Cathodic Protection of Above GroundStorage Tank Bottoms

byDavid H. Kroon

Vice President, Corporate Operations

Corrpro Companies, Inc.

and

Michael Urbas

National Construction Manager

Harco Technologies Corporation

ABSTRACT

The corrosion of steel containment vessels in contactwith soil and water is a natural phenomenon that mustbe arrested to provide for public safety and environ-mental protection. Cathodic protection is a proventechnology for controlling corrosion on the bottoms ofabove ground storage tanks. This paper addresses vari-ous options for cathodic protection of the external(groundside) surfaces of tank bottoms which are incontact with sand pads or native soil. Advanced appli-cation techniques are described for both existing tanksand new construction with secondary containment.

BACKGROUND

Cathodic protection is a proven method of controllingcorrosion of buried, partially buried and submergedmetallic structures. A properly designed, installed andoperated cathodic protection system will eliminate thecorroding areas by passing direct current to the metalsurface. The direct current is discharged from elec-trodes installed in the electrolyte adjacent to or near thestructure being protected. In this manner, corrosion isarrested when the direct current is of sufficient magni-tude and is properly distributed over the entire surfaceto be protected.

There are two basic types of cathodic protection sys-tems which can be used to arrest corrosion. One typeof cathodic protection system is referred to as a gal-vanic or sacrificial anode system. The galvanic anodesystem is based upon the natural potential difference

which exists between the structure being protected andthe auxiliary electrode (anode) which is installed in theelectrolyte. As a result of the potential difference, abattery effect is created and electrical current flowsfrom the anode through the electrolyte to the structure.Materials which are commonly used for galvanic anodesystems are magnesium, zinc and aluminum.

Another method of cathodic protection uses anodes inconjunction with an external D.C. power source. Thistype of cathodic protection system is known as an im-pressed current system. The anodes are installed in thesoil or water and are connected to the positive terminalof a D.C. power source such as a rectifier. The struc-ture to be protected is connected to the negative termi-nal of the power source and electrical current is forcedto flow from the positive terminal to the anodes throughthe electrolyte to the structure. This type of cathodicprotection system uses long life anode materials such ashigh silicon chromium cast iron, graphite, and mixedmetal oxide coated titanium.

There are numerous publications that discuss the prin-ciples of corrosion and the application of cathodic pro-‘eec$o:n ~n~~~~~n~~~si~a~~~~~r~~~~~~ ;;tf:

.new corrosion control technology. The first docu-mented application dates back to the early 19th century.

Each of us has a cathodic protection system in ourhomes. The zinc or magnesium rod installed in domes-tic hot water heaters is a cathodic protection anodewhich provides protective current to the submergedsteel surfaces. The number and/or dimensions of the

anode has a direct bearing on the length of the manu-facturer’s warranty,-i.e. the longer the anode life, thelonger the warranty.

Cathodic protection is an established technology foreconomically preventing leaks. It is required by theEPA for protecting underground steel tanks3, and bythe DOT for protecting underground pipelines4 In ad-dition, API has recently published Recommended Prac-tice (API RP 651) for “Cathodic Protection of Above-Ground Petroleum Storage Tanks.”

EXISTING TANKS

Cathodic protection for corrosion control of existingtank bottoms has been applied using a variety of ap-proaches with varying degrees of success. The use ofsacrificial anodes is typically limited to those applica-tions where the tanks are of small diameter and wherethey are electrically isolated from other undergroundmetallic structures. Impressed current systems havebeen used where more protective current is requiredand where a longer system life is desired.

RECTIF‘IER

ANODES7

NEGATIVE CONNECTION TO TANK

RE

SUPPORT RING

- ANODES -

Figure 1

Distributed Anode System

WELL

JUNCTIONBOX EGATIVE CONNECTION TO TANK

CONCRETE SUPPORT RING

ANODES IN GRAPHITEFILLED COLUMN

EL. - 15’

figure 2

Deep Anode System

Several different types of anode installations to dis-tribute the protective current to the tank bottom arepossible:

* Horizontal or vertical anodes distributedaround the periphery of the storage tank(Figure 1).

* Deep anode systems in long, vertical columns(Figure 2).

