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Corrosion behaviour of vacuum induction-melted Ni-based alloy in sulphuric acid
Corrosion Science 52 (2010) 2323–2330
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Corrosion Science journal homepage: www.elsevier.com/locate/corsci
Corrosion behaviour of vacuum induction-melted Ni-based alloy in sulphuric acid J.H. Chang a, J.M. Chou a,*, R.I. Hsieh b, J.L. Lee c a
Department of Materials Science and Engineering, University of I-SHOU, No. 1, Sec. 1, Syuecheng Rd., Dashu Township, Kaohsiung County 84001, Taiwan, ROC Steel and Aluminum R&D Department, China Steel Corporation, Taiwan, ROC c New Materials R&D Department, China Steel Corporation, Taiwan, ROC b
a r t i c l e
i n f o
Article history: Received 1 November 2009 Accepted 26 March 2010 Available online 31 March 2010 Keywords: A. Nickel B. SEM B. EPMA C. Acid corrosion C. Passive film
a b s t r a c t A vacuum induction-melted (VIM) Ni-based alloy was immersed in 60% H2SO4 solution to investigate its corrosion behaviour and resistance. The results indicate that the microstructure contains a c-Ni solid solution + Ni3Si particles, dendrite Ni3Si, Ni3B, Cr7C3, and CrB. The corrosion started at the zones of the c-Ni solid solution + Ni3Si particles and dendrite Ni3Si. These zones transformed to oxide films and protected the alloy from significant attack. However, the pitting corrosion created paths for acid solution and/or SO4 2 to further attack. Therefore, the corrosion rate decreased and then stabilised at a high value as the immersion time increased. Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction NiCrBSi alloy is frequently used in making hardfacing alloys and is utilised in industries requiring materials resistant to corrosion and wear. The addition of B and Si lowers the melting point of Ni-based alloys, and the addition of Cr can form chromium boride or chromium carbide, both of which increase the alloy’s resistance towards wear. Furthermore, the addition of Cr can also increase resistance towards corrosion [1,2]. Methods frequently used to construct Ni-based hardfacing alloys include laser cladding and high velocity oxygen fuel (HVOF). Researchers have begun to investigate NiCrBSi hardfacing alloys formed using these two methods and have discovered that structures like an c-Ni solid solution, c-Ni + Ni3(B,Si) interdendritic eutectic, borides, and carbides are included [1–5]. Due to the lack of discussion about the corrosion behaviour of NiCrBSi alloy, Nevill and Hodgkiess [6] conducted electrochemical and immersion tests on vacuum furnace-fused NiCrBSi coatings using seawater. These experimental results indicated that the predominant mechanism was crevice corrosion due to attack on the Ni-based matrix. Zhao et al. [7] investigated the electrochemical behaviour of a NiCrBSi alloy crafted using HVOF. The results revealed that the alloy is highly resistant to corrosion in 1 N NaOH. Comparisons made with results from experiments using 1 N H2SO4, 1 N HCl, and 3.5% NaCl showed that corrosion from by Cl is more severe than corrosion from SO4 2 . Porosity and inclusions play important roles in the corrosion behaviour. A large porosity causes the penetration of solu* Corresponding author. Tel.: +886 7 6577711x2201; fax: +886 7 6577469. E-mail address: [email protected] (J.M. Chou). 0010-938X/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.corsci.2010.03.026
tion, while large inclusions lead to local corrosion. The corrosion behaviour of NiCrBSi in 3.5% NaCl solution was further discussed, and the mechanism of corrosion was determined [8]. Many researchers have already conducted research on the corrosion resistance of non-NiCrBSi Ni-based alloys in H2SO4 [9–14]. Liu et al. [10] discovered that passive films form on the surface of Ni-based superalloys when reacting with 0.5 M NaCl + 0.05 M H2SO4. This effect decreases the adhesion of Cl and consequently reduces pitting corrosion. Scherer et al. [12] used scanning tunnelling microscopy (STM), surface X-ray scattering using synchrotron radiation, and electrochemical technology to research the passivation effect of Ni (1 1 1) in 0.05 M H2SO4 (pH 