The Savvy Aviator #19: Thwarting Corrosion
Many of our GA aircraft were not very well corrosion-proofed during manufacture, so it's up to us as owners to keep our airframes corrosion-free. Regular application of corrosion preventive compounds can protect the parts of our airframes that the factory didn't.
I have a soft spot in my heart for the year 1979, because it was the year that Cessna built the T310R that I've owned and flown for the past 18 years. Actually, 1979 was a very good year for general aviation. Manufacturers like Beech, Cessna, Mooney and Piper were assembling craft at a furious pace, delivering about 17,000 new GA aircraft that year. The industry mood was understandably optimistic. Few foresaw that within a few years, demand for new GA aircraft would dry up almost completely -- due to massive changes to the U.S. tax code coupled with a nasty, double-dip recession -- and production would fall off a cliff (to less than 1,000 new aircraft delivered in 1994).
Back in GA's salad days, people bought airplanes much as they did cars. They bought them new, flew them for a few years, then traded them for something bigger, faster or fancier. The aircraft manufacturers designed and built those aircraft in anticipation that they would have a useful life of 10 years or so. At the time, that was not an unreasonable prediction, but it turned out to be terribly wrong. When the production of new airplanes all but stopped in the '80s and '90s, owners had little choice but to keep flying their aircraft build in the '60s and '70s. As a result, the lion's share of airplanes in today's GA fleet are 30 or 40 years old.
Corrosion has taken its toll on many of those aircraft. Because the manufacturers didn't expect them to remain in service more than a decade, most didn't do a very thorough job of corrosion-proofing. Look inside the wings or tailcone or under the floorboards of most '60s- and '70s-vintage airplanes and you'll see mostly bare aluminum. Only the relative handful of aircraft that were ordered as floatplanes received internal corrosion-proofing (with zinc chromate primer).
The industry has learned from its errors. If you look at the new GA airplanes coming from Cessna, Cirrus, Diamond, Lancair, Mooney or Piper, you'll find the factories are paying a lot more attention to corrosion proofing these aircraft. Most of today's production aircraft will probably last as long as anyone wants to fly them.
But that's little consolation to "the rest of us" who own and fly older Spam cans with little or no factory corrosion-proofing. It's up to us to make up for what the manufacturers failed to do 30 or 40 years ago.
What Exactly Is Corrosion?
Corrosion is the destructive attack of metal by electrochemical reaction to the surrounding environment. In order for corrosion to occur, four conditions must be present:
- A metal that will give up electrons ("anode");
- A dissimilar metal that will accept electrons ("cathode");
- An electrical connection between the two metals (typically metal-to-metal contact); and
- A conductive liquid ("electrolyte"), typically condensation (H2O) made conductive by the addition of a bit of salt, acid, or other chemical impurities.
|Corrosion cell, showing the four conditions that must exist for corrosion to occur. (Click graphic for larger view.)|
If you think this sounds a lot like the description of a battery, you're exactly right. A battery produces electricity through controlled corrosion of metallic electrodes (cathode and anode) immersed in an electrolyte (acid or alkali), and continues to do so until the anode is completely corroded.
Corrosion is quite useful when it occurs in a battery, of course -- but in an airframe or engine, it's clearly a very bad thing.
The best-known example of corrosion is the rusting of iron and steel; but perhaps of even greater concern to aircraft owners is the corrosion of the aluminum alloys that make up the majority of our airframes.
Why Airframes Corrode
Pure aluminum is highly resistant to corrosion, because on exposure to air it forms a very thin (virtually invisible) protective layer of aluminum oxide. Aluminum oxide doesn't conduct electricity, so once this tough protective layer forms over the surface of the metal, corrosion is effectively blocked.
Unfortunately, pure aluminum is too soft to be useful as a structural material. Airplanes are actually made of various high-strength aluminum alloys, which consist of mostly aluminum plus small amounts of other metals that improve the strength and toughness of the material.
The most common aluminum alloy used in our airframes is called "2024" and is composed of 95% aluminum, 4% copper, and small amounts of manganese and molybdenum. The resulting alloy is then heat-treated and rolled to create sheet metal (known as "2024-T3" where 2024 specifies the alloy and T3 is the heat-treatment code) that has excellent strength and toughness -- but substantial vulnerability to corrosion.
You see, aluminum is substantially more anodic (i.e., willing to give up electrons) than copper (see the Galvanic Table below). So put these two metals in contact (as they are in 2024 alloy), add some condensation (electrolyte), and voila! -- you have all the necessary conditions for corrosion.
|Galvanic Table, a ranking of metals from least active (top) to most active (bottom). The farther apart two metals are in this table, the stronger the electro-chemical reaction between them when an electrolyte is present. The more active metal (lower in the table) is the one that will corrode.|
|(Most cathodic, noble, and resistant to corrosion)||Platinum|
|Stainless Steel (passivated)|
|Nickel, Inconel (passivated)|
|Copper, Brass, Bronze, Monel|
|Nickel, Inconel (active)|
|Stainless Steel (active)|
|Aluminum (2024 alloy)|
|(Most anodic and vulnerable to corrosion)||Magnesium, Magnesium Alloys|
The key to preventing corrosion of 2024 aluminum is to shield it from electrolytes. To accomplish this, sheets of 2024-T3 are coated ("clad") on both sides with very thin layers of commercially pure aluminum. The resulting material is known as "2024-T3 Alclad" and is the principal material used in our airframes.
