Metal mesh with a low electrical resistance conversion coating for use with aircraft structures

Abstract

A gasket is provided with a tacky polyurethane or polyurea gel body and a metallic mesh skeleton where the skeleton has been coated with a conversion coating. The conversion coating helps reduce corrosion in the gasket and corrosion on the parts that place the gasket under compression, especially where an environment conducive to galvanic corrosion is present. This gasket may then be used in assemblies which include aircraft parts, which parts may be subject to extreme environmental conditions.

Claims

1. A gasket for use between aircraft parts under compression, the gasket comprising: a moisture proof body comprising an elastomeric polymer with a tacky, outer surface configured to releasably contact at least one of the aircraft parts; a metal skeleton encapsulated by the moisture proof body of the gasket; wherein the metal skeleton includes an electrically conductive or semi-conductive conversion coating as an outer surface thereof; and wherein, under compression, the moisture proof body deforms sufficiently to allow an aircraft part to contact the outer surface of the metal skeleton and provides electrical conductivity therethrough.

2. The gasket of claim 1, wherein the moisture proof body is a tacky polyurethane gel and wherein the metal skeleton is an aluminum or aluminum alloy mesh and wherein a resistance of the coated aluminum or aluminum alloy mesh is less than about 2.5 milliohms.

3. The gasket of claim 1, wherein the conversion coating is coated between about 100-250 mg/dm2.

4. The gasket of claim 1, wherein the elastomeric polymer is a polyurethane gel.

5. The gasket of claim 1, wherein the body includes conductive particles or fibers.

6. The gasket of claim 1, including a second skeleton.

7. The gasket of claim 6, wherein the second skeleton is a metallic mesh.

8. The gasket of claim 1, wherein the body is comprised of a polyurea polymer.

9. The gasket of claim 1, wherein the body is comprised of a tacky, deformable, flowable gel.

10. The gasket of claim 1, wherein the metal skeleton has a perimeter and wherein, when under compression, the body extends beyond the perimeter of the skeleton.

11. The gasket of claim 10, wherein the perimeter includes a cure in place injectable polymer.

12. The gasket of claim 1, wherein a resistance of the skeleton is less than about 2.5 m.Math..

13. The gasket of claim 1, wherein the conversion layer comprises a chromium.

14. The gasket of claim 13, wherein the chromium is part of a chromate.

15. The gasket of claim 14, wherein the chromate includes trivalent chromium.

16. The gasket of claim 1, wherein the conversion coating includes a hexafluorozirconate.

17. The gasket of claim 1, wherein the body is a gel and includes corrosion inhibitors.

18. A method of producing a corrosion resistance assembly of aircraft parts comprising: contacting an exterior of a skeleton with a composition comprising a reactive composition to obtain a conversion coating on a top surface coat on the skeleton surface and assembling the skeleton comprising said top surface coat into an aircraft gasket having an elastomeric body and placing the gasket under compression between two parts of an aircraft.

19. The method of claim 18, wherein the skeleton is die cut before contacting.

20. The method of claim 18, wherein before the placing step and after the contacting step, the skeleton is die cut into a preselected shape.

21. The method of claim 18, wherein the reactive composition contains trivalent chromium.

22. The method of claim 18, wherein the reactive composition contains hexavalent chromium.

23. The method of claim 18, wherein the reactive composition contains chromium, either a trivalent or hexavalent chromium, and elastomeric body is a tacky gel body including a polyurethane or polyurea two component sealant.

24. An assembly of aircraft parts comprising: a first aluminum or aluminum alloy part having a first surface; a second aluminum or aluminum alloy part having a second surface; a multiplicity of fasteners for engaging the two aluminum or aluminum alloy parts; a gasket comprising a body comprising an elastomeric polymer; and a metal skeleton encapsulated by the body of the gasket; wherein the metal skeleton includes a conversion coating.

25. The assembly of claim 24, wherein the two parts comprise an aircraft antenna and an aircraft skin.

26. The assembly of claim 24, wherein the two parts comprise an aircraft fuel door surround and an aircraft fuel door.

27. A gasket for use between aircraft parts under compression, the gasket comprising: a moisture proof body having an outer surface comprising an elastomeric polymer; and a metal skeleton encapsulated by the moisture proof body of the gasket; wherein the metal skeleton includes an electrically conductive or semi-conductive conversion coating comprising an outer surface thereof; wherein the moisture proof body and the outer surface is comprised of a tacky, deformable, flowable gel which releasably contacts at least one of the aircraft parts; wherein the resistance of the skeleton is less than about 2.5 m.Math.; and wherein the skeleton contacts the aircraft parts when the body is under compression between the aircraft parts and provides electrical conductivity therethrough.

