High Temperature Hot Corrosion Research Articles

A study on gradient thermal barrier coatings by EB-PVD in a cyclic high-temperature hot-corrosion environment

Gradient thermal barrier coatings (GTBCs) have been produced by electron beam physical vapor deposition (EB-PVD). Their performance was evaluated by isothermal oxidation and cyclic high-temperature hot-corrosion tests. It is found that the GTBCs exhibited better resistance to high-temperature oxidation and cyclic high-temperature hot-corrosion (HTHC) than traditional two-layered TBCs. A dense Al2O3 layer on the bond coat of GTBCs can effectively prohibit inward diffusion of oxidants such as O and S and outward diffusion of Al and Cr. On the other hand, an “inlaid” interface, the formation of which resulted from the oxidation of Al diffusion into the gaps between the columns of bond coat during the fabrication of the GTBCs, contributes to reinforce the adherence of the Al2O3 layer to the bond coat. During fluxing of the Al2O3 layer, S and O diffused into the bond coat. Cracks developed in the surface layer of bond coat by the combined effect of sulfidation of the bond coat and thermal cycling, and finally led to failure of the GTBC.

Failure of gas turbine blades

The first-stage blades of a gas turbine had suffered a severe deterioration after around 10 500 h service. The expected service life was 40 000 h. Failure analysis (visual observations, studies by optical microscopy, scanning electron microscopy (SEM), SEM back-scattered electron (SEM-BSE), EDX, X-ray diffraction (XRD) and dimensional metrology) has been carried out. Blades, manufactured in the nickel superalloy CMSX-4, lost the protective coatings from their tips due to wear. Unprotected surfaces suffered high-temperature hot corrosion (Type-I corrosion). It is concluded that failure was mainly caused by an uneven clearance (out-of-line) between rotor and lining.

Evaluation of NiCrAlYSi overlay coating on Ni3Al based alloy IC-6 after an engine test

Abstract The second stage turbine vanes of a recently developed high performance jet aero-engine were cast using a Ni 3 Al based alloy IC-6, and then coated with NiCrAlYSi by vacuum arc deposition. After a 379 h engine test, the morphology, microstructure, phase structure and chemical composition of the coating were examined and analyzed by scanning electron microscopy with X-ray energy dispersion spectrometer, X-ray diffraction and electron probe microanalysis techniques. The analytical emphasis was placed on the effect of black spots which appeared on the coating surface after the test on the coating and base alloy IC-6. In addition, the origin of black spots was intensely investigated.The results indicated that the black spots were a kind of high-temperature hot corrosion caused by Na 2 SO 4 melt salt from the combusted gases of the engine, the existence of black spots accelerated the internal oxidation of the coating, but had not impaired the base alloy after 379 h engine operation. It may be summarized that NiCrAlYSi coating can provide an effective protection against oxidation and corrosion of base alloy IC-6 at elevated temperature of the present test.

High-temperature oxidation and hot corrosion behavior of two kinds of thermal barrier coating systems for advanced gas turbines

High-temperature oxidation and hot corrosion tests were conducted at 800 to 1100 °C under isothermal and thermal-cycle conditions for two kinds of thermal barrier coating (TBC) systems with different compositions of ceramic top coat: Y2O3-stabiIized zirconia (YSZ) and CaO-SiO2-ZrO2 (C2S-CZ). Qualitative and quantitative failure analyses were carried out to clarify the failure mechanisms of TBC systems. In high-temperature oxidation up to 1100 °C, the YSZ-TBC system was subjected more easily to spalling of the ceramic top coat. This is attributed to the localized oxidation along the ceramic top coat/metallic (NiCrAlY) bond coat interface, as compared with the case of the C2S-CZ-TBC system. Thus, the most significant oxidation damage resulted in the YSZ system under the thermal-cycle condition. On the other hand, for hot corrosion by Na2SO4-NaCI molten salt up to 1000 °C, the C2S-CZ system was more reactive with the molten salt to form a new phase layer composed of both the metallic bond coat constituents, such as aluminum and chromium, and corrosive species such as oxygen at the inner region of the ceramic top coat. Furthermore, effects of both the heat treatment, in particular the atmosphere after plasma spraying, and the chromium content of the bond coat were investigated for each coating system.

