Abstract:
A corrosion inhibiting agent is provided as a finely ground powder that is dispensed into a sheath or other casing enclosing a metal bar, cable, or other tension member. The agents are produced by preparing salts of amines with benzoic acid or nitric acid, drying the salts, and grinding and screening the salts to provide a desired maximum particle size. Also disclosed is a dry fogging system through which the powder is applied to metal tension members enclosed in polymeric sheaths or other fluid tight casings.

Description:
[0001]    This application is a divisional of U.S. patent application Ser. No. 11/559,482, entitled “Corrosion Inhibiting Powders and Processes Employing Powders,” filed Nov. 14, 2006. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    The present invention relates to vapor phase corrosion inhibiting compositions, and more particularly to dry powder inhibitors specifically formulated to provide corrosion protection of metal in recessed areas, e.g. post-tensioning cables inside tubes. 
         [0003]    Vapor phase corrosion inhibiting materials are utilized in a variety of applications for protecting metal from corrosion. One such application is a method of prestressing concrete structures, known as post-tensioning. Post-tensioned concrete systems have been used for decades in the construction of bridges, elevated concrete slabs for parking ramps and garages, and in flooring, walls and columns of commercial buildings. In this form of prestressing, cables, strands, bars, or other members of high strength steel are installed at a job site, usually housed in sheathing or tubes that prevent the steel from bonding to the concrete. After the concrete cures, the steel members are stretched by hydraulic jacks. The tensioned members act upon the concrete slab or other structure to place it in compression, considerably improving the capacity of the structure to withstand tensile and bending forces. 
         [0004]    A persistent problem with post-tensioned structures and systems is corrosion of the metal members, particularly in environments involving exposure to salts and other de-icing materials, acid rain, airborne salts in locations near the ocean, and high humidity. If undetected or untreated, corrosion can weaken tensioned members to the point of breakage. In typical post-tensioned structures where the cables or other members are not bonded to the surrounding concrete, breakage of a tensioned member can create a risk of serious injury and property damage. 
         [0005]    A variety of solutions have been directed to the corrosion problem. For example, U.S. Pat. No. 5,840,247 (Dubois et al.) discloses a process for protecting the tendons embedded in housings by drilling holes in the housings and injecting a corrosion inhibiting liquid solution into the housings while applying a high power pulsating wave to enhance penetration. 
         [0006]    U.S. Pat. No. 5,460,033 (VanderVelde) describes processes for corrosion evaluation and protection of unbonded cables. Holes are drilled in the concrete to expose the tendons, and a dry non-corrosive gas is passed through the conduits enclosing the tendons. The patent notes that if the evaluation of the gas indicates a humidity above sixty percent, corrosion will ensue. The humidity preferably is maintained below forty-five percent, by injection of dry nitrogen gas as needed. 
         [0007]    U.S. Pat. No. 3,513,609 (Lang) shows tendons coated with a polymeric material such as TEFLON (brand name) polymer or an epoxy resin containing up to twenty-five percent finely ground TEFLON polymer. The tendons are coated with a lubricating grease before they are covered with the plastic. 
         [0008]    U.S. Pat. No. 4,442,021 (Bürge, et al.) is drawn to a corrosion protection coating of cement containing up to ten percent corrosion inhibitors. The mixture is applied onto the metallic tendons before their enclosure. 
         [0009]    U.S. Pat. No. 5,770,286 (Sorkin) describes a corrosion resistant retaining seal for end caps. The cap, formed of a polymeric material, contains corrosion resistant material inside the cap. The cap is intended to create a water-tight seal. The patent also describes an “ice pick” method of making a hole in the plastic sheath and injecting grease into the sleeve to displace water and prevent corrosion. 
         [0010]    U.S. Pat. No. 5,540,030 (Morrow) describes injecting a polyurethane resin into the housing to displace water and air and prevent corrosion. 
         [0011]    While the foregoing approaches are acceptable for a variety of applications, none of them is particularly well suited for providing corrosion protection for large scale systems in which the reinforcement members may have considerable length, e.g. exceeding one hundred feet. Drilling holes for injecting anti-corrosive grout or oil becomes prohibitively expensive and time consuming, and corrosion of longer lengths of tensioned members is not adequately addressed by end caps or similarly restricted features. Coating tensioned members directly with anti-corrosive layers or films inhibits corrosion, but is not a practical approach for treating previously installed systems. 
         [0012]    Accordingly, the present invention concerns structures, systems, and processes directed to one or more of the following objects: 
         [0013]    (1) to facilitate corrosion protection of metal tension members having considerable length, without the need to drill multiple holes along the length of the members to be treated; 
         [0014]    (2) to provide a system for treating tensioned reinforcement members in situ in preexisting structures, at low cost and minimal disruption to the structures; 
         [0015]    (3) to provide a system particularly well suited for protecting reinforcement members (either before or after they are tensioned) enclosed in relatively tight tubes or sheaths, or having irregular or varying topographies or otherwise forming relatively small or deep voids where exposed metal surfaces are difficult to reach. 
       SUMMARY OF THE INVENTION 
       [0016]    To achieve these and other objects, there is provided a corrosion inhibition system. The system includes an aerosol that occupies substantially the entire interior volume between an elongate metal tension member and a cover surrounding the tension member. The aerosol includes a dry carrier gas, and volatile corrosion inhibiting chemicals having an affinity for metal surfaces. 
