Aluminum Aerospace Coating May Replace Toxic Chromates
A materials engineering research team at the University of Nevada, Reno has developed an environmentally friendly, self-healing coating for aluminum that could replace toxic chromates. (Source: University of Nevada, Reno/Mike Wolterbeek)
@Ann: Thanks for a good article on a timely topic. It would be interesting to see polarization curves and salt spray testing results for the molybdate coatings.
There is a wide variety of non-chromate conversion coatings, including permanganates, zirconates, titanates, cerates, and others. There are also conversion coatings based on trivalent chromium. Some of these have been around for many years, while others are newer. However, to the best of my knowledge, none of them provide corrosion resistance that is quite as good as that of hexavalent chromate.
Electrodeposited coatings can meet or exceed the performance of chromate, but it is challenging to coat internal passages with electrodeposited coatings, and they don't have the self-healing characteristics of conversion coatings.
The U.S. military has a great on-line database with information about alternatives to chromate conversion coatings.
Chuck, I wish we could get more cost info on some of these new technologies. As usual, not much is known yet until it's in use in industrial quantities. OTOH, the fact that it's non-toxic may be worth a cost differential.
@TJ McDermott: The same thing that makes 7075 and 2024 so much stronger than other aluminum alloys also makes them less resistant to corrosion: copper. The copper-containing intermetallic particles that are responsible for these alloys' high strength create tiny galvanic cells within the material itself.
In general, if you can protect 7075 and 2024 against corrosion, 6061 will be easy.
TJ, I mentioned those specific alloys because the sources did, as the coating was developed specifically for mil/aero applications. But they also stated that this coating works on "all aluminum products." That sounds like all aluminum alloys.
Good discussions. Non-toxic alternatives to chromate coatings of aluminum can move us closer to the overall environmental objectives of such programs as RoHS. Looking at overall lifecycle costs is important when considering a process change as you noted. Chromates consume resources in the disposal of spent coating residues.
Thanks, Dave, for the summary and all those links. Performance is often the main issue with environmentally friendly substitutes, whether it's coatings or fuel, that are candidates for replacing toxic ones. This one sounds like it's pretty close.
As a manufacturer of aluminum connectors for harsh environments the subject of what coating to use for corrosionprotection is a constant. Especially today with RoHS and REACH activities. Needless to say I am imensely interested and was hoping you can provide contact information for Chidambaram or a path to learn more?
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