Combating Corrosion in Aerospace Parts, from Start to Finish

Rusty signs and gates. Copper electrical terminals, slowly turning green. It’s a sad fact of metallurgy that, with a few exceptions, all metals oxidize and—if left unprotected—corrode as a result.

You can blame oxygen, a highly reactive gas that plays a leading role in the phenomenon that eventually takes down even the tallest of buildings and sinks the most massive of ships.

Fortunately, some metals are at least corrosion-resistant, if not completely corrosion-proof. At the top of the list are noble elements like gold, silver and platinum, which are far less susceptible to oxidation, a process referred to as rusting in steel and other iron-bearing metals.

Then there are aluminum, chromium, titanium and nickel. Like their noble cousins, these form a thin layer of metal oxide when exposed to air, protecting their surfaces from further corrosion. That quality, combined with a high strength-to-weight ratio, extreme toughness or combination of the two, is why the metal alloys made from them are favorites in the aerospace industry.

Machining Corrosion-Resistant Alloys

Those same qualities make them notoriously tough to machine, a challenge on which John Giraldo, aerospace engineering projects manager for the Americas at Sandvik Coromant, can offer plenty of advice.

“Our expertise lies in the development of cutting tools that can efficiently and consistently machine these very tough materials and do so in the shortest time possible,” he says. “Materials like Rene 41 and Inconel 718 are not only extremely corrosion-resistant but can withstand very high temperatures, which is why they’re so popular with engine manufacturers, but there’s also plenty of titanium as well as 300-series and PH stainless steel to contend with.”

Richmond offers one caveat to his statement: The cutting fluid must be clean, well-maintained, and kept at the proper concentration level. Further, it should be checked periodically for the passivating compounds that help prevent oxidation in certain metals.

“If you’ve been machining large aluminum aerostructures nonstop for the past couple of years, chances are good that these compounds have been depleted and need a refresh, either with an additive or by changing the fluid out,” he says. 

Unseen Corrosion Risk

Richmond notes that aerospace manufacturers are just as concerned with the environmental corrosion discussed so far as they are with galvanic corrosion, which occurs when dissimilar metals—aluminum and stainless steel, for instance—are left in direct contact with one another, creating a battery effect that gradually destroys the softer of the two metals involved.

Greg Bruce, a senior technical marketing manager with ITW Pro Brands, agrees. He explains that aircraft require regular maintenance, much of which involves inspecting the myriad parts within for both types of corrosion. If corrosion is found, the components are cleaned and treated with a rust inhibitor or lubricant, or sometimes replaced. “If a part’s critical to aircraft function, they’ll just put a new one in,” Bruce says.

Since most of the materials used in modern aircraft are naturally corrosion-resistant, they require little in the way of rust preventatives, he reiterates. The notable exception is where galvanic corrosion occurs, such as in bilge areas and an airplane’s belly. Here, steel alloys are sometimes used, particularly on legacy aircraft like the C-5 Galaxy and C-17 Globemaster, military planes known for their load-carrying capabilities.

“Pretty much everything you can see on any aircraft is rustproof,” adds Tony Reed, brand manager for the ITW Pro’s LPS product line. “It’s all the stuff you can’t see that’s most prone to corrosion, especially in environments that are high in humidity or near saltwater.”

Choosing the Best Lubricant

Whatever the type of corrosion, whichever components are involved, Reed is quick to point out that using the correct lubricant to address the problem is crucial. “LPS 1, 2 and 3 are the crown jewels of the LPS line, and are often called out specifically by several well-known aircraft manufacturers,” he says. 

LPS 1 goes on wet but dries quickly, provides around 30 days of protection and “is really good at displacing moisture from niche areas and crevices, which is an important step before applying a true rust preventative like LPS 2, an oily, non-drying penetrating oil,” he explains.

“LPS 2 is the jack of all trades—a lubricant, penetrant, and corrosion inhibitor that offers a couple years or more of protection,” Reed says. “Then there’s LPS 3. It has limited penetration ability but creates a soft, waxy, self-healing film that’s great for areas where the metal is flexing a little bit, like the structural components I just mentioned. Whatever the application, though, it’s important to choose the correct product, and be sure to reach out for help if you need a recommendation.”

What are your best tips for overcoming corrosion risk? Tell us in the comments below.