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316L stainless steel in supercritical water
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Comparative study on corrosion characteristics of Al2O3/316L and TiO2/316L stainless steel in supercritical water Yuzhen Wang a, Fen Gao a, Jianqiao Yang b, Yitong Zhu a, Changqing Fang a,*, Shuzhong Wang b, Gaoyang Zhao a a
Xi'an University of Technology, Xi'an, Shaanxi, 710048, PR China Key Laboratory of Thermo-Fluid Science and Engineering of MOE, Xi'an Jiaotong University, Xi'an, Shaanxi, 710049, PR China
b
article info
abstract
Article history:
Al2O3 and TiO2 coatings were fabricated on 316L stainless steel by atmospheric plasma
Received 8 May 2017
spraying to improve the corrosion resistance of 316L stainless steel in supercritical water.
Received in revised form
The corrosion characteristics of the samples were evaluated in a batch reactor at 500 C
12 June 2017
and 25 MPa with an oxygen concentration of 1000 mg/L for 80 h. The adhesive strengths of
Accepted 17 June 2017
the coated samples were tested, and the weight changes, morphologies and elements
Available online xxx
distributions of the fresh and corroded samples were analyzed. Results showed that the bond strength of TiO2/316L was 1.5 times than that of Al2O3/316L (26.639 N/mm2). The
Keywords:
surface morphology of Al2O3/316L showed gully erosion with much pores and cracks after
Corrosion
exposed in SCW, which provided channels for oxygen and SCW to get into the substrate
Supercritical water
and also the elements in substrate to diffuse to the surface of the coating. The corroded
Coating
Al2O3/316L suffered significant weight loss, and most of the coatings were peeled off.
Al2O3
However, the surface morphology of TiO2/316L was relatively dense and the thickness of
TiO2
the coating was not found to decrease obviously. © 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Supercritical water gasification (SCWG) is a promising technology for hydrogen production [1,2]. The supercritical water (SCW) is the water beyond the critical point (T ¼ 374 C, P ¼ 22.1 MPa), which has unique physical and chemical properties, including excellent heat-conductivity and diffusivity properties [3]. These properties make SCW an excellent solvent for the both organic substances and gases, providing a homogenous condition for gasification of organics with short
residence time and high efficiency [4e7]. Thus the SCWG technology as a new and highly efficient hydrogen conversion technology have become an important research direction in energy field [8e10]. Recent studies have indicated that hydrogen yields could be enhanced by adding small amounts of oxygen in SCW [11,12]. However, the addition of oxygen would intensify the corrosion of the reactor materials [13]. Researchers have made a lot of studies on corrosion behaviors of the candidate materials (such as nickel based alloy, iron based alloys and titanium based alloy) [14e18]. Generally, nickel based alloys (such as Inconel 625, Hastelloy C276, and
* Corresponding author. E-mail address: [email protected] (C. Fang). http://dx.doi.org/10.1016/j.ijhydene.2017.06.129 0360-3199/© 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Wang Y, et al., Comparative study on corrosion characteristics of Al2O3/316L and TiO2/316L stainless steel in supercritical water, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.06.129
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purity) and TiO2 (200e400 mesh, 99.9% purity) powders (China National Medicines Co., Ltd.) were used as coating materials. Acetone (CH3COCH3, 99.5% purity, Chinese Medicine Group Chemical Reagent Co., Ltd.) and ethanol (C2H5OH, 99.5% purity, China National Medicines Co., Ltd.) were used as cleaning solvent. Hydrogen peroxide solution (H2O2, 30 wt% purity, China National Medicines Co., Ltd.) was used as oxidant.
Inconel 690 alloys) showed a better corrosion resistance performance than iron based alloys (such as 304L SS, 304 SS, 316 SS and 316L SS) [14]. Gao et al. [19] found that 316L stainless steel in SCW showed more severe corrosion at higher temperatures, and the corrosion products were divided into two layers. The inner layer fit closely with the substrate, and the outer layer was easily to peel off. Tang et al. [20] studied the corrosion characteristics of 316L stainless steel when treating urban sewage sludge. Results showed that the corrosion degree of the samples increased with the increasing of oxidation coefficient, and the corrosion rate was up to 0.17 mm/a under acidic conditions. Zhu et al. [21] found that the corrosion rate of 316L stainless steel in SCW at 600 C and 23 MPa was 0.0606 g/(m2h). Therefore, how to enhance the corrosion resistance of the alloys are attracting increasing interest. Preparing coatings on the alloy substrate is one of the effective methods. Zhang et al. [22] deposited the chromium (Cr) coating on two kinds of steels (China Nuclear Steels CNS-I [9Cr] and modified CNS-II [12Cr]) and studied their corrosive properties at 450 C and 25 MPa with an oxygen concentration of 200 mg/L for 1000 h exposure. Results showed that negligible weight of 0.542 mg/dm2 and 13.351 mg/dm2 were gain for the coated CNS-I and CNS-II, respectively, considerably lower than those of the corresponding bare samples (602.17 mg/dm2 and 459.42 mg/dm2, respectively). Khatkhatay et al. [23] found that the TiN coating performed on polished zircaloy-4 tubes can gain significant enhancement of corrosion resistant. Hui et al. [24] reported that P91 steel with Al2O3 coating showed higher corrosion resistance compared with bare P91 steel in SCW. Shao et al. [25] also found that the 304 stainless steel with Cr2O3eAl2O3 coatings had a potential use in severe corrosion conditions. Yang et al. [26] proposed that the plasma-sprayed Al2O3eY3Al5O12 (YAG) eutectic ceramic coating exhibited excellent microstructure and performance stabilities under high temperatures. In this paper, we prepared Al2O3 and TiO2 coatings on 316L stainless steel using atmospheric plasma spraying (APS), respectively. And their corrosion behaviors were investigated in a batch reactor at 500 C and 25 MPa with an oxygen concentration of 1000 mg/L for 80 h. The adhesive strengths of the samples, weight changes, morphologies and components of the fresh and used samples were characterized, and the corrosion depth was also detected.
