idc - A/m2
Iac -
A/m
2
-850 mV DC -950 mV DC -1100 mV DC
-1200 mV DC -1250 mV DC -1300 mV DC
Figure 9. Effect of DC current density on spread resistance.
Figure 10. Correlation between DC and AC current density.
0
20
40
60
80
100
120
140
-20 -15 -10 -5 0
Idc - A/m2V
corr
- m
ym
/yr
0
20
40
60
80
100
120
140
0 50 100 150 200 250 300
Iac - A/m2
Vcorr
- m
ym
/yr
14
0
50
100
150
200
250
0 5 10 15 20
Uac - V
Vco
rr -
mym
/yr
-850 mV DC -950 mV DC -1100 mV DC
-1200 mV DC -1250 mV DC -1300 mV DC
0
50
100
150
200
250
0 100 200 300 400 500 600
Iac - A/m2
Vcorr
- m
ym
/yr
-850 mV DC -950 mV DC -1100 mV DC
-1200 mV DC -1250 mV DC -1300 mV DC
Figure 11. Corrosion rate versus AC voltage. Figure 12. Corrosion rate versus AC current density.
0
50
100
150
200
250
-1400 -1200 -1000 -800 -600
Eon - V CSE
Vco
rr -
mym
/yr
-850 mV DC -950 mV DC -1100 mV DC
-1200 mV DC -1250 mV DC -1300 mV DC
0
50
100
150
200
250
-14 -12 -10 -8 -6 -4 -2 0 2
Idc - A/m2
Vcorr
- m
ym
/yr
-850 mV DC -950 mV DC -1100 mV DC
-1200 mV DC -1250 mV DC -1300 mV DC
Figure 13. Corrosion rate versus on-potential. Figure 14. Corrosion rate versus DC current density.
Equally clear indications of the correlations have been shown for field measurements; figures 15-17.
Figure 15 shows the correlation between AC current density and corrosion rate, while figure 16
shows the same correlation with DC current density.
Figure 15. Correlation between corrosion rate and AC current density – coupon exposed at M/R Ølstykke.
Figure 16. Correlation between corrosion rate and DC current density – coupon exposed at M/R Ølstykke.
0
500
1000
1500
2000
2500
3000
3500
0 50 100 150 200 250 300
Iac - A/m2
Vcorr
- m
ym
/yr
0
500
1000
1500
2000
2500
3000
3500
-12 -10 -8 -6 -4 -2 0
Idc - A/m2
Vcorr
- m
ym
/yr
15
Another example is shown in figure 17. A constant 15 V AC combined with different levels of
rectifier output. At first, the DC on-potential was adjusted to -1150 mV CSE (lower left). This caused
corrosion to occur (upper left). Lower right graph shows the DC current density through. When
corrosion rate increased above 500 micron/yr, the DC potential was adjusted to -850 mV CSE, and
the corrosion stops. The AC current density decreases as well – although the AC voltage was 15 V
constantly, indicating an increase in spread resistance.
0
100
200
300
400
500
600
700
800
900
1000
13-May 02-Jun 22-Jun 12-Jul 01-Aug 21-Aug
Date (2004)
Co
rro
sio
n r
ate
- m
icro
ns/y
r
Corrosion Rate Data
10
100
1000
13-May 02-Jun 22-Jun 12-Jul 01-Aug 21-Aug
Date (2004)
AC
Cu
rre
nt
de
nsity -
A/m
2
AC Current Data
-1.2
-1.1
-1.0
-0.9
-0.8
-0.7
-0.6
13-May 02-Jun 22-Jun 12-Jul 01-Aug 21-Aug
Date (2004)
E
- V
CS
E
ON
OFF
DC Potential Data
0.001
0.01
0.1
1
10
23-May 02-Jun 12-Jun 22-Jun 02-Jul 12-Jul 22-Jul 01-Aug 11-Aug
Date (2004)
Cath
ode D
C d
ensity (
A/m
2)
Direct Cathodic Current Data
Figure 17. Example of elimination of the corrosion when cathodic protection level is lowered. Upper
left: Corrosion rate throughout time. Lower left: DC-on potential, upper right: AC current density,
lower right: DC current density.
The AC voltage of course also has an impact on the corrosion rate. Figures 18 and 19 give some
indication of this. These are results from laboratory investigations in the diluted 0.005 M NaOH –
quartz sand soil box assembly. The corrosion is build up at -1500 mV CSE with 15 V AC. Lowering
the AC to 10 V decreases the corrosion without stopping it (figure 18) whereas the same scenario,
but lowering the AC voltage to 4 volts effectively stops corrosion in this environment.
16
Figure 18. Corrosion builds up at -1500 mV CSE DC potential superimposed by 15 V AC. When AC
is lowered to 10 V, the corrosion decreases but is not eliminated.
Figure 19. Corrosion builds up at -1500 mV CSE DC potential superimposed by 15 V AC. When AC
is lowered to 4 V, the corrosion is eliminated..
Corrosion build up at 15 V ACallowed to relax at 4 V ACDC = -1500 mV (on) vs CSE
0
50
100
150
200
250
300
350
400
450
500
0 24 48 72 96 120 144 168 192
Time (hours)
Vcorr
- m
ym
/yr
0
2
4
6
8
10
12
14
16
AC
Volta
ge (V
)
Corrosion rate based on 12 hour trend
Based on 2 hour trend
AC voltage
Corrosion Rate
Corrosion build up at 15 V ACReduced to 10 V ACDC = -1500 mV (on) vs CSE
0
100
200
300
400
500
600
700
0 24 48 72 96Time (hours)
Vco
rr -
mym
/yr
0
2
4
6
8
10
12
14
16A
C V
olta
ge
(V)
Corrosion rate based on 12 hour trend
Based on 2 hour trend
AC voltage
Corrosion Rate
17
5. Observations on corrosion caused by DC stray currents
It is entirely a different situation when the corrosion risk is attributed to DC stray current alone. A
field location (HNG / KB-1) with DC stray current activity (and no AC interference) had been
chosen from the preliminary spot wise measurements and the existing ER coupons used to make a
quantification of the risk of DC stray current corrosion in the existing soil environment.
