ICMCTF2015 Session A1-3: Coatings to Resist High Temperature Oxidation, Corrosion and Fouling

The components of gas-turbine engines operating in marine environments are highly susceptible to hot-corrosion which has been classified as Type II hot-corrosion attack (650-750°C) and Type I hot-corrosion attack (900-950°C). Even though hot-corrosion has been widely investigated in the last 50 years, several critical questions remain unanswered and new ones have emerged based on new observations as well as the increasing complexity of the alloy systems and sulfate-deposit chemistries. The present work is focused on the well-accepted Type II hot-corrosion mechanism proposed by Luthra [1] for Co-base alloys. A new interpretation of this mechanism of degradation, based on the synergistic fluxing mechanism proposed by Rapp [2], is presented to explain recent results for CoCrAlY and NiCoCrAl alloys ( isothermally exposed at 700 and 800 °C under different atmospheres, including: air and O₂ with 100 and 1000 ppm SO₂). Our observations suggest the rapid dissolution of Co is not the controlling factor in the degradation mechanism, as was proposed by Luthra, since the g-phase which is richer in Co, has not been attacked significantly and therefore Al (present in γ-Al₂O₃) is probably the element responsible of the rapid attack observed which is in good agreement with previous results reported by Chiang et al. [3]

References

1. K.L. Luthra, Low Temperature Hot Corrosion of Cobalt-Base Alloys" Part II. Reaction Mechanism, Metallurgical Transactions A, 13A, 1982 (1853)

2. Y. S. Hwang and R. A. Rapp, Synergistic Dissolution of Oxides in Molten Sodium Sulfate, J. Electrochem. Soc., 137/4, 1990.

3. K.T. Chiang, F.S. Pettit and G.H. Meier, Low-Temperature Hot-Corrosion, High Temperature Corrosion (R. A. Rapp ed.), NACE, Houston, TX.

4. Y. S. Hwang and R. A. Rapp, Synergistic Dissolution of Oxides in Molten Sodium Sulfate, J. Electrochem. Soc., 137/4, 1990.

Commercial chromia forming Ni- and Co-base alloys, as well as some ferritic steels are currently being considered as candidate materials for applications, such as high-efficiency steam turbines and low temperature Solid Oxide Fuel Cells. The service environments for these applications include water containing atmospheres and operating temperatures in the range of 700-800ºC.

Experimental results showed that under these conditions, the alloys exhibited very good oxidation behavior, since a thin, protective chromia layer formed on the surface. Nonetheless, it was observed that long-term oxidation led to substantial microstructural changes in the subsurface layer, mostly enrichment/depletion of intermetallic phases.

Due to the surface scale growth process, chromium is depleted beneath the oxide layer. This decrease in chromium concentration affects the activity of other alloying elements. In the case of, e.g. Nb or W, the activity gradient established provides a driving force for uphill diffusion of these elements towards the scale-alloy interface, and thus for the formation/enrichment of intermetallic phases beneath the oxide scale.

This subsurface enrichment of intermetallic phases can negatively affect the mechanical behavior of the material. Additionally if the oxide layer becomes damaged, e.g. by spallation or particle erosion, a material which is chromium depleted and enriched in elements which form rapid growing oxides will be exposed to the oxidizing atmosphere leading to accelerated failure of the component. Therefore, the development of protective coatings may be necessary.

This presentation will describe the mechanisms of the oxidation-induced microstructural changes.

Thin solar selective film design is a key issue to address in developing carbon free ways to harvest energy in the future. The working temperature of such coatings must be increased from 350°C, the current working temperature of commercial solution, to 650°C to make them cost effective and they must be able to operate in air, not only in vaccum. To allow that, new solar absorber materials must be developed to withstand such temperature for years, without losing their optical properties. Thin films composite materials can be good candidates, especially alumina-platinum (Al₂O₃-Pt) multilayer coatings, because of the good resistance of these materials to heat and oxidation.

In this paper, we will present our work on the design and realization of such coatings. Our absorbers are composed of a substrate, a metallic infrared (IR) reflector and an alternation of thin Al₂O₃ and Pt layers. The number of layers and the thickness of each layer were optimized by optical simulations. Then we used magnetron sputtering to deposit these coating on different substrates: Si substrate to study the intrinsic properties of the coating and metallic alloys substrates (stainless steel and nickel-based alloy) to study the effect of the substrate on the aging of the absorber. Then we made optical characterization, TEM and chemical characterization to study these coating as deposited and after thermal aging at 650°C in air.

By using different kind of IR reflector (molybdenum (Mo) reflector, Pt reflector or no reflector) we demonstrate that the choice of this layer is of great importance for the stability of the whole absorber. We show that Mo reflector is not suitable for applications at high temperature. Best results were obtained with a 7 layer stack, comprising Pt reflector: a solar absorption of α=0.93 and a thermal emissivity of ε=0.43 (calculated for a temperature of 650°C) were measured after ageing at a constant temperature of 650°C in air during 100h.

Steam Electrolysis (SOEC) operated under pressures up to 30 bar at temperatures around 850°C has recently attracted interest as technology for an efficient conversion of renewable energy into liquid fuel (power-to-liquid). The impact of such severe conditions on the oxidation behavior of the interconnector coatings and thus on a reliable operation, is a crucial issue and needs detailed understanding.