Preparation of the coatings The 316L stainless steel samples with the dimension of ɸ25 mm 3 mm size were firstly polished with SiC abrasive papers to remove the oxide layer on the metal. In order to clean the contaminants on the surface of the samples, the samples were degreased ultrasonically in acetone solution, and immersed in ethanol solution for 10 min. Prior to spray, the surface of samples were sandblasted firstly to further improve the bond strength between the coatings and the metal substrate. To reduce the differences of the thermalphysical properties between the coatings and the substrate, a bond coating, made of the superalloy powder (NiCrAl, Shanghai Chemical Trading Co., Ltd.) was then deposited on the substrates. And finally, the Al2O3 and TiO2 coatings were performed on the 316L substrate by APS. A mixture of argon and hydrogen was used as plasma gas. The main parameters of APS were shown in Table 2. After spraying, the samples were dried in a vacuum oven at 100 C for 2 h. The sprayed samples are shown in Fig. 1.
Experimental equipment Fig. 2 showed the schematic diagram of the apparatus used in this study. The reactor was made of Hastelloy C276 alloy, which was designed to withstand a maximum temperature and pressure up to 650 C and 30 MPa, respectively. The reactor was heated by an electric heater with a power of 1.5 kW and the heating rate was about 30 ºC/min (average value). The internal temperature of the reactor was controlled by a temperature indicator and controller (TIC) using Pt (100) as the temperature sensor. Before the experiment, the coated 316L stainless steel samples were fixed on the cooling coil in the reactor, and then the deionized water and a certain amount of hydrogen peroxide (calculated by formula (1)
Materials and methods
Table 2 e Operation parameters of plasma spraying.
Materials and reagents
Experimental parameters
A commercial austenitic stainless steel 316L was used as the substrate material, which was purchased from Tianjin stainless steel Co. Ltd. The chemical compositions of 316L stainless steel was listed in Table 1 [27]. The Al2O3 (200e400 mesh, 99.9%
Voltage(V) current (A) Argon flow (L/min) Hydrogen flow (L/min) Thickness (mm)
Bonding layer
Al2O3 coating
TiO2 coating
70 500 100 10 0.05
72 550 80 15 0.20
72 550 80 15 0.20
Table 1 e Chemical compositions of 316L stainless steel. Elements Proportions (%)
C
Si
Mn
P
S
Ni
Cr
Mo
0.03
1.00
2.00
0.035
0.03
12.0e15.0
16.0e18.0
2.0e3.0
Please cite this article in press as: Wang Y, et al., Comparative study on corrosion characteristics of Al2O3/316L and TiO2/316L stainless steel in supercritical water, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.06.129
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Fig. 1 e The coated samples prepared by APS ((a) Al2O3/316L (b) TiO2/316L). samples were characterized by a scanning electron microscope (SEM, JSM-6390A N) equipped with X-ray energy dispersion spectroscopy (EDS). Prior to the SEM examinations, the samples were coated with platinum by sputtering to improve the electrical conductivity. The elementary compositions of the coatings were analyzed by EDS, and the phase composition was identified by a Philips PW3710 X-ray Diffraction (XRD) with a Cu Ka radiation source.