At this location, intensified measurements of the corrosion rate was established and the datalogger
was programmed to acquire DC potential and –current every 5 seconds in 5 different 20 minutes
periods throughout the day (02:00h, 07:00h, 11:00h, 15:00h and 19:00h).
For each of these 20-minute periods, the DC potential and –current can be plotted against time
(figure 20). From the plot (in practice in a spreadsheet) the maximum, minimum, and average values
as well as RMS and standard deviation were determined. In the same spreadsheet, the positive and
negative current measurements can be identified and directed to separate columns in the sheet, and
based on this procedure, the anodic charge and the cathodic charge for each period of 20 minutes can
be calculated.
Probe-Tag: Bagsværd
Probe Type: PA-0.4-10-0.1-6
Probe serial No.: PA04270009
Test initiated: 29-08-2005
-2.00
-1.50
-1.00
-0.50
0.00
0.50
1.00
1.50
04/10 14:55 04/10 15:00 04/10 15:05 04/10 15:10 04/10 15:15 04/10 15:20
Time
Idc (
A/m
2)
-2.000
-1.800
-1.600
-1.400
-1.200
-1.000
-0.800
-0.600
Eo
n (V
CS
E)
Idc 1 (A/m2)
Eon (V)DC Data
Figure 20. Example of a DC potential and –current plot throughout one period of 20 minutes around
15:00h.
18
From figure 20 is observed that both anodic and cathodic currents are present throughout this period.
The DC potential fluctuates over a wide range both less and more negative relative to the -850 mV
CSE cathodic protection criterion.
A full scale esperiment with the KB-1 pipeline was initiated. The initial rectifier output current
(around 50 mA) was gradually decreased once a week, and the corrosion rate and the intense 20
minutes DC current and potential measurements were realized at these mentioned 5 periods a day.
Figure 21 shows the rectifier current output (dotted line) throughout a 5 week test period. Initial
value 50 mA was decreased to 25 mA after a week, 10 mA after another week, then stepwise back to
the 50 mA. The maximum, minimum and mean potential resulting from the 5 daily intense
measurements are shown in the figure as well.
Probe PA04270009
-1.800
-1.600
-1.400
-1.200
-1.000
-0.800
-0.600
-0.400
-0.200
0.000
29/08/05 05/09/05 12/09/05 19/09/05 26/09/05 03/10/05
Date
Pote
ntial (V
/CS
E)
0
10
20
30
40
50
60
Rectifie
r outp
ut (m
A)
Mean potential
Max potential
Min potential
Rectifier level
Figure 21. Rectifier output and max/min/mean potential detected during experiment with different
rectifier output under DC stray current conditions.
19
Probe PA04270009
0
5
10
15
20
25
30
35
29/08/05 05/09/05 12/09/05 19/09/05 26/09/05 03/10/05
Date
Pote
ntial R
MS
no
ise (
%)
0
10
20
30
40
50
60R
ectifie
r outp
ut (m
A)
Pot noise
Rectifier level
05
1015202530
14/09/05 15/09/05 16/09/05 17/09/05
Date
0102030405060
Figure 22. Potential RMS noise throughout the experimental period. The small graph illustrates the
noise measured in the 5 daily periods for three following days, just to illustrate the low noise at night
and increasing noise during daytime with traffic etc. Apparently, the noise is highest (in these
periods) during the afternoon (black indications).
Figure 23 illustrates the resulting charge calculations. The lower figure is the cathodic charge
released whereas the upper figure illustrates the anodic charge release. In the upper figure, the
corrosion rate measured on an ER coupon is illustrated as well, showing a very good correlation with
the anodic charge release.
Figure 24 shows the correlation between anodic charge (converted into an average anodic current
density) and corrosion rate. The theoretical factor between these quantities in case all anodic charge
causes corrosion is 1.16.
At the location where these measurements were taken, the DC stray current corrosion risk seems to
be well controlled by the 50 mA current output from the rectifier. Further, this experiment has shown
that (at least in this environment and for this stray current pattern) there is some correlation between
anodic charge and corrosion.
20
Probe PA04270009
0
500
1000
1500
2000
2500
3000
29/08/05 05/09/05 12/09/05 19/09/05 26/09/05 03/10/05
Date
Corr
osio
n r
ate
(m
icro
me
ter/
year)
Anodic
charg
e (
C/m
2)
0
10
20
30
40
50
60R
ectifie
r curre
nt (m
A)
Corrosion rate
Anodic charge
Rectifier level
Probe PA04270009
-2500
-2000
-1500
-1000
-500
0
29/08/05 05/09/05 12/09/05 19/09/05 26/09/05 03/10/05
Date
Cath
odic
charg
e (
C/m
2)
0
10
20
30
40
50
60
Rectifie
r curre
nt (m
A)
Cathodic charge
Rectifier level
Figure 23. Comparison between the release of cathodic charge (lower) anodic charge and corrosion
rate (upper).
21
HNG - KTB 1, Hareskov Probe PA04270009
y = 1.0471x + 0.0446
0
0.5
1
1.5
2
2.5
0 0.5 1 1.5 2 2.5
Anodic Current (A/m2)
Corr
osio
n r
ate
- m
m/y
ea
r
Figure 24. Correlation between released anodic charge (converted into average anodic current
density) and the corrosion rate.
22
6. Observations on corrosion under combined AC/DC interference
The risk AC influenced corrosion was shown to increase with increasing CP level in combination
with a certain AC level. The mitigation therefore may include both lowering the AC voltage and the
cathodic protection level. The risk of corrosion due to DC stray current seems to be related to the
anodic current leaving the pipeline at coating faults. The mitigation therefore may include increasing
the cathodic protection level.
When a pipeline is infected by both AC and DC interference, there seems to be a conflict when
optimizing the CP dosage.
In order to illustrate this conflict in practice, test stations have been established on the HNG KB-6
distribution where test stations at the Ølstykke crossing (North) and at the Taastrup crossing (South)
have been equipped with a number of ER coupons and dataloggers.
Distribution pipeline grit with MR stations and test posts
HV power lines
DC traction system
Transmission pipeline grit
Ølstykke crossing
Taastrup crossing
Figure 25. The KB-6 distribution pipeline grit positioning relative to other infrastructure – high
voltage power lines, DC traction systems.
