Not long ago, a man who was training to do structural repairs on aircraft was watching, along with his instructor, as I performed a preflight inspection of an airplane. I pointed out a few of the things I was looking for, but I was a little self conscious, feeling that these guys probably knew how to take the airplane apart and put it back together, while all I knew how to do was ensure that all the bits were attached in the right order. Then the instructor asked the student a simple aerodynamics question which he couldn't answer, plus the student misidentified an aileron, so I realized I wasn't enduring as advanced a scrutiny as I had believed.
A few minutes later I discovered a few spots of corrosion, raising bubbles in the paint on one side of the fuselage. I pointed them out, surely a point of interest for my observers. You don't have to have any training or know that my airplane was made of aluminum to know that corrosion is an undesirable event. It weakens the structure. I heard the instructor ask the student what kind of corrosion it was. Kind? Um ... I didn't know corrosion had names, except that ferrous corrosion is "rust." I knew that the aluminum skin of my airplane was attacked by a combination of moisture, pollution, and the effects of different metals being in contact. I see corrosion forming under the paint, especially at joints and near rivets. The paint gets rough, then bubbles up like badly applied wallpaper. Eventually, or if you press on the bubbles, the paint flakes off, revealing that the aluminum underneath has turned white and flaky. When corrosion forms at an edge, such as at the trailing edge of an aileron, you can see the aluminum lifting into separate layers, like a mille feuilles pastry. If that happens you can crumble the edge of the metal with your fingers. The student wasn't able to answer the question about the type of corrosion, so when I got home I looked it up.
There are a number of different sorts, some variations on the others and often working at the same time in the same spot. Some lists separated out more types of corrosion, or omitted or combined some in my list.
Pure aluminum is corrosion resistant, but pure aluminum is not strong enough for airframes, so aluminum alloys are used in airplane construction. The outer surfaces of the alloy sheeting is often coated in pure aluminum (Alclad) or anodized. The surfaces are also typically primed and painted, but the corrosion still gets in.
General or uniform corrosion, occurs when an exposed surface is consumed equally through reaction with air and moisture. It is the least dangerous sort.
Penetrating pit corrosion is the same kind of degradation to the metal, but instead of being even and on the surface, it occurs when moisture enters localized disruptions to coating, and creates cavities that may go deep down, or cut sideways under the surface.
Galvanic corrosion occurs when two different metals are connected in the presence of moisture, and they become the terminals of a battery, which erodes its own surface through the same principle as electroplating. Because alloys are made of combinations of grains of different kinds of metals, there are always metal-metal boundaries, for intergranular corrosion, even if there is not a steel bolt connected electrically to the aluminum frame. Corrosion products build up along the grain boundaries, making the metal exfoliate into the leaves I described above.
Another form of corrosion is fretting, when parts rub on one another and grind particles from one another. You see this at hinges, and where cowlings rub as the engine vibrates. It creates a fine gray dust that streaks backwards on the airframe, and comes off on your fingertips as a gray smear, almost like grease.
Apparently the structural course was almost over, so the student was almost qualified. I hope he was really good at riveting. Remember: someone graduates at the bottom of every class of doctors, and of every class of maintenance engineers.
After I wrote this, but before I posted it I learned about Connie, a female Canadian aircraft structures mechanic. I've linked to *Corrosion of the Week*, an annex of Connie's main blog, Connielingus. The latter contains some sexual content, but I think you can handle it. There's airplane fixing stuff there, too. I never realized before that repairing airplanes could be romantic. Connie, any corrections or additions to this post are more than welcome.