Appliques providing corrosion protection

Increasingly stringent environmental restrictions make it challenging to apply coatings (i.e., paint) by conventional processes like spray painting because of the volatile solvents and hazardous pigments. The environmental scrutiny is particularly focused upon conventional corrosion protection surface treatments, especially chromated primers and conversion coatings. We apply appliques to provide a vapor barrier over the substrate to provide corrosion protection. We can make curved appliques on a family of molds of different Gaussian curvature and thereby avoid making a "splash" mold of the surface of interest to create the appliques. Using curved appliques reduces ridges, creases, or gaps that sometimes otherwise occur with attempts to a cover a surface with complex curvature with flat (planar) appliques.

TECHNICAL FIELD
 The present invention relates to paint replacement films, especially
 corrosion protection surface coatings in the form of appliques. The
 appliques preferably include a protective film, preferably an elastomer,
 as a topcoat backed with a vapor barrier that is adhered to a substrate,
 like the exterior of an aircraft.
 BACKGROUND ART
 Painting has long been the process of choice for applying coatings to
 surfaces especially those having complex curvature. Painting is generally
 a controllable, reliable, easy, and versatile process. The paint can
 include additives to give the surface desired physical properties, such as
 gloss, color, reflectivity, or combinations thereof. The painting process
 is well understood and produces quality coatings having uniform properties
 even when the surface includes complex curvature. Unfortunately, painting
 is falling under closer environmental scrutiny because they use volatile
 solvents to carry the pigments or because of the pigments themselves.
 Therefore, there is a need to replace the painting process with a process
 that has less environmental impact. Furthermore, while painting is well
 defined, well understood, and common, it remains an "art" where masters
 produce better products than novices or apprentices without necessarily
 being able to account for why or to teach others how.
 Painted surfaces sometimes lack the durability that quality-conscious
 customers demand. The surface must be treated and cleaned prior to
 applying the paint. The environment surrounding the part must be
 controlled during the coating application, often requiring a spray booth,
 Painted coatings are also vulnerable to damage like cracks or scratches.
 Isolated damage may require the repair of a large area, such as forcing
 the repainting of an entire panel.
 Spraying inherently wastes paint and is unpredictable because of the "art"
 involved with the application. Improper application cannot be detected
 until the spraying is complete, then rework to correct a defect usually
 affects a large area even for a small glitch.
 U.S. Pat. No. 4,986,496 by Marentic et al. describes a drag reduction
 article in the form of a conformable sheet material (a decal) with surface
 texturing for application to aircraft flow control surfaces to reduce
 aircraft drag. The material fits on curved surfaces without cracks,
 bubbles, or wrinkles because of the paint-like properties of the basic
 carrier film. Marentic's decals are manufactured flat and are stretched to
 the intended simple curvature. Stretching can be problematic over time if
 the stretched material shrinks to expose a gap between adjacent decals
 where weather can attack the decal-surface interface. Stretching generally
 limits Marentic appliques to surfaces of slowly changing curvature. We
 incorporate this patent by reference.
 Appliques (i.e. decals) are also described in U.S. Pat. No. 5,660,667
 Davis, which we incorporate by reference. Having complex curvature, the
 appliques form complete, bubble-free, wrinkleless coverings on surfaces of
 complex curvature without significant stretching. Davis applies these
 appliques by:
 (a) analyzing and mapping the Gaussian curvature of the surface to be
 covered to identify lines of constant Gaussian curvature;
 (b) identifying geodesic lines on the surface, such that the lines of
 constant Gaussian curvature and the geodesics form a mapping grid on the
 surface;
 (c) analyzing the stretchiness needed to blend between appliques of
 adjacent areas of different Gaussian curvature;
 (d) producing appliques for each Gaussian curvature using a family of
 molds;
 (e) identifying on the surface the grid made up of the lines of constant
 Gaussian curvature and intersecting geodesics; and
 (f) applying appliques of a particular Gaussian curvature along the
 matching line of constant Gaussian curvature on the surface to produce a
 complete, bubble-free, wrinkleless covering on the surface comparable to a
 conventional painted coating and while minimizing stretching of any
 applique to complete the coating.
 Identifying the grid can include physically marking the lines, displaying
 them with an optical template, or simply defining them in a 3-dimensional
 digital data model for the surface.
 The Davis method recognizes that surfaces having the same Gaussian
 curvature can be mapped topologically to correspond. If you have a surface
 of Gaussian curvature 5 ft.sup.-2, for example, instead of making a
 "splash" mold of the surface to make appliques, you mold appliques to
 curvature 5 ft.sup.-2 on a master curvature 5 ft.sup.-2 mold, which, for
 example, might be a sphere. Appliques from the master mold will fit
 bubble-free and wrinkleless on the actual surface.
 Often surfaces must be protected against corrosion. Such protection
 commonly involves surface treatments or primers (i.e. chromated primers or
 conversion coatings) that are relatively expensive because of the
 chemicals involved and the time associated with their application. These
 traditional coatings are relatively heavy, especially when coupled with
 other surface coatings that must be applied over the corrosion protection
 coating to provide color, gloss, enhanced surface durability, abrasion
 protection, a combination of these attributes, or other attributes. The
 chemicals used in conventional corrosion protection coatings often are
 hazardous materials.
 Appliques are of considerable interest today for commercial and military
 aerospace applications. Lockheed Martin and 3M are conducting flight tests
 on paintless aircraft technologies. These appliques (like ours) save
 production costs, support requirements, and aircraft weight while
 providing significant environmental advantages. The Lockheed Martin
 appliques are described in greater detail in the article: "Paintless
 aircraft technology," Aero. Eng'g, November 1997, p. 17, which we
 incorporate by reference. Commerical airlines, like Western Pacific, use
 appliques to convert their transports into flying billboards. We seek
 durable appliques that can replace conventional military or commercial
 aviation paint systems to reduce lifecycle costs, improve performance, and
 protect the underlying surfaces from corrosion.
 SUMMARY OF THE INVENTION
 The present invention combines a surface coating via an array of appliques
 with a vapor barrier to provide corrosion protection. The appliques may
 provide adequate corrosion protection to eliminate altogether conventional
 surface corrosion protection treatments, thereby, saving weight and
 reducing environmental concerns. Alternatively, the combination of
 applique corrosion protection with environmentally friendly but relatively
 inferior, chromate-free conversion coatings may replace the
 environmentally sensitive, traditional corrosion protection techniques
 (i.e., chromated conversion coatings and primers).
 Corrosion on metal surfaces or around metal fasteners in resin composite
 structures produces oxidation that reduces the surface quality and that
 frequently can make the structural integrity suspect. Maintenance to
 correct corrosion or to ensure that it does not occur is costly because it
 is labor-intensive. A more reliable corrosion protection system would find
 widespread acceptance in commercial and military aerospace.
