The cavitation erosion resistance of cobalt alloys is superior to that of the stainless steels, but their cost is considerably higher. The economic factors, together with a better understanding of the factors responsible for the high erosion resistance of the cobalt alloys, have simulated the recent development (Ref 12) of new cavitation-resistant iron-base alloys. The success of this development is evidenced by the erosion data given in Fig. 4 for the new alloys designated IRECA and several iron- and cobalt-base commercial alloys. More comparative data on the erosion rates of various materials are given in the article "Liquid Impingement Erosion" in this Volume. Fig. 4 Cavitation erosion weight loss as function of exposure time measured on various standard and experimental alloys in ASTM G 32 vibratory tests. Source: Ref 12 Surface Coatings and Treatments. The recent trend in materials development has been to specify a component for bulk property requirements and subsequently coat or treat the surface to provide the required environmental resistance. Welded overlays of stainless steels or cobalt alloys are commonly used to provide resistance to cavitation erosion. Several alternative coating techniques that have been investigated in the laboratory in recent years include: arc-sprayed coatings (Ref 24), plasma-sprayed coatings (Ref 25, 26), laser hardening, cladding and alloying (Ref 10, 27), ion implantation (Ref 28, 29), and electroless nickel coatings (Ref 30). Plasma-sprayed coatings and laser treatments are beginning to be applied in practice for this purpose. Of these techniques, plasma spraying (Ref 31) is probably the most commercially well developed and offers flexibility in terms of the types of materials that can be sprayed. In addition, the technique can be carried out in air, in inert gas atmospheres, or in a reduced-pressure environment. There are two major disadvantages of plasma-sprayed coatings. One is that the coating is generally only mechanically bonded to the substrate and therefore does not exhibit very good adhesion. The other disadvantage is the inherent porosity of the coatings. One of the more erosion-resistant coatings produced by plasma spraying is of the "shape memory" alloy, NiTi (Ref 24). Like cobalt and the IRECA stainless steel, NiTi owes its superior properties to a stress-induced phase transformation. Laser surface treatments appear to offer the most possibilities and advantages. By rapidly heating (but not melting) and quenching the surface of a finished component, an extremely hard and resistant surface layer can be induced without requiring any further surface finishing and with little effect on the bulk properties of the part. Alternatively, the composition and properties of the surface can be tailored specifically to the requirements either by melting the surface layers and adding additional alloy components to the base alloy or by depositing a cladding material onto the surface. The advantages of this process are that the clad, alloyed, or heat-treated surface layer is an integral part of the component, which precludes any adhesion problems, and the process can be carried out in the atmosphere, rather than in vacuum. Its major disadvantages are that the processing equipment (laser and manipulation stations) is expensive and the technique cannot readily be executed on internal surfaces, because it is a line-of-sight process. Laser cladding is now being used to provide erosion resistance to marine engine diesel cylinder liners (Ref 32). To the best of the authors' knowledge, the other laser techniques have not yet been applied for the purpose of cavitation erosion resistance. Combined Effects of Cavitation Erosion and Corrosion As described above, cavitation erosion leads to mechanical degradation of engineering materials, whereas corrosion is an electrochemical oxidation, or dissolution, of the material. Because cavitation always takes place in a liquid medium, there is always the possibility of an interaction between mechanical and electrochemical processes, which can produce diverse and complex effects on the materials. The interaction may be synergistic and can lead to increased damage. Alternatively, one mechanism may inhibit or reduce the harmful effects of the other, leading to a reduction in the overall damage. Effect of Cavitation on the Corrosion Process. Cavitation can have a variety of effects on corrosion processes, including: • Removing any protective passive film from the metal surface • Increasing the diffusion rates of reactive dissolved gases to the metal surface • Increasing the