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 been developed to address this threat. Unfortunately, less has been done about internal engine corrosion. We may be totally unaware of any corrosion until the engine is torn apart for an overhaul – an overhaul that may need to be done hundreds of hours earlier because of corrosion damage.
The purpose of this article is to help raise awareness of this problem by presenting some of the current information available including various opinions of industy experts. What is offered here is primarily based on reports already published in the field. Through this discussion, we hope to stimulate more study in the area.
As we discuss corrosion in this article please keep in mind we are dealing only with internal engine corrosion.
What is corrosion?
The average person knows the most common version of corrosion as "rust." The word itself is either a noun or verb. Rust by definition is the result of the oxidation of metal. One engine manufacturer refers to the process of corrosion in aircraft engines as "galvanic corrosion" which is a type of corrosion caused by dissimilar metals being in close proximity of each other. In this process an electrolyte such as water also needs to be present. Almost all metals will oxidize or corrode. Leave one of your tools outside and shortly after a rain you will see rust unless it has a protective coating such as oil or paint. In an aircraft engine, however, it’s much more complicated than that.
Early in this research project, it was clear that there is little agreement on many facts relating to aircraft engine corrosion. There are even facts that go completely against popularly held opinions. The first and most obvious is the popular adage "oil and water don’t mix." More on that later.
Serious interest in the subject of of internal engine corrosion is a relatively recent phenomenon. In the past, many assumed that because the internal parts of the engine were coated with oil, corrosion would not occur. Further, if there was a danger at all, corrosion would raise its ugly head only when the engine was not run for very extended periods of time because the oil drained down to the sump and exposed the metal. Today we know that it can occur within a matter of days between engine operations and that it can even occur with oil coating the metal part.
Who is affected?
Generally every aircraft engine is in danger of corrosion. Some, however, are at a higher risk than others. The four factors to consider regarding which engines are affected relate to the age of the engine, its geographic location, how engines are operated when in use, and most importantly, its frequency of use.
These four engine factors can be broadly separated into two categories: "active" engines and "inactive" engines. We need to note here that Continental further distinguishes "inactive" with its definitions of "temporary storage" and "indefinite storage," the difference being "temporary" is not flown 30 to 90 days and "indefinite" means 90 days and longer.
Let’s explore for a moment, what is considered an "active" engine, or one "run regularly." It requires a very short research effort to quickly get the message that there is no clear definition. A sampling of the time periods used when discussing the subject of aircraft engine corrosion (often from the same document) varies from two days, one hour per week, 100 hours per year, 30 days, 90 days and once every two weeks. Naturally, different circumstances affect the time period but it seems that if one were to insist that experts use only one number, it likely would be 30 days.
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