28. The gasket of claim 27, wherein the conversion layer comprises a chromium.

29. The gasket of claim 28, wherein the chromium is part of a chromate.

30. The gasket of claim 29, wherein the chromate includes trivalent chromium.

31. The gasket of claim 27, wherein the conversion coating includes a hexafluorozirconate.

32. The gasket in claim 1, wherein the gasket leaves no significant visible corrosion after salt fog immersion test according to ASTM B117 after 500 hours.

33. The gasket in claim 1, wherein the resistance of the skeleton is less than 2.5 m.Math. after salt fog immersion tests according to ASTM B117 after 500 hours.

34. The gasket in claim 1, wherein the body includes corrosion inhibitors.

35. The gasket in claim 27, wherein the gasket leaves no significant visible corrosion after salt fog immersion test according to ASTM B117 after 500 hours.

36. The gasket in claim 1, wherein the resistance of the skeleton is less than 2.5 m.Math. after salt fog immersion tests according to ASTM B117 after 500 hours.

37. The gasket in claim 1, wherein the body includes corrosion inhibitors.

38. The gasket in claim 1, wherein the skeleton is an aluminum alloy.

39. The gasket in claim 38, wherein the aluminum alloy is a 6000 series alloy.

40. The gasket in claim 38, wherein the aluminum alloy is a 2000 series alloy.

41. The gasket in claim 38, wherein the aluminum alloy is a 3000 series alloy.

42. The gasket in claim 38, wherein the aluminum alloy is a 4000 series alloy.

43. The gasket in claim 38, wherein the aluminum alloy is a 5000 series alloy.

44. The gasket in claim 1, wherein the conversion coating is on an outer surface of an end of a wire of a skeleton having wire ends.

45. A gasket for use between aircraft parts under compression, the gasket comprising: a moisture proof body comprising an elastomeric polymer with a tacky, outer surface configured to releasably contact at least one of the aircraft parts; a metal skeleton substantially encapsulated by the moisture proof body of the gasket in an uncompressed state; wherein the metal skeleton includes an electrically conductive or semi-conductive conversion coating on an outer surface thereof; and wherein, under compression, the moisture proof body deforms sufficiently to allow the aircraft parts to contact the outer surface of the metal skeleton and provide electrical conductivity therethrough.

46. The gasket of claim 45, wherein, under a nominal electrode pressure of about 200 psi, the conversion coated skeleton has less than 200 milliohms of resistance after 500 hours of exposure at 5% salt fog.

47. The gasket of claim 46, wherein the conversion coating weight is 100-250 mg/dm2.

48. The gasket of claim 46, wherein the conversion coating is a non-chromate conversion coating.

49. The gasket of claim 46, wherein the conversion coating is a trivalent chromium conversion coating.

50. The gasket of claim 46, wherein the skeleton is a cooper alloy skeleton.

51. The gasket of claim 46, wherein the skeleton is a nickel alloy skeleton.

52. The gasket of claim 46, wherein the skeleton is an aluminum alloy skeleton.

53. The gasket of claim 46, wherein the body comprises a gel and the gel includes no corrosion inhibitors.

54. The gasket of claim 46, wherein the body comprises a gel and the gel includes corrosion inhibitors.

55. The gasket of claim 54, wherein the corrosion inhibitors are from a group of non-chromated corrosion inhibitors.

56. The gasket of claim 46, wherein the gasket causes no visible corrosion to the aircraft parts after 500 hours of exposure at 5% salt fog.

57. The gasket of claim 46, wherein the resistance of the conversion coated skeleton is less than 2.5 milliohms after 500 hours of exposure at 5% salt fog.

