Patent Application
20110300406
Metal surface treatment.
The susceptibility of various metals to corrosion has been extensively studied. One field where this is particularly important is the aircraft or airline industry. The exterior of most aircraft are made primarily of metal material, particularly aluminum and titanium. In order to improve the corrosion resistance of metal component parts, particularly, an exterior surface of metal component parts, conversion coatings have been developed. Conversion coatings are generally electrolytic or chemical films that promote adhesion between the metal and adhesive resins. A common electrolytic process is anodization in which a metal material is placed in an immersing solution to form a porous, micro rough surface into which an adhesive can penetrate. Chemical films for treating titanium or aluminum include phosphate-fluoride coating films for titanium and chromate conversion films for aluminum.
Painting of metal surfaces is also of important commercial interest. In the aircraft or airline industry, the exterior metal surface of many commercial and government aircraft are painted at considerable expense. Techniques have been developed, through the use, for example, conversion coatings or sol gel processes to improve the adhesion of paints, particularly, urethane coatings that are common in the aircraft applications. With respect to sol gel coatings, U.S. Pat. Nos. 5,789,085; 5,814,137; 5,849,110; 5,866,652; 5,869,140; 5,869,141; and 5,939,197 describe sol gel technologies, particularly zirconium-based sol gel technologies for treating metal surfaces and adhesion, particularly, paint adhesion.
With respect to metal panels that make up an aircraft, sol gel coatings such as those described in the above-referenced patents have been shown to improve adhesion of epoxy-based and polyurethane paints.
Most panels (e.g., metal panels) that make up, for example, the body of an aircraft are held together by fasteners, particularly rivets. Such fasteners, particularly, the exposed surface of such fasteners must meet corrosion resistance standards mandated by aircraft manufacturers. The fasteners must also be able to maintain a coating, such as a paint (e.g., epoxy-based, polyurethane, polyimide) that may be utilized on the panels that make up the aircraft. One problem that has been identified is that paint that otherwise adheres acceptably to the exterior surfaces of aircraft panels, does not acceptably adhere to the fasteners (e.g., rivets) that join the panels. The condition where paint adherence failure occurs with fasteners in the aircraft industry is known as rivet rash.
In addition to paint adherence, metal panels in the aircraft or airline industry must meet certain corrosion resistance standards. One corrosion resistance standard for conversion coatings of aluminum is a salt spray test in accordance with MIL-DTL-5541. According to this standard, the chemical conversion coated panels undergo salt spray exposure for a minimum of 168 hours and must show no indication of corrosion under examination of approximately 10× magnification. Although not specifically stated in the MIL-DTL-5541 standard, aircraft manufacturers often require that fasteners such as rivets meet certain corrosion resistance standards. One aircraft manufacturer standard for rivets is a salt spray exposure for a minimum of 48 hours without indication of corrosion.
Features, aspects, and advantages of embodiments of the invention will become more thoroughly apparent from the following detailed description, appended claims, and accompanying drawings in which:
A method of coating a metal surface is described. In one embodiment, a method includes forming a first layer including a chemical conversion coating on a metal surface and forming a second layer on the first layer through a sol gel process (e.g., a sol gel film). The method is useful, for example, in treating metal surfaces, particularly surfaces of metal (e.g., aluminum, titanium) fasteners to improve the corrosion resistance and the adhesion properties of the fastener for further treatment, such as for painting.
An apparatus is also described. In one embodiment, an apparatus includes a metal component, such as an aluminum fastener (e.g., rivet) having at least one surface. The at least one surface of the metal component includes a first layer comprising a chemical conversion coating and a second layer derived from a sol gel composition on the first layer. Through the use of a first and second layer, the adhesion properties of the metal component may be improved, particularly, for paint adherence to the at least one surface.
