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Enhancement of cavitation erosion resistance of 316 L stainless steel by adding molybdenum
Accepted Manuscript Enhancement of Cavitation Erosion Resistance of 316L Stainless Steel by Adding Molybdenum D.G. Li, D.R. Chen, P. Liang PII: DOI: Reference:
S1350-4177(16)30353-4 http://dx.doi.org/10.1016/j.ultsonch.2016.10.015 ULTSON 3399
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Ultrasonics Sonochemistry
Received Date: Revised Date: Accepted Date:
27 July 2016 14 October 2016 15 October 2016
Please cite this article as: D.G. Li, D.R. Chen, P. Liang, Enhancement of Cavitation Erosion Resistance of 316L Stainless Steel by Adding Molybdenum, Ultrasonics Sonochemistry (2016), doi: http://dx.doi.org/10.1016/ j.ultsonch.2016.10.015
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Enhancement of Cavitation Erosion Resistance of 316L Stainless Steel by Adding Molybdenum D. G. Li∗ , D.R. Chen and P. Liang State Key Laboratory of Tribology, Tsinghua University, Beijing,100084, China
Abstract: The influence of Mo on ultrasonic cavitation erosion of 316L stainless steel in 3.5% NaCl solution were investigated using an ultrasonic cavitation erosion (CE) facility. The morphologies of specimen after cavitation erosion were observed by scanning electron microscopy (SEM). The results showed that the addition of Mo can sharply decrease the mean depth of erosion (MDE) of 316 L SS, implying the increased resistance of cavitation erosion. In order to better understanding the influence of Mo on the cavitation erosion of 316L SS, the semi-conductive property of passive films on 316 L SS containing different concentrations of Mo were studied by Mott-Schottky plot. Based on Mott-Schottky results and semiconductor physics, a physical model was proposed to explain the effect mechanism of Mo on cavitation erosion of 316L SS. Keywords: 316 L stainless steel; Cavitation erosion; Molybdenum; Energy band bending
1. Introduction Cavitation erosion (CE) is a serious problem which widely occurs in hydraulic components such as pumps, valves, marine propellers, turbines and pipes. The cavitation erosion occurs by the combination of shock loading and fatigue processes incurred from the stress generated by the repeated growth and collapse of bubbles in a high micro-jet with a velocity of 130 m/s [1, 2]. In
∗
Corresponding author. Tel.:+86-10-6279-4946; fax: +86-10-6278-1379. E-mail address:[email protected] 1
such high velocity shock material surface suffer from a serious damage. To solve the cavitation erosion, many kinds of coatings including WC-10Co-4Cr coating [3], Fe-based amorphous coatings [4], oxy-fuel sprayed coatings [5], silver plated coating [6,7] and TiN or TiCN coatings manufactured by PVD method [8-11] are proposed. Other surface treatments such as friction stir processed (FSP) [12], laser gas nitriding [13, 14] and laser surface alloying [15] have also been developed to improve the resistance of cavitation erosion. Although coating and surface modification processes are verified to be effective to enhance the resistance of cavitation erosion. However, coatings or modified surface treatments can not suffer such high and repeating attack of the micro-jet, even the protection of the coatings or the modified surface on the substrate is unsatisfied due to delamination. Therefore, the most basic way to improve the cavitation erosion resistance is to alter the inherent physical and chemical properties of solid-liquid interface. Passivity is a common phenomenon widely existed on the surface of valve metals. An oxide film which is so called passive film can be automatically formed on valve metals in the existence of oxygen. Like titanium, niobium and other valve metals, the passive film can act as the ion barrier to impede the diffusion of the aggressive ions and cations within the passive film, and then the substrate is protected from further corrosion [16]. According to previous papers [17, 28], passivity plays an important role in the cavitation erosion process of the stainless steel. Our group [19, 20] also found that the cavitation erosion resistance of titanium increased with increasing the sample passivity. According to Hu [21], the resistance to cavitation erosion (CE) of Inconel 625 alloy is much superior to that of Inconel 600 alloy in tap water. In actual fact, the compactness of the passive film formed on Inconel625 alloy is much higher than that on Inconel 600 alloy, therefore, it can be concluded that the cavitation erosion resistance of the substrate may be related to the 2
electronic property of the passive film formed no matter on stainless steel and titanium or on nickel-based alloy. To form passive film, electrochemical anodization is a simplest, versatile and widely employed method, and except anodization, adding Mo into stainless steel can also form the more protective passive film on stainless steel [22-26], and how does this passive film affect the cavitation erosion resistance? However, up to now there are limited papers focused on this issue. Herewith, the objective of this work is to investigate the Mo effect on the cavitation erosion of 316 L SS in 3.5% NaCl solution using an ultrasonic cavitation erosion (CE) facility, scanning electron microscopy (SEM) and electrochemical technique.
2. Experimental 2.1 Sample preparation The samples are prepared by melting the mixture of pure Ni (99.9 wt %), pure Cr (99.9 wt %), pure Fe (99.9 wt %), pure Si (99.9 wt %), pure Mo (99.9 wt %) and pure carbon (99 wt %) in a vacuum electric furnace. The molten temperature is maintained at 1923K, after adequate stirring for 30min. When temperature drops to 1723K, the molten alloy is poured into a copper mould with water cooling in the electric furnace to form the blank rod (Φ200×300mm), then, the rode is homogenized for 120min at 1523K, hot rolling into 20-mm-thick plate. Finally, the thick plate is solution annealed at 1323K for 60min, air cooling. The samples are prepared by manufacturing the thick plate into a sample with a size showed in Fig.1. The compositions of samples, determined by chemical analysis, are listed in table1.