1.0). Lee and Min [13] investigated the corrosion properties of HVOF-sprayed Ni–Cr–W–Mo–B alloy coatings by using electrochemical and immersion tests in H2SO4 solutions. The results showed that the compositional difference led to a galvanic effect and localised corrosion occurred at the Ni matrix. The mechanism changed to general corrosion as the compositional difference decreased. Cho et al. [14] not only agreed with the above discussion, but also considered that the microcracks were an important factor influencing the corrosion resistance. In previous work [15], the corrosion behaviour of a VIM Co-based alloy that was immersed in 72% H2SO4 solution was discussed. The galvanic effect would improve the occurrence of pitting corrosion at the cobalt matrix/chromium-rich phase and cobalt matrix/tungsten-rich phase interfaces. Although the formation of a silicon oxide film reduced the corrosion rate, the acid solution and/or SO4 2 could still permeate where the silicon oxide film was not dense enough. The corrosion behaviour of nickel or its alloys should be extensively discussed. However, the microstructure of the VIM NiCrBSi
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Table 1 Chemical composition (in wt.%) of the NiCrBSi alloy powder. C 0.75
Si 4.3
Cr 14.8
B 3.1
Fe 3.7
Ni Bal.
alloy and its corrosion behaviour are rarely investigated. In this study, 60% H2SO4 was employed as an acid solution for immersion tests due to the corrosion product improved the corrosion resistance and its special growth orientation. By examining the microstructure of the VIM Ni-based alloy, the corroded surface, and the solution after the immersion test, the corrosion behaviour of the VIM Ni-based alloy immersed in 60% H2SO4 was defined. 2. Experimental procedures 2.1. Materials and vacuum induction melting process The NiCrBSi alloy chosen in this experiment had a melting point of 970.1 °C and was a spherical powder with an average diameter of roughly 70 lm. The chemical composition is shown in Table 1. According to analytical results from scanning electron microscopy (SEM), X-ray diffraction (XRD), and transmission electron microscopy (TEM), although there were two kinds of morphology after etching, both of them contained a layer structure containing a c-Ni solid solution + Ni31Si12, polygonal Cr2B and dendritic Cr7C3 (shown in Fig. 1). It was determined that the difference between forms was a result of the varied rate of cooling during fabrication of the powder. AISI 4140 steel was used as a substrate and was fashioned into a crucible to receive the Ni-based alloy powder (shown in Fig. 2). The crucible receiving the alloy powder was then placed into chambers with a vacuum degree of 102 Torr, and was heated using a high frequency induction device. When the internal temperature reached 1100 °C, the sample was cooled to 500 °C in vacuum chambers to avoid oxidation. The sample was then air cooled to accelerate the rate of cooling, and the VIM Ni-based alloy fused with AISI 4140 steel was formed.
boride, chromium carbide, and nickel silicide by SEM. An energy dispersive spectrometer (EDS) was used to determine the chemical compositions of each of the phases. In addition, TEM was employed to confirm the structure of the phases observed using EPMA and SEM, as well as to observe smaller extracts that were not previously seen. 2.3. Immersion test To understand the corrosion behaviour of the VIM Ni-based alloy in 60% H2SO4 solution, the AISI 4140 steel substrate was removed before the experiment. The VIM Ni-based alloy was then ground with diamond paste, polished with a suspension of 1 lm Al2O3, and then dehydrated at 80 °C for 24 h. After completing these steps, the mass (W1) and density (D) of the sample were measured at 25 and 50 °C. 60% H2SO4 solution was chosen as the acid, and the immersion time was set between 3 and 72 h. After testing, the corroded sample was dehydrated at 80 °C for 24 h, and the mass of the sample was measured (W2). An inductively coupled plasma-optical emission spectrometer (ICP-OES) was used to analyse the ions that were dissolved in the acid solution. The corrosion rate was calculated using the following equation [16].
