Thickness scaling

A good summary of the behavior of steels in high temperature steam is available (45). Calculated scale thickness for 10 years of exposure of ferritic steels in 593°C and 13.8 MPa (2000 psi) superheated steam is about 0.64 mm for 5 Cr—0.5 Mo steels, and 1 mm for 2.25 Cr—1 Mo steels. Steam pressure does not seem to have much influence. The steels form duplex layer scales of a uniform thickness. Scales on austenitic steels in the same test also form two layers but were irregular. Generally, the higher the alloy content, the thinner the oxide scale. Excessively thick oxide scale can exfoHate and be prone to under-the-scale concentration of corrodents and corrosion. ExfoHated scale can cause soHd particle erosion of the downstream equipment and clogging. Thick scale on boiler tubes impairs heat transfer and causes an increase in metal temperature. 

Manometric and volumetric methods (kinetics) Thermogravimetry (kinetics from very thin films to thick scales stoichiometry) Electrical conductivity of oxides and allied methods (defect structures conduction mechanisms transport numbers) Radioactive tracers and allied methods (kinetics self diffusion markers)... 

Discoid lupus erythematosus discoid patches with adherent thick scales and follicular plugging, atrophic scarring... 

Figure 1.1 Scanning electron photomicrograph of a cross section of a national newspaper comprising 90% spruce and 10% pine thermomechanical pulp (TMP) fibres (45gm 2 and 8fibres thick). Scale bar = 25 fiin.

At temperatures ranging from below room temperature to temperatures of up to 1000 °C, iron reacts chemically with the oxygen of the air to form a surface film of oxide. The films formed at room temperature (at relative humidities below the critical value) are only a few A thick and hence are invisible, but at higher temperatures, thick scales are produced. This type of corrosion involves an oxidation/reduction re-... 

Efficacy Improvement of clinical signs/symptoms of psoriasis (e.g., itching, redness, scaling, psoriatic body surface area coverage) PASI scores are based on plaque thickness, scaling, and redness, adjusted for percentage of affected body surface area quality of life assessments... 

Symptoms Retention hyperkeratosis on extremities better in summer Brown scales all over the body associated features maternal pregnancy abn. Collodion baby generalized scaling large, thick scales ektropion hypohidrosis Intense blistering at birth later verrucous hyperkeratosis, esp. in body folds keratoderma ... 

All work reviewed so far in this subsection concerns thin films with neutral surfaces, but we feel that the general scaling description Eqs. (129)-(133), Fig. 23) should also apply to thin films with symmetric surfaces that both favor the same component (say B, cf. Fig. 5) relative to the other. The additional feature, not present in Fig. 22, then is a shift of the critical volume fraction critCD) with thickness. Scaling considerations [216,224,225] predict for this shift... 

Structuring at the larger than film-thickness scale can include surface grooves and channels and possible incorporation of surface meshes. These act to influence the velocity distribution across the disk allowing the liquid in some cases to detach and reimpact on the surface many times as it crosses the disk. This can be used to introduce a greater variation in the transport rates for the liquid flowing over the disk compared to the more steady conditions on the smooth disk surface. 

For this reason, sahcylic acid is especially useful at anatomic sites where thick scales are present. ... 

Figure 8 Thickness scaling for samples without a post...

Hard water can cause problems. When hard water is heated, calcium carbonate is formed from the reaction of calcium ions and hydrogen carbonate ions in the water. Because calcium carbonate is not soluble in water, it forms thick scales inside water heaters and water pipes. These scales often clog pipes and keep the heater from properly heating the water. 

Figure 9. The film thickness scaled by the effective radius versus the effective capillary number.

Figure 6.5 Effects of growth pressure and V/III ratio of initial GaN seed layers on the surface morphologies, (a) 200 Torr, V/III ratio 2000, (b) 300 Torr, V/III ratio 2000, and (c) 200 Torr, V/III ratio 4000. The 0.5 pm thick seed layers were grown at 1020 °C. The thick scale bar is 10 pm. Reproduced from Moon YT etal., Journal of Crystal Growth, 291(1) 301-308. Copyright (2006), with permission from Elsevier...

Scaling problems tend to occur with moderately hard and very hard waters. Calcium is associated with bicarbonate ions in solution and upon heating these are converted to calcium carbonate, which forms a thick scale on the surface of domestic and industrial boilers, water heating appliance, kettles, pipes, etc. (reaction (19))... 

For fixed sheath voltage and ion current, the sheath thickness scales as 5 oc, i.e., a weak dependence on pressure. 

Typical micrographs after corrosion testing in SOz/air at 650 °C are shown in Fig. 9. The reference alloy 800H shows a thick scale, which had partially spalled oft It can be seen that corrosion attack of the metal occurs mainly via gain boundary attack. In Fe-A110-Cr2, on the other hand, literally no corrosion attack could be detected. As after the carburisation tests, the scale was found to be very thin and it could not be investigated by microprobe analysis. 

Figure 17.3.11 AES depth profiles for GaAs anodized in H3PO4 solutions. Ordinate has been corrected for relative Auger intensities and differential sputtering rates. Abscissa is sputtering time. Thickness scales are approximate. Roman numerals indicate different compositional regions in the oxide layer. Bulk GaAs is at rightmost limit, (a) From electrochemical treatment only.

For mobile or stationary interfaces, the mass transfer boundary layer thickness scales as the one-half power of 3Ba, mix. which represents the diffusivity for mass transfer. One must replace the mass diffusivity by the thermal diffusivity,... 

Several interpretations of this type of behaviour have been provided. For a detailed description and summary, Kofstad and Hauffe" should be consulted. Since the phenomenon is principally one that occurs at low temperatures, a strictly quaUtative description of the various interpretations will be given here, since this phase of the oxidation reaction generally occurs dixring the heating period of an investigation into the formation of thick scales at high temperatures. 

The rapid rate of reaction of iron above 570 °C causes thick scales to develop quickly and, in spite of the relatively high plasticity of the FeO layer, scale-metal adhesion is lost and a porous inner layer of FeO is formed, next to the metal, by the mechanism described earlier. The stresses associated with rapidly growing scale undoubtedly induce physical defects in the outer scale and the penetration of gas molecules, especially those belonging to the CO-CO2 and H2-H2O redox systems, will play a role in scale formation. 

The buckling stress. Equation (5.40), increases as the square of the scale thickness such that, for thick scales, buckling may not be feasible. In this case, shear cracks can form in the oxide and, if Equation (5.39) is satisfied, lead to scale spallation by a wedging mechaiusm, which is shown schematically in Figure 5.35(b). 

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