Hexavalent chromium (CrVI) plating is cheap, fast and effective. It has been used in a wide range of industries to plate other metals until workers discovered the toxic effects on health after exposure to CrVI. The official ban in the EU started in Sept 2017, and operations are only allowed with special permits from Brussels.
Researchers and companies in many industries have been seeking viable alternatives to hard chrome plating for years. Matmatch outlines the results of these efforts and discusses which methods are suitable for the most popular applications.
What is Hexavalent Chromium?
From the 1940’s until quite recently, hexavalent chromium was the most popular hard chrome plating material. The “hexavalent” category includes any chemical compound containing chromium in a hexavalent state of oxidation. Chromium trioxide, a hexavalent chromium compound, is still used under stringent conditions to produce hard chrome plating. During the deposition process, hydrogen and oxygen bubbles release a mist of hexavalent chrome into the atmosphere.
The Advantages of Hexavalent Chromium Plating
The reason why hexavalent chromium has been so popular for hard chrome plating is because of its favorable corrosion resistance properties, high level of hardness, low friction and resistance to abrasive wear. Additionally, it can be applied in a wide range of thicknesses, an important element for reducing fatigue. These properties suit a whole range of applications. Decorative chromium plating gives an attractive glossy finish to otherwise dull metals. Chromium plating is used to increase hardness and abrasion resistance of industrial tools such as hydraulic cylinders and rollers. The plating also facilitates easy cleaning of components. Hexavalent chromium is also fast, simple and cost-effective to produce.
The Health Risks of Hexavalent Chromium
Chromium trioxide is an oxidizer that is also a possible carcinogen. Workers who inhale the mist of hexavalent chrome that is released into the air during the deposition process have reported health problems such as pulmonary cancer, nasal septum, asthma and skin conditions. For these reasons, the US Environmental Protection Agency defined hexavalent chromium as hazardous in 2006 and it is slowly being phased out of use.
Alternatives on the Market
While hard chrome plating suited a wide range of applications, the alternatives must be chosen with more care and depend very much on the application. Safer alternatives include thermal sprays, electroplating and electroless plating with nickel or cobalt-based products, vacuum coatings, heat treatments and weld and laser coatings.
Thermal sprays such as high velocity oxygen fuels are well-suited to applications requiring maximum hardness such as landing gears for aircraft, industrial rolls and most hydraulic rods. They offer a high level of hardness and good wear-resistance and are suitable for rebuilds. Thermal spray is the most commonly used method to replace hard chromium plating as it can be applied to plenty of industrial processes using a wide range of coating materials and equipment.
Electroplating and electroless plating use similar technology to the original chrome plating process and have a wide range of application types. Electroless nickel-based plating is particularly good for internal components.
Vacuum coating methods such as physical vapor deposition (PVD) are expensive, but they do offer the best level of hardness and wear-resistance out there. Due to the cost and complexity, they are most often applied to small items with a high worth. Heat treatments are of course limited to heat-resistant materials only but can be applied in just about any form and size. They are not suitable for delamination or rebuilding. Finally, weld coatings and laser coatings are ideal when a thick coating is required, but they are limited to heat-resistant materials only and require refinishing. Welding is mostly applied to external components.
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