Volume 18 Preprint 6
Monitoring of the External Corrosion of Buried Pipelines
A.I. Marshakov, M.A. Petrunin, V.E. Ignatenko, Ð¢.A. Nenasheva, Ð¢.A. Yurasova, L.B. Maksaeva, A.A. Rybkin
Keywords: pipeline, cathodic protection, corrosion monitoring
The methods for the monitoring of various types of external corrosion of underground pipelines are investigated. The various scenarios where electrical resistance soil corrosion probes can be used for corrosion damage assessment or for the control of cathodic protection effectiveness are described. The possibilities of using hydrogen probes for the prediction of stress corrosion cracking of pipelines are discussed.
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Monitoring of the External Corrosion of Buried Pipelines
A.I. Marshakov, M.A. Petrunin, V.E. Ignatenko, Ɍ.A. Nenasheva, Ɍ.A. Yurasova, L.B.
Maksaeva, A.A. Rybkin
A.N. Frumkin Institute of Physical Chemistry and Electrochemistry Russian Academy
of Sciences, 31 Leninsky prospect, Moscow GSP-1, 119071 Russian Federation.
Email: email@example.com (T.A. Nenasheva)
The methods for the monitoring of various types of external corrosion of
underground pipelines are investigated. The various scenarios where electrical
resistance soil corrosion probes can be used for corrosion damage assessment or
for the control of cathodic protection effectiveness are described. The possibilities
of using hydrogen probes for the prediction of stress corrosion cracking of
pipelines are discussed.
Key words: pipeline, cathodic protection, corrosion monitoring
Currently, the main methods for monitoring the corrosion state of underground
pipelines are electrometric diagnostics with control excavations and in-line
inspection. The use of these methods requires heavy financial expenditures; hence,
the development of PHPORGV IRU POH LQGLUHŃPµ GHPHUPLQMPLRQ RI POH ŃRUURVLRQ
state and protection efficiency of pipelines is attractive. Principally, this problem
can be solved by statistical analysis and mathematical simulation models relating
cases of the corrosion failures of pipelines to their design features, age, operating
conditions, data on geological types, chemical composition and microbial activity
of the soils, and cathodic protection parameters . However, the number of
factors taken into account in such models is too large, and the currently available
databases lack sufficient data for the development of mathematical models of
high reliability. In view of this, the main goals in the development of the theory of
pipe steel corrosion in soil is to determine the main factors in the origination and
growth of various types of corrosion defects and to develop methods for their
monitoring along the route of existing pipelines.
The common goals of corrosion monitoring (CM) of underground pipelines are to
determine the following parameters:
1) The corrosivity of the soil and/or under-film electrolyte toward various types of
corrosion damage to the pipe steel;
2) The corrosion risk for a pipeline segment, namely, the presence of galvanic
macropairs, stray currents, and induced alternating currents;
3) The state of the isolating coating;
4) The efficiency of the pipeline cathodic protection (CP).
The following parameters are of major importance in the nucleation of local types
of corrosion: the state of pipe surface, the presence of non-metallic inclusions and
metallurgical defects in the pipe steel, and the magnitude and frequency of the
mechanical strains in the pipe. It is unlikely that all factors affecting the growth of
various corrosion defects can be monitored on existing pipelines. Therefore, under
actual operation cRQGLPLRQV MQ\ LQGLUHŃPµ GLMJQRVPLŃ PHPORGV RLOO provide a
SURNMNLOLVPLŃ HVPLPMPH RI POH SLSHOLQH·V ŃRUURVLRQ VPMPHB +RRHYHU VPXGLHV RQ POH
mechanisms and kinetics of the underground corrosion of pipe steel will improve
the reliability of such estimates.
An LGHMOµ SMVVLYH SLSHOLQH SURPHŃPLRQ, excluding any contact of the metal with
electrolytes and preventing the origination of any corrosion defects, theoretically
makes it unnecessary to know the corrosion mechanism of pipe steel in soils. The
use of factory-applied coatings has made it possible to significantly reduce
pipeline corrosion damage. However, some mechanical damage to the pipe
insulation during transportation, pipeline assembly and operation is inevitable.
