Abstract:
Corrosion protection in hollow metal structures is attained by placing an injected, foam-in-place, closed cell plastic polymer foam into an interior of a hollow metal structure, thereby preventing the ingress of foreign matter, such as dirt or water.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention pertains generally to corrosion prevention on the surfaces of hollow metal structures, more specifically the placement of a plastic polymer foam material in the box section frame rails of a vehicle to prevent corrosion from salt water collection or condensation in the frame rails. 
     2. Description of the Related Art 
     Corrosion control methods are generally based on the principle of shielding metal surfaces from their environment through the application of fluid or vapor impervious surface layers or coatings. These layers can be formed from a variety of materials including oils and greases, paints, inhibiting surface films, inactive metal overlays, or thickened oxide layers. Coating reliability depends on adhesion strength and resistance to ultraviolet light, heat, mechanical, or moisture induced degradation. 
     Hollow cavities are an intrinsic design feature of metallic structures. For vehicles located near salt water, particularly these cavities tend to collect salt laden moisture which in turn promotes accelerated corrosion of the internal surfaces. Corrosion damage inside structural voids results in premature component failures, significantly shortened lifetimes, and high repair or replacement costs. Internal cavity corrosion is commonly observed in automobiles and trucks causing some vehicles to be removed from service in less than one quarter of their design lifetimes and resulting in high repair costs. For example, the beach vehicles utilized by the Los Angeles County Parks and Harbors Department are retired from service after approximately two years even though they are rinsed with salt-free water daily. 
     The use of corrosion control treatments such as, paints, oils, or inhibiting coatings, for hollow components is often impracticable because of the fact that there is inadequate accessibility for application. Inaccessibility also reduces the effectiveness of cleaning and surface preparations necessary for corrosion product removal prior to the application of coatings. Application of coatings prior to assembly is not always feasible because of possible deleterious effects of welding on the coating. Providing drainage is only effective if all moisture and residue can be eliminated or the interior surfaces have a good corrosion preventive coating. 
     Plastic foams are widely used for building insulation and recreational boat hulls, as a material for flotation devices and as space fillers to avoid maintenance in steel sail boat construction. Although corrosion control is not the primary purpose for foam application in these applications, it has been noted that virtually no corrosion appears on foam encased steel spade rudders, even after prolonged periods of ocean sailing. In the early 1960s foam fillers were explored as a corrosion control method for automotive body applications. This concept was subsequently abandoned because of void and shrinkage problems associated with the foam formulations then available. It has been shown that stainless steel, galvanized iron, and copper appear to be protected by contact with some rigid insulating polyurethane foams, however for mild steel and aluminum the protection afforded is significantly less. It has been suggested that in the latter cases, detrimental species will leach from the foam under conditions of high humidity and condensation and support corrosion attack at the foam metal interface. 
     BRIEF SUMMARY OF THE INVENTION 
     The object of this invention is to provide a technique for preventing or inhibiting corrosion in the frame rails of vehicles subjected to a corrosive atmosphere where there can be already existing levels of internal surface degradation or corrosion. 
     This and other objectives are attained by placing an injected, foam-in-place, closed cell plastic polymer foam into an interior of a hollow metal structure, thereby preventing the ingress of foreign matter, such as dirt or water and corrosion species. The protective foam can be applied to unused equipment or equipment that is partially corroded during operation. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a schematic representation of an enclosed metal region containing a semi-rigid filler material representative of a cured closed cell foam polymer foam. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The foam filling approach to corrosion control, as described below in the preferred embodiment  10 , as shown in FIG. 1, uses an expansive plastic foam as a filler material  12  to infiltrate and occupy enclosed metal cavities, empty structural spaces  14  in order to block corrosive fluid entry and contact with metal surfaces. The properties of the filler material  12  necessary to maintain a fluid or vapor barrier will depend upon the geometry and condition of the cavity  14 . In the worst condition the enclosed metal region  14  contains internal structures which form the sides of clamping or bolting arrangements and drain holes or access points  16  which permit entry or discharge of fluid or semi-fluid materials. For this situation, under ideal conditions, the general spatial and temporal morphology of the filler material  12 , should be such that: (1) it can be deposited at various locations within the confined region by means of injection at preexisting access points  16 , (2) it is able to flow or expand into all unoccupied regions regardless of size or geometry, (3) it is characterized by a curing time for transition from a liquid state to that of a rigid or semi-rigid state. (4) it is sufficiently dense and expansive during curing that a reasonable level of pressure is maintained at the interior surface of the metal structure  14  or interface between metal  18  forming the cavity  14  and filler material  12  and, (5) it possesses sufficient fluid repellence to block contact with the internal surface  18  of the metal structure  14 . In addition, because the filler material  12  represents a post design solution, its presence should not alter the behavior of the structure associated with the specified enclosed region. 
