Sustainable New Technology for the Improvement of Metallic Materials for Future Energy Applications

Abstract

The need for a more sustainable and accessible source of energy is increasing as human society advances. The use of different metallic materials and their challenges in current and future energy sectors are the primary focus of the first part of this review. Cryogenic treatment (CT), one of the possible solutions for an environmentally friendly, sustainable, and cost-effective technology for tailoring the properties of these materials, is the focus of second part of the review. CT was found to have great potential for the improvement of the properties of metallic materials and the extension of their service life. The focus of the review is on selected surface properties and corrosion resistance, which are under-researched and have great potential for future research and application of CT in the energy sector. Most research reports that CT improves corrosion resistance by up to 90%. This is based on the unique oxide formation that can provide corrosion protection and extend the life of metallic materials by up to three times. However, more research should be conducted on the surface resistance and corrosion resistance of metallic materials in future studies to provide standards for the application of CT in the energy sector.

1. Introduction

With the growth of the human population, there is an increasing need for a more sustainable and more easily accessible source of energy [1], bringing prosperity, economic development, security, better health care, welfare, and the overall better social and environmental development of mankind [2]. In recent years, many challenges, such as the distribution of natural resources, growth of the population and its needs, economic instability, new war zones, etc. [3,4] have emerged in energy sources based on oil, gas, and coal. These challenges combined with geo-political challenges and environmental challenges such as greenhouse gases, environmental impact, sustainable development, etc. [2,5], are leading to increased efforts in research and the development of new solutions and options for new and more sustainable energy sources. The energy sources can be classified into natural fossil-based (oil, gas, and coal) and renewable types [6]. It is important to note that nuclear energy can be grouped on its own or as part of one of the previously mentioned groups. This is a highly controversial topic, mainly based on whether conventional or advanced nuclear power is discussed [6,7,8,9]. In this review, nuclear energy will be grouped on its own.

The current prediction of energy sources for the next 20 years (2030–2040) in the European Union (EU), predicted in the year 2020 [10,11], is shown in Figure 1. Currently, the production of energy is still dominated by fossil-fuel-based sources (70%). The nuclear-based sources have consistently maintained a similar ratio, while the renewable sources are constantly gaining an increasing share. For the next two decades, an increasing use of renewable sources for energy production is predicted to increase by 300% in the EU alone by 2030 and by up to 450% by 2040 compared to the current state [10,11]. The nuclear energy source is predicted to remain a stable energy source throughout this period, especially if fusion is included [6,7,8]. Figure 1 and

Table 1. Renewable source preconditions for next 20 years for the EU.

	Current State 2020	2030	2040
Hydroelectric energy Land hydroelectric energy (rivers and lakes) Marine hydroelectric energy (ocean currents, tidal stream, and waves)	13%	10%	9%
Geothermal energy	7%	8%	8%
Biomass	67%	58%	51%
Solar energy Solar thermal Photovoltaic	3%	11%	21%
Wind energy	10%	13%	11%
Combined contribution of the renewable energy and the total energy production	~25%	~35%	~48%

also show the development of renewable energy sources in the EU over the next 20 years and the expected changes in the redistribution of energy sources within different sectors.

Production of Electricity per Year for Different Energy Sector

The production of electricity varies within the different categories of the energy sector, and also, the production costs can vary significantly depending on the energy source (Figure 2 and Table 2). Pricing is highly dependent on a variety of external factors, such as subsidies, various taxes, etc., which also vary depending on geopolitical locations and natural resources.

In addition to the price of electricity, another important factor in energy production is also the so-called capacity factor (also known as CF). The CF is the unitless ratio of the actual electrical energy output over a given period to the theoretical maximum electrical energy output over the same period. The CF can also be thought of as production efficiency. CF is usually calculated over a year in order to average out temporal variations and to represent the realistic values of energy/electricity produced per maximum capacity of an energy source (Table 2). From Table 2, the cost of electricity for renewable energy sources varies due to different economic perspectives and also the source of production and maintenance and repair costs that need to be considered. The next capacity factors show that the most promising renewable energy sources are geothermal [16], hydroelectric [19,20,21], and wind [15,22], based on the lowest possible electricity cost and the highest capacity factor.

Table 2. Energy sectors presented in this manuscript and their cost of production and capacity factor [23].

Type of Energy Sector	Cost (EUR/MWh)	Capacity Factor (%)
Hydroelectric energy Land hydroelectric energy Ocean hydroelectric energy
	53–326	31–66
	130–280	39–45
Geothermal energy	49–353	80–90
Biomass	128	64
Solar energy	27–130	12–30
Wind energy Onshore wind energy Offshore wind energy
	24–67	29–52
	60–130	12–48
Advanced nuclear energy	73	94

Table 2. Energy sectors presented in this manuscript and their cost of production and capacity factor [23].

Type of Energy Sector	Cost (EUR/MWh)	Capacity Factor (%)
Hydroelectric energy Land hydroelectric energy Ocean hydroelectric energy
	53–326	31–66
	130–280	39–45
Geothermal energy	49–353	80–90
Biomass	128	64
Solar energy	27–130	12–30
Wind energy Onshore wind energy Offshore wind energy
	24–67	29–52
	60–130	12–48
Advanced nuclear energy	73	94

To cope with the increasing energy demand and consumption, new energy production pathways are expected to evolve and develop to provide greater energy security, reduce global carbon emissions, and lower the financial cost of energy production. The advancement of production processes in the energy sector will require the development and utilization of new materials. This is where metallic materials (metals and alloys) come into play, many of which have been scarcely used or even unused in energy production and will become the most important players in sustainable energy production (see Figure 1 and Table 1).

The successful implementation of such materials will be particularly crucial in applications where high strength and dimensional flexibility combined with high temperature resistance are required. Today, more than 60 different metallic materials are used in one way or another in energy production (as base materials for reactors, storage and accumulation systems, and supporting infrastructure) [24]. Future energy security (Figure 3) requires a critical awareness of the availability, functionality, substitutability, recyclability, and production of metals and alloys [25,26,27]. Metallic materials are the type of materials that can be newly produced or reused and recycled. Additionally, their properties can be tailored through postprocessing, increasing their flexibility and versatility for various applications. Therefore, the adaptation and development of heat treatment and further processing steps of metallic materials for future applications must be under constant research [25,26,27]. The factors that influence the value of metallic materials are market availability, substitutability, recyclability, and socio-cultural and environmental impacts [26]. In addition, the development and consideration of new materials such as high-entropy alloys and materials for catalysis, energy generation, and storage applications will bring new challenges and benefits to the energy sector [28,29].

This review aims to provide a comprehensive overview of the metallic materials used in the advanced nuclear and renewable energy sectors of the future, including the challenges (environments) to which these materials are and planned to be exposed on a daily basis. Particularly, this review aims to present a possible novel way of processing metallic materials in a more environmentally friendly way, opening the possibility of improving material properties, extending the life cycle of components, and generating lower CO₂ emissions compared to conventional pathways.

The paper is structured in the following order: in Section 2, the future energy sector is presented, where the renewable energy and advanced nuclear energy sources are introduced and discussed. In the same chapter, the challenges and requirements of the metallic materials used in the given environments of the different energy sources are also presented. Section 3 introduces the emerging green technology of cryogenic heat treatment, which has a great potential to reduce the impact of the energy sector on the environment by improving material properties and extending the life cycle of components. Section 4 discusses the outlook for future technology and the energy sector and provides individual guidelines for future materials implementation in the energy sector.

2. Future Energy Sector

The future energy sector is divided into renewable energy sources and advanced nuclear energy based on fusion. The renewable energy sector is divided into hydroelectric energy, geothermal energy, and biomass, solar, and wind energy, with some subdivisions (see Figure 3 and Table 1).

2.1. Energy Sector and Selection of the Right Material

2.1.1. Advanced Nuclear Energy

For future advanced nuclear reactors, three major subtypes are considered: non-water-cooled reactors, advanced water-cooled reactors, and fusion reactors. The latter’s design will be based on the current fission reactors; therefore, the use of similar metallic materials is expected. However, the research into the development of new, more resistant metallic materials and their protection is ongoing [30,31].

The non-water-cooled reactor subtype includes reactors and systems that are still based on fission reaction, but the coolants are either molten salts and are designated as Th-based reactors or high-temperature gases (or cooled with helium, using graphite as a moderator) [30,31]. This category also includes small modular reactors (SMRs), which are fast-cooled reactors based on Na, Pb, and gas cooling [32].

The next category is advanced water-cooled reactors, also based on the fission reaction and using water as a coolant and moderator, i.e., SMRs [30,31]. These reactors are cleaner, fundamentally safer, more fuel efficient, more reliable, and more sustainable than the current generation of reactors [33,34].