rate of removal of the corrosion reaction products from the vicinity of the surface The net effect of cavitation is dependent on the type of corrosion. For example, it has been shown that cavitation can increase the ability of solution-treated stainless steel to become passive, whereas, for the same steel in the sensitized condition, the degree of intergranular corrosion is increased by cavitation (Ref 33). Effect of Corrosion on the Cavitation Process. The corrosion process is electrochemical and can be described by two reactions: the anodic reaction, which involves the dissolution, or oxidation, of the metal, and the cathodic reaction, which usually involves the evolution of hydrogen. As mentioned above, dissolved gases can cushion the implosion of the cavities and reduce their damaging effects. In a situation that involves both corrosion and cavitation, the evolution of hydrogen can therefore have the effect of reducing the mechanical stressing of the metal. Similarly, it is possible (although no evidence has been reported) that solid particles produced by the corrosion process could act as nuclei for cavities, and thereby enhance the onset of cavitation. Cavitation Erosion Testing When testing materials for their cavitation erosion resistance, there is no laboratory experimental equipment that simulates the total situation for a real structural component exposed to cavitating liquids. However, there are a number of laboratory techniques and procedures that can be used to, at least reasonably, rank a series of selected materials on the basis of cavitation erosion resistance. The most commonly used techniques today are flow channels, vibratory (ultrasonic) systems, and cavitating jets, all of which can simulate accelerated cavitation erosion in most materials. However, it is important to note that most real situations involving cavitation also involve corrosion attack (for example, salt water on ship propellers) and other mechanical loading of the materials (for example, the structural load on valve seats or in concrete water channels). Flow Channels. Typical flow channel equipment consists of a closed-loop circulating liquid flow channel with either a test section for scaled components, such as ship propellers, or a venturi restriction with a specimen holder designed to generate cavitation at specific locations near the specimen. This test simulates a flow cavitation situation very well. However, it is difficult to conduct accelerated cavitation erosion testing without changing the cavitation parameters relative to the service envelope of the simulated application. There are several different cavitation and specimen section designs (Ref 5) with the common feature that they are an integral part of the flow channel, which makes specimen changes difficult and more time consuming relative to the other techniques. Vibratory (ultrasonic) equipment consists of an ultrasonic horn that is partly submerged in the liquid, which is contained in a beaker (Fig. 5). The vibration, typically at 20 kHz frequency, generates negative pressure for cavitation nucleation and growth, and positive pressure for cavity collapse in a small, stationary volume of the liquid. The specimen is either mounted on the horn tip (moving specimen) or at a fixed distance (a few millimeters) below the horn tip (stationary specimen). Fig. 5 Vibratory cavitation device in wh ich specimen is either attached to or held below a horn oscillating in the lower kilohertz frequency range. Source: Ref 5 This test device is used for accelerated testing and lends itself to the study of interaction mechanisms with corrosion. Because the cavity size distribution is not the same as in the flow channel equipment, direct comparisons are not advisable. However, because the equipment is easy to use, it is widely applied to cavitation erosion resistance screening. Furthermore, ASTM G 32 describes the equipment and procedures for this test. Cavitating Jet. Two variations of this technique have been described. In type I, a hydraulic pump with an accumulator delivers the test liquid through a sharp-entry parallel-bore nozzle, which discharges a jet of liquid into a chamber at a controlled pressure (Ref 34). In type II, a high-pressure nozzle with an internal center body is used to create the low pressure to initiate the cavitation (Ref 35, 36). Cavitation starts in the vena contracta region of the jet within the nozzle (type I) or at the end of the center body (type II) before ejecting as a cloud of cavities around the emerging jet (type I) or in the center of the jet (type II). The specimen is