"WHMNHUµ ŃRMPLQJV MUH XVed when insulating pipe joints under field conditions, and
the protective properties of coatings degrade over prolonged pipeline operation.
Hence, monitoring the pipe outer surface corrosion remains a problem of current
interest, even where modern isolation pipeline coatings are used.
Determination of the state of the pipeline insulating coating is an important
part of CM; however, this issue will not be covered in this communication because
methods for monitoring the insulation quality are well-known and have been
1.1. Types of corrosion damage and general principles of their
At the first stage of organising the CM, one needs to determine what types of
corrosion damage to the pipeline outer wall are to be monitored. From the
viewpoint of the electrochemical process mechanism, only three types of external
pipeline corrosion can be distinguished: uniform corrosion, crevice corrosion, and
stress corrosion cracking.
Uniform underground corrosion is an arbitrary term because it implies that the
metal surface is equipotential, its structure is uniform, and the corrosion medium
is homogeneous and has a uniform composition. None of the three conditions are
met in the underground corrosion of real-life steel structures. However, the rate of
uniform corrosion determined, e.g., from the mass loss of a check specimen, can
be used for comparative assessment of the soil corrosivity and efficiency of the
Crevice corrosion is the most widespread type of underground corrosion of steel
structures; it is impossible without a potential difference between two areas of a
pipe surface. Differences in the electrode potential appear for various reasons:
stray currents in the soil, operation of macro corrosion cells formed by parts of a
pipeline buried in soils with differing chemical compositions or degrees of
aeration, operation of galvanic elements at the level of ground waters or in a
crevice between a pipe surface and a delaminated insulating coating, etc.
Stress corrosion cracking is currently the most hazardous type of underground
corrosion of high-pressure gas pipelines. The origination and growth of a crevice
in a metal is determined by a combination of three factors: the magnitude of
mechanical stress, the alloy structure and composition, and composition of the
The other known types of local corrosion, such as pitting or intergranular
corrosion, are not typical in underground pipelines. It is possible that such
corrosion defects appear at the early growth stages of a crack or crevice.
Local corrosion defects can be initiated either in a coating defect or under a
delaminated coating. The CM methods will differ depending on the location of the
At the second stage of organising the CM, methods for monitoring the possibility
of the origination and/or growth rate of corrosion sites are defined. There are two
approaches to address this problem. The first one is to use methods that directly
measure the metal mass loss or corrosion penetration depth. This goal is achieved
using probes whose working electrodes undergo corrosion under conditions
VLPLOMU PR POH ŃRUURVLRQ ŃRQGLPLRQV RI POH SLSH·V RXPVLGH RMOOB 7OH VHŃRQG
approach is to monitor the factors that cause the initiation and growth of a
corrosion site, namely: the pipe potential, the chemical composition of the soil (or
under-film) electrolyte, and changes in the metal properties that accompany
corrosion. The second CM approach requires knowledge of the origination reasons
and the growth mechanism of a corrosion site. As this knowledge is accumulated,
ideas about the critical values of the factors being monitored change, and
sometimes the question arises: do we monitor the right factor?
The choice of the factor to be monitored is determined not only by the existence
of an unambiguous functional relationship between this factor and the external
corrosion rate of the pipeline but also by the cost of the measurements. From this
point of view, indirect monitoring methods are generally advantageous.
1.2. Measuring the potential of pipeline segments
Currently, the efficiency of pipeline electrochemical protection is monitored using
the protective potential value. The electrode potential is an indirect parameter
characterizing the metal corrosion rate; hence, the CP protective potential is also
an indirect parameter that characterizes the pipeline protection level. What are the
grounds for this statement?
