**1. Introduction**

The main problem of heat exchanger operation in contact with highly corrosive geothermal fluids is the deposition of fluid salts within heat exchanger tube walls, giving rise to corrosion problems. In combination with fluid pH and aggressive ions concentration such as chlorides, they reduce the lifetime operation of such equipment. Due to these facts and including mass transfer considerations, affordable protection of heat exchangers is a must from the effect of corrosive geothermal fluid. Various materials do exist and are used in heat exchangers in the

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geothermal energy production industries such as steel, copper alloys, and aluminum, depending on cost‐benefit decision making. Nevertheless, proposals do exist to coat these materials with polymeric‐coating systems giving dubious practical results in the past. A possible alternative is the use of hydrophobic coatings, to increase the long‐term aluminum corrosion resistance under severe natural saline conditions encountered in geothermal fluid fields.

#### **1.1. Nanocoatings**

The production of thin films (nano coatings) is a technological field with many applications to elaborate materials with new properties as well as protection of traditional metals, nowadays and in the foreseeable future. Different materials and coatings are combined to produce hybrid or composite materials with special characteristics and properties [1].

The coating thickness range is from tens of nanometers (nm) up to some microns (μm) and in most *grand* scale applications, mono‐layered coatings are used, but fabricating oxide‐oxide, oxide‐metal or other variable metals oxides combination multi‐layers, properties may be improved. Therefore, film thickness control is critical [2, 3].

Coating can be classified due to their manufacturing material interactions, substrate nature, and according to its specific barrier applications in thermal, corrosion and oxidation, wear and diffusion types [4]. Coating nano‐technology concentrates in obtaining thinner films with the same or enhanced protection when compared to conventional coating technologies.

### **1.2. Sol‐gel coatings**

Within the recent deposition technique alternatives, the sol‐gel route technology is promising. In the last two decades, there is an increased widespread use of nanotechnology coatings through sol‐gel methods, formed by way of organic‐inorganic components, eliminating highly damaging and toxic solvents to the environment and the human itself. These coatings protect the metal surface from corrosion by different corrosion protection mechanisms [5].

Sol‐gel synthesis route represents an alternative to generate components or compounds with controlled size, which was difficult to obtain in the past by conventional preparation techniques. This uniform procedure process results not only in particles agglutination but also in a range of products such as fibers, monolithic structures, thin films, or coatings in a wide variety of hybrid or composite materials. The adequate selection of precursor and reactants allows obtaining materials with tailor‐made–designed properties, useful in a wide variety of technological applications such as optics, electronics, biology, medicine, etc. [6]. In general, the sol‐gel route is the formation of a three‐dimensional oxide resulting from hydrolysis and condensation reactions of the molecules from precursors present in a liquid medium under relatively low temperatures and no physical and/or chemical reactions with the substrates, these being advantageous for practical applications.

#### **1.3. Hydrophobic coatings**

Hydrophobic coatings reject an aqueous dissolution or electrolyte. These characteristics may be accomplished by means of encapsulated functionalized species present in the coating or by changing the medium composition. Also, if the more external or outer‐layer morphology or structure of the system is changed or modified, a hydro‐ or superhydrophobic system is promoted [7–10]. The strategies or methodologies contemplating a superficial change through encapsulation systems promised advantageous success; nevertheless, few developments really achieved an effective corrosion protection, since the effect decreases as a function of elapsed time of surface (coating) contact electrolyte [11–13].

To obtain a hydrophobic outcome, a "*Lotus leave*" effect must be generated, which is when the surface attains a roughness level that effectively repels any type of aqueous dissolution. To simulate this effect on coating surfaces, various types of species, such as potassium stearate or calcium hydroxide, porous silica or synthetic urea formaldehyde capsules, styrene‐based copolymers such as methyl‐methacrylate, were encapsulated and incorporated into coating systems, among others [7–14].

Sol‐gel is a versatile method to produce super‐hydrophobic surfaces. The literature reports [15] successive hydrolysis and condensation‐polymerization reactions using ammonia as a catalyzer over aluminum substrate, generating films with high degree of hydrophobicity (contact angles ≈150°). Good corrosion resistance for short periods of immersion was obtained. Incorporation of porous silica particles doped with 3‐amino‐propyl triethoxysilane compounds as hydrophobic agent generates transparent sol‐gel coatings [16]. Also, sol‐gel modifies the polymeric structure with diverse functionalized agents such as fluorinated compounds [17] and/or organic‐inorganic precursors [15], or nano‐fibers or nanotubes incorporation [18] promoting Lotus leaves‐like nanostructures increasing corrosion resistance.

#### **1.4. Aluminum and alloys**

geothermal energy production industries such as steel, copper alloys, and aluminum, depending on cost‐benefit decision making. Nevertheless, proposals do exist to coat these materials with polymeric‐coating systems giving dubious practical results in the past. A possible alternative is the use of hydrophobic coatings, to increase the long‐term aluminum corrosion resistance under severe natural saline conditions encountered in geothermal fluid fields.

The production of thin films (nano coatings) is a technological field with many applications to elaborate materials with new properties as well as protection of traditional metals, nowadays and in the foreseeable future. Different materials and coatings are combined to produce

The coating thickness range is from tens of nanometers (nm) up to some microns (μm) and in most *grand* scale applications, mono‐layered coatings are used, but fabricating oxide‐oxide, oxide‐metal or other variable metals oxides combination multi‐layers, properties may be

Coating can be classified due to their manufacturing material interactions, substrate nature, and according to its specific barrier applications in thermal, corrosion and oxidation, wear and diffusion types [4]. Coating nano‐technology concentrates in obtaining thinner films with the same or enhanced protection when compared to conventional coating technologies.

Within the recent deposition technique alternatives, the sol‐gel route technology is promising. In the last two decades, there is an increased widespread use of nanotechnology coatings through sol‐gel methods, formed by way of organic‐inorganic components, eliminating highly damaging and toxic solvents to the environment and the human itself. These coatings protect the metal surface from corrosion by different corrosion protection mechanisms [5].

Sol‐gel synthesis route represents an alternative to generate components or compounds with controlled size, which was difficult to obtain in the past by conventional preparation techniques. This uniform procedure process results not only in particles agglutination but also in a range of products such as fibers, monolithic structures, thin films, or coatings in a wide variety of hybrid or composite materials. The adequate selection of precursor and reactants allows obtaining materials with tailor‐made–designed properties, useful in a wide variety of technological applications such as optics, electronics, biology, medicine, etc. [6]. In general, the sol‐gel route is the formation of a three‐dimensional oxide resulting from hydrolysis and condensation reactions of the molecules from precursors present in a liquid medium under relatively low temperatures and no physical and/or chemical reactions with the substrates,

Hydrophobic coatings reject an aqueous dissolution or electrolyte. These characteristics may be accomplished by means of encapsulated functionalized species present in the coating or by

hybrid or composite materials with special characteristics and properties [1].

improved. Therefore, film thickness control is critical [2, 3].

these being advantageous for practical applications.

**1.3. Hydrophobic coatings**

**1.1. Nanocoatings**

88 New Technologies in Protective Coatings

**1.2. Sol‐gel coatings**

Aluminum and alloys are widely used in the industrial, architectural, and marine environments due to technical and economic reasons, being the most extended metallic material used, after steel. Nevertheless, they are very reactive and the need for extra protection against corrosion is necessary for certain industrial applications, especially in chloride‐containing environments. To achieve this goal, coating and surface modification technologies were speedily developed in recent times related to chemistry, electrochemistry, metallurgy, and other disciplines. Aluminum and alloy surface modification generally consists of surface roughness generation (etching), anodic oxidation, hybrid or composite coatings, etc. The chemically modified surface samples present super‐hydrophobicity, increasing corrosion resistance of aluminum and its alloys. This result is attributed to the combined effect of nano/microstructures formed over the surface and low‐energy surface material. Chemical etching improves the hydro‐ or superhydrophobic properties of aluminum and alloys.

#### **1.5. Corrosion in geothermal heat exchangers**

In principle, the fluid extracted from the geothermal well is taken to a separator where the mixture, water vapor from the geothermal fluid, is separated. Vapor is sent to the turbine coupled to a generator, where mechanic energy is transformed into electrical energy. The turbine exit is coupled to a condenser helping to increase the cycle efficiency. Finally, the geothermal fluid is reinjected to the well to help recharging.

The main function of heat exchangers metallic tubes or plates is the heat transfer from a heated flow to the feeding water, and for that reason parameters like material thermal conductivity and thickness should be considered. Metallic corrosion promotes thinning of the tubes or plate walls, causing huge economic losses due to operation failures and plant shutdowns as well as malfunctioning of heat exchange processes. Incrustation is the undesirable material accumulation over the metallic elements, calcium carbonates being the most common precipitation, although silica and metallic sulfurous compounds are also common [19]. Both problems present a decrease in equipment performance and efficiency, since overall heat‐exchange coefficient diminishes gradually promoting some component failure.

Geothermal environments present different composition with respect to the type of resources available, low or high enthalpy [19]. However, the composition solely does not depend on this; in low‐enthalpy environments, the main forms of corrosion are generalized (overall) and localized (certain areas) corrosion. Localized corrosion is the most detrimental damage, since it cannot be predicted and is difficult to follow up its evolution and it can produce major damages. This type of corrosion includes galvanic, pitting, crevice; in general, three types of material incrustations may be encountered: silica and silicates, carbonates, and sulfates and sulfites [20]. Silica is in the form of amorphous silica, carbonate incrustations are in the form of low magnesium calcite and sulfates, while crystallized sulfite in many phases predominates as Pb, Zn, Fe, and Cu [20].

The pH control in geothermal water is a crucial factor to control corrosion and silica incrustations; to control the system, HCl or NaOH is added to the geothermal energy source, reactants being expensive. The alternative is to use protective coatings to help diminish incrustations within the tube walls.

The main problem during geothermal plant heat exchanger operation in contact with geothermal fluids is deposition of fluid salts over the tube walls originating multiple corrosion problems which in combination with the fluid pH and aggressive ions concentration (mainly chlorides) notably reduce the useful life of heat exchangers. Therefore, and taking into account heat transfer considerations, it is necessary to protect the heat exchanger metallic elements from the highly corrosive fluid. Different materials are used as heat exchangers in the geothermal industry like steel, copper alloys, or aluminum and proposals are being made to coat the metallic elements with different polymeric systems or schemes [21, 22].