The last category is fusion reactors. Fusion reactors are based on fusion plasmas, which are still in the early stages of research and development. The working conditions of these reactors are much more intense compared to those of the fission reactors, reaching temperatures of several thousand degrees during the plasma generation. Therefore, complex testing and development of metallic materials is required for the construction and operation of such reactors. In fact, there are two types of fusion plasmas being considered for the development of future fusion reactors. The first type is based on strong magnetic fields and is known as magnetic confinement fusion (MFC). The second type is based on compressing the deuterium (DT) fuel and heating it rapidly that fusion occurs before the fuel expands; this method is also known as inertial confinement fusion (ICF). Due to the different primary principle compared to fission, the material for fusion application must have a specific non-equilibrium thermodynamic state, two or more main phases, complex grain boundaries; and dislocation systems. Compared to the traditional fission system/environment, the new challenges for metallic materials used in fusion will mainly focus on adaptation to the higher resistance to neutron irradiation, cladding, and higher temperatures and stresses [30,31].

The benefits of a new, advanced generation of nuclear reactors can bring society lower energy costs, increased production, decarbonization of industry, increased efficiency, and significantly reduced environmental and waste hazards. The development and safety of future nuclear power systems depends not only on the type of fuel but also on the material design. In current and future advanced nuclear power, materials are exposed to high-energy particles, high temperatures, pressure changes, and highly corrosive environments. However, the degradation of metallic materials in this environment is complex due to the different materials used for the plant, the complex and highly variable environmental conditions, and the different loading conditions of the material in various applications [30].

The first factor to consider is the thermal ageing and fatigue of metallic materials that occurs in metals exposed to elevated temperatures. This is a critical aspect for metallic materials used in nuclear energy, as it can lead to a short life cycle of the metallic component. The reason for this is the altered microstructure, resulting from the diffusion activated process, which causes changes in mechanical properties and fatigue (including creep fatigue) [30,31].

The next factor (second) is irradiation, which causes changes in the dimensional stability of metallic materials and thus influences the final properties of the metallic component. This is caused by the following five radiation processes: (i) phase (microstructural) instability induced by neutron irradiation, forming increased precipitation and segregation of alloying elements; (ii) radiation-induced hardening and embrittlement; (iii) volume swelling due to void formation; (iv) high-temperature helium embrittlement caused by helium movement towards grain boundaries; and (v) irradiation creep caused by changes in the crystal lattice due to migration of interstitial atoms and dislocations [30,31].

The third factor is the water environment, where water is the primary reactor coolant. Exposure of metallic materials to water, especially with elevated temperatures, can lead to corrosion of metallic materials, which can cause degradation of properties and lead to component failures. The extent of the corrosion is a product of several factors, such as water pH, water purity, material composition, temperature, gas concentrations, etc.). The type of corrosion mechanisms can be divided into general and localized corrosion. General corrosion mechanisms include uniform corrosion, boric acid corrosion, erosion corrosion, and flow-accelerated corrosion. Localized corrosion mechanisms include crevice corrosion, galvanic corrosion, pitting, environmental assisted cracking, and biological corrosion. In addition, stress corrosion cracking (intergranular and transgranular stress corrosion cracking and low-temperature cracking) can also be present under high-loading conditions. In addition to water, molten salts and liquid metals can also cause corrosion and electrochemical reactions that affect the degradation of the metallic material (see Table 3) [30,31].

2.1.2. Hydroelectric Renewable Energy

Hydroelectric renewable energy currently represents 13% of all renewable energy sources in the EU [21]. Hydropower is based on the movement of water through a turbine, which in turn drives a generator to produce electricity. Hydroelectric power plants can be installed in oceans or on rivers and lakes, which are sources of continental hydroelectric power. Ocean electricity sources can be divided into wave energy, tidal current energy, tidal barrage, ocean thermal air conditioning, and ocean thermal energy conversion (also known as OTEC), where the last two options additionally produce heat.

OTEC relies on the steam from the warm surface water to interact with the turbines. However, OTEC is still at an early stage of development, and the application relies on the cold, deep ocean water, which condenses the steam back into water for reuse. For this application to be viable, there must be a temperature difference of at least 20 K between the two layers of the ocean water (surface/deep layer), which is mostly limited to the tropical regions. OTEC can also be combined with ocean thermal air conditioning [35].

Ocean thermal air conditioning can be used to control the air conditioning of buildings by using cold, deep water to cool the fresh water circulating through a building. The cold water must be between 277 K and 284 K. The same technique can also be used in lakes [36,37].

The next option for harvesting electricity from the ocean is tidal barrages. Tidal barrages are based on the normal hydroelectric concept, where instead of the typical river flow, the tide drives the turbines in the barrage and then generates electricity. However, the tidal difference between high and low tide must be at least 3 m for this technique to be viable [38,39].

The next option for generating electricity is tidal stream (current) energy, which uses similar technology as tidal barrages but relies on tidal currents [40]. In this type of installation, the turbines are anchored to the seabed or can be suspended from a buoy to generate electricity. The challenge of this technique is the preservation of the marine environment when deploying this type of energy solution [41,42,43].

The next type of the energy source is produced by wave power. This type of energy harvesting is based on the prediction of constant wave direction. It is estimated that wave power of solely 2100 TWh per year could be generated by harvesting the naturally occurring waves. To harvest wave energy, cells based on the pressure and then movement of hydraulic pumps are built, which then drive the generator into motion. However, the application of this technology is limited to the areas with constant waves [44,45,46,47]. In development are also hybrid wave and wind energy farms [44,48] that use a combination of the aforementioned techniques. The last type of the ocean-based hydroelectric renewable energy is related to the use of osmotic power, which is based on the salt concentration difference between seawater and fresh river water [49,50,51]. There are two known methods: one is reverse electrodialysis (known as RED) [50,52], and the other is pressure-retarded osmosis (PRO) [53,54].

The continental type of hydroelectric renewable energy can be divided into three parts. The best known and that with the longest tradition is hydroelectric power generation with dams on large rivers or lakes [55]. However, due to its environmental impact, the future of small hydro (also known as SHP) is a promising source of renewable and clean energy that provides significantly lower changes to the environment and disruption of the local ecosystem [49,55,56,57].

The next option for generating electricity from rivers is run-of-the-river (ROR) hydropower, where the natural flow of the river generates electricity, and the flow of the river determines the amount of electricity generated [58,59,60]. This type of source is ideal for streams and rivers that can sustain a minimum flow compared to other types. This type delivers cleaner power and generates less greenhouse gas for equivalent energy production compared to other types. Additionally, because these types do not require a reservoir, there is a reduced influence on the environment and flooding [61].

The last option is a special type of hydroelectric power generation using turbines that are completely submerged and, in some cases, anchored to the river bed [62,63]. The advantages of this type are its high efficiency, low maintenance, and high reliability compared to other hydroelectric types [64]. However, the environmental impact can be controversial, especially on the flora and fauna of the marine/lake/river environments [65].

The metallic materials used in the hydroelectric sector must be able to withstand the different environments (low temperature, high pressure, and highly corrosive environment (including chemical agents)) and also have low density, high strength, high toughness and high resistance to wear (especially abrasion), high resistance to corrosion, and high fatigue resistance [66]. The metallic materials used for the turbines are austenitic stainless steel (over 12% Cr content as an alloy), but the turbine blades can also be made of martensitic stainless steel due to the higher strength of the steel [66,67]. Low-head machine parts are made of weathering steel (Corten steel) and various types of stainless steel [68,69], high-strength steel [56,70], ACSR steel [71], cast steel [68], carbon steel [72,73,74], and ferritic steel [69] (see Section 2.2 for detailed description of metallic materials). Resistance to erosion and cavitation are also important factors to consider for metallic materials used in the hydroelectric sector [66,75,76]. In addition, due to the biologically active environment of hydroelectric plants, biofouling occurs where invasive species grow and accumulate various bacteria [66]. As a result, the metallic materials need to either have self-cleaning capabilities or can be treated with surface specific process to provide resistance to biofouling. Alterative metallic materials that can also be used for turbines and components are also different Al alloys (including Al-Si alloys) [66,71,77], Ti alloys [66,78], Cu alloys [76,79,80], and Ni alloys [81] (see Table 3).

2.1.3. Biomass Renewable Energy

Biomass is a type of energy based on organic sources (animals and plants), the most common sources being plants, waste, and wood. Biomass energy can be used for electricity, heating or even biofuel [82,83,84].

One type of energy production is thermal conversion, where raw materials (paper or waste) are burned (pyrolysis, gasification, anaerobic decomposition, torrefaction, and co-firing) [83,84,85,86,87]. Most of the research and its emphasis has been on the pyrolysis technique, where the combustion of organic material is carried out in the absence of oxygen under temperatures up to 1173 K [85,88]. Different types of catalysts are used for the pyrolysis of different types of source based biomasses (waste, plants, etc.) [89,90].

