ICMCTF 2022 Session A1-2-MoA: Coatings to Resist High-temperature Oxidation, Corrosion, and Fouling II

State of the art MCrAlY coatings are known to protect structural materials against oxidation and corrosion and are part of TBC systems. Due to their mechanical properties at high temperature, they are commonly applied in the hottest sections of turbine engines and other high temperature environments. High costs for the thermal spraying process of MCrAlY coatings drive forwards cost saving alternatives such as galvanic co-deposition of particles. Co-deposition of particles is influenced by numerous factors, such as particle size, kinetic effects, current density, pH related zeta potential or chemical stability of the particles [1]. In this study, nano-Y₂O₃ particles are co–deposited galvanically alongside with nickel and subsequently pack chromized and aluminized. Y₂O₃ is known to improve both oxide scale formation and oxide scale adherence. Another promising effect of Yttria is the reaction with low melting V₂O₅ to form high melting YVO₄, which reduces the aggressiveness and of corrosive salt deposits and thereby reduces corrosion attack [2].

Subsequent chromium and/or aluminum enrichment(s) after co-deposition using a pack cementation process is shown to lead to microstructural refinement compared to coatings without particles and are able to decrease wear up to high temperature. Additional microhardness measurements revealed an increase of approx. 100 HV1 for MMC NiAlY coatings compared to coatings without dispersed nano–Y₂O₃ particles. Depending on the activity of the pack cementation method to be used, differences in the distribution of dispersed particles are observed and discussed. Besides wear, the oxidation resistance as well as corrosion resistance of the manufactured coatings against molten V₂O₅/Na₂SO₄ salt mixtures at 700 °C in 0.1 SO₂/air gas are tested and compared against bare IN617 alloy.

[1]L. Besra, M. Liu,Progress in Materials Science2007, 52, 1–61.

[2]N. S. Bornstein,Vanadium Corrosion Studies1993.

High-temperature Erosion-Corrosion (E-C) is one of the critical issues for the heat exchanger used in a fluidized bed biomass boiler plant. E-C occurs by not only erosion due to impact of sand particles but also high-temperature corrosion in chlorine containing atmospheres. Current coatings widely used for boiler tubes in fluidized boiler plants are Ni-based protective coating (Japan Industrial Standard: JIS SFNi4 and/or SFNi5). However, E-C resistance of those coatings is still not sufficient. Thus, development of coating which have good E-C resistance is strongly required. SFNi4 contains a lot of alloying elements (Cr, B, Si, C, Fe, Co, Mo and Cu). In this study, effect of Mo, Si, Cr and Fe on E-C resistance was evaluated by using model alloys in order to optimize the Fe content. Mo and Si were found to be detrimental for E-C and Fe addition significantly improve E-C resistance. After E-C, oxide scale consisted of thinner NiO on the alloy without Fe, but thicker Fe-rich oxide on the alloy with Fe. Erosion resistance of Fe-rich oxide could be higher than that of NiO, resulting in higher E-C resistance in the alloy with Fe. Based on the results obtained from E-C test, the new coating which is higher Fe content and lower Mo and Si content was proposed and it was confirmed that E-C resistance of this coating was higher than previous coating in the actual plant.

Aluminum slurry coatings are a high-impact and economic technique to protect steels against corrosion at high temperatures and in aggressive media. They are easy to apply using different methods of deposition such as spraying or brushing with a subsequent heat treatment to form the diffusion coating. For optimization as well as for customization to particular applications with different substrate steels and media further development is needed. The use of methodologies and algorithms from artificial intelligence (AI) can significantly accelerate the development and reduce the costs by minimizing the experimental effort. For an AI supported approach the entire system has to be considered and digitalized integrating all components, all process parameters at all processing steps including the models for the different mechanisms such as diffusion.

To digitalize the aluminum slurry coating the entire coating system and its manufacturing process was fully parametrized considering every single parameter having influence. In doing so the coating process was divided into three sections: slurry formulation, slurry deposition and the heat treatment to form the aluminide diffusion layer including the substrate steel data. The parameters comprise the particle size and slurry components, spray characteristics such as distance, particle velocity or angle and heat treatment temperature, time and atmosphere.