Performance of a silicon-modified aluminide coating in high temperature hot corrosion test conditions

The results of burner rig tests for high temperature hot corrosion (HTHC) resistance are discussed for pack and slurry aluminide coatings on IN738. A silicon-modified aluminide coating produced from a slurry had the best HTHC resistance owing to the presence of layered chromium and silicon-rich phases in the outer zone of the coating. Effects of sodium sulfate deposition rate, test temperature, coating thickness, and substrate alloy composition were investigated for the silicon-modified aluminide coating. Increasing sodium sulfate deposition rate led to development of a pitting-type corrosion morphology due to local penetration of the silicon-rich outer zone of the coating. The HTHC rate of the silicon-modified aluminide coating was shown to be essentially equivalent for isothermal test temperature at 885°C and test temperatures incorporating excursions to 1000°C. Coatings of 50 and 75 mm thicknesses showed similar corrosion behavior. The HTHC life of a 50-mm silicon-modified aluminide coating on an IN100 superalloy substrate was approximately 50% lower relative to the coating on IN738. This decrease in life was attributed to the lower chromium content and increased content of molybdenum and vanadium in the IN100 superalloy. The life of a 50-mm silicon-modified aluminide coating on IN100 was equivalent to a 95-mm pack aluminide coating on IN738 in the same test conditions.

Joining ODS materials for high-temperature applications

Oxide-dispersion-strengthened (ODS) high-temperature alloys represent a unique class of powder-metallurgy-based engineering materials. They offer combinations of high-temperature strength, oxidation resistance, and hot corrosion resistance that cannot be obtained in other alloys. The alloys were initially developed for the aircraft gas turbine industry; since then, however, applications have expanded to include industrial gas turbines, equipment for handling molten glass, high-temperature furnace assemblies, and a variety of other industrial components. Internationally, the materials are also of interest for nuclear power systems (both breeder and fusion reactors) since ferritic ODS alloys exhibit both excellent swelling resistance and good elevated-temperature creep resistance. Many of these applications require that the ODS alloys be joined to either themselves or to other materials. The purpose of this paper is to review some of the techniques available for making these joints.

Effects of pretreatments on structure and oxidation behaviour of d.c.-sputtered platinum aluminide coatings

Pt aluminide coatings exhibit better high temperature oxidation and hot corrosion resistance than aluminide coatings. In this study the Pt layer was deposited by d.c. sputtering on Hastelloy-X superalloy instead of by an electroplating method. Laser remelting and diffusion heat treatment were applied to eliminate defects such as leaders existing in the as-deposited Pt layer and to effect the bonding between the Pt layer and the substrate before aluminization. The results of this study showed that the Pt aluminide coating pretreated by diffusion heat treatment and laser surface remelting consisted of a PtAl 2 + NiAl duplex-phase outer layer, an (Ni 2Pt)Al single-phase layer and an interdiffusion zone. The outer duplex-phase layer of the Pt aluminide coating pretreated by diffusion heat treatment consisted of a continuous PtAl 2 phase with NiAl as a second phase. In the laser-remelted Pt aluminide coating, by contrast, a minor PtAl 2 phase was embedded in NiAl. The results of oxidation tests at 1100°C showed that the diffusion-heat-treated Pt aluminide coating exhibited excellent oxidation resistance whereas the laser-remelted Pt aluminide coating failed to provide protection against high temperature oxidation. The latter is deemed to be inferior to a plain aluminide coating.