Biomass also has great potential to be used in the production of steel for reducing the CO₂ impact of the steel industry, but this is still being researched [84].

The main challenges that are facing metallic materials in the biomass sector are corrosion (pitting corrosion, intergranular corrosion, etc.) [89], high-temperature corrosion, and microbial-assisted corrosion [89,91]. The factors that strongly influence all types of the corrosion are fluid dynamics (including different solutions), gas composition (N₂, CO₂, H₂O, Cl₂, H₂, etc. [90]), deposit composition, and temperature (573–1173 K) [89,90].

Other challenges that metallic materials face in biomass energy are abrasive wear [92], material degradation due to small dusty particles [93], and thermomechanical fatigue (high-temperature fatigue) [94,95,96].

The most common metals used in the biomass energy sector are carbon low-alloyed steels [97,98], austenitic and martensitic stainless steels [97], Al and Ni alloys [97], and some specialized non-ferrous alloys, such as nickel-cobalt-aluminum alloys (NCA). All these metallic materials are used for the main structure and various components in the biomass sector [82,83,84] (see Table 3).

2.1.4. Onshore and Off-Shore Renewable Energy

The highest demand and largest source of renewable energy in the global market is currently onshore and offshore wind [99]. Wind energy is based on the conversion of kinetic energy into rotational energy, which is then converted into electrical energy by means of a shaft [100]. It is important to note that some turbine designs can produce more energy compared to others (height, size, etc.). The other way to produce more energy is the so-called yawing technique, where the wind turbine is shifted to face directly into the wind, which can be freely manipulated on demand [101].

Wind energy can be produced both onshore and offshore. Onshore wind energy encompasses the energy produced by the wind on land that comes from the natural movement of air [102]. The advantages of onshore wind energy are cost-effective energy, faster installation and easier maintenance, and a reduced environmental impact compared to offshore energy [102]. However, there are disadvantages to onshore wind energy, such as lower power generation, inconsistent wind, varying wind speeds, and a greater impact on nature [102].

Offshore wind energy, i.e., offshore wind farms, are located out at sea where the wind (sea breeze) has higher speeds and greater consistency [102]. The advantages of this type or, more accurately, placement of wind farms include more space for placement, reduced environmental impact (although this is debatable due to the more complicated and invasive supporting infrastructure), and their greater efficiency compared to onshore turbines. The disadvantages are their higher maintenance and repair costs and higher construction costs [101]. Offshore wind turbines can be anchored to the seabed, known as fixed-bottom turbines, or they can be placed on floating platforms, known as floating turbines [101,102].

Metallic materials are used in wind energy for the main foundation of the structure, tower, gearbox, turbines, bearings, bolts, controllers, casings, and many other components that require high corrosion and wear resistance, resistance to higher loads and dynamic forces (including fatigue), and higher contact pressures [103,104,105]. The material must also be highly resistant to solid particle erosion caused by dust particles [106].

For these reasons, the most common metallic materials used in the wind renewable energy sector (see Section 2.2) are structural steels, stainless steels (austenitic, duplex, and martensitic steels), electrical steels, cast iron, bearing steels, high Cr steels, low C nitrogen steels, and high Si nodular cast iron [103,104,105]. The most common non-ferrous alloys used in this sector are Cu-based alloys and Al-based alloys [103,104,105] (see Table 3).

2.1.5. Solar Renewable Energy

The next renewable energy sector is solar energy, which is based on the system of harvesting solar radiation (photovoltaic) for electricity or collecting solar thermal energy for heating [107]. The photovoltaic (PV) system for harvesting solar radiation is primarily based on solar panels, where solar radiation is absorbed by PV cells in the solar panel. This then creates electrical charges that move and respond to an internal electrical field in the PV cell, causing electricity to flow [108,109]. In addition, photocatalysis can provide additional support for solar energy production and storage by inhibiting the conversion of collected light after exposure when there is insufficient light incoming to the solar panel (night time, low radiance angle, or weather-related obstruction) [110,111].

Solar energy can also be harnessed using concentrating solar thermal power (also known as CSP), which uses a system of mirrors to reflect, concentrate, and convert solar energy into heat or even electricity [108,109].

The advantage of solar energy is that it is the most abundant natural energy source in the world, and solar energy can provide a solid and increasing output efficiency compared to other sources. It has minimal harmful effects on the environment, although this can also be highly controversial [108].

The future of solar energy is also being explored in the context of hydrogen production, which can later be used as a clean energy carrier [112]. H₂ production from sustainable solar energy is a possible environmentally friendly solution for the increasing demand for energy and fuel as well as energy storage and transportation. The production of H₂ from solar energy would be achieved by solar thermolysis and then by electrolysis from solar–thermally produced H₂ and photovoltaic-based hydrogen production [112]. Such systems will also require new adaptations of metallic materials to adapt to new challenges in terms of maintenance and repair in correlation to the high temperatures, hydrogen presence that can cause hydrogen embrittlement, and corrosive environments [112].

The challenges that metallic materials face in the solar energy sector include the corrosion effect between molten salts and thermal storage materials [113], high-temperature corrosion [114], mechanically assisted corrosion [114], localized corrosion (stress corrosion cracking and flow-accelerated corrosion) [114], creep fatigue [115,116], erosion [117,118], oxidation [117], and mechanical properties [119,120].

The most commonly used ferrous alloys in the solar energy sector are austenitic and martensitic stainless steels, carbon steels, Cr-Mo steels, duplex steels, FeCrAl steel, and ferritic-martensitic steels (see Section 2.2). The most-used non-ferrous alloys are Ni alloys, which represent more than 60% of all non-ferrous alloys used in this sector. The other non-ferrous alloys are Al-based alloys, high-entropy alloys (HEA), and Mg-based alloys [121] (see Section 2.2) (see Table 3).

2.1.6. Geothermal Renewable Energy

Geothermal renewable energy is the last renewable energy herein presented. Geothermal energy is a type of thermal energy that originates from the formation of the planet and from radioactive decay of elements [122,123,124]. The Earth’s internal thermal energy flows to the surface by conduction at the rate of 44.2 TW [125] and by radioactive decay at the rate of 30 TW [123]. The output of geothermal energy can currently meet twice the current energy demand from all energy sources (including non-renewable sources), but the challenge lies in the non-renewable energy flow and its extraction [123]. Harvested geothermal energy can be used for electricity or for heating/cooling.

There are several ways to produce electricity directly from geothermal energy. The first and the oldest type is the dry-steam power plant, which is based on the underground steam source [124,126]. The next type is the flash-steam power plant, where the source is underground water (>180 °C) and steam. This is the most common type of electricity source based on geothermal energy [127,128]. The next type is an enhanced geothermal system, which uses fracturing, drilling, and injection to extract fluid from the subsurface, which is then used for heating and electricity generation [129,130]. The next type is the binary cycle power plant, where water is heated underground (100–180 °C), and then, the hot water circulates above ground and heats a liquid organic compound that has a lower boiling point than water. This compound then produces steam, which flows into the turbine and powers the generator to produce electricity [122,124,131].

In addition to electricity, geothermal energy can be used for heating and cooling. Thermal energy can be extracted from low-temperature geothermal plants, co-produced geothermal energy, or by geothermal heat pump [124]. The first type, low-temperature geothermal energy, is based on the extracting energy from the low-temperature pockets (around 150 °C) located a few meters below the surface [132]. The next type is the so-called co-produced geothermal energy, where heat is produced by water that has been heated [133,134]. The last type is the geothermal heat pump, which is installed at a depth of 3 to 90 m. In this system, the temperature difference between both ends of the system is used to transfer energy by either heating or cooling the upper part of the system [135,136].

One of the advantages of geothermal energy compared to other sources is that it can be harvested almost anywhere in the world. Additionally, the power plants can last for decades with proper maintenance, and because there is no seasonal variation in workload, the system can be adapted to different conditions depending on the application and environment [122,124].

The challenges facing metallic materials used for geothermal energy are corrosion (uniform corrosion, pitting corrosion, crack corrosion, stress corrosion cracking, sulfur-assisted corrosion cracking, and galvanic corrosion) [137,138], hydrogen bubbling [139,140], corrosion fatigue [141,142], fatigue [137,138], erosion [140,142,143,144], wear [145], high pressure [144], high temperature [144], and cavitation and decomposition of alloy structure [138]. The most common metallic materials used in the geothermal energy sector are duplex steels, austenitic and superaustenitic stainless steels, martensitic stainless steels, low-alloyed steels, carbon steels, superferritic steels, Cu-based alloys, Ni-based alloys, and Ti-based alloys (see references in Section 2.2) (see Table 3).

