Gases corrosion resistance

Decorative and wear-resistant (decorative/fimctional) coatings Permeation barriers for moisture and gases Corrosion-resistant films Electrically insulating layers for microelectronics Coating of engine turbine blades... 

Heat resistance and gas corrosion resistance depends on chemical, phase compositions and stmcture of an alloy. The local corrosion destmction (LCD) of heat resisting alloys (HRS), especially a cast condition, probably, is determined by sweat of alloying elements. 

Thermal fatigue characteristically results from temperature cycles in service. Even if an alloy is con ectly selected and operated within normal design limits for creep strength and hot-gas corrosion resistance, it can fail from thermal fatigue. 

Corrosion protection of metals can take many fonns, one of which is passivation. As mentioned above, passivation is the fonnation of a thin protective film (most commonly oxide or hydrated oxide) on a metallic surface. Certain metals that are prone to passivation will fonn a thin oxide film that displaces the electrode potential of the metal by +0.5-2.0 V. The film severely hinders the difflision rate of metal ions from the electrode to tire solid-gas or solid-liquid interface, thus providing corrosion resistance. This decreased corrosion rate is best illustrated by anodic polarization curves, which are constructed by measuring the net current from an electrode into solution (the corrosion current) under an applied voltage. For passivable metals, the current will increase steadily with increasing voltage in the so-called active region until the passivating film fonns, at which point the current will rapidly decrease. This behaviour is characteristic of metals that are susceptible to passivation. 

Stainless steel alloys show exceUent corrosion resistance to HCl gas up to a temperature of 400°C. However, these are normally not recommended for process equipment owing to stress corrosion cracking during periods of cooling and shut down. The corrosion rate of Monel is similar to that of mild steel. Pure (99.6%) nickel and high nickel alloys such as Inconel 600 can be used for operation at temperatures up to 525°C where the corrosion rate is reported to be about 0.08 cm/yr (see Nickel and nickel alloys). 

Weak Acid. Stainless steels (SS) have exceUent corrosion resistance to weak nitric acid and are the primary materials of constmction for a weak acid process. Low carbon stainless steels are preferred because of their resistance to corrosion at weld points. However, higher grade materials of constmction are required for certain sections of the weak acid process. These are limited to high temperature areas around the gau2e (ca 900°G) and to places in which contact with hot Hquid nitric acid is likely to be experienced (the cooler condenser and tail gas preheater). 

Chemically Functional. Refractory coatings are used for corrosion-resistant high temperature service in gas turbine and diesel engines, components such as cmcibles, thermocouple protection tubing, valve parts, etc. 

The resulting films are usually hard and scratch-resistant but often show considerable stress (152). Such films can be used as corrosion-resistant coatings on metals or as membranes in gas sensor devices (153,154)... 

The a—and P-aHoys are used where higher strengths are required, such as in shafts, oil and gas weUs, and medical implants. Again, Pd and Ru variations of the basic alloys are available where improved corrosion resistance is needed. Several of the Hsted P-aHoys were developed for implants. These alloys were designed to be free of aluminum and vanadium, which have created some concern related to potential toxicity when used in implants (50). 

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. 

The gas distribution system can be composed of a network of perforated pipe, slotted or vented concrete block, or metal grating. When there are no space hmitations, single-level filters are used. In regions where footprint space is hmited, hke Japan, multiple-deck filter beds have become commonplace. If inorganic compounds are being treated, corrosion-resistant materials of construc tion are used due to the acidic by-products of the bioreaction. 

The most chemical-resistant plastic commercially available today is tetrafluoroethylene or TFE (Teflon). This thermoplastic is practically unaffected by all alkahes and acids except fluorine and chlorine gas at elevated temperatures and molten metals. It retains its properties up to 260°C (500°F). Chlorotrifluoroethylene or CTFE (Kel-F, Plaskon) also possesses excellent corrosion resistance to almost all acids and alkalies up to 180°C (350°F). A Teflon derivative has been developed from the copolymerization of tetrafluoroethylene and hexafluoropropylene. This resin, FEP, has similar properties to TFE except that it is not recommended for continuous exposures at temperatures above 200°C (400°F). Also, FEP can be extruded on conventional extrusion equipment, while TFE parts must be made by comphcated powder-metallurgy techniques. Another version is poly-vinylidene fluoride, or PVF2 (Kynar), which has excellent resistance to alkahes and acids to 150°C (300°F). It can be extruded. A more recent development is a copolymer of CTFE and ethylene (Halar). This material has excellent resistance to strong inorganic acids, bases, and salts up to 150°C. It also can be extruded. 

Nickel and its alloys form another important class of non-ferrous metals (Table 1.3). The superb creep resistance of the nickel-based superalloys is a key factor in designing the modern gas-turbine aero-engine. But nickel alloys even appear in a model steam engine. The flat plates in the firebox must be stayed together to resist the internal steam pressure (see Fig. 1.3). Some model-builders make these stays from pieces of monel rod because it is much stronger than copper, takes threads much better and is very corrosion resistant. 

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