Coat, don’t plate: General Industry Coverage

Hard-chrome plating (HCP) provides parts, such as ones for the aerospace industry, with a wear- and corrosion-resistant surface but requires the use of hexavalent chromium salts. Those salts are genotoxic carcinogens and pose health and environmental hazards, and, therefore, are restricted in the U.S. by the Occupational Safety and Health Administration and in the European Union by REACH (Registration, Evaluation, Authorization and Restriction of Chemicals).

Courtesy of Hardide Coatings

Parts to be coated with Hardide-A are loaded into a CVD coating reactor.

“Hexavalent chromium has been a concern for many years, but the alternatives have always been limited,” said Dan Wilson, business development manager for Hardide Coatings Inc., which offers the Hardide-A coating as an environmentally and worker-friendly alternative to HCP.

Workers applying hexavalent chromium-containing products are exposed the most, particularly when chrome plating because the processing bath produces bubbles, which generate a difficult-to-capture fog, according to Dr. Yuri Zhuk, technical director for U.K.-based Hardide Coatings Ltd.

Hardide-A, on the other hand, is a tungsten carbide-based CVD coating that is applied in a coating reactor, which doesn’t expose workers to environmental hazards. Zhuk added that although spray coatings, such as high-velocity oxy-fuel, are considered the best available alternative to HCP, those are line-of-sight processes that can only be applied to external surfaces because a spray gun can’t fit, for example, into a small bore.

“In the past, hard chrome was often seen as the best option for hardening interior surfaces because spray coatings are difficult or impossible to apply in tight spaces,” Wilson said.

The company stated that the CVD coating’s hardness of 800 to 1,200 HV and standard thickness from 50µm to 100µm (0.002 ” to 0.004 “) match the hardness and thickness of HCP, simplifying the transition without the need for part dimensional changes and drawing redesigns. In addition, Zhuk said Hardide-A offers better corrosion resistance because it passed a salt-spray test for 2,000 hours, whereas HCP failed after 280 hours. The Hardide-A coated surface is also free of microcracks, which are always present in HCP. “These microcracks form channels for corrosive media to attack the substrate,” he said.

Courtesy of Hardide Coatings

A technician measures parts coated with Hardide-A.

The CVD-coated parts have surface finishes as fine as 0.3µm R_a, but can be ground, honed or lapped, if necessary, Zhuk noted.

The company is working on test programs with customers, including helicopter manufacturer AgustaWestland, Airbus and other aerospace OEMs. According to Hardide Coatings, it has successfully completed the first of two test phases to approve Hardide-A as a replacement for HCP on an AgustaWestland helicopter rotor blade component. The first phase involved testing basic properties, such as corrosion resistance and fatigue life, and the second phase will test actual coated components.

To control quality, the company applies Hardide-A at its coating facilities in Bicester, Oxfordshire, U.K.

For more information about Hardide Coatings Inc., Houston, visit www.har dide.com or call (713) 677-3504. CTE

Glossary terms in this article

• corrosion resistance
  Ability of an alloy or material to withstand rust and corrosion. These are properties fostered by nickel and chromium in alloys such as stainless steel.
• fatigue life
  Number of cycles of stress that can be sustained prior to failure under a stated test condition.