Erosion brittle

Figure 8.12 illustrates a solid particle impinging on a surface. It has been found that the erosive wear rate depends upon the impingement angle, a, the particle velocity, vq, and the size and density of the particle, as well as the properties of the surface material. It has also been found that there is a difference in erosive wear properties of brittle and ductile materials. The maximum erosive wear of ductile materials occurs at a = 20°, whereas the maximum erosive wear for brittle materials occurs near a = 90°. Since the impingement angle is probably lower than 90° for these type of flow situations, we might consider only brittle materials, such as ceramics for this application. Let us examine brittle erosive wear in a little more detail first. 

Brittle erosion is the loss of material from a solid surface due to fatigue cracking and brittle cracking caused by the normal collisional force Fn. Materials with very limited capacity for elastic and plastic deformation, such as ceramics and glass, respond to particle impacts by fracturing. The yield stress for brittle failure Fb for normal impacts is about... 

Similarly, for brittle erosions, the wear in volume loss per impact u>b is expressed by... 

Equation (6.16) indicates that the maximum brittle wear occurs at ct = 90°, i.e., normal collision. The variation of the ratio of Eb to Bm (= B( m, b)) with a as a function of Kb is plotted on the basis of Eq. (6.16) as given in Fig. 6.7. The figure reflects that for a given Kb, the degree of abrasive damage due to brittle erosion may be estimated from the given particle flow pattern. 

Figure 6.7. Brittle erosion energy function varied with impact angles and brittle resistance parameters (from Soo, 1977).

Erosion. The abrasive is likely to be gas borne (as in catalytic cracking units), liquid borne (as in abrasive slurries), or gravity pulled (as in catalyst transfer lines). Because of the association of velocity and kinetic energy, the severity of erosion may increase as some power (usually up to the 3d) of the velocity. The angle of impingement also influences severity. At supersonic speeds, even water droplets can be seriously erosive. There is some evidence that the response of resisting metals is influenced by whether they are ductile or brittle. Probably most abrasion involved with hydrocarbon processing is of the erosive type. 

Soft rubber is obtained by adding 2-4% sulfur by adding extra sulfur (25-40%), the rubber can be made into ebonite, which is a hard, brittle material, having a wider range of chemical resistance than soft rubber. Soft ordinary rubber is chemical and erosion resistant, but its thermal resistance is not high (about 80 C). 

This example of aluminium illustrates the importance of the protective him, and hlms that are hard, dense and adherent will provide better protection than those that are loosely adherent or that are brittle and therefore crack and spall when the metal is subjected to stress. The ability of the metal to reform a protective him is highly important and metals like titanium and tantalum that are readily passivated are more resistant to erosion-corrosion than copper, brass, lead and some of the stainless steels. There is some evidence that the hardness of a metal is a signihcant factor in resistance to erosion-corrosion, but since alloying to increase hardness will also affect the chemical properties of the alloy it is difficult to separate these two factors. Thus althou copper is highly susceptible to impingement attack its resistance increases with increase in zinc content, with a corresponding increase in hardness. However, the increase in resistance to attack is due to the formation of a more protective him rather than to an increase in hardness. 

McKinney, Antonucci Rupp (1987) found that the clinical wear of the glass polyalkenoate cement compared favourably with that of the composite resin, but they noted that it was prone to brittle fracture and chemical erosion. 

Surface Damage and Reaction Rates. Erosion of surfaces resulting in higher surface area and removal of inhibiting impurities are two effects of cavitation on solids in liquid media, both of which lead to increased reaction rates. The high temperatures and pressures are sufficient to deform and pit metal surfaces (even cause local melting of some metals) and to fracture many nonmetal lie solids, in particular, brittle materials. 

II.1 In Section 8.2, we saw how erosive wear can be different for brittle and ductile materials. In reality, most materials exhibit behavior that is a combination of... 

If one material has the needed strength, it may be susceptible to erosion if erosion resistant, it may be too brittle. Combinations in the form of liners and plating overcome these deficiencies to an appreciable extent... 

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