Corrosion Control and Prevention

Beginning at the manufacturing level

Types of corrosion
I have seen corrosion in all its many forms and in my experience, galvanic and intergranular corrosion are the most insidious and dangerous types of aircraft corrosion. Galvanic corrosion occurs when two dissimilar metals make contact with one another in the presence of an electrolyte. Galvanic is most troublesome because it can affect electrical and avionics systems’ pin and edge connectors. This type of corrosion can lead to random faults and life-threatening conditions, like uncommanding component and aircraft operations. Intergranular corrosion is hard to detect in the early stages, and if left undetected can lead to catastrophic structural failure. When improperly heat treated T2014 and T7075 aluminum appears to be most susceptible to this type of corrosion.

Most all of us have had Corrosion Physics 101, and have good corrosion control programs in place. The regulatory agencies and original equipment manufacturers (OEMs) have very detailed directions in their documentation on how to prevent, detect, and repair corrosion. I don’t want to dwell on these maintenance practices; rather I wanted to take a look further upstream in the manufacturing chain to see what the OEMs and parts manufacturers are doing to inhibit corrosion. I was curious to know if the OEMs and component manufacturers are using any new materials, manufacturing processes, or new design features to eliminate or prohibit corrosion. I thought if we could get a look at the current processes and materials being used to build today’s airframes, then we probably could estimate the scale of corrosion problems in the near term, as well as a few years down the road.

New materials and machining processes
I had an opportunity to visit with Mark Heasley, president and chief executive officer of Synchronous Aerospace Group out of Santa Ana, CA, about this very topic. Synchronous is a provider of flight-critical machined aluminum and hard-metal components, integrated kits and complex assemblies to the commercial, military, and space markets.

During our conversation Heasley said its companies are very progressive and customer focused. Synchronous is always looking for ways to prevent corrosion downstream in final assembly. Traditionally, the OEM provides Synchronous with standards and specifications for the components it manufactures.

However, Synchronous has taken some steps to prevent corrosion during and after its machining processes. It has virtually eliminated parts and components corrosion by going to a water-based synthetic cooling solution in its machining processes. The synthetic solution helped to reduce heat and subsequent stress to parts and components. After machining, the parts are coated with LPS oil and wrapped in a customer-specified craft paper, further reducing opportunities for corrosion.

I asked Heasley specifically about composites, as Synchronous recently announced that it has acquired Helicomb International Inc., a provider of aerospace composite structures based in Tulsa, OK. Helicomb produces commercial and military aircraft structures manufactured from a broad range of composite and metal-bonded materials for leading aerospace manufacturers, Tier 1 suppliers, and overhaul service providers.

I was curious about the types of graphite composite materials that are being used because they present corrosion opportunities when they come into contact with the traditional alloys used in aircraft manufacturing. There is a misconception that composite materials are not affected by corrosion like aluminums and steels. Composites are not inert; they are excellent cathodes and can generate galvanic corrosion. When the aircraft is operating in marine or other moist environments, composite components can become very reactive. The traditional method for preventing this type of corrosion is to apply a faying surface of sealant or quality primer between metal and composite joints to prevent moisture buildup, thereby reducing galvanic potential.

Heasley said that the company is seeing some changes in airframe materials on the B787, Airbus 350, and the F35 Joint Strike Fighter. These changes include using titanium rather than aluminum in areas that are mated to composite components, thus reducing the potential for galvanic corrosion.

In today’s operating environment, all of us — manufacturers, mechanics, and owners and operators — must aggressively prevent and control corrosion on our expensive aircraft. The down side in today’s economy is that aircraft owners will be reducing their operating hours, resulting in aircraft being parked in all kinds of corrosion-inducing environments. Many aircraft will not be in cozy hangars or dry environments like Tucson or Mojave. The upside is that the maintenance departments will have increased opportunities to conduct detailed visual and nondestructive testing. Then they will have that precious downtime to treat, replace, and repair any corrosion-damaged parts, thus ensuring that the aircraft are safe and ready to fly when the economy cycles up again.

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