         [0017]    Preferably the chemicals are particles having diameters of less than 50 microns. The particles can be suspended in the air or other carrier gas, readily move with the gas, and as a result are distributed throughout the interior volume. The particles are able to gain access to deep recesses and voids within the interior volume. The volatile feature of the chemicals facilitates protection of exposed metal surfaces not accessible by other forms of corrosion inhibiting agents. The particles sublimate to provide a vapor that adsorbs on the exposed metal surfaces, forming a thin, monomolecular protective layer that provides continuous protection against corrosion from exposure to moisture, salt, oxygen, carbon dioxide, or other corrosive elements. 
         [0018]    If the layer is disturbed by moisture or other corrosive components entering the interior volume, the corrosion inhibiting characteristics remain effective. 
         [0019]    In preferred embodiments, the particles consist essentially of the volatile corrosion inhibiting material. The corrosion inhibiting material is formulated in a solid form and dried, then pulverized, and screened to remove particles having diameters larger than the desired threshold size. Suitable volatile corrosion inhibiting agents are selected from the group consisting of cyclohexylammonium benzoate, monoethanolamine benzoate, dicyclohexcyl ammonium nitrate, tolytriazole, benzotriazole, their combinations, and other combinations of corrosion inhibitors such as the amine salts of acids such as sebasic acid and caprylic acid that form solids that can be ground into the desired particle size. 
         [0020]    Another aspect of the present invention is a process for treating an elongate metal structural member adapted to provide structural support while in tension. The process includes the following steps: 
         [0021]    a. generating an aerosol including a dry carrier gas, and multiple solid-phase particles suspended in the carrier gas and including a volatile corrosion inhibiting agent with an affinity for metal surfaces; 
         [0022]    b. introducing the aerosol into an interior of a substantially fluid impermeable casing disposed in surrounding relation to an elongate metal tension member until the aerosol substantially fills an interior volume comprised of the interconnected interstitial voids between the tension member and the casing; and 
         [0023]    c. with the interior volume substantially filled with the aerosol, closing the casing in a substantially fluid-tight manner to contain the aerosol within the interior volume. 
         [0024]    Cables and other tension members can be treated both before and after they are tensioned. Preferably, the aerosol is generated by moving a carrier gas past a collection of the corrosion inhibiting agent particles, to entrain a portion of the particles in the carrier gas. The aerosol is introduced at a positive pressure to the interior volume through an entrance passage near a first end region of the tension member and casing. Simultaneously, the interior volume is evacuated by allowing flow through an exit passage at an opposite end region of the tension member and casing. 
         [0025]    The process is effective over a range of particle concentrations in the aerosol. At the minimum, it has been found advantageous to provide a particulate concentration sufficient to facilitate visual recognition of the corrosion inhibiting particles. In such a cases the aerosol has the appearance of a fog or cloud of dust. Then, the emergence of the aerosol out through the exit passage provides a visible signal that the interior volume is substantially filled with the aerosol. At this stage, the entrance and exit passage are closed to seal the aerosol within the interior volume. 
         [0026]    Another aspect of the present invention is a process for treating an encased tension member in situ. The process includes the following steps: 
         [0027]    a. forming an entrance passage from an exterior of an assembly including a tension member and a fluid impermeable cover to an interior volume between the tension member and the cover; 
         [0028]    b. forming an exit passage from the interior volume to the exterior, spaced apart from the entrance passage; 
         [0029]    c. generating an aerosol including a carrier gas, and multiple solid-phase volatile corrosion inhibitor particles suspended in the carrier gas; 
         [0030]    d. introducing the aerosol into the interior volume through the entrance passage while simultaneously allowing a flow out of the interior volume through the exit passage, to substantially fill the interior volume with the aerosol; and 
         [0031]    e. with the interior volume substantially filled with the aerosol, closing the entrance passage and the exit passage to maintain the aerosol inside the cover. 
         [0032]    The process is particularly well suited for treating previously installed tension members in preexisting structures, particularly when the encased tension members have lengths exceeding 50, 100, and even 150 feet. This is primarily because the only required access to the interior volume inside the cover is an entrance passage formed at one end of the tension member and cover, and an exit passage at the other end. There is no need for intermediate passages for pumping oil or greases into the interior volumes at high pressure. Rather, in accordance with the invention, the aerosol is provided into the interior volume through the entrance passage at low pressure, for example using a conventional air hose at a pressure of less than 100 psi. The particles advance through the interior volume lengthwise of the tension member due to the continued positive pressure, while gases previously present in the interior volume flow out of the interior volume through the exit passage. 
         [0033]    Further aspects of the present invention include a system for treating an elongate structural member in situ, and a corrosion resistant system. 
         [0034]    Thus, in accordance with the present invention, a relatively simple and low cost method of fogging encased tension members can be utilized both before and after the members are initially tensioned, or in the course of normal inspection of previously installed tension members years after a project is completed. In either event, the corrosion protection is enhanced by the capacity of the corrosion inhibiting particles to migrate into deep recesses and voids to reach virtually all exposed metal surfaces. 
     
