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Efficiency of cathodic protection applied to buried LPG tanks
Marcel ROCHE1, André DUCLOS
2, Henri FRANҪOIS
3
1
CEFRACOR, France, marcel.roche@orange.fr 2AD Consult, France, a.duclos@me.com
3Comité Français du Butane et du Propane, France, h.francois@cfbp.fr
Abstract
The buried tanks storing Liquefied Petroleum Gas (LPG) are vessels subject to the regulation
concerning pressure equipment. In spite of the low potential corrosivity of their external environment,
they are protected against corrosion to prevent any leak. Experience feedback demonstrates the
efficiency of an organic coating supplemented by a cathodic protection system. The Comité Français
du Butane et du Propane (CFBP) nevertheless decided to consolidate the demonstration of the
efficiency of the system and to better know the relevance of the various methods of measurements. 3D
digital modelling has been performed using the "Boundary Element Method" (BEM). The main results
of this study are presented. They concern the level of reliability of the "ON" and "OFF" potentials
measurement methods, as well as the advantages offered by the installation of permanent reference
electrodes and/or coupons at fixed locations. The study shows that "probes" constituted of discoid
coupons coupled with reference electrodes measuring the potential in their centre can allow more
representative measurements. The relevance of the detection for coating defects by the DCVG or
ON/OFF CIPS methods applied at the surface of the soil has also been estimated.
A professional guide on “Cathodic protection of tanks under embankments for storage of LPG”
validated by CEFRACOR and published by CFBP in July 2014 is also presented.
Keywords: cathodic protection; buried tanks; LPG; modelling
mailto:marcel.roche@orange.fmailto:a.duclos@me.commailto:h.francois@cfbp.fr
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Introduction
Tanks used for the industrial storage of Liquefied Petroleum Gas (LPG) are generally buried
to be protected from thermal and mechanical damage which could lead to a major accident
scenario. In spite of the low potential corrosivity of these environments, they are protected
against external corrosion by an organic coating completed by a cathodic protection system.
They can be horizontal cylindrical tanks resting on compacted sand beds and possibly on
reinforced concrete supports, and covered by an "embankment" made of a dense inert material
(sand and soil) with a minimum coverage of 1 m. As an alternative, these horizontal
cylindrical tanks can be installed inside reinforced concrete parallelepiped enclosures open on
their upper faces ("sarcophagus"). Another well known solution consists in using spherical
tanks installed "under embankments" of sand covered with soil with a slope of about 75°. An
alternative consists in using reinforced sand made of a mixture of sand and polyester fibers
(e.g. Texsol®), which allows to decrease minimum coverage from 1 m to 0.6 m.
These vessels are subjected to regulations concerning pressure equipment which request
periodical requalification inspections. In France, BSEI Decision No. 13028 of 21 March 2013
of the Ministry of Ecology, Sustainable Development and Energy allows performing these
requalifications without visual inspection of the outer wall of the tank, which avoids removal
of soil and sand. This is accepted provided that specific requirements detailed in the following
professional document are respected: « Dispositions spécifiques applicables aux réservoirs
sous talus destinés au stockage de gaz inflammables liquéfiés » published by the Association
Française des Ingénieurs en Appareils à Pression (AFIAP) [1, 2]. The main provisions of this
document require that cathodic protection is applied by a specialised company using a
personnel certificated in accordance with EN 15257 [3] (e.g. certification from the
CEFRACOR CERTICICATION / Protection Cathodique) or in accordance with an equivalent
international scheme (e.g. Certification by NACE International Institute).
Theory, field measurements and the experience feedback demonstrate the efficiency of this
corrosion prevention system and no corrosion has been reported so far in France. The Comité
Français du Butane et du Propane (CFBP) nevertheless decided to consolidate the
demonstration of the efficiency of the cathodic protection of steel at coating defects and to
better know its limits as well as the relevance of the various methods of measurements. 3D
digital modelling has been implemented, offering a powerful tool to predict the detailed
functioning of cathodic protection, even for complex geometries.
