Abstract:
Disclosed is a method for preventing galvanic corrosion in fastener assemblies employing plated fastener inserts wherein the plating of the insert has an emf potential which is similar to that of the substrate within which the insert is disposed.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    The present invention relates to coated fastener inserts and a method for plating such fastener inserts and, more particularly, to plating fastener inserts to prevent galvanic corrosion in fastener assemblies.  
           [0002]    Fastener assemblies come in a variety of shapes, sizes, designs and materials. Many fastener assemblies include not only a fastener such as a bolt, pin or screw, but also will include a fastener insert to be positioned within a tapped hole.  
           [0003]    The type of fastener insert needed for a particular fastening operation will in large part dictated the type of fastener to be employed. While the present invention is applicable to various fastener assemblies wherein galvanic corrosion is a potential problem, the invention will hereinafter be described with reference to fastener assemblies with metallic helically coiled wire fastener inserts. By way of non-limiting example, certain metallic helically coiled wire inserts useful in association with a threaded fastener are described in U.S. Pat. No. 2,672,070, entitled “Wire Coil Screw Thread Insert for Molded Material” and U.S. Pat. No. 6,224,311 B1 entitled “Coated Fastener Inserts and Method of Producing the Same”, each of which is expressly incorporated by reference.  
           [0004]    Generally speaking, tapped threads are strengthened due to the inherent flexibility of helically coiled wire inserts since the insert provides a more balanced distribution of dynamic and static loads throughout the length of thread engagement. This is especially important when the substrate including tapped holes is formed from a relatively soft metal, i.e., aluminum and magnesium. The inherent flexibility also compensates for variation in lead and angle error.  
           [0005]    Additionally, no stress is initially introduced to the substrate because the helically coiled insert does not exhibit staking, locking or swaging and does not require keying in place. Helically coiled wire inserts allow for the use of smaller bosses, flanges and fasteners than other inserts, thus presenting a cost savings, particularly for high volume applications.  
           [0006]    While such helically coiled wire inserts are generally useful as anchoring mechanisms for threaded fasteners in order to be used in high strength applications, such inserts must be formed from high strength metals such as  302 / 304  stainless steel. The use of stainless steel inserts in association with substrates or fasteners formed from other alloys leads to certain perceived problems such as the possibility of galvanic corrosion occurring over time. By the phrase “galvanic corrosion” it is meant the electrochemical corrosion resulting from the current caused in a galvanic cell between two dissimilar metals in an electrolyte because of the difference in potential (emf) of the two metals.  
           [0007]    Stainless steel fastener inserts have been used in conjunction with zinc chromate primer which is applied to the tapped hole prior to installing the insert in an effort to prevent galvanic corrosion. However, the application of the zinc chromate primer requires strict quantitative controls and is considered labor intensive. The application of the primer is very subjective based on the installer. In the case of fastener inserts for threaded fasteners, the application of too much primer can result in excessive high torque resulting from primer which will eventually harden beneath the cords of the locking coils, restricting their movement. This could result in false clamp load readings and insufficiently torqued bolts. In addition, the installation tools require frequent cleaning to prevent the build up of the primer on the mandrels of the tool, which is undesirable. The application of too little of the zinc chromate primer leads to other problems such as inadequate corrosion protection.  
           [0008]    In view of the foregoing, the present invention relates to a method for preventing galvanic corrosion in fastener assemblies employing a metallic fastener insert and a fastener for use in a tapped hole, said method comprising the steps of:  
           [0009]    a) providing a fastener and a fastener insert for retaining the fastener within said tapped hole;  
           [0010]    b) plating said fastener insert with a metal alloy having an emf potential similar to that of said substrate; and  
           [0011]    c) adjoining the fastener and platted fastener insert within said tapped hole.  
           [0012]    As a result of plating the fastener insert, it is a primary object of the present invention to extend the useful life of fastener applications by preventing galvanic corrosion within a fastener assembly.  
           [0013]    Thus, the present invention also relates to a plated metallic fastener insert comprising a substantially cylindrical metallic body having a plated outer surface, said plating being formed from a metal alloy having an emf potential similar to that of said substrate.  
           [0014]    Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]    The present invention will become more fully understood from the detailed description and the accompanying drawings, wherein:  
         [0016]    [0016]FIG. 1 is a blown apart perspective view of a fastener assembly including a helically wound insert;  
         [0017]    [0017]FIG. 2 is an assembly view in cross-section of the fastener assembly of FIG. 1;  
         [0018]    [0018]FIG. 3 is a perspective view illustrating the process of electroplating a helically wound insert; and  
         [0019]    [0019]FIG. 4 is a cross-sectional view of a convolution taken along line  4 - 4  of FIG. 1 of a plated fastener insert.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0020]    The following description of the preferred embodiment(s) is merely exemplary in nature and is in no way intended to limit the invention, its application, or uses.  
