Coating for aerospace aluminum parts

Details Coating for aerospace aluminum parts Coating for aerospace aluminum parts

1995-12-29

2001-01-09

Jones, Deborah (Department: 1775)

Stock material or miscellaneous articles

Composite

Of silicon containing

C428S447000, C428S457000, C428S650000, C428S654000, C428S704000, C428S687000, C428S626000, C428S036910, C244S126000, C244S133000

Reexamination Certificate

active

06171704

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to protecting aluminum parts from environmental stress. More particularly, this invention relates to coatings for protecting aerospace grade aluminum alloy parts from the effects of high temperature, high salt concentration, water droplet erosion, and other environmental stresses which aluminum aircraft parts are subjected to. The invention is especially suitable for the protection of aircraft parts exposed to high temperatures and water droplet erosion, such as lipskins of nacelles for jet engines and leading edges of wings and tails. The coatings of the invention are easily repairable and retouchable to maintain or restore a physical and aerodynamic virtually new condition and a virtually perfect cosmetic appearance.
“Aircraft” in this specification refers to both fixed wing and non-fixed wing aircraft, such as propeller driven airplanes, jets, and helicopters. When the term “aircraft” or “airplane” is used in this specification in conjunction with a nacelle or a lipskin, or when the term “nacelle” or “lipskin” is used in the specification, it relates to any device in which a nacelle housing for a jet engine is attached, which device may be a device for travelling in air, or may be a jet propelled device for travelling in water, a watercraft, or for travelling on land, a landcraft.
“Leading edge” or “leading edge of wings” in this specification refers to any leading horizontal or vertical membrane on the exterior of an aircraft, including the leading edges of wings and the horizontal and vertical leading edges of tails.
“Cosmetic repairability”, in this specification refers to repair of a damaged area of an aluminum alloy part so that the repaired area is virtually indistinguishable by unaided visual inspection at a distance of 3 meters from an adjacent non-damaged area of the part.
“Repairability”, in this specification refers to repair or restoration of a damaged area of an aluminum alloy part so that the repaired area is restored to, i.e., be virtually equivalent to, from the mechanical and physical point of view, to an undamaged adjacent or non-adjacent area of the part. Another way of expressing this feature is to describe the part as being “aerodynamically” restored to its like-new condition.
BACKGROUND
A “nacelle” is the housing over the jet engine with the forward part being a “nose cowl” made of a composite material attached to an aluminum alloy air inlet ring, which is called a “lipskin”. See FIG.
1
.
Lipskin rings may be up to about 3.66 meters (12 feet) in diameter. Because of the size, weight, and being attached to a composite structure, during the set up, hundreds of rivet holes are drilled and countersunk through the lipskin. During these procedures, the lipskins are invariably scratched, gouged, nicked, or otherwise damaged.
In operation, lipskins and leading edges of wings and tails are subjected to severe environmental exposure, which causes corrosion of these parts. The temperature in flight may vary from a low of −55° C. to a high of 60° C. Also, these parts are subjected to high velocity impact of dirt and runway debris. In flight, water droplet erosion adversely affects the lipskin and the leading edges. Additionally, these parts are subjected to temperatures of 190° C. (375° F.) or higher, up to 232° C. (450° F.), during deicing.
Thus, these leading edges are exposed to a unique combination of severe environmental conditions.
Aerospace aluminum alloys used in lipskins and leading edges are generally of the 2XXX series and contain copper, which provides strength when heat treated. Most members of the 2XXX series and other aerospace aluminum alloys, such as the 6XXX and 7XXX series, soften when exposed to temperatures used to deice a plane (between 121° C. (250° F.) and 190° C. (375° F.)). Temperatures of up to 232° C. (450° F.) may be used for deicing in an emergency situation.
The most common aluminum-copper alloy used for aerospace applications is AA2024, which has 4.4% copper, 1.5% magnesium, 0.6% manganese, and the remainder aluminum. AA2024 is commonly used in the leading edges of aircraft wings and the tail assembly. Leading edges are generally composed of multiple “C” shaped aluminum pieces about 8 inches across and 7-8 feet long. The leading edge may be attached by bolts or riveted to the body of the wing or tail. Leading edges are exposed to high temperatures during deicing, which causes softening of the alloy. However, such softening is generally not critical because the leading edge is not a structural member and, unlike the lipskin, is not a large self-supporting structure attached to a composite structure. The leading edges are subject to corrosion from high temperatures (deicing) and salt fog exposure and to erosion from water droplets from rain, or sleet, for example. Often, the leading edges are clad with Al 1100, pure aluminum, which does not corrode but is very soft. Because of its softness, Al 1100 scratches easily and cannot be repaired cosmetically. The rivets are ground to be flush with the clad aluminum leading edge, with great care being necessary to ensure that the thin cladding, usually about 100 &mgr;m (0.004 inches) thick, is not removed in the grinding process. It requires frequent maintenance, such as frequent polishing to maintain a cosmetic appearance. Polishing, however, is very labor intensive, especially due to the presence of large numbers of rivets, and does not repair scratches cosmetically.
Because AA2219 is heat tolerant (will not lose strength) at temperatures of up to 232° C. (450° F.), it is the most common alloy used in lipskins of nacelles. AA2219 has 6.3% copper, 0.30% manganese, 0.34% of total of vanadium, zirconium, and titanium, and the remainder aluminum. In older jet powered air craft, lipskins were made from sections of clad aluminum alloys joined together to form a ring, which offered some protection from corrosive environmental stresses. In modern manufacture of lipskins, the lipskins are made from one piece or from 2 or more partial-circular pieces. Because of the extreme deep drawing forming stresses these one or two piece lipskins cannot be formed from clad alloys.
The present state of the art method to protect lipskins is by anodizing by sulfuric acid per MIL-A-8625, followed by sealing in boiling water or other sealant to produce a clear or aluminum finish. Generally, following anodizing, lipskins are left unpainted due to the erosion from temperature extremes experienced by the lipskins and to the difficulty in cosmetically repairing the painted surface.
Anodizing protects the lipskin from corrosion, but only for a short time. The anodic coating is very thin and does not have a long erosion life, usually lasting only a few weeks. Thus, the anodic coating protects the lipskin from corrosion during manufacture and setup procedure. However, anodizing does not afford any protection from corrosion due to in-operation environmental stresses.
In addition, damage to the lipskins that occurs during manufacture and set up also damages the anodic coating. Both the erosion of the anodic coating and damage to it from handling during manufacture leave the lipskin unprotected and subject to corrosion. During operation, corrosion is accelerated as corrosion products are washed away by air, which exposes the unprotected lipskin to the corrosive environment.
Such corrosion damage in lipskins and on the leading edges of the wings, even if only on the surface, is unacceptable to commercial airlines because these parts are visible to passengers. Therefore, airlines often will refuse delivery of nose cowls or of nacelles with damaged lipskins and leading edges of wings or will accept these parts from the manufacturer only with a cosmetic concession.
It is known that thicker anodic coatings will offer better protection than thinner anodic coatings. However, increasing the thickness of anodic coatings has not proven feasible because increasing the thickness of the anodic coating leads to reduced fatigue life of the anodized part.
Paint based protection schemes have

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