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Effects of inorganic sealant and brief heat treatments on corrosion behavior of plasma sprayed Cr2O3–Al2O3 composite ceramic coatings
Surface & Coatings Technology 276 (2015) 8–15
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Effects of inorganic sealant and brief heat treatments on corrosion behavior of plasma sprayed Cr2O3–Al2O3 composite ceramic coatings Fang Shao ⁎, Kai Yang, Huayu Zhao, Chenguang Liu, Liang Wang, Shunyan Tao The Key Laboratory of Inorganic Coating Materials, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 201899, PR China
a r t i c l e
i n f o
Article history: Received 11 March 2015 Revised 18 June 2015 Accepted in revised form 21 June 2015 Available online 23 June 2015 Keywords: Cr2O3–Al2O3 coatings Sealing Aluminum phosphate Corrosion resistance Plasma spraying
a b s t r a c t In the present study, Cr2O3–Al2O3 composite coatings were fabricated by atmospheric plasma spraying (APS) technique. Aluminum phosphate with small fraction of Al2O3 nanoparticles was applied as high-temperature sealant to enhance the anti-corrosion performance of the coatings. The inorganic sealant significantly reduced the open pore of the as-sprayed coatings and showed low shrinkage after heat treatment (600 °C, 30 min). Potentiodynamic polarization, electrochemical impedance spectroscopy (EIS) and salt spray tests were used to assess anti-corrosive characteristics of the coatings. The sealants resulted in an augment of corrosion resistance properties of the coatings both before and after heat treatment, indicating effective sealing and good high temperature applicability. The sealed Cr2O3–Al2O3 composite coatings have a potential use in severe corrosion condition. © 2015 Elsevier B.V. All rights reserved.
1. Introduction In the recent period, the booming ocean economy requires advanced corrosion preventing technologies for marine and offshore facilities [1]. Atmospheric plasma spraying (APS) is a versatile technology to produce protective coatings for facilities in extensive applications [2,3]. Various types of wear and corrosion are the most frequent conditions the surface coatings have to withstand [4]. Ceramic coatings such as metal oxides (Al2O3, Cr2O3, ZrO2, TiO2, etc.) are remarkable coating materials to improve wear, cavitation, fretting and corrosion resistance. They are especially useful in applications where high temperature is required simultaneously. However, owing to their lamellar structure and rapid solidification, plasma sprayed coatings usually have plenty of cracks and voids. These structural defects provide paths for corrosive species to corrode the substrate, leading to a deleterious effect when these coatings have to perform in an aggressive environment (e.g., seawater). Thus, post-treatments of the as-sprayed coatings are necessary to reduce porosity and enhance the corrosion resistance of the coatings. Laser remelting [5,6] and high temperature treatments [7] have been applied, but the high thermal inputs may induce microstructural changes of the substrate material, in some cases even lead to cracking of the coatings due to the large residual stress. In consideration of the economic and technical benefits, sealants are widely used [8,9].
⁎ Corresponding author. E-mail address: [email protected] (F. Shao).
http://dx.doi.org/10.1016/j.surfcoat.2015.06.045 0257-8972/© 2015 Elsevier B.V. All rights reserved.
Commercial sealants are usually based on organics such as epoxy, phenolic, silicone and acrylic resin [10]. Moreover, nanoparticles have been dispersed into the polymers to improve the mechanical properties and corrosion resistance [11]. Despite their good permeability, organic sealants have poor wear resistance, and they can hardly meet the high temperature applications (above 500 °C) [12]. In a practical application, the oxide ceramic coatings are frequently subjected to elevated temperature due to the high frictional heat and operating conditions. To overcome this limitation, inorganic sealants were studied. Sol–gel sealing treatments were carried out, the reduction of porosity was however small, indicating insufficient protection [13]. The reasons can be ascribed to the low solid yield and large shrinkage during heat treatment. If the sealant has a high curing shrinkage, especially when it can penetrate and adhere well into the coating, it may create local tensile fields, thus causing easier and larger particle detachments during the wear process [14]. Solid phases, such as fine powders or fibers were used as advanced filler to increasing the solid content [15]. On the other hand, it has been reported that aluminum phosphate sealing leads to obvious pore decrease and better wear resistance when used in Al2O3 and Cr2O3 coatings [16,17]. In earlier studies our group found that Cr2O3–Al2O3 composite coatings show lower porosity and better mechanical properties (such as hardness, fracture toughness, bending strength) than the individual Cr2O3 and Al2O3 coatings [18,19]. The Cr2O3–Al2O3 composite coatings have promising application in some facilities, such as the friction clutch used in shipboard aircraft, where the coatings are often under conditions of severe wear and high temperature due to the high frictional heat. However, the ocean environment brings forward higher demand for corrosion resistance. As far as we know, few research has been done on the corrosion characteristics of
F. Shao et al. / Surface & Coatings Technology 276 (2015) 8–15
plasma-sprayed Cr2O3–Al2O3 composite coatings, much less the inorganic sealing effects. Therefore it is important to study the properties of the sealed Cr2O3–Al2O3 composite coatings before and after high temperature treatments. In this paper, Cr2O3–Al2O3 composite coatings were prepared via APS technique. In addition, aluminum phosphate with a small fraction of Al2O3 nanoparticles was applied as inorganic sealant. The coatings with and without sealing treatment were heated at 600 °C to determine the high temperature applicability. The corrosion behavior was characterized by potentiodynamic polarization, electrochemical impedance spectroscopy and salt spray test. Based on the results of the analyses, an attempt was made to discuss the corrosion process of the Cr2O3– Al2O3 composite coatings. 2. Experimental procedure 2.1. Plasma spraying 304 stainless steel was used as the coating substrate. Prior to deposition, substrates of 30 × 30 × 5 mm3 were sandblasted after ultrasonic cleaning in ethanol. A bond layer NiCr and working layer Cr2O3–Al2O3 (CA hereafter) were produced with a Multicoat™ atmospheric plasma-spraying system equipped with a F4-MB plasma gun (Sulzer Metco AG, Switzerland). Commercial fused and crushed Cr2O3–Al2O3 feedstock powders were available. The median particle diameter (D50) is about 30 μm, and the fraction of Al2O3 phase in the powders is approximately 30 wt.%. The coatings were prepared with the optimized parameters as shown in Table 1. The thickness of the as-sprayed coatings was controlled to be about 350 μm. 2.2. Sealing of the coating The plasma-sprayed CA coatings were sealed by aluminum phosphate with small fraction of Al2O3 nanoparticles. Aluminum phosphate was prepared from the solution of aluminum hydroxide (Al(OH)3) and orthophosphoric acid (85% H3PO4) diluted with about 20 wt.% of deionized water [20]. The Al(OH)3:H3PO4 ratio was 1:4.2 by weight, which corresponded to the molar ratio P/Al of about 3. The solution was mixed and slightly heated on a magnetic stirrer until it became clear. Then, 5 wt.% Al2O3 nanoparticles were added to the solution and stirred to get a well dispersed sealant. The samples were immersed into the sealants at 50 °C for about 30 min with an ultrasonic bath and then standing at ambient temperature and atmospheric pressure for 12 h. Thereafter, the impregnated coatings were heat-treated for 2 h at 100 °C, followed by 2 h at 200 °C, and 2 h at the final curing temperature of 250 °C. The excess sealant was ground off by polishing, ensuring that the true surface of the coating was exposed for microscopic analysis and corrosion tests. The obtained samples were labeled as CA-S. To determine the high temperature resistance, the samples with and without sealing treatment were heated at 600 °C for 30 min and the samples were denoted as CA-SH and CA-H, respectively.