In the first stage of the development of the scientific bases of the method for the
electrochemical protection of steel structures in soils and natural waters, attempts
were made to give a thermodynamic substantiation to the protective potential, i.e.,
to relate this value with the equilibrium potential of iron, E0 (SHE):
Fe ĺ Fe2+ + 2Ɲ, E0 =- 0.44 V + (RT/2F) ln[Fe2+] (1)
At first sight, this approach appears to be justified if one sets a permissible rate
for the diffusion transfer of iron ions to the electrolyte bulk and thus defines the
maximum permissible concentration of iron cations at the metal surface
Baeckmann and Schwenk . However, Equation (1) fails to account for the
participation of electrolyte components in the iron dissolution reaction
Florianovich  and, in particular, the possible formation of hardly-soluble
corrosion products. In fact, considering that iron hydroxide is formed on the metal
surface, the equilibrium potential of the anodic reaction can be determined as:
Fe + 2OH- ĺ Fe(OH)2 + 2Ɲ, E0 = ²0.877 V ² (RT/F) ln[OH-] (2)
Equation (2) should occur upon alkalization of the near-electrode solution layer
due to the cathodic reduction of oxygen or water.
Natural soils contain activators of iron anodic dissolution, e.g., CO3
-) and S2-
(HS-) anions, which shift E0 in the negative direction as their concentration is
increased. Calculation of E0 for the formation of iron carbonate and sulphide gives
by Equations (3) and (4):
Fe + CO3
2- ĺ FeCO3 + 2Ɲ, E0 = ²0.756 V ² (RT/2F) ln[CO3
Fe + S2- ĺ FeS + 2Ɲ, E0 = ²0.95 V ² (RT/2F) ln[S2-] (4)
The thermodynamic substantiation of the protective potential value became even
less justified after the development of the concept of the chemical coupling of
partial corrosion reactions . Chemical coupling implies that a fraction of the free
energy produced by one reaction compensates for the energy consumption by
another (induced) reaction. As a result, the quasi-equilibrium potential of the
induced reaction shifts in the negative direction. A possible mechanism of chemical
coupling of the partial corrosion reactions involves the participation of the
intermediates of the oxidant reduction in the anodic process. Oxygen-containing
oxidants (which form OH- ions upon reduction) accelerate iron dissolution in acidic
media (pH 1-3) at a constant potential, with an acceleration factor of several tens [8-
9]. In weakly- acidic and neutral electrolytes, the accelerating effect of oxygen-
containing oxidants weakens or is not observed. However, even in pH-neutral media,
iron dissolution would be accelerated in the presence of sulphur- or nitrogen-
containing compounds that are reduced to give sulphide ions or ammonia
Thus, the metal dissolution rate is not solely a function of potential because it also
depends on the composition of the corrosive medium and on the rates of other
electrode reactions. The currently accepted range of protective potentials for
pipelines (-0.85 to -1.15 V vs. CSE) has been established empirically, based on
results of long-term practical electrochemical protection of steel structures. The
effect of the corrosion medium composition on the electrochemical kinetics of
iron dissolution is taken into account when establishing the minimum protective
potential, which either increases (in absolute magnitude) to -0.95 V in soils with
microbiological activity or decreases to -0.75 V in low-mineralised high-resistivity
Despite the absence of theoretical substantiations of the protective potential,
monitoring this value along a pipeline versus time is currently the basis for
determining the electrochemical protection efficiency of underground pipelines.
However, this criterion of electrochemical protection efficiency has one significant
drawback: its value does not ensure a quantitative determination of the residual
corrosion rate of the outside pipe wall. As a rule, this results in considerable
financial expense required to maintain too negative protective potentials; however,
in certain cases, the lack of knowledge of the residual corrosion rate may result in
the origination of hazardous corrosion defects on the outside pipe wall in a hole
of the isolating coating.
Furthermore, considerable difficulties exist in the protective potential
measurements of a pipeline segment, primarily due to the compensation of the
ohmic component (IR) in the measured potential value. First, due to the existence
of the IR-component, the potential that is established on the metal in real modes
of electrochemical protection depends on the area of a hole in an isolating coating
. Second, despite the progress in the methods for elimination of the ohmic
component  the results of the polarized potential measurements in the zones of
direct and alternating stray currents are not sufficiently reliable. Third, it is
difficult to measure the polarized potential in high-resistance soils (rocky, dry, or
permafrost) or on pipelines with a heat-insulating coating.