corrosion rateðmm=yÞ ¼
87:6 DW tAD
ð1Þ
mm/y (mm per year): corrosion rate; DW (mg): W1 W2; t (h): immersion time; A (cm2): total surface area of sample; D (g/cm3): density of sample. 2.4. Characterisation of the VIM Ni-based alloy after immersion In order to identify possible changes in the microstructure and the presence of corrosion products, the sample was examined using XRD after corrosion. The working conditions were identical to those used before the immersion test. The corrosion surface as observed using SEM, and the composition was identified using EDS. 3. Results and discussion
2.2. Characterisation of the VIM Ni-based alloy before immersion
3.1. Microstructure of VIM Ni-based alloy before immersion
XRD was used to identify the phases present in the VIM Ni-based alloys with Cu Ka radiation. The scanning speed of the XRD was 1°/min, the scanning range was 20–90°, and the working voltage and current were 45 kV and 40 mA, respectively. X-ray mapping of the electron probe microanalyser (EPMA) was used to observe the distribution of elements in the microstructures. The microstructure etched by the etching solution (nitric acid:hydrochloric acid = 1:4) was employed to observe chromium
Fig. 3 shows the results of X-ray mapping of the VIM Ni-based alloys. The BEI image (Fig. 3a) shows that the microstructure of the VIM Ni-based alloys was separated into two bright gray zones, one gray zone, one dark gray zone, and one black zone. Looking at the spread of each element (shown in Fig. 3b–g), the dendrite bright gray zone contained primarily silicon, iron, and nickel; the other bright gray zone contained chromium, boron, and iron. The gray zone contained nickel, silicon, chromium, and iron. The dark
Fig. 1. The Ni-based alloy powder selected in this study shows different morphology after etching by a solution of weak aqua regia.
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Fig. 2. Schematic illustrations showing the standard AISI 4140 steel crucible and the states between the Ni-based alloy and AISI 4140 steel before and after the induction heating process.
Fig. 3. X-ray mapping of the VIMed Ni-based alloy, which was obtained at 1100 °C, showing (a) BEI morphology, and the dispersion of (b) B; (c) C; (d) Si; (e) Cr; (f) Fe; (g) Ni.
gray zone was formed by chromium and carbon, and the black zone was formed by mostly chromium and boron. The etched microstructure of the VIM Ni-based alloy shown in Fig. 4 shows
that the numerous particles were observed in the gray zone. Results from EDS measurements indicate that these particles are rich in nickel and silicon. According to the XRD results, the
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Fig. 4. The etched microstructure of the VIMed Ni-based alloy.
microstructure of the VIM Ni-based alloy is primarily composed of c-Ni, Ni3B, Ni3Si, Cr7C3, and CrB. Therefore, the above results suggest that the dendrite bright gray zone is Ni3Si and that the gray zone consists of c-Ni solid solution and Ni3Si particles. In addition, the dark gray zone is a Cr7C3 type of chromium carbide, and the black zone is a CrB type of chromium boride.
The bright field TEM images and the results of the selected-area diffraction shown in Figs. 5 and 6 were carried out in many areas of the specimen and compared with the SEM observations. Fig. 5a shows that Ni3Si particles are distributed in a c-Ni solid solution. In addition, both the Ni3Si particles and c-Ni solid solutions are face-centred cubes and have similar lattice constants. High-resolution imaging (Fig. 5b) confirmed that, although in SEM there appeared to be no Ni3Si particles with the c-Ni solid solution, smaller Ni3Si particles exist with diameters under 20 nm. In addition, in some areas, there were either blurry lattice lines or no apparent lattice lines, but this result does not prove that these areas are not crystalline. It is highly probable that these images are influenced by the magnetism of nickel. The selected-area diffraction of c-Ni and Ni3Si in Fig. 5b is a good example of magnetic influence. Fig. 6 shows the bright field image and selected-area diffraction of Cr7C3 and CrB. The experimental results indicate that the structures of Cr7C3 and CrB are both orthorhombic. The lattice constants of Cr7C3 are a = 4.526 Å, b = 7.010 Å, and c = 12.142 Å, and the lattice constants of CrB are a = 2.966 Å, b = 7.866 Å, and c = 2.932 Å. Therefore, the results obtained from TEM further indicate that the results from the SEM and XRD are accurate. On the other hand, Takasugi et al. [17,18] discovered that Ni3Si and Ni3Al have similar structures. In addition, because Ni3Si can easily form an oxidation layer that is resistant to corrosion and oxidation, it can provide these qualities to materials to which Ni3Si is applied. Grosdider et al. [19] showed the transition mechanism of
Fig. 5. The (a) bright field and (b) high resolution images of c-Ni + Ni3Si.
