Corrosion: The never-ending battle

This article has been edited for length. Read the full article in the March 2019 edition of Aviation Maintenance Magazine.

According to the latest report from LMI, the U.S. military’s corrosion cost tracking contractor, the U.S. armed forces rang up an estimated $10.2 billion in corrosion costs for their aviation and missile fleets during FY2016. (That’s just over $1 billion for the Army, $3.4 billion for the Navy/USMC and $5.7 billion for the USAF.) The metric is crude since it doesn’t distinguish aircraft from missile corrosion costs. But it tells us corrosion is an expensive problem.

Corrosion is a readiness as well as an airworthiness and safety issue. The more time an aircraft has to spend at the depot, the less time it’s available for operations, says Randy Boatwright, action officer for the Corrosion Prevention Team at the Naval Air Warfare Center Aircraft Division (NAWCAD).

Naval aviation endures a constant barrage from sea water, and the continuous cycle of humidifying and dehumidifying salt attacks the surface of the airplanes. These factors accelerate the rate of corrosion, explains Dane Hanson, future readiness lead for the organization’s Corrosion Prevention Team.

The military – with its greater challenges and advanced engine technologies – tends to drive lubricant development faster than the commercial market does, says Ed Barnes, a Global Field Engineer with ExxonMobil’s Aviation Lubricants Group. So, demands for new requirements will show up in the military market first.

Interiors as well as exteriors are susceptible. Moisture may accumulate through condensation, leakage from fuel systems and even spills in the galley area. Mark Pearson, operations manager for Lear Chemical Research Corp., recalls an 18-month-old regional jet whose main floor beams had to be replaced because of spills from the forward galley and condensation. Orange juice, coffee, club soda and tomato juice, for example, can find their way into aircraft interiors and start corroding any metal they come in contact with. Other areas to watch are the leading edges of wings, where paint can start to peel back, notes Julie Voisin, aerospace global marketing manager for Sherwin-Williams Aerospace Coatings.

Greases and oils

ExxonMobil’s airframe greases – containing corrosion-inhibiting additives – are used in areas such as flight control actuators.

However, since these greases can “migrate” or be washed off by water, they need to be applied on a “fairly aggressive relubrication” schedule, as directed by the OEM, Barnes says. All told, the company’s greases can lubricate – and protect – some 98% of airframe grease applications, he says.

The company specializes, among other things, in wheel bearing greases, which have to endure a lot of punishment. In this area there has been a move from mil-spec to non-mil-spec, commercially approved products, where a general-purpose military spec is not a good fit for a particular application. Formulations under the multipurpose grease specification, Mil-Prf- 81322G, for example, are “less than ideal” to wheel bearing lubrication, he says.

The problem is that when the wheels are removed – after 100 to 200 landings – there may be very little grease left to protect the bearings. Since the bearings are sealed, grease can’t be reapplied unless the wheel is taken off, usually because of tire wear.

ExxonMobil offers a longer-lasting, non- mil-spec aviation grease, Mobil™ Aviation Grease SHC™ 100, which the wheel bearing and landing gear manufacturers have approved “by name,” based on their testing and experience, Barnes says.

The U.S. Air Force uses Mobil Aviation Grease SHC 100 wheel bearing grease on F-16s and C-130s, among other airframes, he says. The USAF found that the older 81322G grease it was using for F-16s was migrating out of the bearings because of high RPMs during takeoffs and landings. “They tried our Mobil Aviation Grease SHC 100 about 15 years ago,” and found that it had better adhering properties.

Wheel bearings also have been getting attention, primarily over concerns about the effects of runway de-icers. ExxonMobil testing has shown that de-icer chemicals such as potassium acetate and potassium formate are just as corrosive as salt, Barnes says. SAE has finalized a new wheel bearing grease spec, AMS 3058, with “higher corrosion protection expectations” than for the Mil- Prf-81322G spec, Barnes says. The company is developing a product that meets the spec.

Based on where a grease is used, relubrication may be required as frequently as every 100 hours up to every 600-700 hours. A component like a flight control actuator that is used constantly and is exposed to water and wind would require fairly frequent lubrication.

The company also makes aviation engine oils. With an eye to military problems, the company provides a C/I “corrosion-inhibited” formulation approved under the Mil-Prf-23699 spec for turbine oils, Barnes says.

This is something the Navy and Marine Corps have used for a long time, he says. They were willing to trade some cleanliness and coking performance in order to get enhanced corrosion resistance. That’s acceptable to them because they have such an accelerated and aggressive engine maintenance schedule, he adds.

The military services perform off-wing engine maintenance every 2,000 hours or so, he says. More than that would be a very long time for a military engine to be in continuous service. Apart from the corrosion issue, other factors call for relatively short maintenance intervals. Carrier-based airplanes, for instance, experience highly stressful thermal cycles, landing at full power and then immediately shutting down. This causes coking, a deposition of oxidized oil, to form, leaving a residue in the hottest part of the engine, which has to be cleaned out.

“Corrosion: The never-ending battle” first appeared in the March 2019 issue of Aviation Maintenance Magazine. Reprinted with permission.