Two commercially available interconnector materials were selected, one of them pre-coated by a Co containing thin layer. They were coated by a La-Sr-Mn based coating deposited by roll coating and thermal spray. Pure water vapor and pure oxygen, both at opposite extremes of the possible process atmosphere compositions, were selected for the study. Laboratory test autoclaves from Alloy 602 were prepared to conduct the experiments. The autoclaves were closed by welding, inserted into a muffle furnace and connected to the pressured gas supply system. The samples were oxidised for up to 1000 h at 850°C under a pressure of 30 bar. Post-oxidation analysis was performed by field emission scanning electron microscopy (FE-SEM) with EDX element analysis and XRD.

Bismuth titanate (Bi_xTi_yO_z) has received widespread attention due to the fact that during recent times it has found important applications in strategic research fields such as optics and optoelectronic, and more recently studies have shown how their physico-chemical properties may be cast-off in order to be able to use Bi_xTi_yO_z, as an anticorrosive coating.

In this work Bismuth titanate (Bi_xTi_yO_z) coatings were grown on titanium alloy (Ti6Al4V) substrates, using RF magnetron sputtering. The main objectives of the research project were to observe and quantify the evolution of phase formation, the crystallinity and chemical composition and to characterize the behavior of the resistant corrosive layer. The coatings were deposited by changing deposition parameters, such as: electrical power applied at the target, flux of the argon gas and the substrate temperature. The crystalline structure was characterized by X-ray diffraction (XRD) and the chemical composition was analyzed by Rutherford backscattering Spectroscopy (RBS). The corrosion resistance of the coatings was studied by Potentiodynamic polarization test (Tafel extrapolation). Preliminary results revealed a change in its microstructure as a function of the electrical power applied at the target, since they evolved from a completely amorphous phase to a polycrystalline phase. As determined by RBS analysis and by the electrochemical test the activation energy associated with bismuth on the surface and activation energies associated with the titanium in the coating and the substrate showed that corrosion resistance of the coating is far better in the amorphous phases of bismuth titanate than in the polycrystalline phases.

The binary rare earth metal oxides with pyrochlore or fluorite type of structure exhibit remarkable insulating properties which makes them potential candidates for top-coat materials in thermal barrier coatings. In this paper structural and thermal characterization of europium zirconate Eu₂Zr₂O₇, hafnate Eu₂Hf₂O₇ and cerate Eu₂Ce₂O₇ is presented. The feedstock powders were nano-crystalline Eu₂O₃, ZrO₂, HfO₂ and CeO₂. They were mixed in proper molar ratio and homogenized. The calorimetric studies of mixtures were performed. The final product was obtained via high temperature vacuum sintering at 1350°C under 15MPa. Phase composition of obtained materials was analyzed by X-ray diffraction. The internal morphology, especially porosity, presence of impurities and chemical inhomogeneity was also investigated. Thermal properties of obtained materials, including thermal diffusivity, specific heat and coefficient of thermal expansion were measured in the temperature range 25-1400°C. For comparison purposes, same test were carried out for sintered feedstock powders and conventional top-coat material – 8YSZ.

Magnesium alloys have been considered an attractive material in various applications due to their specific properties such as low density, high thermal conductivity and electromagnetic interference resistance. However, their applications are still limited because of the low corrosion resistance due to a high surface reactivity and low standard potential. Although many studies have been carried out to improve the corrosion characteristics and surface properties, it is still important research area.

Dense and uniform hydroxide thin film has been synthesized on magnesium alloys using a hydrothermal method. The alkaline aqueous solution has been used to fabricate protective coating layers. The results turn out that the layers are compact and uniform as non-porous structures and improve the corrosion resistance. The structures are composed of hexagonal nanosheets with a low contact angle on the surface. We also examine the characteristics of the film through the treatment time, temperature and concentration of solutions.

The morphology and structure of the films are examined by field emission scanning electron microscopy (FE-SEM) and transmission electronic microscope (TEM). The structure and orientation of Mg(OH)₂ film are investigated by X-ray diffraction (XRD) analysis. The compositions of the film are studied by Scanning Auger Microscopy (SAM). Potentiodynamic polarization measurements and salt spray tests (SST) are used to determine the corrosion behavior of the layer.

The hypothetic nuclear accidents can create a real danger to the Zr alloys and stability of parts made of these alloys, and especially such as loss of coolant accident (LOCA) and reactivity initiated accidents (RIA). The hydrogen degradation can manifest itself in an appearance of hydride phases resulting in a substantial loss of plasticity, an increase in ductile–brittle transition, sometimes in a decrease in mechanical strength.

This paper describes the coating technology of titanium nitride while performing reactive magnetron sputtering, which leads to reduction of hydrogen permeation through TiN-coated zirconium alloys (Zr1%Nb) of more than two orders of magnitude. Hydrogen permeation was calculated from the kinetic curves of hydrogen sorption at elevated temperature of the specimen (T = 623 K) and pressure (P = 2 atm). Dense titanium (Ti) layers were prepared between TiN protection films and a Zr substrate to improve thermal stability and adhesion between the TiN and the substrate at high temperatures. The applying of a Ti underlayer by vacuum arc deposition and 1,54μm TiN by reactive magnetron sputtering leads to the formation of thermal stable coating under cycling up to 1073 K.