 In addition to the corrosion protection, the vapor barrier can be
 beneficial on aerospace structure to limit the migration of water through
 a structure. For example, with composite honeycomb sandwich structure, a
 vapor barrier applique coating can slow or eliminate the migration of
 water through the laminated face sheets into the honeycomb core.
 Preferred appliques provide corrosion resistance to the underlying surface
 because they incorporate an intermediate vapor barrier. Preferred
 appliques have a 1-8 mil fluoroelastomer or other polymeric film as a
 topcoat (generally 2-6 mil), a vapor barrier typically about 1-4 mil thick
 (generally, 3 mil), and a 2 mil adhesive, typically pressure sensitive or
 thermally activated.
 When making precision coatings that are important for aerodynamic drag and
 other considerations on modern commercial and military aircraft, spray
 painting is a relatively unreliable process because it is difficult to
 control the spray head and spraying conditions to obtain precisely the
 same coating from article to article. One variable in this spray process
 that often is overlooked is the natural variation from article to article
 in the vehicle to which the paint is applied. Such variation results from
 the accumulation of tolerances (i.e., the accumulated variation that
 results from variations within allowable control limits for each part in
 the assembly). The applique method allows better control of the
 manufacture of the coating so that it will have the correct spectral
 properties by distributing pigments, additives, and thin films properly
 throughout the applique and, thereby, over the surface. The benefits of
 appliques are further enhanced if the appliques simultaneously provide
 corrosion protection. Difficulties in precisely manufacturing painted
 coatings to obtain the desired properties can be overcome without the cost
 of either scrapping an entire article because the coating is imperfect and
 inadequate or forcing costly stripping and reapplication of the coating.
 Using appliques allows small area repair of the precision coatings on
 aerospace surfaces by simply cutting away the damaged area and reinserting
 a suitable, fresh applique patch. With paint, the spray transition between
 the stripped area and the original coating in such a repair is
 troublesome. For example, an entire panel usually needs to be re-coated
 with paint to fix a small area defect. Operations like paint spraying,
 surface preparation, masking or otherwise isolating the repair area, and
 the like slow the repainting process.
 For thin appliques, we recommend use of single or double transfer
 protective paper to facilitate their application. One sheet of protective
 paper overlies the surface of the applique that will interface and bond
 with the article. This surface has an adhesive or may have inherent
 tackiness to allow it to stick to the metal or composite aircraft surface.
 The exposed surface may have similar protective paper to reinforce it and
 to protect it during the positioning and transfer with peeloff following
 proper positioning. Identifying information and instructions can be
 painted on the transfer papers to simplify application of the appliques.
 Accordingly, the present invention relates to a corrosion protection
 applique for applying a substantially complete, bubble-free, wrinkleless
 coating to a surface. The applique has a vapor barrier to reduce
 substantially or to eliminate transport of water to the surface and an
 adhesive on at least one face of the vapor barrier for adhering the vapor
 barrier to a surface.
 The present invention also pertains to a paintless coating system for
 replacing conventional paints on metal or composite aerospace parts and
 assemblies, comprising a topcoat, a vapor barrier interfacing with and
 completely underlying the topcoat, and an adhesive for adhering the vapor
 barrier to the parts.
 A method of the present invention replaces conventional painted coatings on
 metal or composite aerospace parts or assemblies with a replaceable,
 resealable protective covering that, preferably, provides significant
 corrosion protection by stopping the migration of moisture. The method
 involves:
 (a) cutting gores of a vapor barrier into a plurality of appliques suitable
 for covering a predetermined surface of the part;
 (b) adhering the gores to the part; and
 (c) optionally, sealing between gores at edge seams to provide a continuous
 vapor barrier between the part and its environment.
 On bare clad A1 2024, the vapor barrier provides equivalent corrosion
 protection to a part having a conventional paint, a chromated conversion
 coating, and a chromated primer meeting military specifications.
 The present invention also relates a method for sealing adjacent appliques
 on a substrate to achieve an essentially continuous vapor barrier. First,
 we define a seam by positioning two appliques on a substrate adjacent one
 another, each applique including a vapor barrier made from a polymer.
 Then, we apply a sealing applique having a vapor barrier over the seam to
 form a lap joint between the sealing applique and the positioned
 appliques. Optionally, we seal edges of the sealing applique with polymer
 to bind the sealing applique to the positioned appliques.
 In one other aspect, the present invention relates to a method for
 essentially stopping the progress of corrosion at a site on an aircraft,
 comprising the step of applying a vapor barrier in the form of an applique
 over the site to eliminate transport of water to the site.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT
 U.S. Pat. No. 4,986,496 teaches the making of flat appliques for covering
 flow control surfaces, and the applique manufacturing techniques are
 applicable to the present invention. U.S. Pat. No. 5,660,667 (Davis)
 describes the manufacture of curved appliques especially suited for use on
 complex (i.e. compound) curved surfaces common in aerospace. We typically
 form a vapor barrier into sheetstock and, then, roll coat the topcoat and
 adhesive onto this film.
 The external film or topcoat 20 (FIG. 2) for the applique 10 is typically
 an organic resin matrix elastomeric composite, particularly a
 fluoroelastomer about 0.001-0.004 inch (1-4 mils) thick. A vapor barrier
 30 (particularly a fluorinated terpolymer, a metallized polymer,
 especially one having an aluminum thin film, or another fluoropolymer),
 and an appropriate adhesive 40, especially a pressure-sensitive or
 thermally activated adhesive (particularly 3M's 966 adhesive) applied as a
 separate layer complete the three layer structure of our preferred
 appliques. The adhesives are commonly acrylic-based materials or rubbery
 polymers or copolymers. The fluoroelastomer should be tough, durable, and
 resistant to weather.
 The adhesive should provide complete adhesion between the vapor barrier and
 the underlying substrate. In addition, it should be slow to absorb water.
 The vapor barrier is the key to our corrosion protection enhancement by
 eliminating the transport of water to the metal surface. The vapor barrier
 should be durable to provide long life in the field. It should be stable
 in hot-wet conditions up to at least about 250.degree. F. It should be
 tatterable so that it will shred to limit the progress of rips and peels
 that occur during use. It should be peelable by stretching for removal,
 when desired, for inspection or replacement, but it should remain adhered
 during flight.
 The topcoat should provide increased durability and hardening to the vapor
 barrier. It can provide anti-static properties to the applique paintless
 coating by including dispersed carbon or graphite fibers. The topcoat
 provides color and gloss through appropriate pigments. It should be
 markable so that removable indicia can be imprinted on the topcoat. It
 should be UV-resistant. Our fluoroelastomer satisfies these criteria as
 well as any materials we have found.