It is practically useless to measure the potential of a pipeline segment to assess
the initiation probability and growth rate of local corrosion defects under a
Thus, to overcome CM problems, it is insufficient to measure the pipe polarized
potential, and it is necessary to search for new methods for assessing the pipeline
corrosion state and CP efficiency.
1.3. Determination of the pipe outside corrosion rate using electrical
Electrical resistance probes (ER probes) are widely used to monitor corrosion rates
in various technical areas. Their operating principle is based on the fact that the
electric resistance of metallic conductors increases as the cross-sectional area
decreases due to corrosion. In the case of pipeline CM, a corrosion probe is placed
in the soil adjacent to the pipe surface and is electrically connected to the pipeline
1.4. Assessment of the possibility of hydrogen induced stress
cracking of a steel pipeline
The possibility of the origination of such hazardous corrosion as hydrogen
induced stress cracking (HISC) of pipe steel can be monitored using hydrogen
probes. The dissolution of atomic hydrogen in steel can result in hydrogen
embrittlement and, hence, in the stress corrosion cracking (SCC) of steel. A crack
grows when the hydrogen concentration in the metal exceeds a certain critical value.
The critical concentration values are reached at critical rates of hydrogen
permeation into steel [16-17]. The rates of hydrogen permeation into steel can be
measured under field conditions using hydrogen probes .
In this paper, the opportunity to assess the pipeline corrosion state and CP
efficiency using ER probes and hydrogen probes are discussed.
2. Experimental procedure
2.1. Design of electrical resistance probes
The corrosion penetration depth (¨d) is calculated either from an increase in the
working element (WE) resistance (for general corrosion) or from the time until WE
rupture (for local corrosion). The general corrosion rate is determined as:
¨d = d0(1 ² R0/R
where d0 is the initial thickness of WE; R0 is the initial WE resistance; and R
WE resistance upon exposure time,
By simultaneously recording the WE and thermal sensor resistances, it is possible to
bring the WE resistance to a standard temperature. Then, the observed changes in
the readings from ER probe can be attributed to corrosion effects.
The surface (S MQG POLŃNQHVV RI POH (5 SURNH·V J( MUH VHOHŃPHG NMVHG RQ POH
following conflicting requirements:
- The d0 value should be large to ensure a long life of the probe. It has been
determined experimentally that the following requirement should be met when the
rate or uniform corrosion is determined: ¨d < 0.5 d0 .
- High sensitivity (or improvement of the probe response time) requires
either a small d0 or high-precision measurements of WE resistance; however, the
latter approach considerably increases the cost of the measuring instrument;
- High reproducibility of ER probe readings requires a sufficiently large area
of metal contact with the heterogeneous medium (soil).
- The power consumption for electrochemical protection with a probe
connected to the pipe is minimised by decreasing S. However, at too low S values,
the probe potential in the real pipeline CP circuits will be more negative than the
metal potential in the coating holes  hence, S should not be smaller than the
maximum area of a coating defect on the pipeline segment being monitored.
- The probe design should allow it to be located as close as possible to the
pipeline; however, installation of a probe directly on the pipe surface generally
makes mounting more expensive.
Furthermore, the dimensions and configuration of the working element should
allow assessment of the crevice growth rate based on the rupture time,
All of the above requirements cannot be achieved in a single sensor. In view
of this, a number of ER probe types have been developed by [12, 13, 19, 20], and the
use of a specific probe in a CM system depends on the intended purpose. Below
described results were obtained by using two types of ER probes (Table 1).
Table 1. Dimensions and uses of WEs in corrosion ER probes.
WE surface area,
cm2 Probe application
DK 100 30 - 330
Monitoring the pipeline corrosion rate in
insulating coating defects; assessment of
cathodic protection performance
VIK 0.1 - 0.2 4.5
Express analysis of soil corrosivity;
assessment of cathodic protection
DK probe is a cylindrical corrosion probe equipped with a built-in resistance
thermometer. The working element of DK probe is a 100 µm thick steel foil band,
3 mm wide, arranged on a plastic tube in the form of a one-layer winding with a
1.5 mm step. The length of WE can be varied from 1 to 11 m. The chemical
composition (wt %) of WE steel is as follows: C: 0.05, Mn: 0.38, Si: 0.03, Cr: 0.05, Cu:
0.15, S: 0.04, P: 0.035, Al: 0.16, Ni: 0.09.