Here table 1
3
2.2 Cavitation erosion tests Cavitaion is produced by an ultrasonic cavitation erosion apparatus (see Fig.1) resonating at 20KHz with double amplitude of 60µm. The shape and dimension of the specimens are shown in Fig.2. All specimen surfaces are prepared by final grinding with 7000# grit abrasive paper. Specimens are degreased by immersion in acetone in an ultrasonic bath and are rinsed with distilled water, then dried and stored in a desiccator. During experiment, the bottom surface of tip is kept below solution level with a 20mm depth. 2.3 Mott-Schottky plot A conventional three-electrode system is used, and the counter electrode is a Pt wire. All of potentials are measured against a saturated calomel electrode (SCE). Mott-Schottky plot is performed on an EG＆G Model 273A potentiostat/galvanostat with an M5210 lock-in amplifier. The sweeping potential region is from -0.2 VSSCE to 1.6 VSCE with a 20mV/s scanning rate. In order to eliminate the capacitance dependence on the frequency, the frequency of 2 KHz is used as the applied frequency in this paper.
Here Fig.1 Here Fig.2
3. Results and Discussion 3.1 Mass lost of 316L SS in 3.5%NaCl solution The influence of Mo on the mass loss of 316L SS is showed in Fig.3a, it can be seen that there is an incubation period for all specimens during ultrasonic cavitation erosion. The incubation 4
period is defined as the initial stage with the zero or negligible cavitation erosion rate compared to later stage [27], and in this paper, the incubation time is determined to time when mass loss becomes 1mg. Therefore, it can be obtained that the incubation time of 316L SS increases from about 1min to 2.43min, 6.26min and 12min with increasing Mo from 2.5% to 4%, 6% and 8%, respectively. After incubation period, the mass loss obviously increases with erosion time, and the mass loss decreases significantly with increasing Mo at one fixed erosion time, implying the increased resistance of cavitation erosion of 316L SS with increasing Mo. In order to quantitively evaluate the influence of Mo on the resistance of cavitation erosion, the erosion loss of specimen is expressed in terms of the mean depth of erosion (MDE), which is defined as:
MDE ( µ m) =
∆m 1000 ρ A
[1]
where ∆m is the mass loss in mg, ρ is the density of the specimen, and A is the referenced area of specimen. Subsequently, MDE is calculated by using Eq. (1), and Fig.3b shows the change of MDE with erosion time. As shown in Fig.1b, MDE increases with increasing erosion time for each specimen. Specimen 1# has the maximum MDE comparing with other specimens at one fixed erosion time. The MDE of specimen 1# is 33.72 µm after 120 min erosion test, which increases about 4 times, 2 times and 1.33 times than that of specimen 4# (8.38 µm), specimen 3# (16.69 µm) and specimen 2# (25.27 µm), respectively. Comparing MDE with Mo concentration, it can be concluded that the resistance of cavitation erosion of 316L SS increases with increasing Mo.
Here Fig.3
5
3.2 Surface morphologies of specimens after cavitation erosion test Fig.4 shows morphologies of four specimens after cavitation erosion test for 5min, it can be observed from Fig.4a that the apparent undulations and pits appear on the surface of specimen 1#, the surface becomes coarse and the appearance of undulation is related to the local plastic deformation. In the incipient stage of erosion, that is the incubation period, the slip bands and plastic deformation in the form of pits occur, and the minimum mass is lost from the surface [28]. While for specimen 2#, the surface roughness caused by the undulations declines, and pits numbers sharply decrease, implying the alleviating of the plastic deformation. The undulations extent and pits numbers continuously decrease in the case of specimen 3#, the surface roughness continue to decrease, and only slight undulations phenomena can be observed in the surface, indicating the continuous enhanced resistance of cavitation erosion. Only one or several pits and no obvious undulations are observed on the surface of specimen 4#, and the surface roughness is almost the same as that of blank specimen, demonstrating the improved capability of anti-plastic deformation. According to the above discussions, it can be concluded that the resistance of cavitation erosion of specimen 4# is the maximum, and specimen 1# the minimum.
Here Fig.4 After cavitation erosion test for 20 min, the surface microstructures of four specimens greatly change, the undulations disappear and pits become bigger and deeper, and the surface roughness increases. As the erosion time increases, the level of plastic deformation increases, repeated impacts of high velocity microjet lead to an increase of stress level in the surface layer, to work-hardening of the surface layer and to the initiation of some micro-cracks. Later on, 6
micro-cracks progress into macro-cracks, they join together and finally they cause the spalling and the mass loss, which leads to the increased surface roughness. The great change in roughness can be frequently associated to the transition between the incubation stage and the maximum erosion rate stage [29]. Fig.5a shows that the intergranular cracking is found in the surface of specimen 1#, which is an indication of spalling and mass loss. While for specimen 2#, undulations are the predominant damage feature, and intergranular cracking is scarce. For specimen 3#, the undulations in width and height continue to decrease, and the roughness of the surface rapidly decreases. Especially for specimen 4#, the surface roughness further decreases, and there are large flat areas existing in surface which are not damaged by the microjet, implying the increased resistance of cavitation erosion. Figs. 