Modelling study
The method
The modelling study has been performed by Computational Mechanics BEASY using a
software based on the "Boundary Element Method" (BEM) for solving the Laplace equations
which govern electrical fields. Complete information on this study has been previously
published [4].The hypotheses introduced into the modelling calculations illustrate the precise
geometry of the structure, the characteristics and location of anodes, the polarisation curves
on the metallic surfaces (a function of the nature of metal, the corrosivity of the electrolytic
environments, the characteristics of the coating if any), and the resistivity of the electrolytes
present between the structure and the anodes. The study assumes intimate contact of the
electrolyte (sand) with the outer wall of the reservoir at any point and takes into account the
steel embedded in concrete of different parts made of reinforced concrete structures (tunnels,
walls). Modeling calculations use assumptions on the cathodic polarisation curves of steel in
the sand or concrete which may not represent the strict reality, knowing that there is no
universally recognized data.
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An innovative aspect was to assess the validity of measurements of "instant OFF" potential
performed with a mobile reference electrode located at the surface of the soil or with buried
permanent reference electrodes as compared with the supposed "true" potential calculated at
the surface of steel at coating defects. The calculated “ON” protection potential is applied to
the tank defect surfaces and a new “OFF” solution is obtained to find the corresponding
distribution of potential in the electrolyte. Application of the protection potential is intended
to reproduce the observed delay between switch-off of the ICCP system and decay of the
potential difference which exists between the metal and the electrolyte just above the metal
surface. The delay results from inertia of the electrochemical reactions which take place at the
metal surface while the Impressed Current Cathodic Protection (ICCP) system is turned on.
All other metal surfaces are “insulated” in the “OFF” solution.
When the coating is assumed permeable to electric current, the potential conditions to
boundary elements have been applied to both the steel surface at coating defects and the
undamaged coated surfaces. Thereby, calculated "instant OFF" potentials concern the external
surface of the coating and not the steel surface under the coating.
Coating defects chosen for the modeling studies
Most of the calculations were made with the assumption of a coating considered totally
impervious to the cathodic protection current. In this case, this current reaches the metallic
surface of the tank only at the places where the coating is considered completely degraded,
leaving a bare metal on a given surface. Assumptions for coating defects were a disc shape
and following surfaces:
- Type D1: 1 cm2 (diameter 1.13 cm), representing the most likely case;
- Type D2: 10 cm2 (diameter 3.6 cm), representing a degraded case;
- Type D3: 100 cm2 (diameter 11.28 cm), representing an exceptionally degraded case.
This assumption of impervious coating except at coating defects leads to a very low
protection current demand to be injected compared to the actual measured intensity in
practice, including for new tanks, in the best electrical insulation of the tank from the
surrounding metal structures. Therefore, certain calculations were performed to investigate
the effect of a coating slightly permeable to the cathodic protection electric current.
Cathodic protection criteria used for assessing the efficieny
For carbon steel exposed to sand or soil, the results fulfilling the conventional criterion of
potential more negative than -850 mV vs. CSE (copper/saturated copper sulfate electrode) are
considered conservative. Otherwise, the results fulfilling the alternative criterion defined by
EN 12954 [5] are considered sufficient: potential more negative than -750 mV for a soil
resistivity between 100 and 1000 Ω.m. Finally, the results fulfilling the alternative criterion
defined by ISO 15589-1 [6], NACE SP01-69 [7] and AS 2832.9-1999 [8] are considered
acceptable: potential decrease of more than 100 mV relative the natural corrosion potential,
for this study -600 mV for a natural corrosion potential of -500 mV. This criterion applies in
particular to the analysis of the potentials calculated by modelling on the steel surface (which
cannot be measured in practice). The criterion of -950 mV, considered in cases where the risk
of bacterial growth is unsignificant (very poorly aerated environments contaminated with
organic matter and sulfates ions) is not relevant here.
