- Email: [email protected]
Microstructure and corrosion behavior of Fe-based amorphous coating prepared by HVOF
Accepted Manuscript Microstructure and corrosion behavior of Fe-based amorphous coating prepared by HVOF Jifu Zhang, Min Liu, Jinbing Song, Chunming Deng, Changguang Deng PII:
S0925-8388(17)32022-4
DOI:
10.1016/j.jallcom.2017.06.046
Reference:
JALCOM 42114
To appear in:
Journal of Alloys and Compounds
Received Date: 6 January 2017 Accepted Date: 4 June 2017
Please cite this article as: J. Zhang, M. Liu, J. Song, C. Deng, C. Deng, Microstructure and corrosion behavior of Fe-based amorphous coating prepared by HVOF, Journal of Alloys and Compounds (2017), doi: 10.1016/j.jallcom.2017.06.046. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT
Microstructure and corrosion behavior of Fe-Based Amorphous coating prepared by HVOF Jifu Zhang a,*, Min Liub, Jinbing Songa,b, Chunming Denga,c, Changguang Denga,b a
Guangdong Institute of New Materials, Guangzhou 510650, China
National Engineering Laboratory for Modern Materials Surface Engineering Technology, Guangzhou 510650, China c
The Key Laboratory of Guangdong for Modern Surface Engineering Technology, Guangzhou 510650, China
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b
Abstract:
Amorphous coatings were deposited by High Velocity Oxygen Fuel (HVOF) spraying
SC
using Fe-based amorphous powder for pitting corrosion resistance. The microstructure and phase component of coatings were characterized by XRD SEM and TEM. It was found that
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although the temperature of supersonic flame was much high than the crystallization temperature of amorphous powder, a coating constituted of nearly amorphous phase and nanocrystalline phase was prepared efficiently. In the process of nanocrystalline formation, the Fe element was observed depleted while Cr element was enriched. The pitting corrosion resistance of the coating was investigated in 3.5wt%NaCl aqueous electrolytes using
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electrochemical cyclic-anodic-polarization measurements. The results showed the pitting potential(Epit) and pitting protection potential(Epp) of Fe-base amorphous coating were obviously high than that of stainless steel and thermal spraying WC coating, implying pitting
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corrosion resistance of parts served in marine environment can be improved by Fe-base amorphous coating.
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Keywords: Amorphous coating; HVOF; pitting corrosion
*Corresponding author. Tel.: +86 20 61086656; fax: +86 20 37238531 Postal address: 363 Changxin Road, Guangzhou 510651, China E-mail address: [email protected] (J.Zhang)
1
ACCEPTED MANUSCRIPT 1. Introduction Marine environment is harsh corrosive environment, which will shorten the service life of many components in the form as wear and/or corrosion, resulting in waster of resources and causing environmental hazards[1]. Surface modification is an important way to enhance the wear resistance and corrosion resistance of many key components with extending service life
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and improving safety and reliability. Hard chromium (Cr-plating) technology is so far one of the most effective methods to coat various critical mechanical components, such as valves, pistons, piston rings, rods, hydraulic components etc., due to the well corrosion resistance and
SC
wear resistance of Cr-plating. Unfortunately, Cr-plating process will cause bad effects on human health because some toxicological substances have to be used in the galvanic process,
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resulting in the increased risk of cancer[2].
Many advanced technologies to alternative Cr-plating are still research hotspot and development direction of modern surface treatment. Such as HVOF coatings are an alternative coating procedure to Cr-plating and can offer many advantages including much higher deposition rates, the elimination of hydrogen embrittlement issues, and the ability to
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repair in the field with systems that have already been deployed[3; 4]. The choice of spray material has a significant impact on the properties of coatings and a host of carbide based alloys including WC-Co, WC-Co-Cr, NiCr-Cr3C2 etc have been commercial application[5; 6]. Recently, the application of amorphous materials to protect key
EP
components in marine has aroused people's attention. In contrast to conventional crystalline metals, the atomic arrangement of amorphous materials exhibits a long-range disorder and
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short-range ordered structure, so they possess excellent physical properties (high permeability, high magnetic inductance, low magnetic loss) and mechanical properties (low elastic modulus, high fracture strength). Especially, the amorphous materials exhibit predominant corrosion resistance than crystalline metals due to the feature without grain boundary, dislocation or interface defects [7]. But the application of bulk amorphous alloy is very difficult, for one reason, the preparation of amorphous material requires cooling the molten metal liquid at an extremely fast cooling rate (~ 105 K/s), the larger the workpiece, the harder to achieve. Another reason is the brittleness of amorphous material is serious and it is sensitive to fracture when made of large pieces. Preparation of amorphous materials into 2
ACCEPTED MANUSCRIPT coatings on industrial components is a good way to utilize the good properties of amorphous materials but avoid the brittle fracture problem. In the present know of amorphous alloy materials, it was considered that Fe-based amorphous alloys have very bright engineering application prospects[8-10], they can be applied to improve the corrosion resistance of many
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components in many fields including oceanic shipbuilding, nuclear, and oil and gas industries.