placed in the path of the jet at a specific stand-off distance from the nozzle tip. The cavities collapse on the specimen, thereby causing erosion of the test material. The advantage of this technique is that it is an accelerated test method that offers the possibility of control and, thereby, allows the possibility of changing most of the cavitation parameters. Furthermore, the cavity size distribution resembles that of a real flow situation more than does that produced in the vibratory system. The technique is currently under consideration by ASTM as a standard test method. Other Techniques. Rotating disk test equipment has been used in earlier cavitation studies (Ref 37, 38, 39, 40). Such equipment consists of a rotating disk with specimen holder and cavitation sites (that is, holes in the disk) submerged in the liquid. The liquid is kept relatively stable in a chamber where the disk is rotated. This technique is no longer in common use; however, it can be used, for example, to simulate cavitation occurring in pump impellers. Means of Combating Erosion Materials Selection and Development. It is clear from the current understanding of deformation mechanisms in metals and alloys, and from the dynamic and localized nature of cavitation loading, that materials selection for erosion resistance should be based on the ability of alloys to absorb the impact energy by a nondestructive strain mechanism, such as twinning, stacking-fault formation, or a stress-induced martensitic-type phase transformation. Unfortunately, most standard mechanical testing is quasistatic in nature, and most cavitation erosion testing comprises weight loss measurements without any determination of the mechanism of material loss. Consequently, mechanical property databases do not usually contain the type of information necessary for appropriate materials selection. Similarly, there has been little effort to develop materials specifically for their erosion-resistant properties. However, the success of the initial research into the development of the iron-base IRECA alloys, which was based on a knowledge of the required deformation mechanisms and understanding of the compositional factors necessary to ensure that the alloy could deform in the appropriate manner, suggests that this is indeed a feasible approach and should be pursued. Coatings and Surface Treatments. Coating technology is one of the more rapidly growing technologies in the field of materials. It is clear that the selection of the base material for its bulk properties and a coating/surface treatment for its resistance to environmental factors is the wave of the future. A combination of the development of materials specifically designed for erosion resistance and the appropriate technique for the application of these materials as a coating would be the optimum solution. Suitable coating techniques also allow for regeneration of parts that have been rendered unusable by erosion. Other measures described below include design, air injection, and control of the operating temperature or pressure. System design represents the best way to either reduce or eliminate cavitation erosion. Therefore, fluid flow systems should be designed to minimize the changes in flow pressure that occur when the velocity is either increased or decreased, usually as a result of constrictions or changes in the direction of the flow. Similarly, the elimination of vibrations or the reduction in their amplitude would reduce the problems of cavitation erosion in many types of machinery. If cavitation cannot be eliminated, the cavitating regions should be designed to allow the cavities to collapse as far away from a solid surface as possible or to decrease the concerted collapse mode of the cavity cluster (Ref 9). Air injection into a cavitating fluid has been shown to be an effective method of reducing the intensity of erosion. The air creates bubbles and partly fills the cavities as they are formed, which prevents their complete collapse, thereby significantly reducing the magnitude of the shock wave emitted or the impact pressure of the microjets. The air also significantly changes the dynamic properties of the liquid, that is, the shock-wave velocity and its attenuation. Control of Operating Temperature or Pressure. It is more difficult to nucleate cavities at temperatures that approach the freezing temperature, T F , of the liquid and more difficult to collapse them at temperatures that approach the boiling temperature, T B . Therefore, cavitation erosion intensity is at a maximum at temperatures in the middle range between freezing and