2.2. Cost of Maintenance and Repairs in Future Energy Sector and Search of the Solutions for Lowering the Costs

Cost of Maintenance and Repairs of Future Energy Sector

Maintenance costs vary for each of the described energy sectors due to the different technologies used to produce electricity or heat (see Figure 4a). For advanced nuclear power, projections are based on current nuclear power sources and can be up to 20% of the initial investment [146,147]. For biomass energy, maintenance costs are estimated to be up to 35% due to the unique environment. However, different technologies require different levels of maintenance and servicing of the components, so maintenance costs can also be as low as 15% of the investment over time [148,149].

Maintenance costs for hydropower can vary from 1.5 to 20% depending on the type of technology used and the environment in which the plant is located [150,151]. The next sector is geothermal sector, where the maintenance costs can reach up to 15% of investment over the course of the life span of a typical power plant [152,153]. In the wind energy sector, the onshore or offshore location of the wind farms plays an important role in maintenance and repair costs, which can vary the costs somewhere between 20% and 30% of the total investment [103,154]. The last sector is solar energy, which has the one of the lowest maintenance costs (up to 10%) compared to all sectors. However, it is important to note that this does not reflect how much energy is actually produced by it (see Table 2) [155].

In order to put the maintenance cost in relation to the actual output of the individual energy source, the maintenance costs are normalized by the CF of the individual energy source, which is presented in Figure 4b. As can be seen, the wind energy has a very wide range due to the high maintenance costs that can be associated with specific maintenance issues of a wind farm. The solar, hydroelectric, and biomass sources show an intermediate influence of the maintenance costs, while geothermal and advanced nuclear energy have the lowest influence of maintenance costs in relation to their effective production capacity. This Figure 4 clearly shows that the improvement of materials will play an important role in the development and improved cost reduction of renewable energy power plants.

2.3. Metallic Mateirals Used in the Energy Sector

Table 3 summarizes the various metallic materials used in the advanced nuclear and renewable energy sectors.

Table 3. The list of metallic materials used in advanced nuclear energy and renewable energy sectors.

Energy Sector	Metallic Materials		Application
	Ferrous Alloys	Non-Ferrous Alloys
Advanced Nuclear Energy		Zr-based alloys (Zircoaloy-4 (Zr-Sn-Fe-Cr), Zirlo, and M4 (Nb-based alloy)) [30,31]	Water reactors [30,31]
Advanced Nuclear Energy	Austenitic stainless steel (AISI 316 [31], AISI 316 SS [31], AISI 316L [156], AISI 316 LN [157], AISI 304 [31,157,158,159,160], AISI 304L [161,162,163,164], AISI 304 N [161], AISI 304 SS [31], AISI 347 [31], AISI 308L [157], AISI 308 SS [31], AISI 310 SS [156,165], AISI 309L [31], AISI 309 SS [31], AISI 321 SS [31], AISI 403 [31], AISI 410 [31]; AISI 347 SS [31], AISI 630 [31], AISI D9 [31], HT-UPS [31], AISI 4340)		Water reactors, piping, pressurizer, steam generator, pump, valve casing, plunger, control rod drive mechanism, and core internal structure [30,31]
Advanced Nuclear Energy	Cast-austenitic stainless steel (CF3, CF3A, CF3M, CF8, CF8A, CF8M, AISI 304 SS, AISI 304L SS, AISI 316 SS, AISI 316L SS, AISI 321 SS, AISI 347 SS) [31]		Primary cooling piping system, reactor coolant, auxiliary system, pump casing, valve bodies, and cooling circuit [30,31]
Advanced Nuclear Energy		Ni-based alloys (600 [161,166,167], 690 [161,168], 625 [31], 718 [31], X-750 [31], 800 [31], 800 H [165], 182 [31], 82 [31])	Piping, steam generators, tubes, and working component in high corrosive environments [30,31]
Advanced Nuclear Energy	Low-alloyed steel (Ferritic steels: A105 [169], A106 GrB [31], A182 [169], A216 GrWCB [31], A302 GrB [169,170], A333 Gr6 [31], SA212 B [169]; A508 Gr3 [169,171,172,173,174], A516 Gr70 [31], A533 A [31], A555 B [31], 15Kh2NMFA [175,176], 08Kh18N10T [177]; bainitic steels: 1Cr1Mo0.25v [31], 2Cr1MoGr 22 [31], NiCrMoV [31]; duplex steel: 2507 [159], DSS [178], Fe20Cr9Ni [179]; carbon steel: AISI 1018 [159])		Steamless piping, gorging, casting, bolting, plate, pressure vessels, piping, and feedwater lines; internal stainless steel cladding; steam generator channel heads [30,31]
Advanced Nuclear Energy	*Fusion RAFM steel (Eurofer97 [180,181,182,183], CLAM [31], Infrafm [31], FB2h [31]; Rusfer [31]; 9Cr-2WVTa [31]) Ferritic steel [31]		First wall at reactor, blanket, shield, vacuum vessel, and divertor [30,31]
Advanced Nuclear Energy		Fusion ODS alloy [31] ODS ferritic alloy [31]
Advanced Nuclear Energy		Fusion other alloys SiC composites [31] W and W-based alloys [184,185,186] Cu-based alloys [186] Pb-Al alloy [187,188] HEA [189] Mo-based alloys [184] Nb-based alloy [184] V-based alloy [184] C-fiber components [190]	Structural and insulating application, joints, and filaments [30,31]
Hydroelectrical Renewable Energy	Austenitic stainless steel (AISI 316/AISI 316L [71,191,192,193], AISI 304/AISI 304L [66,79,194,195], AISI 325 [71], ASTM A743 [79,196], ASTM CF20 [71])		Turbines and other components [66]
Hydroelectrical Renewable Energy		Non-ferrous alloys Al-based alloys [66,71,77] Ti-based alloys [66,78] Cu-based alloys [76,79,80] Ni-based alloys ([81,89])
Hydroelectrical Renewable Energy	Martensitic stainless steel (AISI 410 [197,198,199,200] ASTM F6NM [201,202], 13Cr4Ni [202,203,204], AISI 410T [79], AISI 410 [71], AISI 430 [69], ASTM FV520B [205], ASTM CA6NM [196])		Turbines, shear pins, and other components [66]
Hydroelectrical Renewable Energy	Other steels High-strength steel [56,70,206] ACSR [71,77] Stainless steel [72,77,79,199,206] Cast steel [71] Nitronic steel [202] Carbon steel [72] High-speed steel [207] Electric steel [69] Constructional steel [69]		Supporting systems and other components [66]
Biomass Renewable Energy	Carbon steels and low-alloyed steels (2.25Cr-1Mo [97,98], 5Cr-1Mo [98], 9Cr-1Mo [98,208])		Construction of the plant, pumps, pipes, valves, fittings, and digester tanks [82,83,84,105,209]
Biomass Renewable Energy		Non-ferrous alloys Al-based alloys [97,210,211] Ni-based alloys [97,212] NCA [213]	Specialized components [82,83,84]
Biomass Renewable Energy	Stainless steels Austenitic (AISI 304 L [97,214], AISI 316 L [74]) Martensitic (AISI 409 [74], AISI 410 [74], AISI 416 [74])		Construction of the plant, pumps, pipes, valves, fittings, and digester tanks [82,83,84,105,209]
Biomass Renewable Energy	Cr-steels (12-13Cr [208], 13Cr [208], 14.5Cr [208], 16Cr [208], 12Cr-5Ni-2Mo [208], 11.5Cr-2Mo [208])
Wind and Offshore Wind Renewable Energy	Structural steel (S235J2 [215,216], S355J2 [215], S500G1 [215,216], S35G10 [217], S460 [216], S690 [216], S355 [216], S420M3Z [218], S500M3Z [219])		Foundation, tower, gear, and casing of the wind turbines [105]
Wind and Offshore Wind Renewable Energy	Stainless steel Duplex stainless steel (mostly AISI 2205 [220]) Austenitic stainless steel (22Cr25NiWCoCu [221], AISI 304L [222,223,224], AISI 904L [225,226]) Martensitic stainless steel (mainly from 4xx series, such as AISI 440 C [227,228])
Wind and Off-shore Wind Renewable Energy		Non-ferrous alloys Cu-based alloys [229,230] Al-based alloys mostly from 2xxx and 6xxx series [218,231,232]
Wind and Off-shore Wind Renewable Energy	Other types of steel Electric steels [233] Cast iron [234,235] High-Si nodular cast iron (EN GJS500-14 [235], EN GJS450-18 [235], EN GJS600-10 [235]) Bearing steels (mainly AISI 52100 [227,228]) High-Cr steel [227,228] Low-C nitrogen steel [227,228]
Solar Renewable Energy	Austenitic stainless steel (AISI 304 [236,237], AISI 304L [238], AISI 316 [236,239], AISI 316L [237,240,241], AISI 321 [242,243], AISI 347 [242], AISI 347H [236])		Are used for base in solar-thermal panels, pumps, tanks, and heat exchangers [105]
Solar Renewable Energy	Martensitic stainless steel (AISI 420 [244], EN 1.4903 [236], EN 1.4923 [236], AISI T91 [245], VM12 [246])
Solar Renewable Energy		Ni-based alloy (IN 230 [247], IN 600 [248,249], IN 617 [247,249], IN 625 [236,239,247], IN HT700 [250], IN 800H [251], C-276 [252], XH [249], H230 [249], HR120 [249])
Solar Renewable Energy	Other steels Carbon steels [236,253] Cr-Mo steels [254] Duplex steels [255] FeCrAl steels [241] Ferritic-martensitic P91 [248]
Solar Renewable Energy		Other non-ferrous alloys Al-based alloys, mostly from series 7xxx [256,257,258,259] HEA [260,261] Mg-based alloys, mostly Ti-Y combination [262,263]
Geothermal Renewable Energy	Duplex steels (2205 [264], 2507 [264], 2707 [264])		Heat exchangers, filters, pumps, valves, piping, and condensers [209]
Geothermal Renewable Energy	Austenitic and superaustenitic steels (AISI 304 [265], AISI 304L [264], AISI 310S [264], AISI 316L [264,266], AISI 321 [264], UNS N08031 [266], N08020 [267], N08026 [267], N08825 [267], N08330 [267], S31254 [267])		Construction of the plant [267]
Geothermal Renewable Energy	Martensitic stainless steels (mostly from 4xx series, AISI 400 [268,269], AISI 430 [269], AISI 431 [269])
Geothermal Renewable Energy		Non-ferrous alloys Ti-based alloys [138] Ni-based alloys [138,264] Cu-based alloys [269]
Geothermal Renewable Energy	Other steels Low-alloyed steels [266] Carbon steel [270] Superferritic steels (S44627 [267], S44700 [267], S44800 [267])

