Thermal Spray Processes in Concentrating Solar Power Technology

Abstract

Solar power is a sustainable and affordable source of energy, and has gained interest from academies, companies, and government institutions as a potential and efficient alternative for next-generation energy production. To promote the penetration of solar power in the energy market, solar-generated electricity needs to be cost-competitive with fossil fuels and other renewables. Development of new materials for solar absorbers able to collect a higher fraction of solar radiation and work at higher temperatures, together with improved design of thermal energy storage systems and components, have been addressed as strategies for increasing the efficiency of solar power plants, offering dispatchable energy and adapting the electricity production to the curve demand. Manufacturing of concentrating solar power components greatly affects their performance and durability and, thus, the global efficiency of solar power plants. The development of viable, sustainable, and efficient manufacturing procedures and processes became key aspects within the breakthrough strategies of solar power technologies. This paper provides an outlook on the application of thermal spray processes to produce selective solar absorbing coatings in solar tower receivers and high-temperature protective barriers as strategies to mitigate the corrosion of concentrating solar power and thermal energy storage components when exposed to aggressive media during service life.

Full text

Metals 2021, 11, 1377. https://doi.org/10.3390/met11091377 www.mdpi.com/journal/metals
Review
Thermal Spray Processes in Concentrating Solar Power
Technology
Felice Rubino 1,*, Pedro Poza 1, Germana Pasquino 2 and Pierpaolo Carlone 3
1 Department of Chemical, Energetic and Mechanical Technology, University Rey Juan Carlos, Campus of
Móstoles, Calle Tulipán s/n, 28933 Móstoles, Spain; pedro.poza@urjc.es
2 Universitas Mercatorum, Piazza Mattei, 10, 00186 Rome, Italy; germana.pasquino@unimercatorum.it
3 Department of Industrial Engineering, University of Salerno, Via Giovanni Paolo II, 132, 84084 Fisciano,
Italy; pcarlone@unisa.it
* Correspondence: felice.rubino@urjc.es; Tel.: +34-96-488-8142
Abstract: Solar power is a sustainable and affordable source of energy, and has gained interest from
academies, companies, and government institutions as a potential and efficient alternative for next-
generation energy production. To promote the penetration of solar power in the energy market,
solar-generated electricity needs to be cost-competitive with fossil fuels and other renewables. De-
velopment of new materials for solar absorbers able to collect a higher fraction of solar radiation
and work at higher temperatures, together with improved design of thermal energy storage systems
and components, have been addressed as strategies for increasing the efficiency of solar power
plants, offering dispatchable energy and adapting the electricity production to the curve demand.
Manufacturing of concentrating solar power components greatly affects their performance and du-
rability and, thus, the global efficiency of solar power plants. The development of viable, sustaina-
ble, and efficient manufacturing procedures and processes became key aspects within the break-
through strategies of solar power technologies. This paper provides an outlook on the application
of thermal spray processes to produce selective solar absorbing coatings in solar tower receivers
and high-temperature protective barriers as strategies to mitigate the corrosion of concentrating
solar power and thermal energy storage components when exposed to aggressive media during
service life.
Keywords: clean energy; concentrating solar power; selective solar absorber; sustainable manufac-
turing technologies; thermal energy storage; thermal spray processes
1. Introduction
After photovoltaic (PV), concentrated solar power (CSP) is the second class of solar
technology adopted worldwide to exploit solar energy to produce electricity. CSP plants
use mirrors to reflect and focus solar radiation to heat a thermal fluid [1], which can be
gas, solid particles or fluid steam [2], oil, or molten salts [3–5]), that is used to drive tur-
bines in a power cycle and generate electricity [6,7] (see Figure 1). The thermal fluid can
be directly integrated into the power cycle and expand in the turbines, or used to transfer
the accumulated heat to a separate fluid [6,7]. Depending on the technical solution
adopted for the plant, solar radiation is reflected by mirrors and focused on a pipe (in the
case of a parabolic design, as shown in Figure 1, or linear Fresnel reflector) through which
flows the heated fluid or directed to a single central receiver (the central solar power tower
design or the parabolic dish system) [8]. Compared to the photovoltaic (PV) system, CSP
technology can only be used on a large scale to make energy production economic, due to
their high capital costs, and does not have PV’s modularity. CSP comes, however, with
very intriguing benefits. First, in contrast to PV technology, CSP can incorporate an en-
ergy storage system, which allows for the production of electricity even during night
Citation:
Rubino, F.; Poza, P.;
Pasquino
, G.; Carlone, P. Thermal
Spray Processes in Concentrating
Solar Power Technology.
Metals
2021
, 11, 1377. https://doi.org/
10.3390/met11091377
Academic Editor:
Robert B.
Heimann
Received:
11 June 2021
Accepted:
27 August 2021
Published:
31 August 2021
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Metals 2021, 11, 1377 2 of 30
hours or with no sunlight. Second, being similar to a conventional thermal power plant,
CSP can be coupled with fossil fuel generators to build hybrid plants, in particular with
natural gas power systems [6,9,10].
Figure 1. Scheme of the working principle of a CSP plant equipped with parabolic trough collector
receiver and connected to a power cycle system, adapted from [11].
In turn, however, it suffers from the same thermodynamic limitations and its heat–
electricity conversion efficiency depends on the working temperatures. Until now, indeed,
the heat collected from the sun is not at the same level as the fossil-fuel plants, therefore
the efficiency of CSP plants is lower than that of the conventional thermal plants. Figure
2 below depicts the energy flow and the losses in a CSP plant [12], from the solar radiation
collected by the receiver to the delivery of electricity in the grid. By observing the images,
note that less than half of the total incident energy, approximately 40%, is transferred to
the thermal fluid operating in the power cycle, which results from energy losses due to
the CSP systems associated with the mirrors and the receiver. After that, further reduction
is due to the thermodynamic of the power cycle, usually Rankine, Brighton, or Sterling
cycles; therefore only 40% of the gathered heat is converted into electricity, which depends
on the lower temperatures achievable within the concentrating solar system. It reduces
the global efficiency of the plant to 16% of the total energy available from solar radiation.
In that scenario, increasing the operating temperature of heat thermal fluid (HTF) circu-
lating within the receiver (above 600 °C) and, therefore, the temperatures of the power
cycles could be beneficial to the efficiency of the heat-to-electricity conversion [13].
Figure 2. The efficiency of a typical CSP plant [12].
By analyzing the energy flows depicted in Figure 2, it can be argued that the main
losses are attributed to the capability of the CSP system to collect the energy brought by
the solar radiation; meanwhile, the efficiency of the turbines in CSP plants and conven-
tional thermal plants are comparable. It suggests that losses in solar-to-heat conversion
can be reduced by improving the mirror system and the central receivers. Research in
novel designs of the CSP system components and advanced materials can lead to signifi-
cant improvements in the efficiency of solar plants. In the last few decades, indeed, CSP

Metals 2021, 11, 1377 3 of 30
technologies have been the subject of intensive studies and research activities aiming to
improve the efficiency of the plants and make solar-generated electricity more cost-effec-
tive, as demonstrated by patent documents published in the very recent years [14]. While
these efforts were directed mainly toward different aspects of the CSP, the improvement
in the exploitation of energy from solar radiation and in dispatchability of electricity pro-
duced by CSP plants have encountered particular attention [14]. In the former case, one
of the goals was to increase the amount of energy that can be collected from the sun’s rays
by the receivers, which concurs to almost 50% of the energy losses in CSP plants [11]. In
the latter case, the strategies proposed by designers involved the adoption of a working
fluid operating at higher temperatures to increase the efficiency of the energy storage sys-
tems and the use of high-performance materials to increase the durability and lifetime of
the system components.
Thermal spraying (TS) has encountered a growing interest by researchers and com-
panies active in the solar energy field, which explored the integration of TS technologies
in the production of solar absorber coatings and corrosion barriers within the components
of CSP and thermal energy storage (TES) systems, as an efficient and cost-effective alter-
native to currently employed methodologies. The purpose of this work is to gather the
contributions from published literature and provide an overview on the state of the art
concerning the application of thermal spray technologies within the CSP technologies.
Emphasis is on solar power systems, which can particularly benefit from the properties of
thermal sprayed coatings. Research organizations and companies directly involved in the
development of the CSP systems together with experts working in the surface modifica-
tion sector can be a potential audience of the present work, allowing them to explore novel
solutions and identify new markets in their respective fields of interest. The remainder of
the paper is organized as follows: Section 2 describes the literature on the solar selective
absorber and the application of thermal spray process to manufacture absorbent coatings.
Section 3 reviews the literature on the corrosion mitigation strategies adopted by design-
ers and producers, with emphasis on the protective barrier manufactured by thermal
spraying.
2. Thermal Spray Processes for Manufacturing Solar Selective Coatings
2.1. Introduction
As described previously, concentrating solar power technologies rely on reflectors
(or mirrors) to collect the solar radiation and focus it at a certain level of concentration
onto a line- or point-receiver, also called the absorber, which converts the sun’s rays to
heat. Therefore, the working principles of the active components in a CSP system are
based on the reflection, absorbance, and transmittance of light. These phenomena are in-
fluenced by many factors: the physical state of the materials (gas, solid, or liquid), physical
and chemical properties, surface features and texture, light wavelength, and incidence
angle.
The efficiency of receivers in a CSP plant depends on the amount of energy they can
absorb from solar radiation. Maximizing the absorption of solar energy and reducing the
heat losses from the receiver to the environment (i.e., the thermal emittance) are the key
factors to increase the solar-to-heat efficiency, which constitutes almost 40% of the energy
balance in a CSP plant (Figure 2. The efficiency of a typical CSP plant) [12]. It means that
receivers should have optimized optical properties and microstructure stability in the op-
erative temperature range. In particular, the selective absorbers must be characterized by
high absorptivity in the solar spectrum (i.e., wavelength of 250–2500 nm), to maximize the
solar energy captured, and low emissivity (also referred to as thermal emittance, ε) in the
infrared spectrum (i.e., wavelength of 1.5–2.5 µm) at the operative temperatures to mini-
mize the heat losses. Values of absorptance (α) above 0.95 and ε less than 0.05 are highly
desirable but challenging to attain. The structural materials, e.g., steels and aluminum al-
loys, usually employed in the construction of the elements that compose the CSP plants