58. The gasket of claim 46, wherein the body contains fillers.

59. The gasket of claim 58, wherein the body fillers include conductive fibers.

60. A gasket for use between aircraft parts under compression, the gasket comprising: a moisture proof body comprising an elastomeric polymer with a tacky, outer surface configured to releasably contact at least one of the aircraft parts; a metal skeleton substantially encapsulated by the moisture proof body of the gasket in an uncompressed state; wherein the metal skeleton includes an electrically conductive or semi-conductive conversion coating on an outer surface thereof; wherein, under compression, the moisture proof body deforms sufficiently to allow the aircraft parts to contact the outer surface of the metal skeleton and provide electrical conductivity therethrough; wherein, under a nominal electrode pressure of about 200 psi, the conversion coated skeleton has less than 200 milliohms of resistance after 500 hours of exposure at 5% salt fog; and wherein the gasket causes no visible corrosion to the aircraft parts after 500 hours of exposure at 5% salt fog.

61. The gasket of claim 60, wherein the resistance of the conversion coated skeleton is less than 2.5 milliohms after 500 hours of exposure at 5% salt fog.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a cutaway perspective view of a composite that uses Applicant's conversion coated metallic mesh fabric.

(2) FIG. 2 is a cross-sectional view of a strand of conversion coated metal skeleton having a coated end.

(3) FIG. 3 is a cross-sectional view of an assembly comprising aircraft parts under compression with Applicant's composite therebetween.

(4) FIG. 4 is a flow chart showing steps taken for passivating a metal with a conversion coating of a chromium compound.

(5) FIG. 5 is a cross-sectional view of a strand having an uncoated end.

(6) FIG. 6 is a cross-section of an embodiment of Applicant's composite as part of an assembly comprising aircraft parts under compression.

(7) FIG. 7 is a more detailed flow chart showing steps taken for passivating a metal with examples of a conversion coating of a chromium compound.

(8) FIGS. 8 (cut finished gasket) and 8A (sealed uncoated strand ends) illustrate a coat, pour and cut method of preparing a gasket according to these specifications that leave uncoated strand ends.

(9) FIGS. 9 (cut), 9A (coat), and 9B (pour) illustrate a cut, coat and pour method of preparing a gasket according to these specifications.

(10) FIGS. 10 and 11 illustrate a typical salt fog test assembly (in pieces), front side and opposite or back side, prior to exposure.

(11) FIG. 12 illustrates the test assembly, front and back side, prior to exposure in the salt fog chamber.

(12) FIGS. 13 and 14 illustrate the SurTec skeleton pre-cut (and then conversion coated thus leaving strand ends coated) disassembled after 500 hours of salt fog, front side and opposite side.

(13) FIGS. 15 and 16, close-up of FIGS. 14 and 15 on aircraft side and antenna base side after 500 hours.

(14) FIGS. 17-21 all illustrate results for SurTec post-cut (cutting skeleton after conversion coating thus leaving strand ends uncoated).

(15) FIG. 17 illustrates front and back side of the test assembly assembled together.

(16) FIGS. 18 and 19 illustrate front and back SurTec wire post-cut assembly after 500 hours of exposure.

(17) FIGS. 20 and 21 show aircraft side and antenna base side after 500 hours of salt fog exposure.

(18) FIGS. 22-25 illustrate control, no conversion coating, same aluminum mesh, same article being tested, with FIGS. 22, 23, 24, and 25 showing the corrosion results for the control assembly.

(19) FIGS. 26-30 illustrate results for a post-cut Alodine treated wire mesh skeleton after 500 hours of salt fog exposure.

(20) FIG. 26, front and back of typical test assembly prior to exposure.

(21) FIGS. 27 and 28 disassembled front and back sides of test assembly after 500 hours of exposure.

(22) FIGS. 29 and 30, close-ups of aircraft side and antenna base side after 500 hours of exposure.

(23) FIGS. 31-35 show Alodine treated pre-cut after 500 hours of salt fog.

(24) FIG. 31, typical test assembly, front and back, prior to exposure.

(25) FIGS. 32 and 33, front and back disassembled, after 500 hours of exposure.

(26) FIGS. 34 and 35, front and back (aircraft side and antenna base side) after 500 hours of salt fog exposure.

DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS

(27) FIGS. 1, 2, 3, 5, and 6 illustrate embodiments of applicants composite 10, and assemblies including Applicant's composite which, in some embodiments, comprise a gasket. In one embodiment, the composite may comprise a gasket having a Body 12 (in some embodiments, a tacky gel body) with a metallic skeleton 14, which metallic skeleton contains a thin conversion coating 19 as set forth herein. The skeleton may be a metal mesh skeleton comprising multiple strands 16 of aluminum or aluminum alloy cylindrical wire that are woven together to form the mesh usually an open weave mesh, similar to the mesh of a screen door. The skeleton may also be a metal, such as expanded aluminum. FIG. 2 illustrates that the individual strands (or the metal of a non-strand skeleton) may be coated in some embodiments even across a strand end as when a cut-to-fit gasket mesh is dipped in a bath of a conversion coating solution. FIG. 3 illustrates a use of applicants composite, in this embodiment a gasket having a gel body, between a movable part 22 and a base 24 with the movable part under compression against the base, as by fasteners, for example. Note in the embodiment illustrated, the opposed mating surfaces are flat or slightly curved and match one another and may be metallic, in one case at least one of the parts is aluminum or aluminum alloy. The skeleton and body are typically flexible enough to allow the gasket to fit a slight curvature and the gel is deformable and flowable under compression.

(28) Body 12, typically a tacky polyurethane gel, substantially encapsulates the metal conversion coated skeleton 14 the gel body, in some embodiments, extending above and below the mesh in a first range of about 0 to 50 mil. as seen in FIG. 1 or a second, preferred range of about 10 to 30 mil. Body 12 may also be made of a sealant such as a fast setting, clear, two component polyurea available as Part Number SF2470 from KBS Chemical, Dodd City, Tex. The KBS material is a two-component urethane material designed for use as a watertight, flexible sealant. The low viscosity allows the material to be sprayed. The transparent nature of the sealant allows for inspection through the material, such as the skeleton, without removal. The system demonstrates excellent adhesion to itself (cohesion) and allows for easy removal from the aircraft parts when necessary.

(29) In one embodiment, body 12 may comprise a two-part, cured, tacky, elastomeric polyurethane gel as set forth in the patents incorporated herein by reference, in one case the gel body having a hardness of between about 40 and 150 durometer. When composite 10 is under compression between movable part 22 and base 24, as when fasteners may be torqued down, the gel, being soft, deforms and, typically, some will squeeze out. This allows the movable part to come in contact with the upper surface of the metal mesh, forcing the metal mesh against the lower surface of the base, thereby providing an electrically conductive path between movable part 22 and base 24

(30) Applicant discloses two similar processes and three resulting products, both processes and products resulting in a chemical conversion coating to a metallic skeleton or mesh, which coating is a thin electrically conductive coating but provides for some corrosion resistance.

(31) FIG. 5 illustrates an uncoated strand end. Uncoated strand ends may occur when a coated wire mesh skeleton 14 is die cut as set forth herein and in the patents incorporated herein by reference. FIG. 3 illustrates a conversion coating 19 on the exposed surfaces of the wire forming the mesh skeleton 14.

(32) The thickness and therefore the electrical resistance of the conversion coating may be controlled by the time in which the skeleton is immersed in the conversion coating solution (SurTec 650 or Alodine, for example) with a longer period of time creating the thicker coating.

(33) FIG. 6 illustrates the use of a perimeter sealant between movable part 22 and base 24, wherein composite 10 in one embodiment is slightly undercut leaving a gap, which gap is filled with a perimeter seal 26 such as AvDec Thixoflex Orange, a two part elastomeric cure in place sealant (hardness about 55 to 75 shore oo) or AvDec Thixoblack, a firmer (about 70 to 90 durometer shore A) perimeter sealant.

(34) FIG. 4 illustrates that both conversion coating processes involve the steps of degreasing, chemically etching, then rinsing (together referred to as cleaning) and then chemically converting a surface of a metallic skeleton with a reactive composition that may contain chromium such that the coating contains, in some embodiments, either a trivalent or hexavalent chromium. The chemical conversion provides a chemical conversion coating, in one embodiment, by dipping an etched mesh skeleton in a conversion chemical. The coated metallic mesh is then embedded in a tacky gel (in one case polyurethane) and used as a gasket in a number of settings, some (such as an aircraft antenna) of which involve the use of aircraft parts under compression and subject to EMI (electromagnetic interference).