For steel reinforcement embedded in concrete, the criteria defined by EN ISO 12696 [9] are
used as follows. The results fulfilling an "instant OFF" potential more negative than -720 mV
measured with a Ag/AgCl/KCl 0.5M reference electrode (equivalent to -785 mV vs. CSE) are
considered conservative. Otherwise, the results corresponding to a potential decrease of more
than 150 mV relative to the natural corrosion potential, here -250 mV vs. CSE for a corrosion
potential of -100 mV, are considered acceptable.
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Assumptions for polarisation curves
The cathodic polarisation curves taken as basic assumptions for bare steel (at coating defects)
were established on the following basis:
- Natural corrosion potential of -500 mV vs. CSE (contact with sand in the absence of
development of sulfate-reducing bacteria);
- Linear sections (connected by a curve) of different slopes, first between -500 mV and -981
mV, and second beyond -981 mV (high slope corresponding to hydrogen evolution by
reduction of the molecules water).
Two hypotheses have been taken for the parametric study:
- Current density at the conventional protection threshold (-850 vs. CSE) equal to 20 mA/m2
(Curve No.1), optimistic assumption which may correspond to the situation after a long period
of polarisation,
- Current density at the conventional protection threshold of 100 mA/m2 (Curve No.2), the
most probable hypothesis but conservative.
An alternative polarisation curve, based on the consideration of a semi-logarithmic curve
slope of 130 mV/decade (Tafel law) in the domaine of hydrogen evolution (Curve No.1 bis)
has been used in parallel for a calculation case to obtain more realistic values.
The polarization curves for bare steel used for the modeling are illustrated in Fig. 1 and 2.
Figure 1: Polarisation curves N°1 et N° 2 for bare steel
Figure 2: Polarisation curve N°1 bis for bare steel
Polarisation for tank steel exposed by coating defect
-1200
-1100
-1000
-900
-800
-700
-600
-500
-400
0 20 40 60 80 100 120 140 160 180 200
current density (mA/m2)
Po
ten
tial
(mV
Cu
/Cu
SO
4)
Curve 1 Curve 2
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The steel polarisation curves assuming a current permeable coating were taken homothetic to
the polarisation curve N°1 or N°2, with two homothetic ratios:
- Curve No.1 with a homothetic ratio of 1/10000, i.e. current density at the conventional
threshold of protection taken equal to 0.002 mA/m2 (Curve No.1'');
- Curve No.2 with a homothetic ratio of 1/1000, i.e. current density at the conventional
threshold of protection taken equal to 0.1 mA/m2 (Curve No.2');
- Curve No.2 with a homothetic ratio of 1/10000, i.e. current density at the conventional
threshold of protection taken equal to 0.01 mA/m2 (Curve No.2'').
The polarisation curve of steel embedded in concrete was taken from the work of P. Pedeferri
for concrete not contaminated with chlorides type of pollutants [10]. The simplified form is
given by Fig. 3.
Figure 3: Polarisation curve for steel embedded in concrete
Base case: Horizontal cylindrical tank under embankment with impervious coating and
reinforcements of concrete structures electrically insulated from tank
Configuration
For the parametric study, the resistivities of the native soil and of the soil brought above the
embankment have been taken constant and equal to 100 Ω.m and 50 Ω.m respectively. The
resistivity of the sand has been taken equal to 50 Ω.m or 500 Ω.m, the one of concrete equal
to 500 Ω.m. The steel reinforcements in the concrete tunnel have been considered as
electrically insulated from the tank. A calculation has been performed without the presence of
reinforcement for comparison. Fig. 4 and 5 illustrate this configuration.
