The rapid wastage, leading to ultimate failure, of ship propellers, pump impellers, and other components, as a result of the effects of cavitation is fairly widely experienced and recognized. What may not be so well known is that a specific mode of attack occasionally seen on the water-side surface of diesel engine cylinder liners may also result from the same basic cause.
Cavitation damage has been defined as the dynamic consequence of the hydro-mechanical phenomenon of cavitation, 1 which occurs when a liquid in motion is subject to forces at right-angles to the direction of flow, so that the cohesive strength of the liquid is exceeded leading to the formation of bubbles in those region which fall below the absolute vapour pressure. The bubbles are of only momentary duration and collapse at rapid rates of acceleration to produce shock waves of very high intensity, and the repeated collapse of the bubbles against a metallic surface gives rise to pronounced local damage. Although in many instances this action results in the ultimate production of cavities, the word “cavitation” describes the events which occur within the liquid and not the effect on the metal, i.e., the liquid cavitates with the production of cavitation damage.
Cavitation is generally induced by a rapid increase in the velocity of the liquid, as may arise from the sudden divergence of flow initiated by the presence of objects, or from vibration of adjacent metal surfaces. Cavitation from the latter cause can, therefore, occur when the liquid itself is in a relatively static condition.
A symposium on the subject was held in November, 1965, by British Rail Chemical Research Division, and the subsequent report 2has been largely drawn upon for this article which is a general survey of the subject illustrated by examples from our experience.
Damage to cylinder liners thought to be due to this cause takes the form of deep pitting of a honeycomb nature. It is restricted to certain specific locations on the waterside surface and is found principally on the thrust side of the cylinder liner, i.e., that subjected to the thrust from the piston on the firing stroke, and occasionally on the diametrically opposite, or anti-thrust, side — a typical example is shown inFigure 1 this being associated with ultimate perforation of the liner; a portion of another liner is shown in Figure 2. The pitting may extend for the full length of a liner or be restricted to the lower regions, as seen in the examples, and may in certain circumstances, as shown in Figure 2, result in disruption. In most cases the liner as a whole is relatively free from corrosion and rust deposits. The rate of development of the pits may reach 0.10 in. in 1,000 hours, whereas ordinary corrosion is only of the order of 0.002 in. during the same time. The extent of the pitting varies from engine to engine and may also differ on the liners of any one engine. Cavitation damage is more prevalent in engines in transport service, where speed variation is more pronounced, and the onset in some engines may be associated with the operation of reboring where a significant reduction in the wall thickness has resulted.
Fig. 1 Typical examples of cavitation damage in service.
Fig. 2 Typical examples of cavitation damage in service.
The cavitation damage of liners is considered to be due to vibration of the cylinder wall, initiated by slap of the piston under the combined forces of inertia and firing pressure as it passes top dead centre. The occurrence on the anti-thrust side may possibly result from bouncing of the piston. In the laboratory it has been possible to reproduce the effects of cavitation by vibrations induced by ultrasonic or other transducers.
Although cavitation damage appears to result basically from a mechanical cause, the exact mechanism is not entirely clear and two schools of thought have developed, one supporting an essentially erosive, and the other an essentially corrosive, mechanism and thus the terms “cavitation erosion” and “Cavitation corrosion” have arisen to describe the effect. With the former, it is considered that shock impact on the metal surface produces local stresses in excess of the fatigue limit of the material. Cracks develop and, ultimately, small particles of metal become detached. The fact that cavitation damage can be produced experimentally in materials such as plastics or glass tends to support the view that it is an entirely mechanical phenomenon, as does the evidence that it can also be produced in non-electrolytic environments. On the other hand, there are aspects that lend support to the corrosion hypothesis. It is considered that the impact forces may only serve to remove the protective oxide film normally present on the surface, and that the hare regions thus produced become anodic to the remainder and local corrosion cells are set up, the anodic regions suffering a preferential attack. In addition, the existence of regions of differing stress levels, or zones of cold work resulting from the impacts can also result in small anodic areas. Further, variations in the flow rate over the surface result in different rates of oxygen diffusion and could also promote local corrosion.
Microscopical examination of sections cut from pitted regions of liners affected by cavitation damage show that the relatively mechanically weak phosphide eutectic is often left proud, in the pits. The examination of several pits which developed in the liner shown in Figure 1 showed this feature, as is evident from Figure 3. If erosion was the major factor it is considered that the eutectic should be preferentially removed. This fact, together with the observation that the corrosion products remain in the cavities, suggests that cavitation damage may be due to a mechanism which is primarily of a corrosive nature. It would be reasonable to assume that cavitation results from a combination of these processes, thereby accounting for the fact that different effects are found in different circumstances.
Fig. 3 Section through pitted region showing eutectic standing proud. (×400).
R. W. Wall, in one of the papers in the symposium previously mentioned, refers to laboratory experiments, the results of which indicated that corrosion at metal surfaces is accelerated by vibration and, in the case of iron in water, corrosion occurs in the form of isolated pits rather than in a uniform manner, suggesting that pitting could occur as a result of cylinder vibration only and may not necessarily be a consequence of cavitation in the water. Once pits develop, one would expect them to develop at a fairly rapid rate, due to the combination of a small anode and a large cathode. It is also thought that vibration increases the access of oxygen to the corroded surface, this being utilized in the cathode reaction to reduce the polarising action of hydrogen.