Table 3. The list of metallic materials used in advanced nuclear energy and renewable energy sectors.

Energy Sector	Metallic Materials		Application
	Ferrous Alloys	Non-Ferrous Alloys
Advanced Nuclear Energy		Zr-based alloys (Zircoaloy-4 (Zr-Sn-Fe-Cr), Zirlo, and M4 (Nb-based alloy)) [30,31]	Water reactors [30,31]
Advanced Nuclear Energy	Austenitic stainless steel (AISI 316 [31], AISI 316 SS [31], AISI 316L [156], AISI 316 LN [157], AISI 304 [31,157,158,159,160], AISI 304L [161,162,163,164], AISI 304 N [161], AISI 304 SS [31], AISI 347 [31], AISI 308L [157], AISI 308 SS [31], AISI 310 SS [156,165], AISI 309L [31], AISI 309 SS [31], AISI 321 SS [31], AISI 403 [31], AISI 410 [31]; AISI 347 SS [31], AISI 630 [31], AISI D9 [31], HT-UPS [31], AISI 4340)		Water reactors, piping, pressurizer, steam generator, pump, valve casing, plunger, control rod drive mechanism, and core internal structure [30,31]
Advanced Nuclear Energy	Cast-austenitic stainless steel (CF3, CF3A, CF3M, CF8, CF8A, CF8M, AISI 304 SS, AISI 304L SS, AISI 316 SS, AISI 316L SS, AISI 321 SS, AISI 347 SS) [31]		Primary cooling piping system, reactor coolant, auxiliary system, pump casing, valve bodies, and cooling circuit [30,31]
Advanced Nuclear Energy		Ni-based alloys (600 [161,166,167], 690 [161,168], 625 [31], 718 [31], X-750 [31], 800 [31], 800 H [165], 182 [31], 82 [31])	Piping, steam generators, tubes, and working component in high corrosive environments [30,31]
Advanced Nuclear Energy	Low-alloyed steel (Ferritic steels: A105 [169], A106 GrB [31], A182 [169], A216 GrWCB [31], A302 GrB [169,170], A333 Gr6 [31], SA212 B [169]; A508 Gr3 [169,171,172,173,174], A516 Gr70 [31], A533 A [31], A555 B [31], 15Kh2NMFA [175,176], 08Kh18N10T [177]; bainitic steels: 1Cr1Mo0.25v [31], 2Cr1MoGr 22 [31], NiCrMoV [31]; duplex steel: 2507 [159], DSS [178], Fe20Cr9Ni [179]; carbon steel: AISI 1018 [159])		Steamless piping, gorging, casting, bolting, plate, pressure vessels, piping, and feedwater lines; internal stainless steel cladding; steam generator channel heads [30,31]
Advanced Nuclear Energy	*Fusion RAFM steel (Eurofer97 [180,181,182,183], CLAM [31], Infrafm [31], FB2h [31]; Rusfer [31]; 9Cr-2WVTa [31]) Ferritic steel [31]		First wall at reactor, blanket, shield, vacuum vessel, and divertor [30,31]
Advanced Nuclear Energy		Fusion ODS alloy [31] ODS ferritic alloy [31]
Advanced Nuclear Energy		Fusion other alloys SiC composites [31] W and W-based alloys [184,185,186] Cu-based alloys [186] Pb-Al alloy [187,188] HEA [189] Mo-based alloys [184] Nb-based alloy [184] V-based alloy [184] C-fiber components [190]	Structural and insulating application, joints, and filaments [30,31]
Hydroelectrical Renewable Energy	Austenitic stainless steel (AISI 316/AISI 316L [71,191,192,193], AISI 304/AISI 304L [66,79,194,195], AISI 325 [71], ASTM A743 [79,196], ASTM CF20 [71])		Turbines and other components [66]
Hydroelectrical Renewable Energy		Non-ferrous alloys Al-based alloys [66,71,77] Ti-based alloys [66,78] Cu-based alloys [76,79,80] Ni-based alloys ([81,89])
Hydroelectrical Renewable Energy	Martensitic stainless steel (AISI 410 [197,198,199,200] ASTM F6NM [201,202], 13Cr4Ni [202,203,204], AISI 410T [79], AISI 410 [71], AISI 430 [69], ASTM FV520B [205], ASTM CA6NM [196])		Turbines, shear pins, and other components [66]
Hydroelectrical Renewable Energy	Other steels High-strength steel [56,70,206] ACSR [71,77] Stainless steel [72,77,79,199,206] Cast steel [71] Nitronic steel [202] Carbon steel [72] High-speed steel [207] Electric steel [69] Constructional steel [69]		Supporting systems and other components [66]
Biomass Renewable Energy	Carbon steels and low-alloyed steels (2.25Cr-1Mo [97,98], 5Cr-1Mo [98], 9Cr-1Mo [98,208])		Construction of the plant, pumps, pipes, valves, fittings, and digester tanks [82,83,84,105,209]
Biomass Renewable Energy		Non-ferrous alloys Al-based alloys [97,210,211] Ni-based alloys [97,212] NCA [213]	Specialized components [82,83,84]
Biomass Renewable Energy	Stainless steels Austenitic (AISI 304 L [97,214], AISI 316 L [74]) Martensitic (AISI 409 [74], AISI 410 [74], AISI 416 [74])		Construction of the plant, pumps, pipes, valves, fittings, and digester tanks [82,83,84,105,209]
Biomass Renewable Energy	Cr-steels (12-13Cr [208], 13Cr [208], 14.5Cr [208], 16Cr [208], 12Cr-5Ni-2Mo [208], 11.5Cr-2Mo [208])
Wind and Offshore Wind Renewable Energy	Structural steel (S235J2 [215,216], S355J2 [215], S500G1 [215,216], S35G10 [217], S460 [216], S690 [216], S355 [216], S420M3Z [218], S500M3Z [219])		Foundation, tower, gear, and casing of the wind turbines [105]
Wind and Offshore Wind Renewable Energy	Stainless steel Duplex stainless steel (mostly AISI 2205 [220]) Austenitic stainless steel (22Cr25NiWCoCu [221], AISI 304L [222,223,224], AISI 904L [225,226]) Martensitic stainless steel (mainly from 4xx series, such as AISI 440 C [227,228])
Wind and Off-shore Wind Renewable Energy		Non-ferrous alloys Cu-based alloys [229,230] Al-based alloys mostly from 2xxx and 6xxx series [218,231,232]
Wind and Off-shore Wind Renewable Energy	Other types of steel Electric steels [233] Cast iron [234,235] High-Si nodular cast iron (EN GJS500-14 [235], EN GJS450-18 [235], EN GJS600-10 [235]) Bearing steels (mainly AISI 52100 [227,228]) High-Cr steel [227,228] Low-C nitrogen steel [227,228]
Solar Renewable Energy	Austenitic stainless steel (AISI 304 [236,237], AISI 304L [238], AISI 316 [236,239], AISI 316L [237,240,241], AISI 321 [242,243], AISI 347 [242], AISI 347H [236])		Are used for base in solar-thermal panels, pumps, tanks, and heat exchangers [105]
Solar Renewable Energy	Martensitic stainless steel (AISI 420 [244], EN 1.4903 [236], EN 1.4923 [236], AISI T91 [245], VM12 [246])
Solar Renewable Energy		Ni-based alloy (IN 230 [247], IN 600 [248,249], IN 617 [247,249], IN 625 [236,239,247], IN HT700 [250], IN 800H [251], C-276 [252], XH [249], H230 [249], HR120 [249])
Solar Renewable Energy	Other steels Carbon steels [236,253] Cr-Mo steels [254] Duplex steels [255] FeCrAl steels [241] Ferritic-martensitic P91 [248]
Solar Renewable Energy		Other non-ferrous alloys Al-based alloys, mostly from series 7xxx [256,257,258,259] HEA [260,261] Mg-based alloys, mostly Ti-Y combination [262,263]
Geothermal Renewable Energy	Duplex steels (2205 [264], 2507 [264], 2707 [264])		Heat exchangers, filters, pumps, valves, piping, and condensers [209]
Geothermal Renewable Energy	Austenitic and superaustenitic steels (AISI 304 [265], AISI 304L [264], AISI 310S [264], AISI 316L [264,266], AISI 321 [264], UNS N08031 [266], N08020 [267], N08026 [267], N08825 [267], N08330 [267], S31254 [267])		Construction of the plant [267]
Geothermal Renewable Energy	Martensitic stainless steels (mostly from 4xx series, AISI 400 [268,269], AISI 430 [269], AISI 431 [269])
Geothermal Renewable Energy		Non-ferrous alloys Ti-based alloys [138] Ni-based alloys [138,264] Cu-based alloys [269]
Geothermal Renewable Energy	Other steels Low-alloyed steels [266] Carbon steel [270] Superferritic steels (S44627 [267], S44700 [267], S44800 [267])