The base case of cathodic protection system consists of:
- An impressed current station located near the slope of the side opposite to the tunnel, each
anode being connected by a copper wire of diameter 16 mm2;
- 4 cylindrical anodes of 60 mm diameter and 720 mm length installed in containers of 160
mm diameter and 1050 mm length filled with a backfill of resistivity 0.5 Ω.m and installed
vertically on each side of the tank near the foot of the slope at 5 m of each end of the tank, 1
m below the native ground level.
Alternatively, and for comparison, a calculation was made with anodes installed horizontally
in sand parallel to the tank axis 1 m from its surface on each side 10 m from each of both
ends, two on each side.
Monitoring of the cathodic protection is provided by the installation of a four reference
electrodes in a permanent position located 100 mm from the surface of the tank, two
electrodes under the lower generator at less than 8 m from each end of the tank and other ones
each side of the tank and in its middle.
Polarisation for reinforcement bars
-1200
-1000
-800
-600
-400
-200
0
0 20 40 60 80 100 120 140 160
current density (mA/m2)
Po
ten
tial (m
V C
u/C
uS
O4)
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Figure 4: CP station (T / R), the anodes (A), part of the permanent reference electrodes
(E) and coating defects (D)
Figure 5: Soil types and location of the CP station (T / R), of a part of the anodes (A),
reference electrodes (E) and coating defects (D)
Results
They are summarised in Table 1.
Table 1: Results of the base case
Location of discrete anodes remote close remote
Reinforcement in concrete Without With, isolated from tank
ρ sand (Ω.m) 500 50 500 50
N° of polarisation curve for steel
2 1 1 bis 2
Coating defects 4 D1 and 2 D2 1 D2 4 D1 and 2 D2 4 D1 and 2 D3
I (mA) at CP station 1 0,5 1 2
E (mV) min/max at defect A (Type D2 or D3)
-1036/-983 -1036/-983 -1077/-996 -1030/-984 -1023/-990 -1269/-1070 -1022/-942 -988/-693 -887/-808
E (mV) min/max at defect B
(Type D1) -1111/-1026 -1110/-1025 N.A -1111/-1026 -1071/-1037 -1549/-1296 -1069/-1012 -1075/-1011 -1008/-994
E (mV) min/max at defect C (Type D1)
-1130/-1025 -1129/-1025 N.A -1130/-1025 -1077/-1037 -1590/-1294 -1078/-1011 -1090/-1011 -1010/-994
E (mV) min/max at defect D
(Type D1) -1110/-1026 -1110/-1025 N.A -1111/-1026 -1072/-1038 -1549/-1295 -1069/-1012 -1075/-1011 -1008/-995
E (mV) min/max at defect E (Type D1)
-1110/-1025 -1110/-1025 N.A -1111/-1026 -1070/-1036 -1549/-1295 -1069/-1012 -1075/-1011 -1007/-994
E (mV) min/max at defect F
(Type D2 or D3) -1024/-983 -1025/-983 N.A -1029/-983 -1015/-988 -1234/-1069 -1012/-941 -986/-690 -865/-784
EOn at RE 1 -3263 -3251 -4618 -3262 -1260 -3499 -3498 -2696 -1083
EOn at RE 2 -3263 -3251 -4618 -3262 -1260 -3499 -3498 -2696 -1083
EOn at RE 3 -3263 -3251 -4618 -3262 -1260 -3499 -3498 -2697 -1082
EOn at RE 4 -3262 -3251 -4619 -3261 -1260 -3499 -3497 -2697 -1082
EOff at RE 1 -1035 -1035 -1032 -1035 -1028 -1283 -1009 -880 -871
EOff at RE 2 -1035 -1035 -1032 -1035 -1028 -1283 -1009 -880 -872
EOff at RE 3 -1035 -1035 -1032 -1035 -1028 -1283 -1009 -880 -872
EOff at RE 4 -1035 -1035 -1032 -1035 -1028 -1283 -1009 -880 -872
EOn at surface of
embankment (min/max) -3268/-3250 -3268/-3250 -4618/-4602 -3264/-3262 -1263/-1254 -3502/-3488 -2703/-2668 -1087/-1066
EOff at surface of embankment (min/max)
-1035 -1035 -1032 -1035 -1028/-1027 -1283/-1282 -880/-879 -873/-870
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It should be noted that the presence of the reinforcement, isolated from the tank, has no effect.
