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Microstructures and micro composition developed by Plasma Electrolysis Processing of 316L austenitic steels to obtain Al-containing surface layer
Available online at www.sciencedirect.com
ScienceDirect Materials Today: Proceedings 4 (2017) 6990–6999 www.elsevier.com/locate/procedia
13th International Conference on Nanosciences& Nanotechnologies & 9th International Symposium on Flexible Organic Electronics
Microstructures and micro composition developed by Plasma Electrolysis Processing of 316L austenitic steels to obtain Alcontaining surface layer V.A. Andreia* , E. Coacab, I. Ionitaa, G. Torokc, O.A. Rusub, A. Marinb, M. Mihalacheb, L. Velciub, V. Malinovschid, T. Visane a b
ELSSA Laboratory SRL, Crinului 34, Pitesti,110109, Romania Institute for Nuclear Research , Campului 1, Mioveni, Romania c Budapest Neutron Center d Pitesti University e University „Politehnica”, Bucharest
Abstract The candidate materials for the Generation-IV nuclear power reactors include modified stainless steels, as 316L in order to improve corrosion resistance in extreme conditions (liquid metals, higher temperature than the upper limit of conventional austenitic stainless steels). The aim of this study is to explore in-depth microstructural changes of aluminium containing surface layer developed by Plasma Electrolysis technique on 316L austenitic stainless steel. Complex surface treatments were performed involving anodic oxidation, high-temperature autoclaving in water and micro-arc oxidation in aqueous solution of 0.1 M NaAlO2 and 0.05 M NaOH. The obtained structures were characterized by X-Ray Diffraction, Metallography, Scanning Electron Microscopy and Small Angle Neutron Scattering. Unique surface characteristics were observed due to thermal, chemical, electrical and mechanical effects as a result of Plasma Electrolysis Processing. Corrosion behavior of plasma treated samples was investigated by electrochemical techniques. Some considerations for the further development of the new advanced nuclear materials are presented.
This is an open-access article distributed under the terms of the Creative Commons Attribution-NonCommercial-ShareAlike License, which permits non-commercial use, distribution, and reproduction in any medium, provided the original author and source are credited. * Corresponding author. Tel.: 0040723314800. E-mail address: [email protected] 2214-7853© 2017 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the Conference Committee Members of NANOTEXNOLOGY2016 (13th International Conference on Nanosciences & Nanotechnologies & 9th International Symposium on Flexible Organic Electronics).
V.A. Andrei et al.. / Materials Today: Proceedings 4 (2017) 6990–6999
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© 2017 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the Conference Committee Members of NANOTEXNOLOGY2016 (13th International Conference on Nanosciences & Nanotechnologies & 9th International Symposium on Flexible Organic Electronics). Keywords:plasma electrolytic oxidation; aluminium; 316L stainless steel, XRD, SEM, SANS
1. Introduction 304L and 316L austenitic stainless steels have been used extensively as nuclear structural materials and were selected for Heavy Liquid Metal (HLM) nuclear systems involving lead or Lead Bismuth Eutectic (LBE) [1].The corrosion mechanism of these steels for LBE system is affected by several parameters such as oxygen activity in the liquid metal and the temperature. Adequate oxygen activity in HLM system can minimize the stainless steel dissolution process by forming a protective surface oxide. In order to improve the corrosion resistance of stainless steels exposed to oxygen-containing HLM, at temperatures above 500°C, pulsed electron beam facility (GESA) surface process was employed. GESA, a method designed for material surface treatment ensures the possibility to realize melting depths of materials in the range of 10–100μm [2]. Aluminium was alloyed into the steel surface by GESA facility in order to form an aluminium oxide layer as a result of dissolved oxygen interaction in HLM. The protective surface aluminium alloy obtained by GESA procedure was developed and improved for better corrosion resistance of 316 steel, exposed to Pb and Pb-Bi eutectic, respectively. The procedure itself consists in two stages: (i) Al or Al alloy deposition and (ii) melting of deposited layer on the steel surface using intense pulsed electron beam. The thickness of the aluminium containing layer (10-30 µm) is around the penetration depth of electrons into the steel. Therefore, by applying the GESA procedure, the microstructural properties of the substrate materials (excepting the superficial layer) do not change and is possible to obtain a surface layer with a uniform distribution of Al, controlled thickness, crack-free and adherent to the substrate [1]. There is interest in developing surface engineering techniques, less expensive and more permissive with the geometry of treated components, better than GESA procedure. Other experimental approaches related to achievement of superficial structures based on Al deposited on AISI 316 L steel substrate, which provide corrosion resistance in aggressive environments specific to Lead Fast Reactor type, are presented in Ref.[3–5]. Thereby, electro-deposition from ionic liquids and electrochemical plasma processing techniques are considered in Ref.[3]. Plasma electrolytic oxidation feasibility in aqueous solution of NaAlO2 for preparation of ceramic-like aluminium oxide films on austenitic stainless steels surface was proven in Ref.[4]. Ref.