In most consumer industries, coatings applied by the original equipment manufacturer (OEM) last for the life of the item. Because of the high cost and long life of aerospace and defense equipment and the often harsh operating environment, coatings are frequently stripped and reapplied over the life cycle of the aircraft or vehicle, often on a 5-year cycle that may be repeated over more than 30 years. Coatings are therefore applied not only by the OEM but also by maintenance personnel. This involves many people and coating processes, often under less-than-ideal conditions on substrates that are no longer factory-fresh.
Although industry sectors such as automotive and electrical products have largely eliminated many of the older finishes such as cadmium and hexavalent chromium (or are in the process of doing so), these finishing materials are still used extensively in aerospace and defense applications. Flight safety and weapons system function demand that changes be carefully considered. New technologies must be thoroughly qualified and are usually adopted slowly, with extensive service testing to reveal any hidden safety or failure issues.
Coatings and surface treatments are changing in the aerospace and defense industries due to a number of factors, including environmental, safety and health (ESH) regulations, the need for ever-increasing performance, customer demands for lower costs and changes in how maintenance is contracted. On the ESH front, European rules such as RoHS and REACH are important drivers, but US regulations such as stricter Cr6+ air emission rules and the reduced OSHA Cr6+ permissible exposure limit (PEL) are also having an impact. Coupled with this, legal liability for worker health, air emissions and toxic waste is a serious concern for any manufacturing operation, especially for materials such as cadmium and Cr6+ that are intrinsically dangerous and create risks throughout their life cycle. Although changes in coatings and surface treatments may initially be driven by environmental and health concerns, the closing arguments for change are usually better performance and lower overall maintenance cost. The result is customer demands for better performance and longer times between maintenance. Since aircraft and military equipment are both international businesses, this means that new equipment is increasingly governed by the strictest of the world’s regulations, primarily Europe’s. Changes in how maintenance is contracted—business arrangements such as Leased Power and OEM maintenance contracts—provides yet another driver for more easily maintainable surface treatments.
Dry to Replace Plating?
It is sometimes asserted that liquid-based technology for both plating and painting is old, grimy and passé, to be superseded by modern, clean, dry technology. Dry coatings do obviate the need for waste treatment plants for rinse water and high-volume sludge. But they usually do so at the expense of more complex processes. The simplicity and cost-effectiveness of tank coatings is hard to beat, especially for complex components and non-line of sight areas. But the use of dry (or non-aqueous) methods is indeed growing. The change to dry coating processes is often initiated for environmental reasons but increasingly accepted for improved coating performance. Dry coating processes currently in use or being evaluated include high-velocity oxyfuel (HVOF) thermal spray, which is more wear resistant than hard chrome; powder coating, which offers better quality and performance than paint; vacuum-based processes such as ion and chemical vapor deposition (IVD and CVD, respectively); and electroplated aluminum. The vapor deposition processes and electroplated aluminum are aimed at replacing toxic cadmium while providing better performance. The plating industry is far from fading into the sunset, however. Tank methods are becoming more sophisticated and cleaner, greatly reducing ESH risks while improving process control and product quality. For example, automated tank chemistry maintenance is becoming common, especially for processes such as electroless nickel. Automation makes it possible to closely control more complex chemistries, and automated lines featuring enclosed tanks protect workers from chemicals while improving ventilation and emissions capture. Counterflow and over-tank rinsing can be designed to largely eliminate wastewater discharge, while conforming anodes are increasingly used to improve coating thickness and uniformity.
