The current surface protection methods employed by many original equipment manufacturers (OEMs) pose special environmental and safety concerns. Much of the industry still relies on hard anodizing (designated Dow 17) or a chromate conversion process (designated MILM-3171), which is being used by repair depots. These all involve the use of hexavalent chromium. The electrolyte used to apply the coating comprises sodium dichromate, ammonium acid fluoride, and phosphoric acid which are hazardous to the environment and difficult to recycle.
These operations also produce harmful vapours and hazardous waste in the form of contaminated wastewaters and solid waste, which can be costly to dispose of. There are also safety issues to contend with, such as the formation of hazardous vapours that contain metal salts and carcinogenic substances. The later rise up from heated bath solutions and require additional worker protection measures.
These surface treatment processes are followed with the application of a phenolic resin sealer and then a chromated primer and topcoat, on most surfaces. Robinson has reported that this surface treatment regimen, which incorporates the use of sealers, can increase the resistance of magnesium to salt spray corrosion.
However, regardless of the progress made to improve the corrosion resistance of magnesium alloys, millions of dollars are expended each year on repair and replacement of magnesium alloy components that have corroded or have been damaged. In addition, emerging legislation will be reducing the hexavalent chromium permissible exposure limit from 52 to 1 μg/m3 making the replacement of processes using this compound mandatory.
CORSAIR development will offer a more energy- and material- efficient maintenance route that provides environmental benefits such as the elimination of harmful liquid waste.
The development and qualification of the cold spray process to deposit CP-Al was recommended by the ARL Center for Cold Spray for the purpose of dimensional restoration and protection. The cold spray process was viewed as the best possible method for depositing the aluminum coatings and would be viewed as part of an overall strategy of replacement of the chromate processes (such as Dow 17 and MIL-M-3171 being in use today), eliminating environmental and worker safety issues, while significantly improving performance and reducing life-cycle costs.
While there are efforts currently underway to refine maintenance practices, minimize human exposure and establish acceptable disposal processes, the best way to mitigate the risks associated with the use of these agents is to minimize or eliminate their use.