AC-Corrosion Detection on Electrical Resistance Probes Connected to a Natural Gas Transmission Pipeline

8/10/2019 AC-Corrosion Detection on Electrical Resistance Probes Connected to a Natural Gas Transmission Pipeline

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AC-CORROSION DETECTION ON ELECTRICAL RESISTANCE PROBES CONNECTED

TO A NATURAL GAS TRANSMISSION PIPELINE

N. Kioupis, K. Maroulis Public Gas Corporation of Greece S.A.

ABSTRACT

The results from a field investigation of ac-corrosion detection on ER (Electrical Resistance) -probes,

connected to the polyethylene-coated gas transmission pipeline of Greece, are presented and

analyzed.The most worrying outcome from this investigation is that ac-corrosion can grow very rapidly even

at ac voltage level considered to be too low. These results show the determining factor of the ac-

current density.

Interesting data taken from ER-probes connected at another site of a different pipe section pointed to

ac-corrosion related to loss of cathodic protection as well as corrosion from telluric currents.

Recommendations for further work and remedial measures are proposed.

INTRODUCTION

Nowadays, the problem of the so-called ac-corrosion, i.e. metal loss caused by alternating current at

coating defects of modern pipelines is recognized as a major safety risk and serious threat for the

integrity of pipeline infrastructure. The ac voltage is induced on the pipelines mostly by the HighVoltage (HV) ac power lines that are in proximity. The ac-electrified railways can be an additional

induction source.

However, the ac-corrosion mechanism and the conditions related to it are still not completely

understood. Thus, the ac-corrosion prevention and control is a great challenge for the corrosion

engineers, designers and operators of cathodic protection systems and pipeline owners around the

world. During the last two decades, intensive research for ac-corrosion has been conducted leading to

a breakthrough in cathodic protection criteria and measurement techniques.

In this context, a promising diagnostic tool for ac-corrosion detection and evaluation is the

Electrical-Resistance probes (ER-probes) with dedicated data logger and software. Such probes are

buried and connected to the pipeline in order to record the “fingerprints” of the corrosion conditionof simulated coating defects.



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The problem of ac-interference on pipelines has been intensified due to the crowding of various

energy transmission corridors and transport routes as their space availability is continuously limited

due to the increasing development as well as for economic and environmental reasons. Especially in

areas with uneven soil terrain, like those frequently experienced in Greek territory, the energy

corridors availability is very limited. Thus alternative pipeline routes that could avoid the close

parallelisms with HV power lines can highly increase the complexity and the cost of pipeline

construction, making the selection of those routes practically impossible. In addition, even a

parallelism between a long pipeline and a power line up to a distance of 1000 m [1] (or up to a

distance of 3000 m according to stricter estimations), is to be taken into account for the evaluation of

induced ac voltage effect studies (proximity studies).

Besides, the modern pipelines externally coated with materials of highly insulating properties (e.g.

PE, PP etc) seem to be more sensitive to ac-corrosion, overvoltage effects and stray current

influence. A brief explanation for this higher corrosion and overvoltage effects susceptibility follows.

On the one hand, a well-coated pipeline is a less effective means of grounding the ac-voltage / surges

since it is better isolated from its surroundings by the plastic coating and on the other hand there is a

serious corrosion risk on the tiny coating defects because of the higher ac/dc current density, as the

metal surface area exposed to the corrosive environment is considerably lower in comparison to

coating faults on pipes with porous coatings (e.g. bitumen etc.).

Hence, in the case of well-insulated pipelines special conditions have emerged that impose an

emphasis to be given on specific anti-corrosion protection methods and surge/lightning protection.

Those methods cannot be effective unless the particular conditions of the well-insulated pipelines are

taken into account for surge protection and pipeline grounding measures.

Although grounding measures are usually taken into effect for minimizing the drawbacks of ac-

interference on pipelines they are scarcely effective for the anti-corrosion protection because all

similar methods used to be focused on the electrical safety risks in terms of the safe touch voltage

when an individual comes in contact with the ac-interfered structure. The long-term ac voltage

considered safe is usually between 15 and 65 V depending on the various national regulations but

these orders of ac voltage magnitudes are not low enough to control the ac-corrosion risk.

The attempts for new guidelines and criteria to be applied on the standards for cathodic protection by

taking into account the ac-corrosion effects, has resulted in specification CEN TS 15280 [2] that

replaces the ac-corrosion annex of the standard EN 12954 [3]. In spite of what was the general belief

on the cathodic protection (cp) effect on ac-corrosion, there is increasing evidence supporting that

the high cp-levels rather rise the ac-corrosion rate than reducing it [4]. This makes the situation very

complex in the case of dc-stray currents where recent reports have revealed a rather synergistic effect

of the ac and dc stray currents [5]. Moreover, in case of low or no cp, the ac-corrosion rate can be

accelerated, as evidenced in the present study.

Another secondary effect of the ac-interference, that has been rather overlooked, is that especiallyduring relatively high ac-levels (generally >10V), the dc-potential - at specific areas of the pipeline-

slowly shifts towards anodic direction (as well as to the cathodic direction at other areas of the line

and vice-versa). This effect can also lead to rapid corrosion rates at coating defects of the pipeline.

Specifically in areas with aggressive soil, such as clay, the low frequency anodic excursions of cp

potential can lead to extreme corrosion rates [6].

As the well-coated pipelines are more vulnerable to stray currents (ac/dc) and telluric disturbance the

time-stamped recordings and remote monitoring of cp potentials and currents becomes more and

more important for the efficient management of anti-corrosion protection of such structures. In this

context, the Electrical Resistance probes with data logging and quick presentation of time-stamped

results are one of the most effective means for evaluation of the stray currents impact on pipelinecorrosion.



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EXPERIMENTAL PROCEDURE

The data from two case studies are presented and analysed, i.e. case study no.1 regarding ER-probes

with tag no. 110 and 124 as well as case study no.2 regarding ER-probes no. 35 and 36.

