Abstract:
A method of enhancing corrosion resistance of a hollow vessel. The method includes providing a hollow vessel including a wall defining a cavity, providing a coating tank filled with a liquid coating having charged coating elements, submerging the hollow vessel into the liquid coating, allowing the liquid coating to pass into the cavity through at least one aperture of the wall, coating the exterior surface of the wall with a portion of the coating elements, coating the interior surface of the wall with an additional portion of the coating elements, removing the hollow vessel from the coating tank, draining the liquid coating from the cavity, heating the hollow vessel in an oven, and curing the portion of the coating elements on the exterior surface and curing the additional portion of the coating elements on the interior surface.

Description:
BACKGROUND 
     The present invention relates to corrosion protection for hollow vessels, and more particularly to corrosion, chemical, and fuel resistance in hollow vessels, such as fuel tanks, used with motorcycles. 
     A motorcycle includes a frame and an internal-combustion engine coupled to the frame and operable to propel the motorcycle. The internal-combustion engine runs on fuel supplied from a fuel tank that is also coupled to the frame. The fuel tank is a hollow vessel that includes a wall defining a cavity for storing fuel. The wall includes an interior surface exposed to the fuel and an exterior surface exposed to the environment. The fuel tank is commonly fabricated by welding a series of formed patterns of sheet metal. The interior and exterior surfaces usually include enhanced surface treatments such that the interior surface resists the corrosive effects caused by the fuel and the exterior surface resists the corrosive effects of the environment to maintain an aesthetically pleasing appearance. 
     The enhancement of the interior and exterior surfaces of the fuel tank is typically a two-step process. First, the corrosion resistance of the interior of the tank is addressed. There are many different ways to accomplish the desired corrosion resistance properties. One way is to fabricate the entire tank with stainless steel or nickel plated steel. Although these materials may provide acceptable corrosion resistance, they are significantly more expensive than more commonly-used materials, such as low-carbon steel. A more common way to enhance the corrosion resistance of the interior surface is to use a low-cost, low-carbon steel and to powder coat the interior surface with an epoxy material. 
     The powder coat process includes first cleaning the cavity, and then applying a dry powder to the clean interior surface. The powder is applied using an electrostatic process where the tank is grounded and the cavity is sprayed with the powder, which consists of charged, non-conducting powder particles. The charged particles are attracted to the interior surface and cling to it. The tank is heated in an oven to fuse the particles into a smooth continuous film. The fused epoxy coating improves the corrosion resistance of the interior surface of the tank. 
     After the interior surface of the tank is coated, the corrosion resistance of the exterior surface is addressed. One common way to improve the properties of the exterior surface of the tank is through an electrodeposition or electrocoat (E-coat) process. However, before beginning the E-coat process, the tank must be prepared by the labor intensive process of isolating the interior of the tank to protect the interior of the tank. This is done by blocking all of the holes that allow access to the cavity, for example, by using tooling, plugs, and seals to block the tank&#39;s fuel fill opening, vent holes, and fuel line ports. Despite this blocking operation, the fuel tanks are susceptible to possible leaking during the subsequent processes, which may cause defects in the cavity and increase scrap costs. 
     Once the cavity is isolated, the exterior surface of the tank is washed by immersing the tank into a bath or by spraying the exterior surface with cleansers to remove soils, oil, grease, lubricants, and rust. After the exterior surface is cleaned, a conversion coating is applied to the surface to enhance the adhesion of the subsequent E-coat. The conversion coating is typically a phosphate coating (e.g., iron, zinc, or manganese) and is applied to the tank by immersing the clean tank into a hot processing solution for a period of time dependent upon the bath chemistry and material being used. 
     After the pretreatment with the phosphate conversion coating, the E-coat process is used. E-coat deposition is a process in which positively charged particles are deposited out of a water suspension to coat a conductive part. First the tank is grounded or charged and submerged into a coating tank to begin the cathodic electromechanical process. In this process, the electrically-charged coating binder, pigment, and additives migrate through the water under the influence of an electric field onto the exterior surface. The electrical charge seeks out the path of least resistance and coats the portions of the exterior surface of the tank. As the process continues, the charged particles resume their search for uncoated portions of the tank and begin coating areas that are not as easily reached. The ability to coat these hard-to-reach areas of the tank is known as the coating&#39;s “throw-power”. 