[5] describes the possibility of using processing techniques with plasma electrolysis for substrate activation and for development of Al based deposition. This study aims to characterize micro- and nanostructured transformations induced by plasma electrolytic processing in order to better understand the effects of different process parameters and to further optimize surface treatments. Neutron scattering technique can provide detailed information about structural changes at nano and micro level. 2. Materials and methods AISI 316L stainless steel samples (10x10x1) mm3, were manually grounded up to 1200 mesh SiC paper in order to achieve a fine finish with an average surface roughness of 0.35 mm, which minimizes the mechanical surface damage and allows a good adherence of a coated layer, according to Ref.[10]. Then the samples were cleaned with distilled water and ethanol before treatment. EPP treatments- cathodic polarization in 0.5M AlCl3 in Deep Eutectic Solvent ChCl - ureea 1:2 molar ratio were conducted in the single-compartment two-electrode electrochemical cell containing 500 mL electrolyte. The samples were used as cathodes of the electrochemical cell and the anode was a cylinder made from AISI 316L steel, which surrounded the cathode and has an area of approximately 20 times higher than the cathode area. The temperature of the electrolyte was kept within a range of 90–95◦C using cold water circulating through a heat exchanger.The samples were treated using a programmable DC power supply EA-PS 8400-70 with voltage range of
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V.A. Andrei et al. / Materials Today: Proceedings 4 (2017) 6990–6999
0–400 V and a current range of 0–15 A, under constant voltage conditions. The power supply provided constant voltage with an accuracy of about ±1% of the desired value. MAO treatments of 316L stainless steel surfaces were conducted in the single-compartment two-electrode electrochemical cell containing 500 mL of aqueous solution of.0.1M NaAlO2 and 0.05 M NaOH, prepared using commercial pure reagents and distilled water.The samples were used as anodes of the electrochemical cell and the cathode was a cylinder made from AISI 316L steel, which surrounded the anode and has an area of approximately 20 times higher than the anode area.The temperature of the electrolyte was kept within a range of 40–45◦C using cold water circulating through a heat exchanger. To reduce temperature and concentration gradients in the electrolyte, the solution was magnetically stirred. No more than 2 specimens were coated in a single freshly prepared electrolyte solution. The sample labeling, the applied treatments and the analysis techniques used are presented in the Table 1. Table1.Materials, applied treatments, analysis techniques sample code
Substrate
Treatment parameters
A
AISI 316L
Standard sample – untreated
B1
AISI 316L
B2
AISI 316L Autoclaved in deionized water 600°C, 80 days
EPP:DC power supply, constant voltage conditions, 200V, 4min, Telectrolyte ~90°C, MAO:DC power supply, constant voltage conditions, 320V, 3min, Telectrolyte<45 °C
Analysis techniques XRD, SANS Potentiodynamic Polarization Metallography, XRD, SANS, Potentiodynamic Polarization Metallography XRD, SANS, Neutron Diffraction, Potentiodynamic Polarization
For Metallography, the samples were cut into sections with the ISOMET 4000 device, and then packed into a copper foil and mounted in a thermoplastic raisin with the METAPRESS-A, METKOH covering press. After mounting, the sample was grinded and polished (120-2400 grained abrasive paper) with a BETA GRINDERPOLISHER- BUEHLER device. After polishing, samples were metallographic etched in order to evaluate the structure of the surface layer. The electrolytic etching was performed with oxalic acid 10%, 6V, 10-20 seconds. Scanning electron microscopy (SEM) and elements microanalysis, using the electrons probe was performed using a TESCAN VEGA II LMU microscope. SEM device is equipped with 3 detectors: a secondary electron (SE) detector, a back-scattered electron (BSE) detector, and energy dispersive X - ray spectrometer (EDS). X-ray diffraction measurement(XRD) was performed on a RigakuUltima IV diffractometer, equipped with Parallel Beam optics and a high precision vertical goniometer (θ - θ) with 285mm radius.X-ray diffraction patterns were collected using CuKα radiation: θ- θ scanning mod, accelerating voltage 40 kV, tube current 40mA, scan range 2θ 150 to 1000, step size Δ2 θ =0.050, counting time 2s, graphite monocromator in diffracted beam. For sample B1 diffraction spectra were collected in GIXRD geometry (Grazing Incidence X-Ray Diffraction) at grazing angles of θ=30 and θ =50. Small Angle Neutron Scattering (SANS) measurements were performed on the SANS-diffractometer at Budapest Neutron Center. We have investigated the nanostructure of the surface layer affected as a result of plasma electrolysisprocessing as compared to base metal. We subtracted the SANS contribution of non-treated base sample using neutron beam (with wavelength λ=0.609 nm, Δλ/λ=0.25, beam diameter=8mm). The geometry of experiment is shown in Fig.1.
V.A. Andrei et al.. / Materials Today: Proceedings 4 (2017) 6990–6999
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H=HX
ko
n
S
θ
Y
q
k1 X
D ( X ,Y ) Fig.1. SANS-experiment: neutrons scattered from the sample (S) are registered in two-dimensional detector (D) where ko, k1 are the initial and final wave vectors of neutron In cases the magnetic field Hx can be detected the magnetic scattering also.
They were used in transmission geometry, packages of samples (10x10x1mm3) treated identically in order to increase the effective volume affected by treatment. The packages of samples of stainless steel with surface treatment were fixed and was applied a magnetic field 1.4T. We observed both the isotropic scattering pictures and anisotropic one.