The supersonic flame spraying(HVOF/HVAF) techniques has been proved to be an effective method for rapid and efficient preparation of amorphous coating[11]. By heating the
SC
powder in supersonic flame, the amorphous materials rapidly cooled down on the substrate to form coating. In this study, FeCrMnWMoSi amorphous coating was prepared on stainless
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steel by HVOF spraying. The effect of spraying process and parameters on microstructure of amorphous coating was discussed and the pitting-corrosion behaviors of the coating were tested by electrochemical cyclic-anodic-polarization. 2. Experimental procedure
316L stainless steel with a dimension of 50mm×50mm×8mm were selected as substrate.
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Prior to deposition, the substrates were ultrasonically cleaned in ethanol, and then grit blasted to achieve a roughened surface. FeCrMnWMoSi amorphous powder (as received from Nano steel Company) with particle size of 15-45um was chosen as thermal spray powder, the
shown in Fig.1.
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microstructure and DSC curve measurement of FeCrMnWMoSi amorphous powder was
Exotherm
0.8
DSC / mw.mg
-1
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1.0
0.6 100 µm
0
635.4 C
0.4
0.2 0
690.3 C
0.0
0
200
400
600
800
1000
0
Temperature / C
Fig.1 Microstructure and DSC curve of FeCrMnWMoSi amorphous powder
Fe-base amorphous coatings were prepared by HVOF using a GTV K2 system, with oxygen and kerosene to partially melt the powder particles and spray them onto the steel 3
ACCEPTED MANUSCRIPT substrate. In the process of spraying, the substrate temperature was controlled by compressed air and optimum spraying parameters were taken to deposit the coatings with uniform distribution and thickness. The thermal spray parameters were: Oxygene 900L/min, Powder feed 100g/min, Spray distance 380mm, and three types of Kerosene flux with 18 L/h, 22 L/h
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and 26L/h were controlled to adjust the flame temperature. The supersonic flame temperature and the velocity of the powder particles in the flame flow were detected by SprayWatch 3i. The deposited coatings for performance testing were about 0.3mm.
The microscopic morphologies were characterized by scanning electron microscopy
SC
(SEM, JSM-5910) and transmission electorn microscopy (TEM, JEOL JEM 2100F) equipped with a Tracor EDS. Thin film for TEM observation were cut from as-sprayed coating
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specimens and then were thinned by using in-situ focused ion beam (FIB) lift-out (Quanta 200 3D DualBeam, FEI)and were further thinned for electro transparency in a FIB-SEM(Helios Nanolab DualBeam, FEI). The phase composition of the coatings were identified by X-ray diffractometer (XRD, PhilipsX PertPro) and the thermal stability was investigated by differential scanning calorimetry (DSC) at a heating rate of 0.67 K/s from .
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25-1000
Corrosion behavior was investigated by electrochemical cyclic-anodic-polarization measurements. Prior to electrochemical tests, the specimens were mechanically polished to
EP
mirror finish, then degreased in ethanol, washed in distilled water and dried in air. Electrolytes used are 3.5wt%NaCl aqueous solutions prepared from reagent grade chemicals and distilled water. Electrochemical measurements were conducted in a three-electrode cell
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using a platinum mesh counter electrode and saturated calomel electrode (SCE) reference electrode. Potentiodynamic cyclic-anodic-polarization curves were recorded at a potential sweep rate of 0.5mV/s after immersing the specimens for about 30 min when the open circuit potentials became almost steady. 3. Results Table.1 Representative EDS compositional analysis of the amorphous powder and deposited coating
Element composition (wt.%) Specimens Fe
Cr
Mo
Mn
4
W
B
C
Si
ACCEPTED MANUSCRIPT Powder
54.2
18.16
12.56
1.52
6.40
2.38
0.96
0.91
Coating
52.45
18.26
14.34
2.01
5.87
2.83
2.81
1.44
Silvery white amorphous coatings were obtained with homogeneous appearance, no bubbling or spalling observed on the coating. The composition and content of elements
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detected by EDS were shown in Table.1. It can be seen that the composition of both the coating and powder are Fe, Cr, Mo, Mn, W, B, C, Si and so on, and the element content was similar, no significant O element was detected in the coating, indicating the amorphous
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SC
powder was not oxidized or gasified after heated in the high temperature flame.
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Fig 2 The microscopic view of the amorphous coating prepared by HVOF at various kerosene flux: (a)-(d) 18L/h; (b)-(e) 22L/h;(c)-(f) 26L/h
The kerosene flux has important effect on the micro morphology of the amorphous
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coating as shown in Fig.2. At relatively low kerosene flux (18L/h), un-fused particles deposited on the surface due to the low temperature of the HVOF flame, resulting in a loose out-layer formed on the coating; while when the kerosene flux was too high (26L/h), some over-fused particles would deposited on the surface due to the high temperature of the HVFO flame, which will easily result in some cracks on the superficial coat. Optimum kerosene flux parameter at our experiment was about 22L/h and a much more compact coating has obtained.