23
At these test stations, a high level of both AC and DC interference exist. Figure 26 shows the
corrosion rate throughout 4 months at an ER coupon established at the Taastrup crossing in south.
The characteristic pattern in the first weeks of this period is illustrated in a close up in figure 27. A
certain correlation is observed according to which there is no corrosion at night and elevated
corrosion during daytime. During weekends, the corrosion ceases as well. This behaviour is taken as
traffic related. Unfortunately, there was no DC logging in this period.
Probe-Tag: Høje Taastrup -1
Probe Type: PA-0.4-10-0.1-12
Probe serial No. PA04120004
Test initiated: 12-01-2005
0
200
400
600
800
1000
1200
1400
1600
1800
2000
01/1 08/1 15/1 22/1 29/1 05/2 12/2 19/2 26/2 05/3 12/3 19/3 26/3 02/4 09/4 16/4 23/4 30/4 07/5 14/5 21/5
Exposure time -
Date
Co
rro
sio
n r
ate
- V
CO
RR
µm
/yr
Trend based on 96 measurements
Trend based on 16 measurements
Corrosion Rate
Peaks at 7000 µm/yPeaks at 8000 µm/y
Figure 26. Corrosion rate at coupon 1 – Taastrup crossing – 4 month period.
Probe-Tag: Høje Taastrup -1
Probe Type: PA-0.4-10-0.1-12
Probe serial No. PA04120004
Test initiated: 12-01-2005
0
50
100
150
200
250
300
350
400
450
500
16.1 23.1 30.1 06.2 13.2
Exposure time -
Date
Co
rro
sio
n r
ate
- V
CO
RR
µm
/yr
Trend based on 96 measurements
Trend based on 16 measurements
Corrosion Rate
Figure 27. Daily and weekly variations in corrosion rate – coupon 1, Taastrup crossing.
24
The extremely high corrosion rates that occur medium May and ultimo April (figure 26) seems to be
initiated from and related to some very powerful peaks in the AC voltage (figure 28) probably caused
by fault currents.
Figure 28. The AC voltage throughout the testing period – Taastrup crossing. . Figures 29-32 are examples from the measurements taken at Ølstykke crossing throughout a 5
months period. Fire 30 is the AC voltage in this period. High corrosion peaks are present from the
very beginning of the measuring period and the coupon element was just about to corrode through.
Therefore, the coupon was disconnected from the pipe for a while in order to protect the coupon and
analyze conditions. This period can be identified as the period where the AC voltage is zero (figure
30). The very high corrosion rate seems to occur at AC voltages fluctuating between 2 and 14 V.
Figure 31 shows the corresponding DC data in a period around 16/8, where the high corrosion peaks
are present. DC data were picked up as described for the DC stray current study, i.e. 5 times a day for
20 minutes, sampling every 5 seconds. As can be seen from figure 31, there are extremely high DC
fluctuations. The calm and steady potentials measured at night (short pink lines) showed that no
cathodic protection was active (it had been turned off) – the potential was here around -500 to -600
mV CSE. Despite the absence of CP the corrosion may be attributed to AC. From figure 31 is also
observed that cathodic current densities peak in between 20 and 60 A/m2 Such level is well high
enough to create alkalization even without a steady CP current. Anodic peak values are also very
Probe-Tag: Høje Taastrup -2
Probe Type: PA-0.4-10-0.1-12
Probe serial No. PA04120006
Test initiated: 12-01-2005
0
5
10
15
20
25
30
35
40
45
50
1/1 8/1 15/1 22/1 29/1 5/2 12/2 19/2 26/2 5/3 12/3 19/3 26/3 2/4 9/4 16/4 23/4 30/4 7/5 14/5 21/5
Exposure time -
Date
AC
Vo
lta
ge
- V
AC
vo
lts
AC Voltage
11-03-2005
Peaks at 160 V
28-04-2005
Peaks at 140 V
25
high – 20-60 A/m2. These currents should be a very serious threat in terms of DC stray current
corrosion.
Probe-Tag: 5500 K11A
Probe Type: PA-0.4-10-0.1-6
Probe serial No.: PA04140029
Test initiated: 08-08-2005
Vcorr(Peak)
>10mm/Yr
0
500
1000
1500
2000
2500
3000
3500
4000
17/7 16/8 15/9 15/10 14/11 14/12 13/1 12/2
Dat e
Vcorr
- m
ym
/yr
Trend based on 96 measurements Trend based on 16 measurements
Corrosion Rate
Figure 29. Corrosion rate experienced on coupon 1 – Ølstykke crossing.
Probe-Tag: 5500 K11A
Probe Type: PA-0.4-10-0.1-6
Probe serial No.: PA04140029
Test initiated: 08-08-2005
0
2
4
6
8
10
12
14
16
18
17/7 16/8 15/9 15/10 14/11 14/12 13/1 12/2Date
AC
Vo
lta
ge
- V
AC voltage
AC Voltage
Figure 30. AC-voltage at the Ølstykke crossing.
26
Figure 31. DC measurements on coupon 2 - Ølstykke crossing around 16/08.
Probe-Tag: 5500 K11A
Probe Type: PA-0.4-10-0.1-6
Probe serial No.: PA0510032
Test initiated: 21-09-2005
-80
-60
-40
-20
0
20
40
60
80
30/11 01/12 02/12 03/12 04/12 05/12 06/12 07/12 08/12 09/12 10/12 11/12
Time
Idc (
A/m
2)
-3
-2
-1
0
1
2
3
Idc 1 (A/m2)
Eon (V)
DC Data
Figure 32. DC measurements on coupon 1 - Ølstykke crossing. 3/12. Figure 32 shows DC data in a period around 3/12 (December third), where a violent corrosion attack
is observed. From the potentials measured at night can be observed that the CP system is well
working. Even so, this has not affected a reduction in the anodic (nor cathodic) current peaks, and
corrosion seems to occur due to a combined AC and DC stray current action without really taking
notice of the CP.
