Corrosion needs air, damp and bare metal. Remove any one of these and the corrosion stops, so the problem areas are the places where these are all difficult to deal with.
Coating parts with oil, fuel or a similar liquid will exclude air. Without regular cleaning and application, dirt will get embedded in the oil and, when the oil is washed off, the dirt will stay wet longer than a bare metal surface and so speeds up corrosion. Places where oil has dribbled and was not cleaned up are at risk. For example, if someone puts too much oil into the engine, the extra will be exhausted as a fine mist over several hours of flight and this gets into all sorts of little nooks and corners.
Painting the metal will exclude air. If the metal surface was not prepared right, or there was a tiny amount of corrosion already on the surface, the paint will lift up and let stuff get underneath. Some hydraulic fluids will dissolve the paint and gradually remove it.
Water likes to go through extremely small gaps, because the surface tension effect pulls it into the gap. Aircraft are exposed to water when it rains, in fog or dew, when descending in flight on a humid day, or when the fuel in the tanks is colder than the air's dew point. The water only leaves by evaporation if there is enough air moving past the place where the water accumulated to dry it out. This doesn't happen when it has seeped into the structure or when there is water in a narrow gap between sheets of metal.
Aluminum forms an oxide coat, which protects it a little bit from corrosion because the bare metal is no longer exposed. Vibration can cause rubbing, which removes that oxide so that the corrosion can continue at full speed. Salty humid climates (such as Florida, California and Hawaii) deposit salt on the aircraft, which is also very good at removing the oxide coat and speeding up corrosion. Unfortunately, those three states also have excellent weather for flying, so large numbers of aircraft are at risk of this.
For example, a Cessna 172 has dry wings (there is a separate fuel tank inside the wing), ventilated from the back but not the front, with a seam where two sheets of aluminium are riveted together along the lowest point of the inside of the wing, not normally painted. That area is especially at risk due to all the factors listed above, but you can only inspect it visually by removing all the rivets and taking the wing to pieces. This cannot be done very often.
Many forms of corrosion will eat between the grains of the aluminum metal more quickly than it will eat through the metal itself. This makes it almost impossible to get rid of that kind of corrosion once it starts, since you can sand down to what looks like good metal but you cannot see the tiny corrosion holes that lead deeper and will simply carry on deliverying air and water into the metal as soon as the paint coat has been popped of.
Cracks grow because something is trying to tear or stretch the metal, there is a hole or gap in the area, and the stress concentrated by the hole exceeds the strength of the metal.
Unfortunately, the force that is stretching the metal tends to be something useful, like the weight of the aircraft on the wings. We cannot make that go away, so the only thing to do is avoid concentrating the force around a sharp hole. A crack is a good example of a sharp hole, which is why cracks tend to grow fast. Surface tension sucks water into the crack, encouraging corrosion even if the crack growth itself is stopped by other methods.
In addition to the shapes of the pieces of metal, the pieces are usually held together using bolts, rivets and other fasteners. These all need holes; which are supposed to be round and smooth to keep the concentrating effect as small as possible. Any of those corrosion effects discussed above, as well as the rubbing of a loose fastener, can remove metal from one side of the hole and gradually make it sharper.
When cracks grow from fastener holes, they tend to head for the adjacent fastener (which is the nearest point of weakness). This predictability makes them easier to find, but it is important to remember that they may not be on the outer layer of metal, where they can be seen. Fasteners can hold several layers of sheet metal together, with the crack somewhere inside.
When we find a small crack, we can stop it by drilling a hole at the end to make the tip round and thus less sharp. If we make the tip round enough, the concentration will be low enough that the crack will stop growing due to the force. If the crack is already too big, or the drilled hole is too small, the concentration will simply cause the crack to grow out of the other side of the drilled hole and keep going. Clearly, it is important to find the crack while it is still small.
The FAA issues an Airworthiness Directive (AD) whenever a maintenance issue has been identified that affects safety. These must be complied with, if not already completed. Older aircraft have dozens of such ADs, solving subtle problems that have appeared during the several decades since the aircraft was originally built.
Every aircraft has a series of log books that describe all the work that has been done to it. Unfortunately, these log books may be lost or, many years later, the entries can be hard to interpret unambiguously. If the log book does not unambiguously indicate whether the work for an AD has been performed, it may be necessary to take the aircraft to pieces to check. Often, the disassembly is the bulk of the work in actually completing the AD in the first place, so the owner effectively pays for the work twice.
Sometimes, a simple non-destructive test could determine whether the AD-required change had been made and save people a lot of time and money.
Every aircraft under US registry is required to have an annual inspection, involving a partial disassembly to find hidden damage, worn parts and components that may be close to failure. The search for corrosion requires extensive disassembly or paint removal, either of which accumulates minor damage to the aircraft itself.
The work is labor intensive and most facilities complete a relatively small number of inspections per year, making the purchase of purpose-specific instrumentation prohibitively expensive.
Used aircraft sales purchase is a problematic situation. A new coat of paint increases the value of the aircraft on the market, but can cover up flaws that the buyer would like to know about, so it increases the value of a good aircraft less than the cost of the work. However, for a poor aircraft with lots of flaws, hiding them improves the apparent value of the aircraft enough to justify painting it. This leaves the odd situation that an aircraft in good condition might be sold with an old and worn paint job, to prove that it is in good condition, and the buyer will immediately have it painted once the sale is completed.
The potential buyer of a newly painted aircraft would like to look for corrosion and cracking without stripping the paint off. So, a simple and portable NDE unit is likely to be extremely useful during the prepurchase inspection.