Erosion mechanism

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. 

The behavior of materials, particularly steel, in cavitating fluids results in an erosion mechanism, including mechanical erosion and electrochemical corrosion. The straightforward way to fight cavitation is to use hardened materials, chromium, chrome-nickel compounds, or elastomeric plastics. Other cures are to reduce the vapor pressure with additives, reduce the turbulence, change the liquid s temperature, or add air to act as a cushion for the collapsing bubbles. 

Convincing evidence for a surface erosion process is shown in Fig. 8, which shows the concomitant release of the incorporated marker, methylene blue, release of the anhydride excipient hydrolysis product, succinic acid, and total weight loss of the device. According to these data, the release of an incorporated drug from an anhydride-catalyzed erosion of poly (ortho esters) can be unambiguously described by a polymer surface erosion mechanism. 

The hydrolysis and erosion mechanism is continually repeated until no polymer is left. Seawater-soluble pigment particles, such as CU2O and ZnO, dissolve near the surface of the paint film. As an example, the reactions of CU2O to form CuCU" and CuCf2" are provided ... 

Bacterial attack is an early stage in the degradation of wood exposed in wet or moist conditions. Bacteria can be the dominant form of attack when fungal decay is suppressed by a wood-preserving treatment. Bacteria can attack the cell wall of wood by tunnelling, cavitation or erosion mechanisms (Eaton and Hale, 1993). 

Such a disintegration can only continue when there is an ample supply of water and when kaolinite, quartz and the potassium hydroxide solution are drained off. In such cases when the potassium hydroxide is not drained, the erosion mechanism follows a different path and the clay mineral illite is formed instead of kaolinite ... 

Erosion refers to the dissolution and/or degradation of the polymer to soluble fragments and the progressive weight loss of the matrix. A thorough understanding of the erosion mechanism of particulate carriers is... 

Water-soluble drugs are released primarily by diffusion of dissolved drug molecules across the gel layer, while poorly water soluble drugs are released predominantly by erosion mechanisms. 

A major challenge to experiments is the understanding of erosion mechanisms in the complex environment of a fusion device. Predictions about erosion in ITER by numerical modeling are based on related experimental findings in tokamaks, on laboratory measurements (e.g., ion beams or linear plasma devices) and the theoretical understanding of underlying mechanisms. Most experimental data on erosion yields in tokamaks have been obtained from... 

Therefore improvements of the atomic data base is of importance and might lead to the necessity of re-evaluation of the measurements of erosion yields obtained under tokamak conditions and the corresponding modification of the numerical modeling of erosion in tokamaks [26,27]. Impurity fluxes from wall elements entering the plasma can be generated by a combination of different erosion mechanisms, the main features and open problems of which are discussed in the following. 

In ion beam experiments, carbon materials show an unexpected additional erosion mechanism which dominates the carbon erosion in the temperature range 1200 K 2200 K, called radiation enhanced sublimation (RES) [35,36]. There are doubts whether this effect also exists with very high particle flux densities as they are typical for tokamaks. Test limiter experiments in TEXTOR have demonstrated that RES is not important under such conditions. However, other devices reported a carbon influx by RES and recent measurements indicate also an enhanced erosion of metals at high temperatures under low energy particle impact [34]. Further R D is needed to clarify these issues. 

The calculation of Rero, and therefore also of reg, contains quite some uncertainties. Knowledge about the local plasma parameters, erosion yields and sticking coefficients is required [39]. A possible mix of different erosion mechanisms and a surface layer composition with different materials adds some complexity to the problem. However, the extreme case of erosion mentioned above with a layer of 4.5 m eroded per year is unrealistic, since it is only valid for Rep = 0 or Rero = oo. Instead, experiments indicate that Rero does not deviate very much from unity. Indeed, with values typical for carbon, namely S = 0.75, Yr = 0.015, Yj = 0.02-0.5, c = 0.01-0.03, we obtain Rero in the range of Rero = 0.7-2.7. 

For metals, such as W, an additional erosion mechanism exists due to the formation of a melt layer on the PFC surface, once the surface temperature exceeds the melting temperature of the metal. This layer can reach a width of several tens of microns for W under energy fluxes of 1 M J/m2 sustained during several hundreds of microseconds. The stability of this layer in realistic tokamak geometries and in contact with the plasma is difficult to describe by... 

Erosion due to energetic particle bombardment depends on a number of parameters such as mass ratio of incident particles to surface atoms, particle energy and flux, as well as surface temperature. In the following, the physical understanding of different erosion mechanisms occurring at different wall components in fusion devices will be presented together with supporting data from laboratory experiments. 

Physical phenomena bring about weathering as a result of temperature gradients, abrasion, erosion, mechanical forces, and other similar conditions. 

Estimates of the power of natural sources of SDA injected in to the atmosphere through wind erosion mechanism vary within (1-5) 10 t/year [2, 15, 40]. However, as is noted by Ivlev [4], these figures are probably very much underestimated, and should be considered as the amount of the SDA contribution to the background global concentration of the atmospheric aerosol. 

Degradation of poly(FAD-SA) has also been reported to be nearly zero-order based on the rate of SA release from the polymer. ° However, evidence suggests that devices prepared from these polymers do not necessarily degrade by a purely surface-erosion mechanism. Cumulative release profiles of individual... 

To understand the properties that make polyanhy-drides suitable drug carriers, their chemical, physical, and thermal behavior need to be characterized. This section discusses the methods to determine the chemical structure and composition, the molecular weight, the thermal properties, the phase behavior, the stability, and the erosion mechanism of polyanhydrides. 

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