126.96.36.199 Measures Against Corrosion Damage
In order to prevent corrosion damage without a noticeable effect of mechanical loads, there are many different preventive measures which can be selected to provide optimal corrosion protection in each individual case.Repairing or minimizing damages to corroded parts difficult due to the high safety demands on aircraft engine parts and necessitate sufficiently realistic testing to ensure satisfactory operating behavior.
Storage and Parking:
- Proper storage of parts, units, and engines in containers and packaging with and without conservation. Parts that are stored for a long period (years) should be at least spot-checked at suitable intervals for possible corrosion damage. Special attention must be given to the roller surfaces and bearing tracks of roller bearings. Tarnishing and rough areas in the rolling element are typical signs of standstill corrosion under influence of condensation water or humidity. Use of the prescribed auxiliary materials such as conservation oil and dehumidifiers is necessary. It must be ensured that the packaging provides the required seal.
- Conserving compressors with corrosion-sensitive areas (abradable coatings, Cr-steel or Al-alloy blading in older engine compressor types) before longer standstill periods.
- Connecting the engine to an air-conditioning/dehumidifying unit (Ref. 188.8.131.52-10). This measure is especially suitable for long standstill times in marine environments.
Figure "Dehumidifaction device" (Ref. 184.108.40.206-1 and Ref. 220.127.116.11-2): Corrosion primarily occurs by condensate formation in standing aeroengines. Temperature differences and with this the volume change of the air inside the aeroengine during stand still provide in the mounted condition a sufficient air exchange. The experience shows. that air with a relative humidity below 50% reduces the danger of corrosion essential. Therefore the dehumidification of the air in a standing aeroengine is an effective method to minimize corrosion deteriorations in aeroengines. Such conditions will be achieved by continual blown dry air into the aeroengine (open system, sketch below right) or into the storage room (closed system, sketch below left). As air dehumidification in the practical application a continual operating regenerative system according the ljungstroem principle prevailed (sketch in the middle). Centerpiece ia. a rotating ceramic disk with honeycomb structure channels which by absorption extract humidity from the process air. After a certain rotation the humidity of the gathered air is detracted by a separate flow of heated air („Reactivation Air“) from the disk.
- The constructive design (i.e. the designer) largely decides the corrosion behavior of the engine parts during operation. For example, exposed sharp edges, gaps, and hollow spaces are typical corrosion-sensitive weak points that should be avoided. Otherwise, experience has shown that in later serial operation, repairs or retrofitted improvements will be very costly.
- Use of suitable protective surface coatings. These must be selected not only with regard to corrosion, but also different operating loads such as erosion and resistance to auxiliary materials and cleaning solutions (oil, methanol, fuel, etc.). Ideal coatings have a protective cathodic effect, which ensures that in case of local coating damage (such as through FOD), there is still corrosion protection from the surrounding coating areas.
- As far as possible, corrosion-resistant materials should be used. Coatings that protect against erosion and wear must not have cell action with the base material.
- Preventing metallic contact between materials with different corrosion sensitivity with possible electrolyte contact through non-conducting intermediate layers. Contact between cut carbon fiber layers (if there is not insulating resin layer, the fiber-reinforced carbon material behaves like a noble metal) and corrosion-sensitive metals, such as light metal alloys and steels, presents a special danger of cell action.
- In joined parts made from corrosion-sensitive materials such as steels, gaps and hollow areas should be avoided if possible (for example, in welded constructions). If hollow areas are unavoidable, they should be outfitted with corrosion protection and openings to ensure sufficient air exchange (drying).
Development, Testing, and Verification:
- Realistic testing of corrosion protection systems. This includes parameters such as time periods, load size and progress, temperatures, surrounding media, representative media, and material condition.
Manufacture and Repair:
- Manufacturing processes should be selected and optimized to ensure the least possible risk of corrosion. This includes factors such as galvanic baths, cleaning baths, and etching fluids.
- After “risky” manufacturing processes, proper control measures must be implemented. Depending on the type of expected damage, these may include, for example, control of the bath, inspection of the part surface (visually or microscopically with use of synthetic imprints), or non-destructive crack inspection.
- In case of storage during manufacture, corrosive influences must be avoided by, for example, suitable containers or coverings. Parts in the process of being manufactured should be stored in a position that prevents fluid from collecting and remaining.
- Use of proper tools (chipping, handling, fixing, mechanical cleaning, etc.) that do not deposit any corrosion-promoting foreign material. If corrosive deposits are unavoidable (such as brushing or a peening process with steel shot), the part must be properly cleaned afterward.
Maintenance, Mounting, and Repair:
- Washing compressors in suitable (prescribed) intervals with suitable (prescribed) media.
- Use of mounting tools that do not leave unallowable deposits or damage protective coatings on corrosion-sensitive parts (such as gearbox casings made from Mg alloys).
- If corrosion is discovered, it must be repaired or corrected sufficiently early (in accordance with guidelines).
- Development of proper (practical and sufficiently safe) testing methods and specifications (e.g. within the guidelines of overhaul manuals) to evaluate and separate reparable from irreparably damaged parts (e.g. sample sheets).
- Repairing corroded cross-sections on parts with low mechanical loading (such as deposit welding on housings made of steel or light alloys), or patching housings made from light metal alloys (especially Mg alloys) with filled synthetic resin pastes.
- Sealing porous (e.g. with increased porosity in light metal housings/casings through lacquer-stripping baths) cast housings with suitable impregnation systems such as water glass or synthetic resins.
- Fading out damaged areas and applying suitable coatings.
18.104.22.168-1 “Blow-dry your jet”, Zeitschrift „Flight International”, 18 May 1985, page 49.
22.214.171.124-2 Prospektangaben der Fa. AB Carl Munters in Sollentuna (Schweden) zu Vorrichtungen für “Dehumidification”. 2004.