Technical Insights for Owners and Operators.
by Alex Best, Aero Gas Turbine Specialist
Corrosion is a topic I’ve frequently discussed with operators at conferences and M&O presentations. In this article I will review some of the technical aspects of corrosion as it relates to the materials used in gas turbine engines and the maintenance actions that can be taken to slow its progress. Choosing the right preventative maintenance action and schedule will help avoid runaway overhaul costs, premature engine repair and loss of asset value. Also, let’s remember that some maintenance cost guarantee programs exclude corrosion.
Why do engines corrode – can’t the manufacturers prevent it?
Fundamentally, corrosion is a chemical reaction in which a manufactured metal returns to its natural state as an oxide or some other compound. There is plenty of oxygen in the atmosphere to aid the process when combined with other catalysts such as water, salt or acidic atmospheric pollution. Most metal alloys in a gas turbine engine are subject to corrosion of one type or another. The titanium alloys used in cooler sections of the engine being the rare exception due to the incredibly tough oxide layer that forms on the surface preventing further degradation. (I’m ignoring the possibility of the titanium catching fire which is possible but, fortunately, extremely rare.) Manufacturers do apply coatings to many metals to combat this process. Their effectiveness is a function of the environment in which the aircraft is operated and maintenance activities carried out to mitigate the effects.
Magnesium used in many casings is typically protected by epoxy or enamel paints. Any damage to the protective surface is a potential initiation site for corrosion. Exposed metal will quickly show visible signs of corrosion. From there the condition will progress along the surface under the surrounding coating. Although the chemistry books say an oxide layer will form and prevent further corrosion, this effect is short-lived in the environment inside the engine. Contact with other metallic components can set up a galvanic cell in which the magnesium is invariably the loser. In the presence of water, an unprotected magnesium surface will be seen to bubble and react almost immediately. (We all remember that experiment from high school chemistry class.) The importance of regular treatment, minor repair and paint touch-up can’t be over-stressed. Repair techniques have advanced over the years. On-wing coating touch up and local epoxy repair to replace material in non-structural areas will get most operators through to scheduled overhaul. When that time comes, repair suppliers have the ability to do quite extensive weld build-up and are normally able to save most casings. Keeping salt and other contaminants away from the magnesium is a key factor. Compressor washes are the first line of defense and application of corrosion inhibitors, such as corrosion X, will have a definite benefit in areas that can’t be accessed on-wing. How frequently? This depends on many factors. First and foremost, geographical area. Engines operated in coastal or highly polluted industrial areas will require more frequent attention. Is the aircraft hangered or parked outside? Non-coastal operators who hangar the aircraft can probably limit compressor washes to minor inspection intervals.
Aluminum alloys used on accessories and casings will typically have an anodized finish and, compared with magnesium components, do not normally present a significant corrosion concern.
Ferrous alloys (Steel.) Rusting occurs on all ferrous alloys when exposed to a sufficiently corrosive environment. High strength steels intended for use in bearings and gears will start to corrode within hours if exposed to water rather than the normal coating of oil.
Not all ferrous alloys are so sensitive. Different grades of steel will have varying levels of corrosion resistance, in large part depending on the percentage of chromium in the alloy. However, the designation of a steel as stainless does not equate to “will never corrode.” Generally, the grades of “stainless” steel used on casings and external brackets will tarnish and accumulate a surface corrosion with time. As with Magnesium, time to initiate and speed of progress depends in the operating environment and maintenance activities employed.