Damage to other portions of liners, that which is seen in the piston ring grooves, under flanges, or on the portion adjacent to the lower scaling ring (as seen on the liner shown in Figure 1) is considered to result, not from cavitation, but from differential aeration-a more common cause of corrosion, particularly where stagnant conditions may obtain.
Measures to prevent, or reduce, cavitation damage. should be considered firstly from the aspect of design, attention being given to methods of reducing the amplitude of the liner vibration. Piston slap can be reduced by minimizing the clearance, but attempts to reduce this too far could result in scuffing and increased wear. Other workers have suggested the use of a cam-ground piston with this end in view. Much greater success in reducing the severity of the piston impact has attended work done by the Admiralty Research Laboratories, using a specially designed piston incorporating an annular belt of oil between upper and lower rings, the oil acting as a viscous damper to cushion the motion of the piston and eliminating the tilting at the ends of the stroke.
In certain circumstances it may be possible to alter the design of the liner. In severe cases, longitudinal ribs can be employed or an additional support provided at about the mid-length, in an attempt to avoid resonant conditions. Measures of a palliative nature can be introduced by surface treatment of the liners, and in this respect the use of hard chromium plating or chromium plating on nickel has been found to be effective in certain conditions. Sprayed metals or the use of a ceramic coating based on alumina, applied by flame-spraying, have also been recommended. Other workers favour a resilient coating, such as nylon, but with materials of this type it may be difficult to effect a satisfactory bond and the attendant effect on heat conductivity must also be considered.
Attempts have been made to reduce the severity of attack by attention to the environment. Inhibitors, such as chromates, benzoate/nitrite mixtures, and emulsified oils, have been tried with varying success. Unfortunately, some inhibitors may not be suitable for use at temperatures approaching 100°C. and others may break down under conditions of cavitation; organic compounds, for instance, may be degraded and lead to precipitation.
Cavitation damage tends to fall off as the temperature is raised and, in one case, damage which was shown to be at a maximum at 50°C diminished significantly at 80°C. Raising the temperature of the cooling water is stated to be effective where anti-freeze additions based on ethylene glycol are employed.
Attempts have been made to reduce or prevent cavitation damage by the application of cathodic protection, and this has been found to be effective in certain instances of trouble on propellers. Its application to cylinder liners, however, is particularly difficult, and it is understood that no large measure of success has been obtained. Its effectiveness in any situation must be related to the particular parts played by the electrochemical and the mechanical factors in the case. The fact that cathodic protection has been found to be effective in counter-acting trouble with ships propellers, supports the view that cavitation damage results primarily from an electro-chemical, rather than a simple mechanical cause.
Little success has attended efforts to obtain relief by changes in the type of material. Iron with a higher elastic modulus should assist in reducing damage by modifying the vibration characteristics of the liner. More corrosion-resistant irons alloyed with chromium and nickel would possibly show an advantage, but unfortunately they possess inherently poor wear resistance.
Among other measures that have been advocated is pressurization of the cooling system, and this has effected a cure in certain cases experienced by the French Railways, possibly by preventing the formation of bubbles. It has also been suggested that it would be beneficial to remove the electrochemical factor entirely and to employ fuel oil as a coolant, but obviously a modification of this nature could only be adopted in certain instances.
In one case within our experience, corrosion in a six-cylinder diesel engine driving a small locomotive showed itself on cylinders Nos. 2, 5 and 6 after three years' service. After a further year, the No. 5 liner had to be replaced. The replacement gave only one further year's service, and the new one which was installed showed slight pitting after three months or so. The affected portion of the liners was in all cases adjacent to an internal rib on the engine frame as shown in Figure 4 and it is presumed that the trouble was associated with local turbulence, resulting from restriction in the water spaces. A slight attack was also shown on the frame itself in this vicinity. In this case, a 1% emulsified oil was added to the cooling water, which was also made alkaline, and no recurrence of the cavitation was seen during the subsequent six years.
Fig. 4
In diesel engines trouble from cavitation damage is also found in coolant circulating pumps, main and big end bearings, and in the fuel injection equipment. With bearings, damage is thought to result from flow-induced cavitation, while in pumps and injection equipment it is associated with violent fluctuations that may occur when the flow is suddenly interrupted, It is found mainly in unloaded areas of bearings where the oil film pressure is low and, therefore, results from a cause different from that responsible for the more usual type of failure shown by these components. Its occurrence can be minimised by design changes, so that the oil pressure is maintained, or possibly by a change in material, a tin-base babbitt being more resistant than a lead-base alloy of the same hardness.
In conclusion, it would appear that the precise cause of the type of damage described is not yet known with certainty. Although-it is often ascribed to cavitation within the cooling water the evidence is largely of a circumstantial character based on the similarity between it and the damage shown by propellers and pump impellers. It is fairly certain that it stems basically from vibration of the liner, but whether or not mechanical forces are the primary or only factor is still a matter of doubt. With regard to alleviation of the trouble, it would appear that if the severity of either the mechanical or electrochemical factors is diminished, then the cavitation damage as such may be reduced to more tolerable proportions.
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