6 and 7 show the morphologies of four specimens after cavitation erosion test for 60 min and 120 min, respectively. It can be seen that macro-cracks and micro-holes appear on the surfaces of four specimens, indicating the appearance of the accelerated period of cavitation erosion. The surface roughness in width and height of each specimen sharply increases with increasing cavitation erosion time, transgranular and intergranular fractures developed by heavy plastic deformation appear in the surface. While there are still some flat regions existing in the surface of specimen 4# even after cavitation erosion for 60 min, it implies that the cavitation erosion after testing for 60min is still not entering into the accelerated period. According to the above discussions and the chemical compositions of four specimens, it can be concluded that the resistance of cavitation erosion obeying the following order: specimen 4# > specimen 3# > specimen 2# > specimen 1#, that is the resistance of cavitation erosion for 316 L SS increases with the increment of Mo. 7
Here Fig.5 Here Fig.6 Here Fig.7
3.3 Model of the cavitation erosion of the passive metal in 3.5%NaCl solution The above SEM, mass loss and mass loss results show that Mo can increase the cavitation erosion resistance of 316L SS in 0.35%NaCl solution. The reason may be attributed to the energy band bending of passive films on 316L SS in 3.55NaCl solution. Generally, passive films on stainless steels are mainly composed of inner Cr-oxides and outer Fe-oxides [30-34]. The Cr-oxides have the p-type semi-conductive property, and the Fe-oxides appear the n-type semi-conductive property. As the Femi energy level of the inner Cr-oxides is higher than that of the outer Fe-oxides, the energy band of the inner Cr-oxides bend down, and the energy band of the outer Fe-oxides bend up between the inner film and outer film interface. Similarly, the energy band of the outer film bends up in the outer film/solution interface owing to the Femi energy difference between the outer film and solution. The schematic of the energy band bending is shown in Fig.8. The energy band bending leads to the movement of the positive charge to the outer film/solution interface. Accordingly, the Stern layer is inducted to accumulate the negative charge, and the higher band bending more negative charge accumulation within the Stern layer is. The quantitive illustration of the energy band bending can be evaluated by Efb, and more positive shifting of Efb higher bend bending is [35, 36]. Up to now, it is clarified that the energy band bending can lead to the accumulation of 8
negative charge in Stern layer. How does the energy band bending affect the cavitation erosion? One thing must be illustrated, that is there are plenty of micro-particles existing in the water (as shown in Fig.9), and these micro-particles have the negative charge (see Fig.10). When the micro-bubbles crack, the micro-jet can carry the micro-particles with the high velocity to attack the substrate surface. As the Stern layer is accumulated by negative charges, when micro-particles arrive at the film surface, like poles repel, and the repulsion between micro-particles and the accumulated negative charges is strong enough to make the micro-particles far away from the film surface, and then the substrate is protected. Furthermore, the more negative charges accumulation in the film/solution interface the more evident protection is. Fig.11 shows the Mott-Schottky plots of the passive films formed on four specimens in the 3.5%NaCl solution. It can be seen that all C- 2 ~ E plots can be divided into four regions based on the slopes. The slopes appear positive in the potential regions from 0 VSCE to 0.4 VSCE, implying n-type semiconductor character. The slopes become negative when the sweep potential falls into potential regionⅡ(0.4 VSCE ~ 0.8 VSCE), indicating p-type semiconductor character. When the sweeping potential region is in the range of 0.8 VSCE to 1.2 VSCE(region Ⅲ), the C- 2 ~ E plot reexhibits a positive slope, showing n-type semiconductor property. The appearances of positive and negative plots in all MS plots is related to the compositions of the passive film, in which the straight lines in regions Ⅰand Ⅲ imply the dominant defect in the passive films over these two potential regions are oxygen vacancies or metal interstitials, and the main defect in the passive films over regionⅡ are cation vacancies. Moreover, the slopes of straight lines in three potential regions (Ⅰ,Ⅱand Ⅲ)clearly increase with increasing Mo content, indicating the decreased donor and accept densities. Efb moves to positive direction with increasing Mo, meaning the higher band 9
bending. As noted above, more positive Efb meaning higher band bending and even to accumulate more negative charges in the outer film/solution interface, and then enhancing the repel strength to micro-jet. This is the reason why adding Mo can effectively increase the cavitation erosion resistance of 316L SS in NaCl solution.
Here Fig.8 Here Fig.9 Here Fig.10 Here Fig.11
4. Conclusion The cavitation erosion rate of 316 L SS in 3.5% NaCl solution increases with increasing erosion time, while the erosion rate evidently decreases with increasing Mo at each test time, which implying the enhanced resistance of cavitation erosion of 316L SS with Mo. SEM results show that the roughness of 316L SS after cavitation erosion test increases with increasing test time, but the roughness of specimen 4# in the case of every test time is the minimum, even there are large flat areas existed on the surface of specimen 4# after erosion for 20min.
Acknowledgment This work is financially supported by the National Nature Science Foundation of China (No.51305228).