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2.3. Characterization The phase composition was done using a powder X-ray diffraction (XRD, D/max 2550 V, Rigaku Tokyo, Japan). The microstructure was characterized using a scanning electron microscope (SEM, TM3000, HITACHI, Japan) with affiliated energy-dispersive spectrometer. The accelerated voltage for EDS was 15 KV. The electrochemical corrosion tests of coating samples were performed in a three electrode cell using a platinum mesh electrode as a counter, a saturated calomel electrode (SCE) as reference electrode and the coating samples as the working electrode. The exposed area of the coating samples was 1 cm2. The experiment was conducted in unstirred 3.5 wt.% NaCl solution. Electrochemical systems Ametek 1287 & 1260 were used for the corrosion tests. The corrosion potential and the corrosion current density were obtained through the linear analysis of Tafel approximation. Electrochemical impedance spectroscopy (EIS) curves of the coated samples were acquired at the open circuit potential (Eocp) after 30 min of immersion, which had been determined earlier. The frequency range varied from 10− 2 Hz to 106 Hz. The polarization curves were obtained at a scan rate of 0.2 mV/s and a scan range of 0.2 V–0.4 V vs Eocp. Repeated tests were performed to ensure the reproducibility of the results. The salt spray test was conducted following the Chinese National Standard GB/T 10125-2012. The coatings were exposed to a salt fog generated from a (50 ± 5) g/L NaCl solution with a pH value of 6.5–7.2 at (35 ± 2)°C. The exposure duration lasted for 96 h.
3. Results and discussion 3.1. Phase structure Fig. 1 shows the X-ray diffraction patterns of the as-sprayed CA coating. All peaks can be assigned to a solid solution (Cr, Al)2O3. Since Cr2O3 and α-Al2O3 are isostructural, they can easily form continuous solid solution in which the solute is completely incorporated in the lattice [21]. The peak positions are close to those of pure α-Cr2O3 [ICDD PDF 38-1479]. The only difference is that the location of peaks shifts to higher 2θ angles, towards the positions for α-Al2O3 [ICDD PDF 46-1212] due to the reduction of lattice parameters. While γ-Al2O3 is usually dominant in the Al2O3 coatings deposited by APS, the XRD result of CA indicates that the α-phase is fully stabilized, which can be attributed to the substitution of Al ions in alumina by Cr.
Table 1 Plasma-spraying operating parameters. Parameters
Bond coat
Top coat
Unit
Arc current Primary plasma gas (Ar) Secondary plasma gas (H2) Carrier gas (Ar) Powder feed rate Nozzle diameter Powder injector diameter Powder injector distance Spray distance
600 57 5 3.5 18 6 2.0 5.0 120
650 45 10 3.5 35 6 1.8 5.0 110
A slpm slpm slpm g/min mm mm mm mm
Fig. 1. XRD patterns of the as-sprayed CA coating. The dash line represents the standard line-graph corresponding to α-Cr2O3 [ICDD PDF 38–1479].
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Fig. 2. The polished cross-section morphologies of (a) CA, (b), (c) CA-S coatings; (d) EDS analysis of the region marked with white crosses in (c).
3.2. Morphology and elemental analysis The microstructure of the coating was determined by the crosssectional SEM micrograph of the coatings. As shown in Fig. 2a, the unsealed coating has relatively high porosity and some micro cracks. After sealing, it can be found that the topside of the coating exhibits less porosity, and the sealant has penetrated into the coating to a depth of about 90 μm from the surface (Fig. 2b). The SEM image at higher resolution exhibits the filled defects (Fig. 2c). Fig. 2d is the EDS patterns of range A and B (the inset image) in Fig. 2c. The chemical compositions are listed in Table 2. There is the evident element of phosphorus in the filled defects of CA-S coating (range A in Fig. 2c) while in the dense area B, peaks corresponding to phosphorus were barely observed. Meanwhile, the percent of Al and Cr elements in range A have changed greatly compared with CA and range B. The significantly increased Al should originate from the sealant (aluminum phosphate and the Al2O3 nanoparticles). It strongly indicates that the sealant can be brought into the coatings through the structural defects (e.g., voids, cracks and gaps between the lamellae).