Metals 2021, 11, 1377 4 of 30
(namely heliostats, collectors, and receivers) do not provide the required optical proper-
ties necessary for efficient exploitation of solar energy. For this reason, coatings are used
to cover the surface of these elements to achieve the optimal values of solar absorptance,
reflectance, and thermal emittance, and simultaneously improving (when possible) the
structural integrity of CSP components. In addition, coating materials must be stable in
the air under cyclic loads, have a low-cost, and a large scalability to improve the overall
efficiency of the CSP plant. In PTC systems, the absorbing coatings are already optimized,
and they operate in a vacuum and with very protective conditions. In contrast, receivers
in CPT systems usually operate in the air, therefore severe issues come from the exposure
to reactive environments and the action of erosion, pollution, etc. These factors make the
design of proper absorbing coatings for the receiver in the solar towers more challenging.
Currently, the standard for coating in the central receiver of CPT plants is the Pyro-
mark 2500 paint. It provides a remarkable value of the absorptance from 0.96 at near-nor-
mal incidence to approximately 0.8 at glancing incident angle. However, Pyromark is
characterized by a high thermal emittance that ranges from 0.8 at 100 °C up to 0.9 at 1000
°C and, thus, suffers from significant thermal losses [15,16]. In addition, Pyromark 2500
paint degrades rapidly if operated in air at high temperature. Ho et al. [16] observed a
reduction of several percentage points after 300 h if exposed at the operating temperature
of 750 °C. Therefore, Pyromark coating requires frequent maintenance (annual or bian-
nual) to maintain the performance, determining relatively high operational costs (i.e.,
maintenance and shutdown costs) of the CSP plants. To promote the competitivity of CSP
in the electricity market, new durable absorber materials able to withstand operating tem-
peratures above 650 °C and heating/cooling cycles, cost-effective, and easy to be applied
are needed. At the same time, materials need to be conceived to have the lowest radiative
and convective losses that reduce the thermal efficiency of the receivers, to retain their
structural integrity after thousands of thermal cycles, and to protect the structural com-
ponent from external factors (erosion, action of wind, dust, hail, etc.). Ceramics, metals,
and combinations of both are currently employed as solar selective coatings [17–19].
To be used in CSP applications, the materials should be thermally stable at tempera-
tures above 400 °C and retain their optical properties. Several materials and coating struc-
tures have been developed in the past few decades, aiming to obtain solar absorbers with
higher optical properties and higher thermal stability. According to the most recent stud-
ies [20], three classes of high-temperature coating have been identified depending on the
type of the absorbing dielectric: double cermet solar selective coatings, transition metal
nitride multilayer coatings, and transition metal oxide multilayer coatings. However,
many materials fall within these classes, and describing their advantages and disad-
vantages is beyond the scope of the present review. The readers may read the works of
Kennedy [18], Atkinson [19], and Xu [20] for more exhaustive information about the ma-
terials used and their chemical composition, the typical configurations of the coatings, the
field of application, and the main failure mechanisms.
2.2. Thermal Spray Processes for Selective Absorber
Besides the materials and the structure of the coating, the selective optical properties
have a direct correlation with the manufacturing techniques. Vacuum technologies, such
as physical vapor deposition (PVD) or chemical vapor deposition (CVD), and wet chemi-
cal processing are the methods commonly adopted to develop selective solar coatings
[19,21]. These processes have been mainly applied in PTC, where coatings are required to
have a thickness in the order of microns. In CPT, where thicker coatings are necessary,
these methods are not suitable. Wet chemical processes include painting and sol-gel pro-
cessing and represent the lowest-cost techniques to produce absorber coating. As men-
tioned before, the absorber used most in solar tower applications is the Pyromark black
paint. Indeed, it easy to apply on central receivers and provides the greatest absorptance
even at high temperature, despite it suffers thermal losses due to high thermal emittance
[22]. In addition, several concerns arise regarding the mechanical integrity of these

Metals 2021, 11, 1377 5 of 30
coatings, adhesion on the substrates, and their stability under thousands of heating/cool-
ing cycles.
Vacuum processes are the consolidated technologies to manufacture absorber coat-
ings that have high selectivity in their optical properties. PVD and CVD processes can
produce films starting from a wide variety of metals, ceramics, and compounds; they also
guarantee high reproducibility of the manufactured film owing to the accurate control in
the process parameters [23]. However, they are characterized by high operating cost and
small-scale production, and therefore not suitable for larger plants. Additionally, the op-
tical properties of manufactured coatings degrade faster when exposed to high tempera-
ture and the atmosphere. For these reasons, vacuum processes are convenient in PTC sys-
tems, where the absorbers work in vacuum conditions but are not suitable for central
tower systems. The development of stable and durable selective absorber coatings with
tailored optical properties, production methods, and technologies for large-scale in situ
applications is of paramount relevance to achieve high efficiency in central tower plants
[24,25]. Thermal spray technologies allow application of ceramic and metal coatings on a
wide variety of shapes and sizes of substrates, representing promising alternatives to the
paint coatings and vacuum deposition technologies for the solar absorber. Indeed, ther-
mal spraying is suitable for large-scale production and allows for deposition of coatings
that range from a micron to a few millimeters in thickness. The main advantages of ther-
mal spray processes can be summarized: (i) high stability, (ii) high production efficiency,
(iii) low production costs, and (iv) simple operation. Those aspects make these technolo-
gies promising for solar applications [26]. However, thermal sprayed coatings come with
poorer selective absorption properties when compared to vacuum-deposited absorbers
due to microstructure flaws, surface roughness, and other reasons that are not yet been
fully understood.
The main difference of thermal spraying coatings with respect to those produced
with vacuum technologies is related to the heterogeneity of the material deposited. Ther-
mal sprayed coatings are characterized by a complex microstructure and by open/closed
porosities. Furthermore, they present a higher surface roughness. Several studies have
pointed out the influence of porosities, grain boundaries, internal phases, and roughness
on the optical properties of the coating [27,28]. Therefore, coatings produced by thermal
spray technologies have different optical behavior compared to those deposited with the
vacuum process.
The first attempt to use thermal spray processes to achieve tailored optical properties
was made by Tului et al. [29], who deposited ZnO and Al2O3 mixed powders using con-
trolled atmospheric plasma spray (CAPS). In their work, the authors analyzed the emis-
sivity of the coatings in the visible and near-infrared (NIR) range and tried to establish a
correlation between the amount of alumina content in the initial powder mixture with the
optical behavior of the coatings. Devoted literature indicated the doping effect of the Al
atoms inside the lattice structure of ZnO [30,31] that leads to a reduction of emissivity in
the infrared region, even in low percentages (approximately 3% by weight). Using alu-
mina instead of pure aluminum particles, the authors observed a decrease in the emissiv-
ity with the increase in the % of Al2O3 (from 3% to the eutectic 22% in weight). Process
environment, i.e., air or protective inert gas (or IPS) atmosphere, also influenced the inter-
action with the sunlight. Spraying in an inert atmosphere reduced the rate of oxidation
and led to the depletion of oxygen and, then, the formation of oxygen vacancies in the
ZnO lattice structure, which acted as additional doping [32] and lowered the emissivity
in the visible–NIR region. From the study of Tului et al. [29] emerged that the plasma
spray can be potentially used in solar applications at least for the material system ana-
lyzed. However, emissivity remains high, especially in the middle infrared range, to be
competitive. Therefore, more investigations are required, in particular regarding the op-
timization of the process itself and the thermal stability of the absorber coatings. However,
no other published data about plasma spray and ZnO-based coating have been found in
the literature.

Metals 2021, 11, 1377 6 of 30
After this preliminary study, researchers from the Sandia National Laboratories con-
ducted a thorough analysis on several materials, metals, and ceramics, or their combina-
tion, which were already used in solar applications, by using thermal spray technology
[24,25,33,34]. Ni-25wt%Graphite, Ni-5Al, pure W, WC-20Co, WC-9Co, WC-25Ni, CeO2,
Co-28Mo-17.5Cr-3.5Si, Co-28Mo-17.5Cr-3.5Si metals, and cermet systems were deposited
using the APS process on steels and Ni-alloys substrates. Considering that the ideal selec-
tive optical properties, i.e., α > 0.95 in the visible range and ε < 0.3 in IR region, are hardly
achievable simultaneously in the deposited absorbers, the authors proposed a summariz-
ing index called “factor of merit” (FOM) to aid in comparing the thermally sprayed coat-
ings with the benchmark system [33]. The FOM is defined as:
FOM(W cm) = 60  5 + 
2
(1)
where αsolar, ε80°C and ε2400nm are the solar absorptance, emittance at 80 °C, and emittance at
2400 nm, respectively. The constants 60 and 5, having the units of W/cm2, represent the
energy flux incident on the receiver and the energy flux emitted at 700 °C in an environ-
ment at 20 °C. The index suggests that to improve the efficiency of the central receiver,
maximizing the absorptance is more effective than minimizing the thermal emittance. Ad-
ditionally, SPT receiver materials are opaque to solar energy; therefore, maximizing the
receiver absorptance minimizes the reflectance from the receiver surface. The Pyromark
2500 paint, used as the benchmark, has a FOM of 53.3. The concept behind the application
of APS to produce absorber coatings was to accept systems that can have lower FOM but
are more durable and stable in high operating temperatures. To this scope, the authors
also evaluated the thermal stability of the coating at different temperatures. The as-
sprayed coatings did not perform well; the coatings had absorptance between 0.73 and
0.85 for WC-9Co and CeO2, respectively, and thermal emittance between 0.3 (pure tung-
sten) and 0.62 (Ni-graphite cermet), achieving FOM in 35–48 range. Some materials have
acceptable emittance; however, they also have an absorptance that is too low and then are
not suitable for the receivers in as-sprayed form. Reducing the surface roughness, which
is usually quite high after the plasma spray deposition, further lowered both optical prop-
erties. When the dimension of surface roughness is shorter than the wavelength of the
solar rays, the surface is just the same as a mirror for these rays, and most of the infrared
light is reflected from the surface; therefore, a lower ε also could be achieved [35]. In con-
trast, when the surface roughness is larger than the wavelength of sunlight, the electro-
magnetic wave is trapped by multiple reflections, contributing to better absorption prop-
erties. Smoother surfaces led to a decrease of FOM of the coating by 40%, dropping to as
little as 7% [25,33].
Heat treatment at 600 °C led to an increase in absorptance and emittance of all coat-
ings up to 0.95/0.78 (α/ε) in the case of Ni/graphite cermet and 0.89/0.49 for CeO2 oxide,
which performed the best with respect to the other systems. The heating of coating pro-
duced two distinct phenomena on the surface. From one side, it promotes the formation
of an oxide and, if it is stable, an increase in the coating thickness; conversely, a reduction
in roughness has been observed after the heat treatment. According to that, after the treat-
ment, almost all coatings, regardless of the substrate, experience an increase in the FOM.
The authors argued that changes in the oxide layer on top of the coating have a more
relevant effect on the optical performance than the roughness, which conversely nega-
tively influences light absorption. Ni-5Al metallic coating also proved to be a suitable can-
didate, achieving after the treatment optical properties competitive with the Pyromark
2500 (α/ε equal to 0.89/0.57) with a FOM of 50. Observed behavior has been linked to the
formation of NiO and AlxOy oxides within the coating and the beneficial doping from Al
particles. Pure tungsten and WC-Co coatings performed worse than other materials. In-
deed, tungsten underwent pronounced oxidation with the WOx oxide layer that resulted
in poor adhesion on the substrate. Spalling phenomenon has been observed in the W-
based coatings; the oxide scale together with fractures delaminates due to the different