Probe-Tag: 5500 K11A
Probe Type: PA-0.4-10-0.1-6
Probe serial No.: PA04140040
Test initiated: 08-08-2005
-80
-60
-40
-20
0
20
40
60
80
06/08 13/08 20/08 27/08
Time
Idc
(A
/m2
)
-3
-2
-1
0
1
2
3
Idc 2 (A/m2)
Eon (V)
DC Data
27
Probe-Tag: 5500 K11A
Probe Type: PA-0.4-10-0.1-6
Probe serial No.: PA0510032
Test initiated: 21-09-2005
-80
-60
-40
-20
0
20
40
60
80
14/11 21/11 28/11
Time
Idc
(A
/m2)
-3
-2
-1
0
1
2
3
Idc 1 (A/m2)
Eon (V)
DC Data
Figure 33. DC measurements on ER coupon 1 at Ølstykke crossing around date 21/11.
Figure 33 shows DC measurements from date 14/11 and two weeks ahead. Again, the potentials at
night are quite calm while during the daytime, the current and potential fluctuation is heavy. From
figure 30 can be observed that there is only modest AC voltage present (1-6 V) but still from the
anodic peaks of the DC stray current one would expect corrosion.
The overall conclusion is however, that the severe DC stray current pattern in combination with
occasionally increased AC causes unacceptable corrosion, and further investigations involving line
current measurements to pinpoint the source of DC stray current as well as a good dialogue between
pipeline and traction system operators have been initiated. .
28
7. Evaluation of selected critical parameters
In general, corrosion is a function of electrical / electrochemical and chemical / physical parameters.
For a complete understanding of any corrosion process, one may claim that the relative influence of
all such parameters should be known. A full understanding of the processes is a case for the
corrosion scientist rather than the corrosion engineer, whose principal job is to be able to monitor and
mitigate the corrosion process. However, the better the understanding of the parameters involved,
the better the possibilities for both monitoring and mitigating the corrosion.
The following sections constitute a brief and undoubtedly incomplete discussion of some the
parameters involved in the AC corrosion process, and some of the parameters used to assess the risk.
Included herein is the significance of the parameters in CEN/TS 15280 on AC corrosion likelihood. .
7.1 Electrical parameters
7.1.1 AC voltage, AC current density and Spread Resistance The AC voltage, the AC current density and the spread resistance of a coating fault are
interconnected through Ohm’s law:
UAC (V) = Rspread (Ω.m2) x Iac (A/m2)
The magnitude of the AC voltage is controlled by the induction caused by the parallelism with an AC
power line and the effectiveness of the pipeline grounds. In TS 15280 it is stated that the AC voltage
is the most significant parameter for risk assessment. The pipeline to earth AC voltage will vary
along the pipeline chainage depending on the positioning and effectiveness of the grounds and the
configuration of the interfering structure (length of parallelism, configuration of and current in the
conductors). It is decided in TS 15280 that the AC voltage should not exceed 10 V at any time over
the entire pipeline, and it should not exceed 4 V where the local soil resistivity is below 25 Ω.m. The
extent to which these limits are in line with our own experience is not a question with a straight
forward answer. In figure 18 and 19, the data have been achieved in a soil box where the initial
resistivity has been around 100 Ω.m. It seems to be more evident that the spread resistance rather
than the soil resistivity is the conducting parameter. From figure 9 is observed that the spread
resistance decreases by a factor of at least 4.5 when DC current increases at the coating defect. If the
initial spread resistance corresponds to a soil resistivity of magnitude 100 ohm.m, a decrease in this
spread resistance by a factor of 4.5 corresponds to the creation of a soil resistivity locally at the
coating defect below the 25 ohm.m. The spread resistance, which in TS 15280 has been called the
29
”leakage resistance”, is not attributed a threshold value, and it is not used as parameters for direct
assessment of the corrosion likelihood. It is, however, dependant on the specific soil resistivity, the
electrochemistry at the coating defect, as well as the size and shape of the coating defect. Small
coating defects have lower spread resistances (ohm.m2) as compared with larger coating defects; see
figure 33, and refer to earlier investigations8.
Figure 33. Spread resistance as a function of the size of the coating defect in a certain reference environment8 The trendline in the plot can be written as::
Rs = K . ρs . d (Ω.m2)
where K is a constant dependant on the geometry of the coating fault, d is a measure of the size,
while ρs is the specific resistivity of the soil (in close vicinity of the coating defect and including the
effect of the electrolysis occurring here).
The AC current density is measured on a coupon with known surface area (preferable 1 cm2) and is
an important evaluation parameter in the TS 15280 standard. For AC current densities lower than 30
A/m2, the AC corrosion likelihood is low, in the interval between 30 and 100 A/m2, the likelihood
is medium, an above 100 A/m2, the likelihood is very high. There are no conflict between these
statements and the results obtained in this project. In order to use this parameter however, a
Relative solution resistance as a function of electrode area
Rs/ΩΩΩΩ.cm2 = 140 x (A/cm
2)0.5
1
10
100
1000
0.01 0.10 1.00 10.00
Area (cm2)
Rs
(ΩΩ ΩΩ
.cm
2)
30
continuous logging must be made since the AC voltage varies throughout the day and throughout the
year, the groundwater level may change, the spread resistance may change with water level, CP
dosage etc.
7.1.2 DC potential and DC current density
The effects of the DC parameters have been illustrated through the figures 5-16. Combined with a
certain level of AC, increased level of CP will increase the risk of AC corrosion, and the DC current
density seems to be a very important factor, which in case exceeding some threshold value will be
totally in charge of the spread resistance. The production of OH- causes high pH and low spread
resistance and initiates a wicked circuit; increased AC current density etc.
In TS 15280 a criterion is defined after which the ratio between the AC and DC current density is
calculated. If this ratio is less than 5, the AC corrosion likelihood is low. If the ratio is between 5 and
10, the likelihood exists, and further investigation should be necessary. If the ratio is beyond 10, the
AC corrosion likelihood is considered to be high, and further actions should be taken, e.g. by using
proper earthing. This current ratio criterion should be used with caution, since it may be tempting to
increase the CP level to increase the DC current density in order to reduce this ratio. However, the
CP level must first be adjusted so that excessive CP is not present, and then the criterion may be
used.
By this, the effect of the DC off-potential has also been discussed. It should be controlled to a value
more negative than - but as close as possible to – the CP criterion given by EN 12954 (usually -850
mV CSE) in order to avoid too excessive cathodic protection current concentrated in a coating
defect.
According to our investigations, the spread resistance and the DC current density should be given
higher priorities in the TS 15280 standard compared with the actual case.
