AN ADVANCED COATING system for magnesium-alloy components is being adopted by the US Navy after trials showed its ability to increase dramatically the service life of helicopter gearboxes.

During the two-year trial, the Navy coated a sample batch of its Sikorsky SH-60B helicopter transmissions with Sermatech International's SermeTel 1083/1089 coating system. It found that the average life of the gearbox tails and centre-sections was extended by 75%, resulting in substantial savings in spare-parts costs.

Many helicopter components are made from magnesium alloys because of their high strength-to-weight ratios, but the high galvanic nature of magnesium makes it particularly vulnerable to corrosion. The problem is exacerbated because helicopters tend to be operated at low altitudes, where humidity is higher and airborne particles are present, and in marine environments, where salt levels are high.

"Historically, the US Navy was scrapping about half of the SH-60B transmission housings after a single tour, because of corrosion," says Sermatech.

"Another problem was that a baking varnish applied to the internal housing walls was chipping and flaking off in service, which potentially could have clogged the lubricating-oil passages of the transmission," it adds.

The housings had previously been treated with conventional military-specification epoxy-type paints and primers.

The Sermatech coating system has, also been approved by the US Federal Aviation Administration for use on MBB BO105 and Bell 412 helicopter transmissions and gearboxes.

Source: Flight International