Metals 2021, 11, 1377 7 of 30
thermal coefficients with the underlying layers and the residual stresses arising from the
treatment [24,25,34].
From this preliminary research, it emerged that thermal spray processes can be a
suitable alternative to the vacuum deposition processes or dip-coating method, and the
deposited materials are potentially competitive with the Pyromark 2500. However, be-
sides the optical properties, other important features, such as adhesion, corrosion re-
sistance to the environment, and fatigue strength against thermal cyclic loads, have not
been investigated; therefore, the long-term stability of these coatings has not been fully
demonstrated. Table 1 summarizes the optical properties of the material system.
Table 1. Absorptance and emittance data for coatings with as-sprayed, with 1µ polished, and
with as-sprayed heat-treated surfaces (data from [24,25,33]).
Coating α ε80
FOM
(W/cm
2
)
Material As
sprayed
Smooth
surface (Ra
= 1 µm)
Heat
treat.
As
sprayed
Smooth sur-
face (Ra = 1
µm)
Heat treat.
As sprayed
Smooth sur-
face (Ra = 1
µm)
Heat treat.
Ni-25 Graph-
ite
0.81 0.52 0.93 0.62 0.33 0.78 45 26 52
Ni-5Al
0.63
0.26
0.89
0.39
0.11
0.57
35
12
50
Tungsten
0.69
0.46
0.74
0.29
0.13
0.82
39
28
40
WC-20Co
0.82
0.60
0.55
0.32
46
37
WC-9Co
0.73
0.45
0.53
0.24
40
28
WC-25Ni
0.75
0.60
0.51
0.24
39
35
CeO
2
0.85
0.16
0.89
0.41
0.11
0.49
48
7
50
Co-28Mo-
17.5Cr-3.5Si
0.79 0.86 0.48 0.61 44 48
Co-28Mo-
17.5Cr-3.5Si 0.79 0.86 0.47 0.57 44 48
Sandia researchers investigated thermally sprayed absorbers at higher temperatures
to assess their thermal stability in harsh conditions [34]. Lanthanum strontium manganate
(LSM) coating manufactured by APS was tested at 600, 700, and 800 °C and compared
with the Pyromark 2500. LSM coating, treated with laser after the deposition and before
the heating tests, showed good optical properties at each temperature. LSM showed an
efficiency of 0.89–0.9, equal to that of the Pyromark with α/ε values of 0.96/0.82. In this
case, the selective absorber efficiency was indicated as the ratio of the net radiative energy
absorbed and retained by a surface to the net radiative energy absorbed and retained by
an ideal selective absorber with an absorptance of one and an emittance of zero [34]. In
contrast to Pyromark paint, which showed a pronounced degradation at 800 °C and for-
mation of secondary phase depending on the substrate, LSM experienced a drop in the
absorptance up to 0.94, less than the Pyromark with 0.95, but with a lower emittance, 0.79–
0.80 against 0.885 after 480 h, thus showing a better efficiency in the long term. In addition,
no substrate element or secondary phases were detected after thermal treatment, pointing
out the absence of reaction of coating with the substrates. Therefore, LSM is potentially
more chemically stable than Pyromark over a long operating time. Despite Sandia re-
searchers claiming the competitiveness of the plasma sprayed coatings, no further pro-
gress has been made following the reports. In addition, information is lacking on their
mechanical integrity in actual use conditions, chemical and morphological features, and
optimization of manufacturing processes.
As mentioned above, in contrast to coating produced by vacuum-based techniques,
thermal sprayed coatings showed a heterogenous composition made by secondary

Metals 2021, 11, 1377 8 of 30
phases, melted, and partially melted particles, cracks, and multiscale porosities. The syn-
ergy of these features influences the interaction of absorber material with the sun’s rays.
Studies have been devoted to the characterization of the optical behavior in relation to the
microstructure of the coating and the manufacturing process. Brousse-Pereira and Toru
[27,36] analyzed aluminum/Al2O3 cermet coatings manufactured by the APS process,
highlighting the effect of alumina particles dispersed in the Al matrix on the optical be-
havior. The multiscale microstructure and multiphase nature (that can be seen as made
by homogenous macro areas of single aluminum and alumina, mixed with cermet areas
and porosities) lead to a complex response of the coating to the incident light, which dif-
fers from that of the homogenous material (see Figure 3).
Figure 3. The reflectance of plasma sprayed alumina coating and reflectance of a single crystal of α-
alumina (left axis). The absorption coefficient of α-alumina is computed from extinction index, re-
produced from [27], with permission of ACS 2015.
Toru et al. [27] argued that volume scattering phenomena occurs during light/coating
interaction (see Figure 4). They identified three types of phenomena: (a) diffraction, which
results in a modified direction of light propagation around heterogeneity; (b) refraction
that involves penetration of light in heterogeneity, along with modification of the emerg-
ing direction; and (c) multiple reflections at the interface between heterogeneity and the
matrix medium. They estimated that almost 20% of solar radiation is absorbed by impu-
rities and structural defects.
Figure 4. Illustration of scattering mechanisms that occur in cermets in the transparent region. (1)
Refraction between air and alumina and multiple reflections on aluminum splats; (2) pore scatter-
ing; (3) surface reflections on aluminum splats , reproduced from [27], with permission of ACS 2015.
Brousse et al. [36] claimed that by optimizing the operating parameters and condi-
tions (i.e., plasma power and the resultant particle in-flight speed and temperature) it is
possible to obtain different microstructures and, hence, tuning the optical properties.
Higher plasma power, which leads to higher processing temperature, resulted in reduced
porosities (from 33% to 14%) due to the complete melting and resolidification of the

Metals 2021, 11, 1377 9 of 30
particles and in a smoother surface for the material system analyzed. These features
caused an increase in the reflectance of the surface from 72% to 87% in the IR range, acting
more as a mirror and lowering the absorption. Low plasma power, conversely, seemed to
favor the formation of a more heterogeneous structure (pores, particles with different
shapes and sizes) and a rougher surface. These aspects promote the absorber efficiency:
smother surfaces are more reflective, while surface irregularities cause multiple reflec-
tions on the incident ray, increasing the absorbed part of the radiation and decreasing the
reflectance. In addition, reflectance has been found to increase with the flattening degree
of the sprayed particle: a mix of globular unmelted particles and flat lamellae promotes
the entrapping of the light beam within the coating and promotes the absorption of the
incident radiation. Oxygen and Al203 fractions within the aluminum matrix also play a
key role in the light/coating interaction. APS of Al-boehmite (AlOOH)-produced coatings
have O and Al2O3 content up to 13% and 27% in weight, respectively. The high amount of
dielectric phase dispersed in the metal phase lowered the reflectance up to 15%. In addi-
tion to these promising results, the authors also observed the importance of further inves-
tigations to understand how each phase contributes to the interaction of complex hetero-
geneous coating with solar radiation. Toru et al. [27] further investigated the plasma
sprayed Al/Al2O3 cermets in visible, NIR, and IR spectral regions (from 0.4 to 16 µm wave-
lengths). As also observed by Brousse et al. [36], the amount of alumina and aluminum
metallic phase inside the coating has a key role in the optical behavior. In the visible–NIR
spectral range (0.4–6 µm), which is of interest for solar applications, increasing the %Al in
Al2O3 matrix increases the absorption and reduces the reflectance, but it happened in the
range 0–15 wt% of Al. However, if the Al% further augments, tending toward 100%, the
effect is reversed: a high fraction of easily melted metal leads to the reduction in porosities
that limits the internal scattering. Therefore, increasing the aluminum fraction until a coat-
ing made by 100% of Al is obtained, the specular reflectance is promoted and the coating
acts as a mirror-like surface. Conversely, low aluminum and, hence, high alumina con-
centrations favor a scattered reflectance, promoting the absorption of the radiation. The
authors concluded that, concerning Al/Al2O3 cermet, plasma spray is capable to adjust the
optical properties according to the final applications.
Thermal spray processes have been adopted to deposit other material systems, e.g.,
perovskite oxide (La1−xSrxTiO3+δ) [37], spinel structures [38], and Ni–Al alloys [39]. Zhu et
al. [37] observed that, concerning perovskite, the rate of oxidation occurred during the
deposition and the oxygen content in the coating, which in turn depends on the plasma
power, thus influencing the reflectivity of material in the UV–visible–IR spectral range.
Reflectivity is higher in coatings deposited with low plasma power (intensity current of
600–650 A) (see Figure 5). Low power led to coatings having a high fraction of unmelted
particles, lower density, and high porosities (14%), and cracks. The particles, indeed, re-
tained their initial morphology with a reduced flattening (the thickness of the lamellae
was 4–5 µm) and the thicker splats favor multilayer reflection at the splat interfaces; po-
rosities, in turn, result in a larger scattering coefficient and crystallinity degree. Con-
versely, in high-power plasma, particles are fully melted and the microstructure is com-
posed of highly flattened splats (1–2 µm thick), low porosity (around 9%), and a reduced
crystallinity degree, due to the higher cooling rate of the melted particles than those in
low-power deposition. The authors suggested the influence of the crystallinity/amor-
phous state of the particle on the optical behavior, but it was not fully clarified and quan-
tified. Oxidation also promotes reflectivity: oxygen vacancies are reduced by post-depo-
sition heat treatment and filled by oxide scale, which acts as doping and improves the
absorption [29]. The authors claimed that by optimizing temperature and the time of treat-
ment, together with the spray process parameters, it is possible to adjust the reflectivity
of the coating.

