Anticorrosion and Scale Behaviors of Nanostructured ZrO2

Article pubs.acs.org/IECR

Anticorrosion and Scale Behaviors of Nanostructured ZrO2−TiO2 Coatings in Simulated Geothermal Water Yongwei Cai,* Xuejun Quan, Gang Li, and Nengwen Gao School of Chemistry and Chemical Engineering, Chongqing University of Technology, Chongqing 400054, China ABSTRACT: Corrosion and scaling phenomena often appear simultaneously and act synergistically in geothermal water system. We prepared the nano-ZrO2−TiO2 composite coatings on the AISI type 304 stainless steel with the chemical liquid phase deposition method. Surface morphology, crystal form, and chemical elements of the coatings were investigated with field emission scanning electron microscopy, energy-dispersive X-ray spectroscopy analyzer, Xray diffraction, and thermal gravimetric/differential thermal analyses instruments, respectively. Corrosion behaviors and scale properties were tested by potentiodynamic polarization and electrochemical impedance spectroscopy measurements in the simulated geothermal water at 50 °C. The results reveal that the anticorrosion performance of the ZrO2−TiO2 composite coating is markedly improved compared to the austenitic stainless steels. A pitting corrosion and scale deposition mechanism was proposed for the ZrO2−TiO2 composite coating in the simulated geothermal water. These findings have potential implication for the protection of the austenitic stainless steel against the geothermal water corrosion.

1. INTRODUCTION Geothermal water that buried in the crustal rocks and fluids is the form of heat which can be economically and reasonably exploited.1 Compared with traditional energy sources, geothermal water is a green energy. It has some advantages such as wide distribution, low cost, and direct use, as well as little pollution to environment.2 So it can be widely used in power plants, district heating, bathing, and medical care. However, geothermal water has complicated chemical composition because of the dissolution of the rock, as well as the roles of ion exchange and parsing effect in the movement of the earth for a long time.3 The corrosive components include O2, SO42−, Cl−, H+, sulfide (H2S, HS−, and S2−), etc.4 For another, the fouling components mainly comprise the ions of Ca2+, Mg2+, CO32−, and SO42−.5 For this reason, corrosion and scaling phenomena often appear simultaneously and act synergistically in the geothermal water system. The anticorrosive materials, such as titanium and monel alloy,6 are widely used in the geothermal water system to prevent the corrosion of the equipment and pipelines. However, the expensive prices astrict their wide application for the geothermal water environment. So, it is one of the effective measures, choosing cheaper metal materials for corrosion protection to solve the corrosive problem in the geothermal water. It is worth mentioning that austenitic stainless steels are widely used in the industries due to their excellent corrosion resistance and cheap price. However, these materials have a tendency to undergo localized corrosion in chloride containing environments owing to the breakdown of the passivation layer formed on the stainless steel surface.7 It is widely acknowledged that the anticorrosion properties of the stainless steel can be significantly improved if the stainless steel was coated with the corrosion resistance layer to isolate the corrosive environments.8 © XXXX American Chemical Society

The anticorrosion coatings used in the geothermal energy system include the organic and the inorganic, as well as the composite coatings. The organic coatings, such as PTFE (polytetrafluoroethylene) and PPS (polyphenylene sulfite), usually have large thickness and low thermal conductivity, which can reduce the heat transfer performance of the heat exchangers in the geothermal field. Moreover, the organic coatings are prone to aging, dissolution, and delamination, which consequently result in more serious corrosion.9 Generally, the inorganic coatings, especially, ceramics, show good insulating, tribological, and corrosion resistance properties in aggressive media.10 Therefore, ceramics oxide coatings, like TiO2,11,12 Al2O3,13 SiO2,14 and ZrO2,15 as well as its composite oxide coatings,16−18 are widely applied to improve the anticorrosion properties of the metals. As one kind of the ceramic coating material, TiO2 has high impedance and nontoxic property19 and has been extensively applied for anticorrosion of the metal.20 To date, several techniques have been used for preparation of the ceramic coatings on the metal substrates, such as chemical vapor deposition (CVD),21 physical vapor deposition (PVD),22 plasma spraying,23 electroless plating,24 chemical liquid phase deposition (LPD),25 and sol−gel26 methods. Among them, the LPD method is widely used to fabricate the coatings on the metal surface in heat transfer, anticorrosion, and antiscaling field because this method has many advantages including simple process and low sintering temperature, as well as no limit on the size and the shape of the metal substrate. However, extensive research has been carried out on preparation of the Received: July 30, 2016 Revised: October 4, 2016 Accepted: October 14, 2016

A

DOI: 10.1021/acs.iecr.6b02920 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research

H3BO3 acting as F− scavenger into the solution and forms a more stable complex. Consequently, titanium dioxide particles are formed by a further hydrolysis reaction of [TiF6−n(OH)n]2−, as shown in eq 3.