5
SC
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Fig 3 XRD spectrum of various deposited coatings and powder
The XRD spectrum in Fig.3a showed different coating deposited by various kerosene oil fluxes. Similar to the original powder, all the coatings were exhibited a broad and diffuse peak in the diffraction pattern, without sharply Bragg diffraction peak, this situation was the typical diffraction characteristics of amorphous structure. But the intensity of diffuse peak
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from the coatings were a little higher than that of amorphous powder, especially when the kerosene oil flux increased, implying the crystallization of amorphous powder may occurred in the process of heating in the supersonic flame. (a) 304 stainless steel
300
200
0
Epp
-100
10
10
400 200
-200
Ecorr
-6
600
Epp
0
-5
10
-4
10
2
I, mA/cm
-3
0
Ecorr
-400
10
-2
10
(C) Amorphous coating Epit Epp
800
Epit
EP
Epit
100
-200
1000
400
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E, mV(SCE)
200
1200
(b) WC-CoCr coating
600
-8
-7
10
-6
10
10
-5
-200 -4
10 2
I, mA/cm
-3
10
-2
10
-400
Ecorr -9
10
10
-8
10
-7
10
-6
10
-5
10
-4
10
-3
10
-2
2
I, mA/cm
Fig.4 Cyclic-anodic-polarization curves of (a) 304 stainless steel, (b) WC-CoCr coating, and (c) amorphous coating.
The pitting-corrosion behaviors of the base and various coatings were tested by electrochemical cyclic-anodic-polarization in 3.5wt% NaCl solution. The test results for the 304 stainless steel, the WC-CoCr coating, and the Fe-base amorphous coating HVOF deposited are shown in Fig.4. Ecorr denotes the corrosion potential, Epit is the pitting-corrosion potential, and Epp is the pitting-corrosion-protection potential. In the anodic polarization 6
ACCEPTED MANUSCRIPT process, the three materials showed passivation characteristics, which is when the polarization potential (E) was above the Ecorr, the polarization current density (I) does not change significantly due to passivation film formed on the samples, until the E exceeds the Epit, I increases rapidly, which means that pitting corrosion or breakdown may occur to the
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passivation film. While when the potential is lower than Epp, the I is lower than the passivation current density, which means that the pitting of the passivation film has been suppressed and the sample surface had restored to passivation state. Pitting corrosion is the main failure mode of the passivated metal material in the marine environment. The formation
SC
of the pitting pits will seriously shorten the service life of metal parts.
Table.2 Electrochemical corrosion properties of stainless steel base and various coatings
Ecorr [mv(SCE)]
Epit [mv(SCE)]
Epp [mv(SCE)]
Epp-Ecorr (mv)
Epit-Ecorr (mv)
Epit-Epp (mv)
304 stainless steel
-182.3
125.6
-142.3
40
307.9
267.9
WC-CoCr coating
-336.2
228.7
-8.1
328.1
564.9
236.8
Amorphous coating
-250.4
969.5
919.4
1169.8
1219.9
50.1
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Samples
Epit-Ecorr and Epp-Ecorr can reflect pit initiation and propagation resistances,
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respectively, the larger the values are, the more difficult the pit initiation/propagation to occur. The electrochemical parameters are listed in Table.2. It can be seen that both the Epit-Ecorr and Epp-Ecorr increase remarkably in the order of 304 stainless steel, the WC-CoCr coating, and the amorphous coating, This trend indicates that the tendency of pit initiation increases
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and that of pit propagation decreases from the amorphous coating, the WC-CoCr coating to the 304 stainless steel, indicating both the 304 stainless steel and WC-CoCr coating have bad
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pitting corrosion resistance, while the pitting corrosion resistance can be improved remarkable by Fe-base amorphous coating. 4. Discussion
The DSC curves in Fig.1 display the lowest temperature of amorphous powder to crystal was 635.4
, then at 690.3
and 800
will also begin to crystallize. The crystalline phases
precipitated at that temperature are now not so clear. SprayWatch 3i was used to measure the temperature and velocity of the powder particle in the supersonic flam flow as shown in Fig.4. It can been seen the flame temperature had exceeded 2200
,which was much larger than the
crystallization temperature of Fe-base amorphous powder, but the process of spraying was 7
ACCEPTED MANUSCRIPT transient heating process, the particle flight speed up to 750m/s, the particles flight time was only
0.5ms when the spray distance of 380mm, so the crystallization process would been
greatly inhibited, which was a fast and efficient method for preparing large-area amorphous
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SC
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coatings.