    
     
       IN THE DRAWINGS 
         [0035]    Further features and advantages will become apparent upon consideration of the following detailed description and drawings, in which: 
           [0036]      FIG. 1  is a sectioned elevational view of a concrete structure reinforced with a post-tensioned cable treated in accordance with the present invention; 
           [0037]      FIG. 2  is a sectional view taken along the line  2 - 2  in  FIG. 1 ; 
           [0038]      FIG. 3  is a schematic view illustrating a process for treating metal tension members in the course of forming reinforced concrete structures in accordance with the present invention; and 
           [0039]      FIG. 4  schematically illustrates a process for treating the metal tension members of a prestressed concrete structure in situ according to the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0040]    Turning now to the drawings, there shown in  FIGS. 1-3 , a post-tensioning assembly  16  employed to prestress a concrete slab  18 . The concrete slab maybe a section of a bridge, a parking deck or ramp, wall, floor, or any other structure in which structural sections can be formed of reinforced concrete. 
         [0041]    The assembly includes an elongate tension member in the form of a high-strength steel cable  20  consisting of a center strand  22  surrounded by six peripheral strands  24  wound in a tight helical configuration about center strand  22 . In alternative embodiments, the tension member may be a rod, bar, single strand, or plurality of strands, either unwound or wound in a configuration other than a helical configuration of strands  24 . 
         [0042]    Cable  20  is housed within a sheath  26 . The sheath provides a cover or casing that surrounds the cable over the complete length of the cable contained within slab  18 . Sheath  26  ensures that cable  20  remains unbonded, i.e. free to move axially relative to the slab, to permit stretching the cable to place it under tension to prestress the slab. Sheath  26  typically is formed of a polymeric material, and provides a substantially fluid impermeable barrier between slab  18  and cable  20 . Sheath  26  tends to isolate the cable and the enclosed sheath interior space, i.e. an interior volume  28 , from the outside environment. 
         [0043]    While this fluid isolation provides a degree of protection against corrosion of the steel, corrosive components can and do infiltrate the interior volume. Accordingly, in conventional post-tensioning systems, it is known to inject corrosion inhibiting greases into the interior volume to reduce and counteract such exposure. These greases, however, tend to harden and dry, and even at the outset may fail to reach exposed metal surfaces in deep pockets or crevices of the interior volume. 
         [0044]    Typically, post-tensioning systems employing multiple assemblies such as assembly  16  are installed on a job site, by positioning the cables or other tendons and their surrounding sheaths before the concrete is poured. At their opposite ends, the cables are secured by anchors, as indicated with respect to cable  20  by opposite anchors  30  and  32 . Anchor  30  includes an anchoring body  34  having a frusto-conical central opening  36  surrounding cable  20  and containing several anchoring wedges  38 . Wedges  38 , in the manner known in the art, allow cable  20  to be stretched axially, to the left as viewed in  FIG. 1 , whereupon the wedges converge to secure the stretched cable against slippage relative to anchor  30 . 
         [0045]    In contrast, the opposite end of cable  20  is fixed with respect to anchor  32 . In alternative systems, it may be advantageous or desirable to use anchors such as anchor  30  at both ends, to allow tensioning of the cable at either end of slab  18 . 
         [0046]    The concrete is allowed to cure before the cables of the prestressing system are stretched. With a specific reference to cable  20 , anchors  30  and  32  secure the opposite cable ends, and are adapted to apply compressive forces to the slab to counterbalance the tension of cable  20  when stretched. A hydraulic jack or other equipment (no shown) is used to stretch the cable to the desired tension. Locking wedges  38  maintain the desired tension after the jack is disconnected from the cable. 
         [0047]    One of the problems associated with using grease as the corrosion inhibiting medium is the difficulty in filling the interior volume with the medium, primarily due to its high viscosity. This problem is particularly pronounced in larger reinforced concrete structures, where cables may exceed one hundred fifty feet in length. While multiple access holes can be drilled along the length of the cable, as taught in the aforementioned Morrow &#39;030 and Dubois &#39;247 patents, this approach adds considerable time and cost to the project, and provides more potential paths for corrosive element infiltration. 
         [0048]    In accordance with present invention, the preferred medium for delivering corrosion inhibiting agents to interior volume is an aerosol: more particularly, a non-reactive carrier gas with multiple volatile corrosion inhibiting particles suspended in the carrier gas. 
         [0049]    Corrosion inhibiting chemicals useful for volatizing or sublimating can be prepared by reacting amines with acids. A useful mixture of inhibitors can be formed from cyclohexylammonium benzoate, monoethanolamine benzoate and a small amount amorphous silica. Monoethanolamine benzoate functions well, as does dicyclohexyl ammonium nitrate. Further well-functioning inhibitors include benzotriazole and the monoethanolamine salt of benzo- or tolyltriazole. Sodium nitrate also can be used. 
         [0050]    Suitable corrosion inhibiting agents are formulated by preparing the salts of several amines with benzoic acid or nitric acid, according to the following examples: 
       Example 1 
       [0051]      
         [0000]    
       
