*** Engine Corrosion ***

Corrosion in Engines

Corrosion is one of the most common causes of early engine overhaul or repair. All internal engine components that are manufactured from any steel alloy are susceptible to it and can prematurely fail from it.

The areas of the engine most commonly subject to the detrimental effects of corrosion are the cylinder barrel surfaces, camshaft lobes, tappet faces, and gear teeth. Problems in these parts of the engine can cause varying safety issue's and can very expensive to repair.

Excessive corrosion on a cylinder barrel surface will cause premature wear to itself, the piston rings riding on its surface and the piston that those rings are installed on. This in turn will cause high oil consumption and low compression readings. The only fix is to remove the effected cylinders and overhaul or replace them as necessary.

Corrosion on a cam lobe or tappet face will eventually cause a spalling condition on both surfaces. This results in extreme wear of those surfaces. The operational results of this condition are; low power output, possible engine roughness at high R.P.M. settings and metal in the oil screens or filters. To repair this condition it is necessary to remove the engine from service, completely disassemble it and to repair or replace all effected components before the engine can be reassembled and used again.

If corrosion on the mating surface of a gear tooth is significant enough to cause spalling of that surface, the resultant wear will cause the actual profile of the gear tooth to change. If this happens and the gear is not replaced before the change in profile becomes dramatic, catastrophic engine failure will result. If the aircraft is still in one piece after that, replacement rather than repair, of the engine is often necessary.

It should be stressed that the corrosion itself does not actually cause the part failures but that the corrosion is the catalyst that starts a long chain of events over a several hundred hours of operation that eventually leads to the conditions mentioned.

As an engine sits idle, not being used, it is subject to day to day changes in ambient temperature and humidity. Internal engine components, as well as the oil in the engine itself, are subject to moisture accumulation caused by natural condensation from these changes. If an engine is not run for an extended period of time, the oil film, that was left on any individual component within the engine at the last shutdown, deteriorates to a point that allows the accumulated moisture to make contact with the surface the oil was on. Corrosion of that surface will result. At the same time, accumulated moisture in the engine oil reacts with deposits in the oil that have developed from normal engine operation. This reaction can cause the engine oil itself to become acidic and corrosion on the part that the oil is trying to protect will result.

The only true protection we have against any of these corrosive elements is keeping the engine oil moisture and acid free and physically on the internal parts of the engine. This allows the oil to prevent corrosion from occurring.

Flying the aircraft and engine long enough to develop an oil temperature of at least 160 degrees fahrenheit for 30 to 40 minutes will remove almost all corrosive elements, that build up in the oil from its non use and exposure to the atmosphere. This process also provides a fresh renewed protecting film of oil, that contains little or no moisture or acids, on all internal parts of the engine at shut down. This film of oil does not last forever and if enough time elapses between engine runs, the internal parts of the engine will become subject to corrosion all over again. The only way to prevent this is to fly the aircraft, as prescribed, fairly often. This will repeatedly burn the moisture and acids out of the oil, as well as providing a continuous renewed film of protecting oil.

Another consideration of the ability of the oil to protect against corrosion, is the amount of combustion by products and engine produced deposits that are present in the oil. As previously mentioned, running the engine will remove most of the harmful contaminates present in the oil. However, it will not remove all of them. The longer an oil is run in the engine, the more contaminants it has a chance to absorb. The more contaminants present in the oil increases the likelihood of a reaction with those contaminants that can cause corrosion. Consequently, changing you oil frequently will help reduce the possibility of corrosion as well.

As a general rule, changing your oil at least every 50 hours of operation and flying you aircraft at least once a week will go along way in preventing internal corrosion and it's associated maintenance headaches. If your aircraft is based in an extremely corrosive environment (i.e. high humidity, near the seashore, etc.) or in a extremely favorable anti corrosive area ( stable temperatures, low humidity, desert like conditions, etc.) these intervals can be adjusted accordingly but as a rule of thumb they are a pretty good guide line for preventing excessive internal engine corrosion and the unexpected maintenance costs caused by it.

by Mahlon Russell

Send your questions, to Mahlon at mahlon russell@teledyne.com

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