boiling. Consequently, a change in temperature that is close to either T F or T B will reduce the cavitation intensity and, thus, the degree of erosion. Similarly, increasing the hydrostatic pressure makes nucleation of cavities more difficult, but increases the erosive power of the cavities, whereas decreasing the collapse pressure makes collapse less intense. More comprehensive discussions of the influence of various cavitation parameters on the resulting erosion are given in a number of reviews (Ref 5, 41, 42). The salient conclusions that can be made about cavitation erosion are: • No materials are immune to cavitation erosion, as some are to corrosion; all will eventually erode • Metallic materials that exhibit stress- induced phase transformations have the highest erosion resistance. Further development of alloys specifically for their erosion resistance is to be encouraged • The combination of erosion and corrosion can be either syne rgistic or less harmful than either process alone. Unfortunately, there do not appear to be any general rules; therefore, each combination of material, environment, and erosion conditions must be evaluated • Coating technologies, particularly laser processing, offer great potential both in providing tailor- made erosion resistance to structures that are selected for their bulk properties and in repairing and regenerating eroded surfaces References 1. Lord Rayleigh, Philos. Mag., Vol 34, 1917, p 94-98 2. L. van Wijngaarden, 11th International Congress of Applied Mechanics (Munich), 1964 3. K.A. Mørch, Dynamics of Cavitation Bubbles and Cavitating Liquids, Erosion, C.M. Preece, Ed., Academic Press, 1979, p 309-355 4. M.S. Plesset and R.B. Chapman, J. Fluid Mech., Vol 47, 1971, p 283-290 5. C.M. Preece, Cavitation Erosion, Erosion, C.M. Preece, Ed., Academic Press, 1979, p 249 6. C.M. Preece and I.L.H. Hansson, A Metallurgical Approach to Cavitation Erosion, Advances in the Mechanics and Physics of Surfaces, R.M. Latanision and R.J. Courtel, Ed., Harwood Academic Publishers, 1981, p 191-253 7. B. Vyas and C.M. Preece, Cavitation-Induced Deformation of Aluminum, Erosion, Wear and Interfaces with Corrosion, ASTM, 1973 8. I.L.H. Hansson and K.A. Mørch, Compar ison of the Initial Stage of Vibratory and Flow Cavitation Erosion, 5th International Conference on Erosion by Liquid and Solid Impact, Cavendish Laboratory, Cambridge University, 1979 9. I.L.H. Hansson and K.A. Mørch, The Influence of Cavitation Guide Va nes on the Collapse of Cavity Clusters and on the Resulting Erosion, 11th Symposium of the IAHR Symposium on Operating Problems of Pump Stations and Power Plants (Amsterdam), International Association for Hydraulic Research, 1982 10. C.M. Preece and C.W. Draper, The Effect of Laser Quenching the Surfaces of Steels on Their Cavitation Erosion Resistance, Wear, Vol 67, 1981, p 321 11. E.B. Flint and K.S. Suslick, The Temperature of Cavitation, Science, Vol 253, 1991, p 1397-1399 12. R. Simoneau et al., Cav itation Erosion and Deformation Mechanisms of Ni and Co Austenitic Stainless Steels, 7th International Conference on Erosion by Liquid and Solid Impact, Cavendish Laboratory, Cambridge University, 1987 13. M.J. Kenn and A.D. Garrod, Cavitation Damage and the Tarbela Tunnel Collapse of 1974, Proc. Inst. Civil Eng., Vol 70, 1981, p 65 14. C.M. Preece, S. Vaidya, and S. Dakshinamoorthy, The Influence of Crystal Structure on the Response of Metals to Cavitation, Erosion: Prevention and Useful Applications, ASTM, 1979 15. I.L.H. Hansson and K.A. Mørch, The Initial Stage of Cavitation Erosion of Aluminum in Water Flow, J. Phys., Vol D11, 1978, p 147-154 16. E.H.R. Wade and C.M. Preece, Cavitation Erosion of Iron and Steel, Metall. Trans., Vol 9A, 1978, p 1299- 1310 17. S. Vaidya and C.M. Preece, Cavitation-Induced Multiple Slip, Twinning, and Fracture Modes in Zinc, Scr. Metall., Vol 11, 1977, p 1143-1146 18. S. Vaidya, S. Mahajon, and C.M. Preece, The Role of Twinning in the Cavitation Erosion of Cobalt Singl e Crystals, Metall. Trans., Vol 11A, 1980, p 1139-1150 19. S. Vaidya and C.M. Preece, Cavitation Erosion of Age-Hardenable Aluminum Alloys, Metall. Trans., Vol 9A, 1978, p 299-307 20. R. Schulmeister, Proceedings of the 1st International Conference on Rain Erosion, Royal Aircraft Establishment, United Kingdom, 1965 21. D.A. Woodford, Metall. Trans., Vol 3, 1978, p 1137 22. J.W. Tichler and A.W.J.D. Gee, 3rd International Conference on Rain Erosion, Royal Aircraft Establishment, United Kingdom, 1974 23. T.F. Pedersen, S. Pedersen, and I.L.H. Hansson, Subsurface Deformation Studies of Cavitation Eroded