The working element of the VIK probe consists of a 0.1 - 0.2 µm iron layer thick
vapour-deposited on a dielectric support by decomposition of iron pentacarbonyl.
It is used for the express assessment (within 1 to 2 days) of soil corrosivity and for
monitoring the parameters of the electrochemical protection of pipelines. VIK
probe is characterised by high sensitivity and fast response (approximately 30
min), but its lifetime is as short as several days. The reliability of corrosion rates
obtained using the VIK was confirmed by the gravimetric method using steel
2.2. Design and operating principle of the hydrogen probe
The DH-1 hydrogen probe was developed by .The probe operation is based on
the use of the Devanathan-6PMŃOXUVNL·V ŃHOO. $ SURNH·V 100 P POLŃN VPHHO
membrane is electrically connected to a pipe, and its outside surface has the same
potential as the pipe. Hydrogen formed on the outer membrane surface partially
enters into the metal and diffuses to the internal membrane surface. On the
internal membrane surface, a potential is maintained at which atomic hydrogen is
oxidised. Under steady-state conditions, hydrogen concentration in the metal (CH)
can be calculated from the current of the hydrogen penetration through the steel
membrane, ip (Equation 6):
CH = ip L M/F
where L is the membrane thickness, M is the atomic mass of hydrogen, F is the
is the specific density of the steel, and D is the hydrogen
diffusion coefficient in the steel. Thus, ip can be considered as a criterion of the
steel hydrogen embrittlement hazard.
2.3. Design of the CM point
Typical CM point was equipped with several ER-probes or hydrogen probes,
reference electrode (CSE), data logger with satellite transmitter for collecting and
transmitting of data (Fig. 1). All probes have electric contact with the pipeline
except ER-probes which were used for determination of corrosion activity of soil.
The probes were installed into ground close (not far 5 cm) to pipe surface.
Fig. 1: Design of typical CM point.
2.4. Data collection
IR probes were monitored at least 2 times per day for the first month. Thereafter,
the monitoring frequency was decreased, but it was not less than one reading per
month. Hydrogen probes were monitored at least 4 times per day for the test
3. Results and discussion
3.1. Use of ER probes for the measurement of the "residual"
corrosion rate in the case of CP operation
Figure 2 shows the thickness variation (
d) of working elements in DK probe
(Figure 2a) and VIK probes (Figure 2b) versus time. The probes were connected to
an operating gas pipeline at the drainage point of the cathodic station. The
pipeline polarized potential was -1.15 V (CSE). An approximately constant
the VIK probes is established within 15 hours and that in DK probes, within 2-5
days. The subsequent variations of
d during the entire test (22 days) are
insignificant. The measured steel corrosion rate under these conditions is
negligibly small (below 10
m/year), which proves the efficiency of the pipeline
cathodic protection. The surface area of the working element in DK is
approximately 20 times larger than that in VIK. This is why it takes much longer
for a protective potential to be established on DK probes in comparison with VIK
Fig. 2: Variation of WE thickness of two DK probes (a) and a VIK probe (b) at the
pipeline cathodic protection potential.
3.2. Corrosion rate measurement in a coating defect with
A DK probe was installed on a segment of a working gas pipeline. The CP was
temporarily disconnected. Figure 3 shows the variation in time of the thickness of
the probe working layer. The initial test period (~1 hour) was not taken into
account in the corrosion rate calculations. The corrosion rate measured by the
probe was approximately 0.5 mm/year, which classifies this gas pipeline section as
a high corrosion hazard.
Fig. 3: WE thickness variation in a DK probe with disconnected CP.
The accuracy and reliability of corrosion rate measurements increase as either the
probe exposure time or the measurement frequency increase. Figure 4 shows the
d vs. time plot obtained on a DK probe that was connected to an operating oil
pipeline not equipped with a F3 V\VPHPB 7OH UHVLVPMQŃH RI POH SURNH·V J( RMV
measured every hour. Although the measured
d values are scattered, the
variation of these values in time can be fitted by a linear equation with a high
correlation factor; and the calculated corrosion rate equals 0.2 mm/year. This
corrosion rate remained almost unchanged for 40 days at this pipeline section.