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[31] M.F. Montemor, M.G.S. Ferreira, N.E. Hakiki, M.Da. Cunha Belo, Chemical composition and electronic structure of the oxide films formed on 316L stainless steel and nickel based alloys in high temperature aqueous environments, Corros. Sci. 42 (2000) 1635-1650. [32] M.G.S. Ferreira, J.L. Dawson, Eelectrichemical Studies of the Passive Film on 316 Stainless Steelin in Chloride Media. J. Electrochem. Soc. 132 (1985) 760-765. [33] N.E. Hakiki, M.Da. Cunha Belo, A.M.P. Smiões, M.G.S. Ferreira, Semiconducting properties of passive films formed on stainless steels: Influence of the alloying elements,
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Figure captions Fig.1 Shape and dimension of the specimen, a) lateral view; b) topview Fig.2 Schematic of cavitation erosion equipment, 1: water inlet; 2: cooling bath; 3: sound-proof enclosure; 4: transducer; 5: horn; 6: specimen; 7: beaker; 8: water outlet; 9: ultrasonic generator Fig.3 The dependences of mass loss and mean depth of erosion on test time for four specimens, a) Moss loss; b) mean depth of erosion Fig.4 Microstructures of specimens 1#, 2#, 3# and 4# after cavitation for 5min in 3.5%NaCl solution at ambient temperature, a) 1#, b) 2#, c) 3# and d) 4# Fig.5 Microstructures of specimens 1#, 2#, 3# and 4# after cavitation for 20min in 3.5%NaCl 14
solution at ambient temperature, a) 1#, b) 2#, c) 3# and d) 4# Fig.6 Microstructures of specimens 1#, 2#, 3# and 4# after cavitation for 60min in 3.5%NaCl solution at ambient temperature, a) 1#, b) 2#, c) 3# and d) 4# Fig.7 Microstructures of specimens 1#, 2#, 3# and 4# after cavitation for 120min in 3.5%NaCl solution at ambient temperature, a) 1#, b) 2#, c) 3# and d) 4# Fig.8 Schematic of energy bending of the passive films on stainless steels in aqueous solution Fig.9 SEM images of micro-particles contained in the tap water (a), distilled water (b) and ultra-pure water (c) Fig.10 Zeta potentilas of micro-particles contained in the tap water (a), distilled water (b) and ultra-pure water (c) Fig.11 Mott-Schottky plots of passive films on 316L SS containing various Mo
15
Tables
Table 1 The chemical compositions of 316 L SS containing Mo (wt %) Sample
C
S
P
Si
Mn
Mo
Ni
Cr
Fe
1#
0.02
0.015
0.03
0.8
1.5
2.5
12
17
66.135
2#
0.02
0.015
0.03
0.8
1.5
4
12
17
64.635
3#
0.02
0.015
0.03
0.8
1.5
6
12
17
62.635
4#
0.02
0.015
0.03
0.8
1.5
8
12
17
60.635
16
Figures
b a Fig.1
17
Fig.2
18
35
60 a Specimen Specimen Specimen Specimen
Mass loss / mg
50 40
1# 2# 3# 4#
b Specimen Specimen Specimen Specimen
30 25
1# 2# 3# 4#
MDE / µ
m20
30
15
20
10
10 0
5 0
20
40
60
80
Test time / min
100
120
140
0
0
20
40
60
80
100
Test time / min
120
140
Fig.3
19
a
b
c
d
Fig.4
20
Intergranular cracking a
b
c
d
Fig.5
21
a
b
c
d
Fig.6
22
a
b
c
d
Fig.7
23
Stern layer Double electric layer
Conduction band
Femi energy
Valence band
Efb
Substrate
Fig.8
24
a
c
b
Fig.9
25
4
Total counts
1 2 3
Mean Zeta potential =-23mV
4
4.0x10
4
3x10
1 2 3
4
4
Apparent Zeta potential / mV
4
0
0 -140-120-100-80-60-40-20 0 20 40 60 80100120140
Mean Zeta potential =-13.3mV
1 2 3
1x10
4
1x10
0.0
4
2x10
2x10
2.0x10
c
Mean Zeta potential =-18.6mV
Total counts
4
3x10 b
Total counts
a 6.0x10
4
4
4x10
8.0x10
-140-120-100-80-60-40-20 0 20 40 60 80100120140
Apparent Zeta potential / mV
-140-120-100-80-60-40-20 0 20 40 60 80100120140
Apparent Zeta potential / mV
Fig.10
26
35
4
20
-8
-2
10 C / F ⋅cm
25
-2
30
8Mo 6Mo 4Mo 2.5Mo
15 10 5 0
0.0
0.4
0.8
1.2
1.6
Potential / VSCE Fig.11
27
S1350-4177(16)30353-4 http://dx.doi.org/10.1016/j.ultsonch.2016.10.015 ULTSON 3399
To appear in:
Ultrasonics Sonochemistry
Received Date: Revised Date: Accepted Date:
27 July 2016 14 October 2016 15 October 2016
Please cite this article as: D.G. Li, D.R. Chen, P. Liang, Enhancement of Cavitation Erosion Resistance of 316L Stainless Steel by Adding Molybdenum, Ultrasonics Sonochemistry (2016), doi: http://dx.doi.org/10.1016/ j.ultsonch.2016.10.015
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.
Enhancement of Cavitation Erosion Resistance of 316L Stainless Steel by Adding Molybdenum D. G. Li∗ , D.R. Chen and P. Liang State Key Laboratory of Tribology, Tsinghua University, Beijing,100084, China
Abstract: The influence of Mo on ultrasonic cavitation erosion of 316L stainless steel in 3.5% NaCl solution were investigated using an ultrasonic cavitation erosion (CE) facility. The morphologies of specimen after cavitation erosion were observed by scanning electron microscopy (SEM). The results showed that the addition of Mo can sharply decrease the mean depth of erosion (MDE) of 316 L SS, implying the increased resistance of cavitation erosion. In order to better understanding the influence of Mo on the cavitation erosion of 316L SS, the semi-conductive property of passive films on 316 L SS containing different concentrations of Mo were studied by Mott-Schottky plot. Based on Mott-Schottky results and semiconductor physics, a physical model was proposed to explain the effect mechanism of Mo on cavitation erosion of 316L SS. Keywords: 316 L stainless steel; Cavitation erosion; Molybdenum; Energy band bending
1. Introduction Cavitation erosion (CE) is a serious problem which widely occurs in hydraulic components such as pumps, valves, marine propellers, turbines and pipes. The cavitation erosion occurs by the combination of shock loading and fatigue processes incurred from the stress generated by the repeated growth and collapse of bubbles in a high micro-jet with a velocity of 130 m/s [1, 2]. In
∗
Corresponding author. Tel.:+86-10-6279-4946; fax: +86-10-6278-1379. E-mail address:[email protected] 1
