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The F-15E military jet’s light weight depends on aluminum, which must be treated with corrosion-protecting compounds to make it durable.
COATINGS
Confronting the looming hexavalent chromium ban Critical corrosion-limiting element is proving hard to replace in aerospace applications MARC S. REISCH, C&EN NEW YORK CITY
O
n Aug. 5, 1981, a Boeing 737 took off from Taiwan’s Taipei Songshan Airport with 110 people on board. Fourteen minutes after leaving the ground, the aircraft suffered an explosive decompression and disintegrated. No one survived. Investigators attributed the event in large part to extensive corrosion damage in the lower fuselage. More recently, the 2014 crash of a 28-year-old F-15 fighter jet in Virginia was blamed in part on corrosion. Such failures, thankfully rare, underscore the danger of corrosion in aircraft and the importance of corrosion-protecting surface treatments. However, bans on the aerospace industry’s main weapon against corrosion, compounds based on hexavalent chromium, is looming in Europe. Treatment formulators and paint makers are scrambling to find effective substitutes, but executives say the effort is a challenge. Plating chemical makers such as Coventya, Luster-On, and SurTec and paint makers such as PPG Industries and AkzoNobel have known for some time that the bans were coming. In 2013, the European Union placed a number of hexavalent chromium compounds on a list of substances of very high concern, classifying them as carcinogenic and mutagenic. As a result, under Europe’s Registration,
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C&EN | CEN.ACS.ORG | FEBRUARY 27, 2017
Evaluation, Authorisation & Restriction of Chemicals (REACH) regulations, chrome-plating chemicals such as sodium dichromate may no longer be used after Sept. 21 of this year. Corrosion-resisting paint additives, such as strontium chromate, have a sunset date in January 2019. In addition, the U.S. Occupational Safety & Health Administration recognizes Cr(VI) as carcinogenic. The severe restrictions the agency places on worker exposure to vapors from chrome-plating baths and dust from chromium spray paints are also forcing a shift to alternatives in the U.S. “Hexavalent chromium compounds have been around for more than 90 years,” says Brad Durkin, director of product management at Coventya. Replacing well-characterized older technology with less-wellknown alternatives, especially in critical applications, isn’t easy, he says. So industry groups have applied to the European Chemicals Agency for extensions to use hexavalent compounds for critical applications beyond their sunset dates. The agency has approved Cr(VI) plating treatments for aerospace use through 2024 and some Cr(VI) additives for aerospace paints through 2026. But the agency’s decision must be approved by the European Commission, Durkin points out, and that hasn’t happened yet. In addition, the seven-year
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extensions could be abrogated if acceptable licensee of the Navy’s Cr(III) surface treatCr(VI)-free alternatives are developed, he ment patents. “Most of our customers have says. made the shift to trivalent-based formulaCoventya has been working on plating tions,” says Dieter Aichert, the firm’s global alternatives, mostly based on the less hazsales director. “We ourselves shifted to ardous trivalent form of chromium. Cr(III)- supplying trivalent-based chemicals. For us based substitutes, many for decorative and and for our customers it’s a way of assuring other less critical uses, have been on the the health and safety of workers.” radar since 2012, when it became clear that However, for Cr(VI)-based coatings, European authorities planned to ban Cr(VI) such as for plane exteriors, the effort to compounds, Durkin says. But it has been shift technology is much less advanced. particularly difficult to get equivalent perThe Navy has developed Cr(III)-treated formance using Cr(III), he notes, especially aluminum particles to replace hexavalent for chrome-plating used in heavy-wear apchromium in epoxy primers. The Navy’s plications such as on landing gear. “active aluminum-rich primer” system “is One electroplating method based on equivalent to Cr(VI) systems in some castrivalent chromium uses varying electrical es, in some it is not, and in other cases it’s pulses instead of a steady better,” Matzdorf says. current. Jennings Taylor, The aluminum-rich founder of Faraday Techtechnology isn’t commernologies, developed the cial yet, though Matzdorf technique in 2004 and won says the Navy is working a Presidential Green Chemwith six licensees. Meanistry Award for it in 2013. while, paint makers have He says the process is still been developing their own not commercial. alternative corrosion-inTaylor is working with hibiting primers and paints. Coventya on a Cr(III) AkzoNobel, for instance, formulation that would has patented lithium-based match the wear profile and corrosion inhibition techmicrostructure of a Cr(VI) nology for exterior aerofinish. Once that is done, he space primers and coatexpects a long qualification Joe Ciejka, vice president ings, and some customers process before it is adapted of development and are now using them, the for aerospace use. technology, Luster-On firm says. For structural Luster-On, a supplier applications, Akzo says of plating chemicals, has also developed customers still rely on Cr(VI), though the Cr(III) alternatives that in most cases refirm is developing alternatives. place the more hazardous standard materiPPG is taking a different approach with als. But trivalent chromium is not a perfect its Aerocron electrodeposited epoxy-based replacement. primer system. Dependent on a proprietary “Hexavalent chromium outperforms the nonchromium corrosion inhibitor, Aerotrivalent species in cost and performance,” cron is being used by the U.S. Coast Guard says Joe Ciejka, vice president of developfor parts maintenance, says Mehran Arbab, ment and technology for Luster-On. “The PPG’s global R&D director for aerospace performance issue is an important considcoatings. eration for aerospace components, which In the Aerocron system, components are have to last between 20 and 40 years.” dipped in a treatment tank and then cured Some of Luster-On’s surface treatment in an oven, providing uniform coverage, technology is based on patents licensed even to complex parts. That’s an advantage from the U.S. Navy. The Navy developed its over Cr(VI)-based spray primers that are own Cr(III) recipes because it needed to hand applied, Arbab notes. The Aerocron protect maintenance personnel and wasn’t system also does not pose any of the health satisfied with existing trivalent technology. hazards associated with hexavalent chro“Our formulas give excellent adhesion mium, he says. and corrosion protection on aluminum and PPG and other players in the aerospace aluminum alloys,” says Craig Matzdorf, a coatings sector know they have their work senior materials engineer in the Naval Air cut out for them. “There are a lot of barriers Systems Command and primary inventor to replacing hexavalent chromium, and on the Navy’s seven U.S. patents. Other the longer something has been around, the formulations, such as those based on rare harder it is to change,” the Navy’s Matzdorf earths and magnesium, did not perform to says. But because of the hazards hexavalent the Navy’s standards, he says. chromium use poses, he says change “is The plating specialist SurTec is another part of our job.” ◾
“Hexavalent chromium outperforms the trivalent species in cost and performance.”
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