Results and discussions Surface morphologies and the adhesive strengths of the fresh samples
Fig. 2 e Schematic diagram of the apparatus ((1) electric furnace (2) reactor (3) inlet of cooling water (4) outlet of cooling water (5) pressure gauge (6) temperature sensor (7) temperature indicator and controller, (8) coated samples (9) cooling coil).
according to the oxygen concentration of 1000 mg/L) were added into the reactor. The reactor was then heated at 30 ºC/ min to the desired temperature of 500 C. As soon as the set temperature was reached, the reactor was held for 80 h before the heating was stopped. 2H2 O2 /O2 [ þ 2H2 O
(1)
Characterization of the coated samples The adhesive strengths of the coated samples were measured on a universal testing machine (Mode AG-I 250, USA). The loading rate was 2 mm/min. Each test was carried for three times, and the mean value was taken to eradicate any discrepancies. The surface and cross-section morphologies of the
The surface morphologies of the prepared Al2O3/316L and TiO2/316L were shown in Fig. 3. Results showed that both the coatings were relatively smooth, expect for some little particles on the surface. The EDS analysis of the particles showed that the component was platinum oxide (Fig. 3(c)). This was mainly because that the coating was platinum-sprayed to enhance their conductivity before SEM testing. As the EDS spectrum of the particles on TiO2/316L was similar, we only showed that of the Al2O3/316L's in Fig. 3. The adhesive strengths of the coated samples were showed in Table 3. The results showed that the bond strength of Al2O3/316L and TiO2/316L were 26.639 N/mm2 and 40.607 N/ mm2, respectively. The TiO2 coating exhibited more stronger cohesive force than that of Al2O3 coating. This may have some account with the physical properties of the AlO3 and TiO2 [28]. The Al2O3 powder we used for coating preparation was aAl2O3, which was easily to turned into the metastable phase of g-Al2O3 at the cooling period of the APS process. As the interfacial energy of g-Al2O3 is lower than a-Al2O3 at the interface between liquid and solid phase, more g-Al2O3 would be formed in the Al2O3 coating, which induced to a relatively lower adhesive strength for Al2O3 coating. However, the TiO2 powder, which was in rutile phase, would not have a phase transition during the APS process, and could maintain a relatively high stability [28,29].
Weight changes The weight changes of samples after corrosion test were shown in Table 4. Results showed that Al2O3/316L suffered significant weight loss, while TiO2/316L showed slight weight
Please cite this article in press as: Wang Y, et al., Comparative study on corrosion characteristics of Al2O3/316L and TiO2/316L stainless steel in supercritical water, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.06.129
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Fig. 3 e Surface morphologies of the samples and the EDS spectrum of the particle on Al2O3/316L surface ((a) surface morphology of Al2O3/316L, (b) surface morphology TiO2/316L, (c) EDS spectrum of the particle on Al2O3/316L surface).
Table 3 e The adhesive strengths of the coated samples. Adhesive strength (N/mm2)
Samples
Al2O3/316L TiO2/316L
No.1
No.2
No.3
Average value
26.967 36.328
25.064 41.498
27.886 43.994
26.639 40.607
gain. This indicated that the Al2O3 coating may have been partially peeled off in the severely corrosive environment. Huang et al. [30] also found that the aluminized sample had a slight weight decrease when exposed at 500 C. As for the TiO2/316L sample, some oxides may have been generated on the surface, which will be discussed in the following part.
Surface morphologies of the samples after corrosion test Table 4 e Weights of the samples before and after corrosion test. Sample
Al2O3/316L TiO2/316L
Weight before testing/g
Weight after testing/g
Surface area
Average rate/mg cm2h1
9.1693 9.1690
9.1612 9.1691
4.909 cm2 4.909 cm2
0.0206 0.00025
Fig. 4 displayed the surface morphologies and the elements distributions of the Al2O3/316L and TiO2/316L samples after exposed in SCW at 500 C and 25 MPa for 80 h with an oxygen concentration of 1000 mg/L. Fig. 4 (a) showed that the surface of Al2O3/316L displayed obvious grooving damages with much holes and cracks. These cracks and holes were the channels through which oxygen and SCW to get into the substrate [31]. In addition, the EDS data in Fig. 4 (b) showed that the surface
Fig. 4 e Surface morphologies and the elements distributions of Al2O3/316L after exposed in SCW at 500 C, 25 MPa with an oxygen concentration of 1000 mg/L for 80 h ((a) SEM of Al2O3/316L (b) magnified SEM of micro-region B (c) magnified SEM of micro-region C (d) elements distributions of Al2O3/316L at different regions). Please cite this article in press as: Wang Y, et al., Comparative study on corrosion characteristics of Al2O3/316L and TiO2/316L stainless steel in supercritical water, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.06.129
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Fig. 5 e Possible corrosion mechanisms of the coated samples in SCW.
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of Al2O3/316L was rich in Ni and poor in Al, which was mainly due to the following reasons: 1) the coating had been partially peeled off, and the bond coating or the substrate was exposed. 2) the Ni and other metal elements in the substrate, such as Cr, Fe, Mo, were diffused to the coating surface and reacted with the oxygen to form a mixed layer. Similar results were found on Hastelloy C-276 and Ni-base alloys in SCW [32,33]. For further analysis, the magnified morphologies and the element distributions at different micro-regions around the crack and relatively uncracked coatings were depicted in Fig. 4 (c) and Fig. 4 (d), respectively. The surface around the crack showed a comparatively looser structure, and some apparent holes between the fractured coatings were found. While, the surface
Fig. 6 e Surface morphologies and the elements distributions of TiO2/316L after exposed in SCW at 500 C, 25 MPa with an oxygen concentration of 1000 mg/L for 80 h ((a) SEM of TiO2/316L (b) magnified SEM of micro-region B (c) magnified SEM of micro-region C (d) elements distributions of TiO2/316L at different regions).