such high velocity shock material surface suffer from a serious damage. To solve the cavitation erosion, many kinds of coatings including WC-10Co-4Cr coating [3], Fe-based amorphous coatings [4], oxy-fuel sprayed coatings [5], silver plated coating [6,7] and TiN or TiCN coatings manufactured by PVD method [8-11] are proposed. Other surface treatments such as friction stir processed (FSP) [12], laser gas nitriding [13, 14] and laser surface alloying [15] have also been developed to improve the resistance of cavitation erosion. Although coating and surface modification processes are verified to be effective to enhance the resistance of cavitation erosion. However, coatings or modified surface treatments can not suffer such high and repeating attack of the micro-jet, even the protection of the coatings or the modified surface on the substrate is unsatisfied due to delamination. Therefore, the most basic way to improve the cavitation erosion resistance is to alter the inherent physical and chemical properties of solid-liquid interface. Passivity is a common phenomenon widely existed on the surface of valve metals. An oxide film which is so called passive film can be automatically formed on valve metals in the existence of oxygen. Like titanium, niobium and other valve metals, the passive film can act as the ion barrier to impede the diffusion of the aggressive ions and cations within the passive film, and then the substrate is protected from further corrosion [16]. According to previous papers [17, 28], passivity plays an important role in the cavitation erosion process of the stainless steel. Our group [19, 20] also found that the cavitation erosion resistance of titanium increased with increasing the sample passivity. According to Hu [21], the resistance to cavitation erosion (CE) of Inconel 625 alloy is much superior to that of Inconel 600 alloy in tap water. In actual fact, the compactness of the passive film formed on Inconel625 alloy is much higher than that on Inconel 600 alloy, therefore, it can be concluded that the cavitation erosion resistance of the substrate may be related to the 2
electronic property of the passive film formed no matter on stainless steel and titanium or on nickel-based alloy. To form passive film, electrochemical anodization is a simplest, versatile and widely employed method, and except anodization, adding Mo into stainless steel can also form the more protective passive film on stainless steel [22-26], and how does this passive film affect the cavitation erosion resistance? However, up to now there are limited papers focused on this issue. Herewith, the objective of this work is to investigate the Mo effect on the cavitation erosion of 316 L SS in 3.5% NaCl solution using an ultrasonic cavitation erosion (CE) facility, scanning electron microscopy (SEM) and electrochemical technique.
2. Experimental 2.1 Sample preparation The samples are prepared by melting the mixture of pure Ni (99.9 wt %), pure Cr (99.9 wt %), pure Fe (99.9 wt %), pure Si (99.9 wt %), pure Mo (99.9 wt %) and pure carbon (99 wt %) in a vacuum electric furnace. The molten temperature is maintained at 1923K, after adequate stirring for 30min. When temperature drops to 1723K, the molten alloy is poured into a copper mould with water cooling in the electric furnace to form the blank rod (Φ200×300mm), then, the rode is homogenized for 120min at 1523K, hot rolling into 20-mm-thick plate. Finally, the thick plate is solution annealed at 1323K for 60min, air cooling. The samples are prepared by manufacturing the thick plate into a sample with a size showed in Fig.1. The compositions of samples, determined by chemical analysis, are listed in table1.
Here table 1
3
2.2 Cavitation erosion tests Cavitaion is produced by an ultrasonic cavitation erosion apparatus (see Fig.1) resonating at 20KHz with double amplitude of 60µm. The shape and dimension of the specimens are shown in Fig.2. All specimen surfaces are prepared by final grinding with 7000# grit abrasive paper. Specimens are degreased by immersion in acetone in an ultrasonic bath and are rinsed with distilled water, then dried and stored in a desiccator. During experiment, the bottom surface of tip is kept below solution level with a 20mm depth. 2.3 Mott-Schottky plot A conventional three-electrode system is used, and the counter electrode is a Pt wire. All of potentials are measured against a saturated calomel electrode (SCE). Mott-Schottky plot is performed on an EG＆G Model 273A potentiostat/galvanostat with an M5210 lock-in amplifier. The sweeping potential region is from -0.2 VSSCE to 1.6 VSCE with a 20mV/s scanning rate. In order to eliminate the capacitance dependence on the frequency, the frequency of 2 KHz is used as the applied frequency in this paper.
Here Fig.1 Here Fig.2
3. Results and Discussion 3.1 Mass lost of 316L SS in 3.5%NaCl solution The influence of Mo on the mass loss of 316L SS is showed in Fig.3a, it can be seen that there is an incubation period for all specimens during ultrasonic cavitation erosion. The incubation 4
period is defined as the initial stage with the zero or negligible cavitation erosion rate compared to later stage [27], and in this paper, the incubation time is determined to time when mass loss becomes 1mg. Therefore, it can be obtained that the incubation time of 316L SS increases from about 1min to 2.43min, 6.26min and 12min with increasing Mo from 2.5% to 4%, 6% and 8%, respectively. After incubation period, the mass loss obviously increases with erosion time, and the mass loss decreases significantly with increasing Mo at one fixed erosion time, implying the increased resistance of cavitation erosion of 316L SS with increasing Mo. In order to quantitively evaluate the influence of Mo on the resistance of cavitation erosion, the erosion loss of specimen is expressed in terms of the mean depth of erosion (MDE), which is defined as:
MDE ( µ m) =
∆m 1000 ρ A
[1]
where ∆m is the mass loss in mg, ρ is the density of the specimen, and A is the referenced area of specimen. Subsequently, MDE is calculated by using Eq. (1), and Fig.3b shows the change of MDE with erosion time. As shown in Fig.1b, MDE increases with increasing erosion time for each specimen. Specimen 1# has the maximum MDE comparing with other specimens at one fixed erosion time. The MDE of specimen 1# is 33.72 µm after 120 min erosion test, which increases about 4 times, 2 times and 1.33 times than that of specimen 4# (8.38 µm), specimen 3# (16.69 µm) and specimen 2# (25.27 µm), respectively. Comparing MDE with Mo concentration, it can be concluded that the resistance of cavitation erosion of 316L SS increases with increasing Mo.
