Classifications

• • C—CHEMISTRY; METALLURGY
  • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
  • C23F—NON-MECHANICAL REMOVAL OF METALLIC MATERIAL FROM SURFACE; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL; MULTI-STEP PROCESSES FOR SURFACE TREATMENT OF METALLIC MATERIAL INVOLVING AT LEAST ONE PROCESS PROVIDED FOR IN CLASS C23 AND AT LEAST ONE PROCESS COVERED BY SUBCLASS C21D OR C22F OR CLASS C25
  • C23F13/00—Inhibiting corrosion of metals by anodic or cathodic protection
  • C23F13/02—Inhibiting corrosion of metals by anodic or cathodic protection cathodic; Selection of conditions, parameters or procedures for cathodic protection, e.g. of electrical conditions
  • C23F13/06—Constructional parts, or assemblies of cathodic-protection apparatus
  • C23F13/08—Electrodes specially adapted for inhibiting corrosion by cathodic protection; Manufacture thereof; Conducting electric current thereto
  • C23F13/16—Electrodes characterised by the combination of the structure and the material
• • C—CHEMISTRY; METALLURGY
  • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
  • C23F—NON-MECHANICAL REMOVAL OF METALLIC MATERIAL FROM SURFACE; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL; MULTI-STEP PROCESSES FOR SURFACE TREATMENT OF METALLIC MATERIAL INVOLVING AT LEAST ONE PROCESS PROVIDED FOR IN CLASS C23 AND AT LEAST ONE PROCESS COVERED BY SUBCLASS C21D OR C22F OR CLASS C25
  • C23F13/00—Inhibiting corrosion of metals by anodic or cathodic protection
• • C—CHEMISTRY; METALLURGY
  • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
  • C23F—NON-MECHANICAL REMOVAL OF METALLIC MATERIAL FROM SURFACE; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL; MULTI-STEP PROCESSES FOR SURFACE TREATMENT OF METALLIC MATERIAL INVOLVING AT LEAST ONE PROCESS PROVIDED FOR IN CLASS C23 AND AT LEAST ONE PROCESS COVERED BY SUBCLASS C21D OR C22F OR CLASS C25
  • C23F13/00—Inhibiting corrosion of metals by anodic or cathodic protection
  • C23F13/02—Inhibiting corrosion of metals by anodic or cathodic protection cathodic; Selection of conditions, parameters or procedures for cathodic protection, e.g. of electrical conditions
  • C23F13/06—Constructional parts, or assemblies of cathodic-protection apparatus
  • C23F13/08—Electrodes specially adapted for inhibiting corrosion by cathodic protection; Manufacture thereof; Conducting electric current thereto
  • C23F13/12—Electrodes characterised by the material
  • C23F13/14—Material for sacrificial anodes
• • C—CHEMISTRY; METALLURGY
  • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
  • C23F—NON-MECHANICAL REMOVAL OF METALLIC MATERIAL FROM SURFACE; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL; MULTI-STEP PROCESSES FOR SURFACE TREATMENT OF METALLIC MATERIAL INVOLVING AT LEAST ONE PROCESS PROVIDED FOR IN CLASS C23 AND AT LEAST ONE PROCESS COVERED BY SUBCLASS C21D OR C22F OR CLASS C25
  • C23F2201/00—Type of materials to be protected by cathodic protection

