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Investigating the morphology and corrosion behavior of electrochemically borided steel
Surface & Coatings Technology 200 (2006) 3590 – 3593 www.elsevier.com/locate/surfcoat
Investigating the morphology and corrosion behavior of electrochemically borided steel G. Kartal*, O. Kahvecioglu, S. Timur I.T.U. Chemistry and Metallurgy Faculty, Metallurgical and Materials Engineering Department, 34469 Maslak, Istanbul, Turkey Received 19 August 2004; accepted in revised form 14 February 2005 Available online 12 May 2005
Abstract In this study, the boriding of steels by molten salt electrolysis in borax-based electrolyte at various current densities (50 – 700 mA/cm2), temperatures (800 – 1000 -C), and time (0 – 2 h) was investigated. The optimum condition was determined by taking the thickness, hardness, and morphology of the boride layer into account. Different borided phases can form depending on the amount of the diffused boron from surface to matrix as follows: FeBx (x > 1) ` FeB ` Fe2B ` Fe3B ` Fey B( y > 3) ` Fe. Determination of the optimum conditions for the boriding of DIN EN 10130-99 DC04 low carbon steel are; 10% Na2CO3 + 90% Na2B4O7, 900 -C, 1 h, and 200 mA/cm2. The corrosion tests for the given material were carried out in different media; HCl, H2SO4, HNO3, H3PO4, and HClO4 (10%, vol.). Electrochemically borided low carbon steel showed a corrosion rate of 0.66 10 3 g/cm2/day in HCl, 1.13 10 3 g/cm2/day in H2SO4, 1.59 10 3 g/cm2/day in HClO4, and 3.37 10 3 g/cm2/day in H3PO4. However, it was not resistant to HNO3 with a considerable corrosion rate of 0.3 g/cm2/day. D 2005 Elsevier B.V. All rights reserved. Keywords: Electrochemical boriding; Surface treatment; Molten salt; Corrosion; FeB
1. Introduction The boriding process improves hardness, wear, fatigue, oxidation, and corrosion (towards non-oxidizing dilute acidic solutions, alkali media, and molten metals) properties of surface. Industrial boriding processes can be applied to most ferrous materials such as structural steels; case hardened, tempered, tool, and stainless steels; cast steels; Armco iron; gray and ductile cast irons; and sintered iron and steel [1,2]. Thermochemical boriding is generally preferred in industry because it is technologically simpler and can coat intricate surfaces uniformly although its process time is long (about 5 h). However, electrolytic process provides higher rate of boride layer growth for simpler shape of material in a short time (approx. 30 min) [3,4].
* Corresponding author. Tel.: +90 212 285 33 69; fax: +90 212 285 34 27. E-mail address: [email protected] (G. Kartal). 0257-8972/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2005.02.210
Moissan [1] suggested as early as 1895 that steel should be borided. Since then, industrial application of electrochemical processes has been limited; therefore, there is no more investigation in this area. In these restricted investigations, Segers, Fontana, and Winand [3] investigated the effects of current density and time on coating thickness and surface morphology in molten Na2B4O7 at different temperatures (900 – 1050 -C) and showed that Fe2B or FeB + Fe2B phases was formed depending on the applied current density and temperature. Matiasovsky and his group [4] found that the formation of boride layer was controlled by boron diffusion into the surface. Furthermore, Tkachev, Grigorov, and Katkhanov [5] researched the possible reaction mechanism in electrochemical boriding. Another study was done by Han and Chun [6] who investigated the effect of bath components on layer structure. Boriding, which can considerably enhance the corrosion –erosion resistance of ferrous materials in non-oxidizing dilute acids and alkali media, is increasingly used to this advantage in many industrial applications. Also the borided surface has a high hardness, a moderate oxidation resistance
G. Kartal et al. / Surface & Coatings Technology 200 (2006) 3590 – 3593
2. Experimental techniques 2.1. Cell and furnace The boriding experiments by molten salt electrolysis were carried out in a high frequency furnace containing a graphite crucible with platinum anodes and a steel cathode material. Current was supplied digitally from a direct current source (T0.001 mA). During electrolysis, the temperature of the inner side of electrolyte was controlled by Pt-PtRh13 thermocouple and temperature was controlled by a laser thermometer from the outer surface in order to eliminate the affecting risks of electrical current on thermocouple measurements.