The pipeline and the CP system – case study no.1

The pipeline examined in the case study no.1 is a 45km section of the natural gas transmission main

line of Greece, constructed in the time interval between 1993 and 1995 and buried at a depth of 1 to

2m in the soil. The main line is extended in a south-north direction from progressive distance 0

(south) to 512 km (north) at the Greek – Bulgarian border.

The pipe steel (API 5L X-60) is 762mm (30in.) in outside diameter with wall thicknesses ranging

from 9.52mm to 15.6mm.

It is coated with three-layer extruded polyethylene (factory applied) and heat-shrinkable

polyethylene-compatible sleeves wrapped around every girth weld (field applied). Valves are coated

with a coal tar polyurethane coating.

The pipeline under examination, 45km in length, is electrically separated from the rest line through

buried monolithic isolating joints at both ends (at chainage 2.3km and 47.3km), which are protected

from lightning breakdown through spark gaps. The ground plan of the pipeline can be designed as a

crooked or curved line without branches.Long-term ac-interference on this pipeline is induced by the electromagnetic field generated mostly

from eight 150kV and two 400kV electric power transmission lines.

Impressed current cathodic protection is provided by means of one Transformer/Rectifier (T/R) unit

in potential control mode (set at on-potential around -1,5V). The groundbed comprised magnetite

(Fe3O4) anodes. The T/R unit is equipped with a special low-pass ac-filter preventing ac voltage of

the pipeline from interfering with T/R unit operation.

Previous CP investigations disclosed a disturbance from a cathodic protection system of a foreign

pipeline. However the interference is not considered severe enough to heighten corrosion rates at a

significant level, although more work has to be done to verify it. The reasons for this notion is that

one cannot expect a lot of interference from the foreign pipe CP system because measures had

already been implemented at crossings with this pipe (i.e. the foreign pipeline was wrapped withinsulating PVC at the crossing areas) and the closest anode from the foreign CP system may be at

least 150 m away from the gas pipeline. This closest foreign pipe anode is near the T/R location

(40,1 km) and causes some cathodic interference in this area of the gas pipe though.

There is no supporting evidence that this pipeline is strongly affected by telluric currents. Electric

trains are not present in the proximity area of this pipe section.

The pipeline and the CP system – case study no.2

The pipeline section in the case study no.2 is also a part of the Greek gas main line with the same

pipe and cp technical specifications as those aforementioned for case study no.1.

However this pipe section is longer, extending from 83,3 km up to 377 km (294 km approx. long).

The pipeline in this area is affected both by telluric currents and ac-interference induced mainly bytwelve 150 kV and five 400 kV power lines.

The cathodic protection current was fed by one T/R located at position 198,3km. During the time of

tests, the T/R unit was operating in galvanostatic mode, supplying total cp current of 100 mA.

The overvoltage earthing system – case study no.1

Since construction of the pipeline, grounding wires made of hot-dipped galvanized steel had been

installed at selected locations in parallel to the pipeline at a minimum distance 0.2m from it (Figure

1). The pipeline section under study is equipped with 45 CP test posts, including 18 test posts, named

KG, positioned at its 18 earthing installations.

The external diameter of the earthing wires was minimum 12mm with 70µm zinc layer thickness

corresponding to 500g/m2. The length of the grounding electrode was ranging from 30m to 220m



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depending on the location. More details are given in [7]. In addition, the pipe at valve stations is

grounded through spark gaps.

1.00 m

0.2 m (min)

buried pipe

measuring post

cable to earthing wire

earthing wire

(a) (b)

AC subjected

pipeline

terminal board of test post

earthing

0.2 m (min)

buried pipe

measuring post

cable to earthing wire

earthing wire

AVACF

Figure 1 : (a) Diagram of AVACF and earth wire connections at the CP test post and (b) Earthing

wire installation detail (cross-section)

AVACF

In this pipeline section (case study no.1) due to the problems of the high ac-interference and the

consequent disturbances in the CP system operation that have been analysed and reported previously

[7], a number of specifically designed and manufactured ac conducting dc decoupling devices

(named AVACF) were installed at the 18 places of grounding installations of the pipeline. Those

devices were capable of diminishing the ac voltages at least by one order of magnitude. For instance,most of the time the ac voltage used to be within the range 10-25 V and was reduced to less than 3 V,

as a result of continuous draining of the ac-currents to earth through these devices [8].

The basic technical features of the AVACF devices are the following [8]:

• Max. steady state (long-term) AC voltage: 30Vrms

• Max. steady state DC voltage: 15V

• Max. steady state AC discharge current: 10A

• Steady state AC discharge current at 30VAC

upon tested in a circuit with a 20 Ohm resistor in series: 1,5A

• Steady state DC leakage current at 15VDCupon tested in a circuit with a 20 Ohm resistor in series: 10 µA

• Protection from overcurrent by means of a

slow acting anti-surge high breaking capacity fuse: HBC 16A(T)

• Overall capacitance: 4700 µF

• Surge protection is effected by pre-existing overvoltage arresters with anti-parallel thyristor

circuitry

• Operation temperature range: -20OC to +800C

• Waterproof and corrosion-resistant housing

Some advantages of AVACF are summarised below:



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• Extremely high DC blocking capability (DC leakage current in the order of few microamperes)

so that DC flow to/from earth to be totally prevented. Consequently, the cathodic protection

function, line current and DC potential measurements are almost unaffected

• No increase of DC leakage current with increase of DC voltage

• No requirement for external power source (battery or AC mains)

• Limited on-off CP potential recordings falsification from the AVACF capacitance discharging

effect

•

Execution of on-off potential measurements without having to disconnect AVACF from the protected pipe

• Continuous ac conduction (at any low ac voltage)

• Suitable dimensions to fit inside the existing measuring post housing

• Easy installation and maintenance.

The weak point of AVACF was its fuse, which was frequently blown by surges, thus a frequent

inspection plan was necessary in order to check and replace the fuses.

For this reason an improved dc-decoupling device without fuse has already been prepared.

The overvoltage earthing system– case study no.2.

The earthing arrangement is the same with that aforementioned in case study no.1 except that noAVACF or any other overvoltage arrester is connected. The KG test posts with the corresponding

earthing wires are amounted to 25 spread at various sites along the pipe route. The pipe at valve

stations is also grounded through spark gaps.