3. Results and discussion The results of SEM and Metallography analysis are presented in Fig. 2 and Fig.3. As shown in these images, the different treatments cause different surface morphologies: The results obtained by XRD analysis are shown in Fig.4 ,Fig.5,.and Fig.6. XRD analysis reveals the presence of crystalline structures containing Al: Fe2Al in surface layer of B1 sample, Al2O3 on B2 sample. The results obtained by Metallography, SEM, EDS, XRD are showing that: • EPP - cathodic polarization in 0.5M AlCl3 in Deep Eutectic Solvent ChCl - ureea 1:2 molar ratio, is forming a layered structure; the superficial layer (thick≈100μ) is characterized by the presence of micro-craters and spheroids, containing intermetallic compounds as Fe2Al. • MAO treatment applied on autoclaved 316L steel is forming a layered structure: over magnetite film (thick≈40μ) formed by autoclaving develops a layer composed of Al oxides (prevailing), Fe oxides and Al hydroxide (thick≈30μ) [4]. a Element Al Fe Cr Ni Mn S C O
C [at%] 17.5 49.5 15.6 8.2 1.1 1.9 3.2 3.0
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b
Fig.2a SEM micrograph for B1 sample and element concentrations b. EDS results on the cross section of B1 sample and cross sectional micrographs of B1 sample
a
Element
C [at%]
Al Fe
34.7 3.6
C Cr Si Na O
7.3 0.4 0.1 0.1 53.8
b
Fig.3a SEM micrograph for B2 sample and element concentrations b. EDS results on the cross section of B2 sample and cross sectional micrographs of B2 sample
V.A. Andrei et al.. / Materials Today: Proceedings 4 (2017) 6990–6999
Fig.4. XRD patterns of A sample
Fig. 5.GIXRD patterns of B1 sample
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Fig.6. XRD patterns of B2 sample
The potentiodynamic polarization measurements in 0.5M NaCl aqueous electrolyte (fig.7), show that the corrosion potential, in the case of samples D1, D2occurs to more positive values after treatment so superficial layer provide corrosion protection; the values of corrosion rate computed by the Polarization Resistance method show that the aluminum based thin films obtained ensure some corrosion protection. Polarization curve of D2 sample shows that a decrease in corrosion currents caused by deposition of aluminum oxide films at micro-arc conditions is limited by the high porosity of film. Corrosion potential and corrosion rate as resulted from Polarization Resistance method
Proba A B1 B2
E(I=0) (mV): -334.453 -283.237 -212.148
Co. Rate (mmpy) 0.15 0.12 0.14
Polarization curves for 316L samples (B1, B2 and A) in 0.5M NaCl aqueous electrolyte Fig.7. Electrochemical measurements
The results of SANS analysis of 316L samples are presented in Fig. 8 and Table 2. Data analyses were done by Bersans program package separating the possible magnetic contribution from nuclear one. The scattering intensity of nuclear part is fitted by a sum of Guinier fuction and a fractal part: Y=A+I1*exp(-R2q2/3)+F*qC, where the first part is a simplest approximation of scatterers and the second term represents the density fluctuations at the border of precipitates. (usually that is formed by diffusion processes, so most probably the shape is DLA fractal) The fitted parameters are: R-averaged radius of scattering centre, F- fractal part (inter-granular area ) , Cdimension of fractal structure
V.A. Andrei et al.. / Materials Today: Proceedings 4 (2017) 6990–6999
200 B
316 basic
Data: A316ALI_B Model: Guifract Chi^2 = 37.16983 R^2 = 0.95344
Data: A316BAS_B Model: guifract
100
Chi^2 R^2
= 3.18977 = 0.9092
A I1 R F C
4.12656 960.25849 17.31452 1.67851 -1.38493
Intensity arb. un.
intensity arb un
Nuclear scattering difference 316-316 Ali
100
2000 1000
6997
±0.05822 ±201.61105 ±0.72638 ±0.08403 ±0.07044
A I1 R F C
10
0.37934 32.77896 107.76525 2.36935 -1.97246
±0.44047 ±0 ±0 ±0.27228 ±0.09826
10
1 2 0.06
0.1
1
Q nm
4
0.4 0.1
-1
1
-1
Q nm 2
4
2
C
A+I1*exp(-R *x /3)+F*x
b) Nuclear scattering; difference between B1 and standard 316L(fitted)
a) Nuclear scattering; fit for standard 316L 10000
Magnetic scattering difference 316-316 Ali 100000
Nuclear scattering difference 316-316 Al18
Data: A316ALIM_B Model: Guifract
Data: A316A18_B Model: Guifract
Chi^2 = 189.85034 R^2 = 0.96352 A I1 R F C
100
0.67433 32.77896 107.76525 0.95373 -3.85669
10000 ±0.66886 ±0 ±0 ±0.07817 ±0.05431
10
Intensity arb. un.
Intensity arb. un.