 The topcoat and vapor barrier combination should be durable but also should
 tear and tatter in the event that a peel initiates during flight.
 The appliques can be protected with single or double transfer protective
 paper (50, FIG. 2) to facilitate their application. One sheet of
 protective paper overlies the surface of the applique that will interface
 and bond with the article. This surface has an adhesive or inherent
 tackiness to allow it to stick to the metal or composite aircraft surface.
 For very thin appliques, the exposed surface of the topcoat may also have
 similar protective paper to reinforce it and to protect it during the
 positioning and transfer. We peel off this protective paper following
 proper positioning. Identifying information and instructions about how,
 where, and in what order to apply the appliques can be printed on the
 transfer papers (or directly on the topcoat of the applique) to simplify
 their placement and positioning.
 The benefits of paintless coatings for aerospace include: (1) reduction of
 hazardous materials and waste both during initial application and during
 stripping and replacement, (2) mitigation of corrosion which would in turn
 reduce requirements for corrosion inspection, repair, and replacement, (3)
 potentially increased aircraft life, and (4) significant lifecycle cost
 savings for application and maintenance. As shown in FIG. 11, concurrent
 maintenance can occur on the aircraft 110, in the cockpit, for example,
 while the appliques are inspected, repaired, or replaced. While the
 curvature dictates the size and shape of an applique, a typical applique
 120 applied to the upper wing skin 130 might be rectangular as shown in
 FIG. 12. To replace paint, the appliques cover all, substantially all, or
 merely a part of the aircraft surface where paint would be used. Hot areas
 or areas particularly prone to erosion might require traditional
 treatments or coatings in addition to the common appliques.
 The gores are generally 2-dimensional, flat panels that are sized to
 conform to a 3-dimensional surface, similar to the sections of a baseball.
 During installation, trimming often is required for achieving the final
 fit. The gores may have different thicknesses depending upon their
 intended location on the object. We use thicker gores in areas exposed to
 high wear or in impact zones.
 Decals and appliques normally are manufactured as flat material that is
 flexible and readily bent. Material of this form can easily be applied to
 both flat surfaces and simple curved surfaces such as cylinders, cones,
 and rolling bends. More complicated surfaces involving compound curvature
 can only be covered if the material can be stretched or compressed to
 avoid wrinkling and tearing. If the material is not sufficiently elastic,
 cutting to permit overlapping, or wedge removal, as well as addition of
 darts, can be useful to extend coverage with a nominally flat applique or
 decal material. Such approaches can be time consuming, damaging to the
 applied material, and of questionable use if the material has any
 preferred orientation (as, for example, with riblets.)
 Presuming that a material is somewhat elastic, Davis describes that a decal
 graded according to Gaussian curvature (GC) would be suitable for surfaces
 within a certain range of Gaussian curvature. A given complex curved
 surface can be divided up into zones with corresponding Gaussian curvature
 ranges. Within each zone a single premolded decal can be used. As with
 surfaces suitable for covering with flat materials, each zone could
 involve a great variety of surface shapes subject only to the specified
 range of Gaussian curvature.
 We generally make the appliques entirely from flat (GC=0) material and
 accommodate curvature by the inherent stretchiness and resilience of the
 appliques. Our appliques are primarily made from fluoroelastomers that are
 relatively forgiving and easy to work with. Molded appliques like Davis
 suggests may be desirable for surfaces where the curvature is changing
 rapidly, but they generally are not required.
 Our studies of paintless coatings achievable with appliques included
 evaluation of many different films, coatings, and adhesives. We selected
 vapor barrier films and humidity resistant adhesives for use in our
 paintless coatings. We evaluated the ability of vapor barrier films to
 improve corrosion protection significantly compared with paint. Our
 hypothesis was that corrosion of metal and other surfaces is inhibited by
 preventing the dynamic transport of water to and from the surface.
 Potentially, selective use of our vapor barrier films could prevent or
 mitigate internal aircraft corrosion. They could also slow or prevent the
 migration of water through resin composite laminated face sheets 102 into
 underlying honeycomb core 106, a problem that leads to excessive weight.
 (FIG. 52).
 Our evaluation tests included standard salt immersion and salt spray with
 scribed and unscribed panels. We followed changes in the surfaces as a
 finction of salt exposure using microscopy and electrochemical impedance
 spectroscopy (EIS). These tests indicated outstanding corrosion protection
 (negligible change in the surfaces) of panels coated with a vapor barrier
 adhered to the surface during and following completion of 2,000 hours of
 salt spray exposure, while significant corrosion damage occurred to the
 painted surfaces that we used as a control for comparison. Many of the
 scribed test panels with these appliques over MIL-P-85582 (a chromated
 epoxy) primer showed little or no observable degradation of the surface or
 the scribe line. We also have demonstrated benefits of a paintless coating
 on untreated aluminum of various types to panels which were chemically or
 electrochemically treated, and plan to test performance of the appliques
 when the surface are treated with various non-chromated primers. These
 tests indicate that vapor barriers provide surface corrosion benefits. It
 may be possible to forego primers altogether while maintaining improved
 corrosion protection.
 Davis suggests that flat material can wrinkle or tear when applied to
 surfaces of complex curvature because the material is insufficiently
 compressible or stretchable. While darts or wedge removal, like the
 techniques used in tailoring clothes, does permit some contouring to
 complex curvatures, these tailoring techniques require complicated
 planning and skilled labor to produce a seamless, complete, bubble-free,
 and wrinkleless coating. It, too, wastes material and does not deal with
 the unique irregularities of an actual article. That is, tailoring
 presumes that each article of the same nominal type will have identical
 surface contours. In reality, with hardware as complex as aircraft, each
 aircraft has subtle but significant differences in their surface curvature
 and characteristics. These subtle changes dictate individual tailoring
 rather than mass production.
 If we elect to make curved appliques like Davis recommends, we make an
 article-by-article evaluation of the surface curvature to identify lines
 of constant Gaussian curvature. Otherwise, we analyze the surface
 curvature to design flat gores of appropriate size and shape to cover the
 surface (FIG. 51). This analysis is simplified to some degree if the
 article is designed to permit digital preassembly of solid models of the
 respective parts (as available for Boeing's 777 aircraft), but the
 curvatures can be identified as well using profilometry with conventional
 laser coordinate measuring apparatus, photogrammetry, or the like. Surface
 profiles permit identification of the actual curvature of the surface of
 interest rather than the theoretical curvature that the design data
 suggests. Profilometry likely is necessary for precise coatings. The
 equipment to plot the profile also is useful for the marking of lines of
 constant Gaussian curvature and geodesics on the surface of interest so
 that the respective appliques can be laid down in a "color-by-number"
 process. By "marking," we mean that the locale for each applique is
 identified. Such marking can be done with projection lights or with more
 traditional marking methods (chalklines, pencil, etc.).
