Traditional aerospace finishing processes often involve the use of strong acids or bases and toxic materials such as hexavalent chromium, but the industry is working to replace conventional finishing materials and processes with environmentally friendly alternatives that still meet stringent performance requirements.
To meet these dual objectives, Boeing has developed a family of water-based sol-gel coatings. One formulation, designated Boegel-EPII, is a nonhazardous coating that can be applied to a range of substrates (aluminum, titanium, nickel, steel) using a variety of methods (dip, immersion, spin, brushing, spraying).
A key application for Boegel-EPII is as a surface pretreatment for the exterior of commercial aircraft. The conventional pretreatment is a chromated conversion coating, so the desire was to replace that material with a nonhazardous alternative.
With respect to performance, the significant driver was to improve the adhesion of the exterior paint system, particularly for fasteners. The phenomenon of poor paint adhesion to fasteners, known as “rivet rash,” has been a pervasive problem for the Boeing commercial airplane fleet.
Materials and methods
The sol-gel process uses inorganic or organometallic precursors to form an inorganic polymer sol via a combination of hydrolysis and condensation reactions. The relative rates of hydrolysis and condensation, and the structure and characteristics of the resultant sol are controlled by a number of factors, including the concentration of reagents and catalysts such as acids or bases.
An interconnected solid network is formed through continued condensation driven by drying the coating solution on a metal substrate, where condensation reactions between the sol components and hydroxyl groups on the metal surface attach the gel layer to the substrate.
The baseline Boegel-EPII formulation consists of 1 wt% zirconium n-propoxide and 2 wt% 3-glycidoxypropyltrimethoxy silane in deionized water with glacial acetic acid as a catalyst. Organic functionality of the resulting coating is due to the glycidoxypropyl group on the silane component. It is hypothesized that the difference in condensation rates between the silicon and zirconium components produces a hybrid inorganic/organic layer with a compositional gradient from the metallic surface to the subsequent coating layer.
X-ray photoelectron spectroscopy (XPS) for a Boegel-EPII coating on clad Al 2024-T3 revealed that silicon is found predominantly at the surface of the coating (short sputter times) with its concentration dropping to zero by 100 seconds of sputtering. Zirconium is also found at the surface, but its amount relative to the silicon increases with sputtering time, suggesting that the interface between the aluminum substrate and the Boegel coating is primarily a Zr-O-Al interaction. The long exponential tail for the Zr is likely due to “knock-on,” where sputtered surface species are implanted in the subsurface.
The thickness of the sol-gel coating can be controlled by varying the basic formulation chemistry, with a range of 20 nm to 1 µm achieved depending on the application of interest.
The combination of inorganic and organic polymer fractions in the Si-Zr sol-gel coatings yields unique properties—a hybrid of what would be expected of the individual components. For example, the thin sol-gel coatings are more flexible than an inorganic metal oxide film of similar thickness. This was demonstrated by comparing the flexibility of aluminum alloy panels treated with either the Boegel-EPII coating or a chromic acid conversion coating (CCC). When subjected to a conical mandrel bending test at low temperatures (-65°F), the CCC panel cracked whereas the Boegel-EPII panel showed no cracking or adhesion loss.
Addition of the organic fraction also aids in processability. The Boegel EPII formulation is easily applied from an aqueous solution and forms a film that will cure to completion at room temperature in about 24 h; by applying heat, that time can be considerably shortened, making for efficient parts production.
Testing and implementation
Following the completion of engineering testing and trials on “test tube” fuselage sections to establish application parameters, Boegel EPII was applied to a B777-200LR flight test airplane—the first full-scale production trial on a commercial aircraft.
The performance of the coating on the B777-200LR was assessed after nine months of flight testing. The adhesion of the coating system was excellent, with no exposed rivets. In comparison, a companion flight test B777-200LR that was painted with the conventional chromate conversion coating had 10 exposed rivets in the Section 41 forward fuselage. Although these aircraft had limited flight hours (less than 400 in approximately nine months), the observations of in-service performance were consistent with the results of the laboratory testing.