Fig. 7 e XRD patterns of TiO2/316L ((a) before corrosion test (b) after corrosion test).
Please cite this article in press as: Wang Y, et al., Comparative study on corrosion characteristics of Al2O3/316L and TiO2/316L stainless steel in supercritical water, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.06.129
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Fig. 8 e Cross-sectional structures and the associated composition profiles of the Al2O3/316L and TiO2/316L after exposed in SCW at 500 C, 25 MPa, with an oxygen concentration of 1000 mg/L for 80 h ((a) Al2O3/316L (b) TiO2/316L). structure of the relatively uncracked coating was much denser without significant cracks and pores. The element compositions of the different regions were similar, which further indicating that the elements in 316L stainless steel had diffused into the coating. And the possible corrosion mechanism of the coated samples in SCW was shown in Fig. 5. The surface morphologies and element distributions of TiO2/316L after corrosion test were shown in Fig. 6. Results showed that the surface of the sample was covered with a layer of granular substances, and formed some convex and concave regions. SEM of the convex region under higher magnification found that the bulge showed a smooth circular table shape with some granular substances around it (Fig. 6 (b)). The main component of bulge was TiO2, as shown in Fig. 6 (d). The reason may be that the TiO2 powder was not fully fused during the APS process [34]. The morphology of the concave region in Fig. 6(c) showed that there displayed some discrete particles on the relatively flat surface. And the main component of the concave region was also TiO2 (Fig. 6(d)). Although other elements such as Ni and Mo were also found at the surface of the sample, their content were much lower compared with titanium, which indicating that the TiO2 coating was not seriously cracked. The XRD patterns of TiO2/316L before and after corrosion test were depicted in Fig. 7. Although some other products such as NiFeO4 and NiMoO4 were found on the surface of the coating after corrosion test, the main component of the coating was still TiO2. The formation of these oxides may be the main reason for the gained weight of TiO2/316L found in Section Weight changes.
Corrosion depth of the samples after corrosion test The cross-sectional structures and the associated composition profiles of the Al2O3/316L and TiO2/316L samples after corrosion tests are shown in Fig. 8. Fig. 8 (a) indicated that most of the Al2O3 coating had been peeled off. And the thickness of residual coating was not uniform with a maximum thickness of about 0.05 mm, which further proved that the adhesive strength between the coating and the substrate was poor. However, the TiO2 coating was relatively complete and attached to the substrate closely (Fig. 8(b)). The residual thickness of the TiO2 coating was about 0.2 mm, which was not found to decrease obviously. Results showed
that the TiO2/316L exhibited higher corrosion resistance than Al2O3/316L.
Conclusions The Al2O3 and TiO2 coatings with thicknesses of 0.2 mm were fabricated on 316L stainless steel by APS, and their corrosion behaviors were evaluated in a batch reactor at 500 C and 25 MPa with an oxygen concentration of 1000 mg/L for 80 h. The major conclusions from this work are as follows: 1) The bond strength of TiO2/316L (40.607 N/mm2) was 1.5 times than that of Al2O3/316L (26.639 N/mm2), which was mainly due to the crystal transition of the steady phase of a-Al2O3 to the metastable phase of g-Al2O3 during the cooling period of APS process. 2) The surface morphology of Al2O3/316L showed gully erosion with much pores and cracks, which provided channels for oxygen and SCW to get into the substrate and also the elements in substrate to diffuse to the surface of the coating. The corroded sample suffered significant weight loss, and most of the coatings was peeled off. The residue thickness of the coating was about 0.05 mm. 3) The surface morphology of TiO2/316L was relatively dense. And the residual thickness of the TiO2 coating was about 0.2 mm, which was not found to decrease obviously. The TiO2/316L exhibits relatively better corrosion resistance performance than Al2O3/316L. In the later study, the corrosion resistance of TiO2 coating needs to be further studied in supercritical water for longer time, and preparation of the coating requires further optimization.
Acknowledgment This study was supported by China Postdoctoral Science Foundation (2015M570849); Shaanxi Colleges Association of Science and Technology for Young Talent Lifts (20160115); and Natural Science Foundation of Shaanxi Provincial Department of Education (20161011).