100 δ = FeB ο = Fe2B φ = Fe3B
ο
80 ο
Intensity
(up to 850 -C), and is quite resistant to attraction by molten metals such as zinc [1,2]. The corrosion behavior and properties of electrochemically produced boride layer have not been examined yet, whereas the corrosion properties of boride layer produced by other methods (thermochemical, fluidized bed reactor, etc.) were carried out only in a few studies [1,2,7,8]. Boriding of steels by electrochemical reduction from molten salts was studied and the electrolysis conditions (the composition of electrolyte, current density, temperature, and time) were fixed by taking the hardness, morphology, and thickness of the obtained boride layer into account. In addition, the corrosion resistance of layers, which were produced at optimum conditions, was investigated in dilute acidic solutions.
3591
60
δ
φ φ
φ
40
δ ο δ
20
φ
δ
δ ο
δ
δ
0 0
20
40
2θ
60
80
100
Fig. 1. XRD diagram of borided layer of steel (10% Na2CO3 + 90% Na2B4O7, 900 -C, 200 mA/cm2, 1 h).
Conventional polishing methods were applied to the preparation of the borided surface in order to measure the thickness of the layer at eight different points which were determined before. FeB and FeB2 layer thicknesses were measured one by one and the average value was calculated. The contrast of the phase component of layer was easily discerned so the etching was not always necessary. The composition of boride layer was characterized from surface by thin film X-ray diffraction (XRD) analysis using CuKa radiation (10 kV, 10 mA). Furthermore, hardness values of all of the phases were measured. The resulting boride layer at the optimum conditions was treated for corrosion tests in 10% (vol.) HCl, H3PO4, H2SO4, HNO3, and HClO4 solutions by 48 h.
2.2. Experimental procedure
3. Results and discussion
Throughout the experiments, before inserting cathode material into the electrolyte, the cathode was polished (up to 600) in order to degrease the surface and obtain a defined surface roughness. At the conclusion of each experiment, the cathode was withdrawn from the electrolyte and left in air then current was cut off. The solidified electrolyte on cathode surface was removed in hot boiling water.
Electrolysis experiments were conducted under different conditions (Table 1) for the purpose of determining the optimum parameters of electrochemical boriding low allowed steel, composition of which is given in Table 2. Acceptable surface hardness and a homogeneous, thick layer were established. Experiments indicated that the optimum conditions for the boriding of DIN EN 10130-99
Table 1 Experimental parameters in molten salt electrolysis Group
Parameter
Experimental parameters
Variables
I II III IV V
Time Current density Temperature g = B2O3/Na2O Corrosion resistance
10% NaCl + 90% Na2B4O7, 200 mA/cm2, 900 -C 20% NaCl + 80% Na2B4O7, 1 h, 900 -C 10% NaCl + 90% Na2B4O7, 1 h, 200 mA/cm2 900 -C, 1 h, 200 mA/cm2 10% Na2CO3 + 90% Na2B4O7,1 h, 200 mA/cm2
1 – 5 – 10 – 15 – 30 – 60 – 90 – 120 min 50 – 100 – 200 – 300 – 700 mA/cm2 800 – 900 – 1000 -C 1.25 – 1.5 – 1.75 – 1.87 – 2.245 10% (vol.) HCl – H2SO4 – H3PO4 – HNO3 – HClO4
Table 2 The chemical composition (%) of the steel material C
Mn
P
S
Si
Al
Cu
Cr
Ni
Mo
Ti
N (ppm)
O (ppm)
0.004
0.122
0.003
0.009
0.003
0.04
0.021
0.012
0.03
0.002
0.063
37
45
3592
G. Kartal et al. / Surface & Coatings Technology 200 (2006) 3590 – 3593
DC04 low carbon steel are 10% Na2CO3 + 90% Na2B4O7, 900 -C, and 1 h at 200 mA/cm2. The examination of XRD analysis of electrochemically borided layer (Fig. 1) shows that FeB, Fe2B, and Fe3B phases were formed at the surface. FeB and Fe2B phases established at the surface were also mentioned by other researchers. In electrolytic boriding, Fe3B phase was found and its possible position was determined (Fig. 2). The XRD diagram given in Fig. 1 and the micrograph given in Fig. 2 exhibit a boride layer consisting of differing ratios of FeBx (x 1) compounds and can be formed in an appropriate electrolyte by polarization. During the boriding experiments, different borided phases can be formed depending on the amount of the diffused boron from surface to matrix as follows: FeBx (x > 1) ` FeB ` Fe2B ` Fe3B ` Fey B ( y > 3) ` Fe. The typical micrograph of borided layer (Fig. 3a) and hardness profile (Fig. 3b) on the layer obtained
FeB
Fe2B
Fe3B Steel Substrate
Fig. 2. Micrograph of boride layer (10% Na2CO3, 90% Na2B4O7, 200 mA/ cm2, 900 -C, 1 h).