Fig.5 The measurement results of temperature and velocity of the supersonic flame
In order to further study the deposition process of amorphous coating by HVOF, the microstructure of coating was observed by transmission electron microscopy (TEM) as
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shown in Fig.6. It can be seen from Fig.6a, there are many needle-like precipitates formed in the amorphous coating, and some of which look like “cross”, the grain size was about 300 nm. The selected area electro diffraction (SAED) was performed on various area. The majority of
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the coating area was amorphous structure with ring diffraction pattern (Fig.6b) while there were also obvious diffraction spots of crystal detected from the needle-like area (Fig.6c and
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Fig.6d). The results indicate that the Fe-based coating prepared by HVOF was a mixture structure of amorphous phase and nanocrystalline phase instead of completely amorphous structure. But it can be considered that the main structure of sprayed coating is amorphous phase from the XRD diffraction pattern in Fig.3, and which was the reason of good resistance to pitting corrosion in Fig.4[12]
8
SC
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Fig.6 STEM morphology of the nanocrystalline particle (a) and the SAED pattern from the selected blue frame area: (b) outside the nanocrystalline particle, (c) inside the nanocrystalline particle, (d) crystal area in the amorphous coating
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Fig.7 Element line scan of the needle-like precipitates
Fig.7 shows the elemental line scanning of the needle-like precipitates. It can be seen the
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main composition of the coating were Fe, Cr, W, Mn and Mo elements. When the line passes through the needle-like precipitates, the concentration of Cr element increases, while the concentration of Fe element decreased, means that during the crystallization process, the Cr element was continuously enriched and the Fe element was continuously depleted. It was confirmed that the addition of Cr element will decreases the glass-forming ability of Fe-base bulk glassy alloy, but is very effective in increasing corrosion resistance and improving soft magnetic properties for Fe-base bulk glassy alloy[13; 14]. So the formation of nanocrystals in the coating may be attributed to local segregation of Cr element in the spraying process, but the those nanocrystals rich in Cr element and may have better corrosion resistance than that
9
ACCEPTED MANUSCRIPT of matrix amorphous phase. 5. Conclusions Fe-base coatings prepared by HVOF were basically amorphous structure with many nanocrystals formed in the local region of the coating. During the crystallization process of
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nanocrystals, the Fe element was continuously depleted while the Cr element was continuously enriched. The pitting potential(Epit) and pitting protection potential(Epp) of Fe-base amorphous coating were obviously high than that of stainless steel and thermal spraying WC coating, implying the Fe-base amorphous coating have good resistance to pit
SC
corrosion in seawater or chloride environments, so it’s an excellent coating material for application in marine components instead of tungsten carbide coating.
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Acknowledgement
The authors wish to acknowledge the financial support of the National High Technology Research and Development Program of China (2014BAF08B03) and Guangdong Provincial Science and Technology Project (2015B070701024). References 1062-1093.
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[1] J. Gandra, H. Krohn, R.M. Miranda, P. Vilaça, L. Quintino, J.F.d. Santos, J. Mater. Process Tec. 214 (2014) [2] T. Sahraoui, N.-E. Fenineche, G. Montavon, C. Coddet, J. Mater. Res. 152 (2004) 43-55. [3] N. Cinca, C.R.C. Lima, J.M. Guilemany, J. Mater. Res. 2 (2013) 75-86. [4] X. Guo, M.-P. Planche, H.L. J. Chen, J. Mater. Process Tec. 214 (2014) 456-461. [5] H.S. Sidhu, B.S. Sidhu, S. Prakash, J. Mater. Process Tec. 171 (2006) 77-82.
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[6] K. Kumari, K. Anand, M. Bellacci, M. Giannozzi, Wear 268 (2010) 1309-1319. [7] C.A.C. Souza, J.E. May, I.A. Carlos, M.F.d. Oliveira, S.E. Kuri, C.S. Kiminami, J. Non-cryst. Solids 304 (2002) 210-216.
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[8] S.J. Pang, T. Zhang, K. Asami, A. Inoue, Corros. Sci. 44 (2002) 1847-1856. [9] Z.L. Long, C.T. Chang, Y.H. Ding, Y. Shao, P. Zhang, B.L. Shen, A. Inoue, J. Non-cryst. Solids 354 (2008) 4609-4613.
[10] Y. Huang, Y. Guo, H. Fan, J. Shen, Mater. Lett. 89 (2012) 229-232. [11] M.S. Bakare, K.T. Voisey, K. Chokethawai, D.G. McCartney, J. Alloy Compd. 527 (2012) 210-218. [12] H. Ma, W. Wang, J. Zhang, G. Li, C. Cao, H. Zhang, J. Mater. Sci. 27 (2011) 1169-1177. [13] Z.L. Long, Y. Shao, X.H. Deng, Z.C. Zhang, Y. Jiang, P. Zhang, B.L. Shen, A. Inoue, Intermetallics 15 (2007) 1453-1458. [14] Z. Long, Y. Shao, G. Xie, P. Zhang, B. Shen, A. Inoue, J. Alloy Compd. 462 (2008) 52-59.
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ACCEPTED MANUSCRIPT
Highlights Amorphous coatings were deposited by High Velocity Oxygen Fuel
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(HVOF) spraying using Fe-based amorphous powder for pitting corrosion resistance.
Fe-base coatings were mixed structure constituted of amorphous phase
SC
and nanocrystalline phase.
