Abstract:
An anode is embedded in an electrolyte layer applied to the surface of a structure such as a pipe section to provide an ionic conductive path between the anode and structure to supply cathodic protection to the structure, where the natural environment may not provide a continuous electrolyte. The anode is comprised of a material normally used as a cathodic protection anode material, such as, an expanded valve metal mesh or ribbon having either an electrochemically active coating or noble metal coating, or a sacrificial anode metal alloy. The anode material is made continuous from one end of the structure to the other and may be connected to a common bus wire from one end to the other. The anode and structure to be protected are connected using wires to a DC power supply that causes cathodic protection current flow to the structure in the case of an impressed current system. No separate power supply is needed in the case of a galvanic or sacrificial anode system. A pipeline is formed from a series of interconnected composite pipe sections.

Description:
DISCLOSURE 
     This invention relates to cathodic protection of metal structures such as pipelines, a composite anode-electrolyte pipe section, and methods of making such pipe sections, as well as forming the pipeline, and applying cathodic protection to the pipeline. 
     BACKGROUND OF THE INVENTION 
     Cathodic protection is defined as either (1) the reduction of corrosion rate by shifting the corrosion potential of the electrode toward a less oxidizing potential by applying an external electromotive force or (2) the partial or complete protection of a metal from corrosion by making it a cathode, using either a galvanic or impressed current. Cathodic protection is well established method by which a metal can be protected from corrosion. It is used anywhere the structure and anode are embedded in a common electrolyte, such as, on pipelines, tanks, piers, pilings, bridge decks and substructures. 
     Cathodic protection is accomplished by causing a flow of direct current (DC) between another electrode (called the anode) and the structure (called the cathode). The DC causes the surface of the structure to become polarized, thus stopping or reducing corrosion. 
     In an impressed current cathodic protection system, a rectifier normally converts AC to DC and supplies the current; and the anode is a relatively inert material that can transfer the current to the soil or water, which is the normal electrolyte. In a galvanic system, the anode is an electrochemically active metal compared to the cathode, and the current is a natural occurrence of connecting the anode and cathode together. 
     The chemical reaction at the anode is metal oxidation, with oxygen, or chlorine evolution. Both ions and electrons are formed at the anode. Ions generated by the reaction flow to the cathode via the electrolyte. The electrons flow to the cathode via a connection. Reduction occurs at the cathode, which consumes the electrons. Three common cathodic reactions in cathodic protection are as follows: 
     
       
         O 2 +2H 2 O+4e→4OH −   
       
     
     
       
         O 2 +4H + +4e→2H 2 O 
       
     
     
       
         2H + +2e→H 2   
       
     
     In all cases, both electron and ionic currents are involved in cathodic protection. That is, both a continuous common electrolyte and a metallic connection between the anode and cathode are required. The metallic connection is provided through the rectifier. The soil, water, or other conductive medium provides the common electrolyte. 
     In cathodic protection practice, anodes may be installed in the ground or water where the anodic reactions take place. Anodes can be installed in several different configurations depending on the requirements; these include (1) a single point using shallow anodes, (2) evenly spaced along the structure (distribution system), or (3) in a deep well. Common anode materials include pure metals, alloyed metals, platinum coated valve metals, valve metals having electrochemically active coatings, and certain ceramic materials. 
     If a separation or void in the electrolyte occurs between the anode and cathode, the current will stop, and cathodic protection will fail. Conditions where the electrolyte becomes discontinuous between anode and cathode can and do occur in practice. Examples of situations where this could occur include: coarse backfill materials that are too coarse to permit intimate or uniform contact with the structure surface (e.g., gravel, rocks); where fine backfill is washed away from the environment surrounding the structure from groundwater flow leaving voids against the structure containing only air; and in exposed situations where fine backfill is not possible or practical to place. 
     The application of anode material embedded in concrete for the cathodic protection of steel reinforcement in concrete bridge decks and substructures, piers, and buildings is common. The use of an anode embedded in an acidresistant cement liner containing an anode for the cathodic protection of stainless steel flue gas ducts is disclosed in U.S. Pat. No. 5,290,407. This patent applies to the protection of stainless steel flue gas ducts that are exposed to an environment consisting of a thin-film acidic condensate in an otherwise atmospheric environment. There is a significant problem in providing proper cathodic protection with respect to buried steel structures such as pipelines, where the natural environment consists of rocks or other materials that present voids or prevent full contact between the structure and environment. 