Brief description of ER technique and experimental methodology

The technology for verification of CP effectiveness is based on a specialized ER-concept. The

coupon is formed as a part of an ER (Electrical Resistance) probe (Figure 2) consisting of a metal

plate divided into two parts, one part being exposed to the soil (coupon part), the other part

(reference part) being physically shielded to effectively avoid corrosion (Figure 3).

Figure 2. Picture of ER-probe

Figure 3. Configuration of the coupon and reference element of ER-probe



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The actual thickness of the coupon can de verified by measuring the electrical resistance throughout

time of the coupon since the electrical resistance is a function of the plate dimensions. Comparing

with simultaneous measurements of the electrical resistance of the reference will provide temperature

compensated thickness values. Coupon element thickness at time t Ic(t) is then quantified by a

mathematical algorithm, for instance through:

where (t=0) refers to initial probe conditions.

R C and R R are resistance values of coupon and reference part of the plate.

Figure 4. Illustration of the ER circuitry.

In Figure 4, the basic ER-circuitry is illustrated. Excitation current is passed through the elements,and amplifiers (A1 and A2) pick up the voltages generated across the elements and scale this

properly into a measure of the element resistances, R C and R R .

The slope of a plot of the thickness versus time will provide a measure of corrosion rate. Using the

special data logging unit, time stamped corrosion rate and electrical properties are acquired for

diagnostic purposes.

An ER coupon is used for measurements of corrosion rate at the position where the coupon is placed.

For verification of cathodic protection efficiency of buried structures – for instance pipeline – the

coupon is electrically connected to the structure and placed in the soil adjacent hereto (Figure 5). The

coupon acts as an artificial coating defect and the corrosion rate in this is used to assess the risk ofcorrosion of the structure when exposed through a coating defect to the chemical and electrical

environment in which the coupon is placed.

The use of ER coupons is particularly useful for assessment of the risk of corrosion due to AC or DC

interference (AC-influenced corrosion – corrosion due to DC stray current).

(1)



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Figure 5. Datalogger system with ER coupon used for field investigations

ER-probes are well integrated and commonly accepted tools for monitoring internal corrosion in

process plants. The ER concept has been employed as an alternative to corrosion measurements by

weight loss determination on exposed coupons. The drawbacks of weight loss measurements is

primarily that the coupon has to de withdrawn from the environment, brought to the laboratory for

cleaning and weighing procedures which are costly and time-consuming. The ER concept is on-line

and does not require excavation of the coupon for corrosion evaluation.

Therefore, the ER concept is cost effective and provides a measure of the logical monitoring

parameter – that is corrosion - on top of the traditional electrical measurements made on coupons for

CP verification.

At this point, some problems encountered with the conventional polarization probes are underlined.One of the main disadvantages of conventional probes is that the measurements obtained are usually

instantaneous (taken in sporadic moments of time), thus the information acquired is limited. Usually

there is no information on the corrosion status of the metal probe unless it is excavated and

inspected, practically a difficult process that is rarely implemented and on top of that little info is

collected even in that case. Besides, a lot of the polarization probes can drive to misleading results

because they are usually prone to failure (e.g. leak of the electrode solution that preferentially

corrodes the metal sample and falsifies the environment adjacent to the metal or contamination of the

reference electrode solution from the soil etc.). Eventually, especially the measurements of ac/dc

current to the probe usually require very accurate measurement equipment, suitable measurement

techniques and skilled technicians; otherwise the measurement outcome is frequently in error. Of

course, the ER-probes and data logging methodology avoids all above trouble.

By means of the ER-probe data logger, with a selectable sampling rate (in this case every ½ or 1h

was chosen), measurements of the ac voltage, the ac current density, the coupon and reference

element electrical resistances and calculated thicknesses are obtained. Besides, the spread resistance

of the coupon is calculated by Ohm’s Law through the following equation:

The differentiation of the coupon thickness loss with time gives time-stamped corrosion rates. The

momentary/transient corrosion rates are calculated from the trend of 16 measurements whereas the

average corrosion rates are calculated from the trend based on 96 measurements.

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For the interested reader, more details on the ER-technology concept and equipment can be found in

the literature [9-11].

Assisting data logging equipment

The recording of ac and dc voltage was performed using a cathodic protection datalogger with

512Kbyte data capacity at 30V ac range (input resistance: 1MP) and 3V dc range (input resistance:

2MP, attenuation at 50Hz: 60dB) respectively. This recorder is a two-channel data logger with a

built-in ac-filter allowing simultaneous recording of dc and ac voltage. The ac-filter of the data

logger prevented the dc voltage recordings from being falsified by ac interference. This data logger

facilitated the acquisition of mainly dc potential data since the ER data logger could not take dc data.

Location of ER-probes – case study no.1

Two ER- probes, with tag numbers 110 and 124 respectively, having surface area 0,4 cm2 each and

initial thickness 100µm, of the type illustrated in Figure 2 were buried at a depth of approx. 1 meter

in this corrosive soil mentioned above. Both probes were near each other at distance 1 m between

them but at almost the same distance from the pipe. They were placed close to the gas pipeline, at

position 2,5km where the crossing with an oil pipeline is situated (see map Figure 6). The gas

pipeline burial at that point was around 1,5m. The ER-probes were periodically both connected to or

disconnected from the cathodically protected gas pipeline. The corrosion history and the variouscorrosion rates estimated from the ER-probes data are shown in Tables 1 & 2. It is obvious that both

probes simultaneously were subject to either the effect of combined cathodic protection and ac-

interference (denoted CP + AC) or to natural corrosion (denoted NC) as long as being unaffected

from the combined CP+AC influence when disconnected from the pipe.

The environment chosen for the ER-probes installation, which it is in a region located approx. 30 km

west of Athens is a swampy (and probably saline) soil close to the sea. Particularly, the place of ER-

probes installation is at a distance approx. 400m away from the sea. The bulk soil pH was measured

by means of pH-paper strip at several soil and mud water samples collected from the environment

near the probes. The bulk soil pH was around 8. The bulk soil resistivity was in the order of 2,3

Ohm-m.