TiO2 coatings or film with the LPD method since Nayayama et al. reported earlier for depositing SiO2 film27 and subsequently Deki et al. for TiO2 coating.25 However, the as-prepared pure TiO2 coatings always contain pores and cracks28 which may result in the unfavorable consequences to corrosion at the interface of the coating and the metal substrate. To eliminate the amount of such pores and cracks, another kind of nanoparticle such as ZrO2 has been incorporated to produce ZrO2−TiO2 composite coating based on the following reasons.18 First, the thermal expansion coefficient of ZrO2 is closed to that of the pure metals and alloy steels among the ceramic materials.29 Moreover, ZrO2 reveals excellent properties such as high chemical stability and mechanical strength, as well as outstanding corrosion resistance performance.29 Lin et al.30 successfully deposited continuous ZrO2 thin film on singlecrystal silicon substrates using the LPD method. However, they did not investigate the anticorrosion property of this coating. Abd El-Lateef et al.18 prepared ZrO2−TiO2 composite coating with the sol−gel method. They found that the high corrosion protection efficiency of the composite coating results from mutual influence of TiO2 and ZrO2 film in 1.0 M HCl solution. To the best of our knowledge, there have been few published works on preparation of the ZrO2−TiO2 composite materials with the LPD method. Moreover, little investigation has been done on the corrosion resistance and scale properties of the ZrO2−TiO2 coating in geothermal water. There is no doubt that a considerable amount of research work has been reported on the corrosion and scaling mechanisms of the metal in the geothermal water. Stáhl et al.31 studied the scale and corrosion modeling in a CaCO3− H2O−CO2 system. They believed that corrosion and scaling processes often interact with each other in geothermal systems. Banaś et al.32 explored the corrosion mechanism of the steel alloys in the geothermal H2O−Cl−−CO2−H2S environments. They concluded that the steels containing Cr show lower corrosion rate than the carbon steel, and this lower rate may be ascribed to the microstructural change of the film of the corrosion product. Wu et al.3 investigated the impacts of the fouling ions (Ca2+ and Mg2+) on the corrosion and the fouling processes in the geothermal water. They found that the corrosion rate decreased and the pitting area became smaller when the foulant formed on the steel pipes. Two kinds of substances, the ball-shaped corrosion products and the needleshaped scale, were both detected on the pipe. However, there is a lack of systematic study on the corrosion and scale mechanism of the protective coating in the geothermal water. In short, the objective of this work was to study the potential application of nano-ZrO2−TiO2 as a corrosion protective coating in the simulated geothermal water. First, the novel ZrO2−TiO2 composite coatings were prepared with the LPD method. Then, both the potentiodynamic polarization (Tafel) curves and the electrochemical impedance spectroscopy (EIS) spectra were measured with an electrochemical workstation to assess the anticorrosive behaviors of the prepared coatings in the simulated geothermal water. Finally, one possible corrosion and scale mechanism was presented to deeply analyze the corrosion process of the ZrO2−TiO2 composite coatings.

[TiF6]2 − + nH 2O ⇌ [TiF6 − n(OH)n ]2 − + nHF

(1)

H3BO3 + 4HF ⇌ [BF4 ]− + H3O+ + 2H 2O

(2)

OH

−

[TiF6 − n(OH)n ]2 − ⎯⎯⎯⎯→ [TiF5 − n(OH)n + 1]2 − + F− ←⎯⎯⎯⎯⎯⎯⎯⎯

(5 − n)OH−

⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ [Ti(OH)6 ]2 − + (6 − n)F−

←⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯

(3)

Two chemicals of zirconium sulfate (Zr(SO4 ) 2 and ammonium persulfate ((NH4)2S2O8 are available to prepare the ZrO2 nanoparticles. Two processes occur simultaneously in water solution:30 the dissolution of zirconium sulfate to form sulfate complexes (eq 4) and the hydrolysis of sulfate complexes (eq 5). 7Zr(SO4 )2 + 2H 2O ⇌ 7ZrOSO4 · 5SO3 + 2H 2SO4

(4)

7ZrOSO4 · 5SO3 + 37H 2O ⇌ 7ZrO2 ·5SO3 ·3OH 2O + 7H 2SO4

(5)

The amorphous 7ZrO2·5SO3·30H2O particles tend to be deposited on the substrate and grow into large particles. However, when (NH4)2S2O8 dissolved in water, S2O82− can be revivified into SO42−, and O2 releases from water simultaneously, as the following reaction equations.30 2S2 O82 − + 4e− ⇌ 4SO4 2 −

(6)

2H 2O ⇌ O2 + 4e− + 4H+

(7)

That is, Reaction 5 can be effectively controlled; the smaller 7ZrO2·5SO3·30H2O nucleates more homogeneously and formed small particles. Finally, a continuous film composed of TiO2 and ZrO2 particles may jointly form through heterogeneous nucleation on the substrate. 2.2. Preparation of the ZrO2−TiO2 Coatings. For this study, austenitic stainless steel AISI304 (SS) plates with dimensions of 30 × 20 × 3 mm were prepared as the substrates. Before depositions of the coatings, the steel substrates were pretreated with three procedures: grinding, polishing, and cleaning. First, the substrates were ground with a minielectric die grinder (Zhangqing Trade Co. Ltd., Shanghai, China). Different grinding wheel heads were chosen successively with the materials of boron nitride, shutter wheel, nylon, and rubber to grind off the cutting burr, the scratches, and the oxide. After that, the substrates were polished with the wool wheels daubed with the green polishing paste. Finally, the SS substrates were cleaned in a high-power numerical control ultrasonic cleaner (KQ-400 KDE, Ultrasonic Instrument Co., Ltd., Kunshan, China) with acetone (East Sichuan Chemical Co., Ltd., Chongqing, China), ethanol (Kelong Chemical Reagent Factory, Chengdu, China), and deionized water for 15 min successively and subsequently dried prior to the deposition process. The nano-ZrO2−TiO2 composite coatings were prepared with the LPD method. All chemical reagents applied in the tests were of analytical grade without further treatment. The experimental flowchart is shown in Figure 1. Two precursor solutions of titanium dioxide (solution A) and zirconium

2. EXPERIMENTAL SECTION 2.1. Fundamentals of Coating Formation. For deposition of the TiO2 particles, the following chemical equilibriums have been proposed by Deki et al.25 Eq 1 shows the hydrolysis of [TiF6]2− ion in water solution. Eq 2 denotes the addition of B