Table 2 Chemical composition (atom%) of the four different coatings corresponding to Figs. 2 and 3. Samples
O
Al
Cr
P
CA CA-S (A) CA-S (B) CA-H CA-SH (A) CA-SH (B)
65.0 67.5 65.8 64.5 65.9 68.5
12.2 22.6 12.5 12.1 24.1 11.4
22.8 7.9 21.7 23.4 8.6 20.1
/ 2.0 / / 1.4 /
Fig. 3a, b are the cross-sectional SEM images of CA-H and CA-SH, respectively. Compared with Fig. 2, it can be found the 600 °C heat treatment has no obvious effect on the morphology of coatings except a slightly denser structure. Sealed defects can also be detected in CA-SH coating as shown in Fig. 3c. The EDS results are shown in Fig. 3d and Table 2. Similar effects of sealing on the elements were observed. The increase of Al and P in range A indicates the existence of sealant. There was little shrinkage of the sealant, showing good thermo-stable performance. 3.3. Potentiodynamic polarization Table 3 exhibits the open-circuit potential (Eocp) of the four coating samples when the values were stable after immersion in 3.5% NaCl solution for 30 min. Potentiodynamic polarization curves are shown in Fig. 4. Values of the corrosion potential (Ecorr) and the corrosion current density (Icorr) extracted from these curves by Tafel fitting using CView softwore are also listed in Table 3. Icorr is an important parameter used for evaluating the kinetics of the corrosion reaction. Corrosion protection is inversely proportional to the Icorr [22]. The Icorr of CA and CA-S indicates a reduction in corrosion rate of about 10 times after sealing. CA-H shows a slightly smaller Icorr than CA, which may be due to the denser structure. The Icorr of CA-SH is far less than that of CA-H while almost equal to that of CA-S, signifying the good heat resistance of the sealant. From polarization curves, the open porosity (designated by Po) can be calculated by the following equation: Po ¼
IC 100% IS
ð1Þ
where Ic and Is indicate corrosion current density of the coated
F. Shao et al. / Surface & Coatings Technology 276 (2015) 8–15
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Fig. 3. The polished cross-section morphologies of (a) CA-H, (b), (c) CA-SH coatings; (d) EDS analysis of the region marked with white crosses in (c).
substrate and the bare substrate in the same electrolyte, respectively [8]. Since the sealing efficiency (designated by Si) is related to Po, Eq. (2) can be derived to relate Si and Icorr.
Si ¼
P 0o −P o P 0o
!
" 100% ¼ 1−
Po P 0o
!#
" 100% ¼ 1−
Icorr
!#
I0corr
100% ð2Þ
interconnected porosity was blocked. Further, epoxy resin could penetrate into the Al2O3–TiO2 coating by above 300 μm [26,27]. However, the high temperature resistance of epoxy resin was poor (completely decomposed after heat treatment at 600 °C for 30 min in our experiments). Sol-derived sealant can be used in conditions where the temperature is above 1000 °C, but the porosity of the modified coatings is only decreased minimally (~12% to ~11%) after the sealing treatment [15]. After heat treatment, the coating samples shift the corrosion potential in the negative direction. Compared with CA-H, the Ecorr of CA-SH
where Po and P0o are the open porosity of the sealed and unsealed coatings, and Icorr and I0corr are the corrosion current density of the sealed and unsealed coatings, respectively [23]. The calculated Si of CA-S and CA-SH is shown in Table 3. 89.1% open pores were sealed by modified aluminum phosphate sealant. Further, it was observed that the open porosity in CA-SH was reduced by 85.1% compared with CA-H. The porosity variation between CA and CA-H can also be explained by Eq. (2). The heat treated coating has an ~20% lower porosity, which is consistent with our previous work [24]. Zhang et al. [25] studied the effectiveness of sealing treatment with epoxy resin using similar methods. Epoxy resin sealed Cr2O3–TiO2 coating well and 81% of
Table 3 Corrosion parameters obtained from potentiodynamic polarization measurements of the four different coatings in 3.5% NaCl electrolyte. Samples
Eocp (mV)
Ecorr (mV)
Icorr (μA/cm2)
Rp (KΩ cm2)
Si (%)
CA CA-S CA-H CA-SH
−187 −167 −246 −277
−203 −209 −255 −300
11.0 1.2 8.7 1.3
4.2 39.2 6.5 30.1
/ 89.1 / 85.1
Fig. 4. Potentiodynamic polarization curves of the four kinds of coatings in 3.5% NaCl electrolyte.
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Fig. 5. Nyquist impedance plot for four kinds of coatings. Symbols represent experimental data and solid lines stand for fitted data. The inset image is the equivalent circuit.
was further shifted in the negative direction by about 45 mV. The change in Ecorr depends on the relative effect of the coating on the anodic and cathodic reactions [28]. The negative shift in Ecorr in the present case suggests that the heated coatings preferentially inhibits the cathodic process, especially for the sealed samples. The change trends of polarization resistances (Rp) reveal that CA-S and CA-SH has better corrosion inhibition capability, which are consistent well with the results of Icorr. Rp was calculated based on the Stern and Geary relationship. It is worth noting that unlike CA and CA-H, there was no passivation observed for the sealed coatings (CA-S and CA-SH). This appearance is probably attributed to the fewer permeable defects in sealed coatings [29]. 3.4. Electrochemical impedance spectroscopy (EIS) A number of works revealed that electrochemical impedance spectroscopy (EIS) is a powerful, non-destructive technique for the detection of electrochemical characterization of ceramic coatings in the field of aqueous corrosion science [30–32]. The EIS data of the four kinds of coatings are presented in the Nyquist plots in Fig. 5. The diameters of Nyquist plots increased after sealing, indicating the decreased corrosion. EIS spectra for coatings can be represented by simple equivalent circuit shown in the inset of Fig. 5. Rs stands for the resistance of the solution, Rpo is the resistance to current flow through the pores in the coating, Rt indicates the charge transfer resistance, CPEc is the constant
Fig. 7. Digital photos of the coatings after 96 h of the salt spray test: (a) CA, (b) CA-S, (c) CA-H, (d) CA-SH.
