Solid particle erosion rate

Figure 11. Solid particle erosion rate of several plastics compared to metals (100 m/s, 12S-1S0 pm quarts abrasive, 90 degree impact angle).

In order to elucidate the effect of the whisker orientation on the erosion behavior of material SN-C, erosion tests were carried out in directions both parallel and perpendicular to the whisker orientation. It is apparent that in the highly directional whisker-reinforced silicon nitride material, solid particle erosion in the direction parallel to the whisker orientation resulted in a faster rate of material removal compared to that in the perpendicular direction (Fig. 20.4). 

The effect of ultra-high molecular weight poly-(ethylene) (UHMWPE) the on mechanical and solid particle erosive wear behavior of aramid fabric reinforced-epoxy composites has been investigated [64]. A siUca sand of a size of 150-280 fim was used as an erodent. The erosive wear rate of UHMWPE in aramid-epoxy composite exhibits a lower value in comparison to neat composites. A maximum erosion rate was observed at an impingement angle 60 , and the material behaves in a semiductile manner. 

Erosion is one of several wear modes involved in tribocorrosion. Solid particle erosion is a process by which discrete small solid particles, with inertia, strike the surface of a material, causing damage or material loss to its surface. This is often accompanied by corrosion due to the environment. A major environmental factor with significant influence on erosion-corrosion rates is that of flow velocity, but this should be set in the context of the overall flow field as other parameters such as wall shear stress, wall surface roughness, turbulent flow intensity and mass transport coefficient (this determines the rate of movement of reactant species to reaction sites and thus can relate to corrosion wall wastage rates). For example, a single value of flow velocity, referred to as the critical velocity, is often quoted to represent a transition from flow-induced corrosion to enhanced mechanical-corrosion interactive erosion-corrosion processes. It is also used to indicate the resistance of the passive and protective films to mechanical breakdown [5]. 

Under other circumstances Quid Qow can cause erosion of the surface through the mechanical force of the Quid itself. When solids are present in the liquid they can cause wear or solid particle erosion. In either case, the rate of attack can be accelerated by the combined effects of erosion and corrosion. Erosion corrosion results when the passive Blms that form on alloys are removed and the underlying metal is attacked [25]. The rate can be very rapid. 

Plastics are frequently used for applications requiring erosion resistance, but there does not seem to be much activity or interest in the tribology community of the 1990s. However, there are a number of tests that are applied and have been used to rate erosion resistance of plastics. Erosion, by definition, is progressive loss of material fiom a solid surface due to mechanical interaction between that surfitce and a fluid, a multicomponent fluid, or impinging liquid or solid particles (3). The field of erosion is usually separated into a number of forms of erosion liquid erosion, either continuous stream or droplet, solid particle erosion, slurry erosion, and cavitation erosion. Each have separate laboratory tests. 

Erosion corrosion occurs in an environment where there is flow of the corrosive medium over the apparatus surface. This type of corrosion is greatly accelerated when the flowing medium contains solid particles. The corrosion rate increases with velocity. Erosion corrosion generally manifests as a localized problem due to maldistributions of flow in the apparatus. Corroded regions are often clean, due to the abrasive action of moving particulates, and occur in patterns or waves in the direction of flow. 

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. 

Steady state erosion rates of all target materials eroded by SiC particles at impingement angles of 30° and 90°. Solid bars labeled I and II indicate the erosion rates of material SN-C corresponding to 30° erosion in the direction perpendicular and parallel, respectively, to the whisker orientation. 

As mentioned, the erosion of a solid surface depends on the collisional force, angle of incidence, and material properties of both surface and particles. Although abrasive erosion rates cannot be precisely predicted at this stage, some quantitative account of erosion modes which relates various impact parameters and properties is useful. In the following, a simple model for the ductile and brittle modes of erosion by dust or granular materials suspended in a gas medium moving at a moderate speed is discussed in light of the Hertzian contact theory [Soo, 1977]. 

Elevated speeds have a marked effect on wear, and this is more pronounced if the solution contains some solid particles in suspension. Aluminum forms films of aluminum nitrate or oxide in fuming nitric acid. At low flow rates there is no attack whereas for speeds greater than 1.22 m s the protective layer is removed and erosion-corrosion occurs more readily.16... 

It is possible to determine the four contributions to the total material loss rate by the following experimental principles the total material loss rate Wt is determined by weighing the specimen before and after exposure under combined erosive and corrosive conditions. The sum of Wc and Wce (the corrosion components) can be measured by electrochemical methods during the same exposure (the methods described in Section 9.2 can also be used under erosive conditions). We is determined by weighing the specimen before and after exposure in special tests where corrosion is eliminated by cathodic protection (or possibly by oflier means) but otherwise under the same conditions as in the former experiments. Wc can be measured electrochemically in tests like the original ones but with all solid particles excluded. Finally, the synergy components, Wce and Wec, can be derived from Equation (7.9) and the mentioned experiments. 

Table 7.5 does not give any clear information about critical velocities, but it indicates that such thresholds exist for the copper alloys in the velocity range represented in the table (1.2-8.2 m/s). More specifically, both Figure 7.46 and Table 7.6 show examples of critical velocities for erosion corrosion. The values are not absolute they depend on the composition of the environment, the temperature, geometrical conditions, the exposure history, the exact composition and treatment of the material etc. In connection with Figure 7.46 it can be mentioned that austenitic stainless steels show excellent resistance to erosion corrosion in pure liquid flow at high velocities, while some ferritic [7.42] and ferritic-austenitic steels are attacked less than the austenitic ones if the liquid carries solid particles. The data in Table 7.6 originate from work by Efird [7.43], who interpret his results as follows for each alloy in a certain environment, there exists a critical shear stress between the liquid and the material surface. When this shear stress is exceeded, surface films are removed and the corrosion rate increases markedly. 

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