Diffusion coatings are widely used in high temperature applications to enhance oxidation and corrosion resistance of metals and alloys. Metallic elements (commonly Al, Cr, or Si) are enriched at the surface to form protective oxide scales during exposures at high temperatures.
Al, Cr and Si-based diffusion coatings are mostly accomplished by pack cementation, where the deposition occurs via a gas diffusion process. For the pack cementation process the substrates usually have to be fully embedded into a powder mixture, which is laborious and requires a lot of furnace furniture to be heated up. For Al an alternative slurry process is well established, where the slurry is sprayed on the metallic surfaces by air brush. This simple deposition leads to economic advantages compared to other diffusion coating techniques and also to the possibility to coat large technical parts, weldings or to do local repair works.
The target application of this work focuses on a new generation of solar power plants, where bauxite particles are used as receiver and heat storage medium instead of the state of the art tubes with molten nitride salts. In this case the coating on the outside of the heat transfer tubes, which are in contact with the particles not only has to withstand oxidizing environments at high temperatures but also abrasion due to the impact of the heat carrying particles. For this application Cr- and Si-rich coatings are very interesting, e.g. because of the hard silicide phases, which are more oxidation resistant than the respective Cr-carbides at the target temperatures.
Due to the higher melting points and limited phase formation, the development of Si- or Cr-based diffusion coatings via the slurry process is more complicated, e.g. the use of Cr-Al pre-alloyed powder leads only to Al diffusion.

In this work novel slurry coatings are presented applied by a water-based slurry. The subsequent heat treatment is conducted in an inert Ar atmosphere at temperatures up to 1200° C.
The application of such slurry coatings is demonstrated on Inconel 740 (Ni-base) and Sanicro 25 (austenitic stainless steel). Therefore, different slurry compositions and heat treatment parameters are tested. The coating thicknesses achieved are up to 10 µm on the Inconel 740 and up to 300 µm on the Sanicro 25.
For phase determination and microstructural characterization X-ray diffraction (XRD), scanning electron microscopy (SEM) and electron probe microanalysis (EPMA) are used.
Oxidation exposures in synthetic air at 900°C show the formation of a protective scale and indicate an improved oxidation behavior by the applied coatings of the two investigated substrates.

Solar energy is the ideal energy source to provide heat at medium temperatures with the aim of transitioning to clean, renewable energy sources. Evacuated flat plate solar collectors (EFPCs) are able to convert solar energy directly into heat with high efficiency. Thanks to the high vacuum insulation, the main mechanism of loss in EFPCs is represented only by the radiative losses from the selective solar absorber (SSA). Thermal emittance plays a more important role than solar absorbance for the efficiency of SSA used in EFPCs working at medium temperature [1].

Once the SSA has been optimized for a maximum efficiency [2,3], further improvement can be obtained by reducing the substrate emissivity. We therefore deposited by electron beam low emissive Cu or Ag thin film on an Aluminium bulk substrate to improve the coating performances by reducing its thermal emittance, keeping the economic advantages of using a substrate as cheap and light as Aluminium. The low emissive coating can be used to reduce the thermal emittance of both the selective coating side and the substrate side of SSA. The thermal emittance was measured as a function of temperature through a calorimetric approach [4]. The thermal stability of the coating and the use of thin films of Cr2O3 as a diffusion barrier were also investigated.

[1] F. Cao, K. McEnaney, G. Chen and Z. Ren, Energy Environ. Sci. 7 (2014) 1615-28.

[2] D. De Maio, C. D'Alessandro, A. Caldarelli, D. De Luca, E. Di Gennaro, M. Casalino, M. Iodice, M. Gioffre, R. Russo, M. Musto, Multilayers for efficient thermal energy conversion in high vacuum flat solar thermal panels, Thin Solid Films, 735, 138869, (2021).

[3] D. De Maio, C. D'Alessandro, A. Caldarelli, D. De Luca, E. Di Gennaro, R. Russo, M. Musto, A Selective Solar Absorber for Unconcentrated Solar Thermal Panels, Energies, 14(4), 900, (2021).

[4] R. Russo, M. Monti, F. Di Giamberardino, and V. G. Palmieri, Characterization of selective solar absorber under high vacuum, Opt. Express, 26, (10), A480-A486, (2018).