Since the 1940s, chrome has added a shiny protective coating to metal products from bathroom fixtures to car bumpers. But chrome’s beauty and durability come at a high cost. Though it’s cheap to produce and harmless to consumers, the process used to apply it is dangerous for workers and pollutes the environment.
That process involves running a current through a liquid bath of chromium ions to deposit a thin layer of chrome on the surface of a metal object placed in the bath. The ions, known as hexavalent chromium, are carcinogenic if inhaled, and contact with the liquid can be fatal. Hexavalent chromium can also pollute groundwater; some of the original Superfund cleanup sites involved this type of pollution.
Chrome is “an environmental nightmare,” says Christopher Schuh, an associate professor of materials science and engineering at MIT. “People have been trying to replace it for a very long time.” The problem, he says, is that it’s the only plated metal coating that provides three very desirable properties: hardness, long-lasting shine, and protection from corrosion. So manufacturers have put up with the mess and the safety hazards, which require strict handling precautions. Chromium electroplating has become a $20 billion industry.
Now Schuh and his collaborators have developed an alloy that is safer than chrome–and more durable. The new coating, which is being tested on the bumpers of a fleet of trucks, could also find use in faucet fixtures, engine parts, and other applications.
Knowing that chrome’s nanocrystalline structure makes it harder than steel, Schuh and his group set out to duplicate that structure in a material that could be easily and safely applied by electroplating. They started with nanocrystalline nickel, but nickel on its own, though very hard when first deposited, isn’t as stable as chrome. Its crystals gradually expand, causing the coating to lose its hardness. Using computer models developed to predict material properties, Schuh settled on a nickel-tungsten alloy.
Schuh’s team has shown that nickel-tungsten alloys remain stable indefinitely at room temperature and are highly resistant to decomposition when heated. They can also be made harder and longer lasting than chrome. Recent tests showed that print rolls coated with the new alloy lasted 10 times as long as their chrome-plated counterparts.
The electroplating process is also more efficient with the new alloy, because multiple layers can be applied in one step. That could save money for manufacturers. “Not only do you get rid of the environmental baggage, but you make a better product as well,” says Schuh.