* Angle drilled anode systems extending the an-odes under the tank bottom (Figure 3).

The standard method for determining the effectivenessof cathodic protection is the tank-to-soil potential mea-surement. These measurements are performed using ahigh impedance voltmeter and a stable, reproduciblereference electrode contacting the soil.

The natural potential of a carbon steel tank in contactwith the soil is usually on the order of -0.540 voltswhen measured with respect to a copper/copper sulfateelectrode (CSE). Values which are more positive aretypically indicative of steel structures which have

RECTIF

‘0

tive to the point where little if any potential shift ismeasured, thus indicating a lack of effective corrosionprotection at the center.;. For 120 foot diameter tanksprotected by distributed or deep anode systems, it is notunusual for the potential at the center of the tank to beon the order of 300 millivolts less negative than at thetank rim. However, the angle drilled application tech-nique overcomes this problem by evenly distributingthe current along the tank bottom.

r NEGATIVE CONNECTION TO TANK

RESUPPORT RING

PVC PIPE WITH LOWERAPORTION PERFORATED

Figure 3

Angle Drilled Anode System

undergone corrosion, while values more negative areindicative of new, well coated structures or a structurewhich is under the influence of cathodic protection.

A tank is considered to be effectively protected when apotential measurement at least as negative as -0.850volts CSE is obtained (one of several accepted crite-ria)s. For a true representation of electrical potentialmeasurements, the reference electrode is to be placed asclose as possible to the tank bottom.

The problem associated with monitoring cathodic pro-tection systems on tank bottoms is the inability to placea portable reference electrode in close proximity to theunderside. Most of the testing in the past has reliedupon readings which have been taken at the perimeterof the tank. The placement of the reference electrode atthe tank rim may yield erroneous results because ofpotential gradients created in the soil as a result of thecurrent discharge from the anode. This is particularlytrue when using distributed anodes installed along theperiphery of the tank.

In certain cases, it has been determined that potentialmeasurements at the perimeter satisfy one of severalcriteria, but as the reference cell is advanced towardsthe center of the tank, the potentials become more posi-

Experience with various application techniques for ca-thodic protection of existing tank bottoms has estab-lished that:

* Tank-to-soil potential measurements obtainedat the perimeter of the tank do not indicateactual cathodic protection levels at the center,particularly on larger diameter tanks.

* Potential measurements at the center of thetank and at other areas under the tank are nec-essary if tank bottom cathodic protection lev-els are to be accurately evaluated. This canbe accomplished by boring perforated pipeunder the tank for reference cell access(Figure 4).

PERFORATED PVC PIPE 7

REFERENCE CELL 1 \ /

REFERENCE FC;;;TE, pvC p,pA s=k

Figure4

Monitoring System for Existing Tank

* When testing, the liquid level in the tank mustbe sufficiently high to create intimate contactbetween the tank bottom and the pad. Emptytanks usually result in erroneous test results.

* Certain types of cathodic protection designsmay not be effective in providing adequatecathodic protection to the tank bottoms incertain areas.

* An improved design for existing tanks usesslant or angle drilling techniques to locate an-odes under the tank bottom (Figure 3). Thisdistributes the current to the center and over-comes the tendency for most of the currentfrom distributed or deep anodes to flow to thetank perimeter.

NEW CONSTRUCTION

For new construction without secondary containment,conventional cathode protection anodes can be dis-tributed under the tank bottom (Figure 5) to optimize

RECTIFIER- ANoDESL

P+

NEGATIVE CONNECTION TO TANK

CONCRETE

A N O D E S =

Figure 5

Conventional Anodes Under Tank

REFERENCE CELL7

TEST TEST LEAD CONNECTION

STATION TO TANK

\I- CONCRETESWORT RING

-lPERMANENT REFERENCE CELL

Figure 6

Monitoring System for New Tank

the distribution of cathodic protection current. At thesame time, permanent reference cells can also be placedat the center of the tank (Figure 6) to provide for accu-rate system adjustment and monitoring.

Many new tanks are now constructed with secondarycontainment liners typically made of 60 mil thick highdensity polyethylene (HDPE) installed six to twelveinches under the tank. Since HDPE has a high dielec-tric strength, the liner would prevent protective currentfrom reaching the tank bottom using conventional ca-thodic protection anode configurations.

