Hot Corrosion Behavior and Mechanism of High-Velocity Arc-Sprayed Ni-Cr Alloy Coatings
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Hot Corrosion Behavior and Mechanism of High-Velocity Arc-Sprayed Ni-Cr Alloy Coatings Jie Cheng1 • Yuping Wu1 • Liyan Chen1 • Sheng Hong1 • Lei Qiao1 Zheng Wei1
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Submitted: 31 July 2018 / in revised form: 29 June 2019 ASM International 2019
Abstract High-temperature corrosion affects many components used in coal-fired power plants. A possible solution is the use of protective coatings. In this study, Ni-Cr alloy coatings with different Cr contents (30 at.%, 45 at.%, and 50 at.%) and Ni-Cr-Ti coatings were deposited on 20G boiler steels by high-velocity arc spraying. The hot corrosion behavior of the coatings was characterized in the aggressive environment of Na2SO4 ? 30% K2SO4 molten salt under cyclic conditions at 750C. x-Ray diffraction analysis, optical microscopy, and scanning electron microscopy were used to analyze the phase composition, microstructure, and corrosion products of the coatings. The corrosion kinetic curves of the coatings, established by thermogravimetry, conformed to the classic parabolic law, revealing that the Ni-Cr-Ti coatings exhibited better hot corrosion resistance than the Ni-45Cr or Ni-30Cr coatings. However, among the four tested coatings, the Ni-50Cr coating was found to offer the best protection with the lowest values of mass gain per area (2.9 mg/cm2) and parabolic rate constant (kp, 0.215 9 10-11 g2 cm-4 s-1). This can be explained by the formation of Cr2O3 and NiCr2O4. Therefore, such high-velocity arc-sprayed Ni50Cr coatings could be used as protective layers for hot components in boiler tubes.
& Yuping Wu [email protected] & Sheng Hong [email protected] Jie Cheng [email protected] 1
College of Mechanics and Materials, Hohai University, 8 Focheng West Road, Nanjing 211100, People’s Republic of China
Keywords coatings corrosion mechanism corrosion kinetic curves Cr content high-velocity arc spray hot corrosion Ni-Cr alloys
Introduction Hot corrosion can be defined as accelerated corrosion resulting from the presence of salt contaminants such as Na2SO4, K2SO4, and NaCl that combine to form molten deposits, destroying the protective oxide scale on the coating surface (Ref 1). This phenomenon has been observed in boilers, gas turbines, internal combustion engines, as well as many other components used in coalfired power plants since the 1940s. However, it became a popular topic which attracted much attention in the late 1960s (Ref 2). As an important piece of equipment for coal-fired power plants, boiler tubes are generally exposed to aggressive high-temperature corrosion environments, leading to limited service life. Such deterioration of materials caused by hot corrosion not only increases repair and maintenance costs but also severely threatens the safe operation of power plants (Ref 3, 4). Corrosion problems cannot be eliminated completely, but the related costs can be reduced by more than 30% through use of effective corrosion protection technologies (Ref 5). Among these technologies, thermally sprayed coatings with excellent hot corr
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