Metals 2021, 11, 1377 10 of 30
Figure 5. Reflectivity spectra of LST coatings: (a) LST-1, (b) LST-2, (c) LST-3, and (d) LST-4 (samples
are listed from high to low powers), reproduced from [37], with permission of Elsevier 2015.
Bunmephiphit et al. [39] explored the deposition of Ni-5Al with flame spray process.
The authors claimed that the Ni–Al solar absorber is a good candidate as a solar absorber
material for solar collectors operating at high operating temperatures. As sprayed coat-
ings achieve an average absorptance of 0.77 in the UV–visible–NIR range (from 0.3 to 2.5
µm), but the value was around 0.9 in the UV–visible region, and performed better than
the coatings from Sandia (see Table 1) [24,25,33]. The authors suggested that it could be
due to the high fraction of NiO and Al2O3 oxides dispersed in the Ni and Al metallic
phases (no formation of intermetallic compounds, NiAl, AlNi3, or Al3Ni5, was observed).
However, this was a preliminary analysis; further strategies need to be planned to im-
prove the absorption, and the thermal stability needs to be investigated.
Most recent developments have been directed to the use of spinel structures for solar
applications [40]. Indeed, they possess good intrinsic selective properties, excellent ther-
mal stability, and oxidation resistance. Spinel coatings are usually produced by dip coat-
ing or wet coating (such as electroplating, spraying, or roll-coating [41]) that can deposit
them without altering their composition or chemical structure. Deng et al. [38] investi-
gated the production of spinel coating by means of plasma spray of vanadium tailings
(VT). The high temperatures involved in the deposition process together with the oxygen-
rich atmosphere promoted the reaction of feedstock particles and led to the formation of
AlVO3 and Mg2VO4 spinel structures and Fe3O4 and MnO3 oxides. The as-sprayed coating
showed remarkable optical features, with absorption in the visible range equal to 93.79%,
but a higher emittance of 66.86%. In addition, the relatively high native roughness, meas-
ured at 3.8 µm, is larger than the solar radiation wavelength (0.3–2.5 µm) and favors the
multiple reflections of sunlight beams on the surface, increasing the absorption; however,
it also enhances infrared emission. As observed in other coatings, reducing the roughness
of spinel coatings led to a decrease in the thermal emittance but also in absorptance, with
values of α/ε equal to 0.92/0.49 and a more reflective coating. Reduction of optical prop-
erties was also observed after the thermal treatment. However, the loss in optical perfor-
mance is quite low and determines an acceptable durability of the coating. Surface dura-
bility, in terms of optical properties stability, was estimated by using the PC index, defined
as follows [42]:
PC = + 0.25  
where Δα and Δε are the variation in absorptance and thermal emittance after the heat
treatment, and PC must be less than 5% (PC < 0.05) for the absorber surface to be accepta-
ble. Thermal spray processes have proved to be a viable solution to produce coatings that
can be competitive with Pyromark paint [24,25,33,34,38]. However, many materials that
are commonly used in solar applications are not suitable for deposition with thermal
spray and do not represent a valid alternative; for example, pure alumina or WC-based

Metals 2021, 11, 1377 11 of 30
coatings achieve low optical performance in as-sprayed conditions. For this reason, sev-
eral strategies have been attempted to improve the optical properties and the selectivity
of these coatings. These methods involved the production of coatings with multiscale
structures, multilayered absorbers to tune the selectivity, or post-deposition treatment.
Wang et al. [26] proposed the use of blends of nano-sized and submicro-sized WC
particles (0.7 and 2 µm, respectively) to create a multimodal WC-Co cermet coating utiliz-
ing HVOF process. The multimodal coating was denser and more compact: nano-sized
WC particles fully melted during the process due to the high surface area with respect to
the volume, and filled the pores between coarser WC particles; porosity decreased from
3% and 2% to 1%, and a smoother surface was also obtained. The authors observed that
the absorptance increased from 0.80 and 0.82 for conventional coatings (single scale WC
particles) to 0.87 for multimodal. It was due to the light trapping phenomena inside the
multiphase coatings, consisting of coarse and fine WC; light was reflected between the
submicrometer WC particles and the uniform nanometer WC particles, resulting in more
efficient light-trapping properties (Figure 6).
Figure 6. Sketch map of the formation of coarse and fine WC particles in the multimodal coating,
adapted from [26].
Results were promising, but further optimization of the coating, including particle
size range, distribution, and coating surface morphology, may be necessary to increase
the absorptance, and even more, to make the thermal sprayed WC-Co competitive in solar
applications. The same authors, starting from these results, explored a multilayered ab-
sorber consisting in the multimodal WC-Co absorber coating produced by HVOF, covered
with CuCoMnOx, CuCoMnOx + SiO2 spinel intermediate layer, and SiO2 antireflective lay-
ers (ARC), deposited by dip coating to improve the absorption but also the selectivity [35].
The addition of subsequent layers on the WC-Co absorber progressively increases
the absorptance. The spinel layer (composed of Cu–Mn oxides) has good intrinsic absorp-
tance and improves the absorption of light. However, emittance also rose due to a reduc-
tion in the surface porosities; spinel particles fill the gap between unmelted round-shaped
WC particles on the coating surface, making the surface smoother. The addition of the
other layers, CuCoMnOx + SiO2 transition layer and SiO2 ARC, boosted the optical perfor-
mance, achieving values of α/ε of 0.915/0.290 for the final coating. The behavior is due to
the beneficial effect of the SiO2 inside the spinel structure, which acts as doping, and the
gradient distribution of silicon that gradually changes the refractive index. Subsequent
layers also act as protection to oxidation by providing higher thermal stability. After the
annealing treatment at 500 °C, the optical properties slightly changed with α lowered to
0.901 and ε increased to 0.320. The authors argued that during the heat treatment the ther-
mal stresses form cracks in the ARC layer and weaken the anti-reflection effect. However,
the layers protect the WC/Co by diffusion of oxygen, giving the good thermal stability
observed, but lower the emittance and improve the selectivity of the coating (see Figure
7).

Metals 2021, 11, 1377 12 of 30
Figure 7. Reflectance curves: A—multimodal WC-Co layer; A1—polished WC-Co layer; B1—WC-
Co+ CuCoMnOx layer; C1—WC-Co + CuCoMnOx + CuCoMnOx + SiO2 layer; D—WC-Co + Cu-
CoMnOx + CuCoMnOx + SiO2 + SiO2, reproduced from [35], with permission of Elsevier 2018.
A similar strategy has been proposed by Duan et al. [43] that investigated a tandem
structure (see Figure 8b) made by WC/Co absorber coating manufactured by HVOF pro-
cess plus an aluminum oxide (Al2O3) anti-reflection layer (ARC), produced by the sol-gel
method. The tandem structure presents a good selectivity with α/ε equal to 0.908/0.145
and good stability (0.898/0.172) after annealing at 600 °C for 7 days. The observed im-
provements are due to the beneficial effect of Al2O3. Solar properties improved from
0.746/0.161 of the simple only-WC/Co to 0.827/0.145 when Al2O3 powders are added to the
WC-Co absorber. The light-trapping effect is improved by increasing the light path inside
the materials and the multiple reflections due to the heterogeneity, the multiscale inclu-
sions effect, and alumina optimal optical properties [44,45] (see Figure 8). The ARC Al2O3
led to 0.852/0.153 and 0.908/0.145 for pure WC-Co and WC-Co + Al2O3 coatings, respec-
tively, owing to the anti-reflection effect that the alumina coating provided for improving
absorptance and reducing thermal losses.
Figure 8. Schematic illustration of Co-WC-Al2O3 metal–dielectric composite solar selective absorb-
ing coating: (a) double-layer composite coating; (b) single absorbing coating, reproduced from [43],
with permission of Elsevier 2017.
Similar to what happened with the SiO2 ARC in the multilayered structure developed
in [35], alumina ARC prevents the diffusion of the oxygen in the underlying layer during
the annealing treatment, protecting the absorber from oxidation. Cracks formed on the
alumina layer lowered the anti-reflection effect, thus decreasing the optical performance,
but the tandem structure showed remarkable thermal durability.
A further strategy to improve the solar absorbing properties of the thermal sprayed
absorber was proposed by Gao et al. [46], which conducted post-deposition laser surface
treatment on Ni–Mo and Ni–Mo–Co coatings. The purpose was to reduce the microstruc-
ture defects, correct improper surface roughness, and fix unreasonable phase contents in
thermal spray coatings that cause their poor optical properties. The authors observed that

Metals 2021, 11, 1377 13 of 30
after the treatment the absorptance increased from 0.84 to 0.88 for the Ni–Mo sample and
from 0.75 to 0.83 in the Ni–Mo–Co coating. It was ascribed to the complementary effect of
the Ni + Mo metal volume fraction increasing from 28.6 to 47.3%, and of the reduction in
the Ni-oxide content from 69.7 to 51.4%, promoted by the laser treatment. Surface rough-
ness almost doubled after the laser process, contributing to the increase in the absorptance
[47]. The work of Gao showed promise for improving the optical properties of thermal
sprayed coatings by laser treatment, especially for concentrating solar power. However,
despite the values reached, the proposed approach is not competitive with the usage of
Pyromark, and far from the ideal threshold of 0.96.
In addition to the conventional thermal spray processes, alternative approaches have
been proposed in the past that adopt cold spray [48], laser sintering [47,49], and laser clad-
ding process [50]. Tungsten coating manufactured by laser cladding achieved an absorp-
tance of 0.830 and a thermal emittance of 0.116 at room temperature in the as-sprayed
conditions [47]. The authors argued that the significant increase in absorption of the tung-
sten (bulk material has α equal to 0.49) is due to the formation of tungsten oxides WOx
with various compositions having good intrinsic optical properties [51] and the multiscale
surface texture, with roughness ranging from 0.5 to 1 µm. Indeed, coating absorption ben-
efits from nano- and microroughness result from two complementary effects: (i) if the
roughness is smaller than the wavelength, the surface acts as a graded-index medium ex-
hibiting anti-reflective behavior [52,53]; (ii) when roughness is greater than radiation
wavelength, multireflection occurs, favoring light entrapment. By performing a second-
ary laser scan on the sintered coating without adding new powders, the authors further
improved the optical properties, achieving values of α/εroom/ε80°C of 0.903/0.161/0.245 [49].
The coatings also showed excellent thermal stability after thermal treatment at 650 °C for
36 h. Absorptance slightly decreased to 0.87 and the emittance increased up to 0.49. The
small variations in α/ε were ascribed to the increasing oxidation rate inside the coating
with the formation of other phases (mainly iron oxides due to the steel substrate) and
slight changes in the roughness; however, the protective surface oxide forming during the
sintering avoids an excessive diffusion of the oxygen, retains the optical performance, and
provides the high thermal stability. The FOM was estimated equal to 50, close to the per-
formance of Pyromark 2500 paint. Pang et al. [50] manufactured TiC–Ni–Mo cermet coat-
ings using nano-sized and micro-sized particles. The multiscale cermets proved to be
valid candidates as spectrally selective coatings for high-temperature applications owing
to their excellent selectivity. Cladded coatings indeed reached values of α/ε of 0.8/0.055
and 0.86/0.04 for single (only micro-sized powders of metals and ceramic were used) and
multiscale coatings (using a blend of micro- and nanoparticles), respectively. Multiscale
cermet benefitted from high light entrapping due to the surface morphology and the mul-
tiple interactions between nano- and microphases, as observed for the multimodal WC-
Co coating [26]. Titanium carbide, having good intrinsic optical properties, also contrib-
utes to enhancing absorption in the visible range. The coatings also showed remarkable
thermal stability, barely affected by the thermal cycle (see Figure 9).