2000
Hardness [HV0.1]
1600
1200
800
400
0 0
20
40
60
80
100
120
Distance from the surface [µm]
(a)
(b)
Fig. 3. Typical micrograph of boride layer and hardness profile on the layer (10% Na2CO3 + 90% Na2B4O7, 900 -C, 200 mA/cm2, 1 h).
90
400 10 %HCI 10 % H2SO4 10 % H3PO4
70
10 %HNO3
350
Weight loss [mg/cm2]
Weight loss [mg/cm2]
80
60 Not Borided
50 40 30 20 10
Not Borided
10 % HCIO4
300
Borided
250 200 150 100 50
Borided
Not Borided Borided
0
0 0
10
20
30
40
Time [h]
(a) non-oxidizing acid solution 10 % HCl, H2SO4, H3PO4
50
0
10
20
30
40
50
Time [h]
(b) oxidizing acid solution 10 % HNO3, HClO4
Fig. 4. Weight losses of specimen in acid solution (10% Na2CO3 + 90% Na2B4O7, 900 -C, 200 mA/cm2, 1 h). (a) Non-oxidizing acid solution, 10% HCl, H2SO4, H3PO4; (b) Oxidizing acid solution, 10% HNO3, HClO4.
G. Kartal et al. / Surface & Coatings Technology 200 (2006) 3590 – 3593 Table 3 Corrosion rates in different dilute acidic solutions [10%]
Corrosion rate [g/cm2/day] Non-oxidizing HCl
Borided 0.66 10 Not borided 5.10 10
Oxidizing
H2SO4 3 3
1.13 10 40.2 10
H3PO4 3 3
3.37 10 11.2 10
HClO4 3 3
1.59 10 14 10
HNO3 3 3
0.300 0.315
at optimum experimental conditions are given below. In the investigation of microstructure, the growth of the layer was in the tooth structure and it is known that effective parameters on this structure depend on the growth morphology, composition, and the structural defects. The length of ‘‘peaks’’ and ‘‘valleys’’ of the layer were measured from eight different points and the average values of these measurements are calculated to find the total layer thickness. The total layer thickness was found to be 120 Am for an ideal boride layer and 75 Am for an ideal FeB layer. In Fig. 3, the contrast difference between the FeB and Fe2B layers is clearly visible so the simulation of the hardness profile of phases, which occurred by diffusion of boron, can be easily obtained. Hardness profiles are determined by the Vickers (100 g) method. A typical example is given in Fig. 3b for a two layer deposit. The highest hardness values are obtained for FeB (1890 HV) but the Fe2B layers are also very hard (1572 HV) as compared to the steel substrate (100 HV). It is known that one of the aims of the boriding process is to improve the corrosion resistance of materials by surface treatment. In order to determine the corrosion properties of the electrochemically formed boride layer dissolution tests were applied in 10% (vol.) HCl, H2SO4, HNO3, H3PO4, HClO4 solutions by 48 h. At the end of these tests, the variation of weight loss depending on time was obtained and given in Fig. 4a for 10% (vol.) HCl, H2SO4, H3PO4 solutions, and given in Fig. 4b for oxidizing solution of 10% (vol.) HClO4, HNO3, respectively. As given in Table 3, the corrosion resistance changes according to oxidizing and non-oxidizing solutions.
4. Conclusions & Determination of the optimum conditions for the boriding of low carbon steel are; 10% Na2CO3 + 90% Na2B4O7, 900 -C, 1 h, and 200 mA/cm2.
