Reactions-Ni-Base Alloys
Protective oxidejilms
2Ni + O2 → 2NiO
4Cr + 3O2 → 2Cr2O3
Suljate
2Na + S + 2O2 → Na2 SO4
Na-from NaCl (salt)
S-from fuel
Other Oxides
2Mo + 3O2 → 2 MoO3
2認(rèn) + 3O2 →2認(rèn)O 3
4V + 5O2 → 2V2O5
The Ni-base alloy surface is exposed to an oxidizinggas， oxide nucleiform， and a continuous oxide film forms (Ni) (Cr 2O3， etc.). This oxide film is a protective layer. The metal ions diffuse to the surface of the oxide layer and combine with the molten Na2SO4 to destroy the protective layer. Ni2Sand Cr2S3 results (suljidation):
NaCl (sea salt) → Na + Cl
Na + S(fuel) + 2O2 → Na2SO4
Cl-grain boundaries-causes inter.ranular corrosion
The extent of the corrosion depends on the amount of nickel and chro-mium in the alloy. The oxide films become porous and nonprotective， which increases the oxidation rate (accelerated oxidation).
Catastrophic oxidation requires the presence of Na2SO4and Mo，認(rèn)， and/or V. Crude oils are high in V; ash will be 65% V2O5 or higher. V can be alloyed in metal. A galvanic cell is generated:
MoO3 WO3
cathode anode Na2SO4 V2O5
The galvanic corrosion deletes the protective oxide film and increases the oxidation rate.
The corrosion problem includes: (1) erosion， (2) sulfidation， (3) intergra-nular corrosion， and (4) hot corrosion. The 20% Cr alloys increase oxidation resistance. Sixteen percent Cr alloys (Inconel 600) are less resistant. Cr inalloys reduces grain boundary oxidation， while high Ni alloys tend to oxidize along grain boundaries. Age-hardened gas turbine blades of 10-20% Cr will corrode (sulfidation) at more than 1400 oF. Ni2Sforms in the grain bound-ary. The addition of cobalt to the alloy increases the temperature at whichthe attack occurs. To reduce corrosion， either increase the Cr amount or apply a coating (Al or Al + Cr).
A high-nickel alloy is used for increased strength at elevated temperature， and a chromium content in excess of 20% is desired for corrosion resistance.An optimum composition to satisfy the interaction of stress， temperature， and corrosion has not been developed. The rate of corrosion is directlyrelated to alloy composition， stresslevel， and environment. The corrosive atmosphere contains chloridesalts，vanadium，sulfides， and particulatematter. Other combustion products， such as NO x，CO， CO 2， also contribute to the corrosion mechanism. The atmosphere changes with the type of fuel used.Fuels， such asnaturalgas， diesel .2， naphtha， butane，propane，methane， and fossil fuels， will produce different combustion products that affect the corrosion mechanism in different ways.
Gas Turbine Materials
The composition of the new and conventional alloys throughout the turbine are shown in Table 11-2. This table describes materials used in the GE line of turbines but the materials are common to all brands of high temperature turbine even though there may be some variations in the com-position of the alloys. In the early years of turbine development， increases in blade alloy temperature capability accounted for the majority of the firing temperature increase until air-cooling wasintroduced， which decoupled firing temperaturefrom the blade metal temperature.Also， as the metal temperatures approached the 1600 oF (870 oC)range， hot corrosion of blades became more life limiting than strength until the introduction of protective coatings. During the1980s， emphasis turned toward two major areas:improved materials technology， to achieve greater blade alloy capabilitywithout sacrificing alloy corrosion resistance; and advanced， highly sophis-ticated air-cooling technology to achieve the firing temperature capability required for the new generation of gas turbines. The use of steam cooling to further increase combined-cycle efficiencies in combustors was introduced in the mid to late 1990s. Steam cooling in blades and nozzles will be introduced in commercial operation in the year 2002.
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本文鏈接地址：燃?xì)鉁u輪工程手冊 Gas Turbine Engineering Handbook 2(58)