     Once on the part, the charged materials give up their charge due to neutralization by electrochemically generated OH −  ions. Upon giving up their charge, the coating materials drop out of the water suspension and coalesce as a coating on the exterior surface. The fuel tank is then removed from the coating tank, rinsed, and cured in an oven, after which, the plugs and seals are removed from the openings in the fuel tank. 
     In cases where the plugs and seals fail to adequately isolate the cavity, the cavity becomes at least partially filled with the liquid coating of the E-coat process when the fuel tank is submerged in the coating tank. The presence of the liquid coating in the cavity during the curing process damages the integrity of the powder coat on the interior surfaces. Therefore, the “leakers” are typically scrapped or stripped and completely recoated. 
     SUMMARY 
     The present invention provides a method of enhancing corrosion resistance of a hollow vessel. The method includes providing a hollow vessel including a wall defining a cavity. The wall includes interior and exterior surfaces and at least one aperture providing access to the cavity. The method further includes providing a coating tank filled with a liquid coating having charged coating elements, submerging the hollow vessel into the liquid coating, allowing the liquid coating to pass through the at least one aperture into the cavity, coating the exterior surface with a portion of the coating elements, coating the interior surface with an additional portion of the coating elements, removing the hollow vessel from the coating tank, draining the liquid coating from the cavity, heating the hollow vessel in an oven, and curing the portion of the coating elements on the exterior surface and curing the additional portion of the coating elements on the interior surface. 
     The present invention also provides a novel hollow vessel. The hollow vessel includes a wall defining a cavity, and the wall includes interior and exterior surfaces and at least one aperture providing access to the cavity. The interior and exterior surfaces are coated by the process of providing a coating tank filled with a liquid coating having charged coating elements, submerging the hollow vessel into the liquid coating, allowing the liquid coating to pass through the at least one aperture into the cavity, coating the exterior surface with a portion of the coating elements, coating the interior surface with an additional portion of the coating elements, removing the hollow vessel from the coating tank, draining the liquid coating from the cavity, heating the hollow vessel in an oven, and curing the portion of the coating elements onto the exterior surface and curing the additional portion of the coating elements onto the interior surface. 
     Other aspects of the invention will become apparent by consideration of the detailed description and accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of a motorcycle having a fuel tank embodying the present invention. 
         FIG. 2  is a side view of the fuel tank of  FIG. 1 , illustrating the fuel tank in the upright orientation. 
         FIG. 3  is a bottom view of the fuel tank of  FIG. 1 . 
         FIG. 4  is a top view of the fuel tank of  FIG. 1 . 
         FIG. 5  is a side view of the fuel tank of  FIG. 1 , illustrating the fuel tank in the upside-down orientation above a coating tank containing a liquid coating. 
         FIG. 6  is a view similar to  FIG. 5 , illustrating the fuel tank partially submerged into the liquid coating. 
         FIG. 7  is a view similar to  FIG. 6 , illustrating the fuel tank fully submerged into the liquid coating. 
     
    
    
     DETAILED DESCRIPTION 
     Before any embodiments of the invention are explained in detail, it is to be understood that the invention is not limited in its application to the details of construction and the arrangement of components set forth in the following description or illustrated in the following drawings. The invention is capable of other embodiments and of being practiced or of being carried out in various ways. Also, it is to be understood that the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting. The use of “including,” “comprising,” or “having” and variations thereof herein is meant to encompass the items listed thereafter and equivalents thereof as well as additional items. Unless specified or limited otherwise, the terms “mounted,” “connected,” “supported,” and “coupled” and variations thereof are used broadly and encompass both direct and indirect mountings, connections, supports, and couplings. Further, “connected” and “coupled” are not restricted to physical or mechanical connections or couplings. 
     The motorcycle  10  illustrated in  FIG. 1  includes a frame  12  and an internal-combustion engine  14  coupled to the frame  12  and adapted to propel the motorcycle  10 . The internal-combustion engine  14  runs on fuel supplied from a fuel tank  16  that is also coupled to the frame  12 . 
