Cavitation corrosion damage

Figure 15. Cast-iron pump impeller with cavitation corrosion damage...

To reduce cavitation corrosion damage in cooling circuits, corrosion inhibitors are sometimes added. This underscores the electrochemical nature of cavitation... 

Cavitation damage is a fonn of deterioration associated with materials in rapidly moving liquid environments, due to collapse of cavities (or vapour bubbles) in the liquid at a solid-liquid interface, in the high-pressure regions of high flow. If the liquid in movement is corrosive towards the metal, the damage of the metal may be greatly increased (cavitation corrosion). 

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. 

One other inhibitor type that will prevent cavitation-corrosion is the soluble oil type, which incorporates a light mineral oil plus emulsifiers and adsorption-type inhibitors, such as organic amines. Unfortunately, although effective in controlling cavitation, (possibly by cushioning effects of the adsorbed oil reinforced film) they soften and damage rubber connectors and seals, cause leakage and water loss. When emulsifiers are exhausted, the oil emulsion breaks, and allows oily films to form on heat transfer surfaces. 

Figure 7.50 a) Cavitation corrosion on the propeller of a high-speed passenger ship, h) Close np of the damages. (The material is nickel-aluminium hronze.) (Photos T.E. Hammervold and O Sastre.)... 

Cavitation corrosion is brought about by cavitation damage. It can be seen when the repeated evolution and collapse of tiny air bubbles on material surfaces occurs. Cavitation breaks the stable passive films on materials surfaces, which would easily lead to corrosirMi. 

Low-pressure zones in a fluid system under turbulent flow conditions are at the origin of cavitational wear. When the hydrodynamic pressure at some location temporarily drops below the vapor pressure of the fluid a fraction of the hquid is transformed into gas bubbles. The recurrent formation and implosion of these bubbles leads to material damage by local fatigue. If simultaneously corrosion takes place we speak of cavitation corrosion. 

Particularly susceptible to damage from cavitation corrosion are, for example, ships propellers, condenser pipes, pump impellers, widened segments in pipelines or flow channels. 

Cavitation damage (sometimes referred to as cavitation corrosion or cavitation erosion) is a form of localized corrosion combined with mechanical damage that occurs in turbulent or rapidly moving liquids and takes the form of areas or patches of pitted or roughened surface. Cavitation has been defined as the deterioration of a surface caused by the sudden formation and collapse of bubbles in a liquid. It has been similarly defined as the localized attack that results from the collapse of voids or cavities due to turbulence in a liquid at a metal surface. Cavitation also occurs in areas of high vibration such as on engine pistons and piston liners (Fig. 6.43). 

In some instances, cathodic protection has been successful in reducing or preventing cavitation damage, but because cavitation damage usually involves physical as well as electrochemical processes, it cannot always be prevented by this means. In some cases, inhibitors have been used successfully to limit cavitation corrosion, as in the water side of diesel engine cylinder liners. Cavitation is a problem with ship propellers, hydraulic pumps and turbines, valves, orifice... 

Cavitation corrosion is a form of localized corrosion combined with mechanical damage, that occurs in a rapidly moving liquids and takes the form of areas or patches of pitted or roughened surface. Many of the terms related to cavitation damage need to be distinguished from each other ... 

It is often localized at areas where water changes direction. Cavitation (damage due to the formation and coUapse of bubbles in high velocity turbines, propellers, etc) is a form of erosion corrosion. Its appearance is similar to closely spaced pits, although the surface is usually rough. 

Cavitation Formation of transient voids or vacuum bubbles in a liquid stream passing over a surface is called cavitation. This is often encountered arouna propellers, rudders, and struts and in pumps. When these bubbles collapse on a metal surface, there is a severe impact or explosive effec t that can cause considerable mechanical damage, and corrosion can be greatly accelerated because of the destruction of protective films. Redesign or a more resistant metal is generally required to avoid this problem. 

Indirect attack can also occur because of turbulence associated with flow over and around a deposit. Increased turbulence may initiate attack (see Chap. 11, Erosion-Corrosion and Chap. 12, Cavitation Damage ). 

Erosion and cavitation both can degrade materials simply by mechanical means or by combining the effects of mechanical deterioration and corrosion to produce a synergistic result. However, the mechanisms by which erosion and cavitation operate, and the resulting damage, are quite distinct. 

Cavitation damage in the complete absence of corrosion has been demonstrated (e.g., roughening of polished glass in cavitating distilled water). However, in industrial situations it is highly probable that corrosion is a common contributing factor. 

It is possible to confuse corrosion by a strong mineral acid with damage by cavitation. Both can produce jagged, undercut, spongelike surface... 

Cavitation is more or less continuous, but the cavitation intensity is so low that only the coating of corrosion products is dislodged. Resistance of the corrosion products to cavitation relative to the resistance of the metal to cavitation is low, making the corrosion products more susceptible to damage. 

Examination of surface profiles in these pitted regions under a high-power microscope revealed jagged, undercut profiles free of deposits or corrosion products. This appearance is typical of cavitation damage. 

The pump has experienced graphitic corrosion. Figures 17.10, 17.12, and 17.14 illustrate typical appearances of graphitically corroded cast iron. In addition, cavitation damage (see Chap. 12) has produced severe metal loss in specific areas (see Fig. 17.13). The soft, friable corrosion products produced by graphitic corrosion are susceptible to cavitation damage at relatively low levels of cavitation intensity. 

Graphitic corrosion of the cast iron produced a soft, mechanically weak corrosion product that is highly susceptible to cavitation damage, even at relatively low cavitation intensities. The black coating on the impeller surface is visual evidence of graphitic corrosion. The spongelike surface contours are typical of cavitation damage (see Chap. 12). 

Figure 17.15 Section of a cast iron pump impeller that has suffered graphitic corrosion followed by cavitation damage.

Plate anodes were used for corrosion protection in order to avoid damage due to erosion and cavitation. These consisted of enamelled steel bodies in which a metal oxide-coated titanium anode 1 dm in surface area was fitted. The enamel... 

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