The anode position, remote or close, also has no significant impact. For the same conditions
of resistivity, cathodic protection current and coating defects, a more optimistic assumption
on the cathodic polarisation curve of steel in the sand leads to a better level of protection. This
can represent what happens when protection is applied for several months, leading to a
progressive alkalination to the steel surface moving from curve No.2 to curve No.1. The use
of a polarisation curve with a semi-logarithmic section in the domaine of hydrogen evolution
(curve N°1bis instead of curve N°1) greatly reduces any overpolarisation but does not
significantly change the values in the normal domaine of protection. The most negative
potential calculated (at the periphery of coating defect of the smallest area, 1 cm2) increases
from -1.59 V to -1.08 V vs. CSE.
Efficiency of cathodic protection
Protection against corrosion of steel exposed at coating defects evaluated with the most
conservative protection criteria (-850 mV vs. CSE) turns out to be always achieved if the
current impressed is sufficient for a realistic coating defect surface (1 cm2) and even for a
substantially greater size (10 cm2). For abnormally and very unlikely extended coating defects
(100 cm2), the full protection under this criterion could not be reached in their centre but
remains acceptable with the alternative criterion of decrease of 100 mV potential from the
natural corrosion potential.
Heterogeneity of potential at the surface of coating defects
The potential is not homogeneous on the surface of coating defects, it is less negative in the
centre, the heterogeneity being greater as the surface is larger and the resistivity of the sand is
higher as illustrated in Fig. 6.
Figure 6 : Mapping of potentials at the surface of coating defects
Defect 10 cm2 - ρ = 500 Ω.m Defect 1 cm2 - ρ = 500 Ω.m
Defect 10 cm2 - ρ = 50 Ω.m Defect 1 cm2 - ρ = 50 Ω.m
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Measurement of potentials and relevance of the results
"ON" potentials as well as "instant OFF" potentials measured with permanent reference
electrodes installed near the tank or with a mobile electrode moved to the slope of the surface
are very similar in all modelled cases. The advantage of installing permanent reference
electrodes is essentially to facilitate convenient access and periodic monitoring in
reproducible points. The location of the permanent reference electrodes is not critical.
Modelings confirm that the "ON" potential measurements are very remote from the "true"
potentials calculated at the steel surface at coating defects in all cases, especially when the
resistivity of sand is high. This difference can lead to greatly overestimate the effectiveness of
protection if we limit ourselves to this type of measurement.
"Instant OFF" potentials measured with the permanent reference electrodes or a mobile
electrode moved at the surface of embankment are less distant from “true” potentials
calculated on the surface of steel at coating defects, which validates the advantage of this type
of measurement. However, for a coating with multiple defects of various sizes, these
measurements do not systematically detect a weakness of protection at the centre of very large
defects (100 cm2). The modelling study carried out in the case of a single defect of coating
(10 cm2) also reveals a difference between the "Instant OFF" potential measurements
performed with a reference electrode and the potential calculated at the steel surface of the
defect. The measurement with the reference electrode corresponds to an intermediate value
between the most negative potential (on the periphery of the defect) and the least negative (at
the centre of the defect).
The gradients of “ON” potentials measured at the surface of embankment in front of a coating
defect measured can be significant (a few mV), especially for a defect of significant surface.
Only defects located on parts of the tank shell near the surface are detectable. Fig. 7 illustrates
a case with small defects (D1, 1 cm2) and intermediate defects (D2, 10 cm
2). On the opposite,
"Instant OFF" potential measurements lead to very low potential gradients (some tenths of
mV). The detection of coating defects by Direct Current Voltage Gradient (DCVG) method
applied to the surface of the embankment seems possible especially for large defects not too
far from the surface. ON Close Interval Potential Survey (CIPS) method is also expected to
correctly detect these coating defects.
