CA1330318C

CA1330318C - Cathodic protection of storage tank bottoms

Info

Publication number: CA1330318C
Application number: CA 564467
Authority: CA
Inventors: Robert A. Gummow
Original assignee: Corrosion Service Co Ltd
Current assignee: Corrosion Service Co Ltd
Priority date: 1988-04-19
Filing date: 1988-04-19
Publication date: 1994-06-21
Anticipated expiration: 2011-06-21

Abstract

Abstract of the Disclosure An existing metallic tank with a corroded bottom is improved by replacing the original bottom with a new bottom above the original, and using the original tank bottom to cathodically protect the new tank bottom. A
closed loop of material is removed from the original bottom close to the perimeter thereof, thus electrically isolating the tank from the remainder of the original bottom. An insulated cable is connected to the remainder of the original bottom, and an electrically insulative medium such as polyethylene tape is applied to the inside surface of the remainder of the original bottom in an unbroken pattern which includes the location where the cable is connected, thus shielding portions of the remainder under the insulative medium from being anodically sacrificed. This allows the shielded portions to function as current headers to carry current to the unshielded portions of the remainder of the original tank bottom. Fill material is placed above the original bottom, and a new metallic bottom is installed above the fill material, in electrical communication with the rest of the original tank. A direct current is then impressed between the insulated cable and the rest of the original tank, with the insulated cable being positive.

Description

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_ This invention relates to a method ~or cathodically protecting a new tank bottom by utilizing the old tank bottom as the impressed current anode. The invention also pertains to a tank on which the ~oregoing method has been practiced.
BACKG~OUND OF THIS INVENTION
The corrosion process as it relates to buried, partially buried, and submerged metallic structures is a naturally occurring phenomenon. The principles of this process and the benefits o~ cathodic protection in controlling this type of corrosion are widely known.
Corrosion of steel and other metals is governed by electrochemical principles. In ordar for corrosion to occur, three prerequisites are necessaryo 1) There must be an anode (point of electrical current discharge) and a cathode (point where electrical current is received) with an electrical potential difference between the two.

2) The anode and cathode must be electrically connected through a metallic return path.