 The surface analysis allows us to decide the size and shape of applique
 gores needed to cover the surface of interest. It also allows us to decide
 which appliques will be made from flat sheetstock and which will be molded
 to a complex curvature. We determine the order in which we will apply the
 gores and can apply numbers or other instructions to the applique itself
 or to the transfer paper to order gores in the coating kit. Curved
 surfaces may dictate curved appliques or smaller, flat appliques that can
 accommodate the curvature. We prefer to make each applique as large in
 area as possible while still having the appliques be easily handled by a
 single worker. Large area appliques reduce part count in the kit. Our
 appliques are generally two-four feet wide and five to eight feet long,
 although the size and shape can vary depending on the shape and curvature
 of the surface to which the appliques are applied. One pattern of
 appliques is shown in FIG. 50 wherein the alphanumeric designations
 identify separate gores. Our appliques typically have considerable
 stretchiness, especially if they are thin, so they can conform to curved
 surfaces.
 Gaussian curvature is a surface property for measuring of compound
 curvature. This topic is normally discussed in texts on differential
 geometry and is not widely known in the engineering community. The concept
 is best understood by considering a mathematical plane that includes the
 surface normal vector at a particular point on a curved surface. The curve
 formed by the intersection of the plane with the curved surface is known
 as a normal curve. If the plane is spun around the axis defined by the
 surface normal, an infinite family of normal curves is generated. In some
 particular orientation, a maximum curvature will be obtained. A surprising
 result from differential geometry is that a normal curve with minimum
 curvature occurs when the plane is turned by 90.degree.. These two
 curvatures are known as principal curvatures, and can be used to describe
 the curvatures for other normal plane orientations via a simple formula.
 Each principal curvature can be expressed as the reciprocal of the local
 radius of curvature. The Gaussian curvature is simply the product of the
 two principal curvatures. Two elementary examples help to illustrate the
 concept. For a point on a cylindrical surface, one principal curvature is
 zero (that is, travel along the surface in the direction of the
 longitudinal axis is travel on a straight line). The Gaussian curvature is
 also, zero, since it is the product of the principle curvatures where one
 principle curvature is zero. The Gaussian curvature is also zero for all
 other surfaces that can be formed by bending a flat material, since these
 shapes can be transformed into one another.
 Another simple example is a sphere. The entire surface has a Gaussian
 curvature equal to the inverse square of the radius. Saddle-shaped
 surfaces will have a negative Gaussian curvature since the centers of
 curvature occur on different sides of the surface. In the most general
 case, the Gaussian curvature will vary across a surface. A good example of
 the more general case is a (football-like) prolate ellipsoid, which has
 its highest Gaussian curvature at its ends.
 A decal or applique with a particular Gaussian curvature (GC) can be formed
 on a symmetrical mold such as a sphere (or symmetric saddle). Provided
 that it is flexible, the applique or decal will fit without wrinkling onto
 any other surface with the same GC, even if it is bent and asymmetric. The
 molded material in this case also can be applied on the actual surface in
 any desired orientation rather than in a particular orientation (like a
 jigsaw puzzle piece would require). If the material is able to stretch (or
 to compress), it should be suitable for covering some range of GC values.
 An ellipsoidal mold can be used to create transitional decals which have a
 gradient (i.e., a known variation in GC).
 Premolded appliques can be applied to aircraft markings on complex curved
 surfaces and offer an alternative to painting. While valuable on
 commercial aircraft, appliques are especially well suited to military
 aircraft where there is a need to change camouflage and other low
 signature coverings to suit the theater of engagement. Appliques could be
 commercially valuable in many other areas, such as automobiles, boats, and
 other commercial products.
 Davis describes ellipsoidal mold that has lines of constant Gaussian
 curvature in a symmetrical pattern running from the center to the ends.
 The lines are "straight" lines on the surface that extend parallel to one
 another in a transverse direction on the ellipsoidal mold. The lines
 correspond with global lines of latitude on common maps. Geodesics marked
 on the surface extend longitudinally in graceful curves from pole to pole
 analogous to lines of longitude on global maps. Davis's appliques are
 centered on each constant GC line, and usually are diamond-shaped. We can
 use a similar plotting protocol to position our flat gores in the
 appropriate location and orientation.
 For purposes of this discussion, a geodesic is the shortest line extending
 on the surface between two points. On a sphere, a geodesic would be the
 "great circle" connecting the two points. A geodesic has a curvature
 vector equal to zero and has the principal normal coincide with the
 surface normal.
 Davis's appliques having one nominal GC are placed along the corresponding
 line of constant GC while appliques having a different nominal GC are
 placed along their corresponding lines of constant GC. The bodies of the
 appliques stretch to make the transition between curvatures. The ends of
 an object often are covered with relatively large cup or tulip shapes. The
 various appliques fit together to cover the entire surface without
 wrinkles, gaps, or bubbles.
 Appliques of constant Gaussian curvature can be made on a mold and
 transferred to aircraft, boats, trucks, or the like by placing applique on
 lines of corresponding GC on the surface of interest. Other appliques are
 selected and placed in similar fashion to cover the entire surface. Each
 applique has substantially one Gaussian curvature along one
 characteristic, primary axis and transitional fingers or extensions of the
 applique extending outwardly from the primary axis. The fingers have
 varying Gaussian curvature because they stretch or because of their
 molding for placement along the geodesics.
 The primary size of the appliques depends on the severity of the curvature
 of the surface they will cover. Smaller pieces are required if the
 gradient of the curvature is large, that is, where the GC changes over a
 short distance. Flat appliques of GC 0, of course, can be used for
 cylindrical solids, flat surfaces, and any other large areas of GC 0. A
 family of molds of differing size would supply appliques of positive GC. A
 similar saddle mold family provide corresponding appliques having negative
 GC's.
 The appliques can be applied wet or dry using squeegees, mat knives, rubber
 rollers, wallpaper tools, and the like to place and smooth the films.
 Extracting the trapped air or water with a hypodermic syringe eliminates
 bubbles. Interfacing appliques usually are overlapped 1/4 to 1/2 inch or
 more, but butt joints are possible. The extent of overlap is limited
 because of weight and cost factors but also because the appliques stick
 more securely to the substrate than to one another. Overlaps can be a
 source of peeling in flight, because of the poorer applique-to-applique
 adhesion.