phase element (CPE) of coatings and CPEdl is a double-layer capacitance between the substrate/coating interface [33]. Here, CPE was used as a substitute for the capacitor to fit more accurately the impedance data of the electrochemical capacitance. The fitting curves are presented in Fig. 5 as the solid lines for each spectrum. The element values of Rt and Rpo can indicate the corrosion resistance and protectiveness of coating, respectively [34]. The fitting values of Rpo corresponding to CA, CA-S, CA-H and CA-SH are 146.2, 2252, 183.3 and 1408 Ω, respectively. The relatively low Rpo values of CA and CA-H indicate that the NaCl solution penetrates into the pores of the unsealed coatings. The higher Rpo values of CA-S and CA-SH demonstrate the reduced porosity after sealing. The improvement effect of sealing on the corrosion resistance of coated samples can be further confirmed by the difference of Rt. CA-S and CA-SH have much larger Rt (21,695 Ω and 28,627 Ω) than CA and CA-H (4264 Ω and 5075 Ω), indicating that the electrochemical reaction speed was decreased [30]. In Bode plots the magnitude of the impedance |Z| and phase-angle are plotted respectively as a function of frequency (f). Impedance at
Fig. 6. Bode plots of the four kinds of coatings. Symbols represent experimental data and solid lines stand for fitted data.
F. Shao et al. / Surface & Coatings Technology 276 (2015) 8–15
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Fig. 8. EDS maps of Ni element of (a) CA-H, (b) CA-SH. The scale bar is 2 mm. The detected regions were marked by white rectangle in Fig. 7c and d.
the high frequency represents the performance of the coating in corrosion solution, while at the low-frequency, it is related to the corrosion at the substrate/solution interface [35]. Compared with CA and CA-H, the impedance of CA-S and CA-SH were enhanced over the whole frequency range, as shown in Fig. 6. This indicates that the corrosion resistance of the sealed coating are much greater than that of the unsealed coating. 3.5. Salt spray test CA, CA-H, CA-S and CA-SH were subjected to the salt spray test to further investigate the anti-corrosion performances of the coatings. After 96 h testing, both CA and CA-H exhibit discoloration in the area
marked by the arrows, while there are no visible discoloration in CA-S and CA-SH (Fig. 7). Further, as shown in Fig. 8, additional EDS maps were used to study the corrosion behavior of the samples. There were Ni enrichments in the discoloration area of CA-H (marked by white rectangle in Fig. 7). On the contrary, the phenomenon was not observed in CA-SH. The surface morphology of the coatings was also examined by SEM after 96 h of the salt spray test, as shown in Fig. 9. Since the surface morphologies and EDS results were very similar when the coatings were heated or not, the coatings with heat treatment were discussed here. The EDS analysis result (Fig. 9b) revealed that Ni and Fe elements appeared nearby the discoloration area of CA-H. The chemical compositions are listed in Table 4. It can be concluded that the corrosion
Fig. 9. (a) Top-view SEM image of CA-H after 96 h of salt spray test; (b) EDS results corresponding to the area marked with white rectangle in (a); (c) plane SEM image of CA-SH after 96 h of salt spray test; (d) EDS analysis of the region marked with white cross and white rectangle (the inset image) in (c).
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References
Table 4 Chemical composition (atom%) of CA-H and CA-SH coatings after salt spray test. Samples
O
Al
Cr
P
Na
Cl
Fe
Ni
CA-H CA-SH (A) CA-SH (B)
57.1 70.5 68.9
5.5 10.0 12.1
28.6 3.2 18.1
/ 16.3 0.9
2.2 / /
1.7 / /
1.2 / /
3.7 / /
products generated during the salt spray test. The atom percentage of Ni is higher than Fe. In addition, some EDS results of the around places show 3.5 atom% Ni but unobvious Fe peaks (0.7 atom%). If the electrolyte can easily permeate the NiCr bond coat and reach the substrate, galvanic corrosion would be formed on substrate because the corrosion potential of Ni is higher than that of Fe in NaCl. Fe will be the dominant element in the corrosion products. Based on the above analyses, a possible process for the corrosion of CA and CA-H was proposed as follows: the electrolyte penetrated into the coating following the defects, when it reached the NiCr bond coat, corrosion occurred. With increasing corrosion time, corrosion products such as nickel oxides and hydroxides were accumulated in the NiCr/ceramic interface and appeared on the surface of the top-coat through the pores or cracks. Further, electrolyte may penetrate the bond coat, thus accelerating the corrosion of substrate. In contrast, no obvious peaks of Ni or Fe were detected in CA-SH. Range B shown in Fig. 9c is Cr2O3–Al2O3 coatings while range A is filled with sealant as proved by EDS results (Fig. 9d and Table 4). These results were in complete agreement with those obtained by polarization test and electrochemical impedance spectrum. The sealants prevented the electrolyte from penetrating into the Cr2O3–Al2O3 coatings by closing the available paths. Even after heat treatment, sealed coatings still exhibit good corrosion resistance. 4. Conclusion In the current work, Cr2O3–Al2O3 composite coatings were prepared via APS technique. Aluminum phosphate with small fraction of Al2O3 nanoparticles was applied as high-temperature sealant. The phase composition, microstructure and corrosion properties of the coatings were investigated. Based on the experimental results, the following conclusions can be drawn: (1) The fused Cr2O3–Al2O3 feedstock leads to the complex (Al, Cr)2O3 phase coating. α-Phase is fully stabilized, which can attribute to the plenty Cr2O3 in the fused feedstock. (2) After sealing, the coatings present decreased porosity in the topside. EDS analysis showed that sealant can indeed be brought into the coatings. The sealing treatments significantly improve the corrosion resistance as demonstrated by polarization, electrochemical impedance spectroscopy and salt spray tests. (3) The coatings with and without sealing treatment were heated at 600 °C for 30 min to determine the high temperature applicability. Low volume shrinkage of the inorganic sealant was observed after heat treatment. The sealed coatings exhibit nearly equal anti-corrosion properties before and after heat treatment. The heat treatment has a small effect on the corrosion behavior and the sealing can inhibit the corrosion effectively.
Acknowledgments The study is jointly supported by the National Nature Science Foundation of China (51302299) and Shanghai Nature Science Fund Project (13ZR1446000). Simultaneously, we are grateful to Prof. Shigang Xin and Prof. Yuzhi Zhang for providing the support for this work.
