Corrosion: How does it affect the internal engine?

By Gary Schmidt Corrosion has long been recognized as one of the great enemies of the aircraft owner. Much has been said and written about corrosion in general aviation, but most of it pertains to airframe corrosion. Products and procedures have...

Beginning with the day the brand new engine comes out of the factory, it finds itself on the corrosion hit list and this vulnerability to damage is addressed by the manufacturer. Continental states that if the engine is not run within six months after it leaves the factory, it should be re-treated for protection against corrosion. Its web site states, "Following acceptance testing, all engines are preserved by TCM prior to shipment. This preservation has a shelf life of six months after which the engine must be re-preserved for corrosion protection until the engine is placed into service."

Manufacturers recommend that the new engine break-in be done with a mineral-based oil rather than an ashless dispersant oil. This process creates a varnish-type coating on the metal parts that will protect the metal from rust. The break-in period is normally considered about 50 hours.

Much is also made by manufacturers of the geographic location of the engine or aircraft. Aircraft located in humid regions or coastal areas are considered more susceptible to rust.

Engines that are not run properly can create situations that make them more vulnerable to corrosion. But the most obvious engine in danger of corrosion is the "inactive" engine – one that is not run for extended periods.

What is the risk?

The risk is a shorter engine life. Continental simply says, "Corrosive attack may reduce engine service life." How much shorter, no one knows. Naturally any rust is bad. The problem with measuring the damage corrosion causes is that it does not occur at any constant or measurable rate and it can start and stop and start again. The damage also depends on the surface being attacked and the part being affected. The areas of primary concern are cylinders, pistons rings, valves, valve guides, camshaft, and lifters.

It is clear that internal engine corrosion is prevelant and it is bad. Doing the math with general round numbers, if an engine overhaul costs $30,000, at 2,000-hour TBO, the cost is $15 per hour. Shorten the life of that engine by 500 hours and you have just lost $7,500.

The causes – the debate

Scientists have long known how the process of corrosion occurs. Except for some precious metals such as gold, there are few really stable metals. The molecules that make up the metal want to combine with other molecules and through that process make up a new material with different characteristics. This process can occur either through an electrical or chemical process.

For that corrosion process to occur, several elements must be present. They are an acid such as sulfur or chlorine compounds and moisture which is the electrolyte and may carry the oxygen for "oxidation" or other compound necessary for the corrosion process. Some mention that air is also necessary but only because it is often the vehicle carrying the acid and moisture. Naturally, for this all to happen, we need one other basic item at the center of the entire issue, the metal.

The debate in the industry has centered around the issue of the sources of these elements as well as the issues of when and why the metal is vulnerable to the other elements necessary for the corrosive process.

First let’s analyze where these elements necessary for corrosion originate. No. 1, moisture: When you burn a pound of fuel, you get about a pound of moisture as a result. Most of this is expelled out of the exhaust stacks but some enters the crankcase via "blow by." Some moisture comes from the air that the engine takes in through intake and it is mixed with the combustion moisture. One other minor source is the moisture in the air that is drawn in through the engine breather tube as the engine cools after shut-down. The aircraft engine absorbs this moisture and doesn’t release it unless your oil temperature gauge gets over approximately 180 degrees F. If you have your oil temperature high enough, the moisture vaporizes and exits by way of the crankcase breather.

Naturally, because moisture is a key element to the corrosion process, engine manufacturers emphasize that aircraft located in coastal areas and areas with humid climates are more susceptible than aircraft located in arid regions. The fact is that there are very few areas of the world where the air is so dry that you do not need to worry about corrosion. Everyone needs to consider corrosion regardless of their geographic location.

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