3593
& At the end of the boriding process, different borided phases can be formed depending on the amount of the diffused boron atom from surface to matrix as follows: FeBx (x > 1) ` FeB ` Fe2B ` Fe3B ` Fey B ( y > 3) ` Fe. & The growth of the layer was in the tooth structure and it is known that effective parameters on this structure depend on the growth morphology, composition, and the structural defects. The total layer thickness was found to be 120 Am for an ideal boride layer and 75 Am for an ideal FeB layer. & The hardness of the layer, according to the diffusion amount of boron and the formation of FeBx (x 1) phase, was decreasing from surface (1900 HV) to matrix (100 HV). & The corrosion tests for the given material were carried out in different media such as; HCl, H2SO4, HNO3, H 3 PO 4 , and HClO 4 (10%vol.). Electrochemically borided low carbon steel showed a corrosion rate of 0.66 10 3 g/cm2/day in HCl and 1.13 10 3 g/cm2/ day in H2SO4 and 1.59 10 3 g/cm2/day in HClO4, and 3.37 10 3 g/cm2/day in H3PO4. However, it was not resistant to HNO3 with a considerable corrosion rate of 0.3 g/ cm2/day.
Acknowledgements The authors are pleased to acknowledge the support given by T.R. Prime Ministry The State Planning Organization through the Advanced Technologies in Engineering project in Istanbul Technical University.
References [1] A.G. Von Matuschka, Boronizing, Heyden and Son Inc., Philadelphia, 1980. [2] A.K. Sinha, Heat Treat. 4 (1991) 437. [3] L. Segers, A. Fontana, R. Winand, Electrochim. Acta 36 (1991) 41. [4] K. Matiasovsky, M. Chrenkova-Paucirova, P. Fellner, M. Makyta, Surf. Coat. Technol. 35 (1988) 133. [5] V.N. Tkachev, P.K. Grigorov, B.B. Katkhanov, Met. Sci. Heat Treat. 17 (1975) 348. [6] S.H. Han, J.S. Chun, J. Mater. Sci. 15 (1980) 1379. [7] E. AtNk, U. Yunker, C. Meric¸, Tribol. Int. 36 (2003) 155. [8] T. Wierzchon, P. Bielinski, K. Sikorski, Surf. Coat. Technol. 73 (1995) 121.
Investigating the morphology and corrosion behavior of electrochemically borided steel G. Kartal*, O. Kahvecioglu, S. Timur I.T.U. Chemistry and Metallurgy Faculty, Metallurgical and Materials Engineering Department, 34469 Maslak, Istanbul, Turkey Received 19 August 2004; accepted in revised form 14 February 2005 Available online 12 May 2005
Abstract In this study, the boriding of steels by molten salt electrolysis in borax-based electrolyte at various current densities (50 – 700 mA/cm2), temperatures (800 – 1000 -C), and time (0 – 2 h) was investigated. The optimum condition was determined by taking the thickness, hardness, and morphology of the boride layer into account. Different borided phases can form depending on the amount of the diffused boron from surface to matrix as follows: FeBx (x > 1) ` FeB ` Fe2B ` Fe3B ` Fey B( y > 3) ` Fe. Determination of the optimum conditions for the boriding of DIN EN 10130-99 DC04 low carbon steel are; 10% Na2CO3 + 90% Na2B4O7, 900 -C, 1 h, and 200 mA/cm2. The corrosion tests for the given material were carried out in different media; HCl, H2SO4, HNO3, H3PO4, and HClO4 (10%, vol.). Electrochemically borided low carbon steel showed a corrosion rate of 0.66 10 3 g/cm2/day in HCl, 1.13 10 3 g/cm2/day in H2SO4, 1.59 10 3 g/cm2/day in HClO4, and 3.37 10 3 g/cm2/day in H3PO4. However, it was not resistant to HNO3 with a considerable corrosion rate of 0.3 g/cm2/day. D 2005 Elsevier B.V. All rights reserved. Keywords: Electrochemical boriding; Surface treatment; Molten salt; Corrosion; FeB
1. Introduction The boriding process improves hardness, wear, fatigue, oxidation, and corrosion (towards non-oxidizing dilute acidic solutions, alkali media, and molten metals) properties of surface. Industrial boriding processes can be applied to most ferrous materials such as structural steels; case hardened, tempered, tool, and stainless steels; cast steels; Armco iron; gray and ductile cast irons; and sintered iron and steel [1,2]. Thermochemical boriding is generally preferred in industry because it is technologically simpler and can coat intricate surfaces uniformly although its process time is long (about 5 h). However, electrolytic process provides higher rate of boride layer growth for simpler shape of material in a short time (approx. 30 min) [3,4].