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Surface & Coatings Technology journal homepage: www.elsevier.com/locate/surfcoat
Effects of inorganic sealant and brief heat treatments on corrosion behavior of plasma sprayed Cr2O3–Al2O3 composite ceramic coatings Fang Shao ⁎, Kai Yang, Huayu Zhao, Chenguang Liu, Liang Wang, Shunyan Tao The Key Laboratory of Inorganic Coating Materials, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 201899, PR China
a r t i c l e
i n f o
Article history: Received 11 March 2015 Revised 18 June 2015 Accepted in revised form 21 June 2015 Available online 23 June 2015 Keywords: Cr2O3–Al2O3 coatings Sealing Aluminum phosphate Corrosion resistance Plasma spraying
a b s t r a c t In the present study, Cr2O3–Al2O3 composite coatings were fabricated by atmospheric plasma spraying (APS) technique. Aluminum phosphate with small fraction of Al2O3 nanoparticles was applied as high-temperature sealant to enhance the anti-corrosion performance of the coatings. The inorganic sealant significantly reduced the open pore of the as-sprayed coatings and showed low shrinkage after heat treatment (600 °C, 30 min). Potentiodynamic polarization, electrochemical impedance spectroscopy (EIS) and salt spray tests were used to assess anti-corrosive characteristics of the coatings. The sealants resulted in an augment of corrosion resistance properties of the coatings both before and after heat treatment, indicating effective sealing and good high temperature applicability. The sealed Cr2O3–Al2O3 composite coatings have a potential use in severe corrosion condition. © 2015 Elsevier B.V. All rights reserved.
1. Introduction In the recent period, the booming ocean economy requires advanced corrosion preventing technologies for marine and offshore facilities [1]. Atmospheric plasma spraying (APS) is a versatile technology to produce protective coatings for facilities in extensive applications [2,3]. Various types of wear and corrosion are the most frequent conditions the surface coatings have to withstand [4]. Ceramic coatings such as metal oxides (Al2O3, Cr2O3, ZrO2, TiO2, etc.) are remarkable coating materials to improve wear, cavitation, fretting and corrosion resistance. They are especially useful in applications where high temperature is required simultaneously. However, owing to their lamellar structure and rapid solidification, plasma sprayed coatings usually have plenty of cracks and voids. These structural defects provide paths for corrosive species to corrode the substrate, leading to a deleterious effect when these coatings have to perform in an aggressive environment (e.g., seawater). Thus, post-treatments of the as-sprayed coatings are necessary to reduce porosity and enhance the corrosion resistance of the coatings. Laser remelting [5,6] and high temperature treatments [7] have been applied, but the high thermal inputs may induce microstructural changes of the substrate material, in some cases even lead to cracking of the coatings due to the large residual stress. In consideration of the economic and technical benefits, sealants are widely used [8,9].
⁎ Corresponding author. E-mail address: [email protected] (F. Shao).
http://dx.doi.org/10.1016/j.surfcoat.2015.06.045 0257-8972/© 2015 Elsevier B.V. All rights reserved.
Commercial sealants are usually based on organics such as epoxy, phenolic, silicone and acrylic resin [10]. Moreover, nanoparticles have been dispersed into the polymers to improve the mechanical properties and corrosion resistance [11]. Despite their good permeability, organic sealants have poor wear resistance, and they can hardly meet the high temperature applications (above 500 °C) [12]. In a practical application, the oxide ceramic coatings are frequently subjected to elevated temperature due to the high frictional heat and operating conditions. To overcome this limitation, inorganic sealants were studied. Sol–gel sealing treatments were carried out, the reduction of porosity was however small, indicating insufficient protection [13]. The reasons can be ascribed to the low solid yield and large shrinkage during heat treatment. If the sealant has a high curing shrinkage, especially when it can penetrate and adhere well into the coating, it may create local tensile fields, thus causing easier and larger particle detachments during the wear process [14]. Solid phases, such as fine powders or fibers were used as advanced filler to increasing the solid content [15]. On the other hand, it has been reported that aluminum phosphate sealing leads to obvious pore decrease and better wear resistance when used in Al2O3 and Cr2O3 coatings [16,17]. In earlier studies our group found that Cr2O3–Al2O3 composite coatings show lower porosity and better mechanical properties (such as hardness, fracture toughness, bending strength) than the individual Cr2O3 and Al2O3 coatings [18,19]. The Cr2O3–Al2O3 composite coatings have promising application in some facilities, such as the friction clutch used in shipboard aircraft, where the coatings are often under conditions of severe wear and high temperature due to the high frictional heat. However, the ocean environment brings forward higher demand for corrosion resistance. As far as we know, few research has been done on the corrosion characteristics of
F. Shao et al. / Surface & Coatings Technology 276 (2015) 8–15
plasma-sprayed Cr2O3–Al2O3 composite coatings, much less the inorganic sealing effects. Therefore it is important to study the properties of the sealed Cr2O3–Al2O3 composite coatings before and after high temperature treatments. In this paper, Cr2O3–Al2O3 composite coatings were prepared via APS technique. In addition, aluminum phosphate with a small fraction of Al2O3 nanoparticles was applied as inorganic sealant. The coatings with and without sealing treatment were heated at 600 °C to determine the high temperature applicability. The corrosion behavior was characterized by potentiodynamic polarization, electrochemical impedance spectroscopy and salt spray test. Based on the results of the analyses, an attempt was made to discuss the corrosion process of the Cr2O3– Al2O3 composite coatings. 2. Experimental procedure 2.1. Plasma spraying 304 stainless steel was used as the coating substrate. Prior to deposition, substrates of 30 × 30 × 5 mm3 were sandblasted after ultrasonic cleaning in ethanol. A bond layer NiCr and working layer Cr2O3–Al2O3 (CA hereafter) were produced with a Multicoat™ atmospheric plasma-spraying system equipped with a F4-MB plasma gun (Sulzer Metco AG, Switzerland). Commercial fused and crushed Cr2O3–Al2O3 feedstock powders were available. The median particle diameter (D50) is about 30 μm, and the fraction of Al2O3 phase in the powders is approximately 30 wt.%. The coatings were prepared with the optimized parameters as shown in Table 1. The thickness of the as-sprayed coatings was controlled to be about 350 μm. 2.2. Sealing of the coating The plasma-sprayed CA coatings were sealed by aluminum phosphate with small fraction of Al2O3 nanoparticles. Aluminum phosphate was prepared from the solution of aluminum hydroxide (Al(OH)3) and orthophosphoric acid (85% H3PO4) diluted with about 20 wt.% of deionized water [20]. The Al(OH)3:H3PO4 ratio was 1:4.2 by weight, which corresponded to the molar ratio P/Al of about 3. The solution was mixed and slightly heated on a magnetic stirrer until it became clear. Then, 5 wt.% Al2O3 nanoparticles were added to the solution and stirred to get a well dispersed sealant. The samples were immersed into the sealants at 50 °C for about 30 min with an ultrasonic bath and then standing at ambient temperature and atmospheric pressure for 12 h. Thereafter, the impregnated coatings were heat-treated for 2 h at 100 °C, followed by 2 h at 200 °C, and 2 h at the final curing temperature of 250 °C. The excess sealant was ground off by polishing, ensuring that the true surface of the coating was exposed for microscopic analysis and corrosion tests. The obtained samples were labeled as CA-S. To determine the high temperature resistance, the samples with and without sealing treatment were heated at 600 °C for 30 min and the samples were denoted as CA-SH and CA-H, respectively.