     With reference to  FIGS. 2-4 , the fuel tank  16  is a hollow vessel that includes a wall  18  that defines a cavity  20  for storing fuel. Although the hollow vessel is illustrated as a fuel tank  16 , the hollow vessel could also be another component used on a motorcycle such as an oil tank, a master cylinder housing, a crankcase, a transmission case, or other vessels generally defining a cavity. The wall  18  includes an interior surface  22  and an exterior surface  24  exposed to the environment. The fuel tank  16  includes a canopy  26  on the top side of the fuel tank  16 , and a tunnel  28  that extends along the longitudinal axis  30  of the fuel tank  16  on the bottom side of the fuel tank  16 . The fuel tank  16  includes forward and rear mounting brackets  32 ,  34  that are used to attach the fuel tank  16  to the frame  12  such that the backbone (not shown) of the frame  12  is received within the tunnel  28  on the bottom of the fuel tank  16 . The fuel tank  16  is oriented in the upright orientation (illustrated in  FIG. 2 ) when the fuel tank  16  is connected to the motorcycle frame  12 . 
     The canopy  26  is positioned near the uppermost portion of the fuel tank  16  and includes an aperture  36  providing access to the cavity  20 . The fuel tank  16  also includes a gauge cup  38  and a filler neck  40  on the top surface. The gauge cup  38  provides a mounting location for a fuel gauge  64  ( FIG. 1 ) and includes an aperture  42  allowing access to the cavity  20  such that a portion of the fuel gauge  64  can be passed into the cavity  20  to be in contact with fuel contained within the fuel tank  16 . The filler neck  40  provides a mounting location for a fuel cap  66  ( FIG. 1 ) and includes an aperture  44  that is adapted to allow a fuel pump nozzle to access the cavity  20  to fill the fuel tank  16  with fuel. The fuel tank  16  also includes first and second fittings  46 ,  48  positioned near the lowermost portion of the fuel tank  16 . The fittings  46 ,  48  include apertures (not shown) providing access to the cavity. The fittings  46 ,  48  can provide attachment locations for vent or fuel lines. The canopy aperture  36  is capsule-shaped and is substantially larger than the fuel gauge aperture  42  and the filler neck aperture  44 . The canopy aperture  36  is over twice as large as the fuel gauge aperture  42  and four times as large as the filler neck aperture  44 . In some configurations, the canopy aperture  36  measures between 2 and 4 inches across the minor dimension and between 6 and 10 inches across the major dimension. 
     The illustrated fuel tank  16  is fabricated by welding formed left and right shell patterns  54 ,  56  of sheet metal, such as low carbon steel or other ferrous metals. In other embodiments, the hollow vessel could be molded from a conductive composite material. The interior and exterior surfaces  22 ,  24  include enhanced surface treatments such that the interior surface  22  resists the corrosive effects caused by the fuel and the exterior surface  24  resists the corrosive effects of the environment to maintain an aesthetically pleasing appearance. 
     The enhancement of the interior and exterior surfaces  22 ,  24  of the fuel tank  16  occurs during a single process step. More specifically, the interior and exterior surfaces  22 ,  24  of the fuel tank  16  are both treated together by an electrodeposition or electrocoat (E-Coat) coating process. Initially the fuel tank  16  is washed by immersing the fuel tank  16  into a bath or by spraying the interior and exterior surfaces  22 ,  24  with cleansers to remove soils, oil, grease, lubricants, and rust. After the fuel tank  16  is cleaned, a conversion coating is applied to the interior and exterior surfaces  22 ,  24  to enhance the adhesion of the subsequent E-coat. The conversion coating is typically a phosphate coating (e.g., iron, zinc, or manganese) and is applied to the fuel tank  16  by immersing the clean fuel tank  16  into a hot processing solution for a period of time dependent upon the bath chemistry and material being used. 
     After the pretreatment with the phosphate conversion coating, the E-coat process is used where positively charged particles are deposited out of a water suspension to coat a grounded or negatively charged fuel tank  16 . As shown in  FIGS. 5-7 , the fuel tank  16  is mounted to a fixture (not shown) such that the fuel tank  16  is in an upside-down orientation. The upside down orientation allows the fittings  46 ,  48  to serve as vents allowing a liquid coating  58  to easily enter into the cavity  20  through the canopy aperture  36  as the fuel tank  16  is lowered into the liquid coating  58  contained within a coating tank  60 . This fuel tank  16  configuration allows quick and complete submersion into the liquid coating  58  and provides for the displacement of air with the liquid coating  58 . 