FCC Materials, 6th International Conference on Erosion by Liquid and Solid Impact, Cavendish Laboratory, Cambridge University, 1983 24. A.P. Jardine, Y. H oran, and H. Herman, Cavitation Erosion Resistance of Thick Film Thermally Sprayed NiTi, Proceedings of Symposia on High Temperature Intermetallics, Vol 213, Materials Research Society, 1991, p 815-820 25. X X. Guo, H. Herman, and S. Rangaswamy, Cavitation Erosion of Plasma Sprayed WC/Co, Advances in Thermal Spraying, Pergamon Press, 1986, p 37-41 26. S. Sampath, G.A. Bancke, and H.R. Herman, Plasma Sprayed Ni-Al Coatings, Surf. Eng., Vol 5 (No. 4), 1989, p 293-298 27. R.J. Crisci, C.W. Draper, and C.M. Preece, Cavitation Erosion Resistance of Laser Surface Melted Self- Quenched Fe-Al Bronze, Appl. Opt., Vol 21, 1982, p 1730 28. W.W. Hu, et al., Cavitation Erosion of Ion-Implanted 1018 Steel, Mater. Sci. Eng., Vol 45, 1980, p 263-268 29. C.M. Preece and E.N. Kaufmann, The Effect of Boron Implantation on the Cavitation Erosion Resistance of Copper and Nickel, Corros. Sci., Vol 22, 1982, p 267-281 30. S. Pedersen and I.L.H. Hansson, Nickel Coatings for Cavitation Erosion Resistance of Brass Components, 6th International Conference on Erosion by Liquid and Solid Impact, Cavendish Laboratory, Cambridge University, 1983 31. H. Herman, Plasma-Sprayed Coatings, Sci. Am., Vol 256 (No. 9), 1988, p 112-117 32. W. Amende, private communication, 1989 33. B. Vyas and I.L.H. Hansson, The Cavitation Erosion-Corrosion of Stainless Steel, Corros. Sci., Vol 30 (No. 8/9), 1990, p 761-770 34. A. Lichtarowicz and P.J. Scott, Erosion Testing with Cavitating Jet, 5th International Conference on Erosion by Liquid and Solid Impact, Cavendish Laboratory, Cambridge University, 1979 35. P.A. March, Evaluating the Relative Resistance of Materials to Cavitation Erosion: A Comparison of Cavitation Jet Results and Vibratory Results, Cavitation and Multiphase Flow Forum, FED, 1987 36. P.A. March, Cavitating Jet Facility for Cavitation Erosion Research, Symposium on Cavitation Research Facilities and Techniques, American Society of Mechanical Engineers, 1987 37. J.Z. Lichtman, D.H. Kallas, C.K. Chatten, and E.P. Cochran, Cavitation E rosion Resistance of Structural Materials and Coatings, Corrosion, Vol 17, 1961, p 497-505 38. J.Z. Lichtman and E.R. Weingram, The Use of a Rotating Disc Apparatus in Determining Cavitation Erosion Resistance of Materials, Symposium on Cavitation Research Facilities and Techniques, American Society of Mechanical Engineers, 1964 39. A. Thiruvengadam, A Comparative Evaluation of Cavitation Damage Test Devices, Cavitation Research Facilities and Techniques, American Society of Mechanical Engineers, 1964 40. P. Veerabhadra Rao, Correlating Models and Prediction of Erosion Resistance to Cavitation and Drop Impact, J. Test. Eval., 1976, p 3-14 41. H. Wiegand and R. Shulmeister, Investigations with a Vibratory Apparatus on the Influence of Frequency, Amplitude, Pressure, and Temperature on Material Destruction by Cavitation, Motortechnische Zeitschrift, Vol 29 (No. 2), 1968, p 41-50 42. S. Pedersen, "Cavitation Erosion: Vibratory Cavitation and Cavitation Erosion of Metals," Ph.D. thesis, Laboratory of Applied Physics, Technical University of Denmark, 1986 Liquid Impingement Erosion Frank J. Heymann (retired), Westinghouse Electric Corporation Introduction LIQUID IMPINGEMENT EROSION has been defined as "progressive loss of original material from a solid surface due to continued exposure to impacts by liquid drops or jets" (Ref 1). The operative words in this definition are "impacts by liquid drops or jets": liquid impingement erosion connotes repeated impacts or collisions between the surface being eroded and small discrete liquid bodies. Excluded from this definition are erosion mechanisms due to the impingement of a continuous jet, due to the flow of a single-phase liquid over or against a surface, due to a cavitating flow, or due to a jet or flow containing solid particles although all these can produce erosion (progressive loss of solid material) at least under some conditions. Some of these mechanisms will, however, be discussed briefly in order to distinguish them clearly from the primary subject. The significance of the discrete impacts is that they generate impulsive contact pressures on the solid target, far higher than those produced by steady flows (see the discussion "Liquid/Solid Interaction Impact Pressures" later in this article). Thus, the endurance limit and even the yield strength of the target material can easily be exceeded, thereby causing damage by purely mechanical