Corrosion types cavitation

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. 

Note that localized corrosion having the appearance illustrated in Figs. 12.18 through 12.20 could be associated with brief exposure to a strong acid. In this case, however, all available information indicated that the tubes had never been exposed to an acid of any type. Cavitation was caused by high-frequency vibration of the tubes. The vibration apparently induced a threshold cavitation intensity such that rough or irregular surfaces produced cavitation bubbles, and smooth internal surfaces did not. 

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. 

FYom the multitude of intricate corrosion processes in the presence of mechanical action (friction, erosion, vibration, cavitation, fretting and so on) it is justified to touch upon corrosion types joined under a single failure mode induced by mechanical stresses. These are the stresses that govern the corrosion wear rate of metals during friction. Such processes are usually called corrosion stress-induced cracking in the case that the mechanical action is effective only in one definite direction, or otherwise termed corrosion fatigue in the case that compressive and tensile stresses alternate within cycles. In spite of the differences between the appearance of these corrosion types, they have much in common, e.g. fundamental mechanisms, the causes, and they overlap to a certain degree [19]. 

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. 

Defining the type of fluid. This includes the physical properties (density and viscosity), corrosive properties, and the nominal pressures, temperatures and flow rate. In the case of fluids, it is necessary to know the vapor pressures to check for flashing and cavitation. Also a high temperature increase can severely damage some types of gaskets and packings. 

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. 

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. 

Leaving cavitation corrosion and fretting aside as separate corrosion forms (see Sections 7.10 and 7.11), erosion and abrasion corrosion can be divided into three types, a), b) and c), as described below. The first two types are erosion corrosion, while type c) is to be considered as abrasion corrosion. The three types may overlap each other and partly occur simultaneously in the same system. 

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. 

Cavitation-erosion is the loss of material caused by exposure to cavitation, which is the formation and collapse of vapor bubbles at a dynamic metal-liquid interface—for example, in rotors of pumps or on traihng faces of propellers. This type of corrosion causes a sequence of pits, sometimes appearing as a honeycomb of small relatively deep fissures (see Uhlig s Corrosion Handbook, 2nd edition, R. W. Revie, editor, Wiley, New York, 2000, Fig. 12, p. 261). 

The standard steels of the type SAE 316 (DIN-Mat. No. 1.4401, X5CrNiMol7-12-2) are not suitable for seawater-exposed pipes and fail as a result of pitting and crevice corrosion [155, 156]. The sensitivity to pitting corrosion of these standard steels can be further increased by deposits of maritime bacterial films [157]. Despite these facts, these steels are frequently used as materials for pump parts and have worked well as such because they are cathodically protected by contact with other parts made of less noble materials, e.g. pump casing made of cast iron [130]. [158] reports on tests of the cavitation behaviour of the pump materials GX5CrNiMol9-ll-2 (DIN-Mat. No. 1.4408) in 3% NaQ solution. 

All types of metal can be affected by erosion and cavitation corrosion. 

Loop G-Run 1, had a hot-leg test section consisting of specimens of 2 % Cr-1% Mo, 1 % Cr-1/2% Mo, AISI type-410, and Bessemer steels, joined with welds made with 2 % Cr-1% Mo, % Cr-1/2% Mo, 5% Cr-l/2Co Mo, AISI typc-410, and mild steel bare filler rods. This run was terminated after 938 hr of operation at a 75°C film gradient and a flow of 4 fps in the test section. Examination of the test sc( tion showed that welds made with a 5% Cr-l/2 /o Mo rod were severely attacked, while those made with a 2 j-% Cr-1% Mo rod and a %% Cr-1/2% Mo rod were not attacked. The only other corrosion observed in this run was some. slight attack on welds and base material of AISI type-410 steel. Cavitation pits were also obser cd on the pump impeller. 

The eff ect of velocity on corrosion and mass transfer is not apparent at this time. This is mainly due to a lack of tests in which velocity is the only ariable. Correlation between the pump loops and thermal convection loo))s suggests that velocity has little effect on mass transfer other than on the type of plug formed, provided conditions for cavitation do not exist. However, results from tests now under way should definitely evaluate this variable. 

The housing elements, rotor, and stator are of spheroidal graphite iron or of material that is resistant to cavitation and corrosion. The shaft is of high-quality, heat-treatable steel and the shaft extensions are normally finished to DIN standards although other arrangements are possible. Oil is used as a lubricant for the bearings and radial-lip-type shaft seals or labyrinth seals are normally fitted. Mechanical end-face seals can be provided. 

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