1000
Chi^2 = 86.31098 R^2 = 0.99535 A I1 R F C
1000
35.11316 65332.62599 16.27181 15.19917 -3.46777
±2.35969 ±8022.61189 ±0.3737 ±0.83 ±0.0536
100
1
0.1 0.1
1
4
10 0.1
-1
Q nm 2
-1
Q nm 2
2
1
2
A+I1*exp(-R *x /3)+F*x
C
A+I1*exp(-R *x /3)+F*x
c) Magnetic scattering; difference between B1 and standard 316L(fitted)
d) Nuclear scattering; difference between B2 and standard 316L(fitted)
Fig.8. SANS results for samples 316L
Table 2. Results of neutron scattering techniques:SANS : R,F,C; ND: lattice parameter a[Å]
Parameters derived R [nm] F C
4 C
A (Basic B1 B2 316L 17.314 107.76525 ±0 (*) 16.27±0.37 ±0.73 107.76525 ±0 (**) 1.678 2.36935 (*) 15.2±0.83 ±0.0840 0.953±0.07 (**) -1.385 -1.972±0.098(*) 3.46±0.054 ±0.070 3.856±0.05431(**) *Nuclear scattering; **Magnetic scattering
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In sample B1 the averaged size of scattering particles 2R=214nm with volume fractal structure close to the lamellar one, while the magnetic size is the same 2R=214nm and their surface fractal structure is close to the lamellar form with sharp border (-4). In sample B2 the averaged size of scattering particles 2R=32nm with surface fractal structure with dimension 63.46=2.54. In B2 sample the treatment did not indicated any measurable magnetic structure. The fractal part which represents the area of inter-granular matter is increased what is the other side of dissolution process. The original fractal structure is a volume fractal with dimension of 1.39 what is a hairy structure. At treated area the fractal dimension is around 2, meaning that the volume fractal is more dense and smooth. Also we can conclude that the ratio of fractal part is growing for factor of 2 so the inter-granular fractal volume space is grown for factor of 2. The treatments B1 and B2 led to the formation of the ceramic structure based on Aluminum In the case of B1 and B2 treatments the intergranular area increase from untreated steel, and the volume fractal is more dense and smooth. For use 316L steel in nuclear systems with HLM is necessary to obtain a surface layer with an uniform distribution of Al, of controlled thickness, which is crack-free and adherent to the substrate1. In in the case of EPP [5] high potential between the electrodes leads to concentration of positive ions (as Al3+)that are present in the electrolyte, in the close proximity of the cathode, mostly on the surface of the gas bubbles. Thus, a very high positive charge is present in the close proximity of the cathode. This results in high localized electric field strength between the cathode and the positive charges. When the electric field strength reaches~105V/m or higher, gas space inside the bubbles is ionized and a plasma discharge takes place; the temperature of plasma, locally, can reach as high as 2000°C, which is surrounded by relatively cool electrolyte. Thus, plasma bubble cools down and finally implodes on the metal surface. A combination of mechanical, thermal, chemical and electrical treatment provided by EPT leads to development of unique surface microstructure. It is known[6] that using plasma electrolytic oxidation for anodizing of non-valve metals such as stainless steels, and other practically important metals, is hampered by the fact that at the initial stage of the oxidation, the barrier layer essential for converting the process into spark regime is not formed on the metal surface. The the work [4] was shown that the formation of the dielectric barrier layer suitable for the MAO treatment can be achieved by autoclaving in de-ionized water, to form an iron oxide film without Ni and Cr on the 316L steel substrate. Optimization in plasma electrolysis treatments involves understanding the changes in structure and composition both in thin layers (thickness of tens of Å) using surface science techniques (e.g. XPS, AES) and in layers with thicknesses of tens of microns. This study sought to identify micro and nano - structural changes in layers with thicknesses of tens of microns. It is aimed to identify the microstructural transformations at crystal cell level, eventually the segregation of alloying elements as result of plasma electrolysis treatment. The SANS measurement can exactly show at what circumstances are beginning the phase transformation/segregation process, so we can optimize the applied treatment.
4. Conclusions • • •
We prooved the possibility of using by EPP - cathodic polarization in 0.5M AlCl3 in Deep Eutectic Solvent ChCl - ureea 1:2 molar ratio, to obtain a superficial layer characterized by the presence of micro-craters and spheroids, containing intermetallic compounds as Fe2Al. The potentiodynamic polarization measurements show that by Plasma Electrolysis techniques can be obtained the aluminium based thin films on austenitic stainless steel 316Lwhich improve the corrosion behavior. Small Angle Neutron Scattering (SANS) measurements provides information about the nanostructure of the surface layer affected as a result of plasma electrolysis processing as compared to base metal when using in transmission geometry, packages of samples treated identically in order to increase the effective volume affected by treatment..
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References [1]
V. Engelko, G. Mueller, A. Rusanov, V. Markov, K. Tkachenko, A. Weisenburger, “A. Kashtanov, A. Chikiryaka, A. Jianu, Surface modification/ alloying using intense pulsed electron beam as a tool for improving the corrosion resistance of steels exposed to heavy liquid metals”, Journal of Nuclear Materials 415 (2011) 270-275.