interactions. In some circumstances the damage can also be accelerated by conjoint chemical action. At sufficiently high impact velocities, solid material can be removed even by a single droplet (or other small liquid body). Much of what is currently known about the liquid/solid interactions in liquid impingement has been determined through laboratory experiments and analytical modeling involving single impacts. Liquid impingement erosion in its advanced stages is characterized by a surface that appears jagged, composed of sharp peaks and pits (Fig. 1). A possible reason for this will be given later. Fig. 1 T wo portions of a steam turbine blade that has experienced liquid impingement erosion. The portion on the left was protected by a shield of rolled Stellite 6B brazed onto the leading edge of the blade; the portion on the right is unprotected type 403 stainl ess steel. Note the difference in degree of erosion. Normally such erosion does not impair the blade's function. Both at 2.5× A very comprehensive treatment and review of liquid impact erosion can be found in Ref 2; in particular the chapters therein by Adler (Ref 3) and by Brunton and Rochester (Ref 4). Reference 5 contains some now classic studies that provided the foundation for subsequent work. Many other contributions to this field are found in several ASTM symposium volumes (Ref 6, 7, 8, 9, 10) and in the proceedings of the international "Rain Erosion" and "Erosion by Liquid and Solid Impact" (or "ELSI") conferences (Ref 11, 12, 13, 14, 15, 16, 17). Individual papers from some will be cited in context. Acknowledgements The author would like to thank John E. Field of Cambridge University, George F. Schmitt, Jr. of the Air Force Materials Laboratory and Westinghouse Electric Corporation for supplying photographs. Additional thanks are due to George Schmitt for also supplying information on the current state of rain erosion protection and providing valuable suggestions for improving this article. Occurrences in Practice It is quite difficult to propel liquid droplets to high velocities without breaking them up, and liquid impingement erosion haas become a practical problem primarily where the target body moves at high speeds and collides with liquid drops that are moving much more slowly. Almost all the work done in this subject has been in connection with just two major problems: "moisture erosion" of low-pressure steam turbine blades operating with wet steam, and "rain erosion" of aircraft or missile surfaces and helicopter rotors. Whenever vapor or gas flows carrying liquid droplets impinge upon solid surfaces as in nuclear power plant pipes and heat exchangers, for example erosion can also occur. However, the probable impact velocities and impact angles are such as to make "pure" liquid impingement erosion an unlikely mechanism. It is much more likely that an "erosion- corrosion" mechanism is then involved (see the discussion of "Impingement Attack and Erosion-Corrosion" later in this article). Steam Turbine Blade Erosion. Moisture erosion of low-pressure blades has been a problem throughout steam turbine history, and remains a concern today. In the last stages of the low-pressure turbine, the steam expands to well below saturation conditions, and a portion of the vapor condenses into liquid. Although the condensation droplets are very small, some of them are deposited onto surfaces of the stationary blades (guide vanes), where they coalesce into films or rivulets and migrate to the trailing edge. Here they are torn off by the steam flow, in the form of much larger droplets. These large droplets slowly accelerate under the forces of the steam acting on them, and when they are carried into the plane of rotation of the rotating blades, they have reached only a fraction of the steam velocity. As a result, the blades hit them with a velocity that is almost equal to the circumferential velocity (wheel speed) of the blades, which can be as high as 650 m/s (2100 ft/s) in a modern 3600 rpm turbine. References 18 and 19 describe these processes in detail. The same basic phenomenon can, of course, occur in wet vapor turbines operating with other working fluids, such as sodium or mercury. The principal remedies in modern turbines include extracting moisture between blade rows, increasing axial spacing between stator and rotor to permit droplets to be accelerated and broken up, and making the leading edge of the blade more resistant to erosion. This last remedy has been accomplished by local flame hardening of the blade material, by brazed-on "shields" of Stellite (Fig. 1), or in some cases