Metals 2021, 11, 1377 14 of 30
Figure 9. Absorptance and emittance of the laser-clad cermet coating: (a,b) as-produced condi-
tions; (c,d) after heat treatment at 650 °C for 200 h (graphs are re-elaborated from [50]).
From the analyzed contributions, it emerges that laser processing is a potential alter-
native to vacuum techniques and conventional thermal spray processes. Optimization of
processing parameters and adoption of a multilayer approach, as also seen in [35,43], in-
cluding, for example, anti-reflection layers, could further improve the optical properties
and the selectivity. However, some doubts about the cost-effectiveness of these solutions
remain undebated. Shah et al. [47] gave a rough estimation of the manufacturing cost of a
laser-sintered absorber coating, showing that the technology is competitive with the vac-
uum process, but it refers to small receivers used in parabolic trough collectors, but the
economic validity in SPT plants needs to be estimated. In addition, the in-situ usage of the
laser process is not attainable. Compact plasma and cold spray process, especially in its
low-pressure configuration, in contrast, are opening new scenarios for their use in solar
application [54–57]. Sevillano et al. [48] explored the deposition of Ni–alumina cermet to
produce coatings for solar power generation. The authors were able to produce cermet
coatings with outstanding properties, in terms of hardness, adhesion, low porosity level,
and thermal stability. In addition, optical properties can benefit from the multiscale com-
position of the cold sprayed coating (i.e., high deformed metal particles and ceramic par-
ticles with different dimensions) derived from the fragmentation of the brittle alumina
particles under the high-speed impact [48,58]. Furthermore, similar to what was observed
in plasma sprayed coatings, post-deposition treatments can be beneficial to the optical
properties and mechanical characteristics (e.g., surface hardness, wear resistance), which
also improve the coating durability [59–62].
3. Thermal Spray Coatings for Molten Salt Protection
3.1. Introduction
As mentioned in the previous section, CSP technology can be efficiently integrated
with thermal energy storage (TES) systems to mitigate the intermittency in solar energy
and fulfill the electricity demand even during night hours. CSP technology, indeed, col-
lects solar radiation as heat in a thermal fluid; this heat can be stored for a certain time
without significant losses (the roundtrip efficiency of the TES system currently installed
reaches 96% [12]) to generate electricity when required. The main benefits deriving from
the integration of the TES system are related to:
• Mitigation of the intermittency of the solar energy and better matching the electricity
demand; in addition, the availability of storage allows the power cycle system to op-
erate at a constant rate achieving the highest efficiency, increasing the overall effi-
ciency of the CSP plant;

Metals 2021, 11, 1377 15 of 30
• Transform the solar source into a dispatchable generation source of electricity—the
storage allows to generate and put electricity into the grid in the period of peaks in
the demand and highest prices (usually the night hours);
• Increases the annual capacity factor of the CSP plant.
On the other hand, the addition of the TES system increases the investment and the
operational costs required to install and manage a fully operative CSP plant. The integra-
tion of TES systems is recommended in the case of large-scale plants operating at high
temperatures: central tower plants attain more benefits from the energy storage system
than the parabolic trough collector design. TES systems consist of three main parts: (i) the
storage material (usually steam or molten salts), (ii) the heat transfer equipment, and (iii)
the storage tanks [63,64] (see Figure 10).
Figure 10. Scheme of a CSP system with a two-tank indirect energy storage system, hybridized with
a fossil-fuel backup boiler (optional), adapted from [12].
In a TES system, the heat thermal fluid (HTF) coming from the central receiver or the
parabolic trough collectors can be directly used to heat the water and generate the steam,
or sent to the HTF-to-salt heat exchanger and transfer the accumulated heat to the TES
fluid (usually molten salts). The TES fluid stored at low temperature in the cold tank (usu-
ally between 280 and 290 °C) is heated to the working temperature (from 380 to 550 °C,
depending on the fluid used) and collected in the hot tanks. The fluid stored in the hot
tank is used to heat the fluid operating in the power cycle when the solar energy coming
from the solar field is insufficient to satisfy the electricity demand [64,65].
TES fluids can include water, molten salts, organic solvents, and synthetic oils. How-
ever, in practice, the choice is limited to a few suitable materials able to combine several
properties, such as high heat capacity, low melting point, high boiling point, low reactiv-
ity, and thermal stability [66,67]. In the last few years, molten salts are used increasingly
as heat transfer fluid for the power cycle and as heat storage media for the TES systems,
being able to provide considerable advantages in terms of energy generation efficiency
and allowing for higher operating temperatures, in the range of 300–500 °C up to 800 °C,
depending on the salt mixture adopted [65,66,68–70]. The molten salts that are commer-
cially available and currently implemented in CSP plants can be mixtures of nitrates and
nitrites, chlorides, fluorides, or carbonates [71]. Figure 11 summarizes the salt mixture
used in a CSP system and the operative range of temperature.

Metals 2021, 11, 1377 16 of 30
Figure 11. Melting point and maximum operative temperature of commercial molten salts (data
reported from [72]).
The utilization of molten salts at higher temperatures, however, brings out challeng-
ing issues related to the compatibility with the construction materials, especially in the
case of chloride salts [73–76]. CSP components, indeed, are made of metallic alloys, mainly
carbon steels, stainless steels or, when required, Ni-based superalloys, which are vulner-
able to corrosion attacks from HTF and TES media [14,71]. Corrosion mechanisms com-
monly observed in TES components are “high-temperature corrosion”, “localized corro-
sion” (such as pitting or cracking), and “mechanically assisted corrosion”. They are usu-
ally distinguished in “uniform” or “localized” corrosion; the first phenomenon is pre-
ferred because the rate of corrosion is usually slower and is more easily controllable,
meanwhile localized corrosion phenomena are faster and could lead to an unexpected
catastrophic failure of the structure [66]. Molten salt corrosion in TES involves, first, the
oxidation of the metallic alloy generating protective oxide layers, and subsequently, their
dissolution due to the fluxing action of the molten salts. Cyclic formation, growth, and
dissolution of the oxide layer cause the continuous degradation of the metallic structure.
The occurrence of a specific corrosion mechanism strongly depends on many factors,
e.g., type and composition of the alloy, corrosive medium, working conditions (i.e., tem-
perature, atmosphere, loads, etc.), to cite a few. All influence the kinetics of the reactions
and the corrosion rate. The description of the corrosion mechanisms that occur in TES
system parts and the performance of the materials exposed to the different salt mixtures
is too wide and beyond the main scope of the present manuscript. Readers are referred to
reviews by Ibrahim et al. [72] and Walczak et al. [66]. These works provide detailed insight
into the salt systems used in CSP plants and the corrosion behavior of the structural ma-
terials when exposed to molten salts.
3.2. Corrosion Resistant Coatings in TES System
Considering the susceptibility of metallic parts to corrosion when in contact with
molten salts, the potential failure of TES system components represents a crucial risk for
the CSP plant and a non-negligible part of the overall cost of the plant. It has been esti-
mated that the overall impact of the TES system may reach the 25% of the initial cost of a
CSP plant, where the molten salts contribute almost 50% and the components (tanks, pip-
ing, foundations, etc.) for the remaining half [77]. Preventing the failure or mitigating the
damages of the TES components, therefore, can contribute to keeping the cost of repara-
tion or of shutdown time. Several strategies to prevent or mitigate the corrosion of storage
Solar Salt
Hitec XL
Hitec
LINO3- NaNO3- KNO3
MgCl2- KCl - NaCl
ZNCl2- KCl - NaCl
KF - LiF - NaF
Li2CO3- Na2CO3- K2CO3

Metals 2021, 11, 1377 17 of 30
tanks have been identified in recent years; they may involve the use of a corrosion inhib-
itor added in the electrolyte, cathodic protection, or deposition of a protective coating
having better corrosion resistance than the structural material, to cite some [78,79]. The
addition of a certain amount of graphite, for example, in the nitrate salt mixture reduced
the corrosion rate of carbon steel up to six times if coupled with a graphitization of the
part surface [80]. The application of corrosion inhibitors was also tested to mitigate the
aggressiveness of chloride salts. Mg inhibitor in MgCl2–KCl salt reduced the corrosion
rate by 35% in Fe–Cr–Ni alloys even at the highest temperatures [81,82]. Reduction of im-
purities and in-service monitoring of salt purity also allow designers and engineers to
control the corrosivity of molten salts [83,84]. However, these strategies are not always
economically viable and, to some extent, can be detrimental to the thermal efficiency of
the HTF/TES medium, compromising the overall economic effectiveness of solar energy.
Coating of low-alloy structural steel, on the other hand, represents a suitable and cost-
effective alternative in the design of CSP plants. Indeed, the coatings act as protective lay-
ers, avoiding the oxidation of substrate elements (especially those more prone to oxida-
tion, such as Cr, Ni, and Al) and the depletion of the matrix of its constituents; in that way,
they can enhance the lifetime of these materials by sacrificing their element during usage
[85]. Usage of protective coatings has been suggested not only for low-alloyed steels but
also when metals having high corrosion or oxidation resistance are used, such as stainless
steel or Ni-based alloys. To serve as protection for the base alloys, compositions of the
coatings have to promote oxidation of their constituents depending on the environment.
The coatings should be sufficiently dense and thick to limit the diffusion of the corrosive
species through the coating before the protective oxides form [86].
First attempts to produce protective coatings concerned surface modification strate-
gies by promoting the passivation of surface and formation of the autogenic covering
layer. Gomez-Vidal et al. proposed surface passivation by means of pre-oxidation treat-
ment of alumina-forming alloys (Al-FAs), such as Ni-based Inconel 702, Haynes 22, and
Kanthal APMT [87,88]. These alloys, indeed, are prone to form on the surface a dense and
protective Al2O3 layer having outstanding chemical and thermal stability. Alumina scales
prevent the penetration of aggressive elements from the molten salt bath inside the un-
derneath metal. The protective nature of the passivated surface derives from the for-
mation of α-Al2O3 phase, but other metastable and less protective phases (γ and θ) could
form in the oxidizing environment; therefore specific treatment temperatures also need to
be adopted according to the specific Al-FA employed as base material. The authors tested
the oxidized alumina-forming alloys with molten chlorides (MgCl2–KCl mixture) in both
static and cycling conditions. Each alloy showed the formation of dense and uniform alu-
mina scales (from 5 to 50 µm in thickness) during the oxidation treatment, which were
able to protect the alloys from the molten chlorides under 700 °C isothermal conditions.
The composition of the base material was found to influence the corrosion resistance; the
large amount of Ni in the Inconel 702 seemed to decrease the corrosion, while other ele-
ments, such as Fe, Al, Cr, Ti, and Mo were more prone to dissolve in molten salts. In ther-
mal cycling conditions from 550 to 700 °C, the alumina layers behaved similarly for short
time exposure; in the case of long-term exposure, they became unstable, especially in the
inert atmosphere—Al depleted from the alloys and the scales spalled, enabling the alu-
mina to reform due to the reduced amount of oxygen, and exposing the metallic surface
to the corrosive molten chlorides. The results proposed by the authors seem promising
and suggest that the approach is a viable strategy for corrosion mitigation in CSP. In ad-
dition, the possibility of using an oxygen-rich atmosphere instead of an inert one makes
it more commercially feasible. However, the methodology does not seem sufficiently de-
veloped to be implemented in actual plants, and some concerns need to be addressed. In
short-term tests, the alumina layer remains stable and the corrosion is uniform and, thus,
controllable; however, in long exposure, the alumina layers and the underneath metals
could experience localized corrosion and intergranular attack due to the depletion of con-
stituent elements, especially the Cr-rich alloys, which can lead to a catastrophic failure of