[2] [3]
V. Engelko, B. Yatsenko, G. Mueller, H. Bluhm, Vacuum, Volume 62, Issues 2–3, 15 June 2001, Pages 211–216 J. Konys, W. Krauss, N. Holstein, „Development of advanced Al coating process for future applications as anti-corrosion and Tpermeation barriers”, Fusion Engineering and Design 85 (2010) 2141-2145. V.A.Andrei, E.Coaca, M.Mihalache, V.Malinovschi, M.Patrascu-Minca, Surf. Interface Anal. 2016,48, 654-659 E.I. Meletis, X. Nie, F.L. Wang, J.C. Jiang, „Electrolytic plasma processing for cleaning and metal-coating of steel surfaces”, Surface and Coatings Technology 150 (2002) 246–256 S. A. Karpushenkov, G. L. Shchukin, A. L. Belanovich, V. P. Savenko, A. I. Kulak, , J Appl Electrochem (2010) 40:365–374
[4] [5] [6]
ScienceDirect Materials Today: Proceedings 4 (2017) 6990–6999 www.elsevier.com/locate/procedia
13th International Conference on Nanosciences& Nanotechnologies & 9th International Symposium on Flexible Organic Electronics
Microstructures and micro composition developed by Plasma Electrolysis Processing of 316L austenitic steels to obtain Alcontaining surface layer V.A. Andreia* , E. Coacab, I. Ionitaa, G. Torokc, O.A. Rusub, A. Marinb, M. Mihalacheb, L. Velciub, V. Malinovschid, T. Visane a b
ELSSA Laboratory SRL, Crinului 34, Pitesti,110109, Romania Institute for Nuclear Research , Campului 1, Mioveni, Romania c Budapest Neutron Center d Pitesti University e University „Politehnica”, Bucharest
Abstract The candidate materials for the Generation-IV nuclear power reactors include modified stainless steels, as 316L in order to improve corrosion resistance in extreme conditions (liquid metals, higher temperature than the upper limit of conventional austenitic stainless steels). The aim of this study is to explore in-depth microstructural changes of aluminium containing surface layer developed by Plasma Electrolysis technique on 316L austenitic stainless steel. Complex surface treatments were performed involving anodic oxidation, high-temperature autoclaving in water and micro-arc oxidation in aqueous solution of 0.1 M NaAlO2 and 0.05 M NaOH. The obtained structures were characterized by X-Ray Diffraction, Metallography, Scanning Electron Microscopy and Small Angle Neutron Scattering. Unique surface characteristics were observed due to thermal, chemical, electrical and mechanical effects as a result of Plasma Electrolysis Processing. Corrosion behavior of plasma treated samples was investigated by electrochemical techniques. Some considerations for the further development of the new advanced nuclear materials are presented.
This is an open-access article distributed under the terms of the Creative Commons Attribution-NonCommercial-ShareAlike License, which permits non-commercial use, distribution, and reproduction in any medium, provided the original author and source are credited. * Corresponding author. Tel.: 0040723314800. E-mail address: [email protected] 2214-7853© 2017 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the Conference Committee Members of NANOTEXNOLOGY2016 (13th International Conference on Nanosciences & Nanotechnologies & 9th International Symposium on Flexible Organic Electronics).
V.A. Andrei et al.. / Materials Today: Proceedings 4 (2017) 6990–6999
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© 2017 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the Conference Committee Members of NANOTEXNOLOGY2016 (13th International Conference on Nanosciences & Nanotechnologies & 9th International Symposium on Flexible Organic Electronics). Keywords:plasma electrolytic oxidation; aluminium; 316L stainless steel, XRD, SEM, SANS
1. Introduction 304L and 316L austenitic stainless steels have been used extensively as nuclear structural materials and were selected for Heavy Liquid Metal (HLM) nuclear systems involving lead or Lead Bismuth Eutectic (LBE) [1].The corrosion mechanism of these steels for LBE system is affected by several parameters such as oxygen activity in the liquid metal and the temperature. Adequate oxygen activity in HLM system can minimize the stainless steel dissolution process by forming a protective surface oxide. In order to improve the corrosion resistance of stainless steels exposed to oxygen-containing HLM, at temperatures above 500°C, pulsed electron beam facility (GESA) surface process was employed. GESA, a method designed for material surface treatment ensures the possibility to realize melting depths of materials in the range of 10–100μm [2]. Aluminium was alloyed into the steel surface by GESA facility in order to form an aluminium oxide layer as a result of dissolved oxygen interaction in HLM. The protective surface aluminium alloy obtained by GESA procedure was developed and improved for better corrosion resistance of 316 steel, exposed to Pb and Pb-Bi eutectic, respectively. The procedure itself consists in two stages: (i) Al or Al alloy deposition and (ii) melting of deposited layer on the steel surface using intense pulsed electron beam. The thickness of the aluminium containing layer (10-30 µm) is around the penetration depth of electrons into the steel. Therefore, by applying the GESA procedure, the microstructural properties of the substrate materials (excepting the superficial layer) do not change and is possible to obtain a surface layer with a uniform distribution of Al, controlled thickness, crack-free and adherent to the substrate [1]. There is interest in developing surface engineering techniques, less expensive and more permissive with the geometry of treated components, better than GESA procedure. Other experimental approaches related to achievement of superficial structures based on Al deposited on AISI 316 L steel substrate, which provide corrosion resistance in aggressive environments specific to Lead Fast Reactor type, are presented in Ref.