ENGINES As turbine engine direct operating costs (DOC) escalate,
the requirement for getting the maximum operating value from our turbine engine is becoming an increasing priority. The engine manufacturers offer more exotic component metallurgy in the quest for increased performance as well as reliability. However, there is only so much corrosion resistance that can be manufactured into the parts and components. Eventually, the constant inhalation of this toxic airborne cocktail by our turbine engines will defeat even the most advanced corrosion resistive metal or coating. That is, unless our little friends with the corrosive personalities are stopped before they get the opportunity to create turbine engine mayhem.
Corrosion Know your enemy and it will be defeated. The enemy is airborne pollutants. Corrosion is their aspiration. Subterfuge is their modus operandi. Our turbine engine is the projected target and Pure Water is our best and only weapon. In order to do proper battle, we should learn the enemy’s techniques that are either a direct chemical attack or via electrolysis. Both methods assail our engine components, leaving the parts corroded and structurally weakened. As we learned way back in our early training, corrosion, simply put, is “the deterioration of a material due to direct chemical attack or electrolysis.”
Along with a direct chemical frontal assault, this army of pollutants second weapon of choice is electrolysis. Electrolysis is a chemical decomposition produced by passing an electric current through a liquid or solution containing ions. Any salts (Sodium Chloride or NaCl) are ions. “Electric current” is defined as the transference of
positive and negative electrons. When the sodium chloride framework is dissolved in water, it causes the Na and Cl ions to disintegrate as the ions become surrounded by the water molecules. When salts are deposited on the engine components and then coupled with moisture inherent in the air, shazam! We have electrolysis. Engine materials will be eaten away as a natural result of the electrolysis process. Hydroscopic in nature, salts tend to hold moisture on the surface, promoting corrosion on and within the unprotected metals. Even protective films will be worn down over time by ingested material and chlorides in the air exposing components to corrosive agents. (As a side note, that last explanation of electrolysis really taxed my high school chemistry.)
The hot section components are further compromised because of the exposure to extreme temperatures, as well as the sulfuric compounds, natural products of combustion in fuel. These sulfuric compounds react with the metal components to form sulfides and oxides, leading to corrosion.
Corrosion Control Corrosion prevention and control must be proactive by its very nature. As helicopter maintenance professionals, it is our responsibility to remove the salts and sulfides deposited on the airframe and in the engines before they get the chance to create any mischief.
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HelicopterMaintenanceMagazine.com December 2014 | January 2015
PURE WATER! All engine manufacturers recommend frequent rinses and washes with pure water. If operations are conducted within five to 25 miles of salt water (depending on which engine OEM you talk to), daily pure water rinses are essential, especially for the engine. The rinse activity should take place after the last flight of the day. I know this can be difficult. Come the end of shift, the beer low-level warning light begins to flicker, and waiting for that last helicopter to return plus the wait for the engine to cool down so we can do the rinse seems like an eternity. Couldn’t we wait until first thing in the morning? I’m afraid not — you brush your teeth before you go to bed because you don’t want the decay causing particles hanging around for eight hours unchecked in your mouth, right? If we let a day’s worth of pollution sit on and in our engine, by the time you rinse or wash the engine in the morning it’s too late. Blade decay has begun! This might be a good time to explain the difference
between rinse and wash. An engine rinse is completed using only pure water and usually to remove contaminates. An engine wash starts with a soap and water solution and is followed by a rinse to remove the soap and contaminates. This process is used to not only remove contaminants but also as a power recovery process. Both methods call for the engine to be cool to the touch before proceeding and require a drying run after completion.
What Pure Water Is Used Does Matter Now we need to use the right water. The quality of the water used in rinses and washes is just as important as the quality of the oil we use in the engine. Tap water should be just fine to use since it is fit for human consumption, right? Not a good idea. Tap or drinking water should not be used for engine cleaning purposes. It might contain large quantities of minerals (including chlorides, sulfates, sodium, calcium and magnesium), which is the very stuff we are trying to clean from the engine in the first place. These chemicals can corrode or leave residues in the engine. Don’t see all those chemical parasites in the water? They are there, trust me. Take a look at the picture below. This is a six-inch water main from the municipal water system in a major midwestern city. See the white residue that has almost completely plugged the pipe? That, my friends, is the stuff we are trying keep off our turbine parts, and also the stuff we have been putting into our glass for drinking. Just say no to tap water. If you already suspected that tap water is not up to our
standards for purity (before you saw my graphic example), that might explain the sudden popularity of bottled water among us humans. Based upon the sales figures for bottled water, I would say that we don’t consider tap water all that safe for
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