Metals 2021, 11, 1377 18 of 30
the coating. The formation of a metastable phase of alumina during the oxidation can also
cause the formation of internal cracks, promoting a stress-cracking-corrosion (SCC) mech-
anism. Therefore, particular care needs to be taken during the oxidation treatment to es-
tablish proper conditions, which is not always achievable. This, together with the adop-
tion of costly alloys, could impact the overall cost of the TES system. Finally, despite the
use of an oxygen-rich environment that can favor the performance of the Al-FA, chloride
gases (usually form over the liquid bath) can combine with oxygen and produce a harsh
and highly aggressive atmosphere; therefore, the mitigation strategy needs to take into
account the protection of the surface exposed to the liquid salt and the surface exposed to
the chloride vapors.
The early published literature focused on the corrosion-resistant thermal sprayed
coatings for conventional energy generation plants or engine components, such as turbine
blades or heat exchangers, while the protection against nitrate, chloride, or carbonate salts
in solar energy plants has not been explicitly considered at the beginning. However, these
studies shed light on the behavior of the materials (not only as bulk but also as strongly
heterogeneous systems) under those aggressive conditions, and their outcomes can be
transferred to design production methodology to be adopted in solar energy applications.
First experimentations on the use of thermal spray coatings as a protective layer were
directed toward the development of thermal barrier coating (TBC) in power generation
components, namely cell fuels, incinerators, fossil boilers [85,89,90], or high-temperature
steam tanks [91,92]. Other applications of these coatings may be anti-corrosion barriers
for aeronautical gas turbines [93–95] and high-power diesel engines [96]. In those systems,
the combustion of fuels leads to the formation of ashes (which are mainly compounds of
sulfur-, vanadium-, sodium-, or potassium-forming chlorides, sulfides, etc., depending on
the fuel used) on the surface of the metallic parts. These salts are deposited on the surfaces
in a molten state due to the high operating temperatures (above the melting point of the
salts); in an oxidation environment, they react with the metals and trigger the corrosive
attack, leading to the dissolution of the native protective oxide and then promoting the
transport of the oxidizing species inside the metallic part. Aguero et al. [89] tested the
corrosion resistance of Al–Co–Fe–Cr (57–18–13–11 wt%), Ni–Al (80–20 wt%), and Fe–Cr–
Al (60–30–5) deposited by atmospheric plasma spray and HVOF, in molten carbonate
(Li2CO3 + K2CO3) at 700 °C. The authors observed that Cr-rich ferrous plasma spray coat-
ings performed well as protective layers, remaining stable even after long exposure (1000
h) with no significant alteration of the starting composition and avoiding the substrate to
be attacked by the corrosive agent. The improved corrosion resistance was ascribed to the
iron–aluminide (FeAl) compounds and aluminum oxide scales formed in the coating dur-
ing the deposition. Conversely, NiAl does not form protective nickel aluminides and
shows unalloyed elements, close to the nominal composition of the powders, in the coat-
ing. It led to poor corrosion resistance of the coating and, consequently, the substrate at-
tacked after 300 h of exposition. The study does not provide a quantitative analysis of the
corrosion rate or the penetration of corrosion through the thickness of the coating; how-
ever, in the authors’ opinion, thermal spraying can be a viable technology to improve the
resistance of fuel cell components depositing a protective coating without requiring ex-
pensive heat treatments, and with better performance and lower costs than the conven-
tional aluminide coatings manufactured by ion vapor deposition. Senderowsky et al. [97]
also investigated the corrosion of FeAl intermetallic-based coatings against sodium-based
molten salts (Na2SO4) at 850 °C. Iron aluminides were produced by depositing four differ-
ent powder systems, namely pre-alloyed Fe40Al, Fe25Cr + FeAl-TiAl-Al2O3 mixture,
Fe46Al-6.55Si (at%) and multiphase FexAly, by HVOF facility. The authors observed that
the different composition and morphology of the initial mixtures affected the microstruc-
ture and the resultant behavior as thermal barrier of the coatings. The first mixture led to
the formation of the stoichiometric intermetallic compounds of Fe and Al and Fe- and Al-
oxides (only α-Al2O3) within the coating that gave higher corrosion resistance. In the other
coatings, additional systems in the mixtures led to a pronounced heterogeneity in the

Metals 2021, 11, 1377 19 of 30
chemical composition and the phases; the authors observed the presence of the other non-
protective alumina phases, i.e., γ-, θ- and δ-Al2O3, and different aluminides compounds
in the FexAly coating that suffered severe damages and showed a loss in integrity and a
decohesion from the substrate with a consequent acceleration in the corrosion rate. The
depletion of aluminum in the FeAl matrix resulting from the formation of the aluminum
oxide scales; in addition, this favored the development of FeS sulfides that infiltrated the
thickness through the particle boundaries and reached the substrate. Ti-added and Si-
added coatings, despite a slightly better resistance to the molten salts than the multiphase
coating, experienced a severe intergranular corrosion localized at the splat boundaries due
to the presence of cracks and SiO oxide at the particle boundaries.
FeAl intermetallic coatings can provide good protection in aggressive salts, but not
per se; their performances are, indeed, very sensitive to the processing conditions and
their morphologies, and particular attention has to be given to the choice of the starting
materials and the processing method.
Aluminide-based coatings have also been investigated [98–100]. In these works, the
authors deposited the aluminides by slurry spray process. Pure, iron- and nickel–alu-
minide coatings were tested with solar salt mixture, Na/K/Li carbonates, and Na/K chlo-
rides. Slurry spraying, indeed, represents a low cost and practical method to apply several
types of coatings on a great variety of surfaces, even the inner surface of tubes [99]. Sam-
ples were tested in both static and dynamic regimes at temperatures resembling the actual
operating conditions, namely 580 °C for nitrates, 650 °C for carbonates, and 700 °C for
chlorides. Regardless of the molten salts used to test the aluminide coatings, all systems
showed better performance than uncoated substrates. Concerning solar salt molten ni-
trates, aluminide and nickel–aluminide coatings did not show any remarkable microstruc-
tural change, even after long exposure, and no differences in their behavior were observed
in static and dynamic conditions. It suggests that aluminides have excellent stability with
nitrate at those working loads. Nickel–aluminide showed better corrosion resistance ow-
ing to the formation of a Ni- and Al-rich oxide layer on top of the coating. These stable
oxides formed after the post-deposition heat treatment was applied to the coated system
and required to dry and consolidate the slurry coating on the substrate [98]. Iron–alu-
minides also proved to be able to prevent the corrosion of substrate when exposed to mol-
ten carbonate salts; substrate passed from a corrosion rate of almost 2500 µm/y in the un-
coated conditions to being unaffected when coated. However, in this case, the coating suf-
fered severe corrosion with almost 90% of the thickness oxidized after 1000 h of exposure.
Furthermore, the corrosive attack appeared to be more extended over the coating under
dynamic conditions, with deeper penetration of the corrosion products. The main issue
observed in the system tested, however, related to the nonuniform corrosion experienced
by the coatings in static and dynamic tests, which can lead to an unpredictable progression
of corrosion over a longer time. The authors noted that the problem occurred due to the
low and nonuniform percentage of aluminum within the slurry and suggested the neces-
sity of further investigations considering a greater amount of Al in the aluminide, which
may allow the formation and duration of the Al-oxide protective scales.
From an industrial point of view, slurry aluminide coatings could represent a viable
solution for the mitigation of corrosion in CSP/TES components. The process, indeed, al-
lows for easily coating wide and complex surfaces (e.g., inner surface of storage tank or
internal surface of tubes). However, slurry spray usually requires a secondary deposition
process, such as electrodeposition [98] and a post-deposition treatment, usually a diffu-
sion heat treatment at high temperature (around 700 and 1100 °C, depending on the ma-
terials) and controlled atmosphere (usually low in oxygen), necessary to consolidate the
coating on the substrate and promote the formation of protective stable Fe-, Al-, and Ni-
based oxide compounds [98,99]. This occurrence limits use of this method to small/me-
dium-sized components, increases the processing time, and prevents its use for in situ
maintenance and repair of worn parts. In this scenario, thermal spray processes, especially
in their compact versions, can be a better alternative. In addition, several concerns arise