During the crystallization process of nanocrystals, the Fe element was
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continuously depleted while the Cr element was continuously enriched. The pitting corrosion resistance of Fe-base amorphous coating were obviously high than that of stainless steel and thermal spraying WC
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coating in seawater or chloride environments.
S0925-8388(17)32022-4
DOI:
10.1016/j.jallcom.2017.06.046
Reference:
JALCOM 42114
To appear in:
Journal of Alloys and Compounds
Received Date: 6 January 2017 Accepted Date: 4 June 2017
Please cite this article as: J. Zhang, M. Liu, J. Song, C. Deng, C. Deng, Microstructure and corrosion behavior of Fe-based amorphous coating prepared by HVOF, Journal of Alloys and Compounds (2017), doi: 10.1016/j.jallcom.2017.06.046. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT
Microstructure and corrosion behavior of Fe-Based Amorphous coating prepared by HVOF Jifu Zhang a,*, Min Liub, Jinbing Songa,b, Chunming Denga,c, Changguang Denga,b a
Guangdong Institute of New Materials, Guangzhou 510650, China
National Engineering Laboratory for Modern Materials Surface Engineering Technology, Guangzhou 510650, China c
The Key Laboratory of Guangdong for Modern Surface Engineering Technology, Guangzhou 510650, China
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b
Abstract:
Amorphous coatings were deposited by High Velocity Oxygen Fuel (HVOF) spraying
SC
using Fe-based amorphous powder for pitting corrosion resistance. The microstructure and phase component of coatings were characterized by XRD SEM and TEM. It was found that
M AN U
although the temperature of supersonic flame was much high than the crystallization temperature of amorphous powder, a coating constituted of nearly amorphous phase and nanocrystalline phase was prepared efficiently. In the process of nanocrystalline formation, the Fe element was observed depleted while Cr element was enriched. The pitting corrosion resistance of the coating was investigated in 3.5wt%NaCl aqueous electrolytes using
TE D
electrochemical cyclic-anodic-polarization measurements. The results showed the pitting potential(Epit) and pitting protection potential(Epp) of Fe-base amorphous coating were obviously high than that of stainless steel and thermal spraying WC coating, implying pitting
EP
corrosion resistance of parts served in marine environment can be improved by Fe-base amorphous coating.
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Keywords: Amorphous coating; HVOF; pitting corrosion
*Corresponding author. Tel.: +86 20 61086656; fax: +86 20 37238531 Postal address: 363 Changxin Road, Guangzhou 510651, China E-mail address: [email protected] (J.Zhang)
1
ACCEPTED MANUSCRIPT 1. Introduction Marine environment is harsh corrosive environment, which will shorten the service life of many components in the form as wear and/or corrosion, resulting in waster of resources and causing environmental hazards[1]. Surface modification is an important way to enhance the wear resistance and corrosion resistance of many key components with extending service life
RI PT
and improving safety and reliability. Hard chromium (Cr-plating) technology is so far one of the most effective methods to coat various critical mechanical components, such as valves, pistons, piston rings, rods, hydraulic components etc., due to the well corrosion resistance and
SC
wear resistance of Cr-plating. Unfortunately, Cr-plating process will cause bad effects on human health because some toxicological substances have to be used in the galvanic process,
M AN U
resulting in the increased risk of cancer[2].
Many advanced technologies to alternative Cr-plating are still research hotspot and development direction of modern surface treatment. Such as HVOF coatings are an alternative coating procedure to Cr-plating and can offer many advantages including much higher deposition rates, the elimination of hydrogen embrittlement issues, and the ability to
TE D
repair in the field with systems that have already been deployed[3; 4]. The choice of spray material has a significant impact on the properties of coatings and a host of carbide based alloys including WC-Co, WC-Co-Cr, NiCr-Cr3C2 etc have been commercial application[5; 6]. Recently, the application of amorphous materials to protect key
EP
components in marine has aroused people's attention. In contrast to conventional crystalline metals, the atomic arrangement of amorphous materials exhibits a long-range disorder and
AC C
short-range ordered structure, so they possess excellent physical properties (high permeability, high magnetic inductance, low magnetic loss) and mechanical properties (low elastic modulus, high fracture strength). Especially, the amorphous materials exhibit predominant corrosion resistance than crystalline metals due to the feature without grain boundary, dislocation or interface defects [7]. But the application of bulk amorphous alloy is very difficult, for one reason, the preparation of amorphous material requires cooling the molten metal liquid at an extremely fast cooling rate (~ 105 K/s), the larger the workpiece, the harder to achieve. Another reason is the brittleness of amorphous material is serious and it is sensitive to fracture when made of large pieces. Preparation of amorphous materials into 2
ACCEPTED MANUSCRIPT coatings on industrial components is a good way to utilize the good properties of amorphous materials but avoid the brittle fracture problem. In the present know of amorphous alloy materials, it was considered that Fe-based amorphous alloys have very bright engineering application prospects[8-10], they can be applied to improve the corrosion resistance of many
RI PT
components in many fields including oceanic shipbuilding, nuclear, and oil and gas industries.