Referring to
One suitable coating is a trivalent chromium conversion coating such as LUSTER-ON® Aluminescent, commercially available from Luster-On Products, Inc. of Springfield, Mass. LUSTER-ON® Aluminescent is licensed from the United States Navy under U.S. Pat. Nos. 6,375,726; 6,571,532; 6,521,029; and 6,527,841. LUSTER-ON® Aluminescent includes a trivalent chromium complex and potassium hexafluorozirconate. A suitable thickness of first layer 140 of LUSTER-ON® Aluminescent on a fastener that is an aluminum rivet is, for example, on the order of less than one mil to pass the MIL-DTL-5541 salt spray standard for a fastener (e.g., 168 hour salt spray exposure).
In addition to first layer 140, fastener 100 shown in
Referring to
In one embodiment, for example, the deoxidation may include deoxidizing the fastener in a solution of between approximately 12 percent to 15 percent nitric acid (HNO3) for a period of approximately 30 seconds, followed by a rinse (e.g., a water rinse). Other deoxidizing agents, concentrations, and process times besides those recited here may be used.
At block 320, the fastener is etched with an etching solution. In one embodiment, the etching solution may contain, for example, an alkaline etchant such as DURAETCH™ commercially available from DURACHEM of Lake Elsinore, Calif. In one embodiment, a fastener such as a rivet is exposed to a concentration level of DURAETCH™ at approximately 8.5 ounces per gallon for approximately 15 seconds at approximately 150° F., followed by an aqueous (e.g., double water) rinse. Other etchants, concentrations, and process times and temperatures may be used beyond the specific example stated above.
In one embodiment, the fastener is exposed to a second deoxidization treatment, at block 14, in a manner similar to that set forth above for the deoxidation at block 310. The deoxidation at block 330 may advantageously prepare the surface of the fastener to receive a corrosion inhibiting or resisting coating and increase adhesion of the sol gel coating, to be applied at a later time. The fastener may then be rinsed in, for example, a double water rinse (e.g., rinsing the fastener twice in successive containers of water).
At block 340, in one embodiment, the fastener is exposed to a third deoxidization treatment similar to that set forth above for the deoxidation at block 310. Without wishing to be bound by a particular result or objective of the multiple deoxidation treatments or the requirement for multiple deoxidation treatments, the first and second deoxidation treatments tend to remove smut from the fastener while the third deoxidation treatment prepares the clean surface for subsequent processing.
Following the deoxidization of a metal surface or surfaces, a conversion coating is introduced (block 350) to the metal surface or metal surface of the rivet(s). For an aluminum alloy rivet (e.g., 2017, 7050 alloy rivet), a chemical conversion coating, such as LUSTER-ON® Aluminescent, is applied in accordance with MIL-DTL-5541. Suitable techniques for introducing chemical conversion coating of LUSTER-ON® Aluminescent include immersion, spraying, or drenching the metal surface in a solution of the chemical conversion coating material. In the example of rivets as fasteners, a number of rivets may be placed in a basket, such as a perforated metal basket, and immersed in a chemical conversion coating solution for a few to several minutes (e.g., two to seven minutes).
Following the introduction of a conversion coating, the rivet(s) are rinsed in one or more successive water baths and dried, such as by exposing the rivet to a centrifugal or other drying process, including a standing air dry process. The rivet(s) is/are then brought to room temperature if necessary. Within a specified period, such as within 24 hours, a sol gel film is introduced on an exterior surface of the rivet (block 360). Suitable ways for introducing a sol gel film include immersion coating, spraying, and drenching the rivet(s) in a sol gel solution. In the example where a sol gel coating is applied by immersing, representatively the rivet(s) is/are immersed in a solution including a sol gel for a period of a few to several minutes. In one embodiment, the rivet(s) is/are immersed in a solution including a sol gel for two to three minutes. During immersion, the sol gel solution may be agitated to improve the coating uniformly. The rivet(s) is/are then removed from a sol gel coating solution and centrifuged to remove excess sol gel solution (e.g., centrifuged in a DESCO™ or similar centrifuge for 30 seconds).