Metals 2021, 11, 1377 20 of 30
concerning the mechanical strength of the slurry sprayed coatings when exposed to cor-
rosive ambient and their stability, which have not yet been exhaustively investigated.
Degradation in mechanical properties in harsh environments, indeed, can trigger SSC
phenomena as a combination of intergranular attack mechanism and creep.
Further experimentations have been oriented toward the development of Ni-based
alloy coatings, aiming to exploit their outstanding corrosion resistance with molten salts
[76,101–104]. Ni3Al aluminide coatings were produced by using atmospheric plasma
spray starting from separated Ni and Al powders and exposed to a sodium–vanadium
salt mixture at 900 °C [85] under cyclic conditions. Despite what has been observed by
[89], Ni3Al coating showed compact and intact oxide scales made of Al2O3 and NiAl2O4
spinel (both more protective) dispersed in a less-protective NiO main phase. The authors
did not observe significant corrosion through the thickness of the Ni3Al coating and after
the corrosion cycles, it successfully remained adherent to the substrate that in turn did not
show any remarkable traces of oxidation. The oxides, formed at splat boundaries, reduce
the porosities and make the coating denser, slowing the diffusion of the aggressive ele-
ments from the molten salt bath. Formation of the oxide scales progressively occurs dur-
ing the corrosion cycles leading to a high corrosion rate at the beginning, which decreases
as these oxides stabilized. The uncoated substrate (Ni-based alloy Superni 75) experienced
a continuous cycle of formation and spallation of oxide scales during the hot corrosion
that causes the degradation of the materials. Tristancho-Reyes et al. [90,105] studied the
behavior of NiCrFeNbMoTiAl coating deposited by plasma spray on the low-grade T22
steel with sodium–vanadium (80%V2O5–20%Na2SO4 and 80% Na2SO4–20%V2O5 mixtures)
and potassium–vanadium (80%K2SO4–20%V2O5) molten salts. Depending on the salts and
on the temperatures, the coating showed different behaviors but performed well with val-
ues of corrosion rate far lower than the uncoated T22 steel. Corrosion rate of 0.843 and
0.150 mm/y for 80%V2O5–20%Na2SO4 and 80%V2O5–20%K2SO4, respectively, were meas-
ured. When the coating was exposed to 80%V2O5–20%Na2SO4, the corrosion rate increased
from 0.494 to 0.5191 and 0.933 mm/y when salt temperature was set at 700, 800, and 900
°C, respectively. The values measured are still too high to be accepted in the TES system,
but the deposition process was not optimized, and the thermal conditions were more se-
vere than those applied in hot tanks. The results are promising and can lead the way to
the use of low-alloy steel in both cold and hot tanks, replacing the more expensive stain-
less steels and Ni-alloys.
Ni–Cr systems have also been studied as protective coatings; nickel and chromium
form highly stable oxides when exposed to an oxidizing environment, providing high re-
sistance to aggressive media. A thorough study was conducted by Porcayo-Calderon
[106,107] on the performance of Ni20Cr coating deposited by HVOF and combustion pow-
der spray processes with chloride salts. Regarding the chloride salt, the authors observed
that coating performed better than the uncoated stainless steel 304 substrate, especially at
high temperature. At a temperature of 350 °C, steel and Ni20Cr behaved similarly; the
exposure to the chlorides promoted initial corrosion and the formation of a stable layer of
chromium oxide (Cr2O3) that prevented further aggressive actions. At higher tempera-
tures (from 400 to 450 °C), however, 304SS experienced severe corrosion that led to rapid
degradation of the material. The authors suggested that the “active oxidation” mechanism
is established when the temperature of molten chlorides rose above a threshold. During
active oxidation, the protective FeOx and CrOx dissolved in the salt, forming FeClx and
CrClx gases according to the following reaction:
Cr2O3 + 3Cl2 = 2CrCl3 + 3/2O2
The gaseous metallic chlorides penetrate quickly in the material through the inter-
connected voids, and porosities formed after the depletion of Fe and Cr elements inside
the material (bulk and coating), driven by physical transportation phenomena, which are
faster than a solid diffusion mechanism. The metallic chlorides react with oxygen forming
nonprotective Fe/CrOx oxides and Cl2. The high concentration of Cl2 triggers again the

Metals 2021, 11, 1377 21 of 30
degradation cycle, leading to poor adherence of the oxide scales formed on the steel,
which spall and expose the surface to the aggressive medium [108,109]. Conversely,
Ni20Cr in both conditions remains stable without significant degradation and only slight
penetration of the salts through the surface porosity. Only at the highest temperature does
the coating experience a more pronounced degradation due to the preferential attack by
chlorides to chromium oxide, which causes the formation of cracks inside the coatings.
NiO is more stable and less soluble in chlorides than Fe and Cr, despite CrOx usually being
more protective than Fe and Ni oxides. Additionally, NiClx is more thermodynamically
stable and conditions to dissolve in oxide do not establish. The presence of Ni, more than
Cr, in the coating contributes to the globally higher resistance of the coating compared to
Fe- and Cr-rich alloys [110]. HVOF, therefore, can be addressed as a suitable technology
to manufacture protective coatings in TES applications. However, corrosion protection in
molten salts does not only depend on the material per se, or on the morphology of the
manufactured coating, but is strongly influenced by the chemical stability of the metallic
element and their compounds, such as oxides and chlorides. Contributions of [106,107]
provide a beneficial understanding of the behavior of Ni-, Cr- and Fe-based alloy and
open the way to further improvements related to the investigation of coating stability at
higher operating temperatures (closer to the CSP regime) and the optimization of the ther-
mal spray deposition.
Flame sprayed Ni20Cr coating provided good protection against V2O5–NaSO4 mol-
ten salt mixtures up to 750 °C [111]. The corrosion velocity was found susceptible with the
temperature and exposure time. The corrosion rate is highest at the beginning (namely 1–
2 h) and no remarkable differences are visible between the coated and the uncoated 312SS.
After a long exposure, i.e., 22 h, NiCr coating experienced severe corrosion but was able
to resist much more than the uncoated steel, retarding the corrosive attack. Corrosion rate
established around 15,000 mpy at 750 °C while the steel showed almost 40,000 mpy. The
coating reduces the steel corrosion rate. When uncoated, it is by more than 50%; however,
the value is still high for practical applications in CSP and needs more study before pro-
tection against this corrosive phenomenon can be recommended.
In very recent works, Gomez-Vidal et al. [112,113] proposed the use of MCrAlX
(where M is Ni or Co; X stands for Y, Hf, Si, or Ta) compounds deposited by HVOF or
APS with carbonate and chloride salts (KCO3–Na2CO3 (46.2/53.8 wt%) and KNa2CO3–
Li2CO3 (33.4/34.5/32.1 wt%) mixtures. These alloys were successfully used to protect the
surface of turbine blades from molten salt attacks. The protective effect derives from the
formation of a compact and fully adhered alumina layer on the coating surface when ex-
posed to an oxidizing environment, which provides high-temperature corrosion re-
sistance [114–116]. The authors investigated different MCrAlX systems (that have already
been used as a bond coat in thermal barrier coating) having distinct compositions (Figure
12 summarizes the corrosion rate of the coatings and the uncoated substrate used as a
benchmark). As expected, uncoated alloys suffered severe corrosion at all temperatures,
with values of corrosion rate ranging from 0.5 mm/y at 650 °C to more than 2.5 mm/y at
700 °C. Alloys with high Ni%, such as In800H and SS310, showed better resistance than
other high-grade alloys, namely 321 and 347 stainless steel and In625.
All the coatings allowed for reducing the corrosion rate to a minimum of 34 µm/y.
APS coating Ni–Co–Cr–Al–Hf–Si–Y showed the best corrosion resistance after a pre-oxi-
dation treatment, passing from approximately 1 mm/y of the as-deposited coating to 34
µm/y. It was followed by HVOF coating Co–Ni–Cr–Al–5Y that achieved a corrosion rate
of 46 µm/y after oxidization. Such behavior was also observed with chloride salt mixture
(44.53 wt% NaCl—55.47 wt% KCl). Uncoated alloys were severely attacked by chlorides
with 2.5 and 4.5 mm/y measured corrosion rate. Thermal sprayed coatings reduced the
corrosion rate by one order of magnitude, except for the NiCrAl system, which registered
values at 570 µm/y for NiCoCrAlYTa, 980 µm/y for NiCo–CrAlYHfSi, 1230 µm/y for
NiCoCrAlY, and 1240 µm/y for NiAl in as-deposited conditions. High percentages of Ni
in the coating composition are beneficial to the corrosion resistance with chloride salts;

Metals 2021, 11, 1377 22 of 30
indeed native Ni-oxide formed during the HVOF/APS deposition process is more stable
than Cr and Fe oxides and slows the “active oxidation” mechanism [108,109].
Figure 12. Corrosion rates of bare and coated alloys in Na2CO3–K2CO3–Li2CO3 in bone-dry CO2 and
N2 atmospheres, reproduced from [113], with permission of Elsevier 2016.
Pre-oxidation treatment further decreases the corrosion for all coatings. HVOF
sprayed NiCoCrAlYTa reached a corrosion rate of 190 µm/y reducing the corrosion of
SS310 substrate by 96%. Coatings form corrosion products when exposed to the salts on
the surface and through the thickness, at the interface with oxide scales formed after pre-
oxidation treatment, and near residual pores generated during the thermal spraying (see
Figure 13 and Table 2). These products, rich in Al and Cr-oxides, did not propagate deep
within the coating and remain confined near the surface; the pores are not interconnected
and prevent the permeation of the molten salts through the coating to the substrate, which
did not show evidence of corrosion attacks.
Figure 13. BSE–SEM image of pre-oxidized HVOF–NiCoCrAlYTa (air, 900 °C, 24 h, 0.5 °C/min) and
exposed to NaCl–KCl at 700 °C, showing penetration at the outer surface of the top coating, adapted
from [112].
Table 2. Element percentage from EDS analysis of HVOF–NiCoCrAlYTa pre-oxidized surface after
exposure to NaCl–KCl salt (see Figure 13), data from [112].
EDS
wt%
Phase
a
b
c
O
22.76
31.24
20.12
Al
0.33
2.19
26.08
Si
0.79
1.00
1.15
Cr
48.21
50.09
16.82
Fe
1.10
1.18
0.58
Co
5.48
2.28
12.54
0.0 0.5 1.0 1.5 2.0 2.5
In800H, CO2, 700°C
In800H, CO2, 650°C
In800H, N2, 600°C
SS310, CO2, 700°C
SS310, CO2, 650°C
SS310, N2, 600°C
NiCoCrAlTaY
NiCrAl
CoNiCrAlY
CoNiCrAlY - Oxid. (800°C)
NiCoCrAlHfSiY
NiCoCrAlHfSiY - Oxid. (900°C)
CR, mm/year
34 µm/year
46 µm/year
*
*
All coatings above tested at 700°C in bone-dry CO
2