     SUMMARY OF THE INVENTION 
     The principal aspect of the invention is the use of an anode material embedded in a conductive layer that has been applied to the surface of a metallic structure such that the anode and conductive layer and structure form a composite, and from which a self-contained cathodic protection system can be constructed. The invention is pre-applied to the structure, and the anode, electrolyte, and structure are installed as a composite. The use of a concrete coating on the structure or pipe also serves as an anti-buoyancy mass to keep the structure or pipe from floating, and further serves as a protective casing. 
     The cathodic protection system of the present has the following advantages: 
     1. A continuous backfill surrounding the structure is not necessary since the conductive layer, anode, structure, and power supply (if used) form a self-contained cathodic protection system; therefore, cathodic protection can be applied to structures in environments where a continuous surrounding electrolyte is difficult, expensive, or impossible to achieve. 
     2. The anode provides a continuous distributed current source that provides an even current distribution to the structure. 
     3. A separate anode bed is not necessary since the anode and electrolyte are integrated with the structure. 
     4. Cathodic protection is then possible in areas where the right-of-way is too narrow to use conventional anodes, such as the single point, distribution systems, or deep well systems noted above. 
     It is also an aspect of the invention to provide a process for constructing a cathodic protection system with the structure itself, as well as a process for forming a composite section of the structure which includes the structure section, the anode and the electrolyte. 
    
    
     To the accomplishment of the foregoing and related ends, the invention then comprises the features hereinafter fully described and particularly pointed out in the claims, the following description and the annexed drawings setting forth in detail certain illustrative embodiments of the invention, these being indicative, however, of but a few of the various ways in which the principles of the invention may be employed. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a side elevation of a composite structure or pipe section in accordance with the present invention incorporating a mesh anode in an electrolyte coating; 
     FIG. 2 is a transverse section of the pipe section of FIG. 1 taken substantially from the line  2 — 2  of FIG. 1; 
     FIG. 3 is an enlarged broken side elevation of one end of the pipe section of FIG. 1; 
     FIG. 4 is a side elevation of a pipe section joint showing the anodes of adjacent sections electrically connected, and electrically connected to a parallel bus wire; 
     FIG. 5 illustrates a joint like that of FIG. 4 but incorporating a test station, reference electrodes, and a resistance or polarization probe or coupon; 
     FIG. 6 is a fragmentary perspective view of a composite section embodiment of the present invention utilizing metal ribbons as the electrolyte embedded anode; 
     FIG. 7 is an enlarged transverse section of a suitable ribbon showing its passivating coating; 
     FIG. 8 is a transverse section of a wire which may be used to form a similar grid anode; 
     FIG. 9 is a view similar to FIG. 6 but illustrating a sacrificial anode system embedded in the concrete electrolyte and electrically connected to the metal pipe; 
     FIG. 10 is a schematic illustration of one form of making the composite pipe sections of the present invention; 
     FIG. 11 is a schematic Illustration of another form of making the composite pipe section; and 
     FIG. 12 is a schematic illustration of a still further form of making an anode composite pipe section in accordance with the invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring initially to FIGS. 1,  2 , and  3  there is illustrated a composite pipe section in accordance with the present invention shown generally at  20 . The composite pipe section includes an inner metal pipe such as a conventional steel pipe  22  with a relatively thin dielectric coating  23 . Such steel pipe sections may conventionally be approximately 40 feet in length, and they vary in diameter from a few inches or centimeters to more than a yard or meter. As indicated, at each end of the composite pipe section  20  the metal pipe is exposed as seen at  25  and  26  with the dielectric coating stopping at  27  to expose the ends  25  and  26 . The ends of the pipe sections are normally beveled to permit and facilitate groove/buttweld joining. Other types of pipe sections may be employed such as bell and spigot connections. The basic metal pipe may be seam welded or spiral welded pipe sections cut to length, and treated or beveled at the cut ends. The dielectric coating may be in the form of a wrapping or mastic material. 