Figure 7: Mapping of ON potentials at the surface of embankment
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Alternative cases
Additional calculations were carried out to assess the impact of various factors. Their findings
are summarized below.
Contact of reinforcement with the tank
An accidental electrical contact of the concrete reinforcement with the tank can completely
collapse protection if the impressed current is too low, but this is easily detected by the "ON"
and "OFF" potential measurements carried out with the permanent reference electrodes or
with a mobile electrode moved to the surface of embankment. The protection can be easily
restored by greatly increasing the protection current.
Impact of permeability of the coating to cathodic protection current
The calculations carried out with a coating more or less permeable are summarized in Table 2.
The resistivities of concrete and sand are equal to 500 Ω.m, the skeleton of steel
reinforcements is isolated from the tank and anodes are remote. The protection at coating
defects becomes more difficult to obtain and it is necessary to greatly increase the impressed
current to ensure protection. In this case, the measurement of "instant OFF" potentials carried
out with a mobile reference electrode moved on the surface of embankment or with
permanent electrodes buried near the tank may not be sufficient to ensure detection of a lack
of protection for defects of large surface, including using the alternative criterion of potential
decrease of more than 100 mV relative to the natural corrosion protection. Furthermore, the
DCVG and ON CIPS methods applied on the surface of embankment no longer permit
detection of coating defects, even located near the surface, because the potential gradients
become too weak.
Table 2 : Results with a coating slighly permeable to cathodic protection current
N° of polarisation curve on bare steel 2 - 2 1
N° of polarisation curve on coated steel 2’ 2’’ 1’’
Coating defects 4 D1 and 2 D2 none 4 D1 and 2 D2
I (mA) at CP station 56 10 100
E (mV) min/max at defect A (Type D2) -641/-588 N.A -696/-622 -763/-664 -1079/-1005
E (mV) min/max at defect B (Type D1) -728/-694 N.A -807/-762 -917/-856 -1188/-1092
E (mV) min/max at defect C (Type D1) -751/-710 N.A -815/-764 -945/-874 -1218/-1099
E (mV) min/max at defect D (Type D1) -762/-723 N.A -812/-766 -959/-893 -1211/-1105
E (mV) min/max at defect E (Type D1) -779/-738 N.A -815/-768 -979/-914 -1223/-1111
E (mV) min/max at defect F (Type D2) -672/-611 N.A -694/-625 -805/-697 -1085/-1012
E (mV) min/max on the tank surface -940/-810 -943/-818 -995/-880 -1278/-1073 -1800/-2078
EOn at RE 1 -840 -840 -980 -1134 -1894
EOn at RE 2 -840 -841 -980 -1134 -1894
EOn at RE 3 -924 -925 -993 -1250 -2051
EOn at RE 4 -926 -927 -994 -1254 -2055
EOff at RE 1 -837 -838 -980 -1129 -1889
EOff at RE 2 -837 -838 -980 -1129 -1889
EOff at RE 3 -911 -912 -991 -1228 -2029
EOff at RE 4 -913 -914 -991 -1231 -2032
EOn on the embankment surface (min/max) -1108/-842 -1109/-843 -1027/-880 -1591/-1143 -1881/-2377
EOff on the embankment surface (min/max) -881/-832 -881/-832 -986/-979 -1185/-1124 -1877/-1972
Spherical tank under embankment protected by remote discrete anodes and close anode cables
A 1000 m3 and 12.41 m diameter sphere, covered with "Texsol®" except at the top and
bottom which are covered with sand, is supported by 7 legs made of reinforced concrete with
steel skeleton not electrically insulated from the tank. A "casemate" and a tunnel also made of
reinforced concrete are located under the sphere.