Please cite this article in press as: Wang Y, et al., Comparative study on corrosion characteristics of Al2O3/316L and TiO2/316L stainless steel in supercritical water, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.06.129
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Please cite this article in press as: Wang Y, et al., Comparative study on corrosion characteristics of Al2O3/316L and TiO2/316L stainless steel in supercritical water, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.06.129
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Comparative study on corrosion characteristics of Al2O3/316L and TiO2/316L stainless steel in supercritical water Yuzhen Wang a, Fen Gao a, Jianqiao Yang b, Yitong Zhu a, Changqing Fang a,*, Shuzhong Wang b, Gaoyang Zhao a a
Xi'an University of Technology, Xi'an, Shaanxi, 710048, PR China Key Laboratory of Thermo-Fluid Science and Engineering of MOE, Xi'an Jiaotong University, Xi'an, Shaanxi, 710049, PR China
b
article info
abstract
Article history:
Al2O3 and TiO2 coatings were fabricated on 316L stainless steel by atmospheric plasma
Received 8 May 2017
spraying to improve the corrosion resistance of 316L stainless steel in supercritical water.
Received in revised form
The corrosion characteristics of the samples were evaluated in a batch reactor at 500 C
12 June 2017
and 25 MPa with an oxygen concentration of 1000 mg/L for 80 h. The adhesive strengths of
Accepted 17 June 2017
the coated samples were tested, and the weight changes, morphologies and elements
Available online xxx
distributions of the fresh and corroded samples were analyzed. Results showed that the bond strength of TiO2/316L was 1.5 times than that of Al2O3/316L (26.639 N/mm2). The
Keywords:
surface morphology of Al2O3/316L showed gully erosion with much pores and cracks after
Corrosion
exposed in SCW, which provided channels for oxygen and SCW to get into the substrate
Supercritical water
and also the elements in substrate to diffuse to the surface of the coating. The corroded
Coating
Al2O3/316L suffered significant weight loss, and most of the coatings were peeled off.
Al2O3
However, the surface morphology of TiO2/316L was relatively dense and the thickness of
TiO2
the coating was not found to decrease obviously. © 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Supercritical water gasification (SCWG) is a promising technology for hydrogen production [1,2]. The supercritical water (SCW) is the water beyond the critical point (T ¼ 374 C, P ¼ 22.1 MPa), which has unique physical and chemical properties, including excellent heat-conductivity and diffusivity properties [3]. These properties make SCW an excellent solvent for the both organic substances and gases, providing a homogenous condition for gasification of organics with short
residence time and high efficiency [4e7]. Thus the SCWG technology as a new and highly efficient hydrogen conversion technology have become an important research direction in energy field [8e10]. Recent studies have indicated that hydrogen yields could be enhanced by adding small amounts of oxygen in SCW [11,12]. However, the addition of oxygen would intensify the corrosion of the reactor materials [13]. Researchers have made a lot of studies on corrosion behaviors of the candidate materials (such as nickel based alloy, iron based alloys and titanium based alloy) [14e18]. Generally, nickel based alloys (such as Inconel 625, Hastelloy C276, and
* Corresponding author. E-mail address: [email protected] (C. Fang). http://dx.doi.org/10.1016/j.ijhydene.2017.06.129 0360-3199/© 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Wang Y, et al., Comparative study on corrosion characteristics of Al2O3/316L and TiO2/316L stainless steel in supercritical water, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.06.129
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purity) and TiO2 (200e400 mesh, 99.9% purity) powders (China National Medicines Co., Ltd.) were used as coating materials. Acetone (CH3COCH3, 99.5% purity, Chinese Medicine Group Chemical Reagent Co., Ltd.) and ethanol (C2H5OH, 99.5% purity, China National Medicines Co., Ltd.) were used as cleaning solvent. Hydrogen peroxide solution (H2O2, 30 wt% purity, China National Medicines Co., Ltd.) was used as oxidant.
Inconel 690 alloys) showed a better corrosion resistance performance than iron based alloys (such as 304L SS, 304 SS, 316 SS and 316L SS) [14]. Gao et al. [19] found that 316L stainless steel in SCW showed more severe corrosion at higher temperatures, and the corrosion products were divided into two layers. The inner layer fit closely with the substrate, and the outer layer was easily to peel off. Tang et al. [20] studied the corrosion characteristics of 316L stainless steel when treating urban sewage sludge. Results showed that the corrosion degree of the samples increased with the increasing of oxidation coefficient, and the corrosion rate was up to 0.17 mm/a under acidic conditions. Zhu et al. [21] found that the corrosion rate of 316L stainless steel in SCW at 600 C and 23 MPa was 0.0606 g/(m2h). Therefore, how to enhance the corrosion resistance of the alloys are attracting increasing interest. Preparing coatings on the alloy substrate is one of the effective methods. Zhang et al. [22] deposited the chromium (Cr) coating on two kinds of steels (China Nuclear Steels CNS-I [9Cr] and modified CNS-II [12Cr]) and studied their corrosive properties at 450 C and 25 MPa with an oxygen concentration of 200 mg/L for 1000 h exposure. Results showed that negligible weight of 0.542 mg/dm2 and 13.351 mg/dm2 were gain for the coated CNS-I and CNS-II, respectively, considerably lower than those of the corresponding bare samples (602.17 mg/dm2 and 459.42 mg/dm2, respectively). Khatkhatay et al. [23] found that the TiN coating performed on polished zircaloy-4 tubes can gain significant enhancement of corrosion resistant. Hui et al. [24] reported that P91 steel with Al2O3 coating showed higher corrosion resistance compared with bare P91 steel in SCW. Shao et al. [25] also found that the 304 stainless steel with Cr2O3eAl2O3 coatings had a potential use in severe corrosion conditions. Yang et al. [26] proposed that the plasma-sprayed Al2O3eY3Al5O12 (YAG) eutectic ceramic coating exhibited excellent microstructure and performance stabilities under high temperatures. In this paper, we prepared Al2O3 and TiO2 coatings on 316L stainless steel using atmospheric plasma spraying (APS), respectively. And their corrosion behaviors were investigated in a batch reactor at 500 C and 25 MPa with an oxygen concentration of 1000 mg/L for 80 h. The adhesive strengths of the samples, weight changes, morphologies and components of the fresh and used samples were characterized, and the corrosion depth was also detected.
Preparation of the coatings The 316L stainless steel samples with the dimension of ɸ25 mm 3 mm size were firstly polished with SiC abrasive papers to remove the oxide layer on the metal. In order to clean the contaminants on the surface of the samples, the samples were degreased ultrasonically in acetone solution, and immersed in ethanol solution for 10 min. Prior to spray, the surface of samples were sandblasted firstly to further improve the bond strength between the coatings and the metal substrate. To reduce the differences of the thermalphysical properties between the coatings and the substrate, a bond coating, made of the superalloy powder (NiCrAl, Shanghai Chemical Trading Co., Ltd.) was then deposited on the substrates. And finally, the Al2O3 and TiO2 coatings were performed on the 316L substrate by APS. A mixture of argon and hydrogen was used as plasma gas. The main parameters of APS were shown in Table 2. After spraying, the samples were dried in a vacuum oven at 100 C for 2 h. The sprayed samples are shown in Fig. 1.
Experimental equipment Fig. 2 showed the schematic diagram of the apparatus used in this study. The reactor was made of Hastelloy C276 alloy, which was designed to withstand a maximum temperature and pressure up to 650 C and 30 MPa, respectively. The reactor was heated by an electric heater with a power of 1.5 kW and the heating rate was about 30 ºC/min (average value). The internal temperature of the reactor was controlled by a temperature indicator and controller (TIC) using Pt (100) as the temperature sensor. Before the experiment, the coated 316L stainless steel samples were fixed on the cooling coil in the reactor, and then the deionized water and a certain amount of hydrogen peroxide (calculated by formula (1)
Materials and methods
Table 2 e Operation parameters of plasma spraying.
Materials and reagents
Experimental parameters
A commercial austenitic stainless steel 316L was used as the substrate material, which was purchased from Tianjin stainless steel Co. Ltd. The chemical compositions of 316L stainless steel was listed in Table 1 [27]. The Al2O3 (200e400 mesh, 99.9%
Voltage(V) current (A) Argon flow (L/min) Hydrogen flow (L/min) Thickness (mm)
Bonding layer
Al2O3 coating
TiO2 coating
70 500 100 10 0.05
72 550 80 15 0.20
72 550 80 15 0.20
Table 1 e Chemical compositions of 316L stainless steel. Elements Proportions (%)
C
Si
Mn
P
S
Ni
Cr
Mo
0.03
1.00
2.00
0.035
0.03
12.0e15.0
16.0e18.0
2.0e3.0
Please cite this article in press as: Wang Y, et al., Comparative study on corrosion characteristics of Al2O3/316L and TiO2/316L stainless steel in supercritical water, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.06.129
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Fig. 1 e The coated samples prepared by APS ((a) Al2O3/316L (b) TiO2/316L). samples were characterized by a scanning electron microscope (SEM, JSM-6390A N) equipped with X-ray energy dispersion spectroscopy (EDS). Prior to the SEM examinations, the samples were coated with platinum by sputtering to improve the electrical conductivity. The elementary compositions of the coatings were analyzed by EDS, and the phase composition was identified by a Philips PW3710 X-ray Diffraction (XRD) with a Cu Ka radiation source.
Results and discussions Surface morphologies and the adhesive strengths of the fresh samples
Fig. 2 e Schematic diagram of the apparatus ((1) electric furnace (2) reactor (3) inlet of cooling water (4) outlet of cooling water (5) pressure gauge (6) temperature sensor (7) temperature indicator and controller, (8) coated samples (9) cooling coil).
according to the oxygen concentration of 1000 mg/L) were added into the reactor. The reactor was then heated at 30 ºC/ min to the desired temperature of 500 C. As soon as the set temperature was reached, the reactor was held for 80 h before the heating was stopped. 2H2 O2 /O2 [ þ 2H2 O
(1)
Characterization of the coated samples The adhesive strengths of the coated samples were measured on a universal testing machine (Mode AG-I 250, USA). The loading rate was 2 mm/min. Each test was carried for three times, and the mean value was taken to eradicate any discrepancies. The surface and cross-section morphologies of the
The surface morphologies of the prepared Al2O3/316L and TiO2/316L were shown in Fig. 3. Results showed that both the coatings were relatively smooth, expect for some little particles on the surface. The EDS analysis of the particles showed that the component was platinum oxide (Fig. 3(c)). This was mainly because that the coating was platinum-sprayed to enhance their conductivity before SEM testing. As the EDS spectrum of the particles on TiO2/316L was similar, we only showed that of the Al2O3/316L's in Fig. 3. The adhesive strengths of the coated samples were showed in Table 3. The results showed that the bond strength of Al2O3/316L and TiO2/316L were 26.639 N/mm2 and 40.607 N/ mm2, respectively. The TiO2 coating exhibited more stronger cohesive force than that of Al2O3 coating. This may have some account with the physical properties of the AlO3 and TiO2 [28]. The Al2O3 powder we used for coating preparation was aAl2O3, which was easily to turned into the metastable phase of g-Al2O3 at the cooling period of the APS process. As the interfacial energy of g-Al2O3 is lower than a-Al2O3 at the interface between liquid and solid phase, more g-Al2O3 would be formed in the Al2O3 coating, which induced to a relatively lower adhesive strength for Al2O3 coating. However, the TiO2 powder, which was in rutile phase, would not have a phase transition during the APS process, and could maintain a relatively high stability [28,29].
Weight changes The weight changes of samples after corrosion test were shown in Table 4. Results showed that Al2O3/316L suffered significant weight loss, while TiO2/316L showed slight weight
Please cite this article in press as: Wang Y, et al., Comparative study on corrosion characteristics of Al2O3/316L and TiO2/316L stainless steel in supercritical water, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.06.129
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Fig. 3 e Surface morphologies of the samples and the EDS spectrum of the particle on Al2O3/316L surface ((a) surface morphology of Al2O3/316L, (b) surface morphology TiO2/316L, (c) EDS spectrum of the particle on Al2O3/316L surface).
Table 3 e The adhesive strengths of the coated samples. Adhesive strength (N/mm2)
Samples
Al2O3/316L TiO2/316L
No.1
No.2
No.3
Average value
26.967 36.328
25.064 41.498
27.886 43.994
26.639 40.607
gain. This indicated that the Al2O3 coating may have been partially peeled off in the severely corrosive environment. Huang et al. [30] also found that the aluminized sample had a slight weight decrease when exposed at 500 C. As for the TiO2/316L sample, some oxides may have been generated on the surface, which will be discussed in the following part.
Surface morphologies of the samples after corrosion test Table 4 e Weights of the samples before and after corrosion test. Sample
Al2O3/316L TiO2/316L
Weight before testing/g
Weight after testing/g
Surface area
Average rate/mg cm2h1
9.1693 9.1690
9.1612 9.1691
4.909 cm2 4.909 cm2
0.0206 0.00025
Fig. 4 displayed the surface morphologies and the elements distributions of the Al2O3/316L and TiO2/316L samples after exposed in SCW at 500 C and 25 MPa for 80 h with an oxygen concentration of 1000 mg/L. Fig. 4 (a) showed that the surface of Al2O3/316L displayed obvious grooving damages with much holes and cracks. These cracks and holes were the channels through which oxygen and SCW to get into the substrate [31]. In addition, the EDS data in Fig. 4 (b) showed that the surface
Fig. 4 e Surface morphologies and the elements distributions of Al2O3/316L after exposed in SCW at 500 C, 25 MPa with an oxygen concentration of 1000 mg/L for 80 h ((a) SEM of Al2O3/316L (b) magnified SEM of micro-region B (c) magnified SEM of micro-region C (d) elements distributions of Al2O3/316L at different regions). Please cite this article in press as: Wang Y, et al., Comparative study on corrosion characteristics of Al2O3/316L and TiO2/316L stainless steel in supercritical water, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.06.129
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Fig. 5 e Possible corrosion mechanisms of the coated samples in SCW.
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of Al2O3/316L was rich in Ni and poor in Al, which was mainly due to the following reasons: 1) the coating had been partially peeled off, and the bond coating or the substrate was exposed. 2) the Ni and other metal elements in the substrate, such as Cr, Fe, Mo, were diffused to the coating surface and reacted with the oxygen to form a mixed layer. Similar results were found on Hastelloy C-276 and Ni-base alloys in SCW [32,33]. For further analysis, the magnified morphologies and the element distributions at different micro-regions around the crack and relatively uncracked coatings were depicted in Fig. 4 (c) and Fig. 4 (d), respectively. The surface around the crack showed a comparatively looser structure, and some apparent holes between the fractured coatings were found. While, the surface
Fig. 6 e Surface morphologies and the elements distributions of TiO2/316L after exposed in SCW at 500 C, 25 MPa with an oxygen concentration of 1000 mg/L for 80 h ((a) SEM of TiO2/316L (b) magnified SEM of micro-region B (c) magnified SEM of micro-region C (d) elements distributions of TiO2/316L at different regions).