Fig. 4: Time variation of the WE thickness in a DK probe at a field oil pipeline
segment without CP.
3.3. Soil corrosivity measurement
Probes can be used to assess the soil corrosivity expressed in corrosion rate units.
Express assessments can be performed by means of VIK probes, using a resistance
meter with a measurement accuracy of 0.1 Ohm as the measuring tool.
Figure 5 presents the variation of WE thickness vs. time in three VIK probes at the
free corrosion potential. The corrosion rate calculations were performed without
ŃRQVLGHUMPLRQ IRU POH LQLPLMO PHVP SHULRG a20 PLQ GXULQJ ROLŃO POH SURNH·V J(
acquires a constant temperature and potential. Figure 4 shows that after 20 hours
of testing, the
d significantly exceeds 0.5d0, whereas the
deviates from linearity. This confirms the limited lifetime of the probe, during
which its corrosion rate can be considered constant. The mean corrosion rate
calculated over a period of up to 20 hours equals 45 µm/year.
Fig. 5: Variation of the WE thickness in VIK probes not connected to a pipeline.
Thus, within certain limitations, VIK probes can be used to assess the free
corrosion rate of steel in a specific soil (i.e., its corrosivity), the assessment
duration being one day. If longer-term problems are to be solved, for example, to
assess the effect of seasonal factors (variation of soil electrical resistance,
temperature, humidity, and chemical composition) on the corrosion kinetics of
underground structures, it is recommended to use DK probes that provide more
adequate integral estimates of the steel corrosion rate.
3.4. Assessment of local corrosion rates
The corrosion rate calculated from the electrical resistance variation is an
averaged quantity. For example, Figure 6 provides the thickness profiles of certain
sections of a working element (steel band with d0 = 100 µm) in a DK probe
installed at a distance of 1 km from an operating CP device. Based on the results
of a one-year exposure, the mean corrosion rate was 0.017 mm/year). The
corrosion of the probe's working element is distributed rather uniformly, though
in certain places,
d is up to 50 µm, and the local corrosion rate is approximately
three times higher than the mean value.
The ratio of the penetration depths of the crevice corrosion (¨dL) and the
tentatively uniform corrosion (¨d) depends on many factors, including the
composition of the corrosive medium. Repeated attempts have been made to
assess this ratio for the corrosion of carbon steels in soils; this was also reflected
in certain standards . As noted above, the rate of crevice corrosion can be
MVVHVVHG IURP POH SURNH·V J( UXSPXUH PLPH (Equation 5)
¨dL = d0/
rup GHSHQGV RQ POH SURNH J(·V POLŃNQHVV MQG RLGPOB +RRHYHU ŃRPSMULVRQ
of the ¨dL values determined using probes of the same type provides certain
information on the degree of non-uniformity of the external corrosion on the
pipeline segment being monitored.
Fig. 6: 7OLŃNQHVV SURILOH RI M G. SURNH·V J( MIPHU 12 PRQPOV RI H[SRVXUH MP POH
gas pipeline CP potential.
For example, Table 2 lists average values of ¨d, ¨dL, and their ratios measured
using DK probes on operating field oil pipelines without CP. The pipe walls of
these pipelines had been repeatedly ruptured; as result, the adjacent soil has been
salted with water and has acquired a high corrosivity. Corrosion macropairs
formed in locations where the pipeline crossed the boundaries of natural and
salted soils, which explains the high corrosion rates of the probe WEs. As follows
from Table 2, the local corrosion rates are up to 2.7 mm/year, whereas the ¨dL/¨d
ratio varies from 9.7 to 1.9; the mean ¨dL/¨d value at the monitored pipeline
sections amounts to 3.6. The corrosion rates at the pipeline sections located in
soils with lower corrosivity are several times smaller, but the mean ¨dL/¨d ratio is
also approximately 3. However, upon longer exposure of the probes, the measured
corrosion rates should decrease, and the ¨dL/¨d ratio should increase.
