Erosion corrosion cavitation

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... 

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). 

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. 

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. 

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... 

Noise generation and erosion/corrosion considerations limit the maximum water velocity in pipework systems. Noise is caused by the free air present in the water, sudden pressure drops (which, in turn, cause cavitation or the flashing of water into steam), turbulence or a combination of these. 

Erosion —corrosion, fretting corrosion, impingement attack, cavitation damage stress corrosion cracking, hydrogen cracking, corrosion fatigue... 

Erosion-corrosion in the widest sense of the term will include impingement attack, cavitation damage and fretting corrosion, but since the latter two are dealt with in separate sections (see Sections 8.7 and 8.8) they will not be considered here. 

Applications Plasma spraying is used to apply coatings for protection against wear and corrosion, to prevent erosion or cavitations, and to provide electrical insulation or conductivity. It can also be employed to produce bearing surfaces, abrasive properties or resistance to wetting by molten metals. The coatings can also be applied to facilitate the joining of different 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. °. 

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

FIG. 1—Coolant pump damaged by cavitation-erosion-corrosion. Pitting can be controlled by a properly inhibited antifreeze coolant that is used as directed by the manufacturer. 

When a system consists of a flowing component which impinges on a part or some parts of the structure we encounter erosion-corrosion, or abrasive wear and cavitation. The design must be such that localized constrictions are avoided or sharp directional... 

Toughness can influence the performance of materials under conditions of erosion-corrosion. The soft metals are often more susceptible to erosion corrosion because they are more susceptible to mechanical wear. The toughness is a good criterion for the resistance to the mechanical erosion or abrasion, but this is not necessarily a good criterion to predict the resistance to the erosion-corrosion. Stellite (Co-Cr-W-Fe-C alloy), which has better toughness than 18-8 stainless steel, showed better resistance to cavitation erosion on a water brake.25... 

There are several ways to harden alloys. A certain procedure to increase the resistance to the erosion corrosion is the hardening by solid solution. One adds an element to another to produce a solid solution that is resistant to the corrosion by hardening the metal. The thermal treatment is also a method to harden a metal or alloy, but it changes the microstructure and can induce a greater susceptibility to corrosion. Hardening by cold work is also an important procedure and it is the reason for using stainless steel to resist cavitation erosion. This material, initially hard, attains an even harder surface by cold work and becomes more resistant to attack and erosion. 

A second form of erosion corrosion is the case of cavitation. A type of corrosion familiar to pump impellers, this form of attack is caused by the formation and collapse of tiny vapor bubbles near a metallic surface in the presence of a corrodent. The protective surface film is again damaged, in this case by the high pressures caused by the collapse of the bubbles. 

Ampco-Trode [Ampco], TM for a series of aluminum-bronze arc-welding electrodes and filler rod containing 9.0-15.0% aluminum, 1.0-5.0% iron, balance copper, for joining like or dissimilar metals and overlaying surfaces resistant to wear, corrosion, erosion, and cavitation-pitting. 

In this chapter, we have not discussed materials corrosion under mechanical stresses, such as environmental cracking, stress corrosion cracking, erosion-corrosion, and cavitation-corrosion. 

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