     The outer coating, shown generally at  30 , comprises two concentric sections in the form of an inner section  31  and an outer section  32 . In reality the two sections are a continuum of each other and fully embed the major extent of a mesh anode shown generally at  35  which radially separates the two sections. The mesh anode is concentric with the concrete sections and the pipe. The two sections of concrete in the illustration may be of substantially the same radial extent and the only difference between the inner and outer sections is the size and circumference. The inner section normally extends axially the same extent as the outer section at the mesh anode. The anode however extends further than the ends of the concrete sections to the point  36  which exposes the end of the mesh anode to a short extent as indicated at  38 . The sections of concrete thus normally stop at  39  to expose the anode ends. To complete the composite anode-pipe section bus rings  40  and  41  are spot welded to the projecting ends of the anode mesh at several points around the circumference. The projecting end of the anode is exposed on both radial sides to facilitate attachment of the bus rings. 
     Although the dimensions of the pipe such as its diameter and wall thickness may vary substantially, there are certain minimal dimensions which are considered somewhat important. For example, radial thickness of the inner section of the concrete continuum between the outer part of the pipe and the anode should be approximately at least ¾ of an inch or about 18 to 20 millimeters. Also the axially length of the projecting portion of the mesh anode should be approximately 2 inches or about 5 to 6 centimeters, while the width of the bus bar ring at the outer edge of the anode may be approximately a ½ inch or 1.25 centimeters. For some applications, the outer concrete section  32  may be substantially thicker or heavier. 
     The dimensions may thus vary widely depending upon the applications for the pipe section. The concrete coating sections may be formed with a conventional mixture of sand and Portland cement to form a sprayable, castable or extrudable mixture as hereafter described. The anode material is preferably in the form of a coated valve metal. The mesh is a highly expanded valve metal which has multiple current paths, nodes, and enhanced redundancy. The valve metal is preferably titanium with a mixed metal oxide or noble metal passivating coating. The bus rings at each end are preferably conductor rings or bars of the same valve metal with a similar coating. Such valve metal mesh comes in large sheets or rolls and can easily be applied to the concrete coating to be embedded in the concrete sections substantially concentric with the pipe axis. The mesh anode which is dimensionally stable is the preferred form of anode for an impressed current cathodic protection system for the pipeline to be constructed with the pipe sections seen in FIGS. 1 through 3. For a more detailed disclosure of such mesh anode material reference may be had to Bennett et al. U.S. Pat. Nos. 4,900,410 or 5,639,358, for example. 
     Referring now to FIG. 4 there is illustrated a joint of two composite pipe sections in accordance with the present invention. The exposed metal ends of the two pipe sections are butt welded as indicated at  45 . The pipe sections are then covered with a joint coating seen at  46  which may be a continuation of the coating  23  for each pipe section. With the two pipe sections now joined end-to-end, the bus rings  40  and  41  are joined by shorting bars  48  and  49 . Preferably there are at least two such shorting bars on opposite sides of the connection. They are preferably tack welded to the outer surface of the bus rings as illustrated. The shorting bar  48  near the top of the joint is provided with a spot welded connector or lug  50  which projects upwardly from the shorting bar  48 , and to which is welded at  52  a transition  53  also connected at  54  to a bus line or wire  55 . Other molecular bonds may be employed. Cathodic protection power may be supplied to the bus line  55  by power supplies such as rectifiers  56  above ground and spaced periodically along the pipe line. The shorting bars  48  and  49 , although shown as two in number, may include redundant additional bars spaced symmetrically around the bus rings. Both the connector or lug  50  as well as the transition  53  are preferably of the same valve metal as the mesh anodes. It will be appreciated that not every pipe joint need be provided with the connection to the bus as long as the anodes of adjacent pipe sections are properly electrically connected, yet spaced from the metal of the pipe. After the pipe connection has been formed as seen in FIG. 4, the void between the concrete coatings of the two pipe sections is then filled with concrete as indicated at  58 . 
     Referring now to FIG. 5 there is illustrated a joint like that of FIG. 4 but incorporating a monitoring system at a test station. The joint is formed in substantially the same manner as FIG. 4 with a buttweld at  60  and a continuation of the dielectric wrapping or coating over the joint as illustrated at  61 . The shorting bars indicated at  62  may extend from bus ring to bus ring across the joint. There may be a sufficient number of shorting bars to insure redundancy, but there is not a shorting bar at the top of the joint. This allows room for the positioning of a reference electrode as indicated at  63 , and a resistance-polarization probe or coupon indicated at  64 . The reference electrode is connected through conductor  65  to a contact or connection at the top of test station  66  which is embedded in the ground to project upwardly above ground. The resistance- polarization probe or coupon  64  is also connected to the proper contact at the top of the test station by conductor  67 . The test station also includes connections from the proper probes or poles to the anode as seen at  68  and also to the pipe as seen at  69 . The latter may be made before the joint is completed. 
