Note: Descriptions are shown in the official language in which they were submitted.
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CATHODIC PROTECTION METHODS AND APPARATUS
FIELD OF THE INVENTION
This invention relates generally to the phenomenon of corrosion, and more
particularly, to the protection of metallic structures or surfaces which are
subjected to
conrosive conditions. Of special interest is cathodic protection of above-
ground storage
tank bottoms.
BACKGROUND
It is known that all metallic structures that come in contact with a medium
having
the properties of an electrolyte are susceptible to the phenomenon of
corrosion. Such
corrosion tends to destroy the metallic structure and, depending upon the
particular
corrosive conditions existing, destruction of the metallic structure may occur
within a
longer or shorter period of time. tn many instances.significant damage to the
metallic
structure may occur within a short period of time even though destruction of
the metallic
structure has not yet occurred.
There are a great many structures subject to corrosion damage, including
bridges,
pipes, storage tanks, reinforcing steel of concrete structures, structural
steel and piles. In
most cases the electrolytes for such structures comprise water with dissolved
salts and
moist soils.
Many techniques have been developed to minimize corrosion. Perhaps the most
common method of minimizing corrosion of steel is painting. However, paint is
not fully
effective for underground and immersion conditions because of a gradual
decrease in its
resistance which may result from pin-holing and moisture permeation from the
corrosive
medium to the substrate metal. Corrosion protection in painted steel or steel
containing
structures is therefore often supplemented with another method commonly known
as
cathodic protection. Cathodic protection can also be used for unpainted
surfaces.
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As used herein, the term "cathodic protection" encompasses all manner of
preventing or reducing corrosion of structures in electrolytes such as water,
soil or
chemical solutions using means which are at least partially electrical.
In general, cathodic protection systems operate by utilizing an electrical
current to
oppose a corrosion current between the structure being protected and an
electrolyte.
There are basically two known systems for generating opposing electrical
currents,
''sacrif cial systems" and "impressed current systems." In sacrificial
systems, the current
is supplied by another metal which is galvanically more reactive than the
metal of the
structure. For example, metals such as aluminum, magnesium and zinc are
galvanically
more active than steel and are used as "sacrificial anodes " to protect steel
structures. In
impressed current systems, a consumable metal is used to drain DC current
supplied from
an external source into the electrolyte which will pass to the structure to be
protected.
The parts from which the current is drained are called "anodes " and the
protected
structure is called "cathode." In both sacrificial and impressed current
systems of
cathodic protection, a metallic path between the anode and the cathode is
essential for
flow of current to protect the structure.
The design of cathodic protection systems is influenced by numerous factors,
including the type of metal to be protected, properties of the electrolyte
(chemical,
physical and electrical), temperatures, presence or absence of bacteria, shape
of the
structure, design life, constructability and maintainability. Cathodic
protection has been
achieved by application of various metallic and polymer webs, tapes, wires,
ribbons and
bars to a metallic structure being protected. US 4,992,337 to Kaiser et al. (
1991 }, for
example, describes an improved arc spray process for applying metals or alloys
comprising magnesium, zinc, lithium, and aluminum. The patent also references
an
article by H. D. Steffans entitled "Electrochemical Studies of Cathodic
Protection
Against Corrosion by Means of Sprayed Coatings" in Proceedings 7th
International
Metal Spraying Conference {1974) at p. 123, which describes the arc spray
application
and corrosion testing of zinc, aluminum, and zinc aluminum pseudo alloy
coatings. Still
further, US 4,992,337 references an article by P. O. Gartland entitled
"Cathodic
Protection of Aluminum Coated Steel in Seawater" in Materials Performance June
19$7
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3
at p. 29, which reviews the arc spray coating of steel with aluminum 5 wt %
magnesium,
and summarizes the performance of the coating in seawater.
Cathodic protection using strips or bands of aluminum, zinc, magnesium or
alloys
thereof is described in US 4,496,444 to Bagnulo (1985). Similarly, US
5,411,646 to
Gossett (1995) describes cathodic protection using a braided anode having a
mixed metal
oxide coating, and US 5,547,560 to LeGuyader (1996) describes cathodic
protection of
steels and alloys in seawater using a saturated calomel electrode comprising
an aluminum
based gallium and/or cadmium alloy.
The use of foils in limited circumstances is also known in the field of
cathodic
protection., but none of the prior uses of foils is particularly viable. US
Pat. No.