Effect of yttrium on the stability of aluminide-yttrium composite coatings in a cyclic high-temperature hot-corrosion environment

A composite coating of aluminide-yttrium has shown excellent corrosion resistance in a cyclic high-temperature hot-corrosion environment. To understand the effect of yttrium on the stability of the composite coating, the specimens were prepared with various coating parameters of Y thickness, sequence of post heat treatment and surface condition before Y-ion plating. Performance of the composite coating was evaluated by isothermal oxidation and cyclic high-temperature hot corrosion. Isothermal-oxidation-test results show that the Y in the composite coating helps to form a thick and dense Al2O3 scale which is ductile and resistant to thermal stress. The Y in Al2O3 may act as a donor which leads to an increase in concentration of interstitial oxygen and, thus, increases in oxidation rate. The presence of Y2O3 and (Y, Al) O-type compounds in grain boundaries of Al2O3 and boundaries between the Al2O3 and NiAl effectively prohibits the fast diffusion of oxidants (such as O and S) and Al along grain boundaries. Consequently, it may induce slow diffusion through the matrix, and thus the corrosion resistance of the composite coating under cyclic hot corrosion increases substantially.

イットリア粒子分散強化合金の高温腐食と高温酸化

イットリア粒子分散強化合金の高温腐食と高温酸化

High Temperature Hot Corrosion Control by Fuel Additives (Contaminated Fuels)

Abstract : The potential of fuel additives to minimize corrosion of blade-material has been analyzed by the following series of steps: first, a computer program was employed for an equilibrium-thermodynamic prediction of condensed solution composition; next relevant physico-chemical properties of the molten solution were estimated based on predicted equilibrium composition; then oxide solubility and dissolution rates are calculated for this solution in contact with various solid oxides. Finally, we compare our predicted oxide dissolution-rate results with available experimental hot corrosion rate data, seeking verification of the model for the rate determining process - i.e., oxide dissolution - in hot corrosion. Keywords: Fuel additives, Blade material, Molten solution, Thermodynamic corrosion prediction, Chemical equilibrium.

High-temperature corrosion of alumina-forming coatings for superalloys

The results of recent experiments on the cyclic oxidation and hot corrosion of diffusion aluminide and CoCrAlY overlay coatings on nickel-base superalloys are described. It is shown that platinum additions to aluminide coatings produce a substantial increase in resistance to cyclic oxidation and high temperature (Type 1) hot corrosion but have a rather small effect on resistance to low-temperature (Type 2) hot corrosion. The corrosion of aluminide coatings is also shown to be influenced by the composition of the underlying superalloy. Mechanisms for the above behavior are proposed. The effects of yttrium and hafnium on the cyclic oxidation of CoCrAl are illustrated. The results of scanning and transmission electron microscope characterization of the hot corrosion morphologies of CoCrAlY coatings are discussed and a mechanism to account for the microstructures is briefly described.

Effect of laser surface modification on high-temperature oxidation and corrosion behaviour of alloys and coatings

During laser irradiation, with or without foreign particle injection, the substrate surface can be modified by changing either its alloy composition and structure, or only its structure. It has been shown that laser treatment can extend the metastable solubility, form metastable phases, refine the grain size, eliminate some harmful phases and hence improve the oxidation, corrosion and abrasion properties of the treated materials. As a consequence, laser surface treatment appears to be a very versatile process with good potential for modifying the surfaces of alloys and coatings to improve their resistance to high-temperature oxidation and hot corrosion. In this paper, the positive aspects of laser irradiation in producing surface-modified layers of alloys and coatings to enhance their high-temperature corrosion resistance will be reviewed and the need for further research efforts will be briefly discussed.

Evaluation of the hot corrosion protection of coatings for turbine hot section components

A program was conducted to evaluate the relative corrosion resistance of commercially available coatings in laboratory furnace testing. Three turbine section superalloys were selected, MAR-421, IN-738LC and FS-414, and coating types ranged from simple diffusion aluminides to modified aluminides and a CoCrAlY overlay. Results indicate that the coatings were more protective to IN-738LC than to MAR-M421, with a silicon aluminide and platinum-chromium aluminide affording the best protection overall. The simple aluminide was surprisingly quite resistant in the temperature regimes studied, relative to the more complex aluminides, particularly with IN-738LC. The recommendations for corrosion-resistant coatings (for low temperature and high temperature hot corrosion environments) are as follows: silicon aluminide and platinum-chromium aluminide for MAR-M421; simple aluminide, platinum-chromium aluminide and silicon aluminide for IN-738LC; and platinum-chromium aluminide and silicon aluminide for FS-414.