(35) In one embodiment, the aluminum alloy of the mesh to be coated may be 2, 3, 5 or 6000 series alloy and in one embodiment is 5053 aluminum. Prior to coating, the base metal is mechanically or chemically cleaned such that a water breakfree surface is obtained after rinsing. Abrasives containing iron, such as steel wool and the like should not be used for cleaning. The chemical conversion coating may be applied non-electrolytically by spray, brush, or immersion on the formed aluminum wire, open weave mesh. The chemical conversion coating should be continuous in appearance and visibly discernible in daylight. It should be free from areas of powdery and loose coating, scratches, flaws and other defects for damages which reduce the corrosion protective value of the coating. A cleaner designed to chemically etched aluminum or aluminum alloy may be used. In one case, an acid etch may be used on 5053 aluminum alloy.

(36) In one embodiment, the reactive composition providing the chemical conversion coating is an aqueous acidic solution which contains trivalent or hexavalent chromium. In a second embodiment, the reactive composition providing the chemical coating is Alodine 1201 (a registered trademark of Henkel Surface Technologies) or SurTec 650 ChromitAL (a registered trademark of SurTec International). The former provides for conversion coating comprising a hexavalent chromate and the latter a conversion coating comprising a trivalent chromate. Both materials form protective coatings by a chemical reaction with aluminum or aluminum alloys on the surface of the skeleton. Any suitable aqueous acidic solution containing trivalent or hexavalent chromate or suitable equivalents may be used. A hexafluorozirconate from an aqueous solution of dispotassium hexafluorozirconate, for example, may be used in conjunction with the chromate, or as an additional protective layer.

(37) The weight of the passivation or conversion coated layer 19 depends, in part, on the roughness of the aluminum surface. Passivation layers with good corrosion resistance may, in some embodiments, have a weight of approximately 0.11-0.5 grams per square meter. Immediately upon drying of the treated surface, treated surfaces may be assembled with a two part polyurethane cure in place gel to form a gasket. Drying time may be room temperature up to about 100 C. and, in one embodiment, about 65 C., however, room temperature is preferred.

(38) Spraying, brushing or wiping may also be used to apply the reactive chemical composition. A solution of 20% by volume of SurTec 650, 80% by volume of dionized water may be used. In some embodiments, the optimum coating weight may be between about 100 to 250 mg/dm2 for conductive conversion coatings.

(39) Mil spec MIL-DTL-5541 F describes procedures for coating aluminum to produce a chemical conversion coating. While this mil spec describes the conversion coating of aluminum alloy flat sheet or plate, some of the information and disclosure may be utilized as set forth herein for the chemical conversion of an aluminum mesh or expanded metal or fabric comprised of multiple woven electrically conductive strands.

(40) The conversion coatings may be applied to aluminum or aluminum alloy woven mesh substrates that are not anodized. The coating will inhibit galvanic corrosion. Coating thickness is varied by immersion time and as a result, the electrical conductivity can be controlled, a thinner conversion coating yielding a metal with lower electrical resistance.

(41) When under a nominal electrode pressure of about 200 psi, thin coatings may be tested under MIL-DTL-81706 to have a resistance not greater than 5,000 microhms (5 milliohms) per square inch and 10,000 microhms (10 milliohms) per square inch, after 168 hours of salt spray exposure. A preferable resistance is 2.5 milliohms or less. Other variables may affect resistance values other than coating thickness. These include surface roughness of the specimen, with rough surfaces yielding lower resistance values due to coating fracture. Moreover, if the surface area is not flat, such as in a woven aluminum alloy mesh, the actual contact points will be lower points, and a smaller contact area may result in a higher resistance value.

(42) MIL-C-5541 Class 3 conversion coatings are required to have less than 200 milliohms of resistance when measured at 200 psi contact pressure after 168 hours of exposure to a 5% salt spray. In some embodiments, including those set forth below, the conversion coatings meet these parameters.

(43) SurTec 650Determination of the Layer Weight

(44) Equipment: Analytical balance (+/0.1 mg)

(45) Reagents: 50% vol. nitiric acid (65%)

(46) Procedure:

(47) 1. Passivate a test part with a known surface area in m.sup.2 (preferably)>2 dm.sup.2) in SurTec 650.

(48) 2. Rinse with deionized water and dry with compressed air at room temperature.

(49) 3. Weigh the dry part within 3 h after passivation with the analytical balance (=M.sub.1).

(50) 4. Remove the passivation layer in nitric acid (4 min at 20-25 C.).