3) The anode and the cathode must be in contact with an electrically ionized solution (electrolyte).
When these conditions are satisfied, ener~y in the form of direct current will pass from the negative area to the positive area through the electrolyte (conventional current flow).
It is clear that galvanic corrosion relates very closely to the natural reaction which occurs in a battery. With the flow of electricity from the anode to the cathode, positively charged atoms of metal detach from the surface of the anode and enter into the electrolyte. Conversely, the area known as the cathode is protected from the loss of metal. The amount of `
metal lost varies with the type of material and in the case of iron and steel it occurs at the rate of 20 pounds per ampere year.
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There has been considerable and increasing concern over the integrity of underground storage tanks as well as storage tanks resting on the ground, because of the corrosive reaction ~etween the metal and the electrolyte.
Cathodic protection is a proven method of controlling corrosion of buri~d, partially buried and submerged metallic structuresO In theory, a properly designed, installed and operated cathodic protection system will eliminate the anodic regions which exist on a metal by passing direct current to the metal surface.
The direct current is discharged from electrodes installed in the electrolyte adjacent to or near the structure being protected. In this manner, corrosion will be arrested when the direct current is of sufficient magnitude and is adequately distributed over the entire surface to be protected.
There are two basic types of cathodic protection systems which can be utilized to arrest corrosion. One of these is referred to as a galvanic or sacrificial anode system. The galvanic anode system i~ based upon the natural potential difference which exists between the structure being protected and the auxiliary `
electrode known as an anode, which is installed in tha electrolyte. As a result of the potential difference, a battery e~fect is created, and just as in the case o~
galvanic corrosion, electrical current will ~low from ths anode through the electrolyte to the structure and then return to the anode through a metallic connection between the anode and the structure. Materials which are commonly used for galvanic anode systems are made of magnesium, zinc and aluminum.
Another method of cathodic protection utilizes anodes in conjunction with an external Direct Current (D.C.) power source. This type of cathodic protection system is knawn as an impressed current system. The anodes are installed in soil or water and are connected to the positiv~ terminal of a D.C. power source such as a rectifier. The structure to be protected is connected ~
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to the negative terminal of the source and electrical current is forced to flow from ths positi~e terminal to the anodes through the electrolyte to the structure.
This type of cathodic protection system utilizes anode materials made of high silicon chromium cast iron, graphite, l~ad-silver alloy, and to a certain extent, platinized niobium and platinized titanium.
The application of cathodic protection ~or tank bottoms has been accomplished in a variety of approaches with varying degrees of suc~ess. The use of sacrificial anodes is typically limited to those applications where the tanks ar~ of small diameter construction and where they are effectively isolated from other underground metallic structure~ such that the current requirements to achieve protection are minimal. Impressed current system designs have b~en utilized where current requirements are msre substantial. Several different types of anode in~tallations intended to distribute`the protective current to the tank are possible. For example, in one configuration, horizontal or vertical anodes are installed around the periphery of the storage tank. In another system, horizontal or vertical anodes are installed under the tank bottom. Alternatively, anodes can be installed in deep well configurations located adjacent to the tank.
Each of the foregoing systems has advantages and disadvantages, and the selection of each system is dependent upon design and economic ~actors as well as operational requirements. ;~