[3–5]. Thereby, electro-deposition from ionic liquids and electrochemical plasma processing techniques are considered in Ref.[3]. Plasma electrolytic oxidation feasibility in aqueous solution of NaAlO2 for preparation of ceramic-like aluminium oxide films on austenitic stainless steels surface was proven in Ref.[4]. Ref.[5] describes the possibility of using processing techniques with plasma electrolysis for substrate activation and for development of Al based deposition. This study aims to characterize micro- and nanostructured transformations induced by plasma electrolytic processing in order to better understand the effects of different process parameters and to further optimize surface treatments. Neutron scattering technique can provide detailed information about structural changes at nano and micro level. 2. Materials and methods AISI 316L stainless steel samples (10x10x1) mm3, were manually grounded up to 1200 mesh SiC paper in order to achieve a fine finish with an average surface roughness of 0.35 mm, which minimizes the mechanical surface damage and allows a good adherence of a coated layer, according to Ref.[10]. Then the samples were cleaned with distilled water and ethanol before treatment. EPP treatments- cathodic polarization in 0.5M AlCl3 in Deep Eutectic Solvent ChCl - ureea 1:2 molar ratio were conducted in the single-compartment two-electrode electrochemical cell containing 500 mL electrolyte. The samples were used as cathodes of the electrochemical cell and the anode was a cylinder made from AISI 316L steel, which surrounded the cathode and has an area of approximately 20 times higher than the cathode area. The temperature of the electrolyte was kept within a range of 90–95◦C using cold water circulating through a heat exchanger.The samples were treated using a programmable DC power supply EA-PS 8400-70 with voltage range of
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V.A. Andrei et al. / Materials Today: Proceedings 4 (2017) 6990–6999
0–400 V and a current range of 0–15 A, under constant voltage conditions. The power supply provided constant voltage with an accuracy of about ±1% of the desired value. MAO treatments of 316L stainless steel surfaces were conducted in the single-compartment two-electrode electrochemical cell containing 500 mL of aqueous solution of.0.1M NaAlO2 and 0.05 M NaOH, prepared using commercial pure reagents and distilled water.The samples were used as anodes of the electrochemical cell and the cathode was a cylinder made from AISI 316L steel, which surrounded the anode and has an area of approximately 20 times higher than the anode area.The temperature of the electrolyte was kept within a range of 40–45◦C using cold water circulating through a heat exchanger. To reduce temperature and concentration gradients in the electrolyte, the solution was magnetically stirred. No more than 2 specimens were coated in a single freshly prepared electrolyte solution. The sample labeling, the applied treatments and the analysis techniques used are presented in the Table 1. Table1.Materials, applied treatments, analysis techniques sample code
Substrate
Treatment parameters
A
AISI 316L
Standard sample – untreated
B1
AISI 316L
B2
AISI 316L Autoclaved in deionized water 600°C, 80 days
EPP:DC power supply, constant voltage conditions, 200V, 4min, Telectrolyte ~90°C, MAO:DC power supply, constant voltage conditions, 320V, 3min, Telectrolyte<45 °C
Analysis techniques XRD, SANS Potentiodynamic Polarization Metallography, XRD, SANS, Potentiodynamic Polarization Metallography XRD, SANS, Neutron Diffraction, Potentiodynamic Polarization
For Metallography, the samples were cut into sections with the ISOMET 4000 device, and then packed into a copper foil and mounted in a thermoplastic raisin with the METAPRESS-A, METKOH covering press. After mounting, the sample was grinded and polished (120-2400 grained abrasive paper) with a BETA GRINDERPOLISHER- BUEHLER device. After polishing, samples were metallographic etched in order to evaluate the structure of the surface layer. The electrolytic etching was performed with oxalic acid 10%, 6V, 10-20 seconds. Scanning electron microscopy (SEM) and elements microanalysis, using the electrons probe was performed using a TESCAN VEGA II LMU microscope. SEM device is equipped with 3 detectors: a secondary electron (SE) detector, a back-scattered electron (BSE) detector, and energy dispersive X - ray spectrometer (EDS). X-ray diffraction measurement(XRD) was performed on a RigakuUltima IV diffractometer, equipped with Parallel Beam optics and a high precision vertical goniometer (θ - θ) with 285mm radius.X-ray diffraction patterns were collected using CuKα radiation: θ- θ scanning mod, accelerating voltage 40 kV, tube current 40mA, scan range 2θ 150 to 1000, step size Δ2 θ =0.050, counting time 2s, graphite monocromator in diffracted beam. For sample B1 diffraction spectra were collected in GIXRD geometry (Grazing Incidence X-Ray Diffraction) at grazing angles of θ=30 and θ =50. Small Angle Neutron Scattering (SANS) measurements were performed on the SANS-diffractometer at Budapest Neutron Center. We have investigated the nanostructure of the surface layer affected as a result of plasma electrolysisprocessing as compared to base metal. We subtracted the SANS contribution of non-treated base sample using neutron beam (with wavelength λ=0.609 nm, Δλ/λ=0.25, beam diameter=8mm). The geometry of experiment is shown in Fig.1.
