SAE AIR 4495-1993 Helicopter Powerplant Corrosion Protection.pdf

1、SAE AIRu4495 93 = 7943725 0537358 077 = 400 Commonwealth Drive, Warrendale, PA 15096-0001 Submitted for recognition as an American National Standard I HELICOPTER POWERPLANT CORROS ION PROTECTION FOREWORD Extensive use of helicopters in a wide range of environmental conditions, military or civil appl

2、ications, has driven manufacturers and operators to pay greater attention to corrosion detection/protection on airframe and engine components, including accessories, when they are subjected to adverse operating environments. Component design, choice of materials, methods of simulation for engine ben

3、ch test and development of suitable maintenance procedures are the different subjects which should be carefully addressed to successfully manage operation in a corrosive environment. 1. SCOPE: This SAE Aerospace Information corrosion on helicopter powerp the subsequent consequences on and dependabil

4、ity. Report (AIR) describes the different aspects of ants, on the components that are affected, and the helicopter, engine durability, performance, Guidelines that minimize corrosion during the design stage and during service operation are al so di sassed. 2. REFERENCES: 2.1 U.S. Military Publicatio

5、ns: Available from Standardization Documents Order Desk, Building 4D, 700 Robbins Avenue, Phi 1 adel phi a, PA 191 11-5094. MIL-STD-889B Dissimilar Metals MIL-E-8593 Military Specification, Engines, Aircraft, Turboshaft and Turboprop, General Specification for SA Technical Standards Board Rules prov

6、ide that: This report is published by SAE to advance the state of technical and engineering sciences. The use 01 this report is entirely voluntary, and its applicability and suitability lor any particular use. including any patent infringement arising therefrom, IS the sole responsibility of the use

7、r.“ SAE reviews each technical report at least every five years at which time it may be reaffirmed, revised. or cancelled. SAE invites your written comments and suggestions. Copyright 1993 Society of Automotive Engineers, Inc All rights reserved. Printed in U SA SAE AIR*4495 93 7943725 0517359 TO3 S

8、AE AIR4495 2.2 FAA Publications: Available from Federal Aviation Administration, 800 Independence Avenue S.W., Washington, DC 20591. Federal Aviation Regulation (FAR) Part 33 Amendment 13, August 1990 2.3 Other Publications: Joint Airworthiness Requirements - Engines (JAR-E) Change DEF-STAN-00-971,

9、29 May 1987, General Specification for A Engines 8, 4 May 1990 rcraft Gas Turb ne RTCA DO-lOB/EUROCAE ED 148 - Environmental Conditions and Test Procedures for Airborne Equipments, 1985 3. DESCRIPTION OF THE CORROSION CONDITIONS: Helicopters usually fly close to the earths surface where the atmosphe

10、re contains a greater concentration of potentially corrosive particles. Therefore, helicopter powerplant installations are usually more exposed to corrosion than fixed wing powerplants (except for dedicated military naval uti1 ization). Corrosion currently develops under salt laden atmosphere operat

11、ion (i.e., flight over the sea or inland close to the shore). various sets of ambient conditions and types of missions. The degree of corrosion depends upon the combination of a wide range of ambient conditions and mission profiles. the duration of exposure and the cyclic exposure are major contribu

12、tors. It may also occur under In addition to the concentration of salt in the air, The duration of the mission, the location of the operating base where the helicopter is stationed between missions, arid the maintenance equipment available play an important role in development of corrosion. Up to th

13、e 1970s, design considerations for specific prevention of corrosion were devoted mostly to military navy applications. So called “marinized“ version of engines were developed from existing standard engines. With the development of 1 arge offshore commercial he1 icopter operations for oil rig support

14、, special attention is needed to corrosion effects during any helicopter engine design. This applies for the entire range of engine sizes because light, medium, and heavy helicopters are used for these missions. In addition to navy and offshore oil rig operations, intensive low altitude flights over

15、 highly industrialized urban areas are also potentially conducive to corrosion. -2- SAE AIR*q495 93 m 79V3725 05L73b0 725 m SAE AIR4495 4. DESCRIPTION OF CORROSION EFFECTS: There are four major types of corrosion: a. Surface corrosion, which is a chemical process. b. Stress corrosion, which is a com