2.4. Solutions for Lowering the Costs of Maintenance and Prolonging the Component Durability

As mentioned before, the major maintenance costs are the repair and replacement of materials used in the power plants. It is important to extend the service life of materials and to design materials and components that can be easily and cost-effectively replaced or repaired. A common solution to address these challenges for metallic materials used in different energy sectors is mostly by cathodic corrosion protection in combination with coatings [271,272]. Cathodic protection of the metallic materials is an electrochemical technique to protect and control corrosion of the material [273,274].

Coatings, especially organic coatings, can also be applied without the combination of cathodic protection, which is mainly used for materials that are immersed in water [275,276,277]; often, a combined use is chosen. Cathodic protection can be also achieved by some metallic coatings, such as zinc alloy coatings or by a combination of metallic and organic coatings (see, for example, [278,279]). However, coatings and linings can also be applied alone as a passive corrosion protection or in a so-called duplex system, where both coatings and linings are used simultaneously as a multilayer system [218,280].

Other options for surface treatment to improve resistance to environmental factors and prolong component life include surface treatments such as laser treatment, electron beam, induction heating, plasma nitriding, and selection of the appropriate heat treatment to achieve the desired microstructure [281,282,283,284]. While coatings and surface treatments can be a good technique to overcome many challenges, certain applications that require specialized metallic materials can make this technique very limited. This is particularly an issue when the application is under harsh conditions such as simultaneous high temperatures and high loads, which require either metallic materials that are difficult to coat or specialized coatings and surface treatments that can be very expensive and have limited-service life due to combined wear, erosion, and corrosion effects [285,286].

This requires a holistic approach to material treatment that is not limited to the surface of the material. A common approach for metallic materials is to use conventional heat treatments to tailor individual properties. However, conventional heat treatments typically involve a trade-off where certain properties are improved at the expense of others, typically resulting in metallic materials with high strength but low fatigue and corrosion properties and vice versa [287,288].

As a result, more sophisticated and complex processing and treatment of metallic materials are being explored to overcome such trade-offs. One of the new options, which has also been tested in the steel industry, is the use of cryogenic treatment, which can improve various properties of metallic materials, including corrosion performance, without adding a coating to the surface [289,290,291,292]. A more detailed presentation and explanation of cryogenic treatment and its application to surface and corrosion properties is described in Section 3.

3. Cryogenic Treatments in Energy Sector

The technology of cryogenic treatment (CT) has made tremendous progress in the last 10 years in its application on metallic materials in various sectors ranging from medicine, aerospace, robotics, materials science (including the steel industry), nanotechnology, and mining to even more specialized disciplines [289,293]. The technique has evolved from the first attempts to treat materials at cryogenic temperatures in the 19th century by James Dewar and Karol Olszewski using liquefied gases (nitrogen and hydrogen). Later, the first real scientific observation and documentation of CT was made by NASA (National Aeronautics and Space Administration) in the mid-20th century, when they observed changes in the properties of materials used in space shuttles returning from space [289,293]. The selected aluminum components were harder and more wear-resistant after returning to Earth than they were before the space mission [289,293]. Since then, CT has been slowly adapted with different techniques and applications to metallic materials in order to improve macroscopic and microscopic properties. In the literature, CT can also be called sub-zero treatment, ultra-low temperature processing, or cryo-processing [289,293,294,295,296].

The application of CT in the energy sector can be of particular interest due to the variety of metallic materials that are used in extreme conditions (high-temperature and high-pressure environments, highly corrosive environments, highly abrasive environments, etc.), as discussed in Section 2. However, the application of CT in the energy sector is still in its infancy, mainly due to the slow introduction and development of this treatment scheme and the limited research focus on applications in the energy sector.

3.1. Mechanisms of Cryogenic Treatments

The mechanisms of CTs are based on the type, which is defined by the selected temperature regime for the CT (Figure 5). CT is usually applied after the material has been hardened and quenched and before being tempered, usually for 24 h at a predetermined temperature [289,290,291,292,293,297,298]. The most common and the one with the longest tradition is the conventional cryogenic treatment (CCT), where temperatures as low as 193 K are used [299].

The reason for CCT being the most-used type in the past was the easy availability of media to which the material is exposed, namely dry ice (solid CO₂) [293,299]. In the past, it was also believed that temperatures as low as 193 K were sufficient to transform all the retained austenite (RA) in ferrous alloys to martensite, thereby increasing wear resistance and fatigue strength [293,299]. The transformation of RA to martensite, particularly in steels, was one of the key properties for which CTs were commonly applied, which also propagated the initial research on CT [293,299]. Unfortunately, the negative results of the first experiments with CCT led many companies to abandon the application and development of this treatment (1940s–1950s) [293,299]. This was mainly due to a misunderstanding of the martensitic transformation and its temperatures as well as simplistic and inconsistent treatment procedures [293,299]. It was not until years later, after NASA observations and detailed documentation of the changes at lower temperatures, that the next two types of CT were developed and tested for materials science applications: shallow (SCT) and deep cryogenic treatment (DCT) [293,299].

Shallow cryogenic treatment is defined between 193 K and 113 K. During SCT, more than 50% of the RA is converted to martensite for generally any ferrous alloy that has instable austenite formation during quenching, causing a change in mechanical properties (increased hardness), size reduction of carbides, and increased precipitation of carbides [300,301]. With the positive results of SCT, the research on CT blossomed and led to further research at even lower temperatures, resulting in the development of deep cryogenic treatment.

Temperatures for deep cryogenic treatment are below 113 K and typically go as low as 4 K, which is the temperature of liquid helium. However, the most-used temperature is 77 K, the temperature of liquid nitrogen, which is the most-used medium in DCT due to abundancy of the media and economic reasons. With DCT, for ferrous alloys, most of the RA is converted to martensite (>90%), the precipitation of carbides is increased, grain refinement and precipitation of nanocarbides occurs, and changes to residual stresses are formed [302]. Special mention should be made to a specific type of DCT, the multi-stage deep cryogenic treatment (MCT), where the DCT treatment of the material consists of rapid changes between SCT and DCT temperatures for a predefined time and number of cycles to manipulate predefined properties [303]. DCT performance is influenced by the selected cooling temperature, cooling–warming rate, time the material is exposed to DCT, type of metallic material (ferrous/non-ferrous alloy or type of steel), chemical composition of the metallic material, hardening process, tempering temperature, and also the microstructural phenomena present within the microstructure (such as transformation-induced plasticity (TRIP), austenite reversion transformation (ART), and twinning-induced plasticity (TWIP)) [304,305,306,307,308,309,310].

All types of CT alter the bulk and surface properties of metallic materials. The bulk properties affected by CT are mechanical properties ((micro)hardness [311,312,313,314], toughness [311,315,316,317], strength [318,319,320], and fatigue [307,321,322]) and magnetism [304,323]. The surface properties affected by CT are corrosion resistance [324,325,326,327,328,329,330,331,332], wear resistance [321,333,334,335], roughness [336], and oxide formation [324,325,326,336].

