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...

Note, it states "reducing" and "likelihood." Both carefully chosen words. "Eliminating" could be used without the word "likelihood." Obviously there is no certainty to this recommendation. The other problem is that very few general aviation aircraft owners fly their planes once a week on a regular basis. This recommendation is very impractical. In the same service bulletin, however, the manufacturer provides information on preparing the engine for storage (for protection against corrosion) for an aircraft not flown for 30 days. One can infer from this that they feel that if it is flown at least every 30 days, it does not need corrosion protection.

The same engine manufacturer goes on to state that its engine warranty is based on the engine running at least 20 hours per month. According to Continental an examination of engines that run 20 hours a month show little or no corrosion.

In any event, all can agree that flying regularly is one solution to addressing the corrosion risk. However, that in itself is not the answer. There are other related steps necessary.

The first recommendation is to operate at least 180 degrees F for more than 30 minutes. Here again, recommendations regarding the temperature vary from 165 to 200 degrees and the time varies from 30 to 60 minutes. The consensus seems to be 180 degrees however. This will purge the moisture from the oil through the breather. Less than 30 minutes can actually cause problems by increasing the vapor and acids in the oil according to Continental.

Obviously, for you to be able to do that, you need to know the temperature of the oil during operation. Consequently, it is recommended that you have your oil temp gauge calibrated. If your gauge only has a green area, make your own mark at the point that indicates the engine is at 180 degrees F.

The oil companies have a series of recommendations on the corrosion issue, including change oil often, up to every 25 hours of operation or every six months. They also recommend testing the oil every 60 days. Tests can show if water is present in the oil as well as other metals including lead, by products from the fuel. Changing oil is another way of eliminating moisture that may be present in the oil. This brings up the question "If running the engine at or above 180 degrees doesn’t get rid of all the water, why not?" (Or is this a recommendation that can lead to increased oil sales?)

A common rule of thumb from engine maintenance people is change oil every 50 hours or every six months unless you do not run the engine at 180 degrees, in which case, you should change oil every 25 hours. Continental, on the other hand, suggests changing oil every 25 hours of service if you do not fly 20 hours a month or more. Again, this is an area where there is no hard and fast rules but fresh, clean oil is better than used oil and quite inexpensive compared to the cost of an engine. What provides optimum value for cost is impossible to say. We should note that changing oil frequently will also reduce the corrosion-producing acids (as well as lead deposits) that gradually accumulate in the oil but cannot be eliminated or reduced by running at warmer temps.

Using the right oil is obviously a prudent thing to do. Do the newer oils make a difference? "Yes" says Buldoc who sees improvements in engines he works on. Exxon has spent tens of thousands of dollars to claim it has the oil that does the best job protecting against corrosion. We are not recommending any brand here. We suggest you review the advertising claims of all oil manufacturers yourself and make your own decision. Although multi-vicosity oils are popular, single weight oils absorb less moisture than the multi-grades.

Another suggestion for operating your engine is to keep the engine running lean or at least do not operate it "full rich" more than necessary. A rich mixture results in unburned fuel left in the cylinder. This unburned fuel with its corrosive agents leaks by the rings and ends up in the oil sump. A properly leaned engine also helps increase oil temperature.

Avoiding "sump only" engine preheaters is another recommendation. These heaters tend to vaporize the moisture in the oil and allow it to condense in the cooler parts of the engine which, in turn, provides the moisture necessary for the corrosion process. Engine pre-heaters that heat the entire engine evenly avoid the condensation problem. Buldoc, the engine rebuilder, also strongly recommends not leaving any engine preheater on for lengthy periods, i.e. days or weeks. Clearly warm air holds more moisture than cold air.

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