Erosion, cavitation

Four distinct forms of erosive wear have been identified soHd-particle erosion, Hquid-droplet erosion, cavitation erosion, and slurry erosion. 

The abrasion resistance of cobalt-base alloys generally depends on the hardness of the carbide phases and/or the metal matrix. For the complex mechanisms of soHd-particle and slurry erosion, however, generalizations cannot be made, although for the soHd-particle erosion, ductihty may be a factor. For hquid-droplet or cavitation erosion the performance of a material is largely dependent on abiUty to absorb the shock (stress) waves without microscopic fracture occurring. In cobalt-base wear alloys, it has been found that carbide volume fraction, hence, bulk hardness, has Httie effect on resistance to Hquid-droplet and cavitation erosion (32). Much more important are the properties of the matrix. 

We can aggravate the corrosion effect if misiiligned parts have relative movement, sueh as loose fit bearings or rapid changes in the system. Cavitation, erosion and high fluid velocity advance the corrosion process. 

Fig 8 78 Cavitation erosion resistance ranking of solid polymers... 

Fig. 8.79 (fl) Pump performance characteristics. (6) Cavitation erosion zone, (c) Effect of reducing net positive suction head... 

The final estimation of the value of ay may appear tedious and several assumptions are made in its derivation, but experimental evidence suggests that it may be used with reasonable accuracy to assess the levels of potentially damaging cavitation erosion. In small valves with nominal bores up to 65 mm cavitation inception occurs in intermittent bursts when the value oy is approximately unity. The cavitation becomes continuous and audible as Oy is reduced to about 0.6, but the risk of damage does not become significant until the value falls below 0.4. As a design criterion the condition of light, steady noise has been described by Tullis as the critical level and is sug-... 

Preece, C. M., Cavitation Erosion, Treatise on Materiai Science and Technoiogy, Vol 16, Erosion, Academic Press, London (1979)... 

Lightarowicz, A., Cavilating Jet Apparatus for Cavitation Erosion Testing, Erosion Prevention and Useful Applications, ASTM STP 664, p. 530-549 (1979)... 

Hutton, S. P., Proc. ASME Symposium on Cavitation Erosion in Fluid Systems Boulder, Colorado, USA (1981)... 

This type of damage is dealt with comprehensively in Section 8.8. It can be particularly severe in seawater giving rise to cavitation corrosion or cavitation erosion mechanisms, and hence can be a considerable problem in marine and offshore engineering. Components that may suffer in this way include the suction faces of propellers, the suction areas of pump impellers and casings, diffusers, shaft brackets, rudders and diesel-engine cylinder liners. There is also evidence that cavitation conditions can develop in seawater, drilling mud and produced oil/gas waterlines with turbulent high rates of flow. 

Fig. 9.24 Cavitation erosion in oil and gas production well tubing. Note the severe localised attack arising from a turbulence-creating downhole valve a few metres upstream...

The phenomenon and mechanisms of cavitation-erosion have been considered in Section 8.8 and here it is only necessary to consider laboratory test methods that have been designed to simulate conditions that prevail in practice and which may be used to evaluate the performance of materials. 

In considering these tests it should be remembered that the phenomenon of cavitation-erosion is often accompained by corrosion effects and that a synergistic effect may operate between the mechanically and chemically induced forms of attack. In fact the term cavitation-erosion-corrosion may often be more applicable in describing the requirements of a test procedure. The subject has recently been discussed by Wood etal. °. 

Fig. 19.25 Vibratory cavitation-erosion test using magnetostriction...

Assessment of cavitation-erosion is based on mass loss and the results are expressed as curves showing cumulative mass (or volume) loss V5. the time of the test. Eisenberg, have expressed the cumulative mass loss plot... 

Examples of various vibratory test procedures for studying cavitation-erosion of metals in inhibited engine coolants have been given in an ASTM Special Technical Publication... 

D 2809 1989 Test method for cavitation erosion-corrosion characteristics of aluminium pumps with engine coolants... 

Practice for making and using U-bend stress corrosion test specimens Recommended practice for laboratory immersion corrosion testing of metals Method for vibratory cavitation erosion test Practice for recording data from atmospheric corrosion tests of metallic-coated steel specimens... 

Cavitation erosion With increasing ship speeds, the development of high-speed hydraulic equipment, and the variety of modem fluid-flow applications to which metal materials are being subjected, the problem of cavitation erosion becomes ever more important. Erosion may occur in either internal-flow systems, such as piping, pumps, and turbines, or in external ones like ships propellers (36). 

Wear is the removal of surface material by one of three mechanisms erosion, abrasion, or cavitation. Erosion is the removal of a polymer s surface by abrasive materials carried in a fluid medium. We see this type of wear in plastic pipes used to transport waterborne slurries of minerals in mining operations and in vacuum transfer pipes used to convey powders in a stream of air. Abrasion is the result of two surfaces sliding against each other. We commonly observe abrasion of polymers in the fabrics of our clothes and upholstery. Cavitative wear is caused by voids in a liquid medium collapsing against a surface. It is essentially an impact process. Cavitation is a relatively uncommon cause of wear in polymers. Pump impellers are one of the few applications where polymers must resist this type of wear. 

Hansson I, Morch KA, Preece CM (1977) A comparison of ultrasonically generated cavitation erosion and natural flow cavitation erosion. In Proceedings of the Ultrasonics International Conference, Brighton, UK, pp 267-274... 

Enhanced chemical reactivity of solid surfaces are associated with these processes. The cavitational erosion generates unpassivated, highly reactive surfaces it causes short-lived high temperatures and pressures at the surface it produces surface defects and deformations it forms fines and increases the surface area of friable solid supports and it ejects material in... 

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