ICMCTF2016 Session A1-2: Coatings to Resist High Temperature Oxidation, Corrosion and Fouling

Metal dusting is a severe form of corrosion, which occurs in atmospheres with high carbon activity in the temperature range of 400-900°C (e.g. in coal gasification or syngas production plants). Steels and nickel base alloys both are affected materials because of the catalytic effect of iron and nickel on the dissociation of carbon containing gases, uptake and subsequent graphite nucleation inside the material. Conventional protection is given by the use of alloys with high amounts of oxide scale forming elements or dense oxide scale forming coatings.

During gas turbine operation at elevated temperatures, foreign deposits often accumulate on the surfaces of alloys and coatings and influence their corrosion behavior. In some instances the deposits consist mainly of sulfates and in others, depending on the origin of the deposit, they consist mainly of oxides. The deposits may be solid or liquid and their presence affects the oxidation characteristics of alloys because they separate the alloy from the gas phase responsible for the oxidation reaction. This presentation will involve the effects of prototypical deposits (Na₂SO₄, CaSO₄, CaO, SiO₂ and synthetic CMAS) on the corrosion mechanisms of state-of-the-art superalloys and model alloys with compositions, which are typical of commonly-used coatings. The effects of alloy composition, deposit composition, gas composition and temperature on the kinetics and mechanisms of corrosion will be described. An attempt will be made to describe the features, which are common to the various modes of degradation and those, which are different. Finally, a mechanism for Type II Hot Corrosion, which is significantly different from those, which are generally presumed to operate, will be proposed.

This work was supported by the Office of Naval Research under ONR Contract N00014-10-1-0661, David A. Shifler, Scientific Monitor.

Nowadays, the wide and still increasing field of high temperature applications demands operation temperatures beyond the capacity of commonly used Ni-based single crystal superalloys to increase process efficiencies. Refractory metals are promising materials due to their high melting points and mechanical properties. However, the main challenge of these materials is their low oxidation resistance at elevated temperatures. Aluminum diffusion layers offer a promising solution to overcome oxidation problems by formation of protective oxide scales on the substrate surface during exposure in oxidizing atmosphere. Such aluminum diffusion layers were manufactured on molybdenum, niobium, tantalum, and tungsten using a pack cementation process at 1000 °C for 8 h as well as at 1050 °C for 4 h. Homogeneous diffusion layers of different intermetallic phases with thicknesses up to 49 µm were characterized by optical microscope, EPMA and XRD. The measurements revealed sufficient aluminum reservoir for a protective oxide scale formation. The observed phases were in agreement with phase predictions based on thermodynamic activity calculations. The partial diffusion coefficients of aluminum in the four different refractory metals were determined via Boltzmann-Matano analysis, allowing the prediction of future interdiffusion layer thicknesses.

In addition the oxidation resistance of these coatings was investigated. The samples were exposed to synthetic air at 1300 °C for up to 100 h and their mass change was measured via TGA. After oxidation the samples were again characterized by the methods mentioned above. It was found that the application of the positive halogen effect is necessary in order to enhance the formation of a protective Al₂O₃ oxide layer on niobium, tantalum, and tungsten while on molybdenum substrate the diffusion layer itself is protective. This effect is explained by comparing the growth kinetics of pure, coated, and coated as well as halogen treated samples.

Additively manufactured silver (Ag) nanoparticle films are being considered for sensor applications at high temperature. Such nanoparticle films can monitor critical parameters such as strain and pressure in harsh environment and at hard to reach places in propulsion systems. In this work we have studied the electrical properties and the related microstructural changes of the metal nanoparticle films manufactured using an Aerosol Jet based micro additive manufacturing method. High temperature in-situ impedance experiments are performed to characterize the electrical properties of the film. Impedance spectroscopy analysis was performed at a frequency range of 20Hz-300KHz and temperatures between 24 °C and 500 °C. XRD, SEM studies are performed to understand the oxidation behavior of Ag. The results show that at the time scale of the experiments (several days), the Ag shows minimal oxidation as seen under the impedance measurements. The real part of impedance increases as a function of temperature up to about 150 °C, and then drops upon further heating before rising further. The implications of these results to use Ag nanoparticle inks in high temperature sensors is discussed.

Currently, corrosion protection for aluminium in the aerospace industry is provided by toxic chromate VI-based coatings, used under derogation by the industry. However, due to the high toxicity of hexavalent chromium, environmentally benign alternatives are strongly needed with no viable solutions proposed so far. Hence, there is a strong need in the development of innovative eco-friendly technologies for the replacement of chromate coatings.

The aim of this work is to develop new environmentally friendly sol-gel systems that can be used to coat both free and anodized aluminium, thereby reducing corrosion. The solution proposed here consists of employing the sol-gel process to develop fully densified hybrid organic and inorganic coatings that fulfils the required aerospace anticorrosion standards (ASTM B117). The hybrid sol-gel materials that are central to this research are liquid-phase materials compatible with most coating deposition processes, such as dip-, spin- and spray-coating. They are materials containing organic and inorganic components at a molecular level, synthesized by means of hydrolysis and condensation reactions of inexpensive alkoxide precursors. Key to the development of highly anticorrosive sol-gel coatings is the investigation of the densification process to achieve highly densified coatings. In order to do this, the strategy consists of optimising the sol-gel formulations along with the curing methodologies. Here, the development of dual interpenetrating sol-gel systems are investigated employing both organosilane and transition metal networks.

The relationship between the structure of the nanomaterials and their anticorrosion performances are investigated using a range of characterisation techniques, including EIS, PDS, NSS, SEM, FTIR and DLS. It is shown that the performances of the sol-gel coatings are dependent on the transition metal concentration, the hydrolysis degree and the curing temperature. The leading coatings on blank AA2024-T3 panels exhibited a corrosion resistance of 168 hours in neutral salt spray. It is found that the key issue with the anticorrosion resistance is related to the adhesion of the sol-gel coating on the Aluminium substrate. Therefore, future work consists of investigating the adhesion promotion strategies to improve the corrosion resistance of our sol-gel systems.