V.A. Andrei et al.. / Materials Today: Proceedings 4 (2017) 6990–6999
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H=HX
ko
n
S
θ
Y
q
k1 X
D ( X ,Y ) Fig.1. SANS-experiment: neutrons scattered from the sample (S) are registered in two-dimensional detector (D) where ko, k1 are the initial and final wave vectors of neutron In cases the magnetic field Hx can be detected the magnetic scattering also.
They were used in transmission geometry, packages of samples (10x10x1mm3) treated identically in order to increase the effective volume affected by treatment. The packages of samples of stainless steel with surface treatment were fixed and was applied a magnetic field 1.4T. We observed both the isotropic scattering pictures and anisotropic one.
3. Results and discussion The results of SEM and Metallography analysis are presented in Fig. 2 and Fig.3. As shown in these images, the different treatments cause different surface morphologies: The results obtained by XRD analysis are shown in Fig.4 ,Fig.5,.and Fig.6. XRD analysis reveals the presence of crystalline structures containing Al: Fe2Al in surface layer of B1 sample, Al2O3 on B2 sample. The results obtained by Metallography, SEM, EDS, XRD are showing that: • EPP - cathodic polarization in 0.5M AlCl3 in Deep Eutectic Solvent ChCl - ureea 1:2 molar ratio, is forming a layered structure; the superficial layer (thick≈100μ) is characterized by the presence of micro-craters and spheroids, containing intermetallic compounds as Fe2Al. • MAO treatment applied on autoclaved 316L steel is forming a layered structure: over magnetite film (thick≈40μ) formed by autoclaving develops a layer composed of Al oxides (prevailing), Fe oxides and Al hydroxide (thick≈30μ) [4]. a Element Al Fe Cr Ni Mn S C O
C [at%] 17.5 49.5 15.6 8.2 1.1 1.9 3.2 3.0
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V.A. Andrei et al. / Materials Today: Proceedings 4 (2017) 6990–6999
b
Fig.2a SEM micrograph for B1 sample and element concentrations b. EDS results on the cross section of B1 sample and cross sectional micrographs of B1 sample
a
Element
C [at%]
Al Fe
34.7 3.6
C Cr Si Na O
7.3 0.4 0.1 0.1 53.8
b
Fig.3a SEM micrograph for B2 sample and element concentrations b. EDS results on the cross section of B2 sample and cross sectional micrographs of B2 sample
V.A. Andrei et al.. / Materials Today: Proceedings 4 (2017) 6990–6999
Fig.4. XRD patterns of A sample
Fig. 5.GIXRD patterns of B1 sample
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Fig.6. XRD patterns of B2 sample
The potentiodynamic polarization measurements in 0.5M NaCl aqueous electrolyte (fig.7), show that the corrosion potential, in the case of samples D1, D2occurs to more positive values after treatment so superficial layer provide corrosion protection; the values of corrosion rate computed by the Polarization Resistance method show that the aluminum based thin films obtained ensure some corrosion protection. Polarization curve of D2 sample shows that a decrease in corrosion currents caused by deposition of aluminum oxide films at micro-arc conditions is limited by the high porosity of film. Corrosion potential and corrosion rate as resulted from Polarization Resistance method
Proba A B1 B2
E(I=0) (mV): -334.453 -283.237 -212.148
Co. Rate (mmpy) 0.15 0.12 0.14
Polarization curves for 316L samples (B1, B2 and A) in 0.5M NaCl aqueous electrolyte Fig.7. Electrochemical measurements
The results of SANS analysis of 316L samples are presented in Fig. 8 and Table 2. Data analyses were done by Bersans program package separating the possible magnetic contribution from nuclear one. The scattering intensity of nuclear part is fitted by a sum of Guinier fuction and a fractal part: Y=A+I1*exp(-R2q2/3)+F*qC, where the first part is a simplest approximation of scatterers and the second term represents the density fluctuations at the border of precipitates. (usually that is formed by diffusion processes, so most probably the shape is DLA fractal) The fitted parameters are: R-averaged radius of scattering centre, F- fractal part (inter-granular area ) , Cdimension of fractal structure
V.A. Andrei et al.. / Materials Today: Proceedings 4 (2017) 6990–6999
200 B
316 basic
Data: A316ALI_B Model: Guifract Chi^2 = 37.16983 R^2 = 0.95344
Data: A316BAS_B Model: guifract
100
Chi^2 R^2
= 3.18977 = 0.9092
A I1 R F C
4.12656 960.25849 17.31452 1.67851 -1.38493
Intensity arb. un.
intensity arb un
Nuclear scattering difference 316-316 Ali
100
2000 1000
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±0.05822 ±201.61105 ±0.72638 ±0.08403 ±0.07044
A I1 R F C
10
0.37934 32.77896 107.76525 2.36935 -1.97246
±0.44047 ±0 ±0 ±0.27228 ±0.09826
10
1 2 0.06
0.1
1
Q nm
4
0.4 0.1
-1
1
-1
Q nm 2
4
2
C
A+I1*exp(-R *x /3)+F*x
b) Nuclear scattering; difference between B1 and standard 316L(fitted)
a) Nuclear scattering; fit for standard 316L 10000
Magnetic scattering difference 316-316 Ali 100000
Nuclear scattering difference 316-316 Al18
Data: A316ALIM_B Model: Guifract
Data: A316A18_B Model: Guifract
Chi^2 = 189.85034 R^2 = 0.96352 A I1 R F C
100
0.67433 32.77896 107.76525 0.95373 -3.85669
10000 ±0.66886 ±0 ±0 ±0.07817 ±0.05431
10
Intensity arb. un.