 As described in U.S. Pat. No. 4,986,496, the appliques can include surface
 patterns, and might include plasticizers, extenders, antioxidants,
 ultraviolet light stabilizers, dyes, pigments, emissivity agents (like
 silicon carbide), chopped or continuous fiber reinforcement, or the like,
 to provide the desired color, gloss, reflectivity, or other surface
 characteristics. Chopped fibers can provide improved toughness and
 anti-static properties, for example.
 Generally the pigments are metal flakes, metal oxide particles, or
 organometallic particles, and typically are mixtures of several types of
 material. Suitable aluminum flake pigments include the Aquasil BP series
 of pigments available form Siberline Manufacturing Co. The pigments might
 be glass, mica, metals (like nickel, cobalt, copper, bronze, and the like
 available from Novamet) or glass flake, silver coated glass flake, mica
 flake, or the like available form Potters Industries, Inc. These flakes
 typically are about 17-55 .mu.m for their characteristic dimension. In
 some applications, ceramic pigments may be appropriate. Of course, the
 pigments can be mixed to provide the desired characteristics for the
 coating.
 Titanox 2020 titanium oxide pigments are available from NL Industries.
 Copper oxide or iron oxide pigments are available from Fischer Scientific.
 NANOTEK titania, zinc oxide, or copper oxide pigments are available from
 Nanophase Technologies Corporation. These pigments are generally spherical
 with diameters in the range form about 30 nm (for the NANOTEK pigments) to
 micron sizes.
 Preferred pigments are essentially pure metals (with suitable surface
 conversion coatings) having a thickness of about 1000 .ANG..+-.5-10%
 (i.e., 900-1100 .ANG. and, preferably, 950-1050 .ANG.). These pigments
 otherwise should meet the conventional specifications for paint pigments.
 In that regard the pigments (also called particulates or flakes) must be
 thick enough to provide opacity while producing minimum edge effects
 (scattering). A characteristic dimension, then, for either the length or
 width would be 20-100 .mu.m, and, preferably, 30-50 .mu.m. We target
 particulates of characteristic nominal dimensions of 50 .mu.m.times.50
 .mu.m.times.1000 .ANG. (i.e. 0.1 .mu.m).
 Films of the pure metals of the desired thickness can be prepared by
 sputtering the metal onto two mil thick fluorinated ethylene propylene
 (FEP) sheetstock. Making this film product is done according to the
 conventional processing steps for making food or vacuum bagging materials.
 The method of the present invention removes the metal from the metallized
 film in two, simple and quick immersion steps. First, the metallized roll
 is immersed in a caustic (basic) bath for about 15 sec to loosen the
 metal. Then, we immerse the roll again for about 15 sec in a dilute acid
 solution to neutralize the base and to separate the metal. We brush the
 particulates from the FEP, and precipitate the particulates in the acid
 solution prior to filtering, rinsing, and drying.
 To separate the metal from the FEP, we generally contact the metal with
 counter rotating cylindrical nylon bristle brushes. We sometimes use
 ultrasonic vibration alone or in combination with the brushing. For
 brushes, we prefer 3 inch nylon bristle (0.010) diameter) spiral wound
 brushes available from Richards Brush Company.
 For aluminum thin films, we prefer to use 7 wt % Na.sub.2 CO.sub.3 as the
 base, but can use NaHCO.sub.3, NaCO.sub.3 /NaHCO.sub.3 mixtures, or
 conventional alkaline or alkaline earth hydroxides diluted to about a pH
 of 9.0. The acid solution preferably is 0.01-0.1 N acetic acid at pH
 3.4-3.6, but could be phosphoric acid or a dilute mineral acid.
 For germanium thin films, we prefer to use 2.5 N NaOH as the base with
 acetic acid or with ultrasonic vibration replacing the acid solution.
 The base immersion takes about 15 seconds. Prior to the acid immersion, we
 allow the base-treated metallized film to be exposed to air for about 25
 seconds. The acid immersion lasts about 15 seconds before we brush the
 particulates from the FEP. We tow the metallized roll through the several
 operations in a continuous process, as will be understood by those of
 ordinary skill.
 We monitor the pH of the acid tank with conventional pH or ORP meters and
 add acid as necessary to maintain the desired pH and redox potential.
 We recover the particulates from the acid bath by filtering, rinsing, and
 drying. We size the particulates. Then, we conversion coat the
 particulates using convention aluminum treatments like chromic acid
 anodizing, phosphoric acid anodizing, Alodine treating (particularly using
 either alodine 600 or alodine 1200); cobalt-based conversion coating as
 described in Boeing's U.S. Pat. Nos. 5,298,092; 5,378,293; 5,411,606;
 5,415,687; 5,468,307; 5,472,524; 5,487,949; and 5,551,994; or sol coating.
 The sol coating method creates a sol-gel film on the surface using a mixed
 organozirconium and organosilane sol as described in Boeing's U.S. Pat.
 No. 5,849,110, or U.S. Pat. No. 5,789,085. We incorporate by reference
 these Boeing patents.
 The different treatments can impart different tint to the flakes. Alodine
 imparts a yellow or greenish-yellow tint. The cobalt treatments impart
 blue tints.
 The sol coating is preferably a mixture of organometallics wherein the
 zirconium bonds to the aluminum flake covalently while the organic tail of
 the organosilane bonds with the paint binder. The anodizing treatments
 prepare the surface to achieve adhesion primarily by mechanical surface
 phenomena. The sol coating provides both mechanical adhesion (surface
 microroughening) and adhesion through chemical affinity, compatibility,
 and covalent chemical bonds.
 The topcoat forms a protective film over the vapor barrier, and should be
 selected from suitable materials to retain the corrosion protection
 properties of the applique system. The corrosion protection performance is
 illustrated in FIGS. 3-10 and 22-32 for our preferred vapor barrier. Even
 if the appliques are not optimized for eliminating corrosion, the applique
 coating should still improve lifecycle costs and maintenance by allowing
 simpler coating replacement and zonal overhaul (concurrent maintenance) of
 the aircraft in its regular depot maintenance. Engines can be overhauled,
 for example, on one side of the aircraft while inspection, patching, and
 repair of the paintless coating can proceed on the other side of the
 aircraft as shown in FIG. 11. Normal paint repair requires that the
 aircraft be isolated in a spray booth where other maintenance or
 inspection cannot be conducted simultaneously.