5,167,352 to Robbins, for example, describes the construction of double wall
tanks in
which an outer wall envelope of aluminum foil is installed over a
prefabricated tank.
Robbins' use of aluminum foil is not self supporting, and the physical
strength for the
foil is invariably augmented by a resinous coating applied after the
installation is
completed. The requirement of applying a coating after installation severely
limits the
applicability of the technology to relatively small tanks (less than 100 feet
in diameter)
because the tank must be fabricated and hydrotested before application of the
aluminum
foil envelope . The sequence of events required is: ( 1 ) filling the tank
with water to check
for leaks; (2) emptying the tank; (3) drying the tank interior to prevent
inside corrosion;
(4) wrapping the aluminum foil to the tank bottom to form an envelope; (5)
providing
temporary physical support to the foil during the formation of the envelope
and lifting of
the tank; (6) coating the aluminum foil, seal the overlaps, lift the tank, and
position the
tank on the foundation; and finally (7) removing the temporary physical
support of the
foil with sufficient care not to damage the foil and coating laminate.
Robbins' technology is of limited value for other reasons as well. Among other
things, all overlaps of the aluminum foil must be completely sealed, as the
aluminum foil
is intended as a secondary containment. This greatly increases manufacturing
di~culties. In addition, Robbins' technology cannot be utilized for existing
above
ground storage tanks which require replacement of corroded floors.
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4
Other cathodic protection systems utilize wires and wire meshes in place
of.strips,
bands and foils. US 5,340,455 to Kroon et al. (1994), for example, a
horizontally
disposed cathodic protection anode is positioned between a membrane and the
tank
bottom, the anode being in the form of a matrix, maze or grid of electrically
interconnected coated titanium wires or titanium clad copper wires, and such
wires and
titanium bars or ribbons. The wires are provided with a mixed metal oxide or
noble metal
coating. The bars or ribbons may also be coated. In lieu of the preferred
titanium, other
suitable metals may be used such as aluminum, tantalum, zirconium or niobium,
and
alloys thereof.
Still other systems alter the composition of the foundation. US 5,174,871 to
Russell ( 1992), for example, describes corrosion protection of underground
structures
using a high pH backfill including calcium silicate, calcium nitrate and a
hydroxide such
as calcium hydroxide or aluminum hydroxide.
In short, despite significant work invested over many years in the development
of
cathodic protection systems, uniform protection is not generally possible with
known
systems. Among other things such systems continue to be problematic because:
1. The design calculations are performed with "assumed" resistivity of the
electrolyte because the actual resistivity varies with time and pressure
exerted by the tank
bottom during operation which is not known during the design phase of the
cathodic
protection system:
2. The spacing of the anodes is influenced by the "assumed" resistivity; and
3. There is no proven design method to accurately predict the current
distribution from the ribbon and wire systems on the tank plate. The accuracy
of methods
used to calculate the current distribution from the distributed anodes on the
tank plate is
questionable as well. If the "assumed" resistivity is not correct, adjustment
of the
system may not be possible.
Resistivities of the structure's foundation can range from 10,000 ohm-
centimeters
to 300,000 ohm-centimeters, and variations in resistivities are common from
one location
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to another even within the same foundation. Galvanic anodes, when errrbedded
in the
foundation according to current technologies do not work satisfactorily
because of high
voltage drops between the anode and the structure. Impressed current anodes
can be used
in such high resistivity mediums, but they generate oxygen and chlorine gas
during the
chemical reactions, and these gases collect under the structure. Unless the
oxygen and
chlorine gases are completely purged with inert gas such as nitrogen, pitting
corrosion
occurs on the tank bottoms. Complete nitrogen purging and verification of its
effectiveness is neither practical nor economical.
Still further, tests have shown that installation of impressed current
cathodic
protection designs should not be used in the annular space of double bottom
storage
tanks because oxygen that is generated by impressed current anode systems is
retained
within the closed systems and supports continued corrosion (Reference: Rials
S.R. and
Kiefer J.H., Conoco Inc, Evaluation Of Corrosion Prevention Methods For Above
ground Storage Tank Bottoms, Materials Performance, National Association of
Corrosion Engineers, Jan 1993).
Another problem with existing systems is potential damage to the anodes and
anode connections after they are embedded in with the soil. To prevent
settling of the
structure, surrounding soil generally requires compacting, and compacting
methods can
potentially damage the anodes and anode connections. Compacting methods also
affect
the electrical resistivity which could be different from the electrical
resistivity used in the
design of the cathodic protection system.