Some soil samples were collected but their soil chemical analysis is pending.The particular place for the ER-probes installation was selected because high ac current densities had

been measured in a conventional polarization probe located nearby (chainage 2,3km). That probe

comprised a reference electrode of the type zinc/zinc sulphate combined with a naked metal plate of

1cm2 surface area at its bottom.

Location of ER-probes – case study no.2

The ER-probes with tag numbers 35 and 36, each having surface area 0,4cm2 and initial thickness

100 µm, are buried at position 304,4 km of the main pipeline (see map Figure 6). They are placed

close each other and are both connected to the nearby pipeline. Apart from the ac interference,

telluric currents also affect the pipeline in this area. The pipeline section where the ER-probes of tag

no. 35 and 36 are connected is electrically isolated and far away from the pipeline section where theER-probes tag no. 110 and 124 are connected. Of course, there is no electrical correlation between

the pipe sections of the two case studies.

RESULTS

Corrosion history – case study no. 1

The corrosion history of ER-probes no. 110 and 124 is illustrated in Tables 1&2. In these tables, both

total corrosion rates and ac corrosion rates are depicted since the periods of ac-corrosion (CP+AC)

alternate with periods of natural corrosion (NC) when the probe is left disconnected from the pipe.

Nevertheless, there is a firm basis to assume that the NC periods contribute negligibly to corrosion.

In order to assess the contribution of NC to the total corrosion behaviour of the specimens we

monitored the thickness of the ER-probe during a 4 days period of NC (from 12 until 16/1/2006),followed after a period of corrosion attack on the probes. We were astonished to find that even the



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slightest corrosion could hardly be detected. The same became also obvious when the fuse of the ER-

probe data logger was blown (on 4/1/2006, 14:00) thus leaving the coupon disconnected from the

pipe for 6 days. During this 6 days period of supposed NC, no corrosion was detected too. It seems

that the oxide film on the metal surface produced from the ac-corrosion activity, exhibits a protective

role in terms of natural corrosion. This seems reasonable according to the alkalization theory of ac-

corrosion because in the absence of CP the pH values on the metal surface drop from the very high

values during ac-corrosion process to lower pH values at open-circuit potential (OCP) thus favoring

immunity as a result of the passivity offered from the oxide film. On the contrary, both ac-corrosion

and strong cathodic polarization lead to higher pH values favoring the breakdown of passivity

[5,12]–see Pourbaix diagram (Figure 7). That the oxide film produced from ac-corrosion can be

passive in the OCP state is supported from the visual examination of the corrosion products seen

after retrieval of the probes. The coupons were covered with a black powder/film assumed to be

mostly magnetite ( Fe3O4 ), the known passive ferrous oxide that is believed to be the main corrosion

product arising from ac-corrosion [13].

In the following, the terms shown in Table 1 & 2 and in Figures 8 and 9 are explained.

The cumulative ac-corrosion rates Uac-corr cum, are calculated by dividing the metal thickness loss

by the time from the start of the experiment (18/11/2005) during which the ER-probes had been

experiencing CP+AC only, whereas the cumulative total corrosion rates Utot-corr cum are calculated by dividing the metal thickness loss by the total time from the beginning of the experiment, during

which the ER-probes had been experiencing corrosion during both time periods of CP+AC and NC.

Uac-corr cum (µm/yr) = [100 µm – t f (µm)] / T CP+AC

Utot-corr cum (µm/yr) = [100 µm – t f (µm)] / ( T CP+AC +T NC )

t f denotes the final thickness of the metal probe, while T CP+AC , T NC denote the time from the start of

experiment that the probe is subject to CP+AC and NC respectively.

The corresponding corrosion rates per time period for ac and total corrosion respectively are definedas follows:

Uac-corr per (µm/yr) = [t i (µm)– t f (µm)] / t CP+AC

Utot-corr per (µm/yr) = [t i – t f (µm)] / ( t CP+AC +t NC )

t i denotes the initial thickness of the metal probe at the beginning of every time period, while tCP+AC ,

t NC denote the duration of each time period that the probe is subject to CP+AC or NC respectively.

From the data illustrated in Tables 1&2 and Figures 8&9, at least three distinctive long time phases

of corrosion can be defined.The first phase data are related to the recordings during the very first 7 days of corrosion (18-

25/11/2005). After the initiation of a high-speed corrosion with rather no incubation period,

retardation in the process occurs. Unfortunately no recording is available after the first 7 days

(25/11/2005 – 5/12/2005).

Then after a period of NC (5-22/12/2005), the second phase follows. During this phase, incubation

time (few days duration) can be discriminated, after which the ac-corrosion is promoted during times

of high ac-current density and ac voltage peaks. Then, after the ER-probe data logger fuse is blown

(on 4/1/2006), there follows a 6 days period of no corrosion when the coupon is disconnected from

the pipe (NC).

The third period (10/1/2006-20/1/2006) is characterized by the extremely high corrosion rates that

lead to perforation of the coupons. Unfortunately no ER-probe recording is available during this period except for a period of NC when no corrosion is detected.

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4

5

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The results in Figure 10 show that the corrosion begins with high velocity. However, after the first 7

days, it seems to tend to a plateau indicating a retardation of corrosion speed. This retardation of the

corrosion process can be correlated to the reduction of the average ac voltage (and subsequently the

ac current density too).

The absence of long incubation period is noteworthy too. A likely explanation may be that in this

initial state no corrosion products and high thickness oxide films are present since the incubation

period might be associated with the oxide film present that seems to play a role in the initial

corrosion steps in terms of the time to reach critical high pH enough for ac-corrosion to occur.

The corrosion retardation is also in accordance with the corrosion rates depicted in Figures 8&9

where a slow-down of the average corrosion rates in the mid period can be noticed. It is noteworthy

that, after some time, the corrosion rate is suddenly and dramatically accelerated, as clearly shown in

Figures 8&9. This is in contrast to previous reports that had suggested a decrease in ac-corrosion rate

with time [14]. It is remarkable that during the final time period, both metal specimens of ER-probes

suffer perforation within only 1 day of ac-corrosion duration. Severe metal losses appear i.e. 46,9 µm

and 18,9 µm metal thickness of the probe tag no. 124 and the probe tag.no. 110 respectively, both

vanish within one day. During this period, the corrosion reaches extreme rates maybe higher than 17

mm/yr!! (Figures 8&9). A possible explanation for this behaviour of the specimens may be a sudden150mV shift of dc potential to the negative direction (Figure 11) although such a rapid acceleration

of corrosion is surprising.

