Abstract:
A method for providing enhanced corrosion protection of metallic components and surfaces when exposing the metallic component to a silicate medium while applying an AC (Alternating Current) current.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority to U.S. Provisional application No. 60/874,165, filed Dec. 11, 2006, and which is incorporated herein by reference. 
         [0002]    Additionally, the subject matter of the instant invention is related to the following patents and patent applications: U.S. Pat. Nos. 6,149,794; 6,258,243; 6,153,080; 6,322,687; 6,572,756B2; 6,592,738B2; 6,599,643; 6,761,934; 6,753,039; 6,866,896B2; Ser. Nos. 10/211,094; 10/636,904; 10/713,480; and 10/831,581. The applications have been published as Publication Nos. 20030209290, 20050031894, 20040137239 and 20040222105, respectively. The disclosure of the foregoing patents and patent applications are hereby incorporated by reference. 
     
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
       [0003]    Not Applicable. 
       FIELD OF THE INVENTION 
       [0004]    Enhanced corrosion protection for metallic components and surfaces using a silicate based solution where AC (Alternating Current) current is applied. 
       BACKGROUND OF THE INVENTION 
       [0005]    The development and use of silicates as process solutions for electroless, electrolytic (cathodic or anodic) environments are known. The resulting layer from electroless processes tends to be relatively soluble unless a reducing agent is added. DC (Direct Current) processes where the work piece is the cathode or the anode are dependent upon the substrate and or the type of chemistry involved. Typically DC processes are used to the extent that a favorable reaction takes place on the surface based upon whether or not the reaction requires that electrons be contributed to the resulting layer or given up by the surface. Pulse processing has been demonstrated to facilitate some processes such as chrome plating where efficiency can be increased by repeatedly breaking down the cathodic film, which buildup on the surface that is being plated. In pulse processes the work piece is still net cathodic or anodic based upon the chemistry or surface being deposited. 
         [0006]    Prior art processes have employed electrolytic cathodic process to deposit a beneficial layer with improved corrosion resistance on zinc, zinc alloy plated substrates and zinc diecast components. First the deposition of Silicon is not inherently cathodic in nature. Therefore, without wishing to be bound by any theory or explanation, it is believed that the resulting layer is the result of the pH environment created by the cathodic polarity of the surface, not the direct reduction on the surface due to the availability of electrons. In addition, in a DC process there is a companion reaction, which occurs at the anode. This companion reaction may facilitate the beneficial result and in typical Zinc plating where the Zinc+ ions required for plating can be electrically oxidized at the anode for electro-reduction to zinc metal at the cathode. There is a need in this art for an electrolytic process for treating a metallic surface with silicates in order to improve the corrosion resistance of the metallic surface. 
       BRIEF SUMMARY OF THE INVENTION 
       [0007]    Briefly stated, a method for treating a metallic surface is disclosed. The method comprises contacting a metallic surface with a medium comprising at least one silicate and providing an alternating current to the medium for a time and under conditions sufficient to improve the corrosion resistance of the metallic surface. The silicate can be sodium silicate. The medium can comprise water and at least one silicate. The metallic surface can comprise at least one member selected from the group consisting of zinc, zinc nickel, tin zinc, and zinc iron. In accordance with one aspect of the method, the step of providing the alternating current to the medium comprises contacting the metallic surface with an alternating current while the metallic surface is at least partially in contact with the medium. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         [0008]      FIG. 1  is a schematic of the apparatus and electrical system that can be used in an AC method of the invention. 
           [0009]      FIG. 2  is a graphical representation of the corrosion resistance of articles treated in accordance with the invention. 
       
    
    