These problems are particularly apparent when protecting structures such as
petrochemical holding tanks because of the large surface area being protected,
and
difficulties associated with construction of the structures and foundations.
Thus, there is
still a need for improved cathodic protection systems.
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6
SUMMARY OF THE INVENTION
The invention provides a method of providing cathodic
protection to an exterior metallic surface of a structure which
is subject to corrosion from a medium, comprising: providing an
anode layer which is galvanically more active than that of the
metallic surface, and which is structurally substantially
separable from both the structure and the corrosive medium;
placing the anode layer between the medium and the structure;
and positioning the metallic surface such that the anode layer
has a direct current connection with the metallic surface of
the structure, wherein the anode layer comprises aluminum.
The present invention is directed to apparatus,
compositions and methods which provide cathodic protection to a
structure by placing an anode layer directly between the
structure and its underlying foundation. Structures
contemplated to be protected in this manner include especially
very large structures such as above ground storage tanks.
In one aspect of preferred embodiments, the anode
layer comprises sheets of at least 85% aluminum with other
alloying elements such as magnesium (0.05 to 6%), zinc (0.1
to 8%), Indium (0.005 to 0.03%) and tin (0.05 to 0.2%) added
for the purposes of optimizing current yield, polarization and
ease of manufacturing the sheet. In another aspect of
preferred embodiments, the anode layer comprises at least two
overlapping sheets.
Various objects, features, aspects and advantages of
the present invention will become more apparent from the
following detailed description of preferred embodiments of the
invention, along with the accompanying drawings in which like
numerals represent like components.
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6a
BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1-A is a schematic of a vertical cross-section
of a holding tank and foundation utilizing a SALSA system.
Fig. 1-B is an expanded view of two anode sheets
shown in Fig. 1-A.
Fig. 1-C is a schematic of a weatherproofing sealant
disposed at a junction of a tank plate edge and an anode sheet.
Fig. 1-D is a schematic depicting butt-jointed anode
sheets of an anode layer.
Fig. 1-E is a schematic of a sealant in an overlapped
interface area of two anode sheets.
Fig. 1-F is a schematic of a screw at the overlap of
two anode sheets.
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WO 99118261 ~ PCT/US98I02308
Fig. 2-A is a schematic of a vertical cross-section of a holding tank and
foundation utilizing an alternative SALSAsM system.
Fig. 2-B is an expanded view of three anode sheets shown in Fig. 2A.
Fig. 3 is a schematic of a holding tank, an older corroded tank bottom, and a
new
tank bottom protected with a SALSAS" system.
DETAILED DESCRIPTION
Systems which include apparatus, compositions and methods according to the
inventive subject matter hereinafter may be generically referred to as SALSASM
(Sacrificial Aluminum Sheet Anode) systems. In a preferred embodiment of such
systems
shown in Figures 1A and 1 B, two or more anode layers of aluminum sheets 10
are
positioned below the bottom 22 of a tank 20. The aluminum sheets 10 rest upon
a layer
of sand 30 which contains a plastic liner or other relatively impermeable
moisture barrier
35.
As used herein, the term "corrosive medium" means any medium containing an
electrolyte which is corrosive to, or promotes corrosion of, a metallic
component of a
structure. Corrosive media include foundation media such as river sand, silica
sand,
native soil, clay, crushed rock, gravel and any medium engineered to support
the weight
of the filled tank. Sand is one of the most common foundation mediums, and is
therefore
used in various Figures to indicate the corrosive medium.
As used herein, the term "structure" means any structure having a metallic
surface
or other component which is disposed in long-term contact with an electrolyte,
and is
thereby subject to corrosion. Contemplated structures include petrochemical
storage
tanks, water storage tanks, commercial, industrial and residential buildings,
and bridges.
Specifically included are above ground storage tanks with single bottoms, and
existing
above ground storage tanks with corroded floors, andlor multiple bottoms.
