**1. Introduction**

The devolvement of the energy industry is determined by techno-economical, ecological, legal, but most of all material-technology factors [1–6]. Achieving energetic security and profitability is based on the correct design and application of proper high temperature resistant materials, the application of new and advanced manufacturing technologies, as well as the addition of capabilities for burning alternative fuels such as biomass, non-recyclable waste. In the manufacturing of machines and plants used in energy production, conversion, distribution, fuel processing, and storage high temperature and creep resistant steels are used, as well as more and more often other high temperature resistant metallic materials [2]. Parameters, operational period, and the need for periodic maintenance of machines and elements in the energy industry are based not only on technological and design parameters but mainly on the combined impact of temperature, external forces, mechanical and thermal cycling and chemical composition of the medium on metallic materials.

The use of alternative fuels as an energy resource is a result of the international effort to reduce CO<sup>2</sup> production. However, introducing organic matter and waste to fuel mix results in the production of highly reactive chlorides and fluorides, drastically increasing the rate of power plant elements corrosion [5,7]. The combined application of high temperature and corrosive environments induces a need for a change of used materials of application of protective layers in the power industry. Structural degradation of steel or layer, applied by means of surfacing or thermal spraying, on power industry elements operated in creep range is described by evolving with time set of structural and physicochemical properties depending on applied stress and exploitation temperature. Exploitation conditions of the designed element can lead to the formation and further development of gas pores, voids, and microcracks, leading to premature failure [4]. Structural instability of temperature resistant steels and nickel alloys is influenced by: level of substructural changes i.e., dislocation density evolution, recrystallization and recovery, intermetallic phase and carbide formation and dissolution, change of phase morphology (distribution, shape, size), decomposition of perlite, bainite and martensite, impoverishment of metal matrix in Cr, Mo, W. Aforementioned factors regardless of chromium content in steel or protective layer impact corrosion resistance including adhesion of passive layer, as well as, reduce material strength and resistance to cracking. The stability of this process is based on the chemical composition of the alloy, metal structure in the base state, and decides on the usage of high temperature steel or preventative surfaced/thermal sprayed layer in a given temperature, stress, and other environmental conditions [1,2]. The heat exchanger pipes in waste disposal plants are most susceptible to the corrosive environment. The fact results in a continuous decrease in parameters impacting the operational period and the occurrence of premature failures.

Pressure vessel steel 16Mo3 (1.5415) is a low alloyed chromium-molybdenum steel designed for application in elevated temperature, exhibiting high plasticity and ductility. Molybdenum, as a ferrite promoting element, has a high affinity to carbon. In pressure vessel steels with molybdenum content (e.g., 15Mo3, 16Mo3) there is a possibility of alloying elements carbides formation. Molybdenum content in steel reduces the temperature of perlitic transformation and has nearly no impact on bainitic transformation temperature. Molybdenum alloyed steels posses ferritic-perlitic, ferritic-bainitic, or bainitic microstructure [1,2]. Molybdenum addition in a range of 0.25–1% increases steel creep resistance, hardness, yield strength, and ultimate tensile strength, simultaneously decreasing elongation. Mo as an alloying element has a positive impact on brittle cracking resistance, impact toughness, decreases the critical cooling rate, and increases wear resistance. Moreover, Mo addition slightly decreases plastic workability and machinability. Chromium content in 16Mo3 steel increases corrosion resistance in steam environment. These grades of steels can be successfully applied in energy and chemical industries up to the working temperature of 530 ◦C (e.g., boilers, pressure vessels, hot medium pipelines) [4,5].

One of the methods used to increase the operational period of boiler elements in high temperature and corrosive environments is the usage of complex nickel-based alloy (superalloys) coatings. Nickel-based superalloys are characterized by very high resistance to the above-mentioned conditions compared to typical high temperature resistant steels. This metallic material group is characterized by the following properties: working temperature up to 1250 ◦C, limited susceptibility to cyclic and dynamic loading, resistance to nitrogen, sulfur, and carbon compounds [5,7–10]. A widespread group of nickel superalloys is Inconel. Inconel alloys were developed in the 1940s by a research group of Wiggin Alloy (Great Britan) and are still used in power, aero, and space industries among others [11–16].

Applied on the industrial scale mechanized, automated and robotized surfacing technologies still require complex studies, which lead to achieving high quality protective layers on pressure vessel elements posed by the evolving needs of the power industry. The most widespread fabrication method of Inconel-based protective layers is a consumable electrode in inert gas shielding surfacing—131 (metal inert gas—MIG) [2,10]. However, achieving high quality requires no surface or inner surfacing defects, low iron content in the surfaced layer, and the low heat affected zone (HAZ) depth is difficult or impossible in case of MIG surfacing. This fact induces the need for the application of modern

and advanced coating technologies. The selection of proper surfacing, surfacing or thermal spraying technology for manufacturing nickel-based superalloy layers on pressure vessel steel is multivariable and requires extended studies [17–40].

For this reason, the present work is a study about the possibility of high power density methods application in manufacturing on Inconel 625 protective layers.