The supersonic flame spraying(HVOF/HVAF) techniques has been proved to be an effective method for rapid and efficient preparation of amorphous coating[11]. By heating the
SC
powder in supersonic flame, the amorphous materials rapidly cooled down on the substrate to form coating. In this study, FeCrMnWMoSi amorphous coating was prepared on stainless
M AN U
steel by HVOF spraying. The effect of spraying process and parameters on microstructure of amorphous coating was discussed and the pitting-corrosion behaviors of the coating were tested by electrochemical cyclic-anodic-polarization. 2. Experimental procedure
316L stainless steel with a dimension of 50mm×50mm×8mm were selected as substrate.
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Prior to deposition, the substrates were ultrasonically cleaned in ethanol, and then grit blasted to achieve a roughened surface. FeCrMnWMoSi amorphous powder (as received from Nano steel Company) with particle size of 15-45um was chosen as thermal spray powder, the
shown in Fig.1.
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microstructure and DSC curve measurement of FeCrMnWMoSi amorphous powder was
Exotherm
0.8
DSC / mw.mg
-1
AC C
1.0
0.6 100 µm
0
635.4 C
0.4
0.2 0
690.3 C
0.0
0
200
400
600
800
1000
0
Temperature / C
Fig.1 Microstructure and DSC curve of FeCrMnWMoSi amorphous powder
Fe-base amorphous coatings were prepared by HVOF using a GTV K2 system, with oxygen and kerosene to partially melt the powder particles and spray them onto the steel 3
ACCEPTED MANUSCRIPT substrate. In the process of spraying, the substrate temperature was controlled by compressed air and optimum spraying parameters were taken to deposit the coatings with uniform distribution and thickness. The thermal spray parameters were: Oxygene 900L/min, Powder feed 100g/min, Spray distance 380mm, and three types of Kerosene flux with 18 L/h, 22 L/h
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and 26L/h were controlled to adjust the flame temperature. The supersonic flame temperature and the velocity of the powder particles in the flame flow were detected by SprayWatch 3i. The deposited coatings for performance testing were about 0.3mm.
The microscopic morphologies were characterized by scanning electron microscopy
SC
(SEM, JSM-5910) and transmission electorn microscopy (TEM, JEOL JEM 2100F) equipped with a Tracor EDS. Thin film for TEM observation were cut from as-sprayed coating
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specimens and then were thinned by using in-situ focused ion beam (FIB) lift-out (Quanta 200 3D DualBeam, FEI)and were further thinned for electro transparency in a FIB-SEM(Helios Nanolab DualBeam, FEI). The phase composition of the coatings were identified by X-ray diffractometer (XRD, PhilipsX PertPro) and the thermal stability was investigated by differential scanning calorimetry (DSC) at a heating rate of 0.67 K/s from .
TE D
25-1000
Corrosion behavior was investigated by electrochemical cyclic-anodic-polarization measurements. Prior to electrochemical tests, the specimens were mechanically polished to
EP
mirror finish, then degreased in ethanol, washed in distilled water and dried in air. Electrolytes used are 3.5wt%NaCl aqueous solutions prepared from reagent grade chemicals and distilled water. Electrochemical measurements were conducted in a three-electrode cell
AC C
using a platinum mesh counter electrode and saturated calomel electrode (SCE) reference electrode. Potentiodynamic cyclic-anodic-polarization curves were recorded at a potential sweep rate of 0.5mV/s after immersing the specimens for about 30 min when the open circuit potentials became almost steady. 3. Results Table.1 Representative EDS compositional analysis of the amorphous powder and deposited coating
Element composition (wt.%) Specimens Fe
Cr
Mo
Mn
4
W
B
C
Si
ACCEPTED MANUSCRIPT Powder
54.2
18.16
12.56
1.52
6.40
2.38
0.96
0.91
Coating
52.45
18.26
14.34
2.01
5.87
2.83
2.81
1.44
Silvery white amorphous coatings were obtained with homogeneous appearance, no bubbling or spalling observed on the coating. The composition and content of elements
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detected by EDS were shown in Table.1. It can be seen that the composition of both the coating and powder are Fe, Cr, Mo, Mn, W, B, C, Si and so on, and the element content was similar, no significant O element was detected in the coating, indicating the amorphous
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powder was not oxidized or gasified after heated in the high temperature flame.
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Fig 2 The microscopic view of the amorphous coating prepared by HVOF at various kerosene flux: (a)-(d) 18L/h; (b)-(e) 22L/h;(c)-(f) 26L/h
The kerosene flux has important effect on the micro morphology of the amorphous
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coating as shown in Fig.2. At relatively low kerosene flux (18L/h), un-fused particles deposited on the surface due to the low temperature of the HVOF flame, resulting in a loose out-layer formed on the coating; while when the kerosene flux was too high (26L/h), some over-fused particles would deposited on the surface due to the high temperature of the HVFO flame, which will easily result in some cracks on the superficial coat. Optimum kerosene flux parameter at our experiment was about 22L/h and a much more compact coating has obtained.
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Fig 3 XRD spectrum of various deposited coatings and powder
The XRD spectrum in Fig.3a showed different coating deposited by various kerosene oil fluxes. Similar to the original powder, all the coatings were exhibited a broad and diffuse peak in the diffraction pattern, without sharply Bragg diffraction peak, this situation was the typical diffraction characteristics of amorphous structure. But the intensity of diffuse peak
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from the coatings were a little higher than that of amorphous powder, especially when the kerosene oil flux increased, implying the crystallization of amorphous powder may occurred in the process of heating in the supersonic flame. (a) 304 stainless steel
300
200
0
Epp
-100
10
10
400 200
-200
Ecorr
-6
600
Epp
0
-5
10
-4
10
2
I, mA/cm
-3
0
Ecorr
-400
10
-2
10
(C) Amorphous coating Epit Epp
800
Epit
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Epit
100
-200
1000
400
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E, mV(SCE)
200
1200
(b) WC-CoCr coating
600
-8
-7
10
-6
10
10
-5
-200 -4
10 2
I, mA/cm
-3
10
-2
10
-400
Ecorr -9
10
10
-8
10
-7
10
-6
10
-5
10
-4
10
-3
10
-2
2
I, mA/cm
Fig.4 Cyclic-anodic-polarization curves of (a) 304 stainless steel, (b) WC-CoCr coating, and (c) amorphous coating.
The pitting-corrosion behaviors of the base and various coatings were tested by electrochemical cyclic-anodic-polarization in 3.5wt% NaCl solution. The test results for the 304 stainless steel, the WC-CoCr coating, and the Fe-base amorphous coating HVOF deposited are shown in Fig.4. Ecorr denotes the corrosion potential, Epit is the pitting-corrosion potential, and Epp is the pitting-corrosion-protection potential. In the anodic polarization 6
ACCEPTED MANUSCRIPT process, the three materials showed passivation characteristics, which is when the polarization potential (E) was above the Ecorr, the polarization current density (I) does not change significantly due to passivation film formed on the samples, until the E exceeds the Epit, I increases rapidly, which means that pitting corrosion or breakdown may occur to the
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passivation film. While when the potential is lower than Epp, the I is lower than the passivation current density, which means that the pitting of the passivation film has been suppressed and the sample surface had restored to passivation state. Pitting corrosion is the main failure mode of the passivated metal material in the marine environment. The formation
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of the pitting pits will seriously shorten the service life of metal parts.
Table.2 Electrochemical corrosion properties of stainless steel base and various coatings
Ecorr [mv(SCE)]
Epit [mv(SCE)]
Epp [mv(SCE)]
Epp-Ecorr (mv)
Epit-Ecorr (mv)
Epit-Epp (mv)
304 stainless steel
-182.3
125.6
-142.3
40
307.9
267.9
WC-CoCr coating
-336.2
228.7
-8.1
328.1
564.9
236.8
Amorphous coating
-250.4
969.5
919.4
1169.8
1219.9
50.1
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Samples
Epit-Ecorr and Epp-Ecorr can reflect pit initiation and propagation resistances,
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respectively, the larger the values are, the more difficult the pit initiation/propagation to occur. The electrochemical parameters are listed in Table.2. It can be seen that both the Epit-Ecorr and Epp-Ecorr increase remarkably in the order of 304 stainless steel, the WC-CoCr coating, and the amorphous coating, This trend indicates that the tendency of pit initiation increases
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and that of pit propagation decreases from the amorphous coating, the WC-CoCr coating to the 304 stainless steel, indicating both the 304 stainless steel and WC-CoCr coating have bad
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pitting corrosion resistance, while the pitting corrosion resistance can be improved remarkable by Fe-base amorphous coating. 4. Discussion
The DSC curves in Fig.1 display the lowest temperature of amorphous powder to crystal was 635.4
, then at 690.3
and 800
will also begin to crystallize. The crystalline phases
precipitated at that temperature are now not so clear. SprayWatch 3i was used to measure the temperature and velocity of the powder particle in the supersonic flam flow as shown in Fig.4. It can been seen the flame temperature had exceeded 2200
,which was much larger than the
crystallization temperature of Fe-base amorphous powder, but the process of spraying was 7
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0.5ms when the spray distance of 380mm, so the crystallization process would been
greatly inhibited, which was a fast and efficient method for preparing large-area amorphous
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coatings.