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2.3. Characterization The phase composition was done using a powder X-ray diffraction (XRD, D/max 2550 V, Rigaku Tokyo, Japan). The microstructure was characterized using a scanning electron microscope (SEM, TM3000, HITACHI, Japan) with affiliated energy-dispersive spectrometer. The accelerated voltage for EDS was 15 KV. The electrochemical corrosion tests of coating samples were performed in a three electrode cell using a platinum mesh electrode as a counter, a saturated calomel electrode (SCE) as reference electrode and the coating samples as the working electrode. The exposed area of the coating samples was 1 cm2. The experiment was conducted in unstirred 3.5 wt.% NaCl solution. Electrochemical systems Ametek 1287 & 1260 were used for the corrosion tests. The corrosion potential and the corrosion current density were obtained through the linear analysis of Tafel approximation. Electrochemical impedance spectroscopy (EIS) curves of the coated samples were acquired at the open circuit potential (Eocp) after 30 min of immersion, which had been determined earlier. The frequency range varied from 10− 2 Hz to 106 Hz. The polarization curves were obtained at a scan rate of 0.2 mV/s and a scan range of 0.2 V–0.4 V vs Eocp. Repeated tests were performed to ensure the reproducibility of the results. The salt spray test was conducted following the Chinese National Standard GB/T 10125-2012. The coatings were exposed to a salt fog generated from a (50 ± 5) g/L NaCl solution with a pH value of 6.5–7.2 at (35 ± 2)°C. The exposure duration lasted for 96 h.
3. Results and discussion 3.1. Phase structure Fig. 1 shows the X-ray diffraction patterns of the as-sprayed CA coating. All peaks can be assigned to a solid solution (Cr, Al)2O3. Since Cr2O3 and α-Al2O3 are isostructural, they can easily form continuous solid solution in which the solute is completely incorporated in the lattice [21]. The peak positions are close to those of pure α-Cr2O3 [ICDD PDF 38-1479]. The only difference is that the location of peaks shifts to higher 2θ angles, towards the positions for α-Al2O3 [ICDD PDF 46-1212] due to the reduction of lattice parameters. While γ-Al2O3 is usually dominant in the Al2O3 coatings deposited by APS, the XRD result of CA indicates that the α-phase is fully stabilized, which can be attributed to the substitution of Al ions in alumina by Cr.
Table 1 Plasma-spraying operating parameters. Parameters
Bond coat
Top coat
Unit
Arc current Primary plasma gas (Ar) Secondary plasma gas (H2) Carrier gas (Ar) Powder feed rate Nozzle diameter Powder injector diameter Powder injector distance Spray distance
600 57 5 3.5 18 6 2.0 5.0 120
650 45 10 3.5 35 6 1.8 5.0 110
A slpm slpm slpm g/min mm mm mm mm
Fig. 1. XRD patterns of the as-sprayed CA coating. The dash line represents the standard line-graph corresponding to α-Cr2O3 [ICDD PDF 38–1479].
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Fig. 2. The polished cross-section morphologies of (a) CA, (b), (c) CA-S coatings; (d) EDS analysis of the region marked with white crosses in (c).
3.2. Morphology and elemental analysis The microstructure of the coating was determined by the crosssectional SEM micrograph of the coatings. As shown in Fig. 2a, the unsealed coating has relatively high porosity and some micro cracks. After sealing, it can be found that the topside of the coating exhibits less porosity, and the sealant has penetrated into the coating to a depth of about 90 μm from the surface (Fig. 2b). The SEM image at higher resolution exhibits the filled defects (Fig. 2c). Fig. 2d is the EDS patterns of range A and B (the inset image) in Fig. 2c. The chemical compositions are listed in Table 2. There is the evident element of phosphorus in the filled defects of CA-S coating (range A in Fig. 2c) while in the dense area B, peaks corresponding to phosphorus were barely observed. Meanwhile, the percent of Al and Cr elements in range A have changed greatly compared with CA and range B. The significantly increased Al should originate from the sealant (aluminum phosphate and the Al2O3 nanoparticles). It strongly indicates that the sealant can be brought into the coatings through the structural defects (e.g., voids, cracks and gaps between the lamellae).
