High temperature steam oxidation has been occurred inside the boiler tubes of the superheater and reheater in fossil fuel-fired power plants. Exfoliation of the scale formed by the steam oxidation sometimes makes troubles such as the blockage of the boiler tubes and the erosion of the turbine components. Recently, operating temperature and pressure has been improved due to achieve high efficiency. In Japan, steam conditions have been reached at the ultra super critical steam condition (883K/31.6MPa). These boiler tubes have been mainly made of iron-chromium ferritic and austenitic steels. Recently, Fe-Cr ferritic steels have been widely chosen from the advantages of low costs and low thermal expansion coefficients that close to the oxide scale formed on the alloy surface. But, steam oxidation behavior of the high Cr ferritic steels is not clarified. From 1990's, many researchers have started fundamental researches of the steam oxidation of high Cr ferritic steel, because it needed to clarify the scale formation and control the scale exfoliation. In this paper, high temperature steam oxidation of high chromium ferritic steels are mentioned in the viewpoint of the microstructure evolution of the scale, the effect of Si addition, void formation and hydrogen permeation in the oxide scale.
Corrosion prevention methods of materials are key technologies to increase in power generation efficiency, reduce in environmental pollution and realize maintenance-free in the waste-to-energy plants. Corrosion environment have became severe condition according to social demands such as increase in calolific value of waste, strict pollution regulation etc.. For these trends, corrosion prevention technologies such as the improvement of combustion and the optimization of plant design have been applied. Also, the steam condition of more than 673K/3.9MPa became majure stream in waste-to-energy boiler by using of highly durable corrosion-resistant coatings such as thermal spray, weld overlay etc. for waterwall, also for superheater tube, the use of 310 type stainless steels and High Cr-High Mo-Ni base alloys has been progressed based on the understanding of corrosion mechanisms. This paper describes the majure development and the application of corrosion prevention and corrosion-resistant material technologies progressed for recent thirty years in waste-to-energy plants, and future subjects for corrosion prevention methods.
Thermal barrier coatings (TBCs) are indispensable in the design of hot section components for advanced gas turbines. In particular, yttria stabilized zirconia (YSZ) layers fabricated by electron beam physical vapor deposition (EB-PVD) are suited for high performance applications. The principal reason is their superior tolerance against thermomechanical fatigue due to the unique columnar grain structure. The characteristics of the deposition process and the equipment are briefly explained. In the EB-PVD process, the processing conditions, such as preheating temperatures or rotation speeds of the substrates, strongly influence the microstructures and the crystallographic orientations of the layers. Therefore, controlling these aspects is the key issue to develop the durable TBC layers with excellent functions including thermal insulation. Finally, the state-of-the-art researching prospects concerning the new materials instead of YSZ are surveyed.
A sodium-sulfur battery (NAS battery) cell is comprised of a sodium negative electrode and a sulfur positive electrode separated by a solid electrolyte made of beta-alumina, housed within a cylindrical aluminum container. The container exposed sever corrosive agents, such as molten sulfur and sodium polysulfides, but have to be designed to deliver 4500 charge/discharge cycles over a 15-years operating life. This paper described about the outline of high temperature type NAS battery, plasma sprayed high Cr-Fe alloy coatings and their corrosion resistance against the positive electrode environments used for long service NAS battery.
A coating consisting of duplex layer of β-NiAl and α-Cr layer was formed on Ni-40Cr-3X (X(at%)=Ta, Nb, Ti, Zr, V, Ru, Rh, Re, Ir, and Pt) alloys by electroplating of Ni (12μm thick) and high Al-pack diffusion, followed by heat treatment in air at 1373K for 86.4ks. The coated alloys were oxidized in air for up to 2332.8ks under thermal cycling between 1273K and room temperature. Oxidation amounts increased and then turned to decrease with increasing oxidation time. The critical times became longer in the order of Ta, Ti, Nb, Pt, Ir, V, Re, Rh, and Ru except for Zr which showed no scale spallation. The alloy containing Zr was oxidized in accordance with parabolic rate law showing the rate constant of 1.2×10-10kg2m-4S-1, while the alloy containing Re showed significant spallation of the scale. It was found from the result after oxidation at 1373K for 2338.8ks that the α-Cr layer became thinner from 30 to 10μm for the Ni-40Cr-3 at%Zr alloy, whereas it was almost unchanged to be 20μm for the Ni-40Cr-3 at%Re alloy. It seems to be the reason for α-Cr stability that the α-Cr formed on the Ni-40Cr-3 at%Re alloy contains 8-10 at%Re.