Structure and 900 °C hot corrosion behavior of chromium-modified platinum aluminide coatings

It is well known that the addition of platinum to diffusion aluminide coatings has demonstrated great success in producing protective coatings which delay the onset of high temperature hot corrosion attack in the range 800 – 1000 °C. Recently, it has been found that the addition of chromium to diffusion aluminide and platinum aluminide coatings improves low temperature (700 °C) hot corrosion resistance on IN738 nickel-base superalloy substrates. Therefore, a program was initiated to study the effect of the combined addition of chromium and platinum with varying processing sequences on aluminides in accelerated hot corrosion environments at 900 °C. In this investigation, ten coating types on the nickel-base superalloy substrate IN738 were tested at 900 °C for 200 h. Of these ten coatings, two coatings showed excellent high temperature (900 °C) hot corrosion resistance: (i) Pt-Al (produced by an intermediate temperature, intermediate activity (ITIA) process) and (ii) Cr-Pt-Al (process D). The average measured depth of attack rate was about 0.025 μm h -1 which is about one order of magnitude slower than that of published low temperature (700 °C) hot corrosion data. Scanning electron microscopy was used to examine the microstructures of the coatings. The ITIA Pt-Al coating has a thin (3 μm) surface layer of PtAl 2, with an outer intermediate zone of profuse PtAl 2 particles dispersed in an NiAl matrix and an inner intermediate zone of substrate-element-rich precipitates and carbides in a β-NiAl matrix over an interdiffusion zone. The Cr-Pt-Al (process D) coating has a thick (18 μm) continuous surface layer of PtAl 2, with an outer intermediate zone of a small volume fraction of α-Cr and other substrate-element-rich precipitates dispersed in a β-NiAl(Pt, Cr) matrix and an inner intermediate zone of α-Cr-rich precipitates, refractory metal carbides and NiAl over a typical interdiffusion zone. It appears that an outer dense layer of the PtAl 2 phase backed up with a fairly high level of chromium and platinum near the surface is beneficial in protecting the superalloy substrate against high temperature corrosion attack.

Total corrosion control for industrial gas turbines: High temperature coatings and air, fuel and water management

Contaminants found in air, fuel or water can combine in the turbine hot section to produce corrosion, erosion and/or deposition. The fundamental role of high temperature coatings is to delay the onset of substrate deterioration and attendant effects on component structural integrity. To achieve this, the total concentration of each harmful contaminant in the turbine environment should be determined in order to select the appropriate material condition necessary, coated or uncoated, for the airfoil surface to survive the expected service time successfully. A method is presented whereby the total fuel equivalent concentration of any contaminant from all sources can be calculated. Utilization of this methodology is demonstrated by means of a hypothetical case of a turbine operating under specifically defined conditions. In this example, sodium (plus potassium) concentrations in air and fuel are assumed to be well within industry-accepted standards of 1 ppmw and it is shown that the use of untreated potable water for water injection and evaporative cooling can result in unacceptably high sodium concentrations. Subsequently, the extent and cost of water clean-up can be optimized by assessing the costs of water treatment equipment and the volume of water to be treated for emissions control and evaporative cooling. Field experiences with a turbine of 12 000 horsepower further substantiate the diagnostic advantages of this approach. In three cited cases, the presence of excessive sodium in the turbine hot section resulted in Type I or high temperature hot corrosion degradation of stage 1 blades. The source of contamination was attributed to water-borne sodium, using the methodology described. Corrective action recommended was directed at reducing the sodium concentrations in both the evaporative cooling water and the injected water and with target levels specified to ensure compliance with Solar's specification limit of 1 ppmw (fuel equivalent concentration) of total sodium. A platinum-modified aluminide coating was also applied to first- and second-stage blading. It is demonstrated that, with this approach, total corrosion control can be realized by assessing the source(s) of contamination followed by application of appropriate corrective action to reduce the level of contamination economically.