       [0010]    Corresponding reference numerals will be used throughout the several figures of the drawings. 
       DETAILED DESCRIPTION OF THE INVENTION 
       [0011]    The following detailed description illustrates the invention by way of example and not by way of limitation. This description will clearly enable one skilled in the art to make and use the invention, and describes several embodiments, adaptations, variations, alternatives and uses of the invention, including what I presently believe is the best mode of carrying out the invention. Additionally, it is to be understood that the invention is not limited in its application to the details of construction and the arrangements of components set forth in the following description or illustrated in the drawings. The invention is capable of other embodiments and of being practiced or 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. 
         [0012]    The instant invention relates to using AC current to treat a metallic surface (e.g., to provide the opportunity for all beneficial reactions based upon polarity to occur on the work piece) in a silicate containing medium. The metallic surface can comprise an electroplated article (e.g., a zinc plated fastener), discrete components having a metallic surface, metallic or metallic coated sheets or rolls, among other surfaces. The silicate containing medium can comprise water, at least one silicate, and optionally additives such as silica. An example of a suitable silicate comprises sodium silicate. 
         [0013]    Experiments using AC current instead of DC have demonstrated an improvement in corrosion performance in ASTM-B-117 corrosion testing. Data collected indicates that an increase in hours to 1st white corrosion of 144% (52 to 127) and an 80% increase in the average time to 5% red failure. Samples for this evaluation came from the same Zinc plating batch. The cathodic process using a sodium silicate solution, process times, process temperature, dry conditions, and current densities in accordance with the previously identified Cross-Referenced Patents and Applications were constants. The processes disclosed therein are incorporated herein by reference. The DC cathodic samples were processed using conventional process equipment. The AC samples were produced using an opposed barrel configuration illustrated in  FIG. 1 . 
         [0014]    An experiment wherein the initial current density was tripled yielded an increase in average time to 1st white corrosion of 469%. At 480 hours of exposure only 1 sample out of 20 has reached 5% red rust, whereas all 20 samples from the conventional DC process had failed by 240 hours of exposure. 
         [0015]    The AC process has the advantages of: 1) no dimensionally stable anode is required and thereby minimizes capital costs for setting up the process, and 2) in this configuration work pieces comprise both sides of the electrical circuit; therefore time efficiency is increased in that finished parts can be processed simultaneously. 
         [0016]    The electrical dynamics of AC current are different than the use of pulse rectifiers in that typical pulse sources generate waveforms similar to square waves. In this condition voltage transitions are extremely rapid to a set threshold and remain at the predefined level until rapidly changed again. Conventional AC current is constantly undergoing changes in voltage and current relationships, which can potentially change the chemical response at the surface of the work piece. 
         [0017]    If desired, after exposure to the AC environment, the exposed metallic surface can be rinsed and dried. In some cases, the exposed metallic surface is coated with a film or layer in order to impart improved corrosion resistance, color, and lubricity, among other properties. Examples of suitable coatings can comprise at least one member selected from the group consisting of latex, epoxy, acrylics, and urethanes, among other coatings. 
         [0018]    The configuration of  FIG. 1 . shows the physical components and electrical configuration of the inventive process. A barrel process is shown, however, a rack process could utilize a similar electrical circuit. 
         [0019]    The following Examples are provided to illustrate certain aspects of the invention and do not limit the scope of the appended claims. 
       EXAMPLES 
       [0020]    Zinc plated rivets were treated in accordance with the following Examples. 
         [0021]    Medium/Solution: 9.8% Sodium Silicate in de-ionized Water 
         [0022]    Temperature: 149° F. (65° C.) 
         [0023]    Time 7.5 min 
         [0024]    Barrel size (2) Lusteron 2″×4″ 
         [0025]    Rotation; nominal 12 RPM 
       Example 1 
       [0026]    (Corrosion Test—Group 1) Voltage to achieve 2.0 nominal amps 14 VAC 
         [0027]    Current trend. 0 min. 2.9-3.0 amps, 3.5 min. 0.9-1.5 amps, 5.0 min. 0.8-1.0 amps, 7.5 min. 0.8-0.9 amps. 
         [0028]    Rivet appearance shiny 
       Example 2 
       [0029]    (Corrosion Test—Group 2) 10 amp target could not be reached stopped increasing voltage at 50 VAC 
         [0030]    As voltage was increased a new short term peak could be reached then would diminish. 
         [0031]    Current trend 0 min. 6-9 amps, 1 min. 3-4 amps, 3 min. 2.5-4.0 amps, 5 min. 1.5-3.0 amps. 
         [0032]    Arcing visible inside barrels, Run terminated at 5 minutes after an arcing event. Continuity testing on the barrel danglers indicated infinite resistance. 
         [0033]    Sanded dangler ends to remove buildup continuity restored 
         [0034]    Dangler in neutral barrel black 
         [0035]    Dangler in L1 barrel amber color 
         [0036]    Rivet appearance dull gray. 
       Example 3 
       [0037]    (Corrosion Test—Group 3) Voltage to achieve target 0.6 amps 2 VAC 
         [0038]    Current trend 0 min. 0.6 amps, 1.5 min. 0.4-0.6 amps, 4 min. 0.3-0.6 amps, 7.5 min. 0.3-0.7 amps. 
         [0039]    Rivet appearance shiny 
         [0040]    The following Table summarizes the corrosion resistance of the Group of rivets when measured in accordance with ASTM B-117.  FIG. 2  is a graphical representation of the corrosion resistance of these Groups. 
         [0000]    
       
         
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
             
           
               
                   
               
               
                   
                   
                 Part 
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                 Time 
               
               
                 Group 
                 Group Description 
                 Type 
                 Part. No. 
                 Area 
                 N 
                 n 
                 Min 
                 Avg 
                 n 
                 Min 
                 Avg 
                 n 
                 Min 
                 Avg 
                 (hours) 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 Group 01 
                 Zinc(AC)EMC 
                 Rivets 
                 99020033 
                 Main 
                 20 
                 20 
                 72 
                 127 
                 19 
                 192 
                 289 
                 19 
                 216 
                 322 
                 648 
               
               
                 AC Process 
                 2 amps 
               
               
                   
                 7.5 min. D/R/D 
               
               
                 Group 02 
                 Zinc(AC)EMC 
                 Rivets 
                 99020033 
                 Main 
                 20 
                 17 
                 96 
                 315 
                 3 
                 384 
                 520 
                 3 
                 432 
                 560 
                 648 
               
               
                 AC Process 
                 6 amps 
               
               
                   
                 7.5 min. D/R/D 
               
               
                 Group 03 
                 Zinc(AC)EMC 
                 Rivets 
                 99020033 
                 Main 
                 20 
                 20 
                 24 
                 104 
                 20 
                 144 
                 253 
                 20 
                 144 
                 276 
                 504 
               
               
                 AC Process 
                 0.6 amps 
               
               
                   
                 7.5 min. D/R/D 
               
               
                 Group 04 
                 Zinc Typical 
                 Rivets 
                 99020033 
                 Main 
                 20 
                 20 
                 24 
                 52 
                 20 
                 120 
                 157 
                 20 
                 144 
                 172 
                 240 
               
               
                 DC Process 
                 Line #3 EMC 
               
               
                   
                 22 amps 
               
               
                   
                 7.5 D/R/D 
               
               
                   
               
             
          
         
       
     
         [0041]    In view of the above, it will be seen that the several objects and advantages of the present invention have been achieved and other advantageous results have been obtained. 
         [0042]    As various changes could be made in the above constructions without departing from the scope of the invention, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.