Abstract:
In accordance with exemplary embodiments, a system for isolating magnesium components from fastener devices is provided to prevent corrosion. The system comprises an isolation member of a metal other than magnesium and configured within a hole for receiving the fastener formed in a magnesium component. A compressible sealing washer having a first diameter is positioned over the hole and an isolation washer having a second diameter being less than the first diameter, and consisting of a similar metal as the isolation member, is placed over the compressible sealing washer. The fastener passes through the isolation washer, the compressible sealing washer and the isolation member within the hole, where is it tightened to compress the compressible sealing washer between the isolation member and isolation washer forming a water-tight seal around the hole to resist corrosion of the magnesium component.

Description:
TECHNICAL FIELD 
     The technical field relates generally to the field of corrosion prevention. More particularly, the invention relates to a system for isolation of magnesium components from fastener devices to prevent corrosion. 
     BACKGROUND 
     Magnesium is a useful alternative to aluminum, steel or other structural materials in many applications. For example, magnesium may be used for structural parts in automotive applications. This is due, at least in part, to magnesium&#39;s relatively high strength-to-weight ratio as compared to aluminum. In some instances, a part made from magnesium may be joined to a part made from another metal and/or another metal may be used to join together two magnesium parts. In either case, the magnesium part(s) may be susceptible to corrosion at an interface formed between the magnesium and the other metal when, for example, the interface is exposed to corrosive environments. 
     Corrosion can occur at the interface between devices formed of dissimilar metals due to galvanic action. In general, at the interface between dissimilar metals, the metal with a more negative potential corrodes preferentially. By way of example, when a device formed of magnesium is in physical contact with a device or fastener formed of steel, the magnesium device corrodes near the interface. This corrosion can be compounded if the corrosive environment is a wet environment such as a vehicle undercarriage, engine compartment or closures (e.g., doors) that are regularly exposed to the surrounding environment. 
     A conventional approach to protecting against galvanic corrosion in metals is to provide electrical insulation between the two devices. But insulating materials are not readily employable in certain applications. Other conventional approaches involve coatings. However, coatings can be difficult to apply in some applications and may be prone to wearing away. 
     Accordingly, it is desirable to provide corrosion protection for magnesium components of a vehicle. Also, it is desirable to provide corrosion protection that facilitates magnesium components being utilized in wet environments of a vehicle. Additionally, other desirable features and characteristics of the present disclosure will become apparent from the subsequent description taken in conjunction with the accompanying drawings and the foregoing technical field and background. 
     BRIEF SUMMARY 
     In accordance with exemplary embodiments, a system for isolating magnesium components from fastener devices is provided to prevent corrosion. The system comprises an isolation member of a metal other than magnesium and configured within a hole formed in a magnesium component for receiving the fastener. A compressible sealing washer having a first diameter is positioned over the hole and an isolation washer having a second diameter being less than the first diameter, and consisting of a similar metal as the isolation member, is placed over the compressible sealing washer. The fastener passes through the isolation washer, the compressible sealing washer and the isolation member within the hole, where is it tightened to compress the compressible sealing washer between the isolation member and isolation washer forming a water-tight seal around the hole to resist corrosion of the magnesium component. 
     In accordance with exemplary embodiments, a method for isolating magnesium components from fastener devices is provided to prevent corrosion. The method comprises positioning an aluminum sleeve within a hole formed in the magnesium component that will receive a fastener. A compressible sealing washer is adhered to one end of the aluminum sleeve. Next, the fastener is positioned through an aluminum washer, the compressible sealing washer and the aluminum sleeve. Finally, the compressible sealing washer is compressed by the aluminum sleeve by tightening the fastener and forming a water-tight seal around the hole between the aluminum washer and the aluminum sleeve. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
       The subject matter will hereinafter be described in conjunction with the following drawing figures, wherein like numerals denote like elements, and: 
         FIG. 1  is a pre-assembly exploded view illustration of the fastener isolation system according to exemplary embodiments of the present disclosure; and 
         FIG. 2  is an illustration the assembled fastener isolation system according to exemplary embodiments of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     The following detailed description is merely exemplary in nature and is not intended to limit the subject matter of the disclosure or its uses. Furthermore, there is no intention to be bound by any expressed or implied theory presented in the preceding technical field, background, brief summary or the following detailed description. 