Once a sol gel coating is applied to a rivet(s), the sol gel coating is cured (block 370). In one embodiment, a curing process includes heating the rivet in a preheated oven to a cured temperature. A cure temperature for the sol gel coating solution described above commercially available from Advanced Chemistry and Technology includes exposing the rivet(s) including the sol gel coating to a preheated oven at a 130° F.±10° F. for a sufficient time, typically on the order of 45 to 90 minutes. The following table illustrates curing times for curing a number of rivets at one time (e.g., a number of rivets as a layer in a perforated tray).
Following curing of a layer formed by sol gel process (e.g., a sol gel film), the rivet(s) is/are cooled and a surface of the rivet(s) is/are ready for a coating. Representatively, an epoxy, polyurethane, or polyimide coating may be applied to the surface containing the sol gel film (block 380).
Five 10-inch by 14¾-inch panels of 2024-T3 clad aluminum were prepared each containing four rows of 10 rivets (40 rivets total). Three panels contained 7050 alloy BACR15FV solid rivets and two panels contained 2017 alloy BACR15GF rivets (counter sunk rivet with dome). Each of the five panels had rivets having a sol gel formed film (second layer) over a trivalent chromium conversion coating (first layer). A sixth panel of 2017 alloy BACR15GF rivets had ten rivets with the trivalent chromium conversion coating and no sol gel formed film. All rivets were painted according to Boeing aircraft paint standards. Following painting, the rivets of each panel were subjected to a dry tape scribe test to evaluate paint adhesion.
The rivets were prepared as follows:
7050 Alloy (BACR15FV rivet): Rivets were deoxidized by approximately 15 percent nitric acid solution for approximately 30 seconds; etched for approximately 15 seconds in a solution of DURAETCH™ solution for 20 minutes; deoxidized again in a bath of 15 percent nitric acid solution; immersed in LUSTER-ON® Aluminescent (3 oz/gal, pH 3.5-4) for five minutes at room temperature; rinsed in water; centrifugally dried; immersed in a sol gel solution from AcTeh, AcTech™-131 Parts A-D, for five minutes; and cured at 130° F. for approximately 45 minutes.
2017 Alloy (BACR15GF rivet): Rivets were deoxidized by 15 percent nitric acid solution for approximately 30 seconds; etched for approximately 15 seconds in a solution of DURAETCH™ at 50° F.; deoxided twice more in sequential baths of 15 percent nitric acid solution (approximately 30 seconds each); immersed in LUSTER-ON® Aluminescent (3 oz/gal, pH 3.5-4) for five minutes at room temperature; rinsed in water; centrifugally dried; immersed in a sol gel solution from AcTeh, AcTech™-131 Parts A-D, for five minutes; and cured at 130° F. for approximately 45 minutes.
As noted above, one panel of 2017 alloy BACR15GF rivets did not include a film formed by immersing in a sol gel solution.
Following preparation, the rivets were installed on the six panels and painted with a BMS10-72 Type VIII primer and a BMS10-72 Type III topcoat, color BAC 5289 per D6-1816AU. Each panel was cured at 120° F. for four hours followed by a one week cure at ambient.
Following curing, the rivets were subject to a dry paint adhesion test, per BSS7225 Type I, class 5 (45 degree cross hatch scribes).
The results show superior paint adhesion performance when a coating formed by a sol gel process is formed over a non-hexavalent conversion coating.
In the preceding paragraphs, specific embodiments are described. It will, however, be evident that various modifications and changes may be made thereto without departing from the broader spirit and scope of the claims. The specification and drawings are, accordingly, to be regarded in an illustrative rather than a restrictive sense.
The application claims the benefit of the earlier filing date of co-pending U.S. Provisional Patent Application No. 61/351,670, filed Jun. 4, 2010 and incorporated herein by reference.
| Number | Date | Country | |
|---|---|---|---|
| 61351670 | Jun 2010 | US |