         [0021]    Referring to FIG. 1, there is shown a fastener assembly  10  for use in association with a substrate  12  having one or more tapped holes  14  including inwardly projecting threads  16 . Substrates requiring fastener inserts are generally cast or extruded from a relatively soft metal such as an aluminum or magnesium alloy and can be part of any one of a number of end products such as auto body or aircraft panels and engine blocks, by way of non-limiting example. The fastener assembly  10  includes a fastener  20  and fastener insert  22  insertable within the tapped hole  14 .  
         [0022]    The fastener  20  generally includes a head  24  and a shank  26  having radially outwardly projecting threads  28 . The fastener insert  22  may be of any form capable of retaining the fastener within a tapped hole, but preferably is in the form of a helically wound wire  30  including a body  32  having a plurality of convolutions  34  disposed between first and second ends,  36  and  38 , respectively. At least one of the ends may be provided with a driving tang or notch for tangless embodiments for assistance in the installation or removal process.  
         [0023]    Preferably, the fastener inserts  22  are larger in diameter, before installation than the tapped holes  14  such that upon installation they become firmly secured within the taped holes, as shown most clearly in FIG. 2. Further, when the helically wound inserts are disposed within the tapped holes, the coils provide permanent convolution  600  internal screw threads which accommodate virtually any standard threaded bolt or screw.  
         [0024]    After forming the fastener insert to the desired shape, the insert is optionally cleaned and/or otherwise prepared for plating.  
         [0025]    In this regard, it is preferred that the fastener inserts be free of contaminants such as oil, grease, metal shavings, etc. since contaminants may cause poor adhesion of the plating to the fastener insert. Thus, if necessary, the fastener inserts be cleaned, i.e., degreased prior to plating using a chlorinated or fluorinated solvent, detergents or a high temperature burnout process wherein the part is exposed to temperatures roughly between 400 to 425° C. for ten to twenty minutes to ensure that any petroleum based contaminants are fully carbonized.  
         [0026]    In addition to cleaning the surface, the fastener inserts may be immersed in an acidic deoxidizing composition including inhibitors to suppress hydrogen evolution.  
         [0027]    Once the inserts are prepared for plating, the inserts are electroplated employing known electrochemical plating techniques. For example, for fastener inserts to be disposed in an aluminum alloy substrate, an electroplating bath including a complimentary aluminum composition is provided. Such electroplating bath compositions can be purchased commercially from a variety of sources such as AlumiPlate, Inc. of Minneapolis, Minn.  
         [0028]    A preferred plating process including the steps of aqueous cleaning, acid deoxidation and subsequent plating is described in U.S. Pat. No. 4,417,954. After cleaning and deoxidation, an appropriate aluminum plating composition is prepared in a non-aqueous, apriotic electrolyte wherein the inserts are precluded from exposure to free hydrogen. Under a preferred embodiment, the plating employed will have an emf potential which is ±0.25 v as compared to the emf potential of the substrate. The plating composition is applied to a relatively uniform average dry thickness of between about 0.0002 to about 0.0005 inches and, more preferably, between about 0.0003 to about 0.0004 inches. A thinner plating may not be sufficiently effective at preventing galvanic corrosion in a fastener assembly and thicker platings can interfere with the installation tooling.  
         [0029]    After plating but prior to installation within a tapped hole, for example, the plated fastener insert is optionally coated with a fluoropolymer or chromate composition by dip spraying or air spray techniques.  
         [0030]    Fastener inserts prepared in the foregoing manner were analyzed utilizing a salt spray test. Aluminum plated fastener inserts were installed into tapped holes in aluminum blocks and subjected to salt spray pursuant to ASTM B 117-97 for a period of 500 hours. After 500 hours, the aluminum plated fastener inserts were removed and metallographically inspected. No corrosion was observed on the aluminum block in either the exposed surface or the exposed threads.  
         [0031]    The description of the invention is merely exemplary in nature and, thus, variations that do not depart from the general description of the invention are intended to be within the scope of the invention. Such variations are not to be regarded as a departure from the spirit and scope of the invention. After sectioning, no corrosion was observed between the aluminum block thread and the aluminum plated inserts.