(51) 5. Rinse with deionized water and dry with compressed air at room temperature.

(52) 6. Weigh the dry part again with the analytical balance (=M.sup.2).

(53) 7. Repeat the analysis with a cleaned and deoxidized but notpassivated test part (=M.sub.3 and M.sub.4)

(54) Calculation:
(M.sup.1M.sup.2)/surface area=A
(M.sup.3M.sup.4)/surface area=B
AB=layer weight in g/m.sup.2

(55) Hint: For determination of the layer weight always a fresh solution should be used.

(56) At the latest after 1 m.sup.2/l the solution has to be renewed.

(57) Two examples of conversion coatings are set forth below. These may be prepared by guidelines of the manufacturers, or those set forth in FIG. 7.

Example A (Alodine Coated Mesh, FIGS. 26-35)

(58) Henkel Alodine 1201 Conversion Coating Treatment

(59) Alodine 1201 is a conversion coating (sometimes called passivation) for aluminum or aluminum alloy, the reactive coating composition containing hexavalent (VI) chromium. It is nonflammable, chromic and acid-based and will produce a conversion coating on aluminum and its alloys. The coating formed by Alodine 1201 is gold to tan in color, and it becomes part of the aluminum surface. It offers good paint adhesion and corrosion resistance, but preserves electrical conductivity of the surface. While referred to as a coating, it is (like SurTec 650) actually a chemical treatment to the outer surface of the metal and will not materially change the dimensions of the workpiece. In some embodiments, it may be applied by brush or immersion.

(60) Alumiprep 33 is one product recommended for pre-immersion cleaning. Alumiprep 33 is a nonflammable phosphoric acid-based cleaner that produces a chemically cleaned and corrosion free aluminum surface. After cleaning the metal should be thoroughly rinsed with water. To apply Alodine 1201, the product may be used undiluted or diluted by mixing about 33 parts of Alodine with about 67 parts of water for each 100 parts of bath volume required. Room temperature to about 100 F. is the usual temperature of operation and the time of immersion may be about 2 to 5 minutes, depending upon how much coating is required. Colors of the coating produced will vary from light gold to a dark tan. After treatment, the workpiece should be thoroughly rinsed with water. As an aid to drying, the treated part may be heated or blown off with clean, dry, filtered, forced air, or it may be gently wiped with a dry, clean rag.

(61) Applicant manufactures products that are currently in use solving a wide variety of corrosion problems with non-hazardous polyurethane based products on commercial and military aircraft. This presents the results of salt fog exposure testing performed on various embodiments of Applicant's materials with coated mesh. Applicant's materials with coated mesh were tested by salt fog exposure in accordance with ASTM B117 to simulate accelerated real world application scenarios and to evaluate the effectiveness for corrosion prevention.

(62) Materials Under Test:

(63) Woven MeshAI 5056 18 strand/in each direction, 0.017 inch strand diameter, gel thickness 0.035 in.

(64) Test A1 Coated MeshCut before alodine treatment (strand ends coated) (FIGS. 32-35)

(65) Test A2 Coated MeshCut after alodine treatment (strand ends uncoated) (FIGS. 27-30)

(66) Test A3 Regular MeshConductive antenna gasket (uncoated control) (FIGS. 22-25)

(67) For Alodine Treatment:

(68) Mix 33 parts Alodine to 67 parts deionized water;

(69) Soak mesh sample in Alodine for 2-5 minutes room temperature;

(70) Dunk and rinse sample in deionized water; and

(71) Allow to air dry (room temperature and humidity) for 24 hours.

(72) In-Application Assembly

(73) The test samples were made by applying the test specimen between two test coupons. The test coupons are one 2024 T3 Aluminum coupon, representing typical aircraft skin material, measuring 44 and one 6061 T6 Aluminum coupon, representing typical antenna base material, measuring 33. Each test coupon was scuffed with a Scot Brite pad and cleaned with a paper towel moistened with isopropyl alcohol before use. Four 8-32 stainless steel fasteners were electrically isolated using heat shrink sleeving around each fastener shank and Kapton tape under the steel washers on both sides. The fasteners were tightened to 17 in*lbs and retightened after 15 minutes (see FIG. 12 for a standard test assembly). After the fasteners had been tightened a second time, resistance was measured. Following removal from the salt fog chamber, resistance was measured again.