Fig. 7 e XRD patterns of TiO2/316L ((a) before corrosion test (b) after corrosion test).
Please cite this article in press as: Wang Y, et al., Comparative study on corrosion characteristics of Al2O3/316L and TiO2/316L stainless steel in supercritical water, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.06.129
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Fig. 8 e Cross-sectional structures and the associated composition profiles of the Al2O3/316L and TiO2/316L after exposed in SCW at 500 C, 25 MPa, with an oxygen concentration of 1000 mg/L for 80 h ((a) Al2O3/316L (b) TiO2/316L). structure of the relatively uncracked coating was much denser without significant cracks and pores. The element compositions of the different regions were similar, which further indicating that the elements in 316L stainless steel had diffused into the coating. And the possible corrosion mechanism of the coated samples in SCW was shown in Fig. 5. The surface morphologies and element distributions of TiO2/316L after corrosion test were shown in Fig. 6. Results showed that the surface of the sample was covered with a layer of granular substances, and formed some convex and concave regions. SEM of the convex region under higher magnification found that the bulge showed a smooth circular table shape with some granular substances around it (Fig. 6 (b)). The main component of bulge was TiO2, as shown in Fig. 6 (d). The reason may be that the TiO2 powder was not fully fused during the APS process [34]. The morphology of the concave region in Fig. 6(c) showed that there displayed some discrete particles on the relatively flat surface. And the main component of the concave region was also TiO2 (Fig. 6(d)). Although other elements such as Ni and Mo were also found at the surface of the sample, their content were much lower compared with titanium, which indicating that the TiO2 coating was not seriously cracked. The XRD patterns of TiO2/316L before and after corrosion test were depicted in Fig. 7. Although some other products such as NiFeO4 and NiMoO4 were found on the surface of the coating after corrosion test, the main component of the coating was still TiO2. The formation of these oxides may be the main reason for the gained weight of TiO2/316L found in Section Weight changes.
Corrosion depth of the samples after corrosion test The cross-sectional structures and the associated composition profiles of the Al2O3/316L and TiO2/316L samples after corrosion tests are shown in Fig. 8. Fig. 8 (a) indicated that most of the Al2O3 coating had been peeled off. And the thickness of residual coating was not uniform with a maximum thickness of about 0.05 mm, which further proved that the adhesive strength between the coating and the substrate was poor. However, the TiO2 coating was relatively complete and attached to the substrate closely (Fig. 8(b)). The residual thickness of the TiO2 coating was about 0.2 mm, which was not found to decrease obviously. Results showed
that the TiO2/316L exhibited higher corrosion resistance than Al2O3/316L.
Conclusions The Al2O3 and TiO2 coatings with thicknesses of 0.2 mm were fabricated on 316L stainless steel by APS, and their corrosion behaviors were evaluated in a batch reactor at 500 C and 25 MPa with an oxygen concentration of 1000 mg/L for 80 h. The major conclusions from this work are as follows: 1) The bond strength of TiO2/316L (40.607 N/mm2) was 1.5 times than that of Al2O3/316L (26.639 N/mm2), which was mainly due to the crystal transition of the steady phase of a-Al2O3 to the metastable phase of g-Al2O3 during the cooling period of APS process. 2) The surface morphology of Al2O3/316L showed gully erosion with much pores and cracks, which provided channels for oxygen and SCW to get into the substrate and also the elements in substrate to diffuse to the surface of the coating. The corroded sample suffered significant weight loss, and most of the coatings was peeled off. The residue thickness of the coating was about 0.05 mm. 3) The surface morphology of TiO2/316L was relatively dense. And the residual thickness of the TiO2 coating was about 0.2 mm, which was not found to decrease obviously. The TiO2/316L exhibits relatively better corrosion resistance performance than Al2O3/316L. In the later study, the corrosion resistance of TiO2 coating needs to be further studied in supercritical water for longer time, and preparation of the coating requires further optimization.
Acknowledgment This study was supported by China Postdoctoral Science Foundation (2015M570849); Shaanxi Colleges Association of Science and Technology for Young Talent Lifts (20160115); and Natural Science Foundation of Shaanxi Provincial Department of Education (20161011).
Please cite this article in press as: Wang Y, et al., Comparative study on corrosion characteristics of Al2O3/316L and TiO2/316L stainless steel in supercritical water, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.06.129
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Please cite this article in press as: Wang Y, et al., Comparative study on corrosion characteristics of Al2O3/316L and TiO2/316L stainless steel in supercritical water, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.06.129