Table 2 Chemical composition (atom%) of the four different coatings corresponding to Figs. 2 and 3. Samples
O
Al
Cr
P
CA CA-S (A) CA-S (B) CA-H CA-SH (A) CA-SH (B)
65.0 67.5 65.8 64.5 65.9 68.5
12.2 22.6 12.5 12.1 24.1 11.4
22.8 7.9 21.7 23.4 8.6 20.1
/ 2.0 / / 1.4 /
Fig. 3a, b are the cross-sectional SEM images of CA-H and CA-SH, respectively. Compared with Fig. 2, it can be found the 600 °C heat treatment has no obvious effect on the morphology of coatings except a slightly denser structure. Sealed defects can also be detected in CA-SH coating as shown in Fig. 3c. The EDS results are shown in Fig. 3d and Table 2. Similar effects of sealing on the elements were observed. The increase of Al and P in range A indicates the existence of sealant. There was little shrinkage of the sealant, showing good thermo-stable performance. 3.3. Potentiodynamic polarization Table 3 exhibits the open-circuit potential (Eocp) of the four coating samples when the values were stable after immersion in 3.5% NaCl solution for 30 min. Potentiodynamic polarization curves are shown in Fig. 4. Values of the corrosion potential (Ecorr) and the corrosion current density (Icorr) extracted from these curves by Tafel fitting using CView softwore are also listed in Table 3. Icorr is an important parameter used for evaluating the kinetics of the corrosion reaction. Corrosion protection is inversely proportional to the Icorr [22]. The Icorr of CA and CA-S indicates a reduction in corrosion rate of about 10 times after sealing. CA-H shows a slightly smaller Icorr than CA, which may be due to the denser structure. The Icorr of CA-SH is far less than that of CA-H while almost equal to that of CA-S, signifying the good heat resistance of the sealant. From polarization curves, the open porosity (designated by Po) can be calculated by the following equation: Po ¼
IC 100% IS
ð1Þ
where Ic and Is indicate corrosion current density of the coated
F. Shao et al. / Surface & Coatings Technology 276 (2015) 8–15
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Fig. 3. The polished cross-section morphologies of (a) CA-H, (b), (c) CA-SH coatings; (d) EDS analysis of the region marked with white crosses in (c).
substrate and the bare substrate in the same electrolyte, respectively [8]. Since the sealing efficiency (designated by Si) is related to Po, Eq. (2) can be derived to relate Si and Icorr.
Si ¼
P 0o −P o P 0o
!
" 100% ¼ 1−
Po P 0o
!#
" 100% ¼ 1−
Icorr
!#
I0corr
100% ð2Þ
interconnected porosity was blocked. Further, epoxy resin could penetrate into the Al2O3–TiO2 coating by above 300 μm [26,27]. However, the high temperature resistance of epoxy resin was poor (completely decomposed after heat treatment at 600 °C for 30 min in our experiments). Sol-derived sealant can be used in conditions where the temperature is above 1000 °C, but the porosity of the modified coatings is only decreased minimally (~12% to ~11%) after the sealing treatment [15]. After heat treatment, the coating samples shift the corrosion potential in the negative direction. Compared with CA-H, the Ecorr of CA-SH
where Po and P0o are the open porosity of the sealed and unsealed coatings, and Icorr and I0corr are the corrosion current density of the sealed and unsealed coatings, respectively [23]. The calculated Si of CA-S and CA-SH is shown in Table 3. 89.1% open pores were sealed by modified aluminum phosphate sealant. Further, it was observed that the open porosity in CA-SH was reduced by 85.1% compared with CA-H. The porosity variation between CA and CA-H can also be explained by Eq. (2). The heat treated coating has an ~20% lower porosity, which is consistent with our previous work [24]. Zhang et al. [25] studied the effectiveness of sealing treatment with epoxy resin using similar methods. Epoxy resin sealed Cr2O3–TiO2 coating well and 81% of
Table 3 Corrosion parameters obtained from potentiodynamic polarization measurements of the four different coatings in 3.5% NaCl electrolyte. Samples
Eocp (mV)
Ecorr (mV)
Icorr (μA/cm2)
Rp (KΩ cm2)
Si (%)
CA CA-S CA-H CA-SH
−187 −167 −246 −277
−203 −209 −255 −300
11.0 1.2 8.7 1.3
4.2 39.2 6.5 30.1
/ 89.1 / 85.1
Fig. 4. Potentiodynamic polarization curves of the four kinds of coatings in 3.5% NaCl electrolyte.
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F. Shao et al. / Surface & Coatings Technology 276 (2015) 8–15
Fig. 5. Nyquist impedance plot for four kinds of coatings. Symbols represent experimental data and solid lines stand for fitted data. The inset image is the equivalent circuit.
was further shifted in the negative direction by about 45 mV. The change in Ecorr depends on the relative effect of the coating on the anodic and cathodic reactions [28]. The negative shift in Ecorr in the present case suggests that the heated coatings preferentially inhibits the cathodic process, especially for the sealed samples. The change trends of polarization resistances (Rp) reveal that CA-S and CA-SH has better corrosion inhibition capability, which are consistent well with the results of Icorr. Rp was calculated based on the Stern and Geary relationship. It is worth noting that unlike CA and CA-H, there was no passivation observed for the sealed coatings (CA-S and CA-SH). This appearance is probably attributed to the fewer permeable defects in sealed coatings [29]. 3.4. Electrochemical impedance spectroscopy (EIS) A number of works revealed that electrochemical impedance spectroscopy (EIS) is a powerful, non-destructive technique for the detection of electrochemical characterization of ceramic coatings in the field of aqueous corrosion science [30–32]. The EIS data of the four kinds of coatings are presented in the Nyquist plots in Fig. 5. The diameters of Nyquist plots increased after sealing, indicating the decreased corrosion. EIS spectra for coatings can be represented by simple equivalent circuit shown in the inset of Fig. 5. Rs stands for the resistance of the solution, Rpo is the resistance to current flow through the pores in the coating, Rt indicates the charge transfer resistance, CPEc is the constant
Fig. 7. Digital photos of the coatings after 96 h of the salt spray test: (a) CA, (b) CA-S, (c) CA-H, (d) CA-SH.