Definitions

• This invention relates generally to the phenomenon of corrosion, and more particularly, to the protection of metallic structures or surfaces which are subjected to corrosive conditions.
• Of special interest is cathodic protection of above-ground storage tank bottoms.
• 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.
• cathodic protection systems operate by utilizing an electrical current to oppose a corrosion current between the structure being protected and an electrolyte.
• an electrical current to oppose a corrosion current between the structure being protected and an electrolyte.
• sacrificial systems the current is supplied by another metal which is galvanically more reactive than the metal of the structure.
• metals such as aluminum, magnesium and zinc are galvanically more active than steel and are used as "sacrificial anodes " to protect steel structures.
• 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.
• anodes The parts from which the current is drained are called “anodes” and the protected structure is called “cathode.”
• 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.
• 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) 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.
• 5,167,352 to obbins 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.
• 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.
• other suitable metals such as aluminum, tantalum, zirconium or niobium, and alloys thereof.
• the spacing of the anodes is influenced by the "assumed" resistivity
• 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 to another even within the same foundation.
• Galvanic anodes when embedded 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.
• 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.
• 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, h another aspect of preferred embodiments, the anode layer comprises at least two overlapping sheets.
• 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.
• Fig. 2- A is a schematic of a vertical cross-section of a holding tank and foundation utilizing an alternative SALSA SM 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 SALSA SM system.
• SALSA SM Silicon Aluminum Sheet Anode
• sand 30 which contains a plastic liner or other relatively impermeable moisture barrier 35.
• 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.
• 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, and/or multiple bottoms.
• 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 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.
• 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.
• the deposition could arise by any suitable means, including painting, vapor deposition, thermal spraying, hot-dip galvanizing, electro-depositing, mechanical galvan- izing, plasma coating.
• 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.
• anode layer does not encompass strata which are structurally substantially inseparable from either the structure being protected or the corrosive medium.
• 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.
• 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.
• 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. 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 IB 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 10P such as a polymeric coating (epoxy and acrylic) or a plastic laminate (such as Tedlar TM) on the soil side.
• a factory applied moisture barrier 10P 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 10A preferably would not have any type of coating.
• a moisture barrier 10P 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 10P are optional.
• Sheets in the upper anode layer 1 OB preferably have both sides plain, without any electrically insulating material.
• Anode layer 10B sheets should be placed directly on anode layer 10A sheets, and should have direct metal-to-metal contact with both anode layers 10A sheets and the tank plate.
• FIGS 1-D, 1-E and 1-F show various possible arrangements of aluminum sheets 10 of each anode layer.
• aluminum sheets 10 of any layer can be arranged independent of the method of arrangement of the other layers.
• aluminum sheets 10 of anode layer 10A can be arranged as shown in Figure 1-E and aluminum sheets 10 of anode layer 10B can be arranged as shown in Figure 1-D.
• the anode layer is installed to cover 100% of the top area of the corrosive medium on which the structure would be placed.
• 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.
• a plurality of bottom sheets 10 are placed on a foundation comprising sand 30 (corrosive medium) and a moisture barrier 35.
• 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 10B, 10C 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 10C 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.
• the sheets should extend beyond the tank plate rim by a minimum of l/4th inch for new tanks.
• 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 lC.
• a SALSA SM system is used in conjunction with an existing tank and foundation.
• 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 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 1-F.
• This system substitutes for previously known systems in which about 1 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.
• cathodic protection systems 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.
• 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.
• S ALS A SM systems for large petrochemical storage tanks 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.
• the thickness of the sheets can be varied rather than the number of layers.
• SALSA SM systems as described herein have significant advantages over previously known systems.
• 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 10S at the sheet overlap may also be used to prevent the migration of ground moisture through the overlap.
• the pressure exerted by the floor plates on the aluminum sheet overlaps will prevent moisture migration at the sheet overlaps.
• Another advantage of SALSA SM systems is that aluminum sheets can be 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.
• 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.
• SALSA 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 where 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.
• One 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 SALSA M 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.
• SALSA SM 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.
• 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.
• the SALSA SM system is a better choice when compared to impressed current anode systems because the oxygen generated by SALSA S system is insignificant.
• SALSA 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 SALSA 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 SALSA M systems these considerations are irrelevant because current flow is independent of soil resistivity. Burn-out of anode connections, current attenuation, and electrical grounding are also irrelevant to SALSA systems. Similarly, reference electrodes need not, .and preferably are not installed with SALSA SM systems.
• SALSA M 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 SALSA SM systems.
• Other advantages relate to engineering and construction. For example, engineering time of SALSA M systems is reduced to one or two hours, which is lower than with other systems.
• SALSA SM 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.
• 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 .
• SALSA SM systems also have operating advantages. For example, SALSA SM systems are automatically operational, 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.

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• Chemical & Material Sciences (AREA)
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• Materials Engineering (AREA)
• Mechanical Engineering (AREA)
• Metallurgy (AREA)
• Organic Chemistry (AREA)
• Prevention Of Electric Corrosion (AREA)

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Citations (5)

• 1998
  • 1998-02-06 CN CNB988117134A patent/CN1207443C/zh not_active Expired - Lifetime
  • 1998-02-06 AU AU63202/98A patent/AU746899B2/en not_active Expired
  • 1998-02-06 BR BR9812714-4A patent/BR9812714A/pt not_active Application Discontinuation
  • 1998-02-06 EP EP98907386A patent/EP1100981A4/en not_active Withdrawn
  • 1998-02-06 JP JP2000515046A patent/JP3645180B2/ja not_active Expired - Lifetime
  • 1998-02-06 WO PCT/US1998/002308 patent/WO1999018261A1/en active IP Right Grant
  • 1998-02-06 CA CA002305357A patent/CA2305357C/en not_active Expired - Lifetime
  • 1998-02-06 KR KR1020007003533A patent/KR100362258B1/ko not_active IP Right Cessation

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