Here Fig.3
5
3.2 Surface morphologies of specimens after cavitation erosion test Fig.4 shows morphologies of four specimens after cavitation erosion test for 5min, it can be observed from Fig.4a that the apparent undulations and pits appear on the surface of specimen 1#, the surface becomes coarse and the appearance of undulation is related to the local plastic deformation. In the incipient stage of erosion, that is the incubation period, the slip bands and plastic deformation in the form of pits occur, and the minimum mass is lost from the surface [28]. While for specimen 2#, the surface roughness caused by the undulations declines, and pits numbers sharply decrease, implying the alleviating of the plastic deformation. The undulations extent and pits numbers continuously decrease in the case of specimen 3#, the surface roughness continue to decrease, and only slight undulations phenomena can be observed in the surface, indicating the continuous enhanced resistance of cavitation erosion. Only one or several pits and no obvious undulations are observed on the surface of specimen 4#, and the surface roughness is almost the same as that of blank specimen, demonstrating the improved capability of anti-plastic deformation. According to the above discussions, it can be concluded that the resistance of cavitation erosion of specimen 4# is the maximum, and specimen 1# the minimum.
Here Fig.4 After cavitation erosion test for 20 min, the surface microstructures of four specimens greatly change, the undulations disappear and pits become bigger and deeper, and the surface roughness increases. As the erosion time increases, the level of plastic deformation increases, repeated impacts of high velocity microjet lead to an increase of stress level in the surface layer, to work-hardening of the surface layer and to the initiation of some micro-cracks. Later on, 6
micro-cracks progress into macro-cracks, they join together and finally they cause the spalling and the mass loss, which leads to the increased surface roughness. The great change in roughness can be frequently associated to the transition between the incubation stage and the maximum erosion rate stage [29]. Fig.5a shows that the intergranular cracking is found in the surface of specimen 1#, which is an indication of spalling and mass loss. While for specimen 2#, undulations are the predominant damage feature, and intergranular cracking is scarce. For specimen 3#, the undulations in width and height continue to decrease, and the roughness of the surface rapidly decreases. Especially for specimen 4#, the surface roughness further decreases, and there are large flat areas existing in surface which are not damaged by the microjet, implying the increased resistance of cavitation erosion. Figs. 6 and 7 show the morphologies of four specimens after cavitation erosion test for 60 min and 120 min, respectively. It can be seen that macro-cracks and micro-holes appear on the surfaces of four specimens, indicating the appearance of the accelerated period of cavitation erosion. The surface roughness in width and height of each specimen sharply increases with increasing cavitation erosion time, transgranular and intergranular fractures developed by heavy plastic deformation appear in the surface. While there are still some flat regions existing in the surface of specimen 4# even after cavitation erosion for 60 min, it implies that the cavitation erosion after testing for 60min is still not entering into the accelerated period. According to the above discussions and the chemical compositions of four specimens, it can be concluded that the resistance of cavitation erosion obeying the following order: specimen 4# > specimen 3# > specimen 2# > specimen 1#, that is the resistance of cavitation erosion for 316 L SS increases with the increment of Mo. 7
Here Fig.5 Here Fig.6 Here Fig.7
3.3 Model of the cavitation erosion of the passive metal in 3.5%NaCl solution The above SEM, mass loss and mass loss results show that Mo can increase the cavitation erosion resistance of 316L SS in 0.35%NaCl solution. The reason may be attributed to the energy band bending of passive films on 316L SS in 3.55NaCl solution. Generally, passive films on stainless steels are mainly composed of inner Cr-oxides and outer Fe-oxides [30-34]. The Cr-oxides have the p-type semi-conductive property, and the Fe-oxides appear the n-type semi-conductive property. As the Femi energy level of the inner Cr-oxides is higher than that of the outer Fe-oxides, the energy band of the inner Cr-oxides bend down, and the energy band of the outer Fe-oxides bend up between the inner film and outer film interface. Similarly, the energy band of the outer film bends up in the outer film/solution interface owing to the Femi energy difference between the outer film and solution. The schematic of the energy band bending is shown in Fig.8. The energy band bending leads to the movement of the positive charge to the outer film/solution interface. Accordingly, the Stern layer is inducted to accumulate the negative charge, and the higher band bending more negative charge accumulation within the Stern layer is. The quantitive illustration of the energy band bending can be evaluated by Efb, and more positive shifting of Efb higher bend bending is [35, 36]. Up to now, it is clarified that the energy band bending can lead to the accumulation of 8
negative charge in Stern layer. How does the energy band bending affect the cavitation erosion? One thing must be illustrated, that is there are plenty of micro-particles existing in the water (as shown in Fig.9), and these micro-particles have the negative charge (see Fig.10). When the micro-bubbles crack, the micro-jet can carry the micro-particles with the high velocity to attack the substrate surface. As the Stern layer is accumulated by negative charges, when micro-particles arrive at the film surface, like poles repel, and the repulsion between micro-particles and the accumulated negative charges is strong enough to make the micro-particles far away from the film surface, and then the substrate is protected. Furthermore, the more negative charges accumulation in the film/solution interface the more evident protection is. Fig.11 shows the Mott-Schottky plots of the passive films formed on four specimens in the 3.5%NaCl solution. It can be seen that all C- 2 ~ E plots can be divided into four regions based on the slopes. The slopes appear positive in the potential regions from 0 VSCE to 0.4 VSCE, implying n-type semiconductor character. The slopes become negative when the sweep potential falls into potential regionⅡ(0.4 VSCE ~ 0.8 VSCE), indicating p-type semiconductor character. When the sweeping potential region is in the range of 0.8 VSCE to 1.2 VSCE(region Ⅲ), the C- 2 ~ E plot reexhibits a positive slope, showing n-type semiconductor property. The appearances of positive and negative plots in all MS plots is related to the compositions of the passive film, in which the straight lines in regions Ⅰand Ⅲ imply the dominant defect in the passive films over these two potential regions are oxygen vacancies or metal interstitials, and the main defect in the passive films over regionⅡ are cation vacancies. Moreover, the slopes of straight lines in three potential regions (Ⅰ,Ⅱand Ⅲ)clearly increase with increasing Mo content, indicating the decreased donor and accept densities. Efb moves to positive direction with increasing Mo, meaning the higher band 9
bending. As noted above, more positive Efb meaning higher band bending and even to accumulate more negative charges in the outer film/solution interface, and then enhancing the repel strength to micro-jet. This is the reason why adding Mo can effectively increase the cavitation erosion resistance of 316L SS in NaCl solution.