     In this document, relational terms such as first and second, and the like may be used solely to distinguish one entity or action from another entity or action without necessarily requiring or implying any actual such relationship or order between such entities or actions. Numerical ordinals such as “first,” “second,” “third,” etc. simply denote different singles of a plurality and do not imply any order or sequence unless specifically defined by the claim language. 
     Additionally, the following description refers to elements or features being “connected” or “coupled” together. As used herein, “connected” may refer to one element/feature being directly joined to (or directly communicating with) another element/feature, and not necessarily mechanically. Likewise, “coupled” may refer to one element/feature being directly or indirectly joined to (or directly or indirectly communicating with) another element/feature, and not necessarily mechanically. However, it should be understood that, although two elements may be described below, in one embodiment, as being “connected,” in alternative embodiments similar elements may be “coupled,” and vice versa. Thus, although the schematic diagrams shown herein depict example arrangements of elements, additional intervening elements, devices, features, or components may be present in an actual embodiment. 
     Finally, for the sake of brevity, conventional techniques and components related to vehicle mechanical parts and other functional aspects of the system (and the individual operating components of the system) may not be described in detail herein. It should also be understood that  FIGS. 1-2  are merely illustrative and may not be drawn to scale. 
       FIG. 1  is a pre-assembly exploded view illustration of a fastener isolation system  10  according to exemplary embodiments of the present disclosure. The fastener isolation system  10  contemplates coupling a magnesium (or magnesium alloy) component  12  via a fastener  14  (e.g., bolt, screw or rivet). In the illustrated embodiments, the fastener  14  comprises a bolt having a threaded portion  16  and a tightening portion  18 . The tightening portion  18  may have a general hexagon shape to be tightened with a conventional wrench or socket. Generally, the fastener  14  comprises a dissimilar metal to the magnesium component  12 . Typically, conventional fasteners are made of steel (or a steel alloy) which would react with the magnesium component causing corrosion of the magnesium. 
     Galvanic corrosion often occurs at an interface formed between dissimilar metals when the interface is exposed to corrosive environments. “Corrosive environments,” as used herein, refers to any environment that includes a corrosive element. As an example, and not a limitation, corrosive elements include water, salt, acids, aqueous solutions containing oxygen, aqueous solutions containing chlorides, and the like. In the event that the corrosive environment is a wet environment (e.g., water or aqueous solution), galvanic corrosion may occur more rapidly and over a wider area since electrical conduction between the magnesium component  12  and the fastener  14  is facilitated. That is, galvanic corrosion occurs when the dissimilar metals are electrically connected to each other. The dissimilar metals assume different electrical potentials that tend to cause galvanic current to flow between the metals. In most cases, the less noble metal will act like an anode, and the more noble metal will act like a cathode. The galvanic current flows from the anode to the cathode thereby causing electrochemical dissolution of the anodic metal (i.e., the magnesium component  12 ). 
     According to exemplary embodiments of the present disclosure, the magnesium (or magnesium alloy) component  12  is fitted with an isolation member  20  at the location of receiving the fastener  14  (e.g., hole  22 ). Preferably, the isolation member  20  comprises a metal much less likely to cause galvanic corrosion with the magnesium component  12 , such as aluminum (or an aluminum alloy). As can be seen in the cross-section illustration, the isolation member  20  surrounds the hole  22  formed in the magnesium component  12  and includes extended portions  24  around the hole  22  at the fastener interface surface of the magnesium component  12 . The isolation member  20  is positioned in the hole  22  via a compression (press) fit operation or may be formed in the hole  22  during casting of the magnesium component  12 . In some embodiments, the extended portions  24  of the isolation member  20  are flush with the surface of the magnesium component  12 . Optionally, the extended portions  24  of the isolation member  20  may project slightly above the surface of the magnesium component  12  as indicated by reference numeral  26 . In this embodiment, greater separation from the tightening portion  18  of the fastener  14  to the surface of the magnesium component is achieved without having to employ thick washers. 