The cathodic protection system includes two anode assemblies:
- A set of 7 cylindrical anodes, diameter 60 mm and length 720 mm, installed in containers
diameter 160 mm and length 1050 mm filled with a backfill of resistivity 0.5 Ω.m, installed
horizontally between the 7 concrete legs at 1 m depth;
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- A set of 3 loops of 38 mm diameter cable anode using a conductor polymer installed in the
upper part of the sphere at a distance of 0.2 m.
Fig. 8 illustrates the boundary of the various electrolytes and the location of the coating
defects and anodes. The resistivities of the native soil, "Texsol®", sand and concrete were
taken constant and equal respectively to 100 Ω.m, 100 Ω.m, 500 Ω.m and 1000 Ω.m
Figure 8: Overview of modeling the spherical tank
Monitoring of cathodic protection is provided by the installation of 8 permanent reference
electrodes located 100 mm from the surface of the sphere on the vertical generatrix opposite
to the tunnel (See Fig. 9). Electrode R6 is considered to be either on the surface of the sphere
(6A) or between the sphere and the nearest leg (6B).
Calculations made with a coating impervious to electrical current and automatic current
sharing between anode systems are summarized in Table 3.
Table 3: Results for a spherical tank
N° of polarisation curve 2
Coating defects 4 D1, 2 D2 and 1 D3
I (mA) current output of the cathodic protection station 500
E (mV) min/max at coating defect A (Type D1 -1 cm2 ) in Texsol -1437/-1169
E (mV) min/max at coating defect B (Type D2 - 10 cm2) in sand -836/-694
E (mV) min/max at coating defect C (Type D1 - 1 cm2) in sand -849/-776
E (mV) min/max at coating defect D (Type D2 - 10 cm2) in sand -721/-628
E (mV) min/max at coating defect E (Type D1 - 1 cm2) in sand -969/-874
E (mV) min/max at coating defect F (Type D3 -100 cm2 ) in Texsol -1002/-882
E (mV) min/max at coating defect G (Type D1 -1 cm2) in Texsol -1463/-1181
EOn at RE 1 -2663
EOn at RE 2 -2664
EOn at RE 3 -2665
EOn at RE 4 -2628
EOn at RE 5 -2356
EOn at RE 6A -1506
EOn at RE 6B -1491
EOn at RE 7 -1100
EOn at RE 8 -974
EOff at RE 1 -989
EOff at RE 2 -989
EOff at RE 3 -989
EOff at RE 4 -990
EOff at RE 5 -991
EOff at RE 6A -987
EOff at RE 6B -988
EOff at RE 7 -985
EOff at RE 8 -985
EOn at the surface of embankment (min/max) -2674/-924
EOn at the base of the buried part of the reinforced concrete legs -489
EOff at the surface of embankment (min/max) -995/-985
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Figure 9: Location of reference electrodes
The modelling led to conclusions similar to the base case consisting of a horizontal cylindrical
tank under embankment. However, heterogeneity of potentials at the surface of the various
coating defects is larger and full protection assessed with the usual criteria may not be
achieved at the centre of large defects if they are located in the sand, the resistivity of which
being higher than the one of "Texsol®". Protection is nevertheless achieved when using the
criterion of potential decrease more than 100 mV from the natural corrosion potential. Again,
the measurement of "instant OFF" potential performed with a reference electrode moved over
the surface of the embankment or buried near the tank may be too optimistic. This type of
construction requires significantly greater protection current due to the electrical continuity of
the reservoir with the reinforcement of reinforced concrete legs.
Conclusions and recommendations
1. It is confirmed that the diagnosis of the effectiveness of cathodic protection of a tank under embankment can only be based on measurements of "instant OFF" potentials, "ON"
potentials routine measurements being only aimed at detecting possible operational
problems.
2. A safety margin of about 50 mV on the protection criterion chosen (e.g. -900 mV vs. CSE instead of -850 mV) is recommended to ensure that the value measured with a reference
electrode located at the surface of embankment or permanently buried inform well on the
worst value that can be estimated at the centre of a large coating defect.