Here Fig.8 Here Fig.9 Here Fig.10 Here Fig.11
4. Conclusion The cavitation erosion rate of 316 L SS in 3.5% NaCl solution increases with increasing erosion time, while the erosion rate evidently decreases with increasing Mo at each test time, which implying the enhanced resistance of cavitation erosion of 316L SS with Mo. SEM results show that the roughness of 316L SS after cavitation erosion test increases with increasing test time, but the roughness of specimen 4# in the case of every test time is the minimum, even there are large flat areas existed on the surface of specimen 4# after erosion for 20min.
Acknowledgment This work is financially supported by the National Nature Science Foundation of China (No.51305228).
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[11] S. Münseterer, K. Kohlhof, Cavitation protection by low temperature TiCN coatings, Surf. Coat. Technol. 74-75 (1995) 642-647. [12] M. Hajian, A. Adbollah-Zadeh, S.S. Rezaei-Nejad, H. Assadi, S.M.M. Hadavi, K. Chung, M. Shokouhimehr, Improvement in cavitation erosion resistance of AISI 316L stainless steel by friction stir processing. Appl. Surf. Sci. 308 (2014) 184-192. [13] H.B. Li, Z.D. Cui, Z.Y. Li, S.L. Zhu, X.J. Yang, Surface modification by gas nitriding for improving cavitation erosion resistance of CP-Ti, Appl. Surf. Sci. 298 (2014) 164-170. [14] H.C. Man, Z.D. Cui, T.M. Yue, F.T. Cheng, Cavitation erosion behaviour of laser gas nitrided Ti and Ti6Al4V alloy, Mater. Sci. Eng. A 355 (2003) 167-173. [15] M. Duraiselvam, R. Galun, V. Wesling, B.L. Mordike, R. Reiter, J. Oligmuller, G. Buvanashekaran, Cvitation erosion resistance of Ti6Al4V laser alloyed with TiC-renforced dual phase intermetallic matrix composites, Mater. Sci. Eng. A 454-455 (2007) 63-68. [16] C.O.A. Olsson, D. Landolt, Passive films on stainless steels-chemistry, structure and growth, Electrochim. Acta 48 (2003) 1093-1104. [17] Y.G Zheng, S.Z Luo, W. Ke, Effect of passivity on electrochemical corrosion behavior of alloys during cavitation in aqueous solutions, Wear 262 (2007) 1308-1314. [18] Z. Li, J.S Han, J.J Lu, J.M Chen, Cavitation erosion behavior of Hastelloy C-276 nickel-based alloy, J. Alloys Compd. 619 (2015) 754-759. [19] D.G. Li, J.D. Wang, D.R. Chen, P. Liang, Ultrasonic cavitation erosion of Ti in 0.35% NaCl solution with bubbling oxygen and nitrogen, Ultrason. Sonochem. 26 (2015) 99-110. [20] D.G. Li, J.D. Wang, D.R. Chen, P. Liang, The role of passive potential in ultrasonic cavitation erosion of titanium in 1M HCl solution, Ultrason. Sonochem. 29 (2016) 279-287. 12
[21] H.X. Hu, Y.G. Zheng, C.P. Qin, Comparison of Inconel 625 and Inconel 600 in resistance to cavitation erosion and jet impingement erosion, Nucl. Eng. Des. 240 (2010) 2721-2730. [22] C.M. Abreu, M.J. Cristóbal, R. Losada, X.R. Nóvoa, G. Pena, M.C. Pérez, Comparative study of passive films of different stainless steels developed on alkaline medium, Electrochim. Acta 49 (2004) 3049-3056. [23] V. Vignal, J.M. Olive, D. Desjardins, Effect of molybdenum on passivity of stainlesssteelsin chloride media using ex situ near fieldmicroscopyobservations, Corros. Sci. 41 (1999) 869-884. [24] I. Olefjord, B. Brox, U. Jelvestam, Surface composition of stainless steels during anodic dissolution and passivation studied by ESCA, J. Electrochem. Soc. 132 (1985) 2854-2861. [25] J.W. Schultze, M.M. Lohrengel, D. Ross, Nucleation and growth of anodic oxide films, Electrochim. Acta 28 (1983) 973-984. [26] K. Sugimoto, Y. Sawada, The role of molybdenum additions to austenitic stainless steels in the inhibition of pitting in acid chloride solutions, Corros. Sci. 17 (1977) 425-445. [27] Y. Meged, Modeling of the initial stage in vibratory cavitation erosion tests by use of a Weibull distribution, Wear 253 (2002) 914-923. [28] ASTM Standard Test Method for Cavitation Erosion Using Vibratory Apparatus, Standard G32-09, 2009. [29] M. Pohl, J. Stella, Quantitative CLSM roughness study on early cavitation-erosion damage, Wear 252 (2002) 501-511. [30] D. Shintani, T. Ishida, H. Izumi, T. Fukutsuka, Y. Matsuo, Y. Sugie, XPS studies on passive film formed on stainless steel in a high-temperature and high-pressure methanol solution containing chloride ions. Corros. Sci. 50 (2008) 2840-2845. 13