     After the connections are made and tested, the bottom of the test station may be embedded approximately 2 inches (5 centimeters) into the concrete which is cast around the joint. After the concrete hardens, the test station will be supported in its upright position as shown to project above the surface of the ground. In this manner test stations may be provided periodically along the pipeline. These test stations enable the owner to monitor the effectiveness of the cathodic protections system. 
     The reference electrode may be a conventional copper-copper sulfate, or silver-silver chloride reference cell, while the probe or coupon may be of the same metal or alloy as the pipe. Resistance measurements on the probe or coupon over time are an indication of the corrosion of the coupon or probe, and thus an analog measurement of the effectiveness of the system. 
     Referring now to FIG. 6 it will be seen that there is illustrated a composite pipe section shown generally at  70  which comprises the metal inner pipe  71  which extends axially beyond an inner section of electrolyte shown generally at  72  which separates a concentric grid anode shown generally at  74  from the exterior of the metal pipe. The anode  74  is formed as a reticulate grid of valve metal ribbons such as seen at  76  in FIG.  7 . The ribbon  76  may have a valve metal core  77  such as titanium, and an exterior mixed metal oxide or noble metal coating  78 . Reference may be had to the noted Bennett et al. patents for further details of the valve metals together with the various coatings. The grid anode projects beyond the end of the inner section of the electrolyte  72  and at each exposed end there is provided an annular bus ring  79 . The concrete sections embed the grid anode in a continuum of the electrolyte spaced from the metal pipe. The outer section of the electrolyte  82  forms with the inner section the end face noted at  83  exposing the end of the grid anode at each end of the pipe section. 
     The composite pipe section illustrated in FIG. 6 also includes the dielectric coating, not shown, and the pipe sections are assembled to form the pipeline and the grid anodes interconnected as in FIGS. 4 and 5. 
     The grid anode may also be formed of round section wire shown somewhat enlarged in FIG. 8 at  85 . The wire may include a copper core  86 , a valve metal or titanium sheath  87 , and an outer mixed metal oxide or noble metal passivating coating  88 . A grid may be formed of such wire by tack or spot welding techniques, and then the grid coiled to form a cylinder to be embedded in the inner and outer sections of electrolyte as in FIG.  6 . 
     The embodiments of FIGS. 1-8 represent composite pipe sections and pipe lines for impressed current cathodic protections systems. In each instance the anode is spaced from the metal pipe and the ionic flow through the electrolyte is created by an external power source such as the rectifier. The other major type of cathodic protection is that of sacrificial anodes where the current flow is created by the galvanic action of dissimilar metals, and in such case the sacrificial anode is electrically connected to the pipe to complete the galvanic circuit. However the type of metal used for such sacrificial anodes is significantly different. Aluminum, Magnesium, Zinc, and alloys thereof are commonly employed for such purposes. A particularly useful alloy for forming sacrificial anodes is disclosed in the copending application of Michael Tighe et al. entitled Cathodic Protection Anode and Method For Steel Reinforced Concrete, application Ser. No. 09/050,727 filed Mar. 30, 1998. In such copending application there is described several alloys which may be flame sprayed and which may comprise about 10 to about 50 percent zinc, up to about 9.6 percent indium, with the balance being aluminum. Such alloys make excellent sacrificial cathodic protection anodes where concrete is the electrolyte. 
     In FIG. 9 there is illustrated a composite sacrificial anode pipe section shown generally at  90  which comprises the metal pipe  91 , and inner section of electrolyte  92 , and an outer section of an electrolyte  93  which stops at shoulder  94 . 
     Between the inner and outer sections of electrolyte there are provided longitudinal strips  95  of such sacrificial anode alloy. The strips may be sprayed on the inner section of the electrolyte by flame spraying and extend axially of the composite pipe section. The strips are generally uniformly spaced circumferentially and at each end the strips are exposed as indicated at  96 . The strips are electrically interconnected by a bus ring  97  of the same material and each bus ring is electrically connected at  98  to the metal pipe, completing the galvanic circuit. The pipe sections are joined in the same manner seen in FIG.  4  and the effectiveness of the sacrificial anode system may be monitored in essentially the same manner as in FIG.  5 . 