Unfortunately there was no ac voltage recording during the same period, but we assume that the ac

voltage did not reach much higher values than usual. Of course the increase in corrosion rate by a

combined effect of higher ac voltage and by surplus of cathodic polarisation cannot be ruled out. Note: The negative potential shift was caused by an unknown event, which simultaneously tripped the fuse of the ER-

probe data logger, created a cathodic shift of the gas pipe potential and an anodic shift of the secondary oil pipe potential. These potential values remained in the same order of magnitude during the period that followed the causing

event (see Figure 11 ).

Analysis of ER-probes data from the second phase corrosion period – case study no. 1

The corrosion process (recorded during the second phase) is marked by sharp bursts ofmomentary/transient corrosion rates, which coincide with ac-current density and ac voltage peaks

(Figures 12&13 ). The maximum values of the momentary corrosion rates present a broad dispersion.

Broadly dispersed and different transient corrosion rates exist at the two ER-probes indicating that

the corrosion process is very sensitive to the local soil conditions. However the qualitative features of

the corrosion behaviour of both coupons are the same. In the long run though, it seems that both

probes tend to corrode in a like manner.

The data in Figure 14 illustrate the threshold of ac voltage and ac current density above which the

corrosion is triggered. The corrosion rates data agree with the safety limit of 100 A/m2 for ac current

density. In this example, this ac current density limit corresponds to a safe ac voltage of only 0,5-0,6

Volt. Such a safe ac voltage is much lower than the 4 Volt limit according to specifications [2]

proving that merely the ac voltage is not to be relied upon for ac corrosion risk evaluation. So it is

rather risky to define a safe ac voltage without data from such special corrosion rate recordings.

The particular ac-corrosion process analysed hereto, is also characterized by the small spread

resistance and the lowering of the spread resistance of the coupon upon ac voltage increase. A

tendency to reach a plateau of very low spread resistance at high ac voltage is evident (Figure 15 ).

This behavior of the spread resistance seems to be in accordance to the alkalization mechanism of ac-

corrosion. The rise of ac voltage may increase local pH at the coupon/soil interface therefore

catalysing iron dissolution.

Another interesting observation was that the relation of the transient corrosion rates taken at the peaks of 16-measurements average obeyed a rather linear relationship with the logarithm of the



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corresponding peaks of the ac current density (Figure 16). The explanation for that is rather vague

for the moment. It might be related to the alkalization theory or it could be explained by another

approach as outlined hereafter.

The authors wish to express some initial thoughts about an alternative perspective of ac-corrosion

mechanism relating it to the electronic properties of the oxides formations on the metal surface,

taking into account the amorphous semiconductor theory and ac rectification by the amorphous

ferrous oxide films. This approach might also account for the slow drifts of dc potential associated

with high ac voltage on the pipeline. The amorphous semiconductor theory approach had been found

helpful for the anticorrosion properties of oxide films in connection with corrosion inhibitors effects

in aluminum alloys corrosion [15]. Regarding future work, a hint is given at the anticorrosion

properties of the amorphous oxide films on the metal surface.

Results from case study no. 2

In the following, some interesting results are presented from recordings on two ER-probes of total

surface area 0,8 cm2 (0,4 cm2 each) buried at the position 304.4 km of the main pipeline.

1. AC corrosion related to CP loss

From these results, the importance of cathodic protection potential monitoring and control isestablished. The rectifier being in galvanostatic mode, for an unknown reason - maybe an accidental

contact of the pipe with a foreign grounded structure - the on cp potential dropped to low levels

(around -0,74V) for a two weeks period (Figure 17). At the same time the corrosion rates recorded at

ER-probes installed in the pipeline (at 304,4 km) increased rapidly whereas the transient corrosion

rates were fluctuating in accordance to the ac voltage fluctuations during the same period (Figure

17). The escalation of the momentary corrosion rate at every ac voltage peak, suggests that it is very

likely this corrosion process, which is initiated by the absence of adequate cathodic protection, is

related to ac-interference. It is remarkable that the ac-current density is more or less constant and

well below the threshold of 100 A/m2 (Figure 17). It is also noteworthy that during the period of

normal cathodic protection, no corrosion is detected despite the relatively negative cp-level (median

on-potential -1,34 V with slow negative deviations up to -1,89 V).In this case, the ac-current density hardly plays a significant role in the corrosion process while the

corrosion behaviour of the coupon is characterized by the increase of spread resistance Rs with the ac

voltage (Figure 18). The same trend of the spread resistance with the ac voltage is observed

independent of whether corrosion occurs or not.

This tendency for the spread resistance is supposed to be related to soil constituents (e.g. earth

alkaline cations such as calcium), which by the action of ac voltage can promote the formation of

resistive deposits on the metal surface [9,16].

Consequently, compared to the status recorded in the case study no.1., a completely different

behavior of the ER-coupons is evident in the case study no.2.

The above results suggest that at least two diversified ac-corrosion mechanisms exist.The first mechanism is pertinent to the fall of spread resistance upon ac voltage increase and it seems

to be related to very conductive and aggressive soils. It is possible that the ac-corrosion can rise

dramatically even at low ac voltage level whereas the ac-current density has significant impact on

corrosion rates, being also very sensitive to small variations of the ac voltage due to minimal spread

resistance. With this ac-corrosion mechanism, the CP surplus (cathodic shifting of CP potential) is

believed to give rise to severe corrosion rates.

The second mechanism is characterized by the elevation of the spread resistance when ac voltage is

increased as well as the ac-corrosion triggering by loss of CP. The ac-current density is rather stable

and insensitive to ac voltage variations. However, it seems that ac-corrosion can be activated when

CP is lost (by anodic shifting of CP potential) even when the ac current density is well below

100A/m2.



