Scaling nickel alloys

Carbon steels, low-alloy steels and stainless steels react at elevated temperatures with oxygen in the surrounding air and become scaled. Nickel alloys can also become oxidized, especially if spalling of scale occurs. The oxidation of copper alloys usually is not a problem, because these are rarely used where operating temperatures exceed 260 °C. 

Fluorine cannot be prepared directly by chemical methods. It is prepared in the laboratory and on an industrial scale by electrolysis. Two methods are employed (a) using fused potassium hydrogen-fluoride, KHFj, ill a cell heated electrically to 520-570 K or (b) using fused electrolyte, of composition KF HF = 1 2, in a cell at 340-370 K which can be electrically or steam heated. Moissan, who first isolated fluorine in 1886, used a method very similar to (b) and it is this process which is commonly used in the laboratory and on an industrial scale today. There have been many cell designs but the cell is usually made from steel, or a copper-nickel alloy ( Monel metal). Steel or copper cathodes and specially made amorphous carbon anodes (to minimise attack by fluorine) are used. Hydrogen is formed at the cathode and fluorine at the anode, and the hydrogen fluoride content of the fused electrolyte is maintained by passing in... 

In atomization, a stream of molten metal is stmck with air or water jets. The particles formed are collected, sieved, and aimealed. This is the most common commercial method in use for all powders. Reduction of iron oxides or other compounds in soHd or gaseous media gives sponge iron or hydrogen-reduced mill scale. Decomposition of Hquid or gaseous metal carbonyls (qv) (iron or nickel) yields a fine powder (see Nickel and nickel alloys). Electrolytic deposition from molten salts or solutions either gives powder direcdy, or an adherent mass that has to be mechanically comminuted. 

As Fig. 20.7 shows, if DS eutectics ( DSEs ) prove successful, they will allow the metal temperature to be increased by =100°C above conventional DS nickel alloys, and the inlet temperature by =200°C (because of a temperature scaling effect caused by the blade cooling). Further improvements in alloy design are under way in which existing nickel alloys and DS eutectics are being blended to give a fibre-reinforced structure with precipitates in the matrix. 

Eigelstein, H. C., and E. N. Skinner. The Effect of Composition on the Scaling of Iron-Chromium-Nickel Alloys Subjected to Cyclic Temperature Conditions, ASTM, STP No. 165 (1954). 

Table 7.14 Relative specific scaling rale constant for binary nickel alloys at 900°C ...

There are no significant high-temperature applications for alloys of nickel with iron. The scales formed in air consist of nickel oxide and iron oxide and the latter is usually present in the form of the spinel, NiO-FejOj . In the case of the more dilute nickel alloys, internal oxidation of nickel was Observed S. Substitution of a substantial proportion of nickel by iron results in a deterioration in the oxidation resistance of nickel-chromium... 

The thermal conductivity of cupro-nickel alloys = 50 W/m K and, from Table 9.16, scale resistances will be taken as 0.00020 m2K/W for the water and 0.00018 m2K/W for the organic. 

Chromium compounds Cr203 surface scale Nickel- chromium—iron alloys Nickel-chromium— molybdenum (tungsten) alloys Ni-Cr alloys analytical methods, 6 502-514 composition of metal compared to chromium ferroalloys, 6 501t dispersoid former, 2 325, 327 disposal, 6 519-521 economic aspects, 6 496—500 effect on cobalt alloys, 7 220 effect on stainless steel corrosion resistance, 7 809... 

Concurrently in the U.S., membrane development efforts were sponsored and initiated by the Atomic Energy Commission for gaseous diffusion. Large scale plants were built at Oak Ridge (Tennessee) and subsequently at Paducah (Kentucky) and Portsmouth (Ohio) in the early 1950s with a total capacity of approximately 150 t/d processed. The confidential membranes are believed to be made of nickel and high-nickel alloys but other membrane materials possibly have also been considered. 

IMaterials and Scaling Issues. Two aspects of the basically simple desalination process require special attention. One is the high corrosivity of seawater, especially pronounced in the higher temperature distillation processes, which requires the use of corrosion-resistant, and therefore expensive, materials. Typical materials in use are copper—nickel alloys, stainless steel, titanium, and, at lower temperatures, fiber-reinforced polymers and special concrete compositions (39). It is noteworthy that in quest of a lower initial cost, the use of inadequate materials of constmction in many locations combined with poor operation by virtually untrained hands led to rapid deterioration and failure of plants long before their estimated design life. Adequate experience suggests by now how to avoid such failures. The other aspect is scale formation (40,41), discussed in mote detail below. 

Residence time for supercritical water oxidation systems may be as short as several minutes at temperatures of 600 to 650°C. More than 99.9 percent conversion of EPA priority pollutants such as chlorinated solvents has been achieved in a pilot-scale plant with retention time less than 5 minutes. The system is limited to treatment of liquid wastes or solids less than 200 microns in diameter. Char formation during reaction may impact the oxidation time of the organics, while separation of inorganic salts during the process may be a problem. Typical materials for the reactor are Hastelloy C-276 and Iconel 625 (high nickel alloys), which can withstand high temperatures and pressmes and the corrosive conditions. 

Several complications in the Enichem process constrain the scale of the reactor volume [12]. The reaction is so exothermic that the reactor needs to be cooled to maintain the optimal 130 °C temperature. Vaporization of the reaction mixture removes only a fraction of the water by-product, and concentration of the water in the reactor hampers the selectivity and rate of the reaction. Therefore, the methanol conversion per run is limited to 20%. The presence of HCl necessitates the use of corrosion-resistant reactors, and either glass linings or high-nickel alloys are used. 

For a well designed membrane module of reasonably large scale, the initial cost should be dominated by the cost of the palladium content of the thin metal membrane (assuming a palladium alloy comprises the permselective layer). The module itself will be made of steel, most likely a nickel alloy, with or without significant chromium addition. The cost of the steel and the assembly labor should not exceed the cost of the palladium alloy membrane. 

H.E. Eiselstein, E.N. Skinner, The effect of composition on the scaling ofiron-chromium-nickel alloys subjected to cyclic temperamre conditions, in STP 165, ASTM Interimtional, Philadelphia, PA, 1954, pp. 162-172. 

Smi] Smith, D.B., Chipman, J., Optical Temperature Scale and Emissivities of Liquid Iron-Copper-Nickel Alloys , Trans. AIME, 4(6), 643-644 (1952) (Experimental, Optical Prop., 5) [1957Rav] Ravdel, M.P., Selissky, Ya.R, Transitions in Ternary Ni3pe Solid Solutions , Dokl. Akad. 

For continuous pyrolysis, only metal reactors (steel or nickel alloys) are used and PTFE is fed into the reactor with a screw feeder. The gaseous products are either collected using a scaled up cold trap, or fed directly into other processes [5,13], Continuous systems suffer from a drawback in that it is very difficult to maintain a pressure seal at the PTFE inlet, resulting in the inclusion of air in the reactor atmosphere, which induces PTFE combustion rather than pyrolysis and produces a mixture of mostly undesired products, such as CF4, C02 and CF2O. 

In order to improve the stress corrosion cracking resistance of age hardening nickel alloys (based on Ni, Cr, Mo, Fe) for use in boiling water reactors, alloys with various chromium and molybdenum contents were melted on a laboratory scale and in the form of notched stirrup samples, were tested in high purity water with 8 mg/1 oxygen at 288 °C for 500 h [169]. Within the 500 h test time molybdenum free sam-... 

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