Abstract:
A new structure and method for terminating underfilm corrosion. The method utilizes patterned coatings on metal surfaces creating spatial variations of coating thickness or composition. The resulting structure, or paths of structural variation in the coating, directs the path of filiform growth and promotes entrapment, thereby limiting filiform growth and causing self-annihilation. In the preferred embodiment a stamp is used to impose the desired “paths” of structural variation in the painted coating while the coating is wet.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     This invention relates to the field of corrosion control. More specifically, the invention comprises a structure and method for controlling underfilm and filiform corrosion by the application of patterned coatings on metal surfaces.  
         [0003]     2. Description of the Related Art  
         [0004]     Corrosion is a major concern to industries who utilize steel and aluminum alloys or any other reactive surfaces. Underfilm corrosion (sometimes referred to as filiform corrosion), like other forms of corrosion, is an electrochemical reaction that occurs when metals are exposed to moisture and oxygen in the atmosphere. This kind of corrosion typically occurs under coated surfaces that are exposed to high relative humidity. Underfilm or filiform corrosion leads to the deposition of a multitude of rust trails on metal surfaces, which can be both unsightly and damaging to the surface&#39;s physical properties such as reflectivity. Underfilm corrosion is particularly significant to companies that employ metal-based materials and products that need to endure long-term storage before use or distribution to customers, especially those who employ metal cans for storage of their product.  
         [0005]     Rust filaments have a width up to 4 mm and can extend over several decimeters. Active corrosion occurs only at the filiform head. This region is an oxygen concentration cell for which potential differences of up to 360 mV have been reported. Filiforms progress across the surface in a serpentine or linear fashion and the path of corrosion they leave is commonly referred to as the tail of the filiform. Since filiforms do not cross inactive tails of other filaments, they can become trapped and eventually “die” as the available space decreases.  
         [0006]     Current technology protects metal surfaces with coatings of a generally uniform thickness and composition. While this is sometimes helpful to prevent the onset of corrosion, these homogenous coatings are ineffective to prevent the spread of underfilm corrosion once it has nucleated. The primary goal of the present invention is to control and exterminate corrosion once it has begun.  
       BRIEF SUMMARY OF THE INVENTION  
       [0007]     The present invention comprises a new method and structure for terminating underfilm corrosion. The method utilizes patterned coatings on metal surfaces creating spatial variations of coating thickness or composition. The resulting structure, or paths of structural variation in the coating, directs the path of filiform growth and promotes entrapment, thereby limiting filiform growth and causing self-annihilation. In the preferred embodiment a stamp is used to impose the desired “paths” of structural variation in the painted coating while the coating is wet. 
     
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS  
       [0008]      FIG. 1  is a perspective view, showing a coated metal object.  
         [0009]      FIG. 2  is a perspective view, showing filiform corrosion on a coated metal object.  
         [0010]      FIG. 3  is a perspective view, showing filiform self-entrapment.  
         [0011]      FIG. 4  is a perspective view, showing a coated metal object with varying coating thickness.  
         [0012]      FIG. 5  is a cut-away view, showing a coated metal object with varying coating thickness.  
         [0013]      FIG. 6  is a perspective view, showing a stamp with concave spiral design.  
         [0014]      FIG. 7  is a perspective view, showing a patterned surface with spiral design.  
         [0015]      FIG. 8  is a perspective view, showing pattern-aided filiform entrapment.  
         [0016]      FIG. 9  is a perspective view, showing a surface treated with spiral pattern design.  
         [0017]      FIG. 10  is a perspective view, showing spiral patterns at various phases relative to each other.  
         [0018]      FIG. 11  is a perspective view, showing a patterned surface with a diamond design.  
         [0019]      FIG. 12  is a perspective view, showing a double spiral design.  
         [0020]      FIG. 13  is a perspective view, showing an “s” spiral design. 
     
    
     REFERENCE NUMERALS IN THE DRAWINGS  
       [0000]    
       
           10  metal  
           12  coating  
           14  coated metal object  
           16  rust filament  
           18  trough  
           20  peak  
           22  plateau  
           24  stamp  
           26  concave spiral  
           28  convex spiral  
           30  spiral pattern  
           32  entrapment region  
           34  convex diamond  
           36  diamond pattern  
           38  double spiral  
           40  “s” spiral  
       