(74) Each test assembly was photographed, the resistance was measured, and the samples were placed in the salt fog chamber. The specimen was supported or suspended between 15 and 30 off vertical so that one assembly would not drip on another. The assemblies did not contact each other or any metallic material. Test assemblies were exposed for a duration of 500 hours. At the end of each test duration, the test assembly was removed from the chamber, the salt deposits on surface were rinsed off, the resistance was measured, and the test assemblies were photographed before and after disassembly.

(75) The testing apparatus consisted of a closed fog chamber in which a solution of 5% sodium chloride is atomized by a nozzle thus producing a dense saline fog. The apparatus also contains a salt solution reservoir, a supply or compressed air, specimen supports, and a means for control. The test samples are exposed to this severe corrosive environment. The chamber is a Singleton SCCH 21 chamber and is accepted by ASTM-B-117.

Example B (SUR-TEC 650, FIGS. 10-25)

(76) SurTec 650 ChromitAL TCP Conversion Coating TreatmentSurTec 650 a reactive composition, provides a conversion coating for aluminum alloy such as 2, 5, 6, or 7,000 series aluminum alloy. SurTec 650 provides a coating that is free of hexavalent chromium. It may be disposed as normal waste. Coating may be by immersion. The time of immersion may be between about 1 to 5 minutes at a temperature preferably above 10 C. and below about 50 C. Pretreatment may range from about 30-40 C., temperature will influence bath time (warmer temperature lowers coating time). With a higher bath temperature for immersion, a shorter immersion time is sufficient. For good corrosion protection, for example, at 30 C., about four minutes (range about 2-6 minutes). Passivation (conversion) during dip (immersion) typically occurs at about pH 3.75 to 4 (may be adjusted with sulfuric acid or sodium hydroxide), concentration about 20% at about 30 C. for about five minutes. In one embodiment, the optimum coating weight for a conductive coating of SurTec is about 100 to 250 mg/dm2 square for EMI shielding (ASTM D-4935).

(77) SurTec 650 meets MIL-DTL-81706B and MIL-DTL-5541F for bare metal corrosion (336 h NSS per ASTM B-117). It produces a heat resistant inorganic passivation layer. Further details may be found in the following US patents incorporated herein by reference: U.S. Pat. Nos. 6,375,726; 6,511,532; 6,521,029; 6,527,841. The standard procedure for immersion typically requires degreasing, deoxidizing using, for example, a chromium-free liquid desmutting product (e.g., SurTec 495) and, if silicium is less than 1%, alkaline etching using an appropriate alkaline etching solution (e.g., SurTec 181). Between each step, the surface has to be rinsed. Prior to passivation with SurTec 650, the aluminum surface must be cleaned and deoxidized thoroughly. The surface must be water-break free.

(78) Materials Under Test:

(79) Woven MeshAI 5056 18 strand/in each direction, 0.017 inch, gel thickness about 0.035

(80) Test B1 SurTec Treated Aluminum MeshCut before SurTec treatment (FIGS. 13-16)

(81) Test B2 SurTec Treated Aluminum MeshCut after SurTec treatment\(FIGS. 18-21)

(82) Regular MeshConductive Antenna Gasket

(83) In-Application AssemblySame as Alodine

(84) The process to coat the mesh with SurTec is as follows, this was performed at room temperature:

(85) Using 3-5% alkaline cleaner, soak sample for 5 minutes, dunk sample in deionized water;

(86) Using 2-7% alkaline etcher, soak sample for 1 minute, dunk sample in deionized water;

(87) Using 17% desmutting for aluminum, soak sample for 3 minutes, dunk sample in deionized water;

(88) Using 15% SurTec 650, soak sample for 4 minutes, dunk sample in deionized water;

(89) Allow to dry (ambient temperature) for 24 hours.

(90) See FIGS. 10-25 for typical results, Applicant conductive gasket cut after SurTec treatment before testing and after 500 hours of salt fog exposure. Electrical resistance of the samples was measured at 1.67 m.Math. before testing and 2.85 m.Math. after 500 hours of salt fog exposure. Resistance value change compare to Alodine may be a result of extra gel thickness. Resistance may be measured before and after the salt fog testing by placement of the probes of a milliohmmeter at the points marked A and B in FIG. 12.