Paramount with selection o~ system type is the method of verification that corrosion control has indeed -.
been achieved. Past experience indicates that standard monitoring procedures may not be adequate in analyzing the degree o~ protection being af~orded to a storage ~ `
tank resting on the ground. Experience has shown the ~ollowing~
l. That structure-to-earth potential measurements obtained at the perimeter of the tank do not indicate actual cathodic protection l~v81s at `:
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the cPnter, particularly on larger diameter tanks.
2. That potential measurements at the center o~
the tank and at other areas under the tank are necessary if tank bottom cathodic protaction surveys are to be complete.
3. That the liquid level in a tank is a significant factor in determining the cathodic protection status of the tank bottom at the center.

4. That certain types of cathodic protection designs may not be effective in providing adequate cathodic protection to the tank bottoms in certain areas.

5. That an improved design for a distributed groundbed utilizing slant or angle drilling techniques to locate anodes under the tank bottom can improve the current distribution to the center by reducing the tendency for most of the current from conventional distributed beds to flow to the tank at the perimeter.
The standard method of determining the effectiveness of cathodic protection on any structure is the structure-to-electrolyte potential measurement.
These measurements are performed utilizing a high impedance voltmeter and a stable re~erence electrode :
contacting the electrolyte.
The natural potential o~ a carbon steel structure in contact with an electrolyte is usually on the order of -0.600 volts when measured with respect to a copper~
copper sulfate reference electrode (CU/CuSO4). Values which are more positive are typically indicative o~
steel structures which have undergone corrosion while values more negative are indicative of new~ well coated structures or a structure which is under the influence of cathodic protection.
A structure is considered to be effectively protected when a potential measurement o~ -0.850 volts (Cu/CuSO4) is obtained (one of several accepted ~33~

criteria). For a true representation of electrical potential measurements, the reference electrode is to be placed as close as practicable to the struc~ure under investigation (either directly over or adjacent to).

In view of the problems and difficulties mentioned in the preceding section, it is an object of one aspect of this invention to provide an inexpensive and reliable method for replacing a corroded tank bottom with a new tank bottom, and using the corroded tank bottom as an anode for the cathodic protection of the new tank bottom, utilizing the impressed current technique.
It is an ob~ect of another aspect of this invention to provide a tank having a new bottom, with the bottom being cathodically protected through the use of the old tank bottom as an anode.
More particularly, this invention provides a method for cathodically protecting a replacement bottom in a storage tank made of a metallic material and of which the original bottom has become corroded, the replacement bottom being installed in spaced rPlation above the original botto~ with fill material between the two bottoms, the method comprising the steps:
a) removing a closed loop of said metallic material from the original bottom, the closed loop being close to the perimeter of the original bottom, thus electrically isolating the tank from the remainder of the original bottom within the removed closed loop, b) electrically connecting an insulated cable to said remainder of the original bottom, c) applying an electrically insulative medium to less than all of the inside surface of said remainder of the original bottom, in an unbroken pattern which includes the location where the cable is connected to said remainder of the original bottom, thereby to create shielded portions of said remainder of the original 133~
5a bottom and unshielded portions of said reminder of the original bottom, and thus to protect said shielded portions of said remainder of the original bottom under said medium from being anodically sacrificed so that ' .