Fig. 6. The bright field images of (a) Cr7C3 and (b) CrB.
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the corrosion rate started out at 5.97 mm/y, decreased to 2.98 mm and then stabilised at 2.988 ± 0.13 mm/y. Combining the above results, it was deduced that the VIM Ni-based alloy immersed in 25 °C H2SO4 is more resistant to corrosion than when immersed in 50 °C H2SO4. 3.3. Corroded surface
Fig. 7. The corrosion rate of the VIMed Ni-based alloy immersed in 60% H2SO4 at different temperatures for various immersion times.
the shape of c0 (Ni3Al) and confirmed the shape of the c0 (Ni3Al) transition from sphere, cube, ogdoadically diced cube and then to dendrite. A higher rate of cooling gives a higher chance of forming spherical c0 (Ni3Al), and slower cooling often results in dendrites. The above discussion confirms that different shapes formed by Ni3Si are caused by different rates of cooling. 3.2. Corrosion rate Fig. 7 shows the relationship between the immersion time and the corrosion rate for the VIM Ni-based alloys immersed in 60% H2SO4 at different temperatures. It was observed that the corrosion rate was roughly 1.46 mm/y for the sample immersed in 25 °C H2SO4. When the immersion time reached 12 h, the corrosion rate decreased to 0.25 mm/y. When the immersion time reached 24 h and then 72 h, the corrosion rate first increased to 0.60 mm/y and then eventually remained constant at 0.55 ± 0.10 mm/y. On the other hand, when the alloy was immersed in 50 °C H2SO4,
a
Fig. 8 shows the corroded surfaces and the EDS analysis results of the VIM Ni-based alloys immersed in 25 °C 60% H2SO4 solution for 3 h (Fig. 8a) and in 50 °C 60% H2SO4 solution for 0.5 h (Fig. 8b). As shown in Fig. 8a, only the c-Ni solid solution + Ni3Si particles and dendrite Ni3Si are corroded in the 25 °C H2SO4 solution. By combining the above result with the EDS results from point 1 and point 2, it is confirmed that both of these zones have traces of oxidation. Based on all of these results, it is concluded that both of these zones are not resistant to corrosion in a 60% H2SO4 solution. However, literature sources [20–22] mention that oxides of Ni3Si suppress both corrosion and further oxidation. Based on the EDS results, it is shown that the corrosion rate decrease is due to the formation of oxides. A corroded surface similar to the surface indicated above (Fig. 8b) also displays those properties. According to the four EDS analysis results, it can be suggested that after being immersed in 50 °C 60% H2SO4 for 0.5 h, the c-Ni solid solution + Ni3Si particles and dendrite Ni3Si have higher degrees of oxidation than when they are immersed in 25 °C 60% H2SO4 for 3 h. This result is especially apparent in the dendrite Ni3Si, which transformed to oxide films. A comparison of Fig. 8a and b indicate that increasing the temperature of the solutions increases the corrosion rate, which matches the result in Fig. 7. Fig. 9a indicates that after immersing the VIM Ni-based alloys in 60% H2SO4 solution for 12 h at 25 °C, the c-Ni solid solution + Ni3Si particles and dendrite Ni3Si found on the corroded surfaces had all converted into oxide films. However, the transformation behaviours were different. For the c-Ni solid solution + Ni3Si particles, Ni3Si particles first oxidise and then convert to oxide films following certain orientations (Fig. 9b). There are no specific rules indicating how dendrite Ni3Si oxidises (Fig. 8). As the duration of immersion increased, the density of the oxide films also increased. Using 60% H2SO4 solution at 25 °C as an example, the oxide film
b 4 1 3 2
Ni3Si particles O
Si
Cr
Fe
Ni
1 2
2.84 7.15
3.68 9.82
3.75 -
7.35 1.25
49.84 45.63
3 4
4.84 11.99
7.62 14.19
2.72 -
5.49 -
67.26 58.25
Fig. 8. The corroded surface morphology of the VIM Ni-based alloy immersed in a solution of 60% H2SO4 for (a) 3 h at 25 °C and (b) 0.5 h at 50 °C.
