Cavitation corrosion

Wear, corrosion Fretting, velocity assisted corrosion, cavitation... 

The abrasive wear of plastics occurs as a result of strong adhesive interaction, fatigue, macroshearing, abrasive action, thermal and thermooxidative interaction, corrosion, cavitation, etc. Fillers are involved in these processes because mineral... 

Mechanically assisted degradation can consist of the following types of corrosion erosion-corrosion, water drop impingement corrosion, cavitation erosion, erosive and corrosive wear, fretting corrosion, and corrosion fatigue (CF) (Fig. 1.14). Erosion-corrosion consists of the corrosion process enhanced by erosion or wear. Fretting corrosion consists of the wear process enhanced by corrosion. CF consists of the combined action of fluctuating or cyclic stress and a corrosive environment. 

Carbon dioxide (COj) corrosion Hydrogen sulfide (HjS) corrosion Preferential weld corrosion Erosion and erosion-corrosion Crevice corrosion Flange face corrosion Cavitation Dead-leg corrosion Under-deposit corrosion Microbial corrosion Oxygen corrosion Galvanic corrosion External corrosion Corrosion under insulation (CUI)... 

There are some corrosion cases where other factors, i.e. stress, or other mechanical factors are involved. For example, stress corrosion cracking, corrosion fatigue, hydrogen embrittlement, erosion corrosion, cavitation corrosion, fretting corrosion, etc. can be mentioned. 

Copper Uniform Pitting Crevice Erosion corrosion Cavitation Galvanic Concentration cell... 

Cavitation can also lead to local destruction of protective layers. In rapidly flowing liquid and on solid surfaces that oscillate in the liquids, gas or steam bubbles are produced at sites at which the pressure in the liquid is briefly lowered to vapor pressure due to flow in excess of the critical flow rate threshold. When the pressure is raised again, these bubbles collapse suddenly (implosion) and a jet of liquid hits the material surface at a high rate of speed. This sudden stress load pattern is continuously exposing or creating active surfaces on which increased corrosion (cavitation corrosion) takes place in an aggressive medium. 

Velocity effects include erosion-corrosion, a form of attack caused by high velocity flow cavitation caused at even higher flow by the collapse of bubbles formed at areas of low pressure in a flowing stream and fretting that is caused by vibratory relative motion of two surfaces in close contact under load erosion-corrosion, cavitation, fretting). 

Factors of Filiform Corrosion Water Fine Corrosion Crevice Corrosion Cavitation Erosion... 

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). 

Titanium resists erosion—corrosion by fast-moving sand-laden water. In a high velocity, sand-laden seawater test (8.2 m/s) for a 60-d period, titanium performed more than 100 times better than 18 Cr—8 Ni stainless steel. Monel, or 70 Cu—30 Ni. Resistance to cavitation, ie, corrosion on surfaces exposed to high velocity Hquids, is better than by most other stmctural metals (34,35). 

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. 

Whenever corrosion resistance results from the accumulation of layers of insoluble corrosion products on the metallic surface, the effect of high velocity may be either to prevent their normal formation or to remove them after they have been formed. Either effect allows corrosion to proceed unhindered. This occurs frequently in smaU-diameter tubes or pipes through which corrosive liquids may be circulated at high velocities (e.g., condenser and evaporator tubes), in the vicinity of oends in pipe hnes, and on propellers, agitators, and cen-trifiig pumps. Similar effects are associated with cavitation and impingement corrosion. 

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 ). 

Localized deterioration Corrosion (especially pitting and intergranular attack), erosion, cavitation, mechanical wear, and so on (see Case History 9.8). 

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. 

Cavitation typically produces sharp, jagged, spongelike metal loss, even in ductile materials. The affected region is free of deposits and accumulated corrosion products if cavitation has been recent. 

If the cavitation intensity is low and corrosion is a significant accelerating factor, appropriate inhibitors can be useful. Notable successes have been achieved with diesel engine cylinder liners. 

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. 

Changing the pump metallurgy to a more corrosion- and cavitation-resistant material, such as stainless steel, is a potential solution to this type of problem. Note, however, that all other cast iron pump components that have sustained graphitic corrosion should be replaced to avoid the possibility of galvanic corrosion (see Chap. 16) between retained graphitically corroded cast iron components and new components. 

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. 

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