Carbides corrosion

Table 19 3 Typical Resistance of Impermeable Silicon Carbides Corrosion Rate (mils/yr)...

J. C. Million, C. W. Weber, and P. R. Kuehn, Gas Chromatography of Some Corrosive Halogen-Containing Gases, Report No. K-1639, Union Carbide Corp., Nuclear Division, New York, 1966. 

AISI 321 and 347 are stainless steels that contain titanium and niobium iu order to stabilize the carbides (qv). These metals prevent iatergranular precipitation of carbides during service above 480°C, which can otherwise render the stainless steels susceptible to iatergranular corrosion. Grades such as AISI 316 and 317 contain 2—4% of molybdenum, which iacreases their creep—mpture strength appreciably. In the AISI 200 series, chromium—manganese austenitic stainless steels the nickel content is reduced iu comparison to the AISI 300 series. 

Other methods of metal powder manufacture are also employed for specific metals. Selective corrosion of carbide-rich grain boundaries in stainless steel, a process called intergranular corrosion, also yields a powder. 

Addition of niobium to austenitic stainless steels inhibits intergranular corrosion by forming niobium carbide with the carbon that is present in the steel. Without the niobium addition, chromium precipitates as a chromium carbide film at the grain boundaries and thus depletes the adjacent areas of chromium and reduces the corrosion resistance. An amount of niobium equal to 10 times the carbon content is necessary to prevent precipitation of the chromium carbide. 

Primary alkanolamine solutions require a relatively high heat of regeneration. Also excessive temperatures or localized overheating in reboilers cause the MEA to decompose and form corrosive compounds. An inhibitor system, such as the Amine Guard system developed by Union Carbide, is an effective method of corrosion control (52). Inhibitors permit the use of higher (25—35%) concentration MEA solutions, thus allowing lower circulation rates and subsequendy lower regeneration duty. 

Carbon, Carbides, and Nitrides. Carbon (graphite) is a good thermal and electrical conductor. It is not easily wetted by chemical action, which is an important consideration for corrosion resistance. As an important stmctural material at high temperature, pyrolytic graphite has shown a strength of 280 MPa (40,600 psi). It tends to oxidize at high temperatures, but can be used up to 2760°C for short periods in neutral or reducing conditions. The use of new composite materials made of carbon fibers is expected, especially in the field of aerospace stmcture. When heated under... 

The most widely used austenitic stainless steel is Type 304, known as 18—8. It has excellent corrosion resistance and, because of its austenitic stmcture, excellent ductihty. It may be deep-drawn or stretch formed. It can be readily welded, but carbide precipitation must be avoided in and near the weld by cooling rapidly enough after welding. Where carbide precipitation presents problems. Types 321, 347, or 304L may be used. The appHcations of Types 304 are wide and varied, including kitchen equipment and utensils, dairy installations, transportation equipment, and oil-, chemical-, paper- (qv), and food-processing (qv) machinery. 

Dimethyl and diethyl sulfate are available in a vahety of containers from 0.5-kg glass botdes to tank cars. Diethyl sulfate is somewhat less toxic than dimethyl sulfate and is considered noncorrosive, but dimethyl sulfate is classified as a corrosive Hquid and ICC regulations must be observed. Mild steel to 306 stainless steel is used for large-volume storage. Dimethyl sulfate is manufactured by E. I. du Pont de Nemours Co., Inc., and diethyl sulfate by Union Carbide Corporation. Other producers are Rhc ne-Poulenc and Hoechst-Celanese. 

Chromium carbide is important in powder preparations designed for thermal spray apphcations of corrosion and wear-resistant coatings on tool and machine parts. Lower carbon carbides of chromium are important in hardfacing tods and electrodes for weld-apphed ovedays on machine wear surfaces. However, these carbides are usually formed in situ from Cr and C in the rod and not added as preformed carbides. The properties of Ci2C2 are hsted in Table 2. 

High Temperature. The low coefficient of thermal expansion and high thermal conductivity of sihcon carbide bestow it with excellent thermal shock resistance. Combined with its outstanding corrosion resistance, it is used in heat-transfer components such as recuperator tubes, and furnace components such as thermocouple protection tubes, cmcibles, and burner components. Sihcon carbide is being used for prototype automotive gas turbine engine components such as transition ducts, combustor baffles, and pilot combustor support (145). It is also being used in the fabrication of rotors, vanes, vortex, and combustor. 

Materials made of siHcon nitride, siHcon oxynitride, or sialon-bonded siHcon carbide have high thermal shock and corrosion resistance and may be used for pump parts, acid spray nozzles, and in aluminum reduction ceUs (156—159). A very porous siHcon carbide foam has been considered for surface combustion burner plates and filter media. It can also be used as a substrate carrying materials such as boron nitride as planar diffusion source for semiconductor doping appHcations. 

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