* Corresponding author. Tel.: +90 212 285 33 69; fax: +90 212 285 34 27. E-mail address: [email protected] (G. Kartal). 0257-8972/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2005.02.210
Moissan [1] suggested as early as 1895 that steel should be borided. Since then, industrial application of electrochemical processes has been limited; therefore, there is no more investigation in this area. In these restricted investigations, Segers, Fontana, and Winand [3] investigated the effects of current density and time on coating thickness and surface morphology in molten Na2B4O7 at different temperatures (900 – 1050 -C) and showed that Fe2B or FeB + Fe2B phases was formed depending on the applied current density and temperature. Matiasovsky and his group [4] found that the formation of boride layer was controlled by boron diffusion into the surface. Furthermore, Tkachev, Grigorov, and Katkhanov [5] researched the possible reaction mechanism in electrochemical boriding. Another study was done by Han and Chun [6] who investigated the effect of bath components on layer structure. Boriding, which can considerably enhance the corrosion –erosion resistance of ferrous materials in non-oxidizing dilute acids and alkali media, is increasingly used to this advantage in many industrial applications. Also the borided surface has a high hardness, a moderate oxidation resistance
G. Kartal et al. / Surface & Coatings Technology 200 (2006) 3590 – 3593
2. Experimental techniques 2.1. Cell and furnace The boriding experiments by molten salt electrolysis were carried out in a high frequency furnace containing a graphite crucible with platinum anodes and a steel cathode material. Current was supplied digitally from a direct current source (T0.001 mA). During electrolysis, the temperature of the inner side of electrolyte was controlled by Pt-PtRh13 thermocouple and temperature was controlled by a laser thermometer from the outer surface in order to eliminate the affecting risks of electrical current on thermocouple measurements.
100 δ = FeB ο = Fe2B φ = Fe3B
ο
80 ο
Intensity
(up to 850 -C), and is quite resistant to attraction by molten metals such as zinc [1,2]. The corrosion behavior and properties of electrochemically produced boride layer have not been examined yet, whereas the corrosion properties of boride layer produced by other methods (thermochemical, fluidized bed reactor, etc.) were carried out only in a few studies [1,2,7,8]. Boriding of steels by electrochemical reduction from molten salts was studied and the electrolysis conditions (the composition of electrolyte, current density, temperature, and time) were fixed by taking the hardness, morphology, and thickness of the obtained boride layer into account. In addition, the corrosion resistance of layers, which were produced at optimum conditions, was investigated in dilute acidic solutions.
3591
60
δ
φ φ
φ
40
δ ο δ
20
φ
δ
δ ο
δ
δ
0 0
20
40
2θ
60
80
100
Fig. 1. XRD diagram of borided layer of steel (10% Na2CO3 + 90% Na2B4O7, 900 -C, 200 mA/cm2, 1 h).
Conventional polishing methods were applied to the preparation of the borided surface in order to measure the thickness of the layer at eight different points which were determined before. FeB and FeB2 layer thicknesses were measured one by one and the average value was calculated. The contrast of the phase component of layer was easily discerned so the etching was not always necessary. The composition of boride layer was characterized from surface by thin film X-ray diffraction (XRD) analysis using CuKa radiation (10 kV, 10 mA). Furthermore, hardness values of all of the phases were measured. The resulting boride layer at the optimum conditions was treated for corrosion tests in 10% (vol.) HCl, H3PO4, H2SO4, HNO3, and HClO4 solutions by 48 h.
2.2. Experimental procedure
3. Results and discussion
Throughout the experiments, before inserting cathode material into the electrolyte, the cathode was polished (up to 600) in order to degrease the surface and obtain a defined surface roughness. At the conclusion of each experiment, the cathode was withdrawn from the electrolyte and left in air then current was cut off. The solidified electrolyte on cathode surface was removed in hot boiling water.