Table 2. Uniform corrosion and crevice corrosion rates and their ratios (¨dL/¨d)
measured at pipelines in the absence of CP.
rate, mm/year ¨dL/¨d
1 0.325 1.30 4.0
2 0.316 1.33 4.2
3 1.00 2.68 2.7
4 0.312 0.63 2.0
5 0.112 1.09 9.7
6 0.208 0.39 1.9
7 0.660 1.40 2.1
8 0.275 0.74 2.7
9 0.445 1.25 2.8
Thus, corrosion probes can be used efficiently to achieve the following goals in
monitoring the external corrosion of underground pipelines:
- PHMVXUHPHQP RI UHVLGXMOµ ŃRUURVLRQ UMPHV MP POH SURPHŃPLYH SRPHQPLMO LBHB
determining the performance of CP;
- corrosion rate measurements in a coating defect and ranking of pipeline sections
by corrosion hazard, taking into account the effect of stray currents and currents
of extensive macropairs;
- measurement of soil corrosivity with respect to uniform and crevice corrosion.
3.5. Assessment of the possibility of HISC of a steel pipeline in the
operating zone of cathodic stations
The measurement of the hydrogen permeation rate on operating pipelines was
part of a program of RAO Gazprom in 1995-1997 aimed at investigating the
causes of corrosion failures previously observed on the pipelines of the
Krasnoturyinsk district, Sverdlovsk region, Russia . To this end, control sites
were arranged at CP drainage points, with four to six DH-1 hydrogen probes
installed on each. Figure 7 shows the results of the ip measurements at one of the
sites taken over 1.5 years. One can see that the maximum flux density of hydrogen
into the steel did not exceed 12 - 13 µA/cm2, mainly in the summer months. The
gas pipeline potentials measured without correction for the ohmic drop ranged
from -2.5 to -3.5 V (CSE).
Fig. 7: Plot of the current densities measured by hydrogen probes at control
site; 1 - 4 - parallel gas pipelines.
The critical ip values, at which crack growth by the steel hydrogen embrittlement
mechanism is triggered, were determined by laboratory experiments on specimens
with pre-formed cracks. The ambiguous effect of cathodic polarisation on the
crack growth rate was used for this purpose . Cathodic polarisation would
slow down the growth of corrosion cracks by the mechanism of local anodic
dissolution of metal and accelerate the growth by the hydrogen induced stress
corrosion mechanism HISC.
In laboratory tests, HISC becomes the major crack growth mechanism in ɏ70 pipe
steel at current densities of hydrogen penetration through membrane above 100
µA/cm2 (at L = 100 µm) . The hydrogen probes did not give such readings at
any pipelines in the Krasnoturinsk district. It was thus concluded that cathodic
protection does not stimulate the SCC of underground pipelines.
This conclusion is only correct for steels of this strength grade and for the types
of soils studied. It is possible that in soils containing hydrogen absorption
promoters, e.g., hydrogen sulphide, hydrogen can be accumulated in higher
The use of X80 and X100 steels, as well as consideration for cyclic mechanical
stress, may result in adjustments of the critical rates of hydrogen absorption by
steel at which metal hydrogen embrittlement occurs. Furthermore, some
researchers believe that the SCC of pipe steels in environments with near-neutral
pH results from a synergistic effect of hydrogen and stress on local metal anodic
dissolution [25-27]. It follows that cathodic protection can prevent the
development of stress corrosion, but at potentials near the corrosion potential, an
increase in the concentration of absorbed hydrogen can accelerate the crack
3.6. Promising methods of indirect local corrosion diagnostics under
delaminated pipeline coatings
Monitoring the state of the external pipe surface under a delaminated coating is
the most difficult problem faced in the corrosion monitoring of pipelines. The use
of ER probes was suggested for the monitoring of under-film corrosion .
However, the experience in application of under-film corrosion probes is rather
scarce because the probes have to be installed directly onto a pipe surface, making
it difficult to reproduce the conditions of under-film corrosion growth.
