Scaling high-alloy steels


Calcium hydride is prepared on a commercial scale by heating calcium metal to about 300°C in a high alloy steel, covered cmcible under 101 kPa (1 atm) of hydrogen gas. Hydrogen is rapidly absorbed at this temperature and the reaction is exothermic.  [c.298]

Ga.s—Ga.s He t Exchangers. Gas-to-gas heat exchangers in double absorption plants are built of carbon steel or stainless steel. Typical tube diameters range from 37.5—100 mm, and the tubes mn vertically in a typical sheU-and-tube arrangement. To reduce corrosion and scaling, carbon steel tubes in high temperature service are Alonized, which is a proprietary aluminum alloy coating vapor diffused into the metal surface. Alonizing significantly increases the life of carbon steel with the added benefit of no loss of heat transfer from accumulation of corrosion scale.  [c.188]

Malik reported that, at temperatures between 600 and 850°C, in 101 kPa oxygen, the oxidation rate of Fe-5%A/-C steels (where M was Si, Ti, V, Nb, Ta, Cr, W or Ni) fell as the carbide stability increased. The oxidation of all of the alloys obeyed parabolic kinetics, although some breakaway occurred following an incubation period. This breakaway was attributed to scale disruption, as a result of COj evolution, with the carbon loss being most rapid during the first 5 min. Whilst the amount of carbon loss increased with the carbon content of the alloy, as did the oxidation rate constant, the total carbon loss was very much lower than that available. Those alloys forming a pure carbide phase were found to have a lower oxidation rate than those alloys comprising a solid solution phase or cementite. All of the binary Fe-5%A/alloys displayed a similar reaction rate, which was approximately one order of magnitude lower than that of pure Fe, due to the formation of mixed oxides or spinels in the scale. The Fe-5%A/-C alloys always showed two-layered scales, with an inner mixed oxide or spinel overlaid with Fe O,. The scales formed on the high-carbon alloys were generally more compact and adherent (following initial scale disruption by C loss) due to carbide dispersion improving scale integrity. Malik argued that the carbide-forming elements retard the diffusion of carbon in austenite, reducing the overall scaling rate. In non-carbide-forming alloys, such as Ni and Si, the oxidation rate was greater, due to a higher carbon mobility in the steel.  [c.975]

Magnesium anodes suspended inside a galvanised hot-water tank and in electrical connection with it afford cathodic protection to the zinc, the alloy layer and the steel, at high temperatures as well as in the cold. The magnesium is eventually consumed but it is probable that in the interim a good protective scale will have formed on the inside of the tank, so that the magnesium anode will then no longer be necessary. One of the difficulties of this method, however, is the maintenance of a sufficiently even current distribution over the inside of a tank to protect the whole surface, especially in waters of low conductivity. The method is therefore unlikely to be applicable to soft waters.  [c.60]

The bead and shot mills have overcome some of the limitations inherent with the design of sand mills. The bead and shot mills are similar in constmction and differ only in the fact that bead mills use ceramic or glass media, varying in size from 0.3 to 3 mm, whereas shot mills use similarly sized carbon steel and alloy steel shots in addition to ceramic media. Depending on the appHcation, a horizontal or vertical type of mill is used. The vertical mills are simpler and robust in design, exhibit higher throughput, and are better suited for a wide range of viscosity bases. The media mills in horizontal and vertical configuration have been extensively used in the paint and ink industry for dispersion of pigments. The horizontal mill offers the benefits of uniform distribution of media, ease of starting with large chamber volume, and ease of serviceabiUty. The chamber volume ranges from 0.25 L for a laboratory size mill to as large as 500 L for large-scale industrial installations. The agitator is configured in the form of a series of flat disks with holes or a series of pins. The tip speed varies between 12—20 m/s. The separation of media from the dispersion is accompHshed by cylindrical sieve cartridges or rotating gap separators. AppHcation of the gap separator is usually limited to a media size of 1.00 mm or greater. The slot width (gap) has to be less than half of the average bead diameter. The trend has been toward bead sizes of <0.5 mm to achieve the dispersion quaHty needed. The quaHty of dispersion and the specific energy input is influenced by the size, hardness, and density of the media, tip speed, media loading, and viscosity of the mill base (22). Advances in media mill technology include use of a milling chamber in the form of a narrow annular space allowing extremely high specific energy input (23,24) and novel designs of media separators (22). The quaHty and production costs of the dispersed products manufactured using the media mills have improved significantly in the 1990s, but capital costs of the sophisticated installation has also risen steadily. A significant amount of work has been done to develop a mathematical model of the process in the media mill as appHcable to the dispersion of pigments (25).  [c.513]