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Fig. 9. The corroded surface morphology of the VIM Ni-based alloy immersed in a solution of 60% H2SO4 for (a) 12 h at 25 °C and (b) the magnification of (a).
Fig. 10. The corroded surface morphology of the VIM Ni-based alloy immersed in a solution of 60% H2SO4 for (a) 72 h at 25 °C and (b) 12 h at 50 °C.
formed after immersing for 72 h (Fig. 10a) was denser than films formed after immersing for 24 and 48 h. For films immersed in 60% H2SO4 solution at 50 °C, it only took 12 h to form oxide films (Fig. 10b) similar to the ones found in Fig. 10a. Therefore, these results support the idea that an increase in solution temperature will decrease the time required for corrosion to take place. In addition, the corrosion rate decreased before 12 h and remained stable after 12 h, due to possible influence from the oxide films.
3.4. ICP-OES analysis Table 2 shows the results of ICP-OES analysis of each of the solutions after the immersion experiments. Every element contained in the VIM Ni-based alloys aside from carbon was detected and measured in the solutions. However, the concentration of silicon ions did not increase with immersion time. This result confirms two ideas. First of all, although the corrosion of Ni3B and CrB did not seem to take place when observing the corroded surfaces, both of these two phases were still attacked. Based on the results from X-ray mapping (Fig. 3) and under the premise that only Ni3B and CrB contain boron, the presence of boron ions in solution affirms that at least one of these two phases had been attacked by the solutions. In addition, because the wavelength of carbon ions is close to visible light, they cannot be measured by the instruments used here. As a result, we could not determine whether Cr7C3 had been corroded. Based on Figs. 3 and 8, only the c-Ni solid solution + Ni3Si particles and dendrite Ni3Si contain silicon. After a short period of time, both of these zones were readily corroded. However, after the oxide films were formed, the corrosion rate
Table 2 The ion concentrations (mg/L) of the solutions after testing. Si
Cr
B
Fe
Ni
25 °C 12 24 48 72
0.1 0.1 0.1 0.1
0.8 2.0 4.0 6.0
1.3 1.5 1.9 2.1
0.7 1.2 1.8 2.3
6.9 13.0 20.3 27.0
50 °C 12 24 48 72
0.1 0.2 0.2 0.2
7.0 11.4 26.1 43.4
2.2 2.8 5.2 8.0
2.6 3.3 7.8 12.0
28.3 39.9 92.4 143.1
did not increase as the immersion time increased. These observations indicate that when these two zones are corroded, they enter the solution with chromium. However, after the oxides are formed and silicon is deposited on the corroded surface and because the oxide cannot be further corroded, silicon did not return to the solution. This result explains why the concentration of silicon did not increase when the alloy was immersed for extended periods of time. 3.5. Oxides The results described above show that the oxides formed by Ni3Si play important roles in affecting the corrosion of VIM Ni-based alloys in a 60% H2SO4 solution. Based on the JCPDS Card database and the XRD diffraction figure (Fig. 11) of the VIM
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4. Conclusions It was confirmed through XRD, X-ray mapping, and TEM that VIM Ni-based alloys contain microstructures such as c-Ni solid solution + Ni3Si particles and dendrite Ni3Si, Ni3B, Cr7C3, and CrB. When immersed in 60% H2SO4, the alloy displayed a corrosion rate that decreases with time, but eventually stabilises. However, the resistance to corrosion was higher in a 25 °C solution than in a 50 °C solution. The c-Ni solid solution + Ni3Si particles and dendrite Ni3Si were the first zones to be corroded in the microstructure. At the same time, the oxides formed by these two zones play very important roles in leