Coatings on Copper in Geothermal Water - American Chemical Society

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Fouling and Corrosion Properties of SiO2 Coatings on Copper in Geothermal Water Chen Ning,† Liu Mingyan,*,†,‡ and Zhou Weidong† †

School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, China State Key Laboratory of Chemical Engineering, Tianjin 300072, China

‡

ABSTRACT: Two critical problems in geothermal utilization are corrosion and fouling on the surfaces of plants and fittings, and use of surface coatings is a good method to overcome such difficulties. Silica coatings with film thickness of several hundred nanometers on copper substrates were prepared by the liquid-phase deposition technique and characterized with respect to surface morphology, composition, surface free energy, roughness, and film thickness. Examinations of both fouling in calcium carbonate solution and corrosion in corrosive geothermal water on the surfaces of SiO2-coated samples were performed. The results showed that the fouling rate on the SiO2-coated surface was obviously reduced compared with that on the polished surface. Moreover, the inhibition of corrosion on the modified surface was found even though, after a long immersion time, the corrosion resistance action decreased because of the surface peeling of SiO2-coated samples. The corrosion behavior of the SiO2 coating was further studied with electrochemical measurements by electrochemical impedance spectroscopy. This work suggests a new possibility for solving the fouling and corrosion issues encountered in the use of geothermal water.

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INTRODUCTION Geothermal water is a renewable energy resource and is widely applied in the world because of its advantages of minimal environmental pollution, abatement of carbon dioxide emissions, relatively low cost, widespread distribution, and direct exploitation.1−9 In particular, a significant opportunity for geothermal development is emerging in decreased greenhouse gas emissions compared to fossil-fuel plants.6 The present challenge is to continue to lower the cost of production without compromising safety to remain competitive with other power sources. Among the factors involved in lowering the cost of geothermal utilization, significant fouling and corrosion are two control issues that have not been satisfactorily settled.6,7,10−14 Scaling and corrosion of highly saline and corrosive geothermal water are often observed within plants or in reservoirs in which the cooled fluid is reinjected into formations, thereby decreasing fluid flow by clogging the pipes of the plant and the pores of the rock. The latter occurrence, in particular, can cause irreparable damage to the reservoir by reducing its injectivity.6,7,13 The continuous process of geothermal energy utilization is often interrupted by fouling and corrosion, which can result in huge economic losses.12 Fouling simultaneously results in an increase in fluid resistance, as well as extra energy consumption and wastewater discharge, and an incomplete fouling layer can lead to local corrosion. The most corrosion- and scaling-relevant compounds in geothermal fluids are scales of carbonates, silica, sulfides, or oxides.13,15−17 Several technologies for inhibiting fouling and corrosion have been developed in the past decades according to the fouling and corrosion categories and severity, including crystallizer−clarifiers,6,18 scale inhibitors,6,7,19−23 brine acidifiers,6,7 plant and fitting material selections,6 electrical submersible pumps,6 polyphenylenesulfide-based or epoxy resin coatings,10,13,24−30 and steam cleaning.6 © 2012 American Chemical Society

Crystallizer−clarifier technology has been shown to allow the recovery of base and precious metals from brines.6,18 In a crystallizer−clarifier, iron silicates are purposely precipitated in surface equipment as sludge to prevent fouling of pipelines, brine- and steam-handling equipment, and reinjection wells.31 Scale inhibitors are usually dispersants that keep scales from adhering tightly to piping or equipment surfaces, and very low dosages of these silica inhibitors mitigate hard scale deposits. If used at high dosages, these inhibitors become uneconomical, and they coagulate silica such that deposition of even soft deposits can rapidly reduce brine flow. Great strides have been made in applying inhibitors downhole to control CaCO3 deposition. Simple to extravagant downhole assemblies have been developed to deliver scale inhibitors just below the point of the onset of CaCO3 formation.6,19−23 The brine acidification method is effective in mitigating hisingerite scaling, provided that the brine reinjection temperature is maintained above about 150 °C. Acidifying brine requires carefully controlled pH adjustment to inhibit scale without exacerbating corrosion. The cost of acid is also a concern, as is dissolution of the injection reservoir. However, by adjusting the brine pH just sufficient to mitigate hisingerite scaling, the process can be economical and not detrimental to corrosion or dissolution of the injection formation.6,7,32,33 Corrosion due to hypersaline brines containing traces of oxidizing metals can be controlled by the method of material selection. High alloy well tubular and production piping have been used to mitigate corrosion. Judicious use of highly corrosion-resistant alloys and cement linings has allowed the approach to be applied economically.6 Received: Revised: Accepted: Published: 6001

September 13, 2011 March 27, 2012 April 2, 2012 April 2, 2012 dx.doi.org/10.1021/ie202091b | Ind. Eng. Chem. Res. 2012, 51, 6001−6017

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deposition (LPD). Among them, the first four involve complex processes and require expensive equipments, and they are difficult to apply to substrates with large surface areas and irregular shapes. The first four techniques are relatively costly and are not easy to industrialize. The sol−gel method has no such shortcomings, but the resulting films often have defects such as cracks, exfoliation, and looseness for the application of corrosion inhibition. The advantages of the LPD technique are as follows: (a) The LPD temperature is always low. (b) Because liquids are used, it is possible to deposit coatings on substrates with complex shapes and large areas. (c) The equipment includes water baths and vessels that are simple and inexpensive. (d) The films are relatively dense, uniform, and crack-free. It has been shown that the fouling process is dependent on surface characteristics, in addition to bulk solution properties and operating conditions, and that low-energy surfaces significantly reduce the adhesion force between the metal surface and foulant.37−41 Recently, researchers have become interested in nanomodified surfaces for the inhibition of fouling.42−44 Inorganic TiO2 coatings with different nanometer-scale layer thicknesses and low surface free energies were prepared by the vacuum coating technique, and inhibition of fouling and corrosion on the coated surfaces in boiling pools of CaCO3 solution was obtained.42,44 Nanomodified surfaces have features of longer induction times, lower fouling rates, and ultimately much lower asymptotic fouling resistance and have potential applications in fouling mitigation.43 On the other hand, it is known that coating a thin layer of certain materials on a metal surface is a good way to prevent metals from corroding. Organic coatings have been developed to avoid the corrosion of metal surfaces. However, organic materials have some intrinsic defects, such as easy degradation, susceptibility to solvents and heat, and weak adhesion to metal substrates. Hence, inorganic and composite coatings have been developed. The corrosion-resistance behaviors of silica (SiO2) sol−gel coatings and chemical-vapor-deposited diamondlike carbon (DLC) films on AISI 304 stainless steel substrates with layer thicknesses of 50−300 nm in 3.5% NaCl solution were studied, and good corrosion-resistance performance was obtained, as the coatings function as a physical barrier and prevent the surface from anodizing.45,46 Nanostructured SiO2 films prepared by the LPD method on the surface of carbon steel showed good anticorrosion abilities, but some local defects in the SiO2 films were also found.47,48 The main goal of this work was to explore the antifouling and anticorrosion characteristics of LPD-prepared low-energy inorganic SiO2 coatings on copper substrates with nanometerscale film thicknesses in low- and medium-temperature geothermal waters, as no open literature could be found in this area. In this work, the optimal LPD conditions for preparing silica films on copper substrates were determined; the properties of the silica film in terms of inhibiting fouling and corrosion were investigated using saturated CaCO3 solution and real geothermal water as experimental fluids. This work is expected to provide a new method for preventing the fouling and corrosion of equipment and pipelines in the utilization of geothermal water.

Electrical submersible pumps have proven useful for controlling carbonate scaling. The pumps maintain the brines in the well in a single liquid phase. This prevents brine from flashing and CO2 from exsolving. By maintaining the acid gas in the liquid phase, the brine pH is not allowed to increase so that carbonates remain undersaturated.6 Another protection technique is the application of coatings.10,13,24−30,34 The effectiveness of protective coatings depends on the pretreatment of the surface and the conditions during application. A variety of paints have been tested in geothermal environments.34 Oil-based paints, water-based paints, and combination coatings have been found to exhibit protective performance, although water-based coatings were less satisfactory than two combinations containing an epoxy resin topcoat.10,27 Poly(phenylenesulfide)- (PPS-) based coatings, including PPS-containing polytetrafluoroethylene (PTFE) as an antioxidant additive, silicon carbide (SiC) as a thermally conductive filler, and aluminum oxide-rich calcium aluminate (ACA) as an abrasive wear-resistant filler, have been coated on AISI 1008 carbon steel surfaces to develop antisilica fouling coatings in a wet, harsh geothermal environment, and PTFEblended PPS coatings have a high potential as antisilica fouling barriers on carbon steel heat-exchanger tubes in hightemperature geothermal environments containing silica-rich brine.24−29 Furthermore, nanoscale montomorillonite (MMT) clay fillers became dispersed in the poly(phenylenesulfide) (PPS) matrix. Cooling of this molten exfoliated material led to the formation of a PPS/MMT nanocomposite. When this advanced PPS nanocomposite was used as a corrosionpreventing coating for carbon steel in a simulated geothermal environment at 300 °C, a coating of about 150-μm thickness adequately protected the steel against hot-brine-caused corrosion.30 Many geothermal power plants use steam and turbine washing techniques to mitigate fouling and extend the time between turbine and generator overhauls. Cooling tower water, surface water, aquifer water, and hot well condensates are usually employed for steam and turbine washing.6 In recent years, a variety of numerical codes for simulating geochemical reactions and heat and mass transfer have been used to improve operations, such as CHILLER.6,35 For instance, scale-prediction models are used to determine the flash temperature and pressure setting to control silica scaling. The development and implementation of these tools by production and operations personnel make geothermal energy competitive as a renewable energy resource.36 Even though the methods of inhibiting fouling and corrosion mentioned above have been developed and applied, several problems such as the need for relatively expensive additives, possible secondary pollution, low utilization efficiency, film layer peeling due to different stress forces between substrates and coatings, and relatively low thermal stability restrict their industrial application. With the developments of surface science and nanometer techniques in recent years, nano- or micrometer surface coating methods might provide new opportunities to overcome the drawbacks of existing inhibition measures for fouling and corrosion and to explore new functional surfaces that are less susceptible to fouling and corrosion. Several techniques for the preparation of nano- or micrometer films have been applied in recent studies, including ion implantation, magnetron sputtering, chemical vapor deposition (CVD), physical vapor deposition (PVD), sol−gel deposition, and liquid-phase

1. PREPARATION, CHARACTERIZATION, AND EVALUATION OF COATINGS 1.1. Materials. Substrates of test samples were made of red copper with dimensions of 0.02 m × 0.01 m × 0.005 m or 0.01 6002

dx.doi.org/10.1021/ie202091b | Ind. Eng. Chem. Res. 2012, 51, 6001−6017


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m × 0.01 m × 0.005 m. All chemicals used were analytical reagents. The experiments to evaluate fouling were carried out in a saturated CaCO3 solution prepared by dissolving specified quantities of NaHCO3 and anhydrous CaCl2 in deionized water. Corrosion assessment experiments were conducted using geothermal water taken from the No. 2 geothermal well at Tianjin University, with the chemical composition reported in Table 1.

triangular pyramid was moved electromechanically on the motionless samples. The stylus was perpendicular to the horizontal surface of the samples. The movement of the stylus in the horizontal direction was perpendicular to the surface texture line at all time of the measurements. The radius of the diamond stylus was 5 μm. For all measurements, the same tracing length of 4 mm with a cutoff of 0.8 mm was chosen. The arithmetical mean deviation of the assessed profile, Ra, was calculated as the average value for three different locations on each sample. 1.3.2. Static Contact Angle. Static contact angles were obtained by the sessile drop method with an optical contact angle measuring instrument (OCA20, DataPhysics Instruments GmbH, Filderstadt, Germany), consisting of an image processing system, multiple dosing/microsyringe units, and a temperature control system. The drop image was processed with an image analysis system that calculated both left and right contact angles from the shape of the drop with an accuracy of ±0.1°. All measurements were carried out at a constant temperature of 25 °C with a dosing volume of 2 μL and a freezing time of 10 s. The measurement of each contact angle was repeated three times, and the average value was calculated to achieve higher accuracy. 1.3.3. Coating Thickness. The thicknesses of the SiO2 coatings were determined using a thin-film thickness measurement instrument (SGC-10, TJGD Co., Tianjin, China) based on the principle of white-light interference. When a white-light beam is projected vertically onto the coating surface, reflectance spectra are formed by the interference of the reflected light from the film surface with that from the interface between the film and substrate. The thickness of single- or multiple-layer films is obtained by analyzing the measured and fitted spectra using fitting software. During the measurements, the expected thickness range should be known to make the shape of the fitted and measured curves closest, so that the most accurate value can be obtained. 1.3.4. Morphology, Elements, and Molecular Structure. The morphologies of the SiO2 coating and the fouled and corroded surfaces were characterized by scanning electron microscopy (SEM; Nanosem 430, FEI Co., Hillsboro, OR). Surface chemical elements were analyzed by X-ray photoelectron spectroscopy (XPS; PHI1600, Perkin-Elmer, Wellesley, MA) and energy-dispersive spectrometry (EDS). The molecule structure of SiO2 thin film was analyzed by infrared reflectance spectroscopy (FTS3000, Bio-Rad Laboratories, Hercules, CA). 1.4. Fouling Experiments. Fouling experiments were carried out in prepared saturated CaCO3 solutions to investigate the antifouling performances of test samples. The concentrations of CaCl2 and NaHCO3 in the test solution were 0.6 and 0.9 g·L−1, respectively. The original concentration of calcium ions in the solution was 400 mg·L−1, as measured by complexometric titration with ethylene diamine tetraacetic acid (EDTA).58 The ratio of solution volume to exposed surface area of the sample ranged from 20 to 200 mL·cm−2, because, at such ratios, the influence of fouling products on the scaling rule could be neglected.59 The sample without a SiO2 surface coating was coated with epoxy resin. During fouling experiments, the concentration of calcium ions in the clear solution was measured every 2 h and was kept constant by replacement with fresh solution. The fouling rate was calculated as

Table 1. Chemical Compositions of Geothermal Water in the No. 2 Well at Tianjin University49 component +

K Na+ Ca2+ Mg2+

concentration (mg·L−1) 39.0 776.0 621.2 57.8

component

concentration (mg·L−1)

−

Cl SO42− HCO3− SiO2 TDS

815.4 1993.2 375.3 24.5 4702.4

1.2. Preparation of SiO2 Coatings. Before LPD, the surfaces of the red copper substrates were polished and cleaned successively with a mixture of NaOH and Na2SiO3, absolute ethanol, deionized water, 3% hydrochloric acid (HCl) solution, and deionized water. The mass concentrations of NaOH and NaSiO3 in the mixture were 10 and 5 g·L−1, respectively. After the substrates had been cleaned, they were immersed in the SiO2 deposition solution in the reactor for certain time. Then, they were removed from the reactor and placed in a dryer until completely dried. Finally, they were annealed and cooled in a muffle furnace filled with nitrogen gas. SiO2 coatings on the substrate surfaces with varying thicknesses were obtained by changing the deposition time and/or concentration of H2SiF6 solution. The chemical reaction occurring in the LPD of SiO2 onto cleaned substrates in SiO2-saturated H2SiF6 solution is50,51 H 2SiF6 + 2H 2O → 6HF + SiO2

(1)

During the deposition of SiO2 onto the immersed substrates, a chemical equilibrium from the left side to the right side of eq 1 is set. The deposition rate is increased by addition of boric acid (H3BO3) to the SiO2-saturated H2SiF6 solution, as a result of the reaction H3BO3 + 4HF → BF4 − + H3O+ + 2H 2O

(2)

According to eq 2, the concentration of hydrofluoric acid (HF) in SiO2-saturated H2SiF6 solution will decrease upon addition of H3BO3, and supersaturated SiO2 solution will form according to eq 1. SiO2 films on substrates of Si wafer, glass, gallium arsenide, and SiGe, among others, have been successfully obtained with the LPD method.52−57 The saturation degree of the H2SiF6 solution was examined by adding H2O2 to the solution and observing the color of the solution. After addition of H2O2, unsaturated solutions appear clear, and saturated solutions appear orange in color.53 In this work, the mass concentration of H2SiO3 in H2SiF6 solution was 100 g·L−1. Deionized water was added to the saturated H2SiF6 solution to obtain solutions of different concentrations. 1.3. Characterization of Coatings. 1.3.1. Roughness. Roughness is a critical parameter for characterizing coated surfaces. A stylus roughmeter (JB-8C, YDYQ Precision Instruments Co., Guangzhou, China) was used to measure the surface roughness of the samples. During the roughness measurements, a diamond-tipped stylus with the shape of a 6003


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W2 − W1 S

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potentiostat/galvanostat (PAR273A, Princeton Applied Research, Oak Ridge, TN) linked to a computer for data acquisition and handling through the PowerSuite package. A three-electrode cell was used, in which a saturated calomel electrode (SCE) was used as the reference electrode, a spiral platinum wire as the auxiliary electrode, and geothermal water as the electrolyte. The working electrodes employed were red copper samples, either bare or coated. All but the useful test area of 1 cm2 was masked with epoxy resin. The electrolytic cell was packed into a microwave oven without power, which acted as a Faraday cage to eliminate external electromagnetic interference. The original samples were tested by applying a sinusoidal signal amplitude of ±0.010 V and a frequency range from 105 to 0.1 Hz, and the other samples were measured with a frequency range from 105 to 0.01 Hz. Before EIS measurements, the samples were immersed in the geothermal water for 10 min to reach potential stability. Each test was repeated at least twice, and good reproducibility of the results was observed.

(3)

where ν is the average fouling rate, g·m−2; W1 is the original mass of samples, g; W2 is the mass of samples with fouling, g; and S is the exposed surface area of the sample, m2. The apparatus for the fouling and corrosion experiments is shown in Figure 1. All experiments were carried out at atmospheric pressure.

2. RESULTS AND DISCUSSION The results of preparation and characterization SiO2 coatings, experiments of inhibition of fouling and corrosion of SiO2 coatings were reported and analyzed as follows. 2.1. Preparation and Characterization of SiO2 Coatings. The preparation conditions of the SiO2 coatings investigated here included deposition solution concentration, reaction temperature, and time. The coating characteristics, such as chemical composition, film thickness, surface roughness, and surface free energy, were studied by the methods described in section 1.3. 2.1.1. Effects of Deposition Conditions on Characteristics of SiO2 Coatings. The investigated reaction conditions include H2SiF6 solution concentration, H3BO3 solution concentration, deposition temperature, and time. 2.1.1.1. H2SiF6 Solution Concentration. Figure 2 shows the trend in the film thickness of the SiO2 coatings with the concentration of H2SiF6 solution. The film thickness decreased with increasing concentration of H2SiF6 solution. This downward trend agrees with that reported in the literature.53 Water is not only a solvent but also a reactant. Increasing the water content promotes SiO2 deposition. Higher concen-

Figure 1. Experimental apparatus for the evaluation of fouling and corrosion of SiO2 coatings: (1) iron support, (2) condenser, (3) test sample, (4) flask with three necks, (5) thermometer, and (6) water bath.

1.5. Corrosion Experiments. Corrosion experiments were conducted strictly following the Chinese national standards by immersing samples into geothermal water to investigate the corrosion properties of SiO2-modified samples.60,61 Geothermal water was identified to be a type of liquid medium that can easily corrode metal materials in it but not a type of liquid in which the fouling is likely to occur. It is a proper fluid for performing corrosion studies. SEM and EDS were used for the morphology and composition analyses of corrosion products. Both the weight differentials and the electrochemical impedance spectra were measured to investigate the corrosion behaviors of the coatings. For weight differential measurements, samples with corrosion products obtained at different temperatures and immersion times were first rinsed with deionized water and dried in a drier at 60 °C. Then, the surface was scraped with an eraser and rinsed with deonized water to remove the loose corrosion products. After that, the samples were immersed in 6 mol·L−1 HCl solution for 1 min at 25 °C and then rinsed with deionized water and immediately dried in a drier at 60 °C. Finally, the samples were weighed with an analytical balance with an accuracy of 0.1 mg, and the weight loss was obtained. The corrosion rate was determined by the corrosion depth method and calculated as61 νL =

W0 − Wt 8.76 St ρ

(4)

where νL is the average corrosion rate denoted by the corrosion depth, mm·a−1; W0 is the mass of the sample before the corrosion test, g; Wt is the mass of the corroded sample with corrosion products removed, g; S is the exposed surface area of the sample in the test solution, m2; t is the corrosion time, h; and ρ is the density of copper, g·cm−3. The samples with and without coatings were immersed in geothermal water at 90 °C and removed after different immersion times for electrochemical impedance spectroscopy (EIS) measurements. EIS experiments were performed using a

Figure 2. SiO2 coating thickness as a function of the concentration of H2SiF6 solution. 6004



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Figure 3. Surface morphologies of SiO2 coatings at various H2SiF6 solution concentrations.

2.1.1.3. Solution Temperature. Figure 6 shows the relationship between the thickness of the SiO2 coatings and the deposition solution temperature. As seen in Figure 6, the thickness of the SiO2 film exhibited an initial rise and then

trations of H2SiF6 mean lower contents of water, which can result in a decrease of the deposition rate and the thickness of SiO2 coatings. The corresponding SEM images of the SiO2 films are shown in Figure 3. No obvious particle deposition on the substrate occurred when the H2SiF6 concentration was low, as shown in Figure 3a. The number of SiO2 particles increased with increasing H2SiF6 concentration, as shown in Figure 3c. However, excess H2SiF6 resulted in poor uniformity as well as film cracks, as seen in Figure 3d. 2.1.1.2. H3BO3 Solution Concentration. The effect of the H3BO3 solution concentration on the thickness of the SiO2 coatings can be seen in Figure 4. Because H3BO3 reacts with HF acid to form BF4−, a higher H3BO3 concentration results in a higher SiO 2 deposition rate. However, excess H3BO3 decreased the deposition rate because of the appearance of large SiO2 particles and the generation of a sol−gel in the solution. Large SiO2 particles are difficult to adhere to the surface of substrates, and the presence of a sol−gel inhibits the deposition of SiO2 particles. Figure 5 provides corresponding SEM images of the SiO2 coatings. The addition of H3BO3 accelerates the growth of SiO2 particles, but simultaneously causes different growth rates of SiO2 particles, resulting in SiO2 particles of different sizes.

Figure 4. SiO2 coating thickness as a function of the concentration of H3BO3 solution. 6005



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Figure 5. Surface morphologies of SiO2 coatings at various concentrations of H3BO3 solution.

the deposition solution, usually insignificant below 30 °C, became evident at 50 °C, which was not favorable for the deposition process. Figure 7 shows typical SEM images of SiO2 films prepared at various deposition temperatures. Defects such as particle accumulation and emptiness existed at T = 50 °C, as seen in Figure 7d. At a lower deposition solution temperature (T = 30 °C), a SiO2 film with good quality was obtained. No particles deposited on the substrate when the temperature was 20 °C, as shown in Figure 7a. 2.1.1.4. Deposition Time. Figure 8 shows the thickness of the SiO2 coatings as a function of deposition time. The thickness first increased and then declined, achieving a maximum at 15 h, as shown in Figure 8. In the later period of the deposition process, a layer of large SiO2 particles adhered to the surface, and some peeled off, as shown in Figure 9d,e, so the particles had weak adhension to the substrate and could be easily washed off by water during water flow. By analyzing the SEM surface morphologies of SiO2 films obtained under different deposition conditions, we found that high deposition rates often led to a reduction of the SiO2 film quality, resulting in cracking or peeling, for example. In contrast, dense, orderly, and crack-free SiO2 films were prepared under relatively mild deposition conditions and at lower deposition rates. The optimum deposition conditions were obtained, and SiO2 films with thicknesses of 188.6, 267.9, and 322.7 nm were obtained and used for further study. 2.1.2. Surface Chemical Element Analysis. Elemental compositions and mass-atomic ratios of the SiO2 films were

Figure 6. SiO2 film thickness as a function of deposition solution temperature.

decreased as the solution temperature rose from 35 to 50 °C, because the degree of decomposition of H2SiF6 solution increased when the solution temperature rose. The higher the solution temperature, the more SiO2 particles precipitated from the solution. However, when the solution temperature was higher than 45 °C, some SiO2 particles deposited at the wall and bottom of the reactor. This indicates that, at high solution temperatures, the precipitation of SiO2 particles was too fast to deposit on the substrate surface, thus leading to a decrease of the surface deposition rate. In addition, evaporative losses of 6006



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Figure 7. Surface morphologies of SiO2 films at various deposition solution temperatures.

Figure 11 shows the IR spectrum of a SiO2 coating with a film thickness of 188.6 nm. The absorption peaks around 1087.96, 1021.16, and 798.91 cm−1 are due to the asymmetric and symmetric stretching vibrations of Si−O bonds. The SiO2 film was abundant in Si−O−Si bonds and had an orderly silica network and consequently good chemical stability.54 Another main transmission at 930.73 cm−1 was found in the spectrum and attributed to the stretching vibrations of Si−F bonds. The F contained in Si−F bonds is from H2SiF6 solution. The peak at 1260.39 cm−1 is attributed to −CH3 and might be from organic contamination of the test system. XPS results are shown in Figure 12. A single smooth peak around 103.3 eV is observed, indicating the excitation of the Si 2p core level in SiO2. A symmetric XPS peak without shoulders implies that the oxide was of good chemical purity. Hence, Si element in the coatings was in the form of SiO2. These results imply that SiO2 films of good physical and chemical quality can be properly deposited on substrates by LPD.52,56 2.1.3. Surface Roughness Analysis. The surface roughness of the test samples was measured, and the results are shown in Figure 13. The surface roughness decreased significantly after the polishing treatment, as shown in Figure 13. Surface roughness values of modified samples with SiO2 film thicknesses of 188.6 and 267 nm were close to those of the untreated and polished samples. However, as the SiO2 film thickness increased to 322.7 nm, the roughness rose as a result of the formation of large SiO2 particles on the surface.

Figure 8. SiO2 film thickness as a function of deposition time.

determined through EDS analysis, as shown in Figure 10. It was found that the SiO2 films were mainly composed of Si and O elements, which demonstrates the generation of SiO2 films on the substrate. Copper was from the substrate, and a trace amount of fluorine (atomic ratio of 3.41%) was also found because of the effect of the deposition solution containing fluorine acid. We calculated a ratio of Si/O = 1:1.5 (>1:2), which means that the composition of the coatings was near that of SiO2. The SEM images of the SiO2 films are shown in Figure 9b,c. 6007



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Figure 9. Surface morphologies of SiO2 films at various deposition times.

where γsv, γdsv, and γnd sv are the surface free energy, dispersion component, and nondispersion component, respectively, of the solid surface. According to the Young equation, the relation between γsv and the interfacial energy γsl between solid and liquid is given by

2.1.4. Analyses of Static Contact Angle and Surface Free Energy. According to the literature,62,63 the surface free energy can be described as the sum of a dispersion component, γd, and a nondispersion component, γnd. For a solid, the surface free energy can be expressed as γsv = γsvd + γsvnd

γlv cos θ = γsv − γsl

(5) 6008

(6)



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Figure 10. EDS analysis of a SiO2 coating with a film thickness of 188.6 nm. Figure 13. Surface roughnesses of test samples.

Table 2. Surface Tension Components of Standard Liquids62 a water formamide

γlv (mN·m−1)

γdlv (mN·m−1)

−1 γnd lv (mN·m )

72.8 58.0

21.8 39.0

51.0 19.0

a

Note that the superscripts d and nd represent the dispersion and nondispersion components, respectively.

Table 3. Static Contact Angles and Surface Free Energies of Test Samples

Figure 11. IR spectrum of a SiO2 coating with a film thickness of 188.6 nm.

where θ is the contact angle of the liquid on the solid and γlv is the surface energy of the standard liquid, listed in Table 2. The interfacial energy between liquid and solid, γsl, can be evaluated by the geometric and harmonic mean approach as (7)

Combining eqs 6 and 7, one obtains γlv(cos θ + 1) = 2 γsvdγlvd + 2 γsvndγlvnd

θw (deg)

θf (deg)

untreated polished 188.6 nm 267.9 nm 322.7 nm

53.4 23.9 63.9 60.4 59.0

29.6 45.4 47.5 43.2 33.5

γsv (mN·m−1) 51.0 75.3 41.1 43.9 48.8

± ± ± ± ±

0.4 1.5 1.1 0.3 0.6

calculated the value of the surface free energy, γsv, using eq 5. The results are reported in Table 3. It can be seen from Table 3 that the surface free energy of the polished sample was the highest, followed by that of the untreated sample. Under atmospheric conditions, CuCO3·Cu(OH)2 and CuSO4·3Cu(OH)2 were formed on the untreated surface, leading to low surface free energy.64 The surface free energies of SiO2-modified samples decreased significantly, mainly because inorganic materials often have weaker bonding forces than metals. 2.2. Experiments on the Inhibition of Fouling and Corrosion by SiO2 Coatings. The experimental results of fouling and corrosion inhibition tests of SiO2 coatings including EIS measurements are presented and discussed in this section. 2.2.1. Inhibition of Fouling. In the fouling experimental studies, the morphologies and compositions of the foulants, the deposition rates, and the digital images of the samples were investigated. 2.2.1.1. Morphologies and Compositions of Foulants. Fouling deposition rates on surfaces of test samples immersed in saturated CaCO3 solution for various immersion times and at various solution temperatures were investigated. Figure 14 shows a typical SEM image of a test sample after fouling experiments.

Figure 12. Si 2p core-level XPS results for a SiO2 coating with a film thickness of 188.6 nm.

γsl = γsv + γlv − 2 γsvdγlvd − 2 γsvndγlvnd

sample

(8)

In this study, water and formamide were chosen as standard liquids; the static contact angles on the SiO2 coating surface were measured and are listed in Table 3. Substituting the data into eq 8, we obtained the values of γdsv and γnd sv and then 6009



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Figure 14. (a) Morphology and (b) elemental analysis of the fouling on a SiO2 coating with a film thickness of 267.9 nm immersed in saturated CaCO3 solution at 60 °C for 168 h.

feature of CaCO3. On the other hand, fouling rates on the modified samples were lower than those on the untreated and polished samples. This is partly explained by the low surface free energy of the coated samples, which reduces the adherence of foulants. With an increase in solution temperature, the fouling rates of the modified samples rose slightly. Fouling rates on different test sample surfaces after distinct immersion times at a solution temperature of 60 °C were also experimentally studied, and the data are shown in Figure 17. Digital photographs of test samples before and after fouling experiments are shown in Figure 15b. It was found that the scaling rates of all samples rose when the immersion time increased. In comparison, the fouling rates of modified samples were lower than those of untreated and polished samples, indicating a certain inhibition effect of fouling on SiO2-modified surfaces. In addition, the fouling rate also rose as the film thickness increased, because the size of SiO2 particles increased as the coating thickness increased and coatings with large particles provide more attachment points for fouling deposition. 2.2.2. Inhibition of Corrosion. In the corrosion experimental investigations, the results of morphology and corrosion composition, corrosion product, corrosion rate, and EIS measurements were analyzed. 2.2.2.1. Morphology and Corrosion Composition Analyses. Typical results of SEM morphology and EDS composition analyses of corroded samples are presented in Figure 18. Because of corrosion in geothermal water, the originally smooth surface became rough, and some large fouling particles were formed on it. Four elements (Cu, O, Si, S) with various mass and atom ratios were detected, of which Cu and O had the largest and second largest ratios because of the effects of the copper substrate. According to the mass and atom ratios and related analyses,64,66 copper oxides such as Cu2O are the main corrosion products. The S is mainly from sulfates, and Si might be from silica scaling. 2.2.2.2. Observations of Corrosion Products. Figure 19 shows digital photographs of test samples before and after corrosion experiments. Red and black rust spots formed on the sample surfaces, especially at high solution temperatures or long immersion times. It is shown qualitatively that the surfaces of untreated and polished samples underwent an overall corrosion whereas those of coated samples experienced local

It can be seen in Figure 14a that many crystal particles, mainly composed of calcite,65 were deposited on the sample surface. However, the fouling layer did not cover the sample surface completely. In the EDS analysis (Figure 14b), seven elements were detected. The presence of Ca, O, and C indicates the formation of CaCO3 foulant. Cu is from the copper substrate. Si and some O are from the SiO2 film. Trace amounts of Al are present, possibly as a result of contamination of the test system. The copper was polished and cleaned, and then SiO2 coating was deposited on it. EDS analysis in Figure 10 shows no Al on the coated sample. Hence, the polished surface was cleaned completely by the cleaning method described in this article. After the fouling experiments, trace amounts of Al were detected. The samples used in the fouling experiment were cleaned by the same method. Thus, the Al possibly comes from the fouling experiments or from the characterization process but not from the polishing process. 2.2.1.2. Fouling Observations. To obtain a qualitative understanding of the antifouling effects of SiO2 films, digital photographs of different samples before and after fouling experiments at various solution temperatures (40, 60, and 90 °C) and immersion times (24, 72, and 168 h) are presented in Figure 15. From Figure 15, one can see that the surface color of untreated and polished samples changed from light yellow to dark purple after fouling experiments at a given solution temperature and immersion time, whereas the color of coated surfaces darkened only slightly, which means that the SiO2modified surfaces inhibited CaCO3 deposition to a certain degree compared to that on untreated or polished samples. After fouling experiments, the surfaces of untreated and polished samples were nearly completely covered by the white fouling layers, as the color changed from bright red to white, as shown in the Figure 15c. However, only part of the surface of SiO2-coated samples was covered by a fouling layer, and some was even the same as it was before fouling experiments. Thus, the SiO2 coatings have the ability to prevent fouling formation. 2.2.1.3. Fouling Deposition Rates. Fouling deposition rates of the samples at various temperatures after 168 h of immersion are shown Figure 16. The rate is relatively low at lower temperatures (40 and 60 °C) and climbs sharply at higher temperatures (90 °C), especially for the untreated and polished sample surfaces. This is mainly due to the inverse solubility 6010



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Figure 16. Dependence of fouling deposition rate on test sample at various solution temperatures.

Figure 17. Dependence of fouling deposition rate on test sample at various immersion times.

film on the surface. The lowest corrosion rate of the sample with the thickest coating indicates that the thickest SiO2 coating prevented the corrosive medium from diffusing to the substrate surface. It can also be seen in Figure 20 that the corrosion rates of all samples increased obviously as the solution temperature rose from 40 to 60 °C because, at high temperature, solution temperature dominates the corrosion behavior in addition to the surface properties. When the temperature rose from 60 to 90 °C, the corrosion rates of the samples increased only slightly, except for that of the sample coated with a SiO2 film of 322.7 nm. At relatively high solution temperatures, the corrosion rates of untreated and polished samples were higher than those of coated samples, indicating that the SiO2 coatings improved the corrosion resistance behavior in a short immersion time. The highest corrosion rate of the untreated sample resulted from the formation of microcells on the rough surfaces that can easily lead to a severe electrochemical corrosion. The thickest SiO2 film exhibited the worst corrosion behavior at relatively high temperature because of the peeling of the SiO2 coating, as can be seen in Figure 19b. Sample corrosion rates after a distinct immersion time at 60 °C in geothermal water are shown in Figure 21. It can be seen in Figure 21 that the corrosion rate of the untreated sample was

Figure 15. Photographs of test samples before and after fouling experiments at various temperatures and immersion times in saturated CaCO3 solution.

corrosion, as shown in Figure 19b,c. Coated surfaces were more favorable for corrosion prevention because no obvious darkening of the surface color of the coated samples was observed. However, the phenomenon of delamination or peeling of silica coatings was observed on the modified surfaces after 360 h, as shown in Figure 19c. 2.2.2.3. Corrosion Rates. The corrosion rates of different samples after immersion for 24 h in geothermal water at various temperatures are shown in Figure 20. It can be seen from Figure 20 that the corrosion rates of all samples were near the same low value at low temperature (40 °C). One reason is that the substrate property plays a dominant role in the corrosion behavior at low temperature. The surface of the copper substrate resists corrosion because of the formation of a passive 6011



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Figure 18. (a) Morphology and (b) elemental analysis of corrosion on a SiO2 coating surface with a film thickness of 267.9 nm after being immersed in geothermal water for 168 h at 90 °C.

Figure 20. Dependence of the corrosion rates of different samples on the geothermal water temperature.

Figure 21. Corrosion rates of samples after different immersion times in geothermal water at 60 °C.

Figure 19. Photographs of test samples before and after corrosion experiments at various temperatures and immersion times in geothermal water.

and were lower than those of the coated samples. The great decrease of the corrosion rate of untreated and polished samples might have been caused by the formation of a protective film layer that prevented the substrate from being corroded. The corrosion process of coated samples continued at a relatively high rate after a long immersion time as a result of damage to the coatings.

higher than those of coated samples after 24 h of immersion in geothermal water. However, as the immersion time increased to 168 h, the difference in corrosion rates disappeared, and the corrosion rate of the untreated sample was close to those of the modified samples. After immersion for 360 h, the corrosion rates of untreated and polished samples obviously decreased 6012



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It is worth noting that the present work is still in its early stages from the point of view of long-term anticorrosion effects. However, this is an issue of coating process optimization. Further efforts should be focused on improving the coating technologies to prepare dense and crack-free coatings to overcome the disadvantage of the peeling of coatings. 2.2.2.4. EIS Measurement. Figures 22 and 23 show Nyquist and Bode graphs, respectively, of EIS data for different samples

Figure 22. Nyquist graph of EIS results for different silica coatings at an immersion time of 0 h.

at an immersion time of 0 h. Data at high frequency for the Nyquist plot are given at the right side of Figure 22. From the Nyquist graph, two partial semicircles can be seen, including a large one at low frequency and a small one at high frequency. For different coatings, two semicircles occur without a diffusion line, and the first semicircle is very small. Correspondingly, two peaks are seen in the Bode graph, as shown in Figure 23b. Both the phase angle and the modulus reach maximum values at low frequency, as shown in Figure 23. An equivalent electron circuit with two time constants was used to fit the EIS data of different coatings, which can be expressed with the circuit description model of Rs{Qc[Rp(QdlRct)]}. The calculated results (lines in Figures 22 and 23) show good agreement with the measured data (points in Figures 22 and 23). The parameters Qc and Rp describe one of the time constants and represent the small semicircle at high frequency, which indicates the dielectric characteristics of the silica coatings. Rp depends strongly on the conductivity of the electrolyte in the pinholes of the silica coatings. Hence, it indicates the pinhole characteristics of the coatings. The parameters Qdl and Rct express the other time constant and represent the semicircle at low frequency, which indicates the charge-transfer process at the interface between the coating and substrate. The simulation results are summarized in Table 4. The values of the parameter Rs for different coatings were nearly the same, which indicates the constant resistance of the geothermal water between the working and reference electrodes and the stable cell system. The value of Rp decreases with increasing thickness of the coatings. This indicates that the coating with a thickness of 188.6 nm was denser than others and smaller pinholes inhibited the permeation of the geothermal water into the coating and thus increased the electrical resistance. For the thicker coating of 322.7 nm, looseness and defects due to the

Figure 23. Bode graph of EIS results for different silica coatings at an immersion time of 0 h: (a) modulus, (b) phase angle versus frequency.

peeling of some silica particles, as shown in the SEM images, made the permeation of the geothermal water easier and thus decreased the electrical resistance significantly. The reverse trend was observed for the parameter Rct compared with Rp. The value of Rct mostly depends on the coating thickness. The thicker the coating, the more difficult the charge transfer through the coating. The capacitances Qc and Qdl were very low due to the low capacitive effect, indicating that the silica coating provided effective protection to the copper. The Bode plots for polished and untreated samples are shown in Figure 23. Compared with the values for the silicacoated samples, the modulus and phase angle for the polished and untreated samples were very low. A more obvious difference was observed at low frequency. Uniform passivation was seen on the polished surface during the EIS test: the passivation layer inhibited the corrosion to some extent, so a higher modulus and phase angle were observed than for the untreated sample. Three disordered peaks were observed for the polished and untreated samples as shown in Figure 23b. They were all at low or middle frequency, indicating that many 6013



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Table 4. Optimum Fit Parameters for the EIS Data of the Silica Coatings at an Immersion Time of 0 h sample 188.6 nm 267.9 nm 322.7 nm

Rs (Ω·cm2) 89.13 80.89 82.67

Qc (Ω−1·sn·cm−2) −7

2.337 × 10 9.979 × 10−8 6.386 × 10−8

Rp (Ω·cm2)

nc 0.833 0.918 0.974

2689 1706 307.4

Qdl (Ω−1·sn·cm−2)

ndl

Rct (Ω·cm2)

1.352 × 10 1.714 × 10−7 1.590 × 10−7

1 1 0.973

3.077 × 106 5.365 × 106 1.898 × 107

Qdl (Ω−1·sn·cm−2)

ndl

Rct (Ω·cm2)

0.9593 0.757 1

2.685 × 106 1474 1.605 × 107

−7

Table 5. Fitting Parameters for the EIS Data by the Model Rs(QdlRct)(QcRp) sample 188.6 nm 267.9 nm 322.7 nm

Rs (Ω·cm2) 36.55 61.88 23.74

Qc (Ω−1·sn·cm−2) −6

3.139 × 10 2.510 × 10−7 3.358 × 10−5

Rp (Ω·cm2)

nc 0.6099 1 0.4648

2393 4.981E6 1162

chemical reactions, especially oxidization, occurred on the sample surface and, thus, many microelectric cells formed. The peaks at high frequency disappeared, indicating that geothermal water could easily permeate into the substrates. From Figure 23b, it can be seen that the value of the phase angle was a maximum at the middle frequency and then decreased sharply at the lowest frequency. A corresponding jump can also be seen from Figure 23a. These observations provide evidence for limited diffusion through the corrosion products. In addition to fitting the EIS data with the equivalent electron circuit of Rs{Qc[Rp(QdlRct)]}, an attempt was also made to use the equivalent electron circuit of Rs(QdlRct)(QcRp) to fit the impedance spectra. However, the fitted results were not stable, especially for the coating of 267.9 nm, as shown in Table 5. For the coatings of 188.6 and 322.7 nm, the magnitudes of Q and R were nearly the same as those obtained by the model of Rs{Qc[Rp(QdlRct)]}. Here, the parameters Qdl and Rct indicate the charge-transfer resistance permeating through the coating and substrate interface at pinholes. A completely pure and compact SiO2 layer by itself has good electric isolation performance; the magnitude of the chargetransfer resistance for this thickness is, in theory, about 109 Ω·cm2. However, the coating obtained by the LPD method was porous to a certain degree. This coating layer was so thin that the pinholes had a relatively significant effect on its characteristics. Therefore, the value of the charge-transfer resistance decreased to 106 Ω·cm2. It is not appropriate to consider the coating as an isolation layer. Figures 24 and 25 show the Nyquist and Bode graphs of the EIS data for the silica coating with a thickness of 267.9 nm at different immersion times. The modulus and phase angle decreased as the immersion time increased, especially after 168 h. For immersion times of 24−168 h shown in Figure 24a, three semicircles were observed, indicating three time constants. The semicircle at middle frequency was complete, and the other two were rather incomplete. Correspondingly, the phase angle curves had main peaks at middle frequency, and the peaks at high and low frequency were incomplete and not obvious. The time constant at high frequency is a characteristic of the silica coating. The pinholes in the coating, which provide channels for the permeation of electrolyte, are most sensitive to the corrosive geothermal water. The pinholes were destroyed and became large. Hence, the charge transferred onto the substrate easily, and thus, the semicircles were small and not obvious, even disappearing after long-time immersions. The time constant at low frequency is correlated to the diffusion process caused by the presence of corrosion products. The incomplete semicircle indicates the high capacitive effect. The corrosion products provide a limited diffusion and thus an effective

−7

3.225 × 10 7.048 × 10−7 2.154 × 10−7

Figure 24. Nyquist graph of EIS results for a silica coating with a thickness of 267.9 nm at different immersion times: (a) 24−168 and (b) 360 h.

protection for the substrate. The time constant at middle frequency is attributed to the complex reaction occurring at the surface of the coating. Three splits were found in the main peak at middle frequency, and a sharp jump was observed after 168 h immersion, indicating the formation and delamination of the corrosion products, as well as the dissolution of silica. However, the phase angle and shape of the plots for the 24−168 h did not change much, as shown in Figure 25b, indicating the relative stability of the coating. After 360 h immersion, both the modulus and phase angle decrease greatly compared with that at 0 h, as shown in Figures 24b and 25. The peaks of phase 6014



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metal materials, the prospects of applications of inorganic SiO2 coatings in geothermal utilization is bright.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The authors are grateful to the National Natural Science Foundation of China (Grant 20876106) and Tianjin Research Program of Application Foundation and Advanced Technology (No. 09JCZDJC24100) for financial support.

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Figure 25. Bode graph of EIS results for a silica coating with a thickness of 267.9 nm at different immersion times: (a) modulus, (b) phase angle versus frequency.

NOMENCLATURE n = empirical exponent between 0 and 1 Qc = capacitance at the solution−coating interface, Ω−1·sn·cm−2 Qdl = capacitance at the solution−substrate interface, Ω−1·sn·cm−2 Rct = charge-transfer resistance at the solution−substrate interface, Ω−1·sn·cm−2 Rp = resistance resulting from the coating pores, channels, or cracks, Ω·cm2 Rs = resistance of the electrolyte between the working and reference electrodes, Ω·cm2 S = area of the sample exposed to the solution in fouling or corrosion experiments, m2 t = time, s T = temperature, °C ν = average fouling rate, g·m−2 νL = average corrosion rate denoted by corrosion depth, mm·a−1 W = sample mass, g |Z| = modulus of impedance, Ω·cm2 Zim = imaginary part of impedance, Ω·cm2 Zre = real part of impedance, Ω·cm2

Greek Letters

angle plots at low frequency are more obvious, as shown in Figure 25b, indicating the obvious effect of the corrosion production. In addition, a sharp jump was observed the middle frequency, the same as for the polished sample at 0 h, indicating the partial passivation of the copper and the greater destruction of the silica coating.

ρ = density of the substrate material, g·cm−3 θ = phase angle, deg

Subscripts

3. CONCLUDING REMARKS Inorganic SiO2 coatings with low surface free energies and various film thicknesses on copper substrates were prepared by the LPD method and characterized with respect to surface physics and chemistry properties. Fouling experiments of saturated CaCO3 solution on these coatings indicate that the inhibition function of fouling by SiO2-modified samples is obvious. Experiments to evaluate corrosion on the surfaces of such samples in geothermal water showed that SiO2-modified samples provided good inhibition of corrosion behavior after a short immersion time, even though, after a long immersion time, the corrosion resistance action decreased due to the surface peeling of SiO 2-coated sample. The corrosion mechanism of SiO2 coatings was further discussed in relation to EIS measurements and analyses. Because inorganic SiO2 coatings can overcome the drawbacks of most organic coatings such as low thermal stability and a large stress difference with

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0 = before the corrosion experiment 1 = before the scaling experiment 2 = after the scaling experiment t = after the corrosion experiment with the corrosive product removed

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Coatings on Copper in Geothermal Water - American Chemical Society

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