 The preferred topcoat is a fluoroelastomer, especially a modified CAAPCOAT
 Type III or Type IV rain and thermal resistant fluoroelastomer available
 from the CAAP Company suitable for roll coating in the appropriate colors
 and with appropriate additives as previously described. The preferred
 vapor barrier is a fluoropolymer from 3M, especially a terpolymer derived
 from tetrafluoroethylene, hexafluoropropylene, and vinylidine fluoride
 (THV). Metallized thin-film vapor barriers have also shown promise in this
 application, especially aluminum vapor depositions. The vapor barrier's
 function is to eliminate active transport of water vapor or other
 corrosive agents to the surface. The preferred adhesive is a
 pressure-sensitive acrylic adhesive designated as product 966 or other
 experimental adhesives available from 3M. The adhesive should hold the
 appliques on the surface during normal operation of the vehicle, but
 should be peelable without leaving a residue for replacement of the
 applique inspection of the underlying surface. It should have low
 eletrolytic (ion transporting) properties for the best corrosion
 performance. Additives common used in adhesives to improve tack might
 degrade the corrosion protection. The applique may be re-adhered to the
 surface is some cases, especially if the area uncovered is small.
 Pigments and other additives can be incorporated into the topcoat, vapor
 barrier, or both. An anti-static layer generally is incorporated into the
 exposed surface.
 Seams between appliques in lap joints or butt joints are sealed with a seal
 bead 400 made from topcoat applied like caulk to adhere the adjacent
 appliques together, as shown in FIGS. 40 to 45 for lap and butt joints
 with flat and tapered edge appliques. FIGS. 44 and 45 show sealing
 arrangements using seam tapes. In FIG. 45, the topcoat 20 is removed so
 that the tape adheres to the vapor barrier 30.
 A thicker vapor barrier or multiple vapor barrier layers might assist in
 its retaining its corrosion protection integrity. Typically the vapor
 barrier is about 1 to 4 mils thick, and generally 3 mils. Thicker films
 add weight, but the appliques are still likely to be lighter than multiple
 paint coats that are commonly used today. The appliques initially are
 about the same weight to slightly heavier than an ordinary, single coat,
 paint-primer coating system, like MIL-C-85285 polyurethane over
 MIL-P-25377 epoxy primer.
 The vapor barrier might include a metallized film on one or both surfaces
 (generally, on the surface adjacent the adhesive, if metallization is
 used). Such barriers appear to provide significant corrosion protection,
 perhaps by providing a sacrificial film, but, more likely, by reducing the
 permeability of the organic resin film that otherwise constitutes the
 barrier.
 The appliques have the potential to eliminate the need for chromated
 primers on the substrates. For example, when tested on clad 2024 T3
 aluminum alloy test plaques, the appliques provided equivalent corrosion
 protection to using both a chromated primer and a chromated conversion
 coating on the 2024 aluminum. Comparative results for filiform and salt
 spray tests are shown in FIGS. 33-39. In all our tests, the appliques were
 never equivalent to and typically were better than paint in providing
 corrosion protection.
 We believe that the appliques can be used on most aerospace metals,
 including 2024, 6061, 7075, and other aluminum alloys; all titanium
 alloys; high strength (low carbon) steels like 4130, 4340, and 9310;
 nickel alloys like INCONEL 718, and magnesium alloys protected with a Dow
 conversion coating. Our tests have focused on 2024 and 7075 aluminums,
 which are the standard materials used to assess corrosion protection. In
 addition, the appliques can be used on composite structures. At the
 interface between carbon fiber-reinforced composites and metallic
 structure, the appliques reduce galvanic corrosion by reducing access of
 electrolytes to the metal surfaces. That is, the appliques seal moisture
 and aircraft fluids away from the metals (conductors).
 The substrates are clad. They can be anodized and treated with a chemical
 conversion coating, especially a chromated conversion coating like Alodine
 600, 1000, or 1200. Our tests with nonchromated primers have shown uneven
 or poor corrosion protection performance, but the fault lies with the
 nonchromated primers. We speculate that the primer in these tests is
 attracting and capturing corrosive agents in contact with the metal
 surface. We achieve better results by eliminating the primer altogether.
 Standard filiform corrosion tests show that the corrosion does not progress
 from its original state after the corrosion is covered with an applique.
 This fact means that an applique over the corrosion can stop minor
 corrosion pitting.
 We conducted rain erosion tests at the Univ. of Dayton for the appliques
 and discovered that the best edge seal was filled with chopped fibers to
 improve its strength and resistance to tearing. We also learned that the
 appliques were comparable to or far better than standard coatings. The
 appliques provided protection at 500 mph comparable to special rain
 erosion coatings in some conditions. We noticed delamination between the
 topcoat and vapor barrier on several test specimens. Lap joints and butt
 joints had comparable survivability. Tapered edges out performed flat
 edges. The appliques appear to provide at least the equivalent protection
 as paint even without adding a special erosion coating.
 In patching areas, it may be desirable to create a butt joint with the
 vapor barrier layers while cutting back the topcoat. A thinner vapor
 barrier-topcoat film may fill the area over the vapor barrier where the
 topcoat is selectively cutaway, as shown in FIG. 45. In this way, a vapor
 barrier bridges the gap where the adjacent vapor barriers abutt, thereby
 providing a continuous vapor barrier.
 Edges of the appliques preferably are tapered (FIGS. 40, 41, and 44) to
 improve aerodynamics.
 Repair of the applique coating requires cutting through the appliques,
 preferably without scribing the underlying substrate. To cut the
 appliques, we need a controlled depth, adjustable cutter. Setting the
 depth of cut and holding that depth is a challenge, especially when
 working with depths measured in mils (0.001 in). We control depth using a
 rolling cutter that has a follower wheel to ride on the substrate behind
 the cut to set the depth of cut.
 EXAMPLE 1
 The Electrochemical Impedance Spectroscopy (EIS) system we used for our
 tests included an EG&G Princeton Applied Research () model 273A
 potentiostat-galvanostat, a Schlumberger model SI 1260
 impedance/gain-phase analyzer, and a personal computer. We then measured
 appropriate characteristics using the open circuit potential (OCP). EIS
 measurements applied an alternating voltage of 15 mV for non-painted and
 15 and 40 mV for painted specimens, and took measurements over a frequency
 range of 1.6E-2 to 1.0E+5 Hz with five frequencies, evenly spaced
 logarithmically, per decade.
 The specimens were also tested in model K0235 Flat Cells that included
 a glass cylinder with three electrodes:
 a platinum-clad niobium screen counter
 the test specimen as the working and
 a Ag/AgCl/KCl reference electrode (in a central glass well; a Luggin probe,
 a capillary tube extending nearly to the specimen, is located on one side
 of the well).
 The test area was 16 cm.sup.2. The cell was filled with a fresh 5% NaCl
 solution for each specimen. For each run, the computer tabulates the real
 and imaginary components of impedance (Z' and Z", respectively) for each
 frequency. From this data, we calculate other parameters indicative of
 corrosion. FIG. 13 is a flowchart showing how we manipulate the impedance
 data to calculate the desired parameters.