(91) Some of the environments which Applicant's novel gasket structures may use include antenna gasket (transmitting and/or receiving), for application to the surface of an aircraft, a fuel access door gasket (see U.S. Pat. No. 9,303,447 incorporated herein by reference) that may also be conductive and used with the conversion coated metallic gasket set forth herein.

(92) In addition to the novel conversion coated metallic mesh gasket, Applicant's polyurethane gel, as set forth in the applications herein by reference, may include corrosion inhibitors that are compatible with polyurethane. These may include Hybricor 204, a pigment grade organic/inorganic zinc phosphate silicate corrosion inhibitor available from WPC Technologies, Milwaukee, Wis., or Heucophos SAPP, a strontium aluminum polyphosphate corrosion inhibitor, pigment grade, available from Heubach GmbH, Langelsheim, Germany, or Huecos ZAPP, a zinc aluminum polyphosphate corrosion inhibitor available from Heubach GmbH. These three are known to be compatible with polyurethane (see specification sheets) and are among the known non-chromate corrosion inhibitors used with aluminum.

(93) See FIGS. 17-21 for typical results of the Av-DEC conductive gasket cut after SurTec treatment before testing and after 500 hours of salt fog exposure. Electrical resistance of the samples was measured at 0.38 m.Math. before testing and 0.54 m.Math. after 500 hours of salt fog exposure.

(94) See FIGS. 22-25 (control, no conversion coating) for typical results of the Av-DEC conductive antenna gasket after 500 hours of salt fog exposure.

(95) See FIGS. 26-30 for typical results Av-DEC conductive gasket cut after alodine treatment before testing and after 500 hours of salt fog exposure. Electrical resistance of the samples was measured at 0.92 m before testing and 1.32 m.Math. after 500 hours of salt fog exposure.

(96) See FIGS. 31-35 for typical results of the Av-DEC conductive gasket cut before alodine treatment before testing and after 500 hours of salt fog exposure. Electrical resistance of the samples was measured at 12.53 m before testing and 14.92 m.Math. after 500 hours of salt fog exposure. Resistance value change could be attributed to extra gel thickness.

(97) In one embodiment of Applicant's invention, an aluminum mesh is coated with one of the conversion coatings set forth herein and the gel is mixed with one of the corrosion inhibitors set forth herein. In one embodiment, the mixing of the gel and the referenced corrosion inhibitors may be according to Publication No. US-2016/0017143 (published Jan. 21, 2016), which describes the use of known off-the-shelf non-chromated corrosion inhibitors in tacky polyurethane gels. In another embodiment of Applicant's present invention, the non-chromated corrosion inhibitors are used in the polyurethane gel along with an uncoated or conversion coated metallic mesh, such as aluminum mesh gasket, wherein the mesh is not treated with a conversion coating. One such mesh is 5056AI 18 mesh available from Cleveland Wire or AvDec.

(98) TABLE-US-00001 500 Hour Salt Fog Corrosion and Resistance Summary Corrosion Resistance Delta (qualitative) (measured) Control (untreated) FIGS. 22-25 SurTec Pre-cut FIGS. 13-16 1.18 m .Math. .sup.1 Post-cut FIGS. 18-21 .sup..16 m .Math. Alodine Pre-cut FIGS. 32-35 2.39 m .Math. .sup.1 Post-cut FIGS. 27-30 .sup..40 m .Math. .sup.1Resistance value change could be attributed to extra gel thickness

(99) A copper alloy mesh was also tested with the SurTec conversion coating, applied as set forth with the aluminum alloy mesh. The results were good for both resistance and corrosion prevention, though not quite as good in corrosion prevention as the coated aluminum mesh. The resistance prior to salt fog was 0.48 milliohms and after testing was 1.33 milliohms, with a resistance Delta of 0.85 milliohms. A nickel mesh was tested with similar results.

(100) These tests show that conversion coatings help prevent corrosion of the aircraft parts and still provide a low resistance. Moreover, from a careful examination of the mesh itself, it appears that the mesh of the control has begun to be corroded slightly after 500 hours of salt fog, which was not observed on the coated meshes.

(101) Although the invention has been described in connection with the preferred embodiment, it is not intended to limit the invention's particular form set forth, but on the contrary, it is intended to cover such alterations, modifications, and equivalences that may be included in the spirit and scope of the invention as defined by the appended claims.