Introduced almost fifty years ago on steel compressor cases in J-79 engines, aluminum-ceramic coatings have come to play an essential role in protecting steel components in turbines and turbomachinery. The water-based "spray and bake" inorganic phosphate-base coatings are uniquely resistant to salt corrosion and heat, but pressure is growing to find an alternative to these materials. The slurries contain hexavalent chromium (Cr(Ⅵ)), a known carcinogen. In Europe, REACH initiatives restrict import and use of solutions containing hexavalent chromium. In the US, workplace exposure to Cr(Ⅵ) is limited to a mere 0.005 mg/m~3 (5 micrograms) of Cr(Ⅵ) per person per 8 hour day and employers must monitor the health of those exposed even to such low levels. Aluminum-phosphate slurries clean up with water, but all effluent liquid must be captured and treated to remove Cr(Ⅵ). Solid waste that comes in contact with the slurry or rinse water must be segregated for proper disposal. Efforts to develop a safe, aluminum-phosphate coating without hexavalent chromium began in the early 1990s, but have met with limited success. Recently, a sacrificial, heat resistant aluminum-ceramic has been developed that contains no hazardous materials. This paper will review efforts to develop Cr-free Al-ceramic coatings and explain the promise of a new product that uses a different chemistry to produce an aluminum-ceramic coating that is protective, yet inherently non-hazardous.
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