The passivating stainless steels presented a possibility for developing anodic protection. High-alloy steels, similar to carbon steels, are not capable of being ca-thodically protected in strong acids because hydrogen evolution prevents the necessary drop in potential. However, high-alloy steels can be passivated and maintained in the passive state by anodic protection. C. Edeleanu was the first to demonstrate in 1950 that anodic polarization of the pump housing and connecting pipework could protect a chromium-nickel steel pumping system against attack by concentrated sulfuric acid [38]. The unexpectedly wide range of anodic protection is due to the high polarization resistance of the passivated steel. Locke and Sudbury [39] investigated different metal/medium systems in which the application of anodic protection was relevant. Several anodically protected installations were in operation in the United States by 1960, e.g., storage tanks and reaction vessels for sulfonating and neutralization plants. Not only did the installations have a longer life but also a greater purity of the products was achieved. In 1961 anodic protection was first applied on a large scale to prevent stress corrosion cracking in a caustic soda electrolysis plant in Aswan [40]. Anodic protection for caustic soda tanks has been used on a large scale since the end of the 1960s and electrochemical corrosion protection methods have become of permanent importance for industrial plants (see Chapter 21).  [c.14]

Metal and Ceramic Membranes. Palladium and palladium alloy membranes can be used to separate hydrogen from other gases. Palladium membranes were studied extensively during the 1950s and 1960s, and a commercial plant to separate hydrogen from refinery off-gas was installed by Union Carbide (53). The plant used palladium—silver alloy membranes in the form of 25-p.m thick films. The plant was operated for some time, but a number of problems, including long-term membrane stability under the high temperature operating conditions, were encountered, and the plant was later replaced by pressure-swing adsorption systems. Small-scale palladium membrane systems, marketed byjohnson Matthey and Co., are still used to produce ultrapure hydrogen for specialized appHcations. These systems use palladium—silver alloy membranes, based on those originally developed (54). Membranes with much thinner effective palladium layers than were used in the Union Carbide installation can now be made. One technique is to form a composite membrane comprising a polymer substrate onto which is coated a thin layer of palladium or palladium alloy (55). The palladium layer can be appHed by vacuum methods, such as evaporation or sputtering. Coating thicknesses on the order of 100 nm or less can be achieved.  [c.69]

A faster method of separating components of a mixture is flash chromatography (see Still et al. J Org Chem 43 2923 1978). In flash chromatography the eluent flows through the column under a pressure of ca 1 to 4 atmospheres. The lower end of the chromatographic column has a relatively long taper closed with a tap. The upper end of the column is connected through a ball Joint to a tap. Alternatively a specially designed chromatographic column with a solvent reservoir can also be used (for an example, see the Aldrich Chemical Catalog-glassware section). The tapered portion is plugged with cotton, or quartz, wool and ca I cm of fine washed sand (the latter is optional). The adsorbent is then placed in the column as a dry powder or as a slurry in a solvent and allowed to fill to about one third of the column. A fine grade of adsorbent is required in order to slow the flow rate at the higher pressure, e.g. Silica 60, 230 to 400 mesh with particle size 0.040-0.063mm (from Merck). The top of the adsorbent is layered with ca 1 cm of fine washed sand. The mixture in the smallest volume of solvent is applied at the top of the column and allowed to flow into the adsorbent under gravity by opening the lower tap momentarily. The top of the column is filled with eluent, the upper tap is connected by a tube to a nitrogen supply from a cylinder, or to compressed air, and turned on to the desifed pressure (monitor with a gauge). The lower tap is turned on and fractions are collected rapidly until the level of eluent has reached the top of the adsorbent (do not allow the column to run dry). If further elution is desired then both taps are turned off, the column is filled with more eluting solvent and the process repeated. The top of the column can be modified so that gradient elution can be performed. Alternatively, an apparatus for producing the gradient is connected to the upper tap by a long tube and placed high above the column in order to produce the required hydrostatic pressure. Flash chromatography is more efficient and gives higher resolution than conventional chromatography at atmospheric pressure and is completed in a relatively shorter time. A successful separation of components of a mixture by TLC using the same adsorbent is a good indication that flash chromatography will give the desired separation on a larger scale.  [c.21]

Industrial uses of Na metal reflect its strong reducing properties. Much of the world production was used to make PbEt4 (or PbMc4) for gasoline antiknocks via the high-pressure reaction of alkyl chlorides with Na/Pb alloy, though this use is declining rapidly for environmental reasons. A further major use is to produce Ti, Zr and other metals by reduction of their chlorides, and a smaller amount is used to make compounds such as NaH, NaOR and Na202. Sodium dispersions are also a valuable catalyst for the production of some artificial rubbers and elastomers. A growing use is as a heat-exchange liquid in fast breeder nuclear reactors where sodium s low mp, low viscosity and low neutron absorption cross-section combine with its exceptionally high heat capacity and thermal conductivity to make it (and its alloys with K) the most-favoured material. " The annual production of metallic Na in the USA fell steadily from 170 000 tonnes in 1974 to 86 000 tonnes in 1985 and is still falling. Potassium metal, being more difficult and expensive to produce, is manufactured on a much smaller scale. One of its main uses is to make the superoxide KO2 by direct combustion this compound is used in breathing masks as an auxiliary supply of O2 in mines, submarines and space vehicles  [c.74]


See pages that mention the term Scaling high-alloy steels : [c.1036]   
Corrosion, Volume 2 (2000) -- [ c.7 , c.73 , c.77 , c.79 ]