It is more promising to develop CM methods that allow for the prediction of the
origination and growth of local corrosion defects by analysing the chemical
composition of the soil electrolytes and/or by measuring the partial corrosion
reaction rates on a probe placed into the soil near a pipe wall or into the under-
film electrolyte. The development of such CM methods requires a deeper
understanding of the local corrosion initiation centre and growth mechanism.
Because stress corrosion cracking of pipe steel in soils with near-neutral pH is
currently the most hazardous type of under-film corrosion, researchers have
mainly focused on studies of this phenomenon [28-30].
In particular, the presence of CO2 and H2S in the corrosive medium accelerates
crack growth [28-32] and shortens the time until crack appearance . In
addition, crack colonies on pipe walls are found in anaerobic soils that have pH
values of 5.5-7.5. Therefore, it is considered necessary to monitor the soil
electrolyte pH and oxygen, hydrogen sulphide and carbon dioxide levels.
Because carbonic acid and hydrogen sulphide molecules dissociate to give the
respective ions, it was suggested that the composition of the soil electrolytes be
determined in the field using ion-selective electrodes.
Soils at pipeline segments where crack colonies formed were found to have higher
concentrations of sulphide-containing compounds (HS- and S2- ions) (Figure 8).
Fig. 8: Changes in the concentration of sulphide-containing compounds in a soil
electrolyte at a pipeline segment. Arrows denote the locations of the crack
colonies found upon control excavations.
The oxygen concentration in a soil electrolyte can be determined from the rate of
its cathodic reduction on a special test electrode made of pipe steel. It is also
considered reasonable to determine the hydrogen absorption rate and the kinetics
of pipe steel anodic dissolution in the soil electrolyte at a pipeline being
monitored. The anodic current values at a preset potential and the character of the
current variation versus time can be indications of local steel dissolution, and
therefore show the possibility of the formation of a mechanical stress
concentrator on the pipe surface.
Measurements made using special probes, which include ion-selective electrodes
and electrodes made of pipe steel, should improve the reliability of the existing
method for the discovery of pipeline segments potentially prone to stress
corrosion. Further research on the impact of the composition of the soil
electrolytes on pipe steel SCC should lead to the establishment of new criteria for
determining the possibility of crack initiation and assessment of the growth rate
of cracks in pipe steel under delaminated coatings.
Currently, the most urgent goals of underground pipeline external corrosion
monitoring include: 1) assessment of the CP efficiency by determining the
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in a through coating defect in the absence of CP, and 3) predicting the
development of hazardous corrosion defects (cracks and crevices), primarily under
a delaminated coating. The first two CM goals can be achieved using various ER
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efficiency monitoring that is based on polarized potential measurements. Hence,
corrosion sensors should be installed in areas with unstable temporal variation of
the potential (zones of stray or induced currents) or where measurements of
potential are complicated (high-resistance soils, heat-insulated pipelines). The
main problem that remains unresolved involves the measurement of the efficiency
criterion of the pipeline protection, i.e., the UHVLGXMOµ XQLIRUP ŃRUURVLRQ UMPHB $P
the moment, we can agree with the authors of  that a rate of uniform corrosion
below 25 µm/year measured over a period of one year is an indicator of efficient
CP, but it is impossible to make a final decision on the magnitude of this criterion
without the large-scale use of corrosion sensors.
Ranking pipeline sections based on the use of corrosion sensors provides grounds
for decreasing (in absolute magnitude) the CP protective potential at pipeline
sections with low corrosion susceptibility, or otherwise strengthening the
corrosion protection (by CP installation, changing the operating modes of the
cathodic stations, pipeline re-insulation) at pipeline sections with high corrosion
Prediction of the origination of hazardous corrosion defects under delaminated
pipeline coatings remains the most difficult problem of CM. However, the progress
made in understanding the stress corrosion mechanism in pH-neutral soils and the
development of express methods for determining the chemical composition of soil
electrolytes and electrode reaction rates leading to the growth of cracks, allows
one to expect a substantial improvement in the reliability of the methods for
diagnostics of potentially hazardous pipeline segments.
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