Because pure aluminum is inherently corrosion-resistant, cladding does an excellent job of protecting aluminum alloy sheet metal from corrosion. But Alclad is vulnerable wherever the cladding is breached -- at cut edges and drilled rivet holes, for example. The cladding is also extremely thin -- about .001" thick -- so it's quite easy to breach by scratching. This explains why most airframe corrosion occurs at seams and joints, and why cladding alone is not sufficient to keep corrosion at bay.
|Severe galvanic (dissimilar-metals) corrosion found inside a horizontal stabilizer. (Click for larger version.)|
Another kind of airframe corrosion occurs where dissimilar metals come into contact -- for example, where steel screws are used to fasten aluminum parts, or where a stainless steel firewall is riveted to aluminum structural members. This is known as "galvanic corrosion" or "dissimilar-metals corrosion." This photo shows a particularly severe example, found inside a horizontal stabilizer.
Besides sealing out electrolytes, galvanic corrosion can be prevented by carefully selecting the kinds of metal that are permitted to come into contact with one another. For instance, steel fasteners (screws, bolts, nuts, washers, etc.) are commonly made compatible with aluminum by electroplating them with a thin layer of metallic cadmium -- such fasteners are referred to as "cad-plated." If you look at Table 1, you'll see that cadmium is more anodic than either steel or 2024 aluminum alloy, and so protects them from corrosion.
Zinc is even more anodic than cadmium, and steel parts are often plated with zinc (so-called "galvanized steel") to protect them from corrosion.
Corrosion-Proofing Outside ...
We've seen that aluminum alloys like 2024 innately possess three of the four necessary conditions for corrosion to occur -- namely, an anodic metal (aluminum) and a cathodic metal (copper) in physical contact with one another. Consequently, the only way to prevent such alloys from corroding is to eliminate the fourth necessary condition: the presence of an electrolyte. In other words, moisture and other conductive liquids must be kept away from the metal.
We've also seen one method of doing this: cladding the alloy with a thin coating of commercially pure aluminum that is inherently corrosion-resistant. The corrosion resistance of the pure aluminum cladding may be further enhanced by chemical treatments such as Alodine. But since cutting and drilling the Alclad sheet metal breaches the cladding, additional steps are necessary to protect seams, holes, and un-clad parts from exposure to electrolytes.
This is traditionally accomplished with sealants, and the most commonly used sealant is paint. Modern polyurethane aircraft paints create a thick, impenetrable barrier that effectively keeps moisture away from the metal, and lasts a long time -- 10 years or more. A good paint job is the best defense against airframe corrosion.
Unfortunately, paint only protects the exterior of the airframe. Most manufacturers didn't paint the interior (except for airplanes built as floatplanes) -- and for all practical purposes it's impossible to paint the inside of an airframe once it's all riveted together.
... And Inside
To address this problem, the industry has developed various corrosion preventive compounds (CPCs) that can be applied to the interior airframe surfaces. CPCs provide an effective means for protecting those parts of an aircraft that were not painted or otherwise protected from corrosion by the factory. Unlike paint, CPCs may be applied with little or no surface preparation, and without elaborate equipment or environmental controls. If the airframe is already opened up (e.g., for an annual inspection), CPC treatment typically takes only an hour or two.
Application is usually performed using high-pressure low-volume equipment and thin spray wands that atomize the CPC into such a fine mist or fog that it permeates the inside of the fuselage, wings, empennage and control surfaces, and reaches even areas that are not directly accessible. CPCs are designed to have excellent penetrating action that permits them to wick into lap seams, rivet holes, crevasses, and other hidden areas that are vulnerable to corrosive attack.
There are two basic types of CPCs that are commonly used on GA aircraft. One type, which has been used successfully for decades, creates a thin, waxy film that acts as a sealant to block moisture and other electrolytes from reaching the underlying metal -- much like paint, but much easier to apply (and much less permanent). The most widely used products of this type are LPS-3 and Boeshield T-9.
|Thin-film dielectrics consist of man-made molecules with one end that adheres to metals and the other end that blocks moisture and electrolytes. (Click for larger view.)
The other type of CPC, which first became available in the late 1980s, is known variously as a thin-film dielectric (TFD) or fluid thin-film coating (FTFC). Both ACF-50 (made by Lear Chemical Research Corporation in Canada) and CorrosionX (made by Corrosion Technologies Corporation in Dallas, Texas) employ this technology. These compounds consist of complex, man-made molecules with one end that adheres to metals and the other end that blocks moisture and electrolytes.
Which type of CPC works best? Both types have ardent supporters, and each has its own advantages and disadvantages. The waxy film formers (LPS-3, Boeshield T-9) tend to last longer (three or four years between applications) and don't "weep" for more than a few days. The thin-film dielectrics (ACF-50 and CorrosionX) tend to penetrate better into lap joints and rivet holes, and to do a better job of neutralizing active corrosion cells -- but they need to be reapplied more frequently (generally every two years) and may "weep" for weeks or months after application (particularly if applied a bit too generously).
All four products do an excellent job of corrosion prevention. Unless your aircraft is based in a bone-dry climate or was manufactured in the last 10 years, regular treatment with one of these CPCs should be an indispensable part of your maintenance program. It's not unusual for airframe corrosion to go undetected until it gets so bad that it's uneconomical to repair. An ounce of corrosion prevention is worth a pound (or maybe $20,000) of cure.
See you next month.
Want to read more from Mike Busch? Check out the rest of his Savvy Aviator columns.