 The absolute impedance, .vertline.Z.vertline. (ohms), for example, is
 calculated from
EQU .vertline.Z.vertline.=(Z').sup.2 +(Z").sup.2
 and the phase shift, .phi. (degrees), is calculated from
 ##EQU1##
 Bode plots show .vertline.Z.vertline..multidot.A (ohm.multidot.cm.sup.2,
 where A is the specimen area, usually 16 cm.sup.2) versus frequency and
 phase shift as functions of the input frequency. We used DeltaGraph.RTM.
 software to generate three-dimensional Bode plots as a finction of
 exposure time, when appropriate.
 Boukamp equivalent circuit analysis (ECA) software fits the Z' and Z" data.
 A five-element-circuit model (FIG. 14) is commonly used. R.sub.1 is the
 solution resistance, C.sub.4 is the capacitance of the coating, and
 R.sub.4 is typically called the pore resistance that represents either
 pinhole defects or other inhomogeneities which provide an electrical short
 circuit pathway through the coating to the substrate. C.sub.2 is the
 double layer capacitance, and R.sub.2 is polarization resistance of the
 corrosion process occurring beneath the coating, particularly at pinhole
 defects or other inhomogeneities. In corrosion studies, the polarization
 resistance is inversely proportional to the corrosion rate of the process;
 in other words, the higher the polarization resistance, the lower the
 corrosion rate.
 FIG. 15 shows the series-parallel (SP) circuit model we used. The R values
 are resistors and Q values are constant phase elements (CPE). R.sub.1
 represents the solution resistance. R.sub.2 and Q.sub.2 represent the
 polarization resistance and the non-ideal double layer capacitance of the
 corrosion process, respectively. R.sub.3 and Q.sub.3 represent the
 resistance and the non-ideal capacitance of either the corrosion products
 or the anodization. R.sub.4 and Q.sub.4 represent the resistance and the
 capacitance of the primer.
 The impedance of a constant phase element (CPE) is defined by
 ##EQU2##
 where
 Q is the CPE parameter,
 j=-1,
 .omega. is angular frequency, and
 n is the phase coefficient.
 Using this definition, the CPE unit is mho.multidot.sec.sup.n.
 When n=0, the CPE unit is mho, which is the inverse unit of resistance
 (that is, R=1/Q).
 When 0&lt;n&lt;1, the CPE unit is "CPE mho" (mho.multidot.sec.sup.n), which in
 the SP model is interpreted as a non-ideal capacitance.
 When n=1, the CPE unit is mho.multidot.sec (farad), which is the unit of
 capacitance (that is, C=Q).
 To validate the SP model, we generated three cases of Bode plots using both
 the SP model and the five-element model using selected R and C values. For
 solution resistance, R.sub.1 =30 ohm-cm.sup.2. For break frequency,
 f.sub.2 =(2pR.sub.2 C.sub.2).sup.-1 and f.sub.4 =(2pR.sub.4
 C.sub.4).sup.-1. The cases are summarized in Table 1.
 TABLE 1
 Corrosion-resistant Metal Substrate Coating
 R.sub.2 C.sub.2 f.sub.2 R.sub.4
 C.sub.4 f.sub.4
 Case Rating (ohm .multidot. cm.sup.2) (farad/cm.sup.2) (Hz) Rating
 (ohm .multidot. cm.sup.2) (farad/cm.sup.2) (Hz)
 1 Good 1E+5 1E-4 0.016 Good 1E+6 1E-9 160
 2 High 1E+6 1E-5 0.016 Marginal 1E+5 1E-8 160
 3 High 1E+6 1E-6 0.160 Marginal 3E+5 3E-7 1.8
 FIGS. 16-21 are Bode plots using the SP and five-element models for the
 three validation cases. These graphs show the general correspondence
 between the SP model we selected and the more common five-element circuit
 model. In Case 1, (FIGS. 16 and 17), the corrosion process is nearly
 masked by the coating. If either the polarization resistance R.sub.2 was
 lower or the coating resistance R.sub.4 was greater, the corrosion process
 would probably go undetected beneath the coating. The Bode plots from both
 models are essentially the same. The break frequencies, f.sub.2 and
 f.sub.4, differ by four orders of magnitude. In Case 2, (FIGS. 18 and 19),
 the highly resistive corrosion process is quite evident in the presence of
 the marginal coating. Again, the Bode plots from both models are
 essentially the same.
 Only in Case 3, (FIGS. 20 and 21), does a difference occur between the two
 models; the difference is particularly evident in the phase plot (FIG.
 21). The magnitudes of the Rs and Cs are the same as the other two cases;
 the major difference is that the break frequencies, f.sub.2 and f.sub.4,
 are within an order of magnitude of each other. When break frequencies are
 similar, the R and C values will depend on which model is used. The
 probability of obtaining similar break frequencies as in Case 3 is
 relatively small in view of the wide range of break frequency values for
 the various corrosion processes, corrosion products, and coatings.
 Therefore, the SP model is essentially an equivalent for the five-element
 circuit model.
 Along with the SP model producing similar Bode plots to the common
 five-element model, the SP model allows the primer, applique, topcoat,
 corrosion products, and corrosion processes to be uniquely separated into
 individual R and Q elements that can be easily identified, sorted, and
 monitored with exposure time. Further, the break frequency, f.sub.RQ, of
 each R.sub.i Q.sub.i circuit can be monitored with exposure time. In this
 study, it is an integer identifying an electrical element, either 1, 2, 3,
 or 4. Break frequency is an important intrinsic property that is not
 dependent on surface area. The f.sub.R.sub..sub.i .sub.Q.sub..sub.i is
 taken from the time constant, .tau..sub.R.sub..sub.i .sub.Q.sub..sub.i ,
 of the i.sup.th parallel R.sub.i Q.sub.i circuit where
 ##EQU3##
 When the appropriate units are substituted, the .tau..sub.R.sub..sub.i
 .sub.Q.sub..sub.i unit is
 ##EQU4##
 Since
 ##EQU5##
 and .omega.=2.pi.f, then
 ##EQU6##
 A generalization to be used cautiously is that the f.sub.R.sub..sub.i
 .sub.Q.sub..sub.i in the range of 1E+1 to 1E+5 Hz is associated with the
 anodization, corrosion products, and organic coatings such as primer;
 while an f.sub.R.sub..sub.i .sub.Q.sub..sub.i in the range of 1E-2 to
 1E+l Hz is associated with corrosion processes.
 Once the CPE of the appliques and coating systems is determined from the
 ECA, the dielectric constant can be calculated from the following
 relationship:
 ##EQU7##
 where d is the thickness of the coating and so is the permittivity of free
 space (8.85E-14 farad/cm).
