A research team at the University of Nevada, Reno has developed a new environmentally friendly coating for aluminum to replace the carcinogenic chromate coatings used in aerospace applications. The chromate conversion coatings have been used for more than 50 years to protect aluminum from corrosion.
Attempts to replace chromate coatings with non-toxic coatings have been underway since the 1980s. The awareness on effects of chromates was brought to the forefront in 1993 by the real-life incident involving Erin Brokovich and depicted in the movie released in 2000 of the same name. Although the use of chromates for consumer and automotive applications has been banned, it is still in use by the defense and aerospace industries under various exemptions.
The carcinogenic coatings were exempted from the ban due to unavailability of suitable replacement combined with the high human and financial cost of failure from corrosion. The search for a suitable replacement has been elusive primarily due to one main characteristic of the coating referred to as "self-healing," the ability of the coating to heal itself after being damaged or scratched.
When scratched, the coating components from nearby sites migrate to the damaged region and re-protect the underlying alloy. The new formulation performs comparably to the chromate formula in its ability for self-healing, which is important to the defense and aerospace industry. The coating can be applied to all aluminum products. The new formula creates an environmentally benign, molybdate-based coating that provides corrosion protection to aluminum, used for aircraft and spacecraft. These coatings, when damaged, will re-heal themselves.
The team developed and tested more than 300 coatings before arriving at this formulation. They used a complimentary suite of advanced surface analytical techniques such as Raman microspectroscopy, Fourier transform infrared spectroscopy, energy dispersive spectroscopy, secondary ion mass spectroscopy, and X-ray photoelectron spectroscopy to conclusively prove the presence of molybdate in the scratched region. Further, using electrochemical testing, the team showed the coating re-protected itself via self-healing upon scratch test.