Metals 2021, 11, 1377 23 of 30
Ni
10.81
2.70
20.58
Y
8.56
7.09
0.51
Corrosion performance depends on the composition of the coatings but also on the
manufacturing processes. The pre-oxidization favored the formation of a protective alu-
mina layer (almost 3 µm thick) on the surface of the coating, which separates the metals
from the carbonates reducing the corrosion. It can result in a reduction of 50% w.r.t. the
as-sprayed coatings. However, heating conditions that are too extreme could be detri-
mental to corrosion resistance. HVOF sprayed NiCoCrAlYTa, pre-oxidized at 1000 °C,
indeed, showed a higher corrosion rate than as-deposited coating; the authors argued that
treatment was too close to the maximum operating temperature of the materials (around
1050 °C), leading to a loss of cohesion of particles inside the coating, which accelerated the
corrosion. Additionally, both thermal spray process and pre-oxidation altered the sub-
strate; refinement of grains at the coating/substrate interface, precipitates (AlN), depletion
of Cr (for the SS), boundary ditching, and sensitization due to Cr-carbide precipitation
have been observed. These can promote intergranular attack and the occurrence of local-
ized corrosion (pitting) in the substrate if the coating fails in protecting it.
A novel approach proposed by [117] employed amorphous Fe–Cr (60Fe–40Cr) and
Ni-–Cr (60Ni30Cr5Mo and 60Ni40Cr) coatings, deposited by diamond jet spray as protec-
tion against molten chlorides. The absence of grain boundaries and the lack of crystal de-
fects in the amorphous microstructure, indeed, should prevent the intergranular attack,
occurring due to the dissolution of chromium in chloride solution in Cr-rich alloys, and
increase the corrosion resistance [118]. Manufactured coatings performed well, not show-
ing remarkable corrosion and protecting the substrate from chloride attacks. Coatings ex-
perienced neither a significant degradation nor Cr depletion, despite the amount of chro-
mium present in their composition. The authors ascribed the observed behavior to micro-
structure recrystallization of the coating after exposure to molten salt at 750 °C; they sug-
gested that crystallization could inhibit the interaction between Cr and chlorides, thus
enhancing the corrosion resistance, but the mechanisms behind the low degree of corro-
sive attack were not fully clarified.
Outcomes of the work by Gomez-Vidal and Raiman indicate that protection by the
thermal spray deposition of high-resistant coatings is a viable approach to extend the use
of carbonates and chlorides for a TES system, allowing an increase of the operating tem-
perature from 550 °C (the actual operative limit to employ the nitrates mixtures) to 700/750
°C. Increasing the operating temperature regime, use of low-cost HTF/TES fluid (at least
in case of chlorides), and reduction in the plant maintenance and shutdown costs, derived
from a longer lifetime of the components, make more achievable the goal of the Strategic
Energy Technology (SET) plan and the DOE SunShot program, which aim to reduce the
levelized cost of energy (LCOE) of a solar source and to increase penetration of solar en-
ergy into the energy system [119,120]. However, to date, more studies are required; dep-
osition processes are not fully optimized, and the solutions proposed were mostly tested
in very controlled, laboratory-scale conditions (inert atmosphere static exposure, and high
purity). Information on the behavior of these protective coatings for in-service conditions
lacks the following: testing in flowing salts to exclude saturation of salt with a thermal
gradient, investigating salts with a range of chemistries (including salts with impurities
and redox additive), and analyzing reactive atmospheres. All are necessary, however, to
fully determine the performance of the protective coatings and predict their duration.
4. Conclusions and Future Perspectives
The next generation of concentrating solar power plants is expected to work at higher
temperatures, from 600 °C for actual generation up to 850 °C or more, in order to achieve
higher efficiency. In this regard, one of the main goals for engineers and designers is to
increase the amount of energy collected from solar radiation, maximizing the heat deliv-
ered to the transfer fluid and, at simultaneously, reducing the heat loss at the receiver.

Metals 2021, 11, 1377 24 of 30
Currently, the use of absorbing coatings manufactured by thermal spray techniques has
been shown as a viable alternative, a compromise between vacuum processes (e.g., PVD,
CVD, etc.) and wet chemistry technologies (such as sol-gel dip coating or painting). Ther-
mal spray processes, indeed, provide intriguing advantages, such as high stability of the
coating, very good mechanical performance, high production efficiency with relatively
low capital cost, and ease of scaling from small to large production volumes. In the last
decade, several attempts have been made to extend the use of thermal spraying from con-
ventional surface modification applications to the manufacturing of selective absorbers.
The thermal sprayed cermet coating showed values of optical properties below those
achievable with vacuum processes and far from the Pyromark 2500 paint target, with ab-
sorptance not above 0.85 and thermal emittance at approximately 0.4 or higher. The main
reason behind the observed results is related to the strong heterogeneity of the coatings
deposited by thermal spraying; thermal sprayed coatings, indeed, are characterized by a
complex microstructure made by flat lamellae and partially melted particles, and by open
and closed porosities. Porosities, grain boundaries, internal phases, and surface roughness
influence the interaction between the incident light and the material conditioning the final
optical properties. In addition, the contributions reviewed show that slight variations in
process parameters may have a significant effect on optical performances, more than those
observed on other properties, e.g., mechanical, geometrical, or morphological. The strate-
gies explored to make thermal sprayed absorbers more competitive involve post-deposi-
tion processing, mainly heat treatment, to promote the formation of specific oxide phases
on the coating surface, or multimodal structures obtained by adding in the metal matrix
reinforcement, having distinct sizes from nano- to microscale, or the use of multilayered
coatings. These solutions successfully improve the optical properties of sprayed coatings,
raising the absorptance to 0.9 or more, and with reduced values of emittance. Conversely,
they add complexities to the manufacturing of the coating, and further operative costs.
The efforts of the research reviewed here lead the way to the diffusion of thermal spray in
the CSP technologies, especially for the solar tower systems, where the large scale of the
plants and challenging operating conditions make these processes very intriguing. How-
ever, more work is still needed. First, thermal spray processes have been widely optimized
for a great variety of materials and several applications, such as thermal and electromag-
netic barriers and wear and corrosion resistance, but the interaction between process and
optical performance has only been marginally investigated; more research is necessary for
the solar-absorbing materials. Furthermore, the data collected from these experimenta-
tions are related to laboratory measurements in very controlled conditions; analyses of
coating performance and durability in actual operative conditions (thermal and mechan-
ical cycling stresses, variable night-and-day solar exposure, harsh environments, etc.) are
lack in the current literature and need to be implemented in order to assess the feasibility
of thermal spraying.
Significant improvements in the diffusion of concentrating solar power plants can
also derive from the development of thermal energy storage technologies. In this field,
research is oriented to new materials for both thermal-energy-storage fluids and storage
tanks, able to withstand increasingly higher operating temperatures. In this scenario, the
use of molten salts as a heat transfer fluid or as an energy storage medium has been pro-
posed in order to reach working temperatures between 600 and 800 °C. However, the use
of such aggressive media at those temperatures leads to severe issues related to the integ-
rity of the structural components (namely, piping, heat exchangers, and storage tanks) of
concentrating solar power and thermal energy storage systems, due to the vulnerability
of the construction materials against the corrosive attacks from the molten salts. The way
proposed recently by designers and producers to prevent the critical failures or mitigate
damages in thermal energy storage and concentrating solar power components involves
the use of low-alloyed structural steels coated with high resistant materials. Coatings, in-
deed, act as a protective barrier limiting or slowing the oxidation reactions of the consti-
tutive elements of the substrate, and self-sacrificing to prevent damage to the components.

Metals 2021, 11, 1377 25 of 30
Thermal spray processes have been employed since the 1980s to manufacture corrosion
protective coatings in energy generation plants and engine components, and a wide and
exhaustive literature can be found. However, protection against nitrate, chloride, or car-
bonate salts in solar energy plants by using thermally sprayed coatings has been margin-
ally debated, and only in the last decade devoted investigations have been conducted. To
date, it is not possible to derive a direct comparison between the coatings because the
measured performances are susceptible to the test conditions (time and temperatures,
mainly), the type of test, and the analysis methodology. However, the published contri-
butions indicate that thermal sprayed coatings successfully protect the substrate from cor-
rosion, even if they suffered severe degradation. In some circumstances, the coatings
showed a corrosion rate of around 30 to 40 µm/y in very severe conditions, almost match-
ing the value of 10 µm/y, suggesting this as the threshold to guarantee an uninterrupted
30-year working life of the plant. In other examples, coatings registered a very high cor-
rosion rate (even exceeding 1 mm/y), but they always performed better than the uncoated
substrate in the same conditions, and in all cases, no traces of degradation were detected
in the substrate. The results depicted by the revised contributions are promising and sug-
gest that thermal spray deposition of highly resistant coatings is a suitable approach for
corrosion protection of thermal energy storage components, not only with nitrates but
also with carbonates and chlorides, allowing an increase in operating temperature from
550 °C (the actual operative limit to employ the nitrates mixtures) to 700/750 °C. However,
more studies are needed. Indeed, the solutions proposed were mostly tested in very con-
trolled, laboratory-scale conditions (inert atmosphere, isothermal and static exposure, and
high purity of salts). The behaviors observed are valid for the specific coating/corrosive
medium pair and are related to the particular conditions; therefore, predictions concern-
ing the performance of coatings and their duration are not fully reliable. In addition, esti-
mates of corrosion behavior of the coatings have been made, hypothesizing the occurrence
of uniform corrosion of the coating that is a safe condition, while localized corrosion phe-
nomena (pitting or crevice formation), which can lead to a catastrophic and premature
failure, have been neglected in a first approximation. Furthermore, information about the
behavior of these protective coatings for in-service conditions needs to be collected. Tests
involving flowing salts, which more closely resembles what actually occurs in thermal
energy storage facilities and concentrating solar power plants, and including salt satura-
tion and salts with a range of chemistries (i.e., impurities or redox additives) together with
variable thermal gradients and a reactive atmosphere, need to be carried out.
Author Contributions: Conceptualization, P.P. and G.P.; methodology, F.R. and P.P.; formal anal-
ysis, F.R.; investigation, F.R. and G.P.; resources, P.P.; data curation, G.P.; writing—original draft
preparation, F.R; writing—review and editing, F.R., G.P., P.P., and P.C.; supervision, P.P.; funding
acquisition, F.R. All authors have read and agreed to the published version of the manuscript.
Funding: The authors wish to thank “Comunidad de Madrid” and European Structural Funds for
their financial support to the ACES2030-CM project (S2018/EMT-4319).
Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.
Data Availability Statement: Not applicable.
Acknowledgments: This project received funding from the European Union’s Horizon 2020 re-
search and innovation programme under the Marie Skłodowska-Curie grant agreement No. 754382.
Conflicts of Interest: The authors declare no conflict of interest.

Metals 2021, 11, 1377 26 of 30
Disclaimer: The content of this article does not reflect the official opinion of the European Union.
Responsibility for the information and views expressed herein lies entirely with the authors.
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