16、bined chemical and mechanical process. c. Fretting corrosion, which is a mechanical process. d. Galvanic corrosion, which is due to contact between dissimilar metals. This AIR addresses surface, stress, and galvanic corrosion but does not deal with fretting corrosion because it is a mechanical effec

17、t between two components in close contact where micro or tiny movements may cause material pitting. by ambient conditions. Localized corrosion may occur, which may look 1 ike corrosion caused The following paragraphs address only generalities of the subject because a 1 arge bi bl iography exists on

18、these topics. 4.1 Surface Corrosion: Corrosion can occur on the surface of components which are surrounded by the ambient atmosphere. Some of these areas where corrosion occurs are the outer parts of the engine casings, the accessories fitted around those casings, and the inner airflow path of the e

19、ngine. Entrapment of fine sand particles in engine components can also be a source of corrosi on. The outer surface of casings and the accessories within the helicopter engine compartment are in contact with the air supplied by the compartment ventilation. Due to the shape of engine equipment, and r

20、ecessed areas, air does not circulate freely and moisture becomes trapped. Components which are in the airflow path such as the air intake duct, compressor bl adi ng , combust i on 1 i ner , turbi ne bl adi ng , and i nner surf ace of casings may be affected by corrosion. content, the exposure is si

21、gnificant over a complete mission and during the life of the components as the air mass flow ranges from approximately 1.5 kg/s for small engines up to 14 kg/s for very large engines. The corrosion consists of the degradation of the metallic components into complex oxides. Near room temperature, thi

22、s degradation mainly occurs through electrolytical reactions and the possible contact of dissimilar metals leading to an acceleration of the corrosion. engines, the mechanisms are specific and hot corrosion is generally called “sulfidation“. (fuel) and sodium chloride (sea atmosphere). Even with a l

23、ow corrosive particles In the hot sections of The deciding agent is sodium sulfate formed from sulphur SAE AIRx4495 33 7943725 0537363 bbL SAE AIR4495 4.2 Stress Corrosion: Stress corrosion occurs on components which are subjected to sustained tensile stress. These are mostly rotating components whi

24、ch are in the airflow path. As opposed to surface corrosion, stress corrosion initiates and propagates cracks. not as apparent as with surface corrosion. This particular process is dangerous because the extent of damage is Stress corrosion generally occurs on engine components which operate in the l

25、ow or medium temperature range. parts. Examples are compressors and attachment The damage under stress corrosion should be considered as a propagation phenomenon in an aggressive media starting from surface damage. may be very small and very often from a single superficial corrosion point. This dama

26、ge 4.3 Galvanic Corrosion: Galvanic corrosion is a well known effect of dissimilar metals which form an electrical couple and have a tendency to corrode in contact with an electrolyte such as salt laden atmosphere. This type of corrosion occurs on connectors and accessories where dissimilar metals m

27、ay be used. 5. EFFECT ON COMPONENTS: 5.1 Rotating Engine Components: The major effect of corrosion on engine rotating components is the reduction of the life on those components. to burst if the corrosion damage is not discovered in time by maintenance inspection. Low cycle fatigue on discs could ca

28、use them The corrosion may either initiate a crack from a surface corrosion defect which develops (e.g., under engine start-stop sequences, or from stress corrosion in a recessed area on the surface which develops under static stress). This may happen on rotor discs, compressor or turbine shafts, la

29、byrinth seals, etc. To evaluate the sensitivity of materials to stress corrosion, laboratory tests of stressed specimens in salt spray environment are carried out to determine the K1 SCC (K1 stress corrosion concentration) factor. These effects are major because their consequences relate to aircraft

30、 safety. As such they need to be looked at very carefully at the different stages of the engine life from design to service. -4- SAE AIR*Li495 93 W 79Li3725 0517362 5TB W SAE AIR4495 5.1 (Continued): Corrosion also develops on other components where failure is not a major safety concern but which ma

31、y lead to damage to the engine. turbine blades, for example, may develop corrosion on the profile. from the combined effect of small particle impacts with salt laden atmosphere (which destroy a protective coating) and high temperature (for hot section components). The corrosion usually develops from

32、 a crack initiation lowering the standard initiation life of the material and, therefore, the high cycle fatigue (HCF) limits (so-called vibration corrosion). Corrosion can also take place on rotating components at locations where it would produce only minor effects on engine performance. This would