     
       DESCRIPTION OF THE INVENTION  
       [0037]      FIG. 1  shows a perspective view of a coated metal object, designated coated metal object  14 . Coating  12  is deposited over metal  10 . Metal  10  can be any metal such as iron and aluminum or any of their alloys, and coating  12  can be any type of coating applied to metal or metal alloys, including acrylic.  
         [0038]     Coated metal objects are highly susceptible to underfilm corrosion. Underfilm corrosion begins when the metal substrate is exposed to moisture and oxygen. This can occur because of imperfections in the coating or because of the diffusion of oxygen and water through the coating.  FIG. 2  illustrates typical filiform corrosion on a coated metal object. Rust filaments  16  follow paths that approximately radiate from a point of origin.  
         [0039]     As filiforms grow or propagate, they occasionally have an opportunity to interact. Those skilled in the art know that an active filiform head will not cross an inactive tail of a rust filament. Instead filiform heads “reflect” from the tail and can become entrapped as the space available for the filiform to grow diminishes.  FIG. 3  shows a detail view of the self-entrapment of rust filament  16 . The arrow in  FIG. 3  indicates the direction of propagation of filiform  16 . As rust filament  16  propagates it reflects off both the tails of other filiform and its own tail. After several reflections, rust filament  16  creates an inactive perimeter which it cannot cross and its growth is thereby limited to the region within the inactive perimeter.  
         [0040]     The general concept of this invention is to facilitate filiform self-entrapment by controlling the direction of filiform growth. It has been shown that filiform growth can be controlled by creating spatial variation of the coating thickness.  FIG. 4  shows a coated metal object with varying coating thickness. Coating  12  is profiled in such a way to have a series of troughs  18 , peaks  20 , and plateaus  22 .  FIG. 5  shows a cut-away view of coated metal object  14  and illustrates how rust filaments  16  grow under plateaus  22 . It is noted that rust filaments  16  tend to grow in a relatively straight path under plateaus  22  and do not cross trough  18  regions. Thus, the coating thickness controls the direction of filiform growth.  
         [0041]     One way to create spatial variation of coating thickness involves the application of a micro-patterned polydimethylsiloxane (PDMS) stamps into a drying acrylic film. The PDMS stamps can be made by inexpensive soft-lithography, but other materials and techniques are applicable too.  FIG. 6  shows a perspective view of stamp  24  impressed with the profile of concave spiral  26 .  
         [0042]      FIG. 7  shows a highly-magnified view of a patterned coating created by impressing stamp  24  of  FIG. 6  onto a drying acrylic surface. Coating  12  is uniformly applied to the metal surface, and stamp  24  is pressed onto coating  12  while it is still wet. When the stamp is removed, convex spiral  28  is the resulting design on coating  12 . This pattern remains on the coating after it dries.  
         [0043]      FIG. 8  illustrates how patterns can be used to entrap filiforms. Since rust filament  16  is only active at its head, it generally grows in one direction. When presented with a patterned surface, rust filament  16  follows the path much like a mole follows a tunnel, as demonstrated in  FIG. 5 , where rust filament  16  only grew under plateaus  22 . When rust filament  16  reaches convex spiral  28  it follows the pattern into entrapment region  32 . When the rust filament reaches this point, it has become entrapped and can no longer propagate.  
         [0044]      FIG. 9  shows how the entire surface of coated metal object  14  can be treated with spiral pattern  30 . The application of spiral pattern  30  over the entire surface greatly limits the distance a filiform can grow before being entrapped.  
         [0045]     Since the precise origin of the filiforms and their bearings can seldom be anticipated various rotational offsets are used to “attract” filiforms into the patterns.  FIG. 10  illustrates the rotational offset that can be used in spiral pattern  30 . Each spiral is depicted at a phase angle that is relatively different than that of the adjacent spirals. The variation in phase angle or angular offset of the patterns increases the probability that a filiform will find a path to grow into entrapment region  32  regardless of which direction the filiform is growing.  
         [0046]     While  FIG. 6 ,  FIG. 7 ,  FIG. 8 ,  FIG. 9 , and  FIG. 10  illustrate an Archimedean-spiral design, it is contemplated that any type of relief that induces the self-trapping of filiforms can be patterned on the coating.  FIG. 11  shows how convex diamond  34  can also be used to create diamond pattern  36 . Also,  FIG. 12  shows double spiral  38  and  FIG. 13  shows “s” spiral  40  which could be used in similar patterns. Each of these patterns operates to entrap the filiforms by directing the filiforms to grow in such a way that the filiform ultimately entraps itself. As illustrated in  FIG. 8 , filiforms will similarly follow the pattern of convex diamond  34 , double spiral  38 , and “s” spiral  40  as they grow and ultimately become entrapped in each of their respective entrapment regions.  
         [0047]     As illustrated in the aforementioned examples, “paths” can be created in many different shapes to promote the self-entrapment of filiform. Any path that directs the active head of the filiform to propagate in such a direction that the active head will become substantially surrounded by the inactive tail will work. Each of the aforementioned paths is configured to cause the filiform to propagate in such a direction that the inactive filiform tail creates an inactive perimeter around an entrapment region, where the active filiform head propagates angularly about the entrapment region. When the active filiform head is finally forced to enter the entrapment region, the filiform will become entrapped and will no longer propagate.  
         [0048]     Although the preceding description contains significant detail, it should not be construed as limiting the scope of the invention but rather as providing illustrations of the preferred embodiments of the invention. As an example, it is shown that embedding patterns on coated metal surfaces promotes the self-entrapment of filiform. Other methods for creating structural variations in surface thickness and composition can be used such as screen printing and surface etching.  
         [0049]     In addition, patterns can be embedded in the coating in such a way that its thickness is not affected. For example, light-controlled patterning can be used to vary the coating&#39;s chemical composition or porosity. Since corrosion occurs where moisture and oxygen diffuse through the coating and react with the metal substrate, the direction of filiform growth can be controlled by spatial variation of coating porosity. Patterns of porosity variation can be used much like patterns of thickness variation to promote filiform entrapment as filiforms will follow paths of higher porosity.