         
               
               
               
             
               
               
               
             
           
               
                   
                   
               
               
                   
                 Constituent 
                 Percent by Weight 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 Cyclohexylammonium Benzoate 
                 87 
               
               
                   
                 Monoethanolamine Benzoate 
                 10 
               
               
                   
                 Amorphous Silica 
                 3 
               
               
                   
                   
               
             
          
         
       
     
       Example 2 
       [0052]      
         [0000]    
       
         
               
               
               
             
           
               
                   
                   
               
               
                   
                 Constituent 
                 Percent by Weight 
               
               
                   
                   
               
             
             
               
                   
                 Cyclohexylammonium Benzoate 
                 60 
               
               
                   
                 Monoethanolamine Benzoate 
                 20 
               
               
                   
                 Dicyclohexcyl Ammonium Nitrate 
                 20 
               
               
                   
                   
               
             
          
         
       
     
       Example 3 
       [0053]      
         [0000]    
       
         
               
               
               
             
               
               
               
             
           
               
                   
                   
               
               
                   
                 Constituent 
                 Percent by Weight 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 Cyclohexylammonium Benzoate 
                 55 
               
               
                   
                 Monethanolamine Benzoate 
                 20 
               
               
                   
                 Dicyclohexcyl Ammonium Nitrate 
                 20 
               
               
                   
                 Benzotriazole 
                 5 
               
               
                   
                   
               
             
          
         
       