[31] M.F. Montemor, M.G.S. Ferreira, N.E. Hakiki, M.Da. Cunha Belo, Chemical composition and electronic structure of the oxide films formed on 316L stainless steel and nickel based alloys in high temperature aqueous environments, Corros. Sci. 42 (2000) 1635-1650. [32] M.G.S. Ferreira, J.L. Dawson, Eelectrichemical Studies of the Passive Film on 316 Stainless Steelin in Chloride Media. J. Electrochem. Soc. 132 (1985) 760-765. [33] N.E. Hakiki, M.Da. Cunha Belo, A.M.P. Smiões, M.G.S. Ferreira, Semiconducting properties of passive films formed on stainless steels: Influence of the alloying elements,
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Soc. 145 (1998) 3821-3829. [34] L.V. Taveira, M.F. Montemor, M.Da Cunha Belo, M.G. Ferreira, L.F.P. Dick. Influence of incorporated Mo and Nb on the Mott–Schottky behaviour of anodic films formed on AISI 304L, Corros. Sci. 52 (2010) 2813-2818. [35] Y.H. Qian, Z.Z. Xu, Physics of semiconductor, Higher education press, Beijing, 1999, p.294. [36] LX. Ye, Semiconductor physics, 2nd ed., Higher education press, Beijing, 2007, p.307.
Figure captions Fig.1 Shape and dimension of the specimen, a) lateral view; b) topview Fig.2 Schematic of cavitation erosion equipment, 1: water inlet; 2: cooling bath; 3: sound-proof enclosure; 4: transducer; 5: horn; 6: specimen; 7: beaker; 8: water outlet; 9: ultrasonic generator Fig.3 The dependences of mass loss and mean depth of erosion on test time for four specimens, a) Moss loss; b) mean depth of erosion Fig.4 Microstructures of specimens 1#, 2#, 3# and 4# after cavitation for 5min in 3.5%NaCl solution at ambient temperature, a) 1#, b) 2#, c) 3# and d) 4# Fig.5 Microstructures of specimens 1#, 2#, 3# and 4# after cavitation for 20min in 3.5%NaCl 14
solution at ambient temperature, a) 1#, b) 2#, c) 3# and d) 4# Fig.6 Microstructures of specimens 1#, 2#, 3# and 4# after cavitation for 60min in 3.5%NaCl solution at ambient temperature, a) 1#, b) 2#, c) 3# and d) 4# Fig.7 Microstructures of specimens 1#, 2#, 3# and 4# after cavitation for 120min in 3.5%NaCl solution at ambient temperature, a) 1#, b) 2#, c) 3# and d) 4# Fig.8 Schematic of energy bending of the passive films on stainless steels in aqueous solution Fig.9 SEM images of micro-particles contained in the tap water (a), distilled water (b) and ultra-pure water (c) Fig.10 Zeta potentilas of micro-particles contained in the tap water (a), distilled water (b) and ultra-pure water (c) Fig.11 Mott-Schottky plots of passive films on 316L SS containing various Mo
15
Tables
Table 1 The chemical compositions of 316 L SS containing Mo (wt %) Sample
C
S
P
Si
Mn
Mo
Ni
Cr
Fe
1#
0.02
0.015
0.03
0.8
1.5
2.5
12
17
66.135
2#
0.02
0.015
0.03
0.8
1.5
4
12
17
64.635
3#
0.02
0.015
0.03
0.8
1.5
6
12
17
62.635
4#
0.02
0.015
0.03
0.8
1.5
8
12
17
60.635
16
Figures
b a Fig.1
17
Fig.2
18
35
60 a Specimen Specimen Specimen Specimen
Mass loss / mg
50 40
1# 2# 3# 4#
b Specimen Specimen Specimen Specimen
30 25
1# 2# 3# 4#
MDE / µ
m20
30
15
20
10
10 0
5 0
20
40
60
80
Test time / min
100
120
140
0
0
20
40
60
80
100
Test time / min
120
140
Fig.3
19
a
b
c
d
Fig.4
20
Intergranular cracking a
b
c
d
Fig.5
21
a
b
c
d
Fig.6
22
a
b
c
d
Fig.7
23
Stern layer Double electric layer
Conduction band
Femi energy
Valence band
Efb
Substrate
Fig.8
24
a
c
b
Fig.9
25
4
Total counts
1 2 3
Mean Zeta potential =-23mV
4
4.0x10
4
3x10
1 2 3
4
4
Apparent Zeta potential / mV
4
0
0 -140-120-100-80-60-40-20 0 20 40 60 80100120140
Mean Zeta potential =-13.3mV
1 2 3
1x10
4
1x10
0.0
4
2x10
2x10
2.0x10
c
Mean Zeta potential =-18.6mV
Total counts
4
3x10 b
Total counts
a 6.0x10
4
4
4x10
8.0x10
-140-120-100-80-60-40-20 0 20 40 60 80100120140
Apparent Zeta potential / mV
-140-120-100-80-60-40-20 0 20 40 60 80100120140
Apparent Zeta potential / mV
Fig.10
26
35
4
20
-8
-2
10 C / F ⋅cm
25
-2
30
8Mo 6Mo 4Mo 2.5Mo
15 10 5 0
0.0
0.4
0.8
1.2
1.6
Potential / VSCE Fig.11
27