7.2 Fysico-chemical parameters
7.2.1 Conductivity / resistivity The specific resistivity of the soil is – as already mentioned – the parameter adjusting whether 10 V
AC or 4 V AC is the maximum acceptable level. How this resistivity is measured is not quite clear,
however, it is described as the local resistivity. If this is relative to the pipeline chainage (kilometer),
31
the pipeline depth, or at a defect is not stressed out. However, the measurement should be made in a
soil box using soil from the exact position under investigation. One should take care that there is a
large effect of the soil humidity and for this reason there will be a large effect of the ground water
level and the seasonal variation hereof.
Soil resistivity as function of depth
-250
-200
-150
-100
-50
0
0.1 1 10 100 1000
Resistivity (Ohm.m)
Le
vel (0
=g
rou
nd
)
Frøslev M/R
F-E km 0.9
F-E km 2.9
Ølstykke M/R
Vallensbæk mose
SH km 91.24 -Brøndbyvester BoulevardSK km 94.0 (jernbane)
25 ohm.m limit
Figure 34. Soil resistivity measured as a function of depth – selected test locations.
Figure 34 shows the soil resistivity measured at some of the test stations. One apparently observes
two different kind of tendencies. One showing increased resistivity with increasing depth, probably
indicating that the conducting ions are primarily present in the upper soil layers and drawn further
down with rain. The other one is showing a decreasing resistivity with increasing depth – found in
totally wet peat bogs, where subsurface water movements may distribute ions and metabolites from
biological activity.
In conjunction with the graph (figure 34) it is noted that the measurements have been made by
sampling a certain amount of soil, bringing it back to the laboratory, drying for 24 hours, weighing
out 5 grams and adding hereto 10 ml of water. The conductivity measured in this suspension has
then been inversed to give a measure of resistivity. Specific investigations have shown that this will
32
give a fairly ”true” value compared with a soil box measurements using soil in a totally wet
condition.
7.2.2 Size of the coating defect
As already discussed, the size of the coating defect has importance in relation to the spread resistance
– figure 33.
7.2.3 pH
The soil pH and the pH existing in close proximity of a coating defect are two entirely different
factors. The increased pH which is believed to take part of most AC corrosion possesses is not
connected to the bulk soil, since the surface pH is created by the CP current. The buffer capacity of
the soil, i.e. the capability to neutralize the alkalinity produced by the CP, is a more obvious factor to
consider in this context. In previous studies, the BNE value (base neutralizing effect) has been
compared with occurrence of corrosion. These studies, which are definitely not complete, have not
shown any correlation between low soil buffer capacity and corrosion. Refer to CeoCor 2004
paper9,10. One may be able to observe increased incubation period when buffer capacity is high.
The discussion on pH and buffer capacities / BNE values is not part of TS 15280.
7.2.4 Humidity – ground water level Humidity and ground water levels are quite important factors. Previous studies (Gastra M/R Frøslev)
have shown that coupons that have been buried in a certain depth in the soil initially showed very
high corrosion rates, and then stopped corroding quite suddenly (and vice versa). The explanation
was quite obviously that the ground water level – which was monitored thereafter – change
dramatically by around 1 meter throughout the year – figure 35.
33
-250
-200
-150
-100
-50
0
01/0
9/20
02
10/1
2/20
02
20/0
3/20
03
28/0
6/20
03
06/1
0/20
03
14/0
1/20
04
23/0
4/20
04
01/0
8/20
04
09/1
1/20
04
17/0
2/20
05
28/0
5/20
05
05/0
9/20
05
Date
Level (c
m)
belo
w s
urf
ace
Ground water level
Position of probes
Figure 35. Ground water level at Gastra Frøslev – measured in drainage well over a period over 3
years.
Observations at HNG line KB-6 as well as the Gastra SH pipeline location GKL have shown both
corrosivity and changing ground water levels.
Further research experiments on AC corrosion should include a monitoring of the ground water level.
For general monitoring purposes, the ground water level monitoring may be regarded as an academic
issue.
TS 15280 does mention the correlation between spread resistance and the better conductivity present
in a water saturated soil.
7.2.5 Depth of the coupon or pipeline coating defect The depth of the coupon will play a significant role as already indicated in figure 34. The resistivity
will inevitably vary with the depth, as will the spread resistance and AC current density.
In the South of Jutland, around the Gastra MR Frøslev station, investigations have been made
directly at the station and some 900 meters away from the station. The textural type of soil seemed to
be identical at those two positions. The ground water level at the station however, was found to be
around 1 meter deeper than at km 0.9. Coupon buried to 1 meters depth at the km 0.9 location can
34
therefore be exposed in a totally wet environment, whereas coupons buried in the same depth at the
MR station at the same time is totally dry. While the wet coupons corrode heavily, the dry ones show
no sign of corrosion. In an attempt to compensate for this, a set of new coupons were buried 1 meter
deeper at the MR station in a wet position. It was anticipated that these coupons would corrode,
which they did not. The explanation seemed to be that the conductivity as a function of the depth
were quite lower at the MR station as compared with the km 0.9 location – see figure 34. Therefore,
the initial spread resistance was quite higher at the MR station, and no corrosion could be provoked.
While the humidity is mentioned in TS 15 280 as an important parameter, there is no information on
the correlation with the depth in soil.
7.2.6 Other chemical factors of the soil The effect of other chemical factors in the soil has been studied by other research groups –
particularly by Dr. Stalder in Switzerland. The presence of earth alkaline cations (Ca2+ og Mg2+) is
known to be able to cause an increase in spread resistance by reacting with produced OH-, forming
resistive layers on the steel surface. The spread resistance can be increased by a factor of 100 by
these processes. Presence of CaCO3 in the soil can be detected by adding droplets of acid to a soil
sample and observe if bubbles are created. In such case, CaCO3 is present, and the probability of the
presence of Ca2+ ions high.
If a very high DC current density can damage the formation of resistive layers seems still to be a
question to be answered.
In TS 15280, the above chemical factors are mentioned and highlighted. For laboratory illustrations
of the above, please refer to previous papers12-14.