Bulk properties and, to some extent, selected surface properties have been studied in more detail than others. There are still many unknowns and great potential in surface properties and corrosion resistance, which is also the focus of the following section of this review.

3.2. Energy Sector and Position of Cryogenic Treatments

Cryogenic processing has a great potential in the energy sector due to the use of different materials, from metallic to non-metallic. The application of CT, especially for metallic materials, has a great potential because it improves the properties of metallic materials needed in different energy sectors, from corrosion and wear resistance to mechanical properties and surface modifications [337,338]. At the same time, it does not require the additional application of any other coating treatment to improve the properties (see Section 3.3.1).

However, the application of CT in this sector has not been widespread due to the lack of known test methods and quantification and qualification methods. Only a few attempts have been made to provide systematic guidelines for standards and application of CT for metallic materials [293,306,309,339,340,341]. An additional obstacle was that in the past, there were no large capacity tanks, and no providers of these services or systems were available on an industrial scale, but this is now changing and, in some cases, improving with the establishment of CT-specialized companies, communities, and even patents [294,295,296,332,342,343,344,345,346,347]. CT was also not well transferred to other disciplines, as CT was mainly reserved and developed for improving tools. The research was (and still is) mainly focused on tool steels, such as high-speed steels, hot work tool steels, and cold work tool steels, where the emphasis is on mechanical and wear properties [334,348,349,350,351].

As a result, the majority of other types of steels and alloys have been left out of the focus. There is some limited research on non-ferrous alloys, but even these are mostly related to aluminum alloys used or related to the tooling industry. The study of non-ferrous alloys (Al-, Ni-, and Ti-based alloys) showed the improvement of mechanical properties [352,353,354,355,356,357,358] such as microhardness [352,354,359,360,361], fatigue [352], fracture toughness [362,363], impact toughness [352], and tensile strength [352,354].

The following sections present metallic materials that have been tested by cryogenic treatment, the results of which have the potential to be used in the energy sector.

3.3. Effect of Cryogenic Treatments on Surface, Interface, and Corrosion Properties of Metallic Mateirals Used in the Energy Sector

3.3.1. Metallic Materials Being Tested for the Use in the Energy Sector

There are many metallic materials (ferrous and non-ferrous alloys) that are suitable for use in the energy sector that have already been tested through various cryogenic treatments, and studies have resulted in changes in the microstructure of metallic materials, resulting in changes in the properties of the material. The following ferrous and non-ferrous alloys are used in the following sectors (

Table 4. The list of ferrous alloys that were cryogenically treated and have the potential for use in the energy sector.

Ferrous Alloys	Grades of Steel	Tested Properties	Possibilities of Application in Selected Energy Sector
Austenitic stainless steel	AISI 304 [364,365,366,367,368,369,370], AISI 304L [308,319,371,372,373,374], AISI 304LN [374], AISI 316 [374,375,376,377,378,379,380], AISI 316L [192,341,348,381,382,383,384], AISI 316LN [374,385], AISI 321 [386,387], AISI 347 [388,389]	Hardness, microhardness, wear (abrasive wear), fracture toughness, impact toughness, compressive strength, tensile strength, yield strength, elongation, friction, erosion, strain-hardening exponent, surface roughness, machining of steel, fatigue, residual stress, surface chemistry, and oxidation	In all energy sectors
Martensitic stainless steel	AISI 410 [390], AISI 420 [349,390,391,392], AISI 420 MOD [392], AISI 430 [393,394], AISI 431 [304,309,395], AISI 440C [366], AISI P91 [396], 10Cr13Co13Mo5NiW1VE [397], 13Cr4NiMo [315], 10Cr [398].	Yield strength, elongation, tensile strength, wear, hardness, impact toughness, fracture toughness, magnetism tribocorrosion, electrochemistry, and corrosion resistance (also stress corrosion cracking)	In all energy sectors.
Duplex steels	AISI 2205 [399,400], AISI 2507 [401,402,403,404]	Hardness, wear, machinability, residual stress, and corrosion resistance	Mostly in wind and solar energy
Carbon steels	IS 2062 [405], AISI 1045 [406,407,408,409,410,411,412], AISI 1018 [413]	Hardness, wear, surface roughness, tensile strength, yield strength, ultimate tensile strength, elongation, and residual stress	Steels can be used in hydroelectrical, biomass, solar, and geothermal energy
Other steels	Nitronic steels 40 [414], 50 [415] High-strength steels ASTM A36 [416] Cast steels ASTM A743 [417], SAE J431 G10 [418] ACSR [419] Bearing steel AISI 52100 [326,328,420,421,422] Low-alloyed steels SAE 1008 [423], AISI 4340 [424], AISI 4140 [424] Structural steel S235 [425], S355 [426,427], S460 [428]	Residual tress, hardness, friction, wear, fatigue, impact toughness, corrosion resistance, and machinability	In all energy sectors

).

Table 5. The list of non-ferrous alloys that were cryogenically treated and have potential for use in the energy sector.

Non-Ferrous Alloys	Grades of Alloys	Tested Properties	Possibilities of Application in Selected Energy Sector
Al-based alloy	2xxx series: 2024 [354,429] 3xxx series: A356 [430,431], A390 [432] 6xxx series: 6026 [352,433,434,435], 6061 [353,436,437], 6063 [362] 7xxx series: 7075 [330,331,361,438,439,440,441,442]	Hardness, wear (abrasion), corrosion resistance, tensile strength, machinability, fatigue, strain-hardening coefficient, residual stress, fracture toughness, and corrosion resistance	Mostly in hydroelectrical, biomass, wind, and solar energy
Ni-based alloy	Inconel: 200 [443], 600 [444,445], 617 [446], 625 [447,448,449], 690 [450], 800 [451], 800H [452,453,454] Hastelloy C276 [455], C22 [456,457], X [458]	Fatigue, surface roughness, machinability, durability, impact toughness, microhardness, and tensile strength	In all energy sectors
Other alloys	HEA [459] W-based alloys [460] Cu-based alloys [461] Ti-based alloys Ti6Al4V [451,462]	Microhardness, compressive strength, and plasticity	Mostly in advanced nuclear power (fusion), geothermal, and solar energy

presents the non-ferrous alloys that have been CT-treated and have potential in the current and future energy sectors.

3.4. Effect of Cryogenic Treatments on Metallic Materials Potenitally Used in the Energy Sector

The surface properties that are the focus of this review and that also need more attention in order to carry out more research on them are corrosion resistance and oxide formation, while wear resistance and roughness have been observed and researched by many studies in the cryogenic community (see Section 3).

3.4.1. Oxide Formation

Oxide formation is one of the properties that is seldomly researched and not fully understood in CT. The fact is that most of the studies focus on the corrosion resistance and its improvement by CT, and not many studies strive for deeper understanding of the origin of altered corrosion resistance by CT. A major contribution is provided by passive layers and oxide formation (corrosion products) that can be manipulated by CT and CT-induced changes to the bulk properties of the treated material. The influence of CT on oxide formation has been demonstrated for bearing, high-speed, and cold work tool steels [324,325,326,336]. The oxidation dynamics after the application of CT was mainly studied by Jovičević-Klug et al., where the observations showed a different development of oxides compared to conventional heat treatment (CHT).

Jovičević-Klug et al. 2021 [336] suggested that the chemical composition of the oxide formation directly corresponds to the higher number of precipitates and the higher surface-to-volume ratio of the carbides. Furthermore, the study indicates that the reduced amount of carbide clusters after CT could be directly correlated with the passivation layer and the oxidation state of the surface and the corresponding corrosion products.

In the next study, Jovičević-Klug et al. 2021 [325] suggested that the Cr oxide layer is thicker on the cryogenically treated samples compared to the CHT samples. These observations also suggest that due to the formation of the Cr-oxide-passivation layer on the CT sample, there is no microscopic-related stress corrosion cracking of the matrix, which in turn, combined with the thicker passivation layer, reduces corrosion propagation.

The next factor observed in relation to CT was the formation of Fe oxides. The study by Jovičević-Klug et al. [326] suggested that Fe-oxides form different layers compared to the CT sample, which is attributed to the local excessive corrosion damage in the CHT sample.

The same researchers, Jovičević-Klug et al. 2022 [324], also observed the different layering of the oxides in the samples. The results of ToF-SIMS provided the novel insight that nitrogen from CT is present in greater amounts in the CT samples, which then influences the complex oxide formations (corrosion products), which ultimately influence the corrosion resistance. Nitrogen acts as an exalter for the formation of green rust, which then acts as a precursor for the formation of the next layer (magnetite). As a result, corrosion propagation is greatly retarded due to the higher density and stability of magnetite. The same study also confirmed that the CT-induced passive film is more stable than its CHT counterpart. As a result, the CT-treated sample showed lower corrosion and wear loss, which was also confirmed in extreme environments (elevated temperatures and vibrations).