Low temperature hot corrosion of CoCrAl alloys

Hot corrosion of CoCrAl alloys occurs rapidly at temperature of 600–750°C because of the formation of the low melting Na 2SO 4CoSO 4 eutectic. In this paper the mechanisms and solutions of this low temperature hot corrosion (LTHC) are reviewed. Rapid degradation results from the dissolution of the more noble metal (cobalt) and its oxides (CoO and Co 3O 4) on the surface. On the basis of this mechanism, we have developed CoCr coatings containing about 37.5 wt% Cr or more which are resistant to LTHC and provide adequate resistance against the normal high temperature hot corrosion. Theoretical calculations have also been performed to predict the conditions under which binary K 2SO 4CoSO 4 and ternary Na 2SO 4K 2SO 4CoSO 4 eutectics can form. These calculations suggest that the presence of K 2SO 4 can aggravate the LTHC caused by Na 2SO 4.

Preparation and properties of nickel silicide layers by the diffusion and CVD processes using Si2Cl6 as a source of silicon

Nickel plate was siliconized with a gas mixture of Si2Cl6, H2 and argon in the temperature range 400 to 900° C, and the siliconizing conditions and some of its properties were examined, Appreciable weight increase of the nickel plate was observed above 450° C, which is 200 to 300° C lower than that obtained using SiCl4 as a source of silicon. Siliconizing of the surface and the resistance to high-temperature oxidation and hot corrosion were improved, Nickel silicide layers were also obtained by the CVD process using a gas mixture of Si2Cl6, H2 and argon.

Platinum aluminide coating structural effects on hot corrosion resistance at 900 °C

Studies have shown that platinum modified aluminide coatings can exhibit a range of structural variations depending on initial platinum thickness, prealuminizing heat treatment, aluminizing cycle, and postcoating heat treatment. It had been speculated earlier that such structural variation could account at least in part for some of the reported variation of performance of these coatings. A series of archetype structures was prepared on IN738 by two aluminizing processes: (i) low aluminum activity and (ii) high aluminum activity; and tested at 900 °C under conditions producing so called Type I high temperature hot corrosion. The results of the present investigation showed that the addition of platinum greatly improved the hot corrosion resistance. It is observed that the low aluminum activity process has better corrosion resistance than that of the high aluminum activity process. It is also found that the surface degradation of both aluminizing processes is quite severe when the specimens were given prolonged prealuminizing diffusion heat treatment. Microstructural observation revealed a significant difference in coating thickness in both the processes and it depends on prealuminizing diffusion and the subsequent aluminizing treatment employed.

High temperature corrosion behaviour of ceramic‐based coatings on mild steel

High temperature oxidation and hot corrosion behaviour of some ceramic‐based coatings, e.g. borate, silicate‐chromate and carbide‐oxide on mild steel has been investigated in the temperature range of 400–850°C. The coated steel in general shows much better oxidation and hot corrosion resistance than the uncoated steel specially at higher temperatures. The borate coating has better hot corrosion resistance performance between 700 and 800°C whereas silicate‐chromate is suitable at temperatures above 800°C. The coated steels show parabolic behaviour during oxidation. In presence of Na2SO4, the corrosion rate increases with increasing salt concentration till a maxima is reached. The amount of Na2SO4 corresponding to the maximum corrosion rate decreases with increasing temperature. A self‐sustained fluxing cum sulphidation mechanism has been proposed to explain hot corrosion behaviour of uncoated or coated mild steel.

Reactive sputter-coated reaction-bonded silicon nitride

A coating technique was developed to provide high temperature oxidation resistance and hot corrosion resistance to gas turbine components made of reaction-bonded silicon nitride. Thick coatings of reactively sputtered silicon nitride were deposited onto reaction-bonded silicon nitride surfaces to form an effective oxygen diffusion barrier by eliminating all potential short-circuit paths. To accomplish this, hard crystalline coatings were produced by subsequent heat treatment of the as-deposited silicon nitride. The as-deposited and annealed films were characterized by a variety of methods to determine the phase content, extent of crystallinity, grain size and morphology. With this coating technique, the oxidation of the underlying silicon nitride was substantially reduced in the temperature range 1000–1200°C.