     After the fuel tank  16  is submerged within the liquid coating  58 , the cathodic electromechanical process begins. In other embodiments, an anionic process can be used instead. In the cathodic process, the electrically-charged coating  58  (including binder, pigment, and additives) migrates through the water under the influence of an electric field onto the interior and exterior surfaces  22 ,  24 . The electrical charge initially seeks out the path of least resistance and coats the exterior surface  24  of the fuel tank  16 . As the process continues, the charged particles resume their search for uncoated portions of the fuel tank  16  and begin coating areas that are not as easily reached, such as the interior surface  22  within the cavity  20  of the fuel tank  16 . Due to the configuration of the fuel tank  16  including the enlarged aperture  36  of the canopy  26  and the positions of the canopy aperture  36  and the fitting apertures on the extreme upper and lower points on the fuel tank  16 , the coating has sufficient throw power to substantially evenly coat both the interior and exterior surfaces  22 ,  24  of the fuel tank  16 . 
     Once on the interior and exterior surfaces  22 ,  24 , the charged materials give up their charge due to neutralization by electrochemically generated OH −  ions. Upon giving up their charge, the coating materials drop out of the water suspension and coalesce as a coating on the interior and exterior surfaces  22 ,  24 . After a designated period of time, the fuel tank  16  is raised from the coating tank  60  and the liquid coating  58  drains from the fuel tank  16  through the canopy aperture  36 . The configuration of the fuel tank  16  allows the liquid coating  58  to quickly and completely drain from the cavity  20 . The liquid coating  58  is then rinsed from the interior and exterior surfaces  22 ,  24  and the fuel tank  16  is placed in an oven to cure the coating materials. 
     In the illustrated embodiment, only a single fuel tank  16  is attached to a fixture. However, in other embodiments, fixtures can be manufactured to hold multiple fuel tanks  16  such that more than one fuel tank  16  can be coated at the same time. For example, 2, 3, 4, or even more fuel tanks  16  could be processed together on a single fixture. The fixture can also be wired to provide the necessary electrical charge to the fuel tanks  16 . In addition, the fixtures can be connected to a conveyor such that a significant portion of the E-coat process can be automated. 
       FIGS. 5-7  illustrate the fuel tank  16  being dipped into the liquid coating  58  in the coating tank  60  along a substantially vertical line of motion. It is also understood that the fuel tank  16  can also be submerged into the liquid coating  58  through rotation, horizontal translation, vertical translation, or any combination of these motions. More complex dipping motions may be necessary for more complex cavity configurations where a simple straight up and down dipping motion may leave air trapped in an undercut or non-vented portion of the fuel tank  16  or may provide inadequate drainage. In these instances, rotation, pivoting, spinning, or other complex motion may be used to ensure complete coverage of the coating solution in the cavity  20  and to ensure complete drainage of the coating solution from the cavity  20 . 
     An alternative coating process called autodeposition can also be used in place of the E-coat to coat both the interior and exterior surfaces  22 ,  24  of the fuel tank  16 . Autodeposition does not require electrical charges to be applied to the fuel tank  20  or the coating tank  60 . Autodeposition is a waterborne process that depends on chemical reactions to achieve deposition. The composition of an autodeposition bath includes a mildly acidic latex emulsion polymer, de-ionized water and other ingredients. The chemical phenomenon consists of the mildly acidic bath attacking the steel parts being immersed and causing an immediate surface reaction that releases iron ions. The ions react with the latex in solution causing a deposition on the surface of the interior and exterior surfaces  22 ,  24  of the steel parts. The newly deposited organic film is adherent yet quite porous. Thus, the chemical activators can rapidly diffuse to reach the internal and external surfaces  22 ,  24  of the metal, allowing continued coating formation. 
     Thus, embodiments of the present invention can, among other things, eliminate powder epoxy material costs, associated tooling and operational costs, eliminate the costs of Ni-clad and stainless steel components, eliminate plugging of the tanks including the tooling, plugs, seals, and associated labor, and eliminate E-coat “leakers” and its associated scrap costs. Various features and advantages of the invention are set forth in the following claims.