The above examples show that there is a need for research on oxide formation as the basis for successful tailoring of corrosion resistance and prolonged component life of treated materials. The studies only focused on tool and bearing steels, which means that other steels such as high-Cr steels, stainless steels, duplex steels, and non-ferrous alloys are still potential research areas with great opportunities for the application of CT to manipulate oxide formation and modify corrosion resistance. To date, no similar studies or research have been conducted or found for non-ferrous alloys. Section 3.4.2 discusses corrosion resistance.

3.4.2. Corrosion Resistance

The influence of CT on corrosion resistance has not only been investigated in relation to tool steels, but many studies have also tested other ferrous (bearing steels and stainless steels) and non-ferrous (mostly Al-based alloys) alloys. The first part focuses on the corrosion testing of ferrous alloys, while the second part focuses on the non-ferrous alloys in relation to CT.

Corrosion Resistance of Ferrous Alloys

The studies on tool steels showed that corrosion resistance is influenced by CT [329,392,463,464,465,466]. The corrosion resistance of bearing and tool steels can be improved by up to 65% in an alkaline environment, with the improvement depending on the steel type and heat treatment strategy [326]. This was also observed by Senthilkumar 2014 [327], who found that in alkali conditions, CT improves corrosion resistance, which was postulated to be a result of formation of more stable passive film. Furthermore, in extreme alkaline environments, such as elevated temperatures and vibrations, the CT-treated samples (tool steels) suggested improvement of corrosion resistance by 90% in the study of Jovičević-Klug et al. 2022 [324]. Also, the study by Jovičević-Klug et al. 2021 [326] showed that in an alkaline environment, the formation of pits is modified by CT (for tool and bearing steels). The study showed that pits in CT specimens expand only in the exposed upper part and decrease continuously deeper into the material. It was suggested that this is due to the confinement of the corrosion attack to the grain boundaries and the exposure of the pit opening to the oxidative media, which is limited by the change in orientation of the crack with respect to the sample surface. In addition, the 2021 study by Jovičević-Klug et al. [325] also showed that in the alkaline environment, the CT samples did not show any stress corrosion cracking of the passivation layer, and the presence of Mo in the steel allowed the continuous growth of the protective Cr oxide layer, which reduced the formation and growth of pits. The results show that CT samples have a 3× slower corrosion rate of pitting corrosion, which can be directly correlated to the slower material degradation and prolonged functionality of the metallic material.

Only a few studies have been conducted on stainless steels and a few other types of steels that are more commonly used in energy sector applications. The studies showed different results of CT on the corrosion resistance of steels used in the energy sector. A study by Wang et al. 2020 [467] showed that there is an increase in corrosion resistance for high-strength stainless steel. On contrary, a study by Baldissera and Delprete 2010 [468] postulated that CT has no effect on austenitic stainless steel. Another study by Cai et al. 2016 [469] indicated that for austenitic stainless steel, CT could improve corrosion resistance, which is suggested through Cr-carbide precipitation at the austenite grain boundary, which then reduces the intergranular corrosion. For martensitic stainless steels, CT has been shown to improve corrosion resistance in correlation with both the general and pitting corrosion, as was shown by Ramos et al. 2017 [366]. Another explanation for the higher pitting corrosion potential was proposed by He et al. 2021 [470], in which pitting corrosion was reduced by increased carbide precipitation and Si segregation at the interface boundaries between M₂₃C₆ and martensite in the matrix. For structural steels, a 95% improvement in corrosion resistance was determined by Ramesh et al. 2019 [392], which is suggested to be a consequence of uniform and homogenous carbide precipitation and microstructure modification.

The above literature review shows that there has been some research on corrosion enhancement with CT but only on a limited selection of ferrous alloys. Furthermore, the review shows that there is a great need for research on the corrosion resistance of ferrous alloys used in the energy sector in combination with CT. Such research could open up new avenues and applications for CT to improve corrosion resistance alone or in combination with coatings, which could further expand the energy sector from both an economic and sustainable point of view.

Corrosion Resistance of Non-Ferrous Alloys

The corrosion resistance of CT-treated non-ferrous alloys has been mainly focused on the Al-based alloys of the 2xxx [471], 5xxx [355], and 7xxx [330,331,440] series. The study by Cabeza et al. 2015 [471] on Al-based alloys from the 2xxx series suggested that CT improves the resistance to stress corrosion cracking due to changes in compressive residual stresses. Another study by Aamir et al. 2016 [355] showed that for the 5xxx Al-alloy, the corrosion resistance is increased due to the minimization of dislocation densities and noncontinuous distribution of the β-phase. From the 7xxx series, the tested representative was the 7075 Al-alloy. A study by Ma et al. 2021 [440] showed an improvement in corrosion resistance after the application of CT, which was attributed to the increased precipitation of the η′ phase. They postulated that the grain boundary from the η′ resulted in short chains of carbides, which then blocked corrosion channeling, thus enhancing the corrosion resistance of the alloy. Similar observations were also made by Su et al. 2021 [331]. Ma et al., from their study in 2022 [330], additionally showed that the optimized combination of aging and CT can influence the rate of the corrosion improvement when CT is applied.

Compared to ferrous alloys, research on non-ferrous alloys is also considered to be lacking and is mostly focused on specific alloys, mainly aluminum alloys. The review clearly confirms the lack of research on non-ferrous alloys, which have a great potential for use in the future energy sector. The lack of research can be particularly evident in the case of Ni alloys and corrosion resistance in combination with CT, which are one of the main non-ferrous alloys used in different energy sectors due to their versatility. Other non-ferrous alloys such as Cu-based, Mg-based, V-based, W-based, etc., are also completely excluded from the studies, and therefore, this could be another potentially interesting niche to study in more depth the influence of CT on these alloys, which could be applied to the future energy sector. Furthermore, in most cases, the reasons for improved or sometimes reduced corrosion performance are based on speculation. Fundamental research is needed to elucidate the reasons for the effects of CT on corrosion performance.

4. Economic and Ecological Aspects and Future Role of Cryogenic Treatment in Future Energy Sector

While it is clear that the application of CT to ferrous and non-ferrous alloys has great potential due to its versatile effect on bulk and surface properties, the next question that comes to mind is the economic and environmental aspects of its application. CT uses mostly liquid N₂ as a coolant media, which is highly considered as a viable option for the conventional heat treatment of metallic materials. After CT treatment, LN₂ evaporates to become nitrogen gas (N₂) and becomes part of the air (78% of air consists of N₂). It leaves no harmful residue to the industry and the environment and no health hazards compared to other processing/machining techniques [369]. Therefore, it is considered as a recycling and environmentally friendly approach to improve the materials. Furthermore, it is suggested in some works, see, e.g., Hong and Broomer [369] and Dosset [472], showing a cost reduction of about 50%; however, more insight into the economic advantages is expected in the near future, which is expected to cause an increasing interest in the energy sector. In conclusion, CT shows great potential to improve the corrosion performance of materials, and the process is environmentally unharmful.

Based on all these facts, CT has a bright future in the future energy sector, where advanced knowledge of more economical and ecological impacts on the environment is being considered. Not only that, but with the trend of diminishing natural resources and more recycling options, CT also has an answer, as no additional treatment is required. CT is also expected to play an important role in emerging materials for energy applications and storage (HEA and catalytic materials), which have not yet been explored and applied.

5. Conclusions

This review first provides an overview of the metallic materials used in the energy sector and then of the application of CT on metallic materials used in different energy sectors. In addition, this review also presents a synopsis of the current work and results on surface properties and corrosion, with critical comments to provide a future possibility for metallic materials in relation to CT and oxide formation and corrosion resistance. The review also highlights which materials should be prioritized for CT testing due to lack of research but are of high importance for applications (or are already in use) in different energy sectors.

The main conclusions of the study can be summarized as follows:

• The energy sector has a great demand for the improvement of metallic materials;
• Available green and cost-effective CT technology has been proven to effectively improve the bulk and surface properties of metallic materials;
• CT improves corrosion resistance by up to 90% depending on metallic materials and environmental conditions;
• CT also produces a unique sequence of oxide formation that effectively influences the improved corrosion resistance of cryogenically treated metallic materials;
• The result of CT is a reduction in material degradation and a possible 3-fold increase in the service life of the treated metallic material;
• Further detailed and systematic investigation of the effectiveness of CT is required, using both experiments and modeling of both ferrous and non-ferrous alloys. Combined with detailed microstructural investigations, the mechanisms responsible for changes in metallic material properties can be clearly identified, and standards for the application of CT in the energy sector can be established.