7.2.7 Soil texture The effect of soil texture (grain size distribution) has been incompletely investigated in collaboration
with the Geographical Institute, University of Copenhagen. Soil samples from areas where corrosion
has been found has been compared with samples from area where corrosion is absent. In a range of
case, one can illustrate that whether or not corrosion occur in a sediment is not (exclusively)
connected to the texture. While it seems quite clear that presence of solid particles as diffusion
barriers are requisites in an AC corrosion case, it is not still clear if different textures cause different
35
corrosion scenarios. This question is of course to be seen in view of humidity condition, conductivity
issues etc.
Further, it is usually claimed that fine particles like clay have more chemical substances attached to
the surface as compared with coarse grained sand and gravel, and therefore that the conductivity is
better in fine grained sediment. This seems to be a plausible statement. However, the buffer capacity
of the soil is also caused by chemical substances attached to the particles, and this quantity would be
affected as well.
>2 1-20,5-10,25-0,5
0,125-0,250
0,063-0,125
0,020-0,063
0,002-0,020
<0,002
Pos
AP
os C
Pos
E
0
20
40
60
Fraction (%)
Grain size (mm)
Position
Pipeline 1
Figure 36. Corrosion (red) versus no corrosion (white) in three identical sediments.
36
8. Summary and conclusions
AC influenced corrosion can be shown to involve the formation of very high pH at a coating defect
combined with the potential vibration caused by superimposed AC. Since cathodic protection (CP)
increases the pH at a coating defect, it is essential not to apply the CP in excessive amounts as the
AC corrosion risk will increase. This is in contradiction to DC interference corrosion, which will
require an extra amount of CP, and a conflict therefore seems to exist between CP requirements for
AC corrosion mitigation and CP requirements for mitigation of corrosion caused by DC stray
currents.
Clear correlations have been established between CP current density and the spread resistance of the
coating defect. Above a threshold level, the CP (DC) current density and consequent pH increase
results in a significant lowering of the spread resistance. In turn, the lowering of the spread resistance
results in an increase of the AC current density, which combined with a pH increase and potential
vibration results in increased corrosion rate. Under such conditions, corrosion rates of several
millimeters per year can be measured on coupons. In field and in lab, it has been demonstrated that
corrosion can be reduced or even effectively stopped by reducing the CP level. In other words, the
results clearly show an unintended detrimental effect of excessive CP under such circumstances.
A single test post at which low AC voltages levels but high DC interference levels occur has received
special attention. In a particular study, the cathodic protection level of the pipeline (rectifier current
output) was gradually lowered in steps of one week, and the corrosion rate on coupons were
measured and compared with the electrical parameters describing the CP level and DC interference
condition. An increased corrosion was quantified by the coupons a quite accurate correlation with the
amount of current escaping the coupons under anodic interference peaks could be observed. This
study clearly shows that under DC interference conditions, it is wise to maintaining a certain level of
CP.
Pipelines experiencing both AC and DC interference constitute a dilemma when it comes to
optimized CP. The survey methods applied for AC and DC interference corrosion respectively have
been used on a pipeline experiencing both AC and DC interference. The interference patterns have
been clarified and compared with the corrosion rate of the coupons. It seemed to be impossible to
dose a suitable amount of CP in this case. The peaks in cathodic current density seemed to be severe
enough to create a critically high pH, which combined with the AC caused potential vibration creates
37
a classic AC corrosion case with very high corrosion rates. For the purpose of protecting against the
anodic DC current peaks a certain level of CP should be applied, but in such severe interference
cases, the actual level of CP seemed to be less important, the corrosion proceeds anyway.
The effect and role in AC corrosion of selected parameters have been discussed in brief terms. The
parameters include both electrical parameters like AC voltage and – current density, DC potentials
and –current density, and spread resistance, as well as physico-chemical parameters like
conductivity/soil resistivity, pH, presence of ground water, soil texture and selected chemical
substances. The discussion is made in view of the results obtained in the present project, as well as in
previous projects and from outside the Danish sector. The points are discussed in view of the recently
published CEN/TC 15280 document and AC corrosion likelihood. Among conclusions are that a
coupon DC current density and spread resistance combined with the pipeline AC voltage seems to be
the best indicators of the corrosion risk.
9. Perspectives
The CEN/TS 15280 document recommends to a large degree the use of coupons for quantification of
the various parameters that are used for assessment of the AC corrosion likelihood. For this reason, it
has been a Danish viewpoint in the ad hoc group that formed the document that techniques that could
also quantify the extent of corrosion on the coupon should be implemented in the document.
The whole idea of using coupons for the risk analysis seems today to be the best suited way (or
perhaps the only option), although one may question the extent to which a coating fault simulated in
a coupon represents an actual coating fault at the pipeline surface. The question could also be turned
upside down, in the sense that one could ask why a coupon should not represent a coating fault, as
long as a careful registration of the environment in which the coupon is placed is made along with
appropriate electrical descriptions of the coupon behavior. One could further ask what is the
difference in behavior of two different coating defects. Today, a NACE recommended practice
(RP0104-2004) on use of coupons for CP monitoring purposes exists, but it is not particularly
focusing on nor necessarily applicable for aspects relating the assessments of AC/DC interference
corrosion. In the CEN standards, the use of coupons is widespread, but no official document deals in
details with the possibilities and/or limitations when using coupons.
38
For the above reasons it has been suggested that a new working group activity is initiated in CeoCor
sector A, dealing with the issue, and a proposal for a GERG project which include activities on
coupon research has been forwarded through the Danish Gas Technological Centre.
Other interesting spin offs from the project has included some premature ideas on optimized cathodic
protection by closer CP control and thoughts on what could be called “dynamic corrosion control”. A
rectifier system, which measures the off-potential frequently throughout the day and adjusts the
rectifier current output accordingly to meet the pre-programmed CP requirement, is now operating in
HNG pipelines. It has been observed here that adjustments within 200 mV in the on-potential are
sometimes necessary within just a 24h period in order to keep a constant off-potential.
New approaches to effective CP under interference conditions could be born within the above
frames, for instance by quantifying the interference patterns continuously and dosing CP
accordingly. If high DC interference is present during daytime and high AC is present at nighttime
(caused by export of power for instance), the CP requirements in those two situations are quite
different.
39
10. References
1. EN 50162:, Protection against corrosion by stray current from direct current systems.
2. CEN/TS 15280, Evaluation of AC corrosion likelihood of buried pipelines – application to
cathodically protected pipelines,
3. EN 12954, Cathodic protection of buried or immersed metallic structures – General principles and application for pipelines.