After the conclusion of flight testing, the airplane was treated with peroxide paint stripper and repainted in the customer’s decorative livery. As anticipated from the engineering testing, it took longer to strip the Boegel-EPII test airplane than a conventionally painted airplane. The coating system, which varied in thickness from 100 to 300 µm, was removed with a total of four applications of stripper and 29 hours of dwell time.
Future work will focus on development of paint strippers that are more compatible with the Boegel-EPII conversion coating to reduce the stripping time and effort.
Based on the results of the engineering testing, manufacturing testing, and production trial, Boegel-EPII was qualified for use by Boeing D6-1816 “Process Document for Decorative Finishing of Airplane Exteriors.” Implementation of the technology for all commercial models was completed in 2008, and fleet surveys are under way to assess the in-service performance. Though not a statistically significant sampling, results to date suggest reduced levels of rivet rash for the majority of airplanes surveyed, with all occurrences of adhesion failure limited to high erosion areas.
Depending on size of the aircraft, between 100 and 400 gal of chromated coatings and rinse water per plane are eliminated by replacing the conventional chromated conversion coating.
Boegel EPII also is being implemented for the P-8A Poseidon and has been demonstrated on KC-135 and F-15 aircraft. And it is used as a pretreatment for painting titanium on B777, B787, and F22 aircraft and is widely used as a pretreatment for adhesive bonding of aluminum and titanium.
Applications beyond adhesion
The initial application of the Si-Zr sol-gel coating has been as an adhesion promoter; however, by incorporating various additives and pigments, multifunctional features can be built into the structure.
For the application of corrosion control, inhibitors can be added to the matrix to provide corrosion protection of alloy surfaces. The inhibitors can be trapped in the pore structure of the sol-gel matrix or chemically incorporated into the backbone structure of the matrix, which plays a critical role in the effectiveness of the corrosion inhibitor.
When 0.01% cerium oxalate was added to the Boegel-EPII matrix, the resulting coating gave some corrosion protection after 24 h of exposure to a salt spray environment per ASTM B117. When the concentration of cerium oxalate was increased to 0.05%, the network-forming capability of the Boegel-EPII matrix was affected and the resulting coating gave very poor protective capability.
As expected, addition of potassium chromate resulted in excellent corrosion properties, with no corrosion noted after seven days of exposure to a salt spray environment. (This is not proposed as a route to corrosion protection; rather, the testing with potassium chromate was performed as a control with which to compare the performance of nonchromated inhibitors.)
Pigments such as indium tin oxide, carbon black, and nickel flake can be added to the sol-gel matrix to produce conductive coatings. This is useful for applications such as dissipation of static charges on the exterior of nonconductive surfaces. A range of coatings were prepared as a concept demonstration. Several of the coatings gave no response, indicating that there was no electrical conductivity (infinite resistivity).
Of the formulations tested, the nickel flake version had the lowest surface resistivity, though these formulations were not optimized. The behavior of the carbon black samples is consistent with percolation phenomenon, where as the loading increases the conductivity passes through a sharp and abrupt rise over a very narrow concentration range, known as the percolation threshold, with further increases in loading past this threshold causing little increase in the conductivity.
The sol-gel matrix withstands high temperatures and can be used to provide oxidation protection. As an example, half of a Ti-6Al-4V alloy panel was painted with a thin Boegel-EPII coating layer and the other half was left bare. After 336 h exposure to 700°F in an air environment, the bare right half of the titanium alloy panel underwent significant oxidation, as noted by a blue-colored titanium oxide layer that had formed on the surface. The left half protected by the sol-gel coating did not undergo substantial color change, retaining its metallic grey color.
This article is based on SAE International technical paper 2009-01-3208 by Kay Y. Blohowiak, Joseph H. Osborne, and Jill E. Seebergh of Boeing Research & Technology, The Boeing Co.