12

2. Telluric currents corrosion

Another interesting case, observed at the same coupons at the same place, is this briefly described in

the following. By the use of the same ER-probe technique we could detect corrosion emanating from

telluric activity that was completely differentiated from and independent of ac-corrosion (Figure 19 ).

Note that at the same place (304,4 km) where the ER-probe was buried, although a sudden increase

of the ac-voltage from 2 to 12 V was recorded, it was not accompanied with ac-corrosion.

Nevertheless, the brief period of higher corrosion rate (during the late hours of 12/6/2006 and the

first morning hours of 13/6/2006) coincides with an increase of telluric activity indicated by the Kp

index which is a number - ranging from 0 to 9 - indicating telluric activity intensity [17].

The above results show the complexity of the problem implying the cathodic protection engineer

should be particularly cautious that every case should be treated separately. The electrical parameters

of the interference, the cathodic protection status and the soil physical and chemical properties can

have a significant impact on the corrosion behaviour at coating defects of pipelines.

Further actions – Remedial measures

The remedial measures suggested for the efficient control of the ac-corrosion process might be a

series of actions such as those briefly outlined below:

CIPS, DCVG, and similar surveys that help find and repair coating defects in aggressive soils Repair of coating defects with new methods and materials that prevent the metal from coming

into contact with corrosive constituents of the environment

Installation of additional grounding sites

Improvement of the spread resistance of the grounding electrodes

Installation of more effective ac conducting dc decoupling elements

Closest monitoring of the cathodic protection operation (by special data loggers, CP remote

monitoring systems etc.)

ER-probe corrosion monitoring at longer durations

Proximity effect studies (i.e. ac-voltage interference theoretical calculations) based on more

accurate models [18,19]

Efforts to comply with the rules of CEN/TS 15280

Chemical analysis of aggressive soils as well as corrosion products analysis

Intelligent pigging

Bell-hole excavations for inspection of the pipeline with NDT techniques, pit depth gage etc.

Lab experiments that can render light to the mechanism of ac-corrosion

Application of Pipeline Integrity Management (PIM) and External Corrosion Defect

Assessment (ECDA) guidelines [20].

The main countermeasure to combat ac-corrosion is the ac (voltage) - mitigation through effective

ac-conducting dc-decoupling devices. Regarding such devices, particular attention must be given on

the ac-conduction at any value of low ac voltage in combination with negligible dc current leak

otherwise may be ineffective in terms of both ac-corrosion control and undisturbed function ofcathodic protection.

As a next research step, the role of the soil constituents in ac-corrosion a well as the mechanism of

the influence of the ac voltage on spread resistance are recommended as topics of further

investigation.

CONCLUSIONS

The present case studies exhibit results taken from ER-probes recordings in the cathodic protection

system of the Greek gas transmission main pipeline. In the case study no.1 a severe ac-corrosion case

is disclosed. The results indicate the important role of the ac current density in aggressive soils. The

ac-corrosion can grow rapidly even at ac voltage levels on the pipeline that are considered low

(around 1 Volt). Under these circumstances, the ac-corrosion mechanism is characterized by thetendency of the spread resistance to decrease upon ac voltage increasing.



13

Interesting results obtained from ER-probes connected at another place in a different pipe section

(case study no.2) pointed to a different ac-corrosion, which is related to loss of cathodic protection.

Those data imply a distinctive ac-corrosion mechanism, which is marked by the elevation of spread

resistance upon ac voltage increasing in spite of the aforementioned ac-corrosion mechanism.

The ER-probe technique is sensitive enough to detect even small corrosion caused by telluric

currents and differentiate it from ac-corrosion.

Eventually, the ER-probes technique can be an invaluable tool for the evaluation of the effectiveness

of the ac-mitigation measures. Besides it is a promising diagnostic tool that helps locate areas of the

pipeline prone to ac-corrosion that deserve special attention. Critical ac voltage and cp levels for ac-

corrosion to occur can be identified from analogous evaluations and the remedial measures can be

directed properly.

Generally, an integrated corrosion risk evaluation and several inspections, anti-corrosion and

rehabilitation actions have to be taken for a successful outcome in the fight against pipeline

corrosion. The routine CP inspection plans have to be modified by adopting new technologies (e.g.

for CP remote monitoring, corrosion monitoring and ac-mitigation) and by applying the

contemporary guidelines of pipelines integrity management.

ACKNOWLEDGEMENTS

The authors wish to acknowledge the cooperation and advice from Dr. Lars Vendelbo Nielsen

(Metricorr), Mr. Henrik Rosenberg (BALSLEV Consulting Engineers A/S) and the professors of

National Technical University of Athens, Mr. G. Batis and Mrs. N. Kouloumbi.

REFERENCES

1.W.von Baeckmann, W. Schwenk, W. Prinz (Eds.), Handbook of Cathodic Corrosion

Protection:Theory and Practice of Electrochemical Protection Processes, Gulf Publishing

Company, Houston TX, 1997.

2. Technical Specification prCEN/TS 15280

Evaluation of a.c. corrosion likelihood of buried pipelines–Application to cathodically

protected pipelines, July 2005.

3. EN 12954 Cathodic Protection of Buried or Immersed Metallic Structures - General

Principles and Applications for Pipelines. CEN – Brussels 2001.

4. L.V.Nielsen, P.Cohn, B.Baumgarten , Investigating AC and DC stray current

corrosion, CeoCor 2005.

5. 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, CeoCor 2006.

6. N.Birbilis, L.J.Holloway, M.Forsyth, Technical Note: Simulated Transient Loss of

Cathodic Protection for Buried Pipelines, Corrosion NACE, Vol. 61, No.5, 2005, pp.498-

501.

7. N.Kouloumbi, G. Batis, N. Kioupis, P. Asteridis, Study of the effect of AC-interference

on the cathodic protection of a Gas Pipeline , ACMM, Vol. 49, No. 5, 2002, pp. 335-345.



14

8. =. Kioupis, N. Kouloumbi, G. Batis, P. Asteridis Study of the AC- Interference and

AC-mitigation on the cathodic protection of a Gas pipeline, CeoCor 2003.