     There are a number of ways the composite pipe sections of the present invention can be made. In FIG. 10 there is illustrated a process where the metal pipe section indicated at  101  is supported at each end on roller nests  102  and  103 , at least one of which rollers is powered for rotation. The rollers are supported on trolley  104  which is supported on rails  105  and  106  by wheels  107  and  108 , respectively. In this maner the pipe section is supported at each end for both rotation about its axis and for movement longitudily of its axis. The pipe section movement may be programed to traverse the pipe by a nozzle or applicator  110  which applies the inner electrolyte coating  111  in a uniform fashion. After the inner coating is formed, the anode shown generally at  113  is spooled onto the inner section of the electrolyte to form a continuous cylinder concentric with the pipe section. Once the concentric cylinder is formed the anode material is cut, and the supports for the pipe section ends then program the pipe section both to rotate and move axially so that the applicator  110  will form the uniform outer section of the electrolyte. Suitable removable templates may be employed to form the ends of the inner and outer sections of the electrolyte. When the outer section is completed, the templates may be removed to expose the anode ends and the bus rings are then applied. 
     Instead of spooling on the valve metal impressed current anode  113  as seen in FIG. 10, the outer surface of the inner section of the electrolyte may be sprayed with the sacrificial anode alloy material as indicated by the flame spray nozzle  120  with the alloy being fed to the nozzle by wire  121  from spool  122 . The apparatus of FIG. 10 can thus also form the sacrificial anode of FIG.  9 . Once the sacrificial anode strips are in place, the outer section of the electrolyte is placed thereover. The anode strips are then connected by the bus ring  97 . 
     In FIG. 11 there is illustrated an extrusion process wherein the metal pipe section  125  moves axially over a pilot  126  and through an extrusion die  127 . The extrusion die includes a concrete plenum or chamber  128  which is supplied with concrete under pressure by line  129  from pump  130 . The fine sand and Portland cement mixture enters through the supply lines  131  and  132 , respectively, to be funneled at  133  into mixer  134 . The appropriate amount of water or other additives is added at  135 . The movement of the pipe together with the pressure of the pump  130  causes the inner and outer sections  137  and  138  of the electrolyte to be formed concurrently. The electrolyte may be drawn through a heating collar indicated at  139  to accelerate the cure of the electrolyte. 
     The anode shown generally at  142  is supported by removable fixture  143  which is mounted on the end of the pipe section. The anode is drawn with the pipe section as it moves in the direction of the arrows  145 , and as it is encased by the extruded concrete. The anode may be drawn over a smooth seal indicated at  146  supported from the right hand end of the extrusion apparatus as illustrated. The opposite end of the anode may be supported on a similar fixture or template as that shown at  143 . 
     FIG. 12 illustrates a manufacturing process for a composite pipe section where the metal pipe  150  is positioned vertically on a pilot  151 . A removable template fixture  152  is provided at the bottom of the apparatus and includes a slot  153  in which is seated the end of the tubular anode  154 . A conical funnel indicated at  156  includes a cylindrical spout  157  which telescopes over the template or fixture  152 . The funnel is supported as indicated at  160  for elevation or vertical movement and is guided vertically by rail  161  which extends parallel to the axis of the pipe. The rail may extend both through a guide sleeve  163  at the top of the funnel as well as the roller set  164  connected to bracket arm  165  extending from the cylindrical spout  157  of the funnel. In its lowermost position the funnel is filled with sufficient electrolyte material such as a Portland cement and sand, or concrete mixture  167 , and is then slowly elevated to form concurrently the inner and outer sections  168  and  169  of the electrolyte casing embedding the anode. When the casing is completed, the pipe section is removed. The removal of the templates at the top and bottom then expose the anode ends. The bus rings are then applied and the composite pipe section is completed. 
     It can now be seen that there is provided a composite pipe section which can be utilized to provide cathodic protection for pipelines where cathodic protection might otherwise be impossible. Not only does the composite pipe section facilitate the installation of the cathodic protection system, whether an impressed current or sacrificial system, but the composite pipe section also protects the pipeline, and in situations where required, the weight of the electrolyte, and particularly the outer section thereof, can be designed to overcome any buoyancy problems which might be encountered in the installation of the pipeline. 
     To the accomplishment of the foregoing and related ends, the invention then comprises the features particularly pointed out in the claims, these being indicative, however, of but a few of the various ways in which the principles of the invention may be employed.