     
         [0054]    It is advantageous that the mixtures of inhibitor powders are dried and screened to an average particle size of about 0.2 mm. The screened particles are then subjected to a further size-reduction stage, specifically pulverizing/grinding in a model DPM-1 Dense Phase Mill pulverizing system available from CCE Technologies, Inc. of Cottage Grove, Minn. Then, the particles are screened further using a screen pore size of 50 micrometers, such that the resulting powder is made up of particles with diameters less than 50 microns. The screening to remove larger particles is particularly useful to prevent formation of piles or blocks that might interfere with flow of the gas and particles through the interior volume. 
         [0055]    To facilitate loading the aerosol into interior volume  28 , entrance and exit passages are disposed at the opposite ends of the sheath and cable. An entrance passage  40  is provided in the form of gaps between adjacent wedges  38 . At the opposite end where cable  20  and anchor  32  are integrally coupled, an exit passage  42  is formed through concrete slab  18 . 
         [0056]    When interior volume  28  is filled with the aerosol, the entrance and exit passages are sealed to contain the aerosol. The volatile particles sublimate, resulting in a corrosion inhibiting vapor that adsorbs on the exposed metal surfaces, forming a thin, molecular layer that provides both cathodic and anodic protection. 
         [0057]      FIG. 3  illustrates a fogging process used to load the corrosion inhibiting aerosol into interior volume  28 . An aerosol generator  48  is used to introduce the aerosol into the internal volume through entrance passage  40  under a positive pressure. Generator  48  includes a container  50  of the particles and a nozzle  52  coupled to receive air under pressure from a source not shown, e.g. a conventional air hose. The flow of air toward and through nozzle  52  creates a negative pressure that draws the corrosion inhibiting particles upwardly into the conduit and through the nozzle. 
         [0058]    The aerosol proceeds axially through interior volume  28 . The flow of the aerosol may be laminar or more turbulent, depending largely upon the shape of the internal volume. In either event, as the aerosol advances through the interior volume, the air or other gas previously in the volume is displaced, and leaves the volume through exit passage  42 . 
         [0059]    The introduction of the aerosol continues until the aerosol substantially fills the interior volume. This event is detectable visually, upon observing that the aerosol, rather than the air or other gas previously in the interior volume, is leaving the volume through the exit passage. 
         [0060]    The fogging process is effective over a range of particle concentrations. A preferred minimum particle concentration is the concentration at which the aerosol is easily observed leaving the interior volume, thus to facilitate determining when the interior volume is filled. Particle concentrations preferably are kept below levels at which the particle density might interfere with or impede flow through the interior volume. 
         [0061]    After fogging, the entrance and exit passages are closed to contain the aerosol in the interior volume. Fogging can be employed before cable  20  and the other cables in the post-tensioning system are tensioned, and/or at a later stage such as after tensioning and sealing. 
         [0062]    An important factor influencing aerosol flow is particle size. By providing particles with diameters less than a predetermined threshold, preferably 50 microns, particles can be generated at higher densities without impeding the aerosol flow, and the smaller particles are better suited to reach relatively inaccessible areas. 
         [0063]    One advantage of the present invention is the capacity to treat post-tensioning assemblies in previously installed reinforced concrete structures.  FIG. 4  illustrates a tension cable  54  surrounded by sheath  56  embedded in a concrete slab  58 . Cable  54  acts through anchors  60  and  62  to apply compressive forces to the concrete slab. Cable  54  is attached integrally to anchor  60  and secured to anchor  62  through wedges or other structure that permits axial movement to stretch the cable, as before. Anchor  62 , and an end region of cable  54  extending beyond anchor  62 , are enclosed by an end cap  64 , for example of the type disclosed in U.S. Pat. No. 5,770,286. Anchor  60  likewise, may be covered with an end cap, although this is not illustrated. 
         [0064]    Corrosion inhibiting treatment of cable  54  begins with formation of opposite end entrance and exit passages in fluid communication with an interior volume  66 . The entrance passage  68  is formed by removing end cap  64 , and may also require removal of the grease from between adjacent wedges. 
         [0065]    The exit passage is drilled through the concrete and sheath, as indicated at  70 . At this stage, the corrosion inhibiting aerosol is introduced into the internal volume, as before. The passages can be functionally reversed if desired, with the aerosol provided under positive pressure through passage  70 , with displaced gasses leaving through the gaps between the wedges. In either event, once the internal volume is filled with the aerosol, passage  68  is closed and sealed, using an end cap if desired, and passage  70  is closed and sealed with a corrosion inhibiting grout. 
         [0066]    In cases where there are no end caps, the entrance and exit passages are formed by drilling through the concrete and sheath, and sealed with corrosion inhibiting grout after the aerosol is introduced. 
         [0067]    Thus in accordance with the present invention, corrosion inhibiting agents are applied through a fogging process that distributes a particulate suspension of a corrosion inhibiting agent throughout an enclosed space surrounding a cable, bar or other tension member providing post-tensioning or other structural support. The process is relatively simple and low cost, yet provides substantially complete coverage of exposed metal surfaces for effective and long-term corrosion protection. The fogging process can be integrated into the fabrication of reinforced concrete structures and other structural components, or may be applied in situ to previously completed structures.