4. L.V. Nielsen, Monitoring cathodic protection efficiency and AC induced corrosion using new
high-sensitive electrical resistance technology, Proc. Eurocorr 2001, Riva del Garda (2001),
paper no. 20
5. L. Vendelbo Nielsen, Field and Laboratory Detection of AC Corrosion Using High-Sensitive
ER-Technology, Paper 796, Proc. ICC2002 conference – Grenada Spain, 2002.
6. L.V. Nielsen and K.V. Nielsen, Differential ER Technology for Measuring Degree of
Accumulated Corrosion as well as Instant Corrosion Rate. Proc. NACE 2003, Paper 03443,
2003.
7. Lars Vendelbo Nielsen & Folke Galsgaard, Sensor technology for On-Line monitoring of AC
corrosion along pipelines, NACE 2005, paper 05375.
8. L.V. Nielsen, and P. Cohn, AC-Corrosion and Electrical Equivalent Diagrams, Proc CeoCor,
2000,
9. L. V. Nielsen, B. Baumgarten , H. Breuning-Madsen , P. Cohn , H. Rosenberg, Detection and
mitigation of AC induced corrosion in pipelines, Proc. Nordic corrosion conference,
Reykjavik 2004.
10. L.V. Nielsen, B. Baumgarten, P. Cohn, On-site measurements of AC induced corrosion:
Effect of AC and DC parameters - A report from the Danish activities, CeoCor conference
June 2004 Dresden, sector A.
11. L.V. Nielsen, B. Baumgarten, P. Cohn, Investigating AC and DC stray current corrosion,
CeoCor conference June 2005 Malmö.
12. Thermodynamical Considerations on the Local Chemistry Formed at the Steel-Soil Interface
of Cathodically Protected Pipelines, Technical Note, PCO / LVN.
40
13. AC Corrosion Rates of Cathodically Polarised Steel Exposed in a Scaling, Neutral-pH Soil
Solution, Technical Note, PCO / LVN.
14. AC Corrosion Rates of Cathodically Polarised Steel Exposed in a Non-Scaling, Neutral-pH
Soil Solution, Technical Note, PCO / LVN.
41
Appendix 1. Description of project and activities
Parties: HNG I/S, Gastra A/S, Naturgas Midt Nord I/S, Naturags Fyn I/S, DONG
Distribution A/S, MetriCorr, Aps, Balslev consulting engineers.
General background In situ measurements at Ølstykke M/R Station have shown that AC induced corrosion at ER coupons connected to the pipe system may be reduced and perhaps even eliminated if the CP dosage is reduced hereby reducing alkalinity production. However, this may conflict with the degree of CP necessary for prohibiting corrosion due to DC stray current from the adjacent railway electrical system. Optimum CP will in this case normally be ensured by dosage of CP to a level ensuring that the amount of cathodic current supplied by the rectifier will at any time be larger than part time anodic DC stray currents supplied by the railway system. Suggestions for investigations that may contribute to a clarification of the conflict are therefore invited. Theoretical considerations
• In proximity of a coating fault to which CP cathodic current is flowing, the pH is expected to be elevated compared with remote soil. According to the Pourbaix diagram, an anodic peak current in the pH region 9-12 should displace the potential towards (and perhaps into) the passive domain.
• However, it is known that passivity requires a formation of ferrous oxides, and this is a process that lasts an amount of time – a time constant can be attributed to the process. If assumed that creation of passivity is fast in comparison with a time period of anodic DC stray current, the steel is expected to passivate. In this case, the anodic DC stray current would not be expected to create ferrous dissolution (corrosion); the anodic current is escorted by some other process. Oxygen production requires a rather high anodic potential compared with the cathodic protection potential, but at very reduced conditions this may be an option?
• Under circumstances where a passive film may decompose, for instance under the presence of chlorides, an anodic DC stray current may be able to initiate pitting corrosion. Pitting is likewise a process to which a time constant can be associated. The pitting process can be characterized by a critical pitting potential above which pitting is initiated, and a re-passivation potential below which the pitting is blocked and passivation re-established.
In relation to the above considerations, the questions seem to be:
I. Can it be confirmed on a broader experimental basis that the AC corrosion risk is reduced if the CP dosage is decreased?
II. Would it be possible to adjust this CP dosage so as to avoid both AC induced corrosion and
DC stray current corrosion…
42
…either by
a. The CP is not dosed unnecessarily higher than needed for compensation of the DC stray current peak.
…or by
b. Keeping a balance in the CP dosage so that pH is not elevated to a degree that induces
AC corrosion, however high enough to guide the coating defect in to the passive region during an anodic stray current. Is this potential balance dependant on chloride contaminants?
III. How is the above affected by specific soil conditions like level of groundwater, pH, presence
of CacO3, conductivity of the soil, chloride and BNE value (base neutralizing capacity)? Suggestions for actions Point I can be investigated from different angles:
1. In situ measurements where ER probes are established on locations where AC induced corrosivity have been demonstrated. A number of probes should be established both below and above ground water levels. These probes should be varied with respect to area of the coating defect. AC corrosion detectors that measure AC parameters, spread resistance and corrosion should be installed in the measuring posts. The rectifier potential should be altered between moderate/low CP and high CP dosage.
2. Laboratory investigations in AC corrosive soil where a constant AC voltage is maintained
whilst the DC cathodic potential is varied. Two different AC loads for each of which the DC level is changed within 5 levels. High sense ER measuring the corrosion rate.
Point II as well can be investigated from different angles:
3. Balslev consulting engineers have pointed out 19 locations along the HNG pipeline system where DC stray currents are measurable. Soil samples should be collected from these sites and characterized with respect to chalk, chloride, pH, conductivity and ground water level. These will subsequently be categorized (wet/dry, acidic/alkaline etc.). Two ER coupons are installed and the corrosion is followed during a one-year period. In this period, the CP-level is changed from low via moderate to high, and spotwise ER measurements, spread resistance measurements. At the end of each CP condition, a measuring campaign is completed in which a close-up logging of the current and potential relations is performed. During these campaigns the probes are shortly de-coupled from the pipeline system so as to verify the open circuit potential drift under the chemical conditions formed due to the relevant CP condition. Optionally, the probes are characterized by Ramlog correal measurements.
