**3. Electrochemical evaluation**

*2.2.3. Thermal conductivity*

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

94 New Technologies in Protective Coatings

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.

To evaluate the performance and efficiency of corrosion protection coatings, it is common to use electrochemical methods, electrochemical impedance spectroscopy (EIS) being widely applied. The working electrode was immersed in the test solution similar to a geothermal fluid containing 3580 ppm Na and 6693 Cl until a steady‐state open‐circuit potential (*E*oc) was measured. Electrochemical impedance spectroscopy measurements as a function of time were made using an ACM potentiostat, carried out at corrosion potential *E*oc, by using a small sinusoidal signal with an amplitude of ±10 mV, in a frequency interval of 50 mHz to 20 KHz recording six points per decade. The EIS results were represented using Nyquist and Bode plots, and the effect of time of exposure of the aluminum and coated samples to the corrosion media was also evaluated. Also, electrochemical noise measurements (ENMs) were taken with a second electrode at the tip of a platinum wire and the reference electrode.

The parameters for electrochemical noise were a second Pt tip electrode 1 or 0.5 s per sample obtaining 1024 measurements. The electrolyte was (3580 ppm Na and 6693 ppm Cl) similar to a geothermal fluid (see **Figure 6**).

**Figure 6.** Electrochemical measurements as a function of time: EIS charge transfer resistance and electrochemical noise resistance.

From the electrochemical results obtained EIS charge transfer resistance and electrochemical noise resistance, a similar pattern is revealed. After 720‐h immersion, a decrease was observed for both techniques, suggesting an increase in the corrosion rate for the coatings tested. A decrease of one order of magnitude was observed [26].

## **4. Conclusions**

An example of hydrophobic coatings for geothermal heat exchangers was presented. The coating characterization was obtained and coating evaluation as a function of time was presented. Good coating behavior was obtained for both hydrophobic coatings tested.