As used herein, the term "anode layer" means any galvanically active stratum
which is structurally substantially separable from both the structure being
protected and
the corrosive medium. This definition is quite broad, including, for example,
a stratum
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which comprises a foil, sheet or plate, or an assembly thereof, even if such
stratum is
bolted or welded to the structure. Embodiments may have one or more such anode
layers, and such layers may be placed contiguously with or without overlaps,
with or
without staggered arrangement, with or without sealant in the overlaps, and
with or
without mechanical fasteners at the overlaps. Further examples of contemplated
anode .
layers are: a plastic sheet or other barrier upon which a metal or metallic
composition is
deposited; an aluminized plastic sheet; and a zinc coated steel sheet. In such
anode
layers, the deposition could arise by any suitable means, including painting,
vapor
deposition, thermal spraying, hot-dip galvanizing, electro-depositing,
mechanical galvan-
izing, plasma coating. It is contemplated that at least one of anode layers in
preferred
embodiments will be manufactured from a metal which is galvanically more
active than
the structure it is protecting. Aluminum is preferably used for an anode layer
because
most tank bottoms are comprised primarily of steel, and aluminum is
galvanically more
active than steel. While other metals (which term is used herein to include
alloys) may be
galvanically more active than aluminum when in contact with iron and iron
including
metals, aluminum has additional advantages such as low cost, light weight, and
malleability.
It is important to note that the term ''anode layer'' as used herein does not
encompass strata which are structurally substantially inseparable from either
the structure
being protected or the corrosive medium. Thus, for example, an anode layer as
contemplated herein would not encompass aluminum paint which is spray painted
directly onto the bottom of a structure being protected. Such paint would
presumably be
strongly adhered onto the structure, and would therefore not be structurally
substantially
separable from the structure. On the other hand, the term ''anode layer" would
include
aluminum paint intermixed with the upper surface of a foundation of packed
sand,
because the anode layer is still substantially separable from the remainder of
the sand.
In Figures 1A and 1B the aluminum sheets 10 are each preferably approximately
36 or 48 inches wide by about 0.020 inches thick, and comprise aluminum alloys
3003,
3004, 3005, 3105, 5005, 5010, 7006, 7011, 7075 and 7178 (ASTM B-209) with a
minimum of approximately 85% aluminum content. It should be appreciated,
however,
that other numbers of sheets, sheet sizes, and sheet compositions may be
utilized.
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9
In other embodiments, the sheets may contain different percentages of
aluminum.
Still further, while the sheets in many instances should be flat and smooth,
it is
contemplated that such sheets may contain some degree of corrugation,
embossing or
other surface pattern to increase friction in earthquake prone zones. With
respect to
widths and lengths, the sheets may vary considerably from the present
standards provided
by the aluminum suppliers.
The aluminum sheets 10 of Figs. 1A and 1B are preferably laid on the
foundation
with staggered joints under the plates of the tank bottom 22. Sheets in the
lowest anode
layer 10A are Laid directly on the foundation, and may have a factory applied
moisture
barrier I OP such as a polymeric coating (epoxy and acrylic) or a plastic
laminate (such as
Tedlar TM) on the soil side. The top side of anode layer I OA preferably would
not have
any type of coating. A moisture barrier 1 OP is not necessary for performance
of this
system, however, it is included because it is a very low cost item and is
expected to
prolong the anode life. It is contemplated that all such moisture barriers 1
OP are optional.
Sheets in the upper anode layer 1 OB preferably have both sides plain, without
any
electrically insulating material. Anode layer I OB sheets should be placed
directly on
anode layer 10A sheets, and should have direct metal-to-metal contact with
both anode
layers I OA sheets and the tank plate.
Figures 1-D, I-E and 1-F show various possible arrangements of aluminum
sheets 10 of each anode layer.
In Figure 1-D, aluminum sheets 10 are placed next to each other without
overlaps.
In Figure 1-E, aluminum sheets 10 are placed in an overlapping pattern with an
optional moisture resistant sealant lOS in the overlap area.
In Figure l-F, aluminum sheets 10 are placed in an overlapping pattern and the
overlap is mechanically fastened with screw IOF.
It is important to note that the aluminum sheets 10 of any layer can be
arranged
independent of the method of arrangement of the other layers. For example,
aluminum
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sheets 10 of anode layer 10A can be arranged as shown in Figure 1-E
and~alum~um .
sheets 10 of anode layer lOB can be arranged as shown in Figure 1-D.
In preferred embodiment of the invention, the anode layer is installed to
cover
100% of the top area of the corrosive medium on which the structure would be
placed.
However, anode layers can be installed to cover less than 100% of the top area
of the
corrosive medium if partial cathodic protection of the structure is
acceptable.
In Figures 2A and 2B, a plurality of bottom sheets 10 are placed on a
foundation
comprising sand 30 (corrosive medium) and a moisture barrier 35. As discussed
above,
each of the sheets in the lowest anode layer 10A is preferably approximately
0.020 inches
thick, and are preferably installed with a coated side down and bare/uncoated
side up.
