**2. Coatings**

Examples of hydrophobic coatings were obtained from sol‐gel, or dip‐coating methods, over aluminum substrate. The hybrid‐coating systems used were based on silane polymer solutions or compounds. Evaluation and characterization were done using electrochemical techniques and characterization through scanning electron microscope (SEM), water‐drop contact angle, and thermal conductivity measurements.

#### **2.1. Electrochemical etching**

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

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

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

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

Examples of hydrophobic coatings were obtained from sol‐gel, or dip‐coating methods, over aluminum substrate. The hybrid‐coating systems used were based on silane polymer solutions or compounds. Evaluation and characterization were done using electrochemical techniques and characterization through scanning electron microscope (SEM), water‐drop contact

component failure.

90 New Technologies in Protective Coatings

as Pb, Zn, Fe, and Cu [20].

within the tube walls.

schemes [21, 22].

**2. Coatings**

angle, and thermal conductivity measurements.

Commercial aluminum (see **Table 1**) rods were cut in cylinders 2 cm height to expose approximately 1 cm<sup>2</sup> area to the corrosive electrolytes, which after their electrical connection by means of a copper wire were encapsulated in an epoxy resin at room temperature. Before experiments, the electrodes' surface was prepared abrading with 600 grade emery paper and rinsed with distilled water, and after cleaning, the electrodes were degreased with acetone and finally dried in warm air flow. The purpose is twofold: to render the metal surface ready for coating application improving the coating adhesion and second to improve corrosion resistance of the metal surface. The electrochemical arrangement was a three‐electrode cell using a graphite rod as a counter‐electrode and a silver/silver chloride as a reference electrode.

The aluminum etching is as follows: the electrode was immersed in NaOH pH 11 solution for about 3 min. Afterwards, the electrodes were removed and cleaned again with de‐ionized water, dried and immersed again, this time in a HCl pH 3 solution, promoting anodic dissolution, polarizing up to −100 mV(Ag/AgCl2) potential and left constant at this potential for 30 min. The presence of chloride ions promotes the formation of micro‐/nanopits or pores over the metal surface. The samples were ultrasonically cleaned with de‐ionized water, dried, and ready for coating application.

**Figure 1** presents SEM micrographs (cross section) where micro/nanostructure can be observed over the aluminum surface, after electrochemical etching and anodic dissolution. Rough and convex irregular and attacked areas and pitted surface are clearly observed. These structures may be favoring the capacity to trap air bubbles supporting hydrophobicity with an appropriate surface roughness. The structure is irregular and disordered and micro/nanoparticles and pores can be observed.

In the presence of chloride ions, the surface structure promotes the appearance of a larger quantity of pits and nanopores at high potentials (above ‐100 mV). This is presented in general and a detailed view is seen in **Figure 2**, where a multitude of pits and pores over the anodized surface can be observed. The anodized compact film surface diminishes corrosive anions attacking the aluminum substrate [23].

#### **2.2. Coatings**

Then, the as‐prepared samples were immersed in 5mM perfluoro cthyl trioxi sylane {CF<sup>3</sup> (CF<sup>2</sup> )5 (CH<sup>2</sup> ) 2 ‐Si(OCH<sup>2</sup> CH<sup>3</sup> )3 } (PTES) solution obtained from Sigma Aldrich. The solution prepared was a mixture of de‐ionized water and ethanol (volume ratio 1:1) at 60<sup>o</sup> C for 2 h [24]. The coating was applied through two‐cycle dip coating technique. The first one was immersion for 1 min and then removed and was allowed to dry, and then a second 30‐s immersion was done to seal the pores present on the coated sample.


**Table 1.** Aluminum substrate composition.

**Figure 1.** Micrograph after electrochemical aluminum etching in acid media.

**Figure 2.** SEM micrograph showing pores over aluminum in acid media in the presence of chloride ions.

Another coating was prepared via sol‐gel synthesis, combining one inorganic and another organic compound. The first one consists of a combination of zirconium tetra‐n‐propoxide with ethyl aceto‐acetate using nitric acid pH 5, as a catalyzer for the reaction. The organic part was prepared mixing 3‐glycyloxypropyltrimethoxysilane (GPTMS) with 2‐ propanol (SA nomenclature) also in the presence of HNO<sup>3</sup> . During the sol‐gel synthesis, a commercial fluoride compound tetra deca‐flouro hexane (TDFH) *Chemguard* from Sigma Aldrich (HA nomenclature) was incorporated in different weight proportions (10, 20, and 30%). The sol‐gel coating was deposited using the spin‐coating technique, at 2800 rpm for 28 s.

#### *2.2.1. Coating thickness*

After substrate preparation and coating application, film thickness measurements were taken using a coating thickness tester. The average values obtained from five measurements are presented in **Table 2**.

The highest thickness obtained and observed was the PTES coating, while the sol‐gel coatings were 10 times thinner. From these coatings, the higher thickness obtained was the SA coating and the CH 30%, with the highest fluoride compound percentage incorporated. As an example, **Figure 3** presents the general and detailed view (cross section) of the aluminum PTES‐coated sample, showing good adhesion conditions over the metal substrate.

#### *2.2.2. Hydrophobicity*

For each sample, the contact angle (right and left) was measured and the average was obtained. When a surface shows a contact angle greater than 90°, it represents a hydrophobic surface. The PTES‐ and CH 20%‐coated samples clearly present hydrophobicity (see **Table 2**). The image is presented in **Figure 4**.


**Table 2.** Coating thickness measurements.

Another coating was prepared via sol‐gel synthesis, combining one inorganic and another organic compound. The first one consists of a combination of zirconium tetra‐n‐propoxide with ethyl aceto‐acetate using nitric acid pH 5, as a catalyzer for the reaction. The organic part was prepared mixing 3‐glycyloxypropyltrimethoxysilane (GPTMS) with 2‐

**Figure 2.** SEM micrograph showing pores over aluminum in acid media in the presence of chloride ions.

a commercial fluoride compound tetra deca‐flouro hexane (TDFH) *Chemguard* from Sigma Aldrich (HA nomenclature) was incorporated in different weight proportions (10, 20, and 30%). The sol‐gel coating was deposited using the spin‐coating technique, at 2800 rpm

. During the sol‐gel synthesis,

propanol (SA nomenclature) also in the presence of HNO<sup>3</sup>

**Figure 1.** Micrograph after electrochemical aluminum etching in acid media.

92 New Technologies in Protective Coatings

for 28 s.

**Figure 3.** SEM micrograph showing PTES coating: (a) general and (b) detailed view.

**Figure 4.** Contact angle of coating.

#### *2.2.3. Thermal conductivity*

One method to evaluate the thermal conductivity (heat transfer) of polymer coatings, which is important to use in heat exchangers, is the heat plate method to measure the temperature distribution of thermal conductivity. This is according to ISO 8302:1991 standard [25]. Its principle consists in generating a unidirectional heat flow through the samples, as flat plates

**Figure 5.** Thermal conductivity of bare aluminum and PTES‐coated sample.

conducting heat. As an example, **Figure 5** presents the thermal conductivity as a function of time of PTES coating over the aluminum substrate and compared with the blank (bare metal) sample. The coating presents favorable thermal conductivity conditions when compared with aluminum.