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naturally dried at room temperature. Finally these specimens were calcined in the muffle furnace (Aixi Furnace Factory, Shanghai, China) at different temperatures with constant heating rate. 2.3. Analysis of the Prepared Coatings. 2.3.1. SEM. The surface morphologies of the prepared coatings were achieved with a field emission scanning electron microscopy (FE-SEM, MERLIN Compact, ZEISS, Germany). Moreover, the chemical compositions of the coatings were characterized by an energy dispersion spectroscopy (EDS, Genesis2000XMS, EDAX Company, US) attached to SEM. 2.3.2. XRD. Crystal form of the coatings was performed through an X-ray diffractometer (XRD-7000S, Shimadzu Co., Japan) using Cu Kα1 radiation (λ = 0.154 nm) at 30 kV, current of 20 mA. In order to increase the sensitivity of the radiation signals, the data were collected with a slow scan rate of 1°·min −1 (in 2θ) in a scan range of 10−90°. 2.3.3. TG-DTA. In order to further examine the thermal stability of the nano-ZrO2−TiO2 composite coating materials, thermal analysis of the precursor particles deposited in the solution C was carried out using a thermal gravimetric/ differential thermal analyses instrument (TG/DTA, STA2500, NETZSCH Scientific Instruments Trading Co., Ltd., Shanghai, China) with an alumina crucible in nitrogen atmosphere to further characterize the collected sediments which were formed simultaneously with the deposition of the coating. Before carrying out the thermal analysis, the sediments were dried at 103 °C in the digital display blast drying oven with a stainless inner container (SK101, Shengke Equipment Co., Ltd., Shanghai, China) for 10 h. The scavenging rate of nitrogen gas in the instrument was 50 mL·min−1, the heating rate of this operation was 22 °C·min−1, and the temperature ranged from 28.9 to 1100 °C. 2.4. Corrosion Performance Tests. Electrochemical tests were performed in a three-electrode cell (CHX400, Chuxi Industrial Co., Ltd., Shanghai, China) using a computer controlled PGSTAT 128N potentiostat (Metrohm Autolab, Switzerland). A three-electrode cell configuration was applied to the electrochemical measurements.33 A saturated calomel electrode (SCE) acted as reference and a platinum sheet as auxiliary electrode. The coating specimen served as the working electrode. The distance between the auxiliary electrode and the working electrode remained the same for all the measurements. The specimens were vertically immersed in the three-electrode cell containing 250 mL with the simulated geothermal water at 20.0 ± 0.5 °C. The simulated geothermal water used for the corrosion tests was prepared by mixing the chemical reagents (analytically pure) with deionized water. The compositions of the simulated geothermal water were modeled after the geothermal water derived from central China,3,34 as listed in Table 1. Before the corrosion tests were performed, a defined area (1 cm × 1 cm) of each specimen was exposed to the corrosion media. Both of the backs and the edges of the specimen were encapsulated by a mixture of epoxy resin and polyamide with a volume ratio of 2:1. Then the overlays on the specimen surfaces

Figure 1. Flowchart of the coatings preparation and the evaluation technology in the hot simulated geothermal water solution.

dioxide (solution B) were first prepared to deposition of the composite ZrO2−TiO2 coatings. Solution A was obtained by dissolving hexafluorotitanate ammonium ((NH4)2TiF6, Guangfu Fine Chemical Research Institute, Tianjin, China) and boric acid (H3BO3, Chengdu Kelong Chemical Reagent Factory, China) in deionized water. Solution B was then acquired by dissolving zirconium sulfate (Zr(SO4)2, Zhanwang Chemical Reagents Plant, Wuxi, China) and ammonium persulfate ((NH4)2S2O8, Kelong Chemical Reagent Factory, Chengdu, China) in deionized water. Finally, solution C was obtained by mixing the solutions of A and B at various compositions and diluting them with deionized water. The substrates treated above were dipped into solution C for coating preparation. Then solution C was kept at a constant temperature. After the suitable reaction time, the specimen was taken out from solution C, then carefully rinsed with deionized water, and

Table 1. Chemical Compositions of the Simulated Geothermal Water Modeled after the Geothermal Water Derived from Central China3,34 composition −1

concn (g·L )

MgCl2·6H2O

Na2SO4

NaHCO3

CaCl2

KCl

NaCl

0.328

0.113

0.332

0.284

0.117

3.641

C


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Figure 2. FE-SEM images of nano-ZrO2−TiO2 composite coatings amplified by 50,000 times: (a) ZTA, (b) ZTB, and (c) ZTC.

were solidified at 60 °C for 4 h in the digital display blast drying oven (SK101, Shengke Equipment Co., Ltd., Shanghai, China). The as-prepared specimens were immersed in the simulated geothermal water solution with a constant temperature of 50 ± 0.1 °C for corrosion at different times. The corrosion solutions were replaced every 2 days in order to keep the concentration constant. Besides, the solutions were neither agitated nor degassed during the tests. Before testing, the specimens were previously immerged in the corrosion solution for 10 min to establish the steady-state potential.33 Each electrochemical measurement for the same specimen was performed three times to guarantee the reproducibility of the test results. In the paper we showed only the most representative results.35 2.4.1. Potentiodynamic Measurements. Tafel analyses were carried out in the simulated geothermal water to quantitatively determine the corrosion rates of the as-prepared specimens. Tafel curves were measured under the testing parameters of a scan rate of 0.01 V·s−1 and a scan voltage range of −1.50 to 0.60 V.33 The open circuit potential (OCP) were first established to balance the rates of the anodic and cathodic processes. The corrosion potential (Ecorr) was confirmed from the intersection of the cathodic and the anodic branches of the Tafel curves, and the corrosion current density (icorr) was obtained using the Tafel extrapolation method.36,37 The polarization resistance (Rp) was estimated using the SternGeary equation38,39 Rp =

CR (mm/year) =

D(g/cm 3) ·V

(9)

where M is the molecular weight, V is the valence, 3270 is constant, and D is the density. 2.4.2. Electrochemical Impedance Spectroscopy. The electrochemical impedance spectroscopy (EIS) was performed at the different immersion times in a frequency range of 0.01 Hz−100 kHz, with a 10 mV sinusoidal applied potential. ZSimpWin software was applied to fit the EIS data with an appropriate equivalent circuit (EC). All experiments were performed at open circuit potential (OCP). All electrochemical tests were conducted in a Faraday cage to avoid external electromagnetic interference42 and repeated three times to confirm the reproducibility of the test results.

3. RESULTS AND DISCUSSION 3.1. SEM and Mechanical Properties Analyses. With regard to a protective coating, besides the excellent anticorrosion property of the coating material itself, the coating may also isolate the metal substrate from the corrosive medium so that the electrochemical corrosion reaction is prevented at the coating/substrate interface.18 For this reason, a good anticorrosive coating should be dense and compact to a feasible extent so that the corrosive media cannot pass through. In this case, we explored the surface morphology image of three typical coatings of ZTA, ZTB, and ZTC, as presented in Figures 2a, 2b, and 2c, respectively. It could be observed that all the coatings showed a continuous and crack-free morphology on the SS substrates. We can infer that these coatings may provide good corrosion protection for the stainless steel substrate. Furthermore, the coatings consist of nanoparticles with a size range of 20−30 nm. It should be noted that the oblique stripes in Figure 2c are the grinding scratches on the SS substrate.

βa βb 2.303(βa + βb)icorr

3270·icorr(A/cm 2)·M(g)

(8)

where βa and βb are the slopes of the anodic and the cathodic Tafel curves (ΔE/Δlog|i|, V·dec−1), respectively. The corrosion rate (CR) was calculated as40,41 D


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Figure 3. EDS analysis of the nano-ZrO2−TiO2 composite coatings: (a) a selected typical region for EDS analysis of the specimen ZTA; (b) EDS spectra in part a for ZTA; (c) a selected typical region for EDS analysis of specimen ZTB; (d) EDS spectra in part c for ZTB; (e) a selected typical region for EDS analysis of ZTC; and (f) EDS spectra in part e for ZTC.

The mechanical properties of the coating are also a crucial role in the corrosion area.43 Chen et al.44 and Piwoński et al.43 indicated that the ZrO2−TiO2 composite coating manifests the excellent tribological and adhesive properties to the metal substrate. Vickers hardness HV500 of the ZrO2−TiO2 composite coatings in different mass ratios is between 1500 and 1600.45 The friction coefficient is 0.14−0.20 under 0.5 N applied load, and the antiwear life is larger than 5,000 sliding cycles against the AISI 52100 steel ball and the Si3N4 ball in dry sliding contact.44 Piwoński et al. evaluated the adhesion capability of the composite ZrO2−TiO2 coating by measuring the width of the wear scars after microtribological experiments. The results indicated that the width of the frictional traces of the composite ZrO2−TiO2 was 45 μm which was narrower than for the bare TiO2 coating with the normal load of 80 mN.43 These

The EDS analysis was performed to analyze the components of three deposited coatings (Figure 3). The result indicated that these specimens contain the elements of Fe, Cr, and Ni (SS substrate), as well as Ti, Zr, and O, which confirm that ZrO2− TiO2 composite coatings were successfully prepared by the LPD method. The fact that the elements in the SS substrate having been detected implied that these composite coatings prepared with the LPD method are quite thin. The atomic content of the zirconium element of ZTA, ZTB, and ZTC is 1.97%, 0.99%, and 2.97%, as well as the element content ratio Zr/Ti being 0.87, 0.44, and 0.94, respectively. The results showed that the ZrO2 and TiO2 compositions of specimens ZTA, ZTB, and ZTC have a mass percentage of 57.31% and 42.69%, 40.43% and 59.57%, and 59.19% and 40.81%, correspondingly. E


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Industrial & Engineering Chemistry Research improvements in adhesion property attributed to the existence of hard ZrO2 nanoparticles with agglomerates form in the composite coating which tightly adhered to the substrate.43 3.2. XRD Measurements. The XRD profiles for the coatings after sintering at different temperatures for 2 h in air and the SS substrate were presented in Figure 4. The diffraction

Figure 5. TG/DTA curves for the precipitate after preparation of the nano-ZrO2−TiO2 composite coating.

titanium dioxide. The last range of 590−767 °C with mass loss of 25.64% probably may relate to the formation process of ziroconium dioxide, which is confirmed by the aforementioned results of XRD analysis. Further investigations of these results by DTA analysis show a great endothermic effect at the peak value of about 100 °C, which corresponds to the evaporation of the absorbed water and some structural water in the sample. As the temperature increases, the remaining water, chemically bonded inside the pores of the coating particles, can be discharged. The endothermic effect at the peak value of 400 °C could be due to the conversion of titanium dioxide from amorphous to anatase. Furthermore, the endothermic effect at a peak value of 528 °C probably relates to the phase transition of the sediments. Both the endothermic and exothermic peaks are observed in the last weight loss region from 600 °C to about 800 °C. Such reactions are potentially attributed to formation of ziroconium dioxide and iron−titanium oxides. 3.4. Electrochemical Analyses. 3.4.1. Tafel Analyses. Figure 6 presents the corrosion protection behaviors of the coatings of ZTA, ZTB, and ZTC, as well as the SS substrate in the simulated geothermal water under the potentiodynamic polarization conditions. The results show that the Tafel

Figure 4. XRD patterns of the nano-ZrO2−TiO2 composite coatings with different annealing temperature at a scan rate of 1°·min−1 and a scan range of 10−90°.

peaks of the coating were not detected at the sintering temperature of 500 °C except for the characteristic peaks of SS (43.7°, 50.9°, 74.8°, JCPDS PDF 33-0397) which indicates that the coating is amorphous and quite thin. Cai et al.33 demonstrated that the anatase phase crystal of titanium dioxide was found at the sintering temperature of 500 °C. Comparing the two results, it can be seen that the presence of the zirconium dioxide substance inhibits the formation of the titanium oxide crystals. After the coating was annealed at 600 °C, the diffraction peaks at 2θ of 24.2°, 24.4°, 33.1°, 39.3°, 40.8°, 49.3°, 54.1°, 57.5°, 62.3°, and 71.8° were observed for the coating. These peaks correspond to the crystals peaks of zirconium dioxide (2θ of 24.1°, 24.5°, and 54.1°, JCPDS PDF No. 65-1023) and iron−titanium oxide (Fe9TiO15, 2θ of 33.2°, 35.6°, 40.9°, 49.5°, 57.5°, 62.4°, and 72.0°, JCPDS PDF No. 54-1267). This result indicates that mixing of the oxides of TiO2 and ZrO2 can produce new crystallographic phases, which is consistent with the conclusion drawn from Tomar et al.46 However, the characteristic peaks of anatase-phase crystals of titanium dioxium (2θ of 25.3°, JCPDS PDF No. 21-1272)33 are not found in Figure 4. Furthermore, the peak intensities increase and the peak width becomes narrow at 800 °C, which is due to the coating gradually crystallizing. 3.3. TG/DTA. Figure 5 displays TG/DTA curves for the sediments after preparation of the nano-ZrO2−TiO2 composite coating. The TG curves demonstrate a total 76.8% mass loss for the initial temperature of 28.9 °C up to 767 °C. The weight loss occurs in four stages. The first stage happens in the range of 28.9−128 °C with mass loss of 4.13% and concerns the removal of the physically absorbed water from the nanoparticles. The second weight loss in the temperature range of 128−413 °C with mass loss of 13.76% is related to the crystalline temperature range of titanium dioxide from amorphous to anatase. The third one in the range of 413−590 °C with mass loss of 33.32% likely corresponds to the formation of rutile

Figure 6. Tafel curves of the different samples recorded at a scan rate of 0.01 V·s−1 in the simulated geothermal water at 50 °C. F


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the simulated geothermal water. Figure 2 displays that all deposited coatings were all homogeneously covered with a nanogranular surface. We can conclude that the nano-ZrO2− TiO2 composite coatings provide a more powerful protection for the SS substrate as a result of the compact and the uniform features of the coating surfaces. Figure 8 demonstrates the scan results of the Tafel polarization curves for the specimen ZTB after the different

polarization curves in the anode regions bend to the low current ranges which indicate that the mental substrates come into the passivation state and the corrosion reaction is in a diffusion control stage. The anode curves of the Tafel polarization have almost no straight line sections. However, there exist the straight line segments in the cathode curves of the Tafel polarization. Accordingly, the corrosion current density (icorr) can be determined by Tafel’s extrapolation method.36 As with specimen ZTB for example, the corrosion current density (icorr) was obtained by finding the intersection point of the tangent line from the line segment of the cathode region (−1.13 to −0.86 V) and the horizontal line at the corrosion voltage (Ecorr = −0.643 V), as shown in Figure 7.

Figure 8. Tafel curves of the ZTB coating obtained with a scan rate of 0.01 V·s−1 at the different corrosion time in the simulated geothermal water at 50 °C.

corrosion time in the simulated geothermal water at 50 ± 0.1 °C. Table 3 displays the electrochemical parameters achieved from the analysis of the Tafel polarization curves. The shape of both polarization curves of cathode and anode for this system is practically independent of the corrosion time (Figure 8). After the immersion time of 720 h, higher current densities and less noble corrosion potential are found, from 4.53 × 10−6 A·cm−2 and −0.643 V to 1.61 × 10−5 A·cm−2 and −0.797 V, respectively. The results show that the anticorrosion property of the coating declines gradually with the corrosion time increasing. 3.4.2. EIS Measurements. 3.4.2.1. Corrosion Comparison. The corrosion protection properties of the coated specimens were additionally estimated using EIS. EIS is well-known for intensively prediction of the corrosion resistance. Employing EIS data, it is possible to deeply explore the anticorrosive characteristics of the coatings.47 Bode and Nyquist plots were usually used for identification of the coating performance and better description of the coating impedance behaviors. The EIS measurements have been conducted at the open circuit potential. Experimental results obtained by EIS measurements are presented in the Nyquist plot (Figure 9a) and the Bode plot (Figure 9b). As can be seen from Figure 9a, the Nyquist plot for the specimen ZTB features a higher diameter semicircle than that of the SS substrate. Meanwhile, the impedances of the coating ZTB are greater than those of the SS substrate (Figure

Figure 7. Determination of Ecorr (vs SCE) and icorr by Tafel’s extrapolation method (specimen ZTB).

Accordingly, the tangent line of the anode curve can be achieved by making a straight line from the intersection point to the tangency point of the anode curve. Table 2 lists the values of the electrochemical parameters of Ecorr, icorr, βa, βb, Rp, and CR. Compared to the bare SS substrate, three coating specimens result in a remarkable displacement of the Tafel polarization curves toward lower current densities accompanied by moving the corrosion potential toward the positive values, as seen in Figure 6 and Table 2. The corrosion current density (icorr) of the coatings of ZTA, ZTB, and ZTC is declined about 1 order of magnitude compared with that of the SS substrate (Table 2). Furthermore, the polarization resistance (Rp) is about 20.9, 23.1, and 15.1 times higher than that of SS, respectively. At the same time, the annual corrosion rate (CR) increases by 85.4%, 90.7%, and 90.1% respectively. Generally, a higher Ecorr and Rp and a lower icorr and CR indicate a better corrosion protection.41 For this reason, the coatings have more excellent corrosion resistance properties compared to the SS substrate in

Table 2. Electrochemical Parameters Achieved by Tafel Extrapolation Methoda item SS ZTA ZTB ZTC a

Ecorr vs SCE/V −0.812 −0.660 −0.643 −0.790

βa/V·dec−1 0.318 1.588 0.733 0.434

βb/V·dec−1 0.248 0.590 0.504 0.397

icorr/A·cm−2 4.85 7.16 4.53 4.78

× × × ×

−5

10 10−6 10−6 10−6

Rp/(Ω·cm2)

CR/mm·a−1

1247.7 26095.2 28772.3 18846.4

0.369 0.054 0.034 0.036

The data are normalized by apparent surface area. G


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Industrial & Engineering Chemistry Research Table 3. Potentiometric Measurement Results for the ZTB Coating with Different Corrosion Timea corrosion time/h 0.5 360 720 a

Ecorr vs SCE/V −0.643 −0.782 −0.797

icorr/A·cm−2 −6

4.53 × 10 6.63 × 10−6 1.61 × 10−5

βa/V·dec−1

βb/V·dec−1

Rp/Ω·cm2

0.737 0.627 0.141

0.508 0.338 0.429

28772.3 14332.7 2854.0

The data are normalized by apparent surface area.

3.4.2.2. Coating Decay. Corrosion resistant persistence is an important quality of the anticorrosion coatings. Therefore, the pathway to coatings failure can be monitored by making periodic EIS detections. In this research, four corrosion time series of 0.5, 168, 360, and 720 h were chosen to assess the changes in the coating resistance, the pore resistance, and the double layer resistance in the simulated geothermal water solution. EIS data of the coatings immersed for different times are shown in Figure 10 in the form of Nyquist and Bode plots. However, it is generally known that the EIS spectra are sensitive to many aspects, and it is hard or even impossible to analyze all features in detail.49 Therefore, we neglect the small features in the quantitative analysis. At the immersion time of 0.5 h, the impedance modulus |Z| at the low frequency region displays very high values, and the corresponding phase angle is close to 70°. However, with the elapse of time, the corrosion solution gradually penetrates into the coating, the maximum phase angle reduces, and the impedance modulus decreases, as shown in Figure 10b. Both of these results demonstrate that the anticorrosion properties of the coating gradually decrease as the immersion time increases. On the other hand, the impedance value at 0.01 Hz, which reflects the total corrosion protection provided by the coating,42 is notably reduced after the immersion time of 168 h. This result suggests an acceleration of the corrosion process under the coating during the early stage of the immersion in the simulated geothermal water. Figure 10c displays a movement of phase angle to highfrequency region after the immersion time of 360 h, which indicates an enlarging area of the coated SS exposed to the corrosion media.50 From a general view, the phenomena of the impedance magnitude at the low and the mid frequencies in the Bode plot continuing to drop demonstrate that the corrosion medium has permeated to the coating (Figure 10c). A third time constant appears which attributes to the corrosion process at the coating/substrate interface and leads to the substrate begin to corrode.50 These results indicate that the protective effects of the composite coating are greatly reduced under the seeping of the corrosion electrolyte. As the corrosion solution penetrates the coating and reaches the coating/substrate interface, the microcell corrosion reaction would be formed in the interface area, and the time constant of the impedance spectrum would increase. So the third time constant appears at the frequency of 1000 Hz after the corrosion time of 360 h. This result could be probably correlated to the diffusion process caused by the presence of the fouling and corrosion products layer.51 This layer may have a protective effect to inhibit the further corrosion of the substrate.52 However, the substantial accumulation of the fouling and corrosion products will lead to the expanse of the coating volume. Consequently, the adhesion of the coating on the substrate will be greatly reduced, and even the coating will peel off from the substrate, and then the corrosion rate of the substrate accelerates.53 After the corrosion time of 720 h, the largest phase angle of the EIS was less than 45° both in the high and the low frequency areas. The results

Figure 9. Nyquist and Bode diagrams of the EIS data for the polished AISI 304 stainless steel and the ZTB coating obtained in the simulated geothermal water at 50 °C: (a) Nyquist plot and (b) Bode plot.

9b). Generally, the larger the semicircle diameter (charge transfer resistance) and the higher values of the impedances at low frequencies48 indicate the specimen ZTB has a lower corrosion rate and better anticorrosion ability.18 The EIS spectrum for bare SS demonstrates one time constant, which corresponds to the corrosion process occurring at the metal surface (Figure 9b). For specimen ZTB, two time constants can be found (Figure 9b). The one in the higher frequencies region can be ascribed to the protective coatings. The other at the lower frequencies region is probably relevant to the EIS information feedback of the corrosion process occurring at the coating/SS interface.10 H


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demonstrate that anticorrosion properties of the coating degenerate finally. 3.4.2.3. Electric Circuit. To quantificationally study the anticorrosion properties of the coatings during the immersion process, the impedance spectra of ZTB coating are fitted to the appropriate equivalent circuit. Several configurations of equivalent electric circuit models were tested, and the best fit between the simulated spectra of the coatings and the experimental ones was obtained with a circuit description code (CDC) of R(RQ) (RQ) (RQ) model,54 as presented in Figure 11, which proposed the introduction of three different

Figure 11. Equivalent circuit for fitting the EIS data obtained from Figure 10 (WE: working electrode, RE: reference electrode).

time constants. A constant phase element (CPE), Q, was commonly applied for the equivalent circuit to replace the idea electrical capacitance in the actual corrosion environment. The CPE is a usual diffusion-related element and describes the behavior of the coatings having heterogeneities of the mesostructured and/or of the chemical composition.55 The equivalent circuit (EC) shown in Figure 11 was employed to fit the EIS spectra data by ZsimpWin software. Impedance parameters obtained using this EC model are given in Table 4. It is indicated that a good superposition between the experimental data (msd) and the simulated line (cal) is observed by fitting the EIS spectra data with this model (Figures 10a−10c), and the uncertainty of the parameters in Table 4 is less than 4%, which demonstrates the reliability of the fitting results.55 As presented in Figure 11, Rs is the solution resistance between the working electrode and the reference electrode, which depends not only on the resistivity of the electrolyte medium but also on the morphology of the area in which current is carried.55 The solution resistance Rs remains substantially constant (Table 4), which means that the electrochemical test system is stable.55 Generally, one semicircle in the Nyquist diagram corresponding to one phase angle in the Bode plot demonstrates a one time constant in the EIS spectrum. This one time constant corresponds to one parallel circuit in the EC model.10 The first time constant at low frequency in the Bode plot is adapted to the corrosion process taking place at the interface of solution/metal in the pinholes of the coating.42 This time constant can be expressed with a parallel subcircuit (Qdl, Rct) in the EC model, as shown in Figure 11. Rct is the charge transfer resistance at the coating/ electrolyte interface and implies the penetration resistance of the ions and water through the coating pinholes.33 The second time constant at the frequency range of 1.0−10.0 Hz corresponds to the nano-ZrO2−TiO2 composite coating. This time constant is shown as a coating resistance Rcoat in parallel with a capacitance Qcoat of the coating layer. Rcoat could represent the antipenetrating properties of the coating, which is a significant parameter to estimate the corrosion resistance of the coating. The coating capacitance Qcoat compactly concerns the antipermeability into the coating.51 The high values of Rcoat and the low values of Qcoat imply the best anticorrosion

Figure 10. Evolution of Nyquist (a) and Bode diagrams (b and c) of the specimen ZTB after the different corrosion time in the simulated geothermal water at 50 °C. Symbols are corresponding to the experimental data (msd) and full lines are for the fitting curves (cal). Optimum fit result using the model in Figure 11: (a) Nyquist plot, (b) lg|Z| vs f, qnd (c) −θ vs f. I


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Table 4. Optimum Fit Parameters for the Specimen ZTB after the Different Immersion Time Using the Model in Figure 11a corrosion time/h 0.5 168 360 720 a

Rs/Ω·cm2 17.38 22.57 18.95 13.94

Qf/Ω·sn·cm−2 4.77 4.93 5.35 5.96

× × × ×

−4

10 10−4 10−4 10−4

nf

Rf/Ω·cm2 Qcoat/Ω·sn·cm−2

0.51 0.56 0.77 0.81

112.1 88.58 64.75 63.74

2.54 8.88 9.54 10.55

× × × ×

−4

10 10−4 10−4 10−4

Rcoat/Ω·cm2

ncoat 0.95 0.74 0.69 0.69

11928 7142 4618 4498

Qdl/Ω·sn·cm−2 1.52 4.74 5.66 6.61

× × × ×

−4

10 10−4 10−4 10−4

ndl

Rdl/Ω·cm2

0.76 0.81 0.63 0.71

9.29 × 10 2217 734.2 471.3

5

χ2 2.1 6.5 2.4 1.9

× × × ×

10−4 10−5 10−4 10−4

The data are normalized by apparent surface area.

ability.37 Due to the continuous diffusion of Cl− ions at the metal/layer interface, the coating resistance Rcoat gradually decreases and the coating capacitance Qcoat gradually increases as the increasing of the immersion time. Besides, the third time constant, (Rf, Qf), at the high frequency could be probably correlated to the diffusion processes caused by the presence of the fouling products56 of the simulated geothermal water. With regard to the corrosion resistance of specimen ZTB in the hot simulated geothermal water, Tafel analysis indicates that the polarization resistance Rp is 28772.3 Ω·cm2 (0.5 h in Table 3). On the other hand, the coating resistance Rcoat obtained by EIS analysis is 11928.0 Ω·cm2 (0.5 h in Table 4). The results show that the values of the Rp and Rcoat are in the same order of magnitude. However, the relative error between them is 58.5%. The deviation may come from the different calculation methods. Rp obtained from Tafel analysis is affected by the selection of the line segments of the strong anodic and cathodic polarization regions, whereas Rcoat required from the EIS characterization is affected by the selected equivalent circuit and the fitting quality of the EIS data. 3.4.3. Corrosion and Scale Mechanism. The corrosion and scale mechanism was introduced to further investigate the damage process of the coating in the simulated geothermal water (pH value: 6−7), as shown in Figures 12a and 12b. Although the composite ZrO2−TiO2 coating is relatively dense, the corrosive electrolyte solution is able to penetrate the coating with the increase of the immersion time because of the very small radius of water molecules (0.2 nm) and chloride ion (0.181 nm).57 Once the chloride ions contact with the substrate, a certain concentration of chloride ions easily adsorbs onto the passive film of the stainless steel and edges out oxygen atoms in the passivating films. Then the chloride ions combine the cations (Fe, Cr, Ni) of the passivation film to form soluble chlorides.58 Finally, the corrosion pits are produced on the stainless steel surface. The metal surfaces in the corrosion pits are in the activated state with negative potential. Conversely, the coating surfaces outside of the pits are in the passivated state with positive potential. Therefore, the microgalvanic battery forms between the coating and the steel substrate, which results in corrosion. The electrochemical reactions take place, as shown in eqs 10−13.57 Anodic dissolution reaction in the pits:57 Fe → Fe2 + + 2e−

(10)

Cr → Cr 3 + + 3e−

(11)

Ni → Ni 2 + + 2e−

Cathode reaction outside of the pits: O2 + 2H 2O + 4e− ⇌ 4(OH)−

Figure 12. Schematic representation of the corrosion and scale mechanism on the coating: (a) the tortuous path of O2 and Cl− through the protected coating at the initial stage and (b) coating damage and scale formation processes after long time immersion.

metal ions are produced. The chloride ions outside of the pits continually transfer into the corrosion pits to balance the electrically neutral and more metal atoms are dissolved.59 Therefore, the cycle repeats, and the diameters and the depths of the pits increase constantly. Finally, the coating collapses, and the perforations are formed. The chlorides formation and hydrolysis reactions are shown in eqs 14−17.41

(12) 57

(13)

Fe2 + + 2Cl− ⇌ FeCl 2

Meanwhile, the soluble chlorides are easy to hydrolyze, and the partial solution in the pits became acidic. Consequently, the passsivating films on the stainless steel are dissolved, and excess

Cr J

3+

−

+ 3Cl ⇌ CrCl3

soluble

(14)

soluble

(15)


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Industrial & Engineering Chemistry Research Ni 2 + + 2Cl− ⇌ NiCl 2

soluble

FeCl 2 + 2H 2O ⇌ Fe(OH)2 + 2H+ + 2Cl−

scale shapes on the coating surface by SEM (Zeiss supra 55). SEM results display that some part of the protective coating is destroyed (Figure 13). Figures 14a and 14b display the detailed

(16) (17)

From another aspect, scale is also easily formed in the geothermal area because the geothermal water usually has a very high mineralization degree. The inorganic ions (such as Ca2+, Mg2+, CO32−) are liable to gather to form crystallization fouling on the coating surface. Moreover, in the existence of CO2, the corrosion tendency of steel is under the control of the corrosion products, such as iron carbonate (siderite).60 The scaling reactions for the formation of calcium carbonate confirm to the following equilibrium reactions:61,62 2HCO−3 + Ca 2 + ⇌ CaCO3 ↓ +CO2 + H 2O

(18)

2HCO−3 + Mg 2 + ⇌ MgCO3 ↓ +CO2 + H 2O

(19)

CO2 + H 2O ⇌ H 2CO3

(20)

H 2CO3 ⇌ H+ + HCO−3

(21)

Fe(OH)2 + H 2CO3 ⇌ FeCO3 + 2H 2O

(22)

According to the above analysis, we propose a model for the pitting corrosion and scale deposition mechanism in the simulated geothermal water, as shown in Figures 12a and 12b. Figure 12a shows the initiation stage of pits occurs on a surface defect and a crystal nucleus forms at the localized sites, which may be due to the coating defects63 or the microstructural crystalline scale points. It is assumed that many anodic and cathodic reactions take place at the localized sites.64 Figure 12b shows the pit growth and the coating collapse after corrosion. So we speculate that the corrosion and scale processes can promote each other. On one hand, the corrosion products provide more crystalline cores for the scale forming; on the other hand, more scale points promote more microcorrosion cells emerging. 3.4.4. Morphologies of Corrosion Products and Scale Deposition. To validate the proposed corrosion and scale mechanism, the surface topography and the components of the coating ZTB were inspected after the corrosion time of 720 h in the simulated geothermal water at 50 °C. Figure 13 shows the overall appearance of the pitting hole as well as the different

Figure 14. SEM image of a collapsed pitting corrosion hole for the coating surface after exposed time of 720 h in the simulated geothermal water at 50 °C: (a) amplified by a factor of 5,000 and (b) amplified by a factor of 10,000.