33、 preclude the components from being acceptable for rebuild at repair or overhaul. Compressor or This is This can be the case for costly parts, such as turbine discs, where defects due to corrosion on their balancing flanges would fail repairability limits. 5.2 Static Engine Components: The outer sur

34、faces of the engine and accessory casings, along with their flange and attachment elements, are exposed to the ambient corrosive atmosphere. nuts, and bolts, as they have recessed areas and “niches“ where highly corrosive air tends to stagnate. The most sensitive elements are the assemblies, such as

35、 flanges, Such a condition accelerates the attack on poorly protected or locally damaged surfaces. exposed. Components such as combustion and turbine casings are 5.3 Systems, Accessories: The most sensitive system components to corrosion are the electrical circuit components, mainly the connectors.

36、The joining of dissimilar materials, the use of copper, and the dimensions of pins, plugs, wires, etc. favor corrosive action. The result is that the system becomes inoperative by short with connector. The mechanical and hydromechanical unit sensitivity except that the exposed cas al so become corro

37、ded. 6. CORROSION PROTECTION: The above considerations have caused the to establish design principles to design possible. small n the components do not have he same ngs, like the engine casings, may helicopter powerplant manufacturers as many corrosion-free components as -5- SAE AIR*4475 93 7743725

38、05133b3 434 SAE AIR4495 6. (Continued): It is obviously less costly and easier to take the necessary action at the source than to modify parts on a complete engine fleet. effects are very difficult to predict precisely, careful maintenance instructions and actions are required. Since corrosion 6.1 D

39、esign Stage: The ideal situation for the designer would be to use corrosion resistant or stainless steel material for all parts where a corrosion threat exists. Due to many other requirements, such as performance (need of suitable characteristics at high temperature), high life limits and weight, th

40、ere are trade-offs which may obligate the designer to choose materials which are vulnerable to corrosion. the selection process of each new material. This consideration should be of prime importance in 6.1.1 Rotating Components: chosen very carefully to cope with the objective life limit, the burst

41、requirements, resistance to corrosion, and cost. The materials for parts in the airflow path should be On compressors, titanium alloys are widely used because, in most cases, it remains the best combination of mechanical resistance, corrosion resistance, and weight. evidence that there is no fire ha

42、zard either by direct contact or by other conditions as developed in the airworthiness standards and in the appropriate literature. All of these factors are combined to optimize the choice. Most of the time, blading requires coating when an iron based material is used instead of titanium. However, t

43、his choice must be supported by the For hot section components, nickel base alloys are mainly used. engines require higher and higher creep characteristics, the chemistry of the alloys is developed to the prejudice of hot corrosion resistance. Therefore, the blading of turbines must generally be coa

44、ted. Modified aluminides or MCrAlY coatings (where M is for Ni and/or Co) are widely applied. The processes are quoted in 7.1.6 (more particularly low pressure plasma spraying - (LPPS - and physical vapor deposition -PVD-) . As modern The main difficulty regarding blade coating is the proper adheren

45、ce with the parent material without affecting the basic characteristics of the blade material. When this is achieved, another difficulty occurs in operation. Wear or impact of foreign particles which break the surface, creates spots which are then unprotected from corrosion. -6- SAE AIR*4495 93 W 79

46、43725 0537364 370 SAE AIR4495 6.1.2 Static Components: which are exposed to ambient atmosphere in the airflow path and the entire surface of casings in the engine compartment. The static parts most exposed to corrosion are those For the components in the flow path, the precautions to be taken are si

47、milar to those for rotating components, therefore, the choice proper of materi al is essent i al . Any discontinuity of the surface should be carefully designed not only to prevent corrosive particles from being trapped, but also for the obvious aerodynamic purpose of minimizing drag. aerodynamics s

48、hould match with corrosion-free design. are areas where it is impossible to avoid discontinuities, such as joint surfaces where flanges contact, temperature probe locations air bleed openings, etc. When the material is not fully resistant to corrosion or not coated, the detail design must be conside

49、red carefully. As with rotating components, coatings are also used on guide vanes, stators, and diffuser blades in the cold section, when titanium or stain ess steel is not chosen. This is one area where good Nevertheless, there One basic principle for designing corrosion-free static components is to avoid contact of two different materials, where it is not absolutely proven that they will not develop galvanic corrosion. This effect is well known and the compatibility of materials is currently established; if not, appropriate laboratory testing should be carried out. The use of mag