Thickness scaling temperature

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. 

The distinction between a him and scale is not well defined, but it is usual to use the former when referring to a thin continuous layer of reaction product (visible or invisible) whilst the latter is normally used for thick high-temperature layer (always visible). 

Fig. 4a-e. Snapshots of configurations taken from the production stages of simulations of GB(3.0, 5.0, 1, 2) at scaled temperatures a 3.00 b 2.19 c 1.49 d 1.00 e 0.50. The molecules are represented hy lines which are shorter than the molecular length the thick lines show the director and their lengths are proportional to the orientational order parameter, P2, for the configuration... 

For the accumulation of heat, scale surface temperature increased with the increase of scale layer thickness. The thicker the layer was, the temperature was closer to gases temperature the thinner the layer was, the temperature was closer to wall surface temperature. Obviously, if the scale layer was thin enough, the scale temperature was low enough and... 

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-... 

Figure 8.5 Typical in-plane distributions of diffuse x-ray intensity around the PbTiOs 304 peak, for various film thicknesses and temperatures. Darker shade indicates higher intensity (log scale). Elongation of central Bragg peak is due to asymmetric resolution function.

Figure 46. Photocurrent transients for hole transport in TTA (40 wt.%)-doped polycarbonate containing as an added impurity l,l-bis(p-diethylaminophenyl)-4,4-diphenyl-l,3-butadiene (BD) at various concentrations. The current (vertical) scale is arbitrary. The samples were 16 pm thick, the temperature 298 K, and the field strength 3.75 x 10 V cm . (Reprinted with permission from Ref [74j].)...

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. 

If thick steel (which stays in the furnace for a long time) is heated in a hot furnace, the scale becomes mushy, if not liquid. Semimolten scale has caused many erroneous temperature measurements in steel heating furnaces. Scale is an insulator. Its conductance is lower in its solid form, but the high reflectivity of the molten form causes it to act as an insulator. If the scale is not shiny or glossy, optical pyrometers and radiation pyrometers measure scale temperature, but not steel temperature pyrometers indicate a temperature somewhere between furnace ceiling temperature and scale temperature, but not steel temperature. Shiny scale (semimolten) reflects radiation nearly eliminating heat transfer to the load. 

Gauge gaj [ME gauge, fr. ONE] (15c) (gage) n. (1) Any instrument that measures and indicates such quantities as thickness, pressure, temperature, or liquid level. (2) The thickness of a plastic sheet or film, usually given in mils or mm. (3) Any of the standard wire and sheet-metal scales in which the gauge numbers are inversely related to wire diameter or sheet thickness. (4) The number of wales per inch in a knit fabric. (5) On spinning or twisting frames, the distance from the center of... 

Logarithmic Reaction Kinetics Logarithmic reaction kinetics are most often associated with low-temperature service tests, the initial stages of oxidation of certain materials, and with thick scale behavior where internal scale cavities or precipitates interfere with diffusion mechanisms [3]. The rate equation describing logarithmic reaction kinetics (Fig. 3) is... 

With a CaS04 scale of 0.15 mm thick, the temperature gradient across the scale is approximately 200 K in order to maintain the rate of formatiOTi of steam. This raises the external temperature of the steel from 325°C in the absence of any scale to about 525°C. This exceeds the safe temperature for steel in an air atmosphere, and at the steam pressure generated, it will result in tube failure. 

Figure 5.11 Measured phase diagram of 5CB on an oxidized silicon wafer using X-ray reflectivity (adapted from Van Effenterre ]. Vertical scale, temperature °C horizontal scale, thickness nm. Triangles LB values full circles, UB values.

A low O2 condition is produced at a corrosion interface in the presence of protective scales, and complex corrosion reactions such as chlorination, sulfidation and oxidation occur below the corrosive deposit layer. Thick scales have pores and cracks due to temperature fluctuations and the vaporization of chlorides. As the thickness increases, the scales easily peel off from the surface. In particular, severe thermal cycles or increased gas velocities due to soot blowing accelerate the breakdown and spalling of the scale. Also, as a result of continuously repeated variations of gas conditions on the scales, the balance of chlorination, sulfidation and oxidation reactions at the corrosion interface and in the scales is forced to be changed by the penetration of O2. An increase of the partial pressure of O2 ( /qj ) temporarily halts the chlorination and sulfidation reactions. Therefore, a multi-layered scale stracture is produced. The presence of multi-layered oxides formed by corrosion resistant elements such as chromium, nickel, aluminum, silicon and molybdenum increases the protective effect of the scales against the... 

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