     To limit water pooling around or intrusion into the hole  22 , a compressible sealing washer  28  is employed. The compressible sealing washer  28  is compressed by an isolation washer  30  when the fastener  14  is tightened. In some embodiments, the isolation washer  30  comprises the same or similar material (e.g., aluminum or an aluminum alloy) as the isolation member  20 . The compressible sealing washer  28  may be of any water resistant material, such as silicon, rubber, butyl, foam or adhesive impregnated mesh. In some embodiments, the compressible sealing washer  28  includes an adhesive layer  32  for fixation about the hole  22  for ease of assembly. In exemplary embodiments, the compressible sealing washer  28  has a diameter  34  that is slightly larger than the diameter  36  of the isolation washer  28 . This assures a water-tight seal around the hole  22 , while providing height isolation between the tightening portion  18  of the fastener  14  and the surface of the magnesium component  12 . That is, the thickness of the compressible sealing washer  28  (when compressed) and the thickness of the isolation washer  30  provide height separation for corrosion resistance. In some embodiments, the minimum height separation distance is approximately three millimeters. 
     For assembly, the various components and members are coaxially aligned (as indicated by reference numeral  38 ) and the threaded portion  16  of the fastener  14  passes through the isolation washer  30 , the compressible sealing washer  28  and the isolation member  20 . For tightening, the threaded portion  16  of the fastener  14  is turned (rotated) against a retainer (e.g., nut)  40 . In some embodiments, the threaded portion  16  of the fastener  14  may be received by another component (not shown) having mating threads formed therein facilitating coupling to the magnesium component  12 . 
       FIG. 2  is an illustration the assembled fastener isolation system  10  according to exemplary embodiments of the present disclosure and wherein like reference numbers refer to like components. In the illustrated embodiment of  FIG. 2 , the fastener  14  is a screw and the retainer  40 ′ comprises a tapped extrusion within a vehicle body member  42  (for example, a frame of a vehicle door latch). Other types of fasteners and retainers are, of course, possible. As noted above, the compressible sealing washer  28  and the isolation member  20  provide sufficient separation  48  of at least three millimeters. Accordingly, in this embodiment, the isolation washer  30  has a thickness  42  of 2 millimeters and compressible sealing washer  28  has a thickness  44  of 1.5 millimeters, that is compressed (as indicated at  46 ) by 0.5 millimeters when the fastener  14  is tightened. Thus, a separation  48  of three millimeters is achieved. Additionally, the isolation member  20  provides fastener-to-magnesium isolation through the hole  22  and to the retainer  40 ′ via the projecting portions  24  of the isolation member  20 . In this way, corrosion of the magnesium component  12  is reduced or eliminated by the fastener isolation system  10  of the present disclosure. 
     Accordingly, a fastener isolation system is provided for use in a vehicle that may be employed in wet environments of a vehicle such as the engine compartment, passenger door areas and for vehicle under-carriage applications. 
     While at least one exemplary embodiment has been presented in the foregoing summary and detailed description, it should be appreciated that a vast number of variations exist. It should also be appreciated that the exemplary embodiment or exemplary embodiments are only examples, and are not intended to limit the scope, applicability, or configuration of the disclosure in any way. Rather, the foregoing summary and detailed description will provide those skilled in the art with a convenient road map for implementing the exemplary embodiment or exemplary embodiments. It should be understood that various changes can be made in the function and arrangement of elements without departing from the scope of the disclosure as set forth in the appended claims and the legal equivalents thereof.