Corrosion coating: Chromate replacement

Coating analysis

 

Fourier transform infrared and Raman microspectroscopy, as well as other techniques have been used to test a putative, non-carcinogenic metal coating material based on molybdate to replace chromate for corrosion protection of aluminium in the automobile, aerospace and engineering industries.

Researchers at the University of Nevada, in Reno, USA, have developed what they refer to as an environmentally friendly way of coating aluminium to protect engineering components from corrosion. The coating could replace chromate coatings, which have been used in aerospace and other applications for more than half a century. Details of the work were presented to the international Pacific Rim Meeting on Electrochemical and Solid-State Science in Hawaii in October.

Lead scientist Dev Chidambaram, who has built on fourteen years work to develop the coating, explains that, "There is no question that this will be able to replace the chromate-based coating. Even though the coating formulation is yet to be optimized, the coating has shown exceptional performance."

Materials scientists have been attempting to find ways to replace chromate coatings with non-toxic alternatives since the 1980s. However, there was significant negative publicity surrounding chromates wrought in 1993 by Erin Brokovich and the subsequent dramatisation of events in the eponymous 2000 movie. Carcinogenic chromates are now banned in consumer and automotive products, but their use in the defence and aerospace industries is exempted in the absence of viable alternatives. The risk-benefit concerning their toxic effects being offset by the impact corrosion of aerospace components can have on passenger safety being an important consideration in their continued use.

The main obstacle until now for finding a viable replacement is that chromate coatings can "self heal" if they are damaged or scratched whereas no previous alternative has had that property. When scratched, the coating components from nearby sites can migrate to the damaged region and re-protect the underlying alloy. Now, Chidambaram and colleagues have demonstrated how a new molybdate-based formulation can perform comparably well to chromate coatings and undergo self-healing. The current formulation is the result of testing of more than 300 new coating materials.

The team used a raft of advanced surface analytical techniques including Raman microspectroscopy, Fourier transform infrared (FTIR) spectroscopy, energy dispersive spectroscopy (EDS), secondary ion mass spectroscopy (SIMS) and X-ray photoelectron spectroscopy (XPS) to demonstrate that molybdate can migrate into scratched regions of a coated sample of aluminium. Additional electrochemical tests also showed the how the self-healing diffusion occurs.

Self-healing surfaces

The Reno team included graduate student David Rodriquez, who carried out the extensive testing necessary on the coating materials, as well as summer intern Roshan Misra. The researchers are still endeavouring to optimize the coating formulation for even better corrosion protection.