Electrolysis experiments were conducted under different conditions (Table 1) for the purpose of determining the optimum parameters of electrochemical boriding low allowed steel, composition of which is given in Table 2. Acceptable surface hardness and a homogeneous, thick layer were established. Experiments indicated that the optimum conditions for the boriding of DIN EN 10130-99
Table 1 Experimental parameters in molten salt electrolysis Group
Parameter
Experimental parameters
Variables
I II III IV V
Time Current density Temperature g = B2O3/Na2O Corrosion resistance
10% NaCl + 90% Na2B4O7, 200 mA/cm2, 900 -C 20% NaCl + 80% Na2B4O7, 1 h, 900 -C 10% NaCl + 90% Na2B4O7, 1 h, 200 mA/cm2 900 -C, 1 h, 200 mA/cm2 10% Na2CO3 + 90% Na2B4O7,1 h, 200 mA/cm2
1 – 5 – 10 – 15 – 30 – 60 – 90 – 120 min 50 – 100 – 200 – 300 – 700 mA/cm2 800 – 900 – 1000 -C 1.25 – 1.5 – 1.75 – 1.87 – 2.245 10% (vol.) HCl – H2SO4 – H3PO4 – HNO3 – HClO4
Table 2 The chemical composition (%) of the steel material C
Mn
P
S
Si
Al
Cu
Cr
Ni
Mo
Ti
N (ppm)
O (ppm)
0.004
0.122
0.003
0.009
0.003
0.04
0.021
0.012
0.03
0.002
0.063
37
45
3592
G. Kartal et al. / Surface & Coatings Technology 200 (2006) 3590 – 3593
DC04 low carbon steel are 10% Na2CO3 + 90% Na2B4O7, 900 -C, and 1 h at 200 mA/cm2. The examination of XRD analysis of electrochemically borided layer (Fig. 1) shows that FeB, Fe2B, and Fe3B phases were formed at the surface. FeB and Fe2B phases established at the surface were also mentioned by other researchers. In electrolytic boriding, Fe3B phase was found and its possible position was determined (Fig. 2). The XRD diagram given in Fig. 1 and the micrograph given in Fig. 2 exhibit a boride layer consisting of differing ratios of FeBx (x 1) compounds and can be formed in an appropriate electrolyte by polarization. During the boriding experiments, different borided phases can be formed depending on the amount of the diffused boron from surface to matrix as follows: FeBx (x > 1) ` FeB ` Fe2B ` Fe3B ` Fey B ( y > 3) ` Fe. The typical micrograph of borided layer (Fig. 3a) and hardness profile (Fig. 3b) on the layer obtained
FeB
Fe2B
Fe3B Steel Substrate
Fig. 2. Micrograph of boride layer (10% Na2CO3, 90% Na2B4O7, 200 mA/ cm2, 900 -C, 1 h).
2000
Hardness [HV0.1]
1600
1200
800
400
0 0
20
40
60
80
100
120
Distance from the surface [µm]
(a)
(b)
Fig. 3. Typical micrograph of boride layer and hardness profile on the layer (10% Na2CO3 + 90% Na2B4O7, 900 -C, 200 mA/cm2, 1 h).
90
400 10 %HCI 10 % H2SO4 10 % H3PO4
70
10 %HNO3
350
Weight loss [mg/cm2]
Weight loss [mg/cm2]
80
60 Not Borided
50 40 30 20 10
Not Borided
10 % HCIO4
300
Borided
250 200 150 100 50
Borided
Not Borided Borided
0
0 0
10
20
30
40
Time [h]
(a) non-oxidizing acid solution 10 % HCl, H2SO4, H3PO4
50
0
10
20
30
40
50
Time [h]
(b) oxidizing acid solution 10 % HNO3, HClO4
Fig. 4. Weight losses of specimen in acid solution (10% Na2CO3 + 90% Na2B4O7, 900 -C, 200 mA/cm2, 1 h). (a) Non-oxidizing acid solution, 10% HCl, H2SO4, H3PO4; (b) Oxidizing acid solution, 10% HNO3, HClO4.
G. Kartal et al. / Surface & Coatings Technology 200 (2006) 3590 – 3593 Table 3 Corrosion rates in different dilute acidic solutions [10%]
Corrosion rate [g/cm2/day] Non-oxidizing HCl
Borided 0.66 10 Not borided 5.10 10
Oxidizing
H2SO4 3 3
1.13 10 40.2 10
H3PO4 3 3
3.37 10 11.2 10
HClO4 3 3
1.59 10 14 10
HNO3 3 3
0.300 0.315
at optimum experimental conditions are given below. In the investigation of microstructure, the growth of the layer was in the tooth structure and it is known that effective parameters on this structure depend on the growth morphology, composition, and the structural defects. The length of ‘‘peaks’’ and ‘‘valleys’’ of the layer were measured from eight different points and the average values of these measurements are calculated to find the total layer thickness. The total layer thickness was found to be 120 Am for an ideal boride layer and 75 Am for an ideal FeB layer. In Fig. 3, the contrast difference between the FeB and Fe2B layers is clearly visible so the simulation of the hardness profile of phases, which occurred by diffusion of boron, can be easily obtained. Hardness profiles are determined by the Vickers (100 g) method. A typical example is given in Fig. 3b for a two layer deposit. The highest hardness values are obtained for FeB (1890 HV) but the Fe2B layers are also very hard (1572 HV) as compared to the steel substrate (100 HV). It is known that one of the aims of the boriding process is to improve the corrosion resistance of materials by surface treatment. In order to determine the corrosion properties of the electrochemically formed boride layer dissolution tests were applied in 10% (vol.) HCl, H2SO4, HNO3, H3PO4, HClO4 solutions by 48 h. At the end of these tests, the variation of weight loss depending on time was obtained and given in Fig. 4a for 10% (vol.) HCl, H2SO4, H3PO4 solutions, and given in Fig. 4b for oxidizing solution of 10% (vol.) HClO4, HNO3, respectively. As given in Table 3, the corrosion resistance changes according to oxidizing and non-oxidizing solutions.
4. Conclusions & Determination of the optimum conditions for the boriding of low carbon steel are; 10% Na2CO3 + 90% Na2B4O7, 900 -C, 1 h, and 200 mA/cm2.
3593
& At the end of the boriding process, different borided phases can be formed depending on the amount of the diffused boron atom from surface to matrix as follows: FeBx (x > 1) ` FeB ` Fe2B ` Fe3B ` Fey B ( y > 3) ` Fe. & The growth of the layer was in the tooth structure and it is known that effective parameters on this structure depend on the growth morphology, composition, and the structural defects. The total layer thickness was found to be 120 Am for an ideal boride layer and 75 Am for an ideal FeB layer. & The hardness of the layer, according to the diffusion amount of boron and the formation of FeBx (x 1) phase, was decreasing from surface (1900 HV) to matrix (100 HV). & The corrosion tests for the given material were carried out in different media such as; HCl, H2SO4, HNO3, H 3 PO 4 , and HClO 4 (10%vol.). Electrochemically borided low carbon steel showed a corrosion rate of 0.66 10 3 g/cm2/day in HCl and 1.13 10 3 g/cm2/ day in H2SO4 and 1.59 10 3 g/cm2/day in HClO4, and 3.37 10 3 g/cm2/day in H3PO4. However, it was not resistant to HNO3 with a considerable corrosion rate of 0.3 g/ cm2/day.
Acknowledgements The authors are pleased to acknowledge the support given by T.R. Prime Ministry The State Planning Organization through the Advanced Technologies in Engineering project in Istanbul Technical University.
References [1] A.G. Von Matuschka, Boronizing, Heyden and Son Inc., Philadelphia, 1980. [2] A.K. Sinha, Heat Treat. 4 (1991) 437. [3] L. Segers, A. Fontana, R. Winand, Electrochim. Acta 36 (1991) 41. [4] K. Matiasovsky, M. Chrenkova-Paucirova, P. Fellner, M. Makyta, Surf. Coat. Technol. 35 (1988) 133. [5] V.N. Tkachev, P.K. Grigorov, B.B. Katkhanov, Met. Sci. Heat Treat. 17 (1975) 348. [6] S.H. Han, J.S. Chun, J. Mater. Sci. 15 (1980) 1379. [7] E. AtNk, U. Yunker, C. Meric¸, Tribol. Int. 36 (2003) 155. [8] T. Wierzchon, P. Bielinski, K. Sikorski, Surf. Coat. Technol. 73 (1995) 121.