Cavitation corrosion attack

Figure 25. Concentrated zone of loss, absence of corrosion products, and sharply defined boundaries identify cavitation-corrosion attack on cylinder liner...

Cavitation-Corrosion. This form of corrosion attack, which causes localized failures of cylinder liners in Diesel engines, has been responsible for serious maintenance problems in power units of trucks, buses, railway locomotives and marine transport. The concentrated area of attack, and severity of corrosion are clearly shown in Figure The areas of greatest... 

Corrosion within pump and piping systems is another problem, and general uniform attack is common. Pitting, crevice corrosion, intergranular corrosion, dealloying, galvanic corrosion, and cavitation corrosion are also possible depending on the environment. 

The various forms of corrosion can be classified by their various causes. These are uniform corrosion attack (UC), bimetallic corrosion (BC), crevice corrosion (CC), pitting corrosion (PC), grain boundary corrosion (GBC), layer corrosion (LC), stress corrosion cracking (SCC), cavitation corrosion (CC), and hydrogen embrittlement (HE). 

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). 

Acidic pH values will lead to general corrosion the other factors will generally favor localized attack. Cavitation-corrosion can be encountered in the pumps or at other locations where turbulent or high-velocity flow may occur. Stage heaters and economizers are designed to increase the feed-water temperature, which will increase the operating efficiency of the entire system, and, as the temperature... 

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 —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. 

At very high water speeds cavitation-damage (Section 8.8) is sustained by any meti high-speed bronze propellers, for instance, may suffer seriously. This form of attack is mainly mechanical, although an element of true corrosion may be present, and is not specifically associated with sea water. 

Locomotive diesels As larger volumes of coolant are required in railway locomotives than in road vehicles, the cost of inhibition is proportionally greater. An additional factor is the possibility of cavitation attack of cylinder liners. These considerations place a restriction on the choice of inhibitors. In the past, chromates have been used at concentrations of up to 0-4%, but their use presents handling and disposal problems. Chromates cannot be used with ethanediol antifreeze solutions. A IS I borate-metasilicate at a concentration of 1 % has been used in the UK. Nitrate is added to this to improve inhibition of aluminium alloy corrosion. Tannins and soluble oils are also used, but probably to a lesser extent than in the past. The benzoate-nitrite formulation (formerly BS 3151) is effective and has been used by continental railways . ... 

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. °. 

Titanium. Unlike other metals, titanium normally does not pit, is not susceptible to stress corrosion, is free from local corrosion under fouling organisms, is free from impingement and cavitation attack at velocities which attack copper-base alloys, and is not susceptible to sulfide attack in contaminated sea water. Experiments with water velocities at 20 to 50 feet per second show no attack on titanium. 

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. 

Airborne or splash zone attack is not normally a problem at freshwater facilities however, air pollution can cause potential problems. Under certain flow conditions, such as turbulent flow or cavitation, fresh water can cause severe corrosion to submerged metallic elements. Ice damage also can limit the effectiveness of coatings on bulkhead walls and support piling. 

As with other types of erosion, the superposition of a corrosion process also has to be taken into account where damage by cavitation occurs. Removal of material by corrosion after destruction of protective covering layers often represents the more intensive attack, and the corrosion resistance of the material is then the dominant property. 

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