9. L.V.Nielsen, K.V.Nielsen, B.Baumgarten, H.Breuning-Madsen, P.Cohn, H.Rosenberg,

AC-induced corrosion in pipelines: Detection, characterisation and mitigation, NACE

2004 paper No. 04211.

10. L.V.Nielsen, K.V.Nielsen, Differential ER-technology for measuring degree of

accumulated corrosion as well as instant corrosion rate, NACE 2003 paper No. 03443.

11. L.V.Nielsen, F. Galsgaard, Sensor technology for on-line monitoring of AC-induced

corrosion along pipelines, NACE 2005 paper No. 05375.

12. L.V.Nielsen, P.Cohn, On-site measurements of AC induced corrosion - Effect of AC

and DC parameters, CeoCor 2004.

13. CeoCor Booklet, AC Corrosion on Cathodically Protected Pipelines - Guidelines for

risk assessment and mitigation measures, APCE, 2001.

14. R.A. Gummow, R.G. Wakelin, S.M. Segall, AC Corrosion – A challenge to PipelineIntegrity , MP, Vol. 38, No.2, 1999, pp. 24-30.

15. L.Kobotiatis, N.Kioupis, P.G.Koutsoukos, Electronic Properties of Passive Filmsgrown on Al 7075 in solutions containing oxalate and chromate, Corrosion NACE, Vol.53, No.7, 1997, pp.562-571.

16. F.Stalder et al., AC corrosion on cathodically protected pipelines, CeoCor 2000.

17. BALSLEV Report no.4460500G302, Stray Current Investigation Project for theNatural Gas Pipeline, 2005.

18. G. C. Christoforidis, D.P. Labridis, P.S. Dokopoulos, N. Kioupis AC- interference ona Gas Pipeline caused by nearby power lines in a complex right-of-way –Comparisonbetween measurements and calculations, CeoCor 2004.

19. G. Lucca, L. Di Biase, M. Moro, A.C. Corrosion on buried pipelines: a probabilisticapproach, CeoCor 2003.

20. ASME B31.8S-2001 Managing System Integrity of Gas Pipelines.



15

Figure 6. Map of the Greek Natural Gas Transmission System where the regions of the ER-probes of

both case studies are shown.

Figure 7 . Potential-pH (Pourbaix) diagram of iron in water showing the region of ac-corrosion risk.



16

PROBE 110Initial

Thickness

(µm)

Final

Thickness

(µm)

CP+ AC

Duration

(days)

NC

Duration

(days)

total corrosion rate

per time period

(µm/yr)

ac corrosion rate

per time period

(µm/yr)

cumulative total

corrosion rate

(µm/yr)

cumulative ac

corrosion rate

(µm/yr)

from 18/11/2005

until25/11/2005from 25/11/2005

until 22/12/05

from 22/12/05

until 10/01/06

from 10/01/06

until 12/01/06

from 12/01/06

until 16/01/06

from 16/01/06

until 20/01/06

365

236

404

365

54

425

0

65,7 18,9

118

23289 65,7 14 6

93 89 10 17

TIME PERIOD

100 93 7

2 0 529 8978541

1725 6899 570 1074

0

365

8541

607

146

365

_ 18,9 18,9 0 4 493 897

18,9 0 1 3

Table 1. Corrosion history of ER-probe no. 110

PROBE 124Initial

Thickness

(µm)

Final

Thickness

(µm)

CP + AC

Duration

(days)

NC

Duration

(days)

total corrosion rate

per time period

(µm/yr)

ac corrosion rate

per time period

(µm/yr)

cumulative total

corrosion rate

(µm/yr)

cumulative ac

corrosion rate

(µm/yr)

from 18/11/2005

until 25/11/2005

from 25/11/2005

until 22/12/05

from 22/12/05

until 10/01/06

from 10/01/06

until 12/01/06

from 14/01/06

until 16/01/06

from 18/01/06

until 20/01/06

193

199

TIME PERIOD

100 87,5 7

18381,5 71,5 14

0 652

397

6

87,5 81,5 10 17

587

81

652

336

346 5874490

46,9 0 4

46,9 2 0 4490

261

219

652

107446,9 0 1 3 17119

_

4280 570

0 323

71,5

652

46,9

Table 2. Corrosion history of ER-probe no. 124



17

CORROSION RATE PER TIME PERIOD

652219 261

4490

17119

365 146607

8541

17119

0

2000

4000

6000

8000

10000

12000

14000

16000

18000

TIME PERIODS WITH AC CORROSION INFLUENCE

C O R R O S I O N R A T E

( µ m / y r )

PROBE 110

PROBE 124

7 days18/11/05-5/12/05

10 days25/11/05-5/12/05

14 days22/12/05-4/1/06

2 days10/1/06-12/1/06

1 day16/1/06-17/1/06

PENETRATION

Figure 8. AC-Corrosion rates per time period (ER-probes no.110 and no.124)

CUMULATIVE AC CORROSION

365

236

404

897 897

1074

652

397

336

587 587

1074

0

200

400

600

800

1000

1200

CUMULATIVE TIME WITH AC CORROSION INFLUENCE

C O R R O S I O N R A T E ( µ m

/ y r )

PROBE 110

PROBE 124

7 days 17 days 31 days 33 days 33 days 34 days

PENETRATION

Figure 9. Cumulative AC-Corrosion rates (ER-probes no.110 and no.124)



18

Probe-Tag: 110

Probe Type:

Probe serial No.:

Test initiated: 18/11/2005

0

200

400

600

800

1000

1200

1400

1600

1800

2000

16/11/05 20/11/05 24/11/05

Date

V c o r r - µ m / y r

Trend based on 96 measurementsTrend based on 16 measurements

Corrosion Rate

0,0

0,2

0,40,6

0,8

1,0

1,2

1,4

1,6

1,8

2,0

16/11/05 20/11/05 24/11/05Date

A C V

o l t a g e - V

AC voltage

AC Voltage

Ff

Probe-Tag: 124

Probe Type:

Probe serial No.:


0

500

1000

1500

2000

2500

3000

3500

4000

4500

16/11/05 20/11/05 24/11/05

Date

V c o r r - µ m / y r

Trend based on 96 m easurementsTrend based on 16 m easurements

Corrosion Rate

Figure 10. Corrosion rates and ac voltage during initial phase (ER-probes no.110 and no.124)



19

Figure 11. Recording of the sudden dc potential shifts of gas and oil pipeline on 4/1/2006 14:00

(blue line: gas pipe, red line: oil pipe)



20

Probe-Tag: 110

Probe Type:

Probe serial No.:


0

1000

2000

3000

4000

5000

6000

7000

8000

9000

18/ 12/2005 22/ 12/2005 26/12/2005 30/12/ 2005 3/1/2006 7/1/2006

Date

V c o r r - m y m / y r

Trend based on 96 m easurementsTrend based on 16 measurements

Corrosion Rate

Probe-Tag: 124

Probe Type:

Probe serial No.:


0

1000

2000

3000

4000

5000

6000

7000

8000

9000

18/12/2005 22/12/2005 26/12/2005 30/12/2005 3/1/2006 7/1/2006

Date


Trend based on 96 m easurementsTrend based on 16 m easurements

Corrosion Rate

Figure 12. Corrosion rates during the second phase (ER-probes no.110 and no.124)



21

Probe-Tag: 110

Probe Type:

Probe serial No.:


0,0

0,2

0,4

0,6

0,8

1,0

1,2

1,4

1,6

1,8

18/12/2005 22/12/2005 26/12/2005 30/12/2005 3/1/2006 7/1/2006Date

A C

V o l t a g e - V

AC voltage

AC Voltage

Probe-Tag: 110

Probe Type:

Probe serial No.:


1

10

100

1000

18/12/2005 22/12/2005 26/12/2005 30/12/2005 3/1/2006 7/1/2006Date

A C

c u r r e n t

- A / m 2

AC current density

Probe-Tag: 110

Probe Type:

Probe serial No.:


0,001

0,01

0,1

1

10

18/12/2005 22/12/2005 26/12/2005 30/12/2005 3/1/2006 7/1/2006Date

S p r e a d R e s i s t a n c e

- O h m . m

2

Spread resistance

Spread resistance

(Leakage resistance)

Figure 13. AC parameters and spread resistance of the coupons during the same period of corrosion

depicted in Fig.12.



22

Correlation between ac voltage and ac current density

y = 0,0022x + 0,3192

R

2

= 0,9746

y = 0,0041x + 0,2224

R2

= 0,9427

0

0,2

0,4

0,6

0,8

1

1,2

1,4

1,6

1,8

2

0 100 200 300 400 500 600 700

AC CURRENT DENSITY (+ /m2)

A C V O L T A G E ( V )

PROBE 110

PROBE 124

Figure 14. The ac voltage vs. ac current density on ER-probes no. 110 and no. 124 (the slope of each

line is indicative of the average spread resistance – also note the safe upper limit of the ac current

density)

Rs vs. ac voltage- probe 110

0

0,005

0,01

0,015

0,02

0,025

0,03

0 0,2 0,4 0,6 0,8 1 1,2 1,4 1,6

ac voltage (V)

R s ( O h m

m 2 )

Figure 15. Reduction of spread resistance with ac voltage



23

MOMENTARY CORROSION RATE vs. AC CURRENT DENSITY

y = 8861.7Ln(x) - 49348

R2

= 0.9992

y = 2594.7Ln(x) - 13017

R2

= 0.9549

0

1000

2000

3000

4000

5000

6000

7000

8000

9000

10 100 1000

AC CURRENT DENSITY (A/m2)

C O R R O S I O N R A T E ( µ m / y )

PROBE 110

PROBE 124

Figure 16. Linear relationship of momentary corrosion rates with the ac current density (logarithmic

scale)



24

Probe-Tag: 35

Probe Type:

Probe serial No.:


0

500

1000

1500

2000

2500

3000

13/6/05 17/6/05 21/6/05 25/6/05 29/6/05 3/7/05 7/7/05 11/7/05

Date



Corrosion Rate

Probe-Tag: 35

Probe Type:

Probe serial No.:


0

1

2

3

4

5

67

8

13/6/05 17/6/05 21/6/05 25/6/05 29/6/05 3/7/05 7/7/05 11/7/05Date

A C

V o l t a g e - V

AC voltage

AC Voltage

Probe-Tag: 35

Probe Type:

Probe serial No.:


1

10

100

1000

13/6/05 17/6/05 21/6/05 25/6/05 29/6/05 3/7/05 7/7/05 11/7/05Date

A C

c u r r e n t d e n s i t y -

A / m 2

AC current

AC current density

Figure 17. AC-corrosion related to CP loss (ER-probe no.35). Corrosion rate and ac parameters at

the coupon(304.4km); AC voltage(blue line) and DC potential(red line) at T/R site 198,3km.



25

Rs vs. ac voltage - probe 35

0

0,05

0,1

0,15

0,2

0,25

0 2 4 6 8 10 12 14

ac voltage (V)

R s ( O h m

m 2 )

Figure 18. Elevation of spread resistance with ac voltage



Probe-Tag: 35

Probe Type:

Probe serial No.:


0

50

100

150

200

250

300

350

400

450

9/6/05 13/6/05 17/6/05

Date

V c o r r - m y m

/ y r


Corrosion Rate

Probe-Tag: 35

Probe Type:

Probe serial No.:


0

2

4

6

8

10

12

14

9/6/05 13/6/05 17/6/05Date

A C

V o l t a g e - V

AC voltage

AC Voltage

Probe-Tag: 35

Probe Type:

Probe serial No.:


1

10

100

1.000

9/6/05 13/6/05 17/6/05Date

A C

c u r r e n t - A / m 2

AC current

AC current density

Figure 19. Telluric currents corrosion (ER-probe no.35). Corrosion rate and ac parameters at the

coupon (304.4km); Telluric intensity Kp index.

Documents

AC-Corrosion Detection on Electrical Resistance Probes Connected to a Natural Gas Transmission Pipeline