Why the concern about low aircraft and engine time on an older airplane? Wouldn’t that mean less wear and tear? What does low usage have to do with corrosion? What about the anti-corrosion treatments?
Others may have had a different experience, and specific airframes can also differ (for example, one that was kept in a climate-controlled hanger in Arizona, versus one on a ramp tiedown next to a Florida beach.). My experience has been that the more a plane has been used, the less corrosion it has had (whether airframe or engine). The airframes and engines with the most serious corrosion problems are those that sit largely unused for months or years. If one of these planes is being considered for purchase, the pre-purchase inspection needs to be particularly thorough for corrosion detection. This includes the airframe in places such as the wings, spars, and tail structure. It includes the engine, in places such as the cylinders and camshaft. A high-age but low-time original or rebuilt engine needs to have the cam and lifters examined as part of the pre-purchase inspection. In my opinion this remains true even if the engine was properly preserved for long-term storage. The proper prep work just reduces the likelihood that problems will be found; it doesn’t eliminate the need to check. Note that this is not simply an economic consideration; it is a safety of flight issue. There have been many reports of serious engine problems that occurred on that first flight home from a purchase, in the case of low-time, high-year airplanes..
Most planes never dry out completely on the ground, especially when tied down outside. While a hangar helps, in many places the humidity still gets quite high inside a hangar. Temperature fluctuations below the dew point put considerable condensation on aluminum. A dramatic demonstration of this occurs when you make a landing in a humid area, following several hours of cruise flight at an altitude where OAT is in the fifties or below. The part of the fuel tank that still contains fuel will be covered in moisture on the outside of the wing. This often goes unnoticed, as it is usually on the underside of the tanks. The cold fuel keeps that part of the structure below the dew point, while the rest of the structure slowly warms during the descent.
The airframe manufacturer designed in drain holes that are supposed to prevent water accumulation throughout the airframe, and thereby prevent potential ice damage (from expansion due to freezing) or corrosion. Many of these holes become blocked by debris over the years. Part of a good Annual Inspection is checking and clearing all those little holes. Despite the holes, you can find places in any airframe that are unlikely to drain completely on the ground. In many cases the airframe doesn’t really sit at the correct attitude on the ramp, to enable complete drainage, and therefore only drains well in flight.
In nearly all cases, the airframe is dependent on the high-speed airflow of flight, and the lower humidity of altitude, to ever get completely dry (unless it lives in the arid Southwest). This is true regardless of the ambient temperature, as humidity is relative. That means that the more the plane flies, the drier the airframe stays over the long run. The same thing is true of the engine. It usually does not get hot enough to dry out the oil, unless it is flown. Especially following cold starts, the engine deposits moisture in the oil. This is due to condensation of the water vapor that is a byproduct of combustion. Ground running will not get the oil hot enough, for long enough, to get the water boiled out of the oil. If it stays in the oil, one side effect is the formation of sulphuric acid. We are reliant on the oil’s anti-corrosion additives to neutralize these acids. The additives have an easier time of it, and will last longer, if the oil stays moisture-free.
For what it is worth, I am a believer in Corrosion X or ACF 50 treatment. I don’t think it needs to be as frequent as is often stated. Unless the plane is in a corrosion-prone environment, you can go several years between treatments. This is especially true if you follow up the first application with a compound like Boeshield T9 or LPS-3, which leaves a longer-lasting protective film. I don’t like using the coating products first, as I like to be certain that the “creeping function” of the Corrosion X/ACF 50 has a chance to get started without anything interfering with it. Retreatments with the lighter compounds, sometime down the road, will restore protection anywhere the heavier compound has been breached (or failed to reach).
If you don’t want to pay for professional spraying, you can do a reasonable job with spray cans, while the plane is apart for the Annual. These compounds are somewhat messy, as they weep from nearly everywhere for months (or longer). Nonetheless, it is comforting to see a corrosion prevention compound clearly making it from between riveted lap joints, instead of wondering whether moisture is trapped in those same areas. I have never seen a recurrence of filiform corrosion on a treated airframe. Filiform often forms adjacent to joint edges, where paint films are usually too thin.