Nickel alloys used in the combustion and turbine sections, while inert at room temperature and resistant to many chemicals, have their own issues at their typical working temperatures. Sulfidation and thermal oxidation being the two most relevant to this discussion. Sulfidation is a chemical process that attacks temperature resistant alloys with a high chromium content. This is most often seen on turbine blade airfoils but can occur in other locations. There are two types, operating in two different temperature ranges. Type I sulfidation operates at 1,470 F to 1,740 F (800 C to 950 C) whereas Type II low temperature hot corrosion is prevalent in the range of 1,240 F to 1,380 F (670 C to 750 C,) give or take a few degrees depending on which research you read. Type II is the variety most commonly seen in small gas turbines. The process starts with airborne sea salt (sodium chloride) combining with Sulphur from the fuel to produce sodium sulphate salts (Na2SO4.) If these salts are allowed to accumulate, they will become molten in the sulfidation temperature ranges and leach the chromium content from the blade alloy. With the depletion of the chromium, oxidation attack of the alloy follows quickly. Manufacturers do apply coatings to increase resistance. Diffused aluminum in the surface of the blade material gives good resistance in most applications but it has its limits. There are more exotic options. Some manufacturers offer an optional platinum aluminide for operators in areas particularly prone to the issue. The significant cost increase however does not make a good business case for most corporate jet operators. Irrespective of the type of coating, there will be an incubation period during which the blade coating provides a barrier. Pitting of the coating becomes the first indication of sulfidation attack. Once the coating is compromised the condition will worsen more rapidly as the chromium depleted base material begins to oxidize, bulge and eventually start to flake away. Since sulfidation is temperature dependent, it will be seen in the turbine stages that operate at these temperatures. In larger turbine engines with airfoil cooling, the cooled blades may operate at temperatures above the sulfidation range. If this is the case, the sulfidation may occur in the cooler stages farther downstream, in the LP turbine perhaps. The good news, if such exists, is that the sodium sulphate salts are very soluble in water. Therefore, water washing can be effective in removing them from the engine gas-path. Think of a desalination wash as one that removes sodium chloride from the compressor and sodium sulphate salts from the turbines.
Washing a little more frequently than you think is necessary will do no harm.
Every little helps. Provided it is done properly.
Care needs to be taken when carrying out compressor and turbine washes. Allowing water or cleaning products to enter the engine oil system will cause gears and bearings to rust very rapidly, as I have mentioned. A prime culprit is failing to properly seal breather discharge lines that exhaust into the bypass duct. If this occurs it must be addressed immediately by flushing the oil system. Otherwise rust will start to form within hours and progress rapidly to a point where there is no alternative but to remove the engine for repair. Manufacturers’ manuals will indicate which openings into the engine need to be sealed prior to the wash, so ensure that the recommended steps are followed.
It is also good practice to ensure the engine is dried after the wash. Run it for a few minutes to blow out any water. Heat dissipated after shut-down will also help to ensure everything is dry before installing the covers. Engines with abraidable coatings on blade paths are particularly at risk from a slipshod compressor wash job. If wash water pools and the engine is inactive for a week or two the residual water can soak into the porous plasma coating. (I’m talking about the grey powder metallic coatings on the Boost, IP and HPC areas, not blue or black material surrounding the fan blades.) Corrosion will subsequently cause it to swell, become embrittled and delaminate from the casing. Swelling can reduce the tip clearance to the point that contact between the casing and the blade tips lock the rotor.
How do I avoid additional corrosion related maintenance cost?
Even if you have the “Gold” maintenance cost protection plan, check the fine print. Although some manufacturers advertise that their program will cover corrosion “no questions asked” many programs will list it among the exclusions. Originally intended to ward off costs associated with grossly negligent maintenance practices I have seen a gradual change in approach. Eager customer reps may try to boost their sales margins by adding a charge for any parts rejected with the word “corrosion” on the tag. There are ways to push back if the additional charges are unreasonable.
- Understand your operating environment and act accordingly.
- Take the appropriate precautions when washing.
- Always make sure the engines are thoroughly dried after a compressor wash. Start them up and run for a few minutes to get them fully dried out before putting the covers on.
- Have the maintenance techs check the engine maintenance manual for procedures to apply a barrier type corrosion inhibitor such as Corrosion X.
- Be aware of engine preservation recommendations if the aircraft will be inactive.
At the end of the day, some corrosion is inevitable but reasonable care will help avoid unnecessary component rejection at overhaul. A good record of the maintenance activities will help ward off additional charges from your program provider at the time of overhaul. If you need help, let me know.
I hope this has explained some of the science behind the terminology allowing you to make more informed decisions that ultimately save money and protect your investment. If you have any comments or questions please feel free to email me.
This article is protected by Copyright and may not be reproduced or published in whole, or in part, without the written permission of Alex Best Consulting.