1

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)

6

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

8

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

9

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;

10

- 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

11

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].

12

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/