Intensity arb. un.
1000
Chi^2 = 86.31098 R^2 = 0.99535 A I1 R F C
1000
35.11316 65332.62599 16.27181 15.19917 -3.46777
±2.35969 ±8022.61189 ±0.3737 ±0.83 ±0.0536
100
1
0.1 0.1
1
4
10 0.1
-1
Q nm 2
-1
Q nm 2
2
1
2
A+I1*exp(-R *x /3)+F*x
C
A+I1*exp(-R *x /3)+F*x
c) Magnetic scattering; difference between B1 and standard 316L(fitted)
d) Nuclear scattering; difference between B2 and standard 316L(fitted)
Fig.8. SANS results for samples 316L
Table 2. Results of neutron scattering techniques:SANS : R,F,C; ND: lattice parameter a[Å]
Parameters derived R [nm] F C
4 C
A (Basic B1 B2 316L 17.314 107.76525 ±0 (*) 16.27±0.37 ±0.73 107.76525 ±0 (**) 1.678 2.36935 (*) 15.2±0.83 ±0.0840 0.953±0.07 (**) -1.385 -1.972±0.098(*) 3.46±0.054 ±0.070 3.856±0.05431(**) *Nuclear scattering; **Magnetic scattering
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In sample B1 the averaged size of scattering particles 2R=214nm with volume fractal structure close to the lamellar one, while the magnetic size is the same 2R=214nm and their surface fractal structure is close to the lamellar form with sharp border (-4). In sample B2 the averaged size of scattering particles 2R=32nm with surface fractal structure with dimension 63.46=2.54. In B2 sample the treatment did not indicated any measurable magnetic structure. The fractal part which represents the area of inter-granular matter is increased what is the other side of dissolution process. The original fractal structure is a volume fractal with dimension of 1.39 what is a hairy structure. At treated area the fractal dimension is around 2, meaning that the volume fractal is more dense and smooth. Also we can conclude that the ratio of fractal part is growing for factor of 2 so the inter-granular fractal volume space is grown for factor of 2. The treatments B1 and B2 led to the formation of the ceramic structure based on Aluminum In the case of B1 and B2 treatments the intergranular area increase from untreated steel, and the volume fractal is more dense and smooth. For use 316L steel in nuclear systems with HLM is necessary to obtain a surface layer with an uniform distribution of Al, of controlled thickness, which is crack-free and adherent to the substrate1. In in the case of EPP [5] high potential between the electrodes leads to concentration of positive ions (as Al3+)that are present in the electrolyte, in the close proximity of the cathode, mostly on the surface of the gas bubbles. Thus, a very high positive charge is present in the close proximity of the cathode. This results in high localized electric field strength between the cathode and the positive charges. When the electric field strength reaches~105V/m or higher, gas space inside the bubbles is ionized and a plasma discharge takes place; the temperature of plasma, locally, can reach as high as 2000°C, which is surrounded by relatively cool electrolyte. Thus, plasma bubble cools down and finally implodes on the metal surface. A combination of mechanical, thermal, chemical and electrical treatment provided by EPT leads to development of unique surface microstructure. It is known[6] that using plasma electrolytic oxidation for anodizing of non-valve metals such as stainless steels, and other practically important metals, is hampered by the fact that at the initial stage of the oxidation, the barrier layer essential for converting the process into spark regime is not formed on the metal surface. The the work [4] was shown that the formation of the dielectric barrier layer suitable for the MAO treatment can be achieved by autoclaving in de-ionized water, to form an iron oxide film without Ni and Cr on the 316L steel substrate. Optimization in plasma electrolysis treatments involves understanding the changes in structure and composition both in thin layers (thickness of tens of Å) using surface science techniques (e.g. XPS, AES) and in layers with thicknesses of tens of microns. This study sought to identify micro and nano - structural changes in layers with thicknesses of tens of microns. It is aimed to identify the microstructural transformations at crystal cell level, eventually the segregation of alloying elements as result of plasma electrolysis treatment. The SANS measurement can exactly show at what circumstances are beginning the phase transformation/segregation process, so we can optimize the applied treatment.
4. Conclusions • • •
We prooved the possibility of using by EPP - cathodic polarization in 0.5M AlCl3 in Deep Eutectic Solvent ChCl - ureea 1:2 molar ratio, to obtain a superficial layer characterized by the presence of micro-craters and spheroids, containing intermetallic compounds as Fe2Al. The potentiodynamic polarization measurements show that by Plasma Electrolysis techniques can be obtained the aluminium based thin films on austenitic stainless steel 316Lwhich improve the corrosion behavior. Small Angle Neutron Scattering (SANS) measurements provides information about the nanostructure of the surface layer affected as a result of plasma electrolysis processing as compared to base metal when using in transmission geometry, packages of samples treated identically in order to increase the effective volume affected by treatment..
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