Fig.5 The measurement results of temperature and velocity of the supersonic flame
In order to further study the deposition process of amorphous coating by HVOF, the microstructure of coating was observed by transmission electron microscopy (TEM) as
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shown in Fig.6. It can be seen from Fig.6a, there are many needle-like precipitates formed in the amorphous coating, and some of which look like “cross”, the grain size was about 300 nm. The selected area electro diffraction (SAED) was performed on various area. The majority of
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the coating area was amorphous structure with ring diffraction pattern (Fig.6b) while there were also obvious diffraction spots of crystal detected from the needle-like area (Fig.6c and
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Fig.6d). The results indicate that the Fe-based coating prepared by HVOF was a mixture structure of amorphous phase and nanocrystalline phase instead of completely amorphous structure. But it can be considered that the main structure of sprayed coating is amorphous phase from the XRD diffraction pattern in Fig.3, and which was the reason of good resistance to pitting corrosion in Fig.4[12]
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Fig.6 STEM morphology of the nanocrystalline particle (a) and the SAED pattern from the selected blue frame area: (b) outside the nanocrystalline particle, (c) inside the nanocrystalline particle, (d) crystal area in the amorphous coating
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Fig.7 Element line scan of the needle-like precipitates
Fig.7 shows the elemental line scanning of the needle-like precipitates. It can be seen the
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main composition of the coating were Fe, Cr, W, Mn and Mo elements. When the line passes through the needle-like precipitates, the concentration of Cr element increases, while the concentration of Fe element decreased, means that during the crystallization process, the Cr element was continuously enriched and the Fe element was continuously depleted. It was confirmed that the addition of Cr element will decreases the glass-forming ability of Fe-base bulk glassy alloy, but is very effective in increasing corrosion resistance and improving soft magnetic properties for Fe-base bulk glassy alloy[13; 14]. So the formation of nanocrystals in the coating may be attributed to local segregation of Cr element in the spraying process, but the those nanocrystals rich in Cr element and may have better corrosion resistance than that
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ACCEPTED MANUSCRIPT of matrix amorphous phase. 5. Conclusions Fe-base coatings prepared by HVOF were basically amorphous structure with many nanocrystals formed in the local region of the coating. During the crystallization process of
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nanocrystals, the Fe element was continuously depleted while the Cr element was continuously enriched. The pitting potential(Epit) and pitting protection potential(Epp) of Fe-base amorphous coating were obviously high than that of stainless steel and thermal spraying WC coating, implying the Fe-base amorphous coating have good resistance to pit
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corrosion in seawater or chloride environments, so it’s an excellent coating material for application in marine components instead of tungsten carbide coating.
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Acknowledgement
The authors wish to acknowledge the financial support of the National High Technology Research and Development Program of China (2014BAF08B03) and Guangdong Provincial Science and Technology Project (2015B070701024). References 1062-1093.
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[1] J. Gandra, H. Krohn, R.M. Miranda, P. Vilaça, L. Quintino, J.F.d. Santos, J. Mater. Process Tec. 214 (2014) [2] T. Sahraoui, N.-E. Fenineche, G. Montavon, C. Coddet, J. Mater. Res. 152 (2004) 43-55. [3] N. Cinca, C.R.C. Lima, J.M. Guilemany, J. Mater. Res. 2 (2013) 75-86. [4] X. Guo, M.-P. Planche, H.L. J. Chen, J. Mater. Process Tec. 214 (2014) 456-461. [5] H.S. Sidhu, B.S. Sidhu, S. Prakash, J. Mater. Process Tec. 171 (2006) 77-82.
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[6] K. Kumari, K. Anand, M. Bellacci, M. Giannozzi, Wear 268 (2010) 1309-1319. [7] C.A.C. Souza, J.E. May, I.A. Carlos, M.F.d. Oliveira, S.E. Kuri, C.S. Kiminami, J. Non-cryst. Solids 304 (2002) 210-216.
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[8] S.J. Pang, T. Zhang, K. Asami, A. Inoue, Corros. Sci. 44 (2002) 1847-1856. [9] Z.L. Long, C.T. Chang, Y.H. Ding, Y. Shao, P. Zhang, B.L. Shen, A. Inoue, J. Non-cryst. Solids 354 (2008) 4609-4613.
[10] Y. Huang, Y. Guo, H. Fan, J. Shen, Mater. Lett. 89 (2012) 229-232. [11] M.S. Bakare, K.T. Voisey, K. Chokethawai, D.G. McCartney, J. Alloy Compd. 527 (2012) 210-218. [12] H. Ma, W. Wang, J. Zhang, G. Li, C. Cao, H. Zhang, J. Mater. Sci. 27 (2011) 1169-1177. [13] Z.L. Long, Y. Shao, X.H. Deng, Z.C. Zhang, Y. Jiang, P. Zhang, B.L. Shen, A. Inoue, Intermetallics 15 (2007) 1453-1458. [14] Z. Long, Y. Shao, G. Xie, P. Zhang, B. Shen, A. Inoue, J. Alloy Compd. 462 (2008) 52-59.
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Highlights Amorphous coatings were deposited by High Velocity Oxygen Fuel
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(HVOF) spraying using Fe-based amorphous powder for pitting corrosion resistance.
Fe-base coatings were mixed structure constituted of amorphous phase
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and nanocrystalline phase.
During the crystallization process of nanocrystals, the Fe element was
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continuously depleted while the Cr element was continuously enriched. The pitting corrosion resistance of Fe-base amorphous coating were obviously high than that of stainless steel and thermal spraying WC
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coating in seawater or chloride environments.