phase element (CPE) of coatings and CPEdl is a double-layer capacitance between the substrate/coating interface [33]. Here, CPE was used as a substitute for the capacitor to fit more accurately the impedance data of the electrochemical capacitance. The fitting curves are presented in Fig. 5 as the solid lines for each spectrum. The element values of Rt and Rpo can indicate the corrosion resistance and protectiveness of coating, respectively [34]. The fitting values of Rpo corresponding to CA, CA-S, CA-H and CA-SH are 146.2, 2252, 183.3 and 1408 Ω, respectively. The relatively low Rpo values of CA and CA-H indicate that the NaCl solution penetrates into the pores of the unsealed coatings. The higher Rpo values of CA-S and CA-SH demonstrate the reduced porosity after sealing. The improvement effect of sealing on the corrosion resistance of coated samples can be further confirmed by the difference of Rt. CA-S and CA-SH have much larger Rt (21,695 Ω and 28,627 Ω) than CA and CA-H (4264 Ω and 5075 Ω), indicating that the electrochemical reaction speed was decreased [30]. In Bode plots the magnitude of the impedance |Z| and phase-angle are plotted respectively as a function of frequency (f). Impedance at
Fig. 6. Bode plots of the four kinds of coatings. Symbols represent experimental data and solid lines stand for fitted data.
F. Shao et al. / Surface & Coatings Technology 276 (2015) 8–15
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Fig. 8. EDS maps of Ni element of (a) CA-H, (b) CA-SH. The scale bar is 2 mm. The detected regions were marked by white rectangle in Fig. 7c and d.
the high frequency represents the performance of the coating in corrosion solution, while at the low-frequency, it is related to the corrosion at the substrate/solution interface [35]. Compared with CA and CA-H, the impedance of CA-S and CA-SH were enhanced over the whole frequency range, as shown in Fig. 6. This indicates that the corrosion resistance of the sealed coating are much greater than that of the unsealed coating. 3.5. Salt spray test CA, CA-H, CA-S and CA-SH were subjected to the salt spray test to further investigate the anti-corrosion performances of the coatings. After 96 h testing, both CA and CA-H exhibit discoloration in the area
marked by the arrows, while there are no visible discoloration in CA-S and CA-SH (Fig. 7). Further, as shown in Fig. 8, additional EDS maps were used to study the corrosion behavior of the samples. There were Ni enrichments in the discoloration area of CA-H (marked by white rectangle in Fig. 7). On the contrary, the phenomenon was not observed in CA-SH. The surface morphology of the coatings was also examined by SEM after 96 h of the salt spray test, as shown in Fig. 9. Since the surface morphologies and EDS results were very similar when the coatings were heated or not, the coatings with heat treatment were discussed here. The EDS analysis result (Fig. 9b) revealed that Ni and Fe elements appeared nearby the discoloration area of CA-H. The chemical compositions are listed in Table 4. It can be concluded that the corrosion
Fig. 9. (a) Top-view SEM image of CA-H after 96 h of salt spray test; (b) EDS results corresponding to the area marked with white rectangle in (a); (c) plane SEM image of CA-SH after 96 h of salt spray test; (d) EDS analysis of the region marked with white cross and white rectangle (the inset image) in (c).
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References
Table 4 Chemical composition (atom%) of CA-H and CA-SH coatings after salt spray test. Samples
O
Al
Cr
P
Na
Cl
Fe
Ni
CA-H CA-SH (A) CA-SH (B)
57.1 70.5 68.9
5.5 10.0 12.1
28.6 3.2 18.1
/ 16.3 0.9
2.2 / /
1.7 / /
1.2 / /
3.7 / /
products generated during the salt spray test. The atom percentage of Ni is higher than Fe. In addition, some EDS results of the around places show 3.5 atom% Ni but unobvious Fe peaks (0.7 atom%). If the electrolyte can easily permeate the NiCr bond coat and reach the substrate, galvanic corrosion would be formed on substrate because the corrosion potential of Ni is higher than that of Fe in NaCl. Fe will be the dominant element in the corrosion products. Based on the above analyses, a possible process for the corrosion of CA and CA-H was proposed as follows: the electrolyte penetrated into the coating following the defects, when it reached the NiCr bond coat, corrosion occurred. With increasing corrosion time, corrosion products such as nickel oxides and hydroxides were accumulated in the NiCr/ceramic interface and appeared on the surface of the top-coat through the pores or cracks. Further, electrolyte may penetrate the bond coat, thus accelerating the corrosion of substrate. In contrast, no obvious peaks of Ni or Fe were detected in CA-SH. Range B shown in Fig. 9c is Cr2O3–Al2O3 coatings while range A is filled with sealant as proved by EDS results (Fig. 9d and Table 4). These results were in complete agreement with those obtained by polarization test and electrochemical impedance spectrum. The sealants prevented the electrolyte from penetrating into the Cr2O3–Al2O3 coatings by closing the available paths. Even after heat treatment, sealed coatings still exhibit good corrosion resistance. 4. Conclusion In the current work, Cr2O3–Al2O3 composite coatings were prepared via APS technique. Aluminum phosphate with small fraction of Al2O3 nanoparticles was applied as high-temperature sealant. The phase composition, microstructure and corrosion properties of the coatings were investigated. Based on the experimental results, the following conclusions can be drawn: (1) The fused Cr2O3–Al2O3 feedstock leads to the complex (Al, Cr)2O3 phase coating. α-Phase is fully stabilized, which can attribute to the plenty Cr2O3 in the fused feedstock. (2) After sealing, the coatings present decreased porosity in the topside. EDS analysis showed that sealant can indeed be brought into the coatings. The sealing treatments significantly improve the corrosion resistance as demonstrated by polarization, electrochemical impedance spectroscopy and salt spray tests. (3) The coatings with and without sealing treatment were heated at 600 °C for 30 min to determine the high temperature applicability. Low volume shrinkage of the inorganic sealant was observed after heat treatment. The sealed coatings exhibit nearly equal anti-corrosion properties before and after heat treatment. The heat treatment has a small effect on the corrosion behavior and the sealing can inhibit the corrosion effectively.
Acknowledgments The study is jointly supported by the National Nature Science Foundation of China (51302299) and Shanghai Nature Science Fund Project (13ZR1446000). Simultaneously, we are grateful to Prof. Shigang Xin and Prof. Yuzhi Zhang for providing the support for this work.
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