3. It is recommended to design a "probe" formed by a disc-shaped steel coupon associated with a reference electrode connected through an electrolytic bridge for measuring as
accurately as possible the potential at the centre of the coupon. The surface of the coupon
should represent the largest coating defect being feared for the type of coating used on the
tank. The calculations leading to measurements of "instant OFF" potentials which are more
negative than the potential calculated at the centre of the largest coating defects advocate in
favour of the advantage of such a "enhanced probe" (especially in case of a current-
permeable coating).
4. Trials of such “enhanced probes” on some existing tanks should allow interesting comparisons with "instant OFF" potential measurements carried out at the surface of the
embankment. This type of "enhanced probe" would also simulate calibrated artificial
defects at well known locations for helping validation (or not) of detection of coating
defects on tanks by DCVG and CIPS methods.
5. In addition, the use of Electrical Resistance Probes (ERP) could also help to improve knowledge of the quality of protection [11].
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6. It is recommended to perform an assessment of the permeability to the cathodic protection current of the coating systems used on buried LPG tanks, because the protection at the
coating defects and their detection is made more difficult when the coating is permeable.
CFBP Professional guide on “Cathodic protection of tanks under embankments for
storage of LPG”.
A professional guide on “Cathodic protection of tanks under embankments for storage of
LPG” validated by CEFRACOR has been published by CFBP in July 2014 [11]. It may be
ordered through the CFBP web site. This guide takes into account the modelling study
presented here.
References
1. Décision BSEI n° 13-028 du 21 mars 2013 relative à la reconnaissance d’un cahier
technique professionnel pour le contrôle en service des réservoirs sous talus destinés au
stockage de gaz inflammables liquéfiés
2. Cahier technique professionnel de l’Association Française des Ingénieurs en Appareils à
Pression (AFIAP) définissant les « Dispositions spécifiques applicables aux réservoirs sous
talus destinés au stockage de gaz inflammables liquéfiés », Révision de mars 2013 (Edition
initiale juin 2004) [1] Guide professionnel CFBP « Protection cathodique des réservoirs sous
talus destinés au stockage de GPL », 2014, http://www.cfbp.fr/ 3. EN 15257: 2007, Cathodic protection - Competence levels and certification of cathodic
protection personnel
4. M. Roche, A. Duclos, H. François, L’apport de la modélisation dans l’étude de l’efficacité
et du contrôle de la protection cathodique des réservoirs de GPL sous talus, 6èmes Journées
Protection Cathodique CEFRACOR, Antibes – Juan-les-Pins, 24 - 26 June 2014
5. EN 12954: 2001, Cathodic protection of buried or immersed metallic structures - General
principles and application for pipelines
6. ISO 15589-1: 2015, Petroleum and natural gas industries – Cathodic protection of pipeline
transportation systems - Part 1: On-land pipelines, Nov. 2003
7. NACE SP01-69-2013: Control of External Corrosion on Underground or Submerged
Metallic Piping Systems, NACE International, Houston, Texas, USA
8. AS 2832.2-1999, Australian Standard – Cathodic protection of metals – Part 2: Compact
buried structures
9. EN ISO 12696: 2012, Cathodic protection of steel in concrete
10. Cathodic Protection, L. Lazzari, P. Pedeferri, Polipress, January 2006, Milano, Italy
11. Report on Corrosion Probes in Soil or Concrete, NACE International Publication 05107,
Aug. 2007
12. Guide professionnel CFBP - Protection cathodique des réservoirs sous talus destinés au
stockage de GPL, Ref.533, Juillet 2014, Comité Français du Butane et du Propane, 8 Terrasse
Bellini, 92807 Puteaux cedex, France, http://www.cfbp.fr/
http://www.cfbp.fr/http://www.cfbp.fr/
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