Sacrificial anode systems using zinc or magnesium rib-bons or rods have been previously used for cathodicprotection. These are placed between the liner and thetank bottom but generally suffer from limited systemlife.

It was desirable to develop a new impressed currentcathodic protection technique that would have the cur-rent capacity for large diameter tank bottoms and thatcould be designed for an operating life of up to 50years. An economical system design was therefore developed using mixed metal oxide coated titanium rib-bon. The ribbon is linearly extended in strips above the

secondary containment liner. Titanium conductor barsare resistance welded perpendicular to the ribbon toform an anode grid (Figure 7). The grid evenly dis-tributes current to the tank bottom through multiplepower feeds. It can be placed as closely as four inchesto the tank bottom and still maintain proper protectionlevels, even with the ribbon spacing extended to sixfoot centers (Figure 8). Because of the even currentdistribution provided by this design, current require-ments for cathodic protection of the tank bottom are re-duced, which in turn reduces the operating costs overthe life of the system.

TRANSVERSE TO

Figure 7

Titanium Anode Grid

PERMANENT ZINC REFERENCE 1 TANK BOTTOM 1 I- TANK WALL

Testing of the anode grid systems on a number of tankshas clearly established that:

Cathodic protection is required for corrosionprotection of tank bottoms with secondarycontainment liners. Even if the liner is per-fectly sealed at the ring wall, condensationforms on the tank bottom due to temperaturedifferentials. This results in a corrosive envi-ronment.

The titanium anode grid system provides ex-cellent levels of corrosion protection at re-duced operating costs.

With only factory sealed electrical connec-tions under the tank, the probability of systemmalfunction due to splice failure is negligible.

System designs can satisfy design life re-quirements of up to 50 years.

CONCLUSION

The application of cathodic protection is an economicaland effective means of controlling corrosion on tankbottoms. The success of the cathodic protection systemis dependent upon proper design, selection of installa-tion type and effective monitoring.

For existing tanks without secondary containment lin-ers, angle drilled impressed current anodes have beenproven to be the best approach. Access to the center of

+t

............................................................................ RING WALL.........................................................................................................................................................................................................8”

..........................

IJUNCTION BOX

........................COMPACTED SAND BACKFILL

.......................................................................................................................................................................................................................................................... - CONDUIT TO............................................................................................................................. RECTIFIER#lo HYPALON INSULATED STRANDED COPPER

..................................................................... . ...

LEAD WIRE ROUTED TO CONDUIT .

TITANIUM ANODE RIBBON INSTALLED

APPROX. 4" ABOVE MEMBRANE ON COMPACTEDSAND BACKFILL LAYER. SECONDARY CONTAINMENT MEMBRANE

Anode Grid Cross Section

the tank for system testing should be provided by thesimultaneous installation of slotted pipe.

For new construction with HDPE secondary contain-ment liners, the preferred application technique uses atitanium anode grid. Accurate cathodic protectionmonitoring can be achieved by including the installationof one or more permanent reference cells.

REFERENCES

1.

2.

3.

4.

5.

Romanoff, M. “Underground Corrosion” NationalBureau of Standards Circular 579, April, 1957.

Uhlig, H.H. Corrosion and Corrosion Control,John Wiley & Sons, Inc., 1963.

Environmental Protection Agency, Title 40, Codeof Federal Regulations, Parts 280 and 281,September 23, 1988.

Department of Transportation, Title 49, Code ofFederal Regulations, Parts 192 and 195, asamended June 30, 1971.

National Association of Corrosion Engineers, Rec-ommended Practice RP-01-69, “Control of Exter-nal Corrosion on Underground or SubmergedMetallic Piping Systems,” revised 1983.

While statements contained in this publication are believed to be accurate, they are offered as suggestionsonly and no warranty or representation is intended. Drawings and materials depicted herein are theproperty of CORRPRO COMPANIES, INC., and are not to be used in whole or in part to assist in makingor to furnish any information fix the making of drawings, prints, apparatus or parts thereof. All propri-etary information is the exclusive property of CORRPRO COMPANIES, INC., and is protected by copy-right.

Copyright 1990, CORRPRO COMPANIES, INC.