4. Laboratory investigations where CP of ER probes exposed in a soil environment leads to a
build-up of an alkaline environment. At certain intervals an anodic polarization is realized with size and duration that corresponds a “typical” stray current. DC parameters (current /potential) are registered. For a number of CP-level/stray current combinations the corrosion rate is recorded by the high sense equipment. In parallel investigations the probes
43
are connected to a larger steel structure that has not been cathodically polarized and for this reason does not passivate by anodic polarization. The current distribution between this structure and the probes is registered and it is investigated whether the corrosion rate is affected by the coupling procedure.
Each of the 4 phases is reported continuously. Implementation The above identified 5 points of action are realized according to the below time-schedule.
Activity 1 2 3 4 1 2 3 4
1. In situ AC corr.
2. Lab investigation AC corr.
3. In situ DC stray current corr.
4. Lab investigations DC stray
5. Final report
2004 2005
Phase 1 (AC investigations in the field) is initiated in direct continuation of the present investigations along the HNG line. Phase 2 (AC investigations in lab) is initiated directly subsequent to budget approval, and will continue for the proceeding 3 months Phase 3 (DC stray current in field) involves a preliminary investigation of the soil conditions after which the actual field measurements can be initiated around 4th quarter of 2004. Phase 4 (DC stray current investigations in laboratory.) Initiated directly after the AC investigations in lab (phase 2) and will proceed for 3 months.
44
Appendix 2. Papers and presentations – now and then.
Previous internal reports and papers prepared in regi of DONG in collaboration between Peter Cohn
and Lars Vendelbo Nielsen:
1. Thermodynamical Considerations on the Local Chemistry Formed at the Steel-Soil
Interface of Cathodically Protected Pipelines
2. pH Gradients Existing in Proximity of Cathodically Charged Steel Buried in Sediment
3. EIS Investigation of the Randles Circuit Elements for Carbon Steel Exposed in Artificial
Soil Solution
4. Comparison of EIS and Ramlog Measurements of Spread Resistance and Polarisation
Impedance for Steel Exposed in Artificial Soil Solutions with and without Scaling Capacity
at 50 Hz AC
5. Effects of 50 Hz AC on the DC Polarisation Behaviour of Steel Exposed in Artificial Soil
Solutions
6. AC Corrosion Rates of Cathodically Polarised Steel Exposed in a Scaling, Neutral-pH Soil
Solution
7. AC Corrosion Rates of Cathodically Polarised Steel Exposed in a Non-Scaling, Neutral-pH
Soil Solution
8. AC Corrosion Rates of DC Polarised Steel Exposed in a High-pH Solution
9. The Effect of Superimposed Alternating Current on Hydrogen Absorption by Cathodically
Polarised Steel Exposed in a Non-Scaling Artificial Soil Solution
Papers presented at conferences:
10. K. Vendelbo Nielsen & L. Vendelbo Nielsen, Measurement of Accumulated Corrosion and
Instant Corrosion Rate Using a New Differential Electrical –Resistance Technique, Internal
paper VN-Instrument, 1998.
11. L.V. Nielsen, and P. Cohn, AC-Corrosion and Electrical Equivalent Diagrams, Proc
CeoCor 2000,
45
12. L.V. Nielsen, Monitoring cathodic protection efficiency and AC induced corrosion using
new high-sensitive electrical resistance technology, Proc. Eurocorr 2001, Riva del Garda
(2001), paper no. 20
13. L.V. Nielsen, and P. Cohn AC-corrosion in pipelines: Field experiences from a highly
corrosive test site using ER corrosivity probes, Report given at CeoCor 2002.
14. L. Vendelbo Nielsen, Field and Laboratory Detection of AC Corrosion Using High-
Sensitive ER-Technology, Paper 796, Proc. ICC2002 conference – Grenada Spain, 2002.
15. L.V. Nielsen and K.V. Nielsen, Differential ER Technology for Measuring Degree of
Accumulated Corrosion as well as Instant Corrosion Rate. Proc. NACE 2003, Paper 03443,
2003.
16. L.V. Nielsen et al, AC-Induced Corrosion in Pipelines: Detection, Characterisation and
Mitigation, Proc NACE 2004, Paper 04211, 2004.
17. L. V. Nielsen, B. Baumgarten , H. Breuning-Madsen , P. Cohn , H. Rosenberg, Detection
and mitigation of AC induced corrosion in pipelines, Proc. Nordic corrosion conference,
Reykjavik 2004.
18. L.V. Nielsen, B. Baumgarten, P. Cohn, On-site measurements of AC induced corrosion:
Effect of AC and DC parameters - A report from the Danish activities, CeoCor conference
June 2004 Dresden, sector A.
19. A. Kulkarni, L.V. Nielsen, H. Rosenberg Characteristics of AC-Induced corrosion in
pipelines and concepts for mitigation and monitoring. Corcon, NACEIndia, Dew Delhi
2004.
20. L.V. Nielsen, Role of Alkalization in AC induced corrosion of pipelines and consequences
hereof in relation to CP requirements, NACE 2005, paper 05188.
21. Lars Vendelbo Nielsen & Folke Galsgaard, Sensor technology for On-Line monitoring of
AC corrosion along pipelines, NACE 2005, paper 05375.
22. L.V. Nielsen, B. Baumgarten, P. Cohn, Investigating AC and DC stray current corrosion,
CeoCor conference June 2005 Malmö.
46
23. Lars Vendelbo Nielsen and Folke Galsgaard, Techniques for investigating AC and DC stray
current corrosion along pipelines, Proc. UK Corrosion 2005.
24. H. Rosenberg, A. Kulkarni, L.V. Nielsen, Pipeline operation and Maintenance with focus
on Corrosion Protection, Corcon, NACEIndia, Chennai 2005.
Planned papers/presentations CeoCor 2006 – Luxemburg:
25. L.V. Nielsen, B. Baumgarten, P. Cohn, H. Rosenberg, A field study of line currents and
corrosion rate measurements in a pipeline critically interfered with AC and DC stray
currents.
26. L.V. Nielsen, P. Cohn, Use and limitations of coupons for corrosion monitoring
applications (A working group start-up).