Most GA aircraft were not very well corrosion-proofed during manufacture, so it's up to us as owners to keep our airframe corrosion-free. Regular application of corrosion preventive compounds can protect the parts of our airframe that the factory didn't.
What Exactly Is Corrosion?
Corrosion is the destructive attack of metal by electro-chemical 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.
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, 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 airframe.
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 float planes) -- 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. We use Corrosion X.
One application every two to three years will save you thousands of dollars on corrosion removal. Stop corrosion before it starts and if it has already started, keep it from growing.