by shields of tool steel or weld-deposited hardfacing. Tests on many blade and shield materials are reported in Ref 20 and 21. The base material for present-day low-pressure blades is usually a 12% Cr martensitic stainless steel, a 17Cr-4Ni precipitation-hardening stainless steel, or, more rarely, a titanium alloy. Recently, success has been claimed for new "self-shielding" blade alloys that harden under the action of the impacts. One such alloy is Jethete M152, a martensitic steel containing about 11% Cr, 2.9% Ni, 1.6% Mo, and 0.3% V. Other new approaches that have been investigated include plasma-deposited Stellite and an ion-plated chromium-tin multilayer coating; however, it is doubtful that relatively thin coatings can provide long-term protection. The evaluation and prediction of steam turbine blade erosion is very complex; recent contributions include Ref 22 and 23. Aircraft Rain Erosion. Rain erosion became a major problem in the 1950s, when military aircraft reached transonic and supersonic speeds. The impact of rain drops, 2 mm (0.08 in.) or more in size, on unprotected aluminum alloy surfaces, optical and infrared windows, and radomes caused severe erosion which seriously limited operational time in rain storms. This resulted in many government-funded research projects into erosion mechanisms as well as development and evaluation of protective coatings. Reference 24 gives an overview of the rain erosion problem, with special reference to radomes. The current status concerning remedies has been summarized as follows by Schmitt (Ref 25): Protection of aircraft radomes and composite surfaces is accomplished with two classes of elastomeric coatings polyurethanes for widespread lower temperature applications, and fluorocarbons where elevated temperatures (above 177 °C, 350 °F) or special requirements (camouflage colors, thermal flash protection) are involved. For applications where su personic rain erosion is a concern, the inherent erosion resistance of the base materials combined with streamline geometry to reduce impact angles is the most often used approach. Another aerodynamic technique is to utilize the shock waves to shatter and fragment the raindrops into very small pieces that produce less damage. For hemispherical domes where impact angles must be large (near normal), protective coatings of boron phosphide, germanium carbon and diamond are being pursued, but they are restricted to velocities less than Mach 2. At extremely high velocities, protection at high impact angles may require metal tips or sacrificial layers even though a performance penalty must be paid. In many cases, lack of adequate materials and potential catastrophi c failure simply precludes operation at high supersonic speed in rainy environments. References 8, 9, 10, 11, 12, 13, 14, 15, 16, and 17 contain numerous papers on evaluation of materials and coatings for rain erosion applications. Among the most recent are investigations of infrared window materials, slip-cast fused silica, and hard carbon-coated germanium in Ref 17; polyurethane and fluoroelastomer coated composite constructions, composite materials for radomes, new materials for radomes, and infrared windows (including polyethersulfone, polyetherimide, polyetherketone, and germanium) in Ref 16; and slip-cast fused silica, boron-aluminum composites, composite and honeycomb structures, polytetrafluoroethylene (PTFE) and polymethylmethacrylate (PMM) in Ref 15. As mentioned earlier, rain erosion also poses a threat to missile surfaces and helicopter rotors. Figure 2 shows the catastrophic failure of a missile dome due to rain erosion effects. Fig. 2 Rain erosion effects on a Maverick missile dom e made of coated zinc sulfide that was exposed for 10 s at a speed of about 210 m/s (690 ft/s). The dome itself suffered catastrophic damage, and erosion is also seen on the filled elastomeric mounting ring. Courtesy of G.F. Schmitt, Jr., Materials Laborat ory, Wright Research and Development Center, Department of the Air Force Relationship to Other Erosion Processes Continuous Jet Impingement. Impingement of a high-velocity continuous jet can cause material removal, and that fact has led to the development of jet-cutting technology used in quarrying, mining, and material cutting. While there is some overlapping with erosion research, much of the literature is found in the "American Water Jet Conferences" and the "International Conferences on Jet Cutting Technology." [...]... equations for maximum erosion rate and for incubation period are given in Ref 39 and 41, based on an interlaboratory test program sponsored by ASTM Technical Committee G-2 on Wear and Erosion These are: log Re = 4.8 log V - log NER - 16. 