Two or more additional anode layers l OB, l OC of aluminum sheets, also
approximately
0.020 inches thick, but with both sides plain ( without electrically
insulating material on
any side) are then installed on top of the lowest anode layer 10A in a
staggered pattern,
with overlap preferably about 24 inches. The final anode layer of sheets l OC
are in
contact with the tank bottom 22. In this manner most or all of the foundation
is covered
in aluminum sheets, and most or all of the tank bottom 22 is in contact with
aluminum
sheets.
In especially preferred embodiments, the sheets should extend beyond the tank
plate rim by a minimum of 1 /4th inch for new tanks. If desired, an optional
weather
resistant caulking compound 36 may be applied in the corner area on the
weather
exposed side of the tank plate rim and the aluminum sheet to prevent any rain
water from
entering into the interface area of the anode layer and the tank floor plate
as shown in
Figure 1 C.
In Figure 3, a SALSAsM system is used in conjunction with an existing tank and
foundation. Here, a tank 130 has a shell 131 and a previously installed,
generally
corroded floor 132. A dielectric barrier (usually a 40 to 80 mils thick
polyethylene sheet
or a monolithic coating) 134 is laid on top of the corroded floor 132, and a
layer of sand
136, preferably from about four to about six inches thick, is placed on top of
the
dielectric barrier 134. One or more anode layers of aluminum sheets 140 are
then placed
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on top of the sand 136, and finally a new tank bottom 150 is placed on top of
the-
aluminum sheets 140. The arrangement of these aluminum sheets 140 can be in
substantially the same manner as the aluminum sheets 10 shown in Figures 1-D,
1-E and
I-F. This system substitutes for previously known systems in which about I
inch of sand
is placed on top of a dielectric barrier. A conventional cathodic protection
system
consisting of mixed metal oxide anodes (ribbon, grid or coils) is placed on
the sand, then
a subsequent layer of about five inches of sand is placed on top of the
anodes.
Storage tank bottoms, when positioned on a corrosive medium, have natural
electrical potentials which are more corrosive at the center and relatively
less corrosive
towards the outer edge of the tank bottom. To compensate for this variation in
corrosive
potentials, cathodic protection systems should be designed to provide more
cathodic
protection current at the center of the tank than the cathodic protection
current for the
perimeter of the tank bottom, and this can be accomplished by providing a
higher mass of
the anode at the center than at the perimeter. To provide such varied anode
quantities,
1 S SALSAsM systems for large petrochemical storage tanks, such as those
having a footprint
diameter of at least 100 feet, may advantageously comprise 3 anode layers
within 25 feet,
two anode layers from within 25 to 40 ft, and one anode layer within 40 feet
to 50 feet of
the radial distance measured from the center of the tank. In alternative
embodiments, the
thickness of the sheets can be varied rather than the number of layers. Some
of the
parameters which affect the design of such variation in anode mass are tank
size,
thickness of the anode sheet, foundation soil, amount of rain fall and its
drainage away
from the tank, and design life of the tank.
SALSAsM systems as described herein have significant advantages over
previously known systems. Among other things, aluminum sheets are essentially
impermeable to moisture, and therefore prevent migration of ground moisture to
the
structure except, potentially, at the sheet overlap. An organic sealant l OS
at the sheet
overlap may also be used to prevent the migration of ground moisture through
the
overlap. However, even without such sealant l OS, the pressure exerted by the
floor plates
on the aluminum sheet overlaps, will prevent moisture migration at the sheet
overlaps.
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Another advantage of SALSAsM systems is that aluminum sheets can b~ placed in.
direct contact with up to 100% of the tank plate, providing uniform protection
independently of the foundation resistivity. The degree of contact between
aluminum and
tank plate is especially enhanced by the conformability of aluminum at the lap
joints of
floor plates of the tank when the tank is filled due to the considerable
weight brought to
bear on the tank bottom.
Another advantage is that in SALSAsM systems, there is no need for complete
fabrication and pressure testing of the tank before the anode installation.
Instead, the tank
floor can be fabricated directly on top of the anode sheets, or alternatively,
the anode
installation and the tank floor fabrication can be performed in increments.
SALSAsM systems are also advantageous in that structures being protected may
have, but do not require, application of inorganic or organic coatings.