     Referring to FIG. 1, a filler material  12  is introduced into a cavity  14  region through existing or selectively drilled access points  16  in a liquid or highly amorphous state. The foam material  12  is applied through a drain hole or selectively drilled access point  16  into a hollow metal structure  14  having an interior surface  18 , preferably without a paraffin coating that has been applied by a manufacturer as a rust proofing. The interior surface of the hollow metal structure  14 , however, may be in a corroded state. Immediately following disposition, the filler material  12  begins to expand wherein foaming and hardening will occur. The evolution rate of this gas and availability of moisture containing air, as influenced by the venting conditions through existing or selectively drilled access points  16 , can determine the structure and morphology of the final product. The filler material  12  assumes a rigid outer skin at surfaces that are in the proximity of either access points  16  or areas where there is an open venting. The internal cell structure can develop over a period of several days, and is a function of the pre-expansion conditions within the confined region or cavity  14  which may not be uniform. And finally, there is some level of bonding at the interface  18  between the filler material  12  and the metal surface  18 . 
     The preferred filler material  12  is a polymeric diisocynate polyol resin mixture, such as Foam Plus®, manufactured by Insta Foam of Jolliet, Ill., however other materials that meet the criteria described herein may be utilized, such as a two-part polyurethane foam. The preferred mixture cures with a closed-cell morphology, has good metal bonding characteristics and is available in pressurized containers with a plastic tube extension for easy application into poorly accessible areas. Following deposition, expansion, and curing the expanded filler material  12  fills the hollow metal structure  14 . The interior morphology depends upon the size of the cavity  14  and the availability of external access providing moisture to set the foam and venting to allow release of the expansion gas. If there is inadequate external access there is insufficient moisture for proper curing and volume for gas expansion so that voids or channels can form. Despite the formation of voids for the most part a small sized cellular structure forms at metal contact points. Where the filler material  12  does not expand, a thin transparent material forms on the metal structure  14 . 
     The general morphology of a one part foamed filler material  12  within an enclosed cavity region  14  will tend to be different from that of a partially enclosed region or a region that is defined by materials through which the diffusion of gases can readily occur. Under ideal conditions curing and expansion rates are complementary and the final product will consist of a uniformly sized cell material. However, under less than ideal conditions, various factors can influence the morphology of the cured material and possibly its utility for water displacement and exclusion. Some of these factors include: 
     (1) Set-up rate. To insure a morphologically uniform product the rate of solidification immediately following deposition will decide the tolerable separation distances between access points  16  and deposition locations within the cavity  14 . 
     (2) Venting. For single component expansive foams, poor venting during the curing process restricts the escape of the gaseous expansion agent and access of moisture which can result in the formation of voids or a liquefied internal phase. These cells may preferably absorb and harbor, rather than displace environmental fluids. 
     (3) Bonding. Poor metal-polymer bonding could result in wicking of fluids along the interface. 
     (4) Curing. For single component expansive foams, restricted access to air delays the curing process and results in an unexpanded liquified phase which eventually hardens. 
     For precorroded surfaces, a pressure wash, such as water, removes loose corrosion products and debris and provides an improved surface for bonding. 
     The corrosion control techniques described in this invention will prevent the accumulation of foreign materials, such as dirt and water, within a hollow metal structure, such as a vehicular frame rail and thereby prevent corrosion of this component. The techniques described may be applied to new construction or retrofitted to apparatuses already in service. Laboratory tests indicate that injection of a closed-cell foam filler material  12  offers a promising retrofit solution to corrosion control inside structural cavities  14  that are exposed to periodic seawater immersion and may already possess a level of internal surface degradation. 
     Although this invention has been described in relation to an exemplary embodiment thereof, it will be understood by those skilled in the art that still other variations and modifications can be affected in the preferred embodiment without detracting from the scope and spirit of the invention as described in the claims.