65 + 0 .67 log d + 0.57 J - 0.22 K log N0 = -4 .9 log V + log NOR + 16. 40 -0 .40 J where Re and N0 are the rationalized erosion rate and incubation period, respectively,... Erosive-Corrosive Wear in Steam Extraction Piping, Paper 87-JPGC-Pwr-35, American Society of Mechanical Engineers, 1987 30 P.A Coulon, Erosion-Corrosion in Steam Turbines Part II: A Problem Largely Resolved, Lubr Eng., Vol 42 (No 6) , 19 86, p 35 7-3 62 31 N Henzel, W Kastner, and B Stellwag, Erosion Corrosion in Power Plants under Single- and Two-Phase Flow Conditions Updated Experience and Proven Counteractions,... Abrasion-corrosion (b) Scouring wear, with wear areas equal (left) and unequal (center and right) (c) Crushing and grinding (d) High-velocity erosion (e) Low-velocity erosion (f) Saltation erosion (g) Cavitation Abrasion-corrosion wear is the result of any metal-to-metal rubbing in the presence of abrasive solids in a corrosive liquid This is the most destructive and misunderstood mode encountered when handling... well as the costly liquid end of the pump (Fig 4 and 5) Slurry-throttling valves and parts that are downstream also experience this type of rapid wear, as do the impellers and cut-water mechanisms of centrifugal pumps Fig 4 Typical worn and washed-out valve Fig 5 Typical worn and washed-out slurry valve seat Low-velocity erosion is usually a low-rate wear mode that occurs when there is a flow of slurry... Nickel Phosphate Potash Pyrite Quartzite Rutile Salt brine Sand and sand fill Sea bottom Shale Serpentine Sewage, digested Sewage, raw Sodium sulfate Soda ash tailings Sulfur Tailings (all types) Tar sand Waste, nickel Waste, coal 6, 8 57 10 41 28, 37, 64 , 79, 122, 157, 234 7, 30 14 22, 30, 39, 43, 46 113 4 64 , 71, 134 76 10 31 68 , 74, 84, 134 1, 2 194 99 10 11 51, 68 , 85, 1 16, 138, 149, 2 46 11 53, 59 134... Cambridge, England, 1983, p 1 8- 1 to 1 8- 6 37 J.E Field, J.P Dear, and J.E Ogren, The Effects of Target Compliance on Liquid Drop Impact, J Appl Phys., Vol 65 (No 2), 15 Jan 1989, p 53 3-5 40 38 F.J Heymann, On the Time Dependence of the Rate of Erosion due to Impingement or Cavitation, STP 408, ASTM, 1 967 , p 7 0-1 10 39 "Standard Practice for Liquid Impingement Erosion Testing," G 73, Annual Book of ASTM Standards,... Vol 32 (No 9), 1989, p 37 5-3 81; and A Model of Erosion of Metals by Liquid Impact II, Kernenergie, Vol 32 (No 12), 1989, p 47 1-4 76] 51 F.N Mazandarany and F.G Hammitt, "Russian and Eastern European Literature on Cavitation Resistant Steels," Report MMPP-34 4-4 -T, University of Michigan, Aug 1 969 52 A Akhtar, A.S Rao, and D Kung, Cavitation Erosion of Stainless Steel, Nickel and Cobalt Alloy Weld Overlay... Materials Science and Technology, Vol 16, Erosion, C.M Preece, Ed., Academic Press, 1979, p 185 -2 48 5 F.P Bowden, Organizer, Deformation of Solids by the Impact of Liquids (and Its Relation to Rain Damage in Aircraft and Missiles, Blade Erosion in Stream Turbines, Cavitation Erosion), Philos Trans R Soc A, Vol 260 (No 1110), 1 966 6 Symposium on Erosion and Cavitation, STP 307, ASTM, 1 962 7 Erosion by... University of Cambridge, England, 1983 17 J.E Field and J.P Dear, Ed., Proceedings of the Seventh International Conference on Erosion by Liquid and Solid Impact (ELSI-VII), Cavendish Laboratory, University of Cambridge, England, 1987 18 G.C Gardner, Events Leading to Erosion in the Steam Turbine, Proc Inst Mech Eng., Vol 178, Part 1 (No 23), 1 96 3-1 964 , p 59 3 -6 23 19 M.J Moore and P Schulpher, Conditions... 25 References 1 "Standard Terminoloy Relating to Wear and Erosion," G40, Annual Book of ASTM Standards, ASTM 2 C.M Preece, Ed., Treatise on Materials Science and Technology, Vol 16, Erosion, Academic Press, 1979 3 W.F Adler, The Mechanics of Liquid Impact, Treatise on Materials Science and Technology, Vol 16, Erosion, C.M Preece, Ed., Academic Press, 1979, p 127 -1 83 4 J.H Brunton and M.C Rochester, . corrosion-erosion, 10 to 50 m/s (35 to 165 ft/s); erosion- corrosion, 50 to 200 m/s ( 165 to 65 5 ft/s); and erosion, >200 m/s (> ;65 5 ft/s). Henzel et al. (Ref 31) give an update on experience and. Engineers, 1 964 40. P. Veerabhadra Rao, Correlating Models and Prediction of Erosion Resistance to Cavitation and Drop Impact, J. Test. Eval., 19 76, p 3-1 4 41. H. Wiegand and R. Shulmeister,. polyetherimide, polyetherketone, and germanium) in Ref 16; and slip-cast fused silica, boron-aluminum composites, composite and honeycomb structures, polytetrafluoroethylene (PTFE) and polymethylmethacrylate