Organic coatings
are required on structures for most other known cathodic protection systems,
and are
subject to damage during welding of structure components and attachments for
interior
cathodic protection system on the floor. The first problem can be mitigated by
leaving the
structure edges bare. However, the bare surfaces require more cathodic
protection current
while the coated surfaces require less current, and it is difficult to satisfy
the additional
cathodic protection requirements by known cathodic protection systems. The
second
problem relates to damage to the organic coating v~ihere attachments to fasten
the interior
floor anodes are welded. Such damage under the floor generally cannot be
repaired,
carbonized coating acts as a cathode and will promote corrosion of the floor
from the
foundation side if sufficient cathodic protection current is not available.
Organic coatings which are required with other cathodic protection
technologies
on tank plates are also problematic in that they may be cathodically disbonded
from the
substrate metal at cathodic protection voltages below -1.2V (measured with
copper-
copper sulfate reference electrodes). For safety reasons, tanks containing
flammable
fluids should be fully grounded and should eliminate electrical isolation at
nozzles to
avoid electrical sparking during lightning. In such cases, the grounding
system draws a
substantial amount of current from the conventional cathodic protection
systems, and
only a small portion of such current would be available for corrosion
protection. One
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possible solution is to increase the current output from the rectifier, but,
when the current
output from the rectifier is increased coated areas of the structure close to
the anode may
be subjected to cathodic disbondment due to excessive cathodic protection
voltage.
Disbonded coatings generally shield the substrate metal from cathodic
protection
currents, thereby reducing the adequacy of the cathodic protection. This kind
of problem
is eliminated with SALSAsM systems both because an organic coating is not
required on
the structure, and because the anode sheets are in direct contact with the
uncoated
structure.
SALSAsM systems have particular advantages compared with systems utilizing
zinc containing anodes such as zinc ribbon. Zinc anode systems are limited to
operating
temperatures less than 140° F because zinc reverses its polarity at
temperatures from
140°F to 250°F when moisture is present. Zinc promotes corrosion
of steel when its
polarity is reversed and therefore can not be used for protection of tanks
with hot
hydrocarbons (residuum) which operate at temperatures of about 250°F.
Aluminum has
no such reverse polarity and can be used at all temperatures up to
1,200°F. Also,
impressed current anode systems should not be used under hot structures
because higher
current output which is required at higher temperatures also generates a
higher amount of
oxygen. The pitting corrosion of the structure increases with increased
presence of
oxygen. Again, the SALSASM system is a better choice when compared to
impressed
current anode systems because the oxygen generated by SALSAsM system is
insignificant.
As discussed in part above, SALSAsM systems have several advantages compared
with other systems by virtue of their independence from soil or other
foundation
conditions. For example, while other systems may not prevent ground moisture
from
reaching the structure, the anode sheets in SALSAsM systems can be entirely
impermeable to water. Similarly, design of other systems is based on the
assumptions
that the soil resistivity is uniform under the structure, and that all of the
steel areas of the
tank will receive equal current density. In SALSAsM systems these
considerations are
irrelevant because current flow is independent of soil resistivity. Bum-out of
anode
connections, current attenuation, and electrical grounding are also irrelevant
to SALSAsM
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systems. Similarly, reference electrodes need not, and preferably are not
installed-with _
SALSAsM systems.
SALSASM systems have still other advantages relative to previously known
systems. For example, impressed current systems generate stray currents which
can
damage the rebar of the tank ring wall, while such problems do not exist with
SALSAsM
systems. Other advantages relate to engineering and construction. For example,
engineering time of SALSASM systems is reduced to one or two hours, which is
lower
than with other systems. SALSASM systems also do not require skilled labor for
installation, and such systems help improve schedules for shipment of tank
plates to
project sites because painting operations for the structure are reduced. In
addition,
suitable aluminum sheets are readily available or have short lead time.
Electrical cables,
test stations, etc. are not required. Still further, anode installation and
floor construction
are concurrent rather than sequential. This saves 2 to 3 weeks of construction
time .
SALSAS"' systems also have operating advantages. For example, SALSAsnn
systems are automatically operationai, and protect the floor plate as soon as
the floor
plate is laid on the aluminum sheets. 'There is no need for start up
procedures for cathodic
protection or temporary protection. Still further, test stations are not
required because the
entire surface of the structure is isolated from the foundation soil.
Thus, specific embodiments and applications of SALSASM systems have been
disclosed. It should be apparent, however, to those skilled in the art that
many more
modifications besides those already described are possible without departing
from the
inventive concepts herein. The inventive subject matter, therefore, is not to
be restricted
except in the spirit of the appended claims.