 Fluoropolymer (FP) and polyurethane (PU) appliques were applied,
 respectively, to 3-in.times.6-in panels of clad 2024-T3 A1 that were
 chemical conversion coated with Alodine 600. Prior to the application, at
 one end of the panel, the surface was scribed with an ".times."; the
 length of the leg from the center point of the .times. was 0.75 in. The FP
 applique was placed over the .times. scribe to simulate patching a damaged
 area: this was not done for the PU applique.
 Two application methods were used to apply the FP applique to the
 Alodine-treated surface. In the wet application method, the surface is
 lightly sprayed with water. The FP applique is then placed on the surface.
 The water allows the applique to be easily positioned on the surface, and
 is acceptable provided that care be taken to remove excess water from
 beneath the applique. Otherwise, trapped water will produce bubbles. The
 PU applique, which served as a baseline, was applied using the dry
 application method. The appliques were placed over the entire surface
 including the scribe and was sealed along the edge with a fluoroelastomer
 to eliminate seepage of the salt solution from the edge.
 FIGS. 22-25 show Bode plots of our wet and dry appliques as a barrier and
 as a patch over a scribe. These plots are similar to the data presented in
 FIGS. 3-10. The increase in impedance (.vertline.Z.vertline.) with
 decreasing frequency is attributed to the electrical properties (i.e.,
 resistance and capacitance) of the applique as a barrier coating. The
 negative phase of nearly 90 degrees (FIG. 23) shows the very capacitive
 nature of the applique. The patched scribe behaved the same as the
 barrier. Over the 53 days of exposure, the impedance remained essentially
 constant except for the slight tapering off at very low frequencies.
 Retention of the impedance indicates that hardly any corrosion occurred
 under the appliques during the duration of the test. The appliques were
 among the best treatments available to prevent corrosion.
 FIGS. 24 and 25 are Bode plots of our applique applied wet as a barrier and
 as a patch over a scribe. The method of application had no significant
 effect on the applique as a barrier coating to prevent corrosion.
 FIGS. 26 and 27 are Bode plots of the polyurethane applique control applied
 dry. At 4 days, the impedance increase with decreasing frequency is
 attributed to the electrical properties of the applique. The smooth
 increase in impedance begins to reduce at about 10 Hz. The resistance of
 the coating is lower than our preferred appliques that function as a vapor
 barrier. Correspondingly, the negative phase also decreases much sooner.
 With continued immersion time, the impedance of the coating continues to
 decrease. A second rise in impedance is observed that is particularly
 evident in the phase plot. For example, at 51 days, the phase decreases to
 a minimum at 100 Hz, rises to a maximum, and then decreases to a minimum
 again. The second increase in the impedance and phase maximum is
 attributed to corrosion beneath the polyurethane applique.
 FIGS. 28-32 show the ECA results (the derived parameters, FIG. 13) of our
 appliques that include the break frequency, resistance-area, constant
 phase element and the n parameter. In FIG. 28, the break frequency for our
 preferred fluoropolymer applique was in the vicinity of IE-2 whereas the
 break frequency of the polyurethane control was IE+2 Hz. The break
 frequency for the corrosion occurring beneath the coating was about 5 Hz.
 The lower break frequency of our preferred fluoropolymer applique is
 attributed to the higher resistance to corrosion. Though much scatter
 exists in the data, the break frequencies are not significantly dependent
 on time.
 In FIG. 29, the resistance (often called the pore resistance) of the FP
 applique is 1E+11 ohm.multidot.cm, which is very high for any of the
 organic coatings commercially available; it is also not dependent on
 application method. The resistance value for the our applique of 1E+7
 ohm.multidot.cm is more typical of the commercial organic coating systems.
 The better corrosion resistant barrier coatings will normally have a
 resistance greater than 1E+7 ohm.multidot.cm. Our applique has a
 resistance several orders of magnitude greater than the commercial organic
 coating systems indicating a very good potential for corrosion barrier
 applications. In addition, the polarization resistance of the corrosion
 process beneath the coating is significantly high, which indicates that
 corrosion is proceeding slowly, if it is occurring at all.
 A very small constant phase element (CPE) of 1E-10 CPE mho/cm.sup.2 and "n"
 parameter of nearly 1.0 shown in FIGS. 30 and 31 represent the capacitance
 of the appliques. The CPE value of 1E-6 to 1E-9 CPE mho/cm.sup.2 and n
 parameter of 0.6 to 1.0 represents the non-ideal capacitance of the
 corrosion process occurring beneath the polyurethane control applique. The
 large scatter for the polyurethane control results from the difficulty in
 deconvoluting the EIS data in the presence of the impedance high.
 FIG. 32 plots the dielectric constant (DE) of the appliques versus time.
 The DE for the appliques magnitude of 2 to 3 is about the same as
 Teflon.RTM.. How the appliques were applied (wet v. dry) had no
 significant effect on the DE. The DE of the polyurethane control applique
 is slightly lower than the reported values of 4 to 8. Our preferred
 appliques were quite stable while immersed in the salt solution over the
 test period.
 EXAMPLE 2
 We also tested our preferred appliques against a conventional coating used
 for painting commercial aircraft. For a control, we used a BMS 10-60
 polyurethane topcoat over a BMS 10-79 epoxy primer on 3-in.times.6-in
 2024-T3 clad aluminum panel treated with Alodine 600, a chemical
 conversion coating. The coated panels were exposed to a salt spray
 environment in accordance with ASTM B117. Periodically, the panels were
 removed for visual examination and EIS testing.
 FIGS. 46 and 47 are Bode plots of the painted coating as a function of time
 up to 24 days. After 5 days of salt spray exposure, the impedance
 increases with decreasing frequency to about 1 Hz. The increase in
 impedance begins to taper off. At longer exposure times, the impedance
 begins to taper off at higher frequencies. The barrier coating resistance
 decreased with exposure time.
 FIGS. 48 and 49 are Bode plots of our applique for 11 days of testing. The
 impedance increased with decreasing frequency. The impedance remained
 constant. The negative phase of nearly 90 degrees shows the retention of
 the capacitive nature of the applique. In comparison to the painted
 coating, the applique was significantly more resistant to the salt spray
 exposure, showing essentially no corrosion under our appliques because the
 appliques are a vapor barrier.
 While we have described preferred embodiments, those skilled in the art
 will readily recognize alterations, variations, and modifications that
 might be made without departing from the inventive concept. Therefore,
 interpret the claims liberally with the support of the full range of
 equivalents known to those of ordinary skill based upon this description.
 The examples are given to illustrate the invention and not intended to
 limit it. Accordingly, limit the claims only as necessary in view of the
 pertinent prior art.