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~-~ 33~3~8 such shielded portions can function to carry current to said unshielded portions of said remainder of the original bottom, and d) impressing a direct currsnt between said insulated cable and the tank, the insulated cable being positive.
Further, this invention provides a method of replacing a corroded bottom of a tank made of a metallic material, and of cathodically protecting the replacement bottom, comprising the steps:
a) removing a closed loop of said metallic material from the original bottom, the closed loop being close to the perimeter of the original bottom, thus electrically isolating the tank from the remainder of the original bottom within the removed closed loop, b) electxically connecting an insulated cable to said remainder of the original bottom, c) applying an electrically insulative medium to ~:
less than all of the inside surface of said remainder of 20 the original bottom, in an unbroken pattern which ~ `
includes the location where the cable is connected to said remainder of the original bottom, thereby to creatP
shielded portions of said remainder of the original ~:
bottom and unshielded portions of said reminder of the 25 original bottom, and thus to protect said shielded portions of said remainder of the orignal bottom under said medium from being anodically sacrificed so that such portions can function to carry current to the unshielded portions of said remainder o~ the original 30 bottom, d) placing fill material above the original bottom, e) installing a new bottom of a metallic material .
above the fill material, the new bottom being in 35 electrical communication with the rest of the original tank, and f) impressing a direct current between the insulated cable and the rest of the original tank, the insulated cable being positive.
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Finally, this invention provides a metal storage tank made of a metallic material and having an original bottom and a replacement bottom above the original bottom, the replacement bottom being in electrical communication with the tank, fill material between the two bottoms, the original bottom being intarrupted by virtue of a removed closed loop of material, the closed loop being close to the perimeter o~ the original bottom ~ and defining a remainder of the original bottom which is electrically isolated from the rest of the tank, an insulated cable electrically connected to said remainder of the original bottom, an electrically insulative medium applied to less than all of the inside surface of :~ said remainder of the original bottom in an unbroken . 15 pattern which includes the location where the cable is ~ connected to said remainder of the original bottom, thereby to create shielded portions of said remainder o~
the original bottom and unshielded portions of said remainder of the original bottom, and thus to protect ~:
said shielded portions of said remainder of the original bottom under said medium from being anodically sacrificed so that such shielded portions can function to carry current to said unshielded portions of said remainder of the original bottom, and means for impressing direct current between said insulated cable . and the tank with the insulated cable being positive.
GENERAL DESCRIPTION OF THE DRAWINGS
One embodiment of this invention is illustrated in : the accompanyin~ drawings, in which like numerals denote like parts throughout the several views, and in which:
. Figure 1 is a vertical sectional view through the bottom portion of a tank to which a new tank bottom has been applied in accordance with this invention: and Figure 2 is a plan view of the original t~nk bottom, showing its preparation to act as an impressed current anode for the cathodic protection of the new tank bottom.
D~,TAILED DESCRIPTION OF THE DRAWINGS
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Attention is first directed to Figure 1, which shows a portion of an existing tank side wall at 10, the tank having had an original tank bottom 12 with a periphery 22 adjoining the side wall 10 (ignoring the broken lines at 20). The tank is made of an electrically conductive material, such as a metal. In accordance with one aspect of the method o~ this invention, a new -~ tank bottom 14 is installed above and in spaced relation - from the original tank bottom 12, with sand or other granular fill material 16 between the two bottoms.
However, before the fill material 16 and the new tank bottom 14 are put into place, certain operations are carried out on the existing or original tank bottom 12 T as will now be explained with reference to Figure 2.
The first step is to remove a closed loop 20 of the conductive material from the original bottom 12, the closed loop being close to the perimeter 22 o~ the original bottom. This operation electrically isolates`
the tank from the remainder of the original bottom within the removed closed loop 20. In both figures, the said remainder of the original tank bottom is identi~ied ¦ by the numeral 24, to distin~uish from the original entire bottom 12. The remainder 24 extends only to the edge of the closed loop 20, whereas the original bottom 12 extends to the perimeter 22.
An insulated cable 26 is electrically connected at points 28 to the remaindar 24 of the original bottom 12.
This can be done by welding, silver-soldering, or any other appropriate step.
Next, an electrically insulative medium in strip form, such as polyethylene tape, is applied to the inside surface (the upper surface in Figure 1) of the remainder 24 of the original tank bottom, in an unbroken pattern which includes all locations 28 where the cable 26 is connected to the remainder 24 of the original tank bottom. The unbroken pattern illustrated in Figure 2 by ~.
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~-` way of example is a cross formation having four arms 30 extending at 90 intervals from a central cross location -` 32. The connection location 28 for the cable 26 is at the end of one of the arms 30. This operation shields those portions of the remainder 24 of the bottom which :~ are located under the insulative medium from being , anodically sacrificed during the cathodic .''',, .
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.~ g protection. This means that these protected portions can function as current headers, thus carrying ; electrical current to the unshielded portions of the ` remainder 24.
S Following the steps just outlined, sand or other granular fill material 16 is placed abo~e the original - tank bottom, as seen in Figure 1, whereupon a new tank bottom 14 is welded into place against the original tank side wall.
If desired, one or more reference electrodes 34 can be positioned within the sand or other granular fill - material 16, for monitoring purposes. The reference electrodes 34 are connected by insulated cables 36 to a monitoring apparatus (not shown) of conventional kind.
The cable 36 can pass through the old wall below the new tank bottom 14.
The final step in cathodically protecting the replacement or new tank bottom 14 is to impress a dirèct current between the insulated cable 26 and the remainder 24 of the tank 10, with the insulated cable 26 being positive.
The tank 10 illustrated in the figures is a ~ cylindrical tank with a circular hori~ontal section.
;~ This means that both of the bottoms 12 and 14 are -~ 25 substantially circular. In Figure 2, the removed closed loop 20 is a circular annulus located close to the periphery 22 of the original bottom 12. It will, o course, be appreciated that the shape of the removed closed loop 20 will vary depending upon the cross-sectional shape of the tank in question, and that the expression ~closed loop" does not necessarily connote ~;
circularity.
It will thus he appreciated that the invention described above precludes the need for and expense of conventional cathodic protection anode materials by physically modifying an existing tank bottom and using it as the impressed current anode. The power supply utilized can be a simpl~ constant voltage type, or the automatically controlled type, both of which are well .", , ~

known. The negative of the power supply may be connected to the existing tank shell or the new tank bottom by an insulated cable (not illustrated) at any convenient location. As described above, when the power supply is energized, the remaining existing tank bottom 24 functions as an impressed current anode, distributing protective current to the new tank bottom. It is important that the removed closed loop 2~ of material be wide enough to prevent preferential protective current discharge at the edge of the tank bottom anode (the remainder ~4 of the original tank bottom). The minimum radial width of the removed closed loop 20 will vary depending upon the surrounding earth and the nature of the fill material 16, however as a general rule~ the radial dimension of the removed closed loop 20 should not be less than the vertical separation between the original bottom 12 and the new bottom 16.
The system described herein has the advantage of improved current distribution and substantially lower cssts, by comparison with conventional or known methods.
Furthermore, much of the cathodic protection fabrication can be done by the tank fabricator, rather than requiring specialized cathodic protection personnel.
While one embodiment of this invention has been illustrated in the accompanying drawings and described hereinabove, it will be evident to those skilled in the art that changes and modifications may be made therein, ~ without departing from the essence of this invention, as ;; set forth in the appended claims.

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Claims

1. A method for cathodically protecting a replacement bottom in a storage tank made of a metallic material and of which the original bottom has become corroded, the replacement bottom being installed in spaced relation above the original bottom with fill material between the two bottoms, the method comprising the steps:
a) removing a closed loop of said metallic material from the original bottom, the closed loop being close to the perimeter of the original bottom, thus electrically isolating the tank from the remainder of the original bottom within the removed closed loop, b) electrically connecting an insulated cable to said remainder of the original bottom, c) applying an electrically insulative medium to less than all of the inside surface of said remainder of the original bottom, in an unbroken pattern which includes the location where the cable is connected to said remainder of the original bottom, thereby to create shielded portions of said remainder of the original bottom and unshielded portions of said remainder of the original bottom, and thus to protect said shielded portions of said remainder of the original bottom under said medium from being anodically sacrificed so that such shielded portions can function to carry current to said unshielded portions of said remainder of the original bottom, and d) impressing a direct current between said insulated cable and the tank, the insulated cable being positive.

2. The invention claimed in claim 1, in which the closed loop is substantially an annulus.

3. The invention claimed in claim 1, in which said electrically insulative medium is polyethylene tape applied in a cross formation of which the center 11a substantially coincides with the center of the remainder of the original bottom.

4. A method of replacing a corroded bottom of a tank made of a metallic material, and of cathodically protecting the replacement bottom, comprising the steps:
a) removing a closed loop of said metallic material from the original bottom, the closed loop being close to the perimeter of the original bottom, thus electrically isolating the tank from the remainder of the original bottom within the removed closed loop, b) electrically connecting an insulated cable to said remainder of the original bottom, c) applying an electrically insulative medium to less than all of the inside surface of said remainder of the original bottom, in an unbroken pattern which includes the location where the cable is connected to said remainder of the original bottom, thereby to create shielded portions of said remainder of the original bottom and unshielded portions of said reminder of the original bottom, and thus to protect said shielded portions of said remainder of the orignal bottom under said medium from being anodically sacrificed so that such portions can function to carry current to the unshielded portions of said remainder of the original bottom, d) placing fill material above the original bottom, e) installing a new bottom of a metallic material above the fill material, the new bottom being in electrical communication with the rest of the original tank, and f) impressing a direct current between the insulated cable and the rest of the original tank, the insulated cable being positive.

5. The invention claimed in claim 4, in which reference electrodes are installed in the fill material for monitoring purposes.

6. The invention claimed in claim 5, in which the closed loop is substantially an annulus.

7. The invention claimed in claim 5, in which said electrically insulative medium is polyethylene tape applied in a cross formation of which the center substantially coincides with the center of the remainder of the original bottom.

8. A metal storage tank made of a metallic material and having an original bottom and a replacement bottom above the original bottom, the replacement bottom being in electrical communication with the tank, fill material between the two bottoms, the original bottom being interrupted by virtue of a removed closed loop of material, the closed loop being close to the perimeter of the original bottom and defining a remainder of the original bottom which is electrically isolated from the rest of the tank, an insulated cable electrically connected to said remainder of the original bottom, an electrically insulative medium applied to less than all of the inside surface of said remainder of the original bottom in an unbroken pattern which includes the location where the cable is connected to said remainder of the original bottom, thereby to create shielded portions of said remainder of the original bottom and unshielded portions of said remainder of the original bottom, and thus to protect said shielded portions of said remainder of the original bottom under said medium from being anodically sacrificed so that such shielded portions can function to carry current to said unshielded portions of said remainder of the original bottom, and means for impressing direct current between said insulated cable and the tank with the insulated cable being positive.

9. The invention claimed in claim 8, in which the closed loop is substantially an annulus.

10. The invention claimed in claim 8, in which the electrically insulative medium is polyethylene tape applied in a cross formation of which the center substantially coincides with the center of the remainder of the original bottom.

11. The invention claimed in claim 8, in which the fill material contains at least one reference electrode for monitoring purposes.