Internal combustion engines that are inactive have a tendency to rust internally. This is particularly true for horizontally opposed aircraft engines. Engines that have low operating time since overhaul are particularly susceptible. The corrosion (rust) is the result of having in combination: an exposed metal surface; a corrosive agent such as acid which forms in oil; oxygen; and moisture.
The corrosive agent and oxygen are found in the air and will rust a bare metal surface if it is not protected. When an engine runs it introduces more corrosive agents and moisture into the internal engine cavities. This is the consequence of burning fuel. The corrosive attack can be a direct chemical attack or an electrolysis between two metals with a common electrolyte (moisture).
Moisture is a common component needed for both types of corrosion to occur. Moisture comes from burning fuel such as gasoline. When burned, a pound of fuel will result in a pound of moisture in the exhaust gases. The induction air is also moisture laden which moisture is added to the combustion moisture produced in the cylinder. Some of the moisture goes by the piston rings with "blow-by" and ends up in the crankcase. Another minor source of moisture is moisture laden air that is drawn in through the engine breather as the engine cools after being shut down. As a result of these factors, the resultant humidity in an inactive engine can approach 80% and often reaches 100%.
A small amount of corrosive agent is found in the air. The rest is the result of the combustion process. The corrosive agent can be acid which is formed when oil oxidizes due to long time exposure to heat. It may be sulfur from poor quality crude oil used in the engine, or chlorine compounds found in the air.
Exposed engine surfaces are particularly susceptible to this corrosion. Such surfaces result when the engine is overhauled and everything is cleaned to remove buildups of carbon and oil residue. This also removes the varnish layer which protects parts from rust. When being overhauled, many engine surfaces are freshly ground which exposes bare metal.