The effect of added Nb to improve the intergranular corrosion resistance of SCH2 heat resistant cast steel was investigated by exposure test in the public waste incinerator and laboratory test. Nb was added up to 2.5wt% to SCH2 cast steel. Nb carbide precipitated at the grain boundaries and precipitates were identified as NbC and Nb6C5 by X-ray diffraction. The addition of Nb to the cast steel resulted in good intergranular corrosion resistance after the exposure test and the laboratory test. This result shows that the intergranular corrosion that was caused by preferential corrosion of Cr carbide developed along the grain boundaries was inhibited by precipitation of Nb carbide. X-ray mapping shows that the NbS is formed near the corrosion front, but NbS is protective as it is well known that NbS grows late and as it is dense. From this mechanism, the cast steel with Nb added showed good corrosion resistance.
The effect of alloying elements on the corrosion resistance of Ni-base cast alloys in chlorine-containing high temperature environment was investigated. Previous works have shown that corrosion rate peak are emerged at around 900°C in chlorine-containing atmospheres and related to the behavior of the chloride inside the alloy. In this work, it was found that addition of Al to the alloys reduced the peak and improved corrosion resistance. The role of Al is considered the formation of Al2O3 scale and the prevention of Cl accumulation inside the alloy. It was also found that addition of Si to Al added-alloys improved corrosion resistance. Based on the obtained result in the laboratory test, base alloys (Ni-33Cr-15W-1Si-0.3C) and Al-added alloys (Ni-33Cr-10W-2Si-2Al-0.3C) were exposed in an actual incinerator at around 900°C for one year. The corrosion amount of the Al-added alloys was much smaller than that of the base alloys. The test proved that addition of Al is promising for practical applications.
Three austenitic stainless steels and two Ni base heat resistant alloys were exposed to simulated environment of a waste pyrolysis and gasification treatment system for times of up to 1000 hours at 973K. These tested materials show not only hot corrosion but also oxidation. The oxide layers were consisted only of the chromium and silicon oxide, and not formed of iron and nickel oxide. The oxidation mechanism in the tested environment was discussed in point of the comparison oxygen partial pressure of the environment with the dissociation pressure of those oxides.
Effect of heat treatment and ceramic coatings to HVOF (JP-5000) spray coatings for prevention of high temperature corrosion in waste incineration plants have been investigated by laboratory and field tests. 1) Alloy 625 spray coating layer by JP-5000 diffused to base metal by heat treatment. The layer indicated good high temperature corrosion resistance in compared with SUS347HTB and SUS310J1. 2) Ceramic (ZrO2-Y2O3/Al2O3) top coating on alloy 625 and CoNiCrAlY layer by JP-5000 is confirmed improve the quality of metallic layers and penetration restrain for corrosive matter into the coating layers at gas temperature of 950-1050°C in field test. 3) Diffusion between the metallic coating layers and base metal are observed by heat treatment in air in the metallic/ceramic duplex coating layers. Also, good corrosion resistance was observed at gas temperature of 550-620°C by field test in the ceramic coating layer. 4) Although there are some problems such as increased coating times and its cost, application of heat treatment and ceramic (ZrO2-Y2O3/Al2O3) double layers coating for metallic coating layer to prevention of high temperature corrosion in waste incineration plants are considered to be effective because of noticeable improvement effect for high temperature corrosion resistance.
Thermal barrier coating (TBC) systems, which have been inevitably introduced to the critical hot-section components such as blades and vanes of the advanced gas turbines, one required to assure the improved durability and reliability under the complicated modes of thermal-mechanical-chemical loadings. In the present paper, the stress rupture property evaluation and failure analysis were conducted for TBC system with the Y2O3 partially-stabilized ZrO2 top-coat in air and two kinds of the corrosive environments. Static creep loading was found to bring about the typical creep rupture behavior for TBC system similar to the no-coated Mar-M247 alloy in air regardless of aggressive environment. On the contrary, it was revealed that dynamic fatigue and creep-fatigue loadings tend to bring about a significant rupture life reduction of TBC system both in air and in corrosive environments, because the penetration crack pre-existing in the top-coat tends to provide a nucleation site for the fatigue crack in air, and the corrosion-fatigue crack in hot corrosive environment. In molten salt environment, furthermore, characteristic manner of crack propagation behavior was observed to occur such as crack branching and crack tip blunting, which should be rather effective for suppressing the crack propagation. Effect of the corrosive environments on such the crack behavior was discussed mainly from the viewpoint of dynamic fatigue crack propagation.