topography of the collapsed pitting corrosion hole. The coating collapsed, and the corrosion products are deposited into the holes after corrosion (Figure 14b). Besides, much crystallization scale is found around the corrosion hole and on the coating surface (Figure 14a). The chemical compositions of the coating surface after corrosion were also analyzed by the EDS method (Oxford INCA spectrometer). Three regions which are around the corrosion hole, as well as without the hole, were chosen to detect the corrosion and scale compositions in more detail. Figure 15a shows a selected region for the EDS analysis of the pitting corrosion of the collapsed hole. Figure 15b demonstrates the EDS spectrum around the corrosion hole. The results indicate that the corrosion scales include pronounced chlorine, calcium, carbon, oxygen, and sodium signal and less intense signals of potassium and magnesium. Undoubtedly, the crystalline scales are due to the formation of NaCl, KCl, CaCl2, and MgCl2 salts, which come from the simulated geothermal water. Figure 16 shows the micromorphology and the structure of the scale deposited on the coating region without the corrosion hole. The coating surface is covered with two different scale forms, cubic and needle-like crystals. The crystal distribution is

Figure 13. Morphology of the whole area of the coating after the corrosion time of 720 h in the simulated geothermal water at 50 °C (amplified by a factor of 1,000). K


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Figure 15. EDS spectrum of the corrosion products and scale: (a) a selected region of the collapsed pitting corrosion hole for EDS analysis and (b) EDS spectra in part a.

Figure 16. Micromorphology of the scale deposited on the coating region without the corrosion hole.

Figure 17. Structure of the scale deposited on the coating region without corrosion hole: (a) selected region 1 for EDS analysis; (b) EDS spectrum of region 1; (c) selected region 2; and (d) EDS spectra of region 2.

more scattered and reveals relatively small scale. Two selected regions have been chosen to reveal the EDS spectrum of the scale compositions of the different regions (Figures 17a and 17c). The result indicates the scale is mainly calcium carbonate (Figures 17b and 17d). Figure 17a shows that the cubic crystal phase is calcite, and Figure 17b indicates that the needle-like one is aragonite.65

FE-SEM results demonstrate that the prepared nano-ZrO2− TiO2 coatings are compact and crack-free and consist of nanoparticles with a size range of 20−30 nm. XRD results indicate that the coatings are amorphous at the sintering temperature of 500 °C. The new crystallographic phases composed of zirconium dioxide and iron−titanium oxide are available at the calcined temperature of 600 °C. The peak intensities increase and the peak width tends to narrow with the sintering temperature increasing from 600 to 800 °C. Tafel analyses indicate that the nano-ZrO2−TiO2 composite coatings have more excellent corrosion resistance properties

4. CONCLUSIONS In this study, we have prepared the nano-ZrO2 −TiO 2 composite coatings on the AISI type 304 stainless steel with the chemical liquid phase deposition method. The following concluding remarks can be drawn from the above results and discussion: L


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Rf = resistance caused by the presence of the fouling products, Ω cm2 Rp = polarization resistance, Ω cm2 Rs = solution resistance between the reference electrode and the working electrode, Ω cm2 V = valence Z = impedance in Nyquist plot, Ω cm2 Zim = imaginary of impedance of EIS spectra, Ω cm2 Zre = real part of impedance of EIS spectra, Ω cm2

compared to the austenitic stainless steels in the simulated geothermal water. Furthermore, the anticorrosion property of the coating declines gradually with the corrosion time increasing. EIS experiments also demonstrate that the ZrO2−TiO2 coating has a lower corrosion rate and better anticorrosion ability. The coating decay tests demonstrate that the impedance modulus of the coating at low frequency is high and the correspondent phase angle is close to 70° at the beginning immersion time. With the immersion time increasing, the anticorrosion properties of the coating gradually decrease. The third time constant in the Bode plot appears after a corrosion time of 360 h. These results could be correlated to the presence of the fouling and corrosion products layer. A pitting corrosion and scale deposition mechanism in the simulated geothermal water shows that the corrosion and scale processes can promote each other. SEM results indicate some part of the coating is destroyed and collapsed after the corrosion time of 720 h in the simulated geothermal water, and much crystallization scale is found around the corrosion hole and on the coating surface. SEM and EDS spectrum results demonstrate that the scale is mainly calcium carbonate. The cubic crystal phase is calcite, and the needle-like one is aragonite. In short, since the nanostructured ZrO2−TiO2 composite coatings have excellent anticorrosion properties, we expect these coatings might lead to the industrial applications in the protection of the austenitic stainless steel against the geothermal water corrosion.

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Greek letters

βa = slope of the anodic polarization curve, V dec−1 βb = slope of the cathodic polarization curve, V dec−1 θ = Bragg X-ray diffraction angle of the crystal, ° λ = X-ray wavelength (Cu K = 0.154 nm) Abbreviations

AUTHOR INFORMATION

Corresponding Author

*Phone: +86 23 6256 3221. Fax: +86 23 6256 3221. E-mail: [email protected].

■

Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The authors are pleased to acknowledge the financial support provided by the General project of Chongqing foundation and frontier research project, China (Grant No. kj14000912) and the Project Supported by Scientific and Technological Research Program of Chongqing Municipal Education Commission, China (Grant No. 14jcyjA90009). The authors thank Dr. Yaoqiong Wang who provided many conveniences for using the electrochemical workstation. The authors also thank Dr. Tiantao Zhao for his useful suggestions and comments.

CDC = circuit description code CPE = constant phase element CR = corrosion rate CVD = chemical vapor deposition EC = appropriate equivalent EDS = energy dispersion spectroscopy EIS = electrochemical impedance spectroscopy FE-SEM = field emission scanning electron microscopy LPD = liquid phase deposition OCP = open circuit potential PPS = polyphenylene sulfite PTFE = polytetrafluoroethylene PVD = physical vapor deposition SCE = saturated calomel electrode SS = austenitic stainless steel AISI304 TG/DTA = thermal gravimetric/differential thermal analyses XRD = X-ray diffractometer ZTA,B,C = ZrO2−TiO2 composite coating

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NOTATION Ccoat = coating capacitance, Ω−1 sn cm−2 D = density, g cm−3 Ecorr = corrosion potential, V icorr = corrosion current density, A cm−2 M = molecular weight, g mol−1 Qcoat = constant phase element of the coating layer and coating defects, Ω−1 sn cm−2 Qct = constant phase element modeling the electric double layer system, Ω−1 sn cm−2 Qf = constant phase element caused by the presence of the fouling products, Ω−1 sn cm−2 Rcoat = coating resistance, Ω cm2 Rct = charge transfer resistance at coating/electrolyte interface, Ω cm2 M


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