Corrosion resistance is a major issue for the U.S. Department of Defense, with an estimated annual cost of $10–20 billion. In aircraft, corrosion is the major reason for overhaul of airframe items such as landing gear. However, all our traditional corrosion control materials are coming under severe pressure because of their ESH effects. We might even go so far as to enunciate a basic Corrosion Principle: “Any material active enough for corrosion control will be a health and environmental hazard.” Any anti-corrosion coating must continue to work even when damaged in service. For this reason, we use surface treatments that are sacrificial and self-healing; barrier coatings are rarely adequate. The primary corrosion control materials used in aerospace and defense, cadmium plating and hexavalent chromates, are both prominently featured on almost every list of toxic materials worldwide. Eliminated from most products, the only places where they are still widely used are aerospace and defense. Chromates are the most common corrosion inhibitors, used almost universally as self-healing treatments for aluminum (chromate conversion coatings); for corrosion-resistant zinc, cadmium and aluminum electroplate; and in primers, sealants, and every application that requires corrosion resistance and good metal-polymer bonding. Unfortunately, Cr6+ is highly toxic. Fortunately, the automotive industry, driven by the European ELV and RoHS rules, has largely eliminated the use of Cr6+ products. As a result, good alternatives are available on the market. DoD has also developed a trivalent (Cr3+) treatment specifically for aluminum, which is the most common skin material for aircraft. Available chromate alternatives include Cr3+, usually in combination with other inhibitors such as vanadium. For the short term this is the most obvious alternative. However, many companies treat all chromium alike, because Cr3+ faces the same disposal issues even though it avoids ESH problems during application. Chromium-free options are gaining market presence, including formulations based on rare-earth metals, vanadates, permanganates and so on. According to our Corrosion Principle, however, all of these are likely to elicit regulatory attention once they become widely used. Most of the chromate conversion alternatives use nanoparticles to inhibit permeation. This is an effective approach, but concern over nanoparticles is also increasing. A different tactic is to promote paint adhesion so that the primer and paint system can protect the surface more thoroughly. There are now a number of adhesion promoters on the market that have been found to be effective on aircraft. The major source of Cr6+ in aircraft and defense equipment finishes is the chromated primer. Several new non-chromium primers are now available. At the present time most aerospace users are moving cautiously into non-chromium finishing systems, keeping Cr6+ in either the substrate finish or the primer. However, the ultimate aim of most users is to move to an entirely non-chromate finishing system eventually.
Cadmium provides corrosion resistance for high-strength steels used in landing gear, fasteners and most high-strength structural components on all types of defense equipment because of a unique combination of properties. The metal’s open circuit potential, for example, is just a little more negative than steels, affording sacrificial corrosion protection without rapid dissolution of the coating. Cadmium gives fasteners lubricity and galling resistance. An alternative with a different lubricity would require changing all of the thousands of existing torque specifications for fasteners on all military equipment and aircraft – a monumental (and monumentally expensive) task. Cadmium corrosion products are low-volume, preventing corroded fasteners from “freezing.” Cadmium alternatives, long used in other industries, are just beginning to permeate aerospace and defense. One often forgotten problem is that high-strength steels must be plated with low-hydrogen-embrittlement (LHE) cadmium, not bright cadmium. For aerospace, cadmium alternatives must usually be modified to eliminate hydrogen embrittlement and re-embrittlement as well as ensuring that they do not cause fatigue. There are several alternatives. Aluminum is an excellent cadmium alternative. It has better corrosion and hydrogen embrittlement performance than cadmium, but it cannot be plated from an aqueous bath. It must be deposited either by the aerospace-qualified IVD vacuum process; by thermal spray, which is used for some landing gear; by non-aqueous electroplating such as the AlumiPlate process used on the F-35); or by CVD, a relatively high-temperature process. Zinc-nickel alloy electroplate is also gaining popularity. Bright zinc-nickel is widely used in the automotive sector but cannot be used in aerospace because it causes embrittlement. A recent development is an alkaline LHE zinc-nickel for use on high-strength steels, which is undergoing aerospace qualification. Some aircraft have successfully used ceramic-metallic paints (SermeTels), while others have experienced significant problems with this approach. Until recently these materials have been chromated, but new Cr6+-free versions have entered the market from a variety of suppliers. The most widely used approach, however, has been to adopt corrosion-resistant materials such as titanium and stainless steels. Most modern hydraulic actuators are made from 15-5PH stainless, while many fasteners are now stainless steel or titanium. Corrosion-resistant fasteners hold modern turbine engines together, but they cannot be used on aluminum aircraft skins and frames because they cause galvanic corrosion. Composites, used to reduce weight in new aircraft such as the Boeing 787 and the F-35 Joint Strike Fighter, also eliminate the need for cadmium.
Wear, Erosion, Abrasion
Traditionally, the aerospace and defense industries have relied on engineering hard chrome for two purposes: to provide wear resistance for items such as hydraulic rods and cylinders and bearing journals, and to rebuild worn components to print dimensions. The initial driver for hard chrome replacement was the Clean Air Act reduction in allowable Cr6+ emissions from plating plants. As a result, HVOF thermal spray of tungsten carbide/cobalt-chrome (WC/Co-Cr) or other metal-ceramic composites is now specified for all new-design aircraft landing gear (including the Boeing 787, Airbus A380 and F-35), as well as for many hydraulic actuator rods. The process is used both by OEMs and repair shops. The wear performance and seal leakage of superfinished WC/Co-Cr are far better than that of engineering hard chrome. In addition, airlines found that overhaul turnaround time was far shorter with HVOF, where spraying takes an hour or two. Overhauling hard chrome coatings frequently involves 24 hr for plating and a further 24 hr to hydrogen bake. Adoption of HVOF technology was accelerated by the generation of all the critical performance data by the U.S.-Canadian Hard Chrome Alternatives Team (HCAT), which was funded through an international project between the U.S. DoD, the Canadian Department of National Defense (DND) and Industry Canada. However, HVOF cannot be used for most internal diameters, such as inside landing gear outer cylinders, which are frequently hard chrome or thin dense chrome-plated for wear resistance. There are very few options currently available for this type of application, but they include electroless nickel (EN), which is widely available and aerospace-qualified and nanophase cobalt-phosphorus (Co-P) alloy, developed specifically as a chrome alternative for internal diameters but not yet qualified. Overall, there is a significant increase in the use of EN to replace hard chrome. Recent improvements in the EN process make it more reliable and able to be plated to high thickness. Although EN can often be used as-deposited, maximum hardness is achieved by heat treating at about 660°F, which exceeds the allowable temperature for shot-peened high-strength steel. Having two completely different processes for finishing the exterior and interior of aircraft components presents a logistical problem for manufacturers, and it is likely that eventually one approach will shake out for both.
In the immediate future, ESH issues will tend to drive aerospace and defense manufacturers to adopt technologies similar to those used in other industries. Aluminum and zinc-nickel alloy coatings will tend to replace cadmium, while Cr3+ and non-chromium corrosion inhibitors will be widely adopted. Some of the biggest problems will occur with the simplest items. Fasteners, for example, must meet a broad range of requirements without changes to assembly specifications. The industry still lacks one of the most important types of coating—one that will resist wear while providing sacrificial corrosion protection. Because aerospace and defense manufacturers tend to be early adopters of high-performance coatings, we expect to see new coating systems coming into use over the next 10–15 years. Nanomaterials have some major advantages in terms of corrosion and wear. But potentially serious ESH issue for coatings that use them is likely to impact development of coatings with built-in nanostructures. The industry is always looking for “smart coatings” that will adapt to damage or changes in their environment, releasing corrosion inhibitors when needed, repairing themselves after damage, or indicating when they have become worn or compromised. Absent any major breakthroughs, these are concepts likely to take many years to reach fruition.
Much of the work discussed in this article was performed under the auspices of HCAT. HCAT is funded by a variety of sources, including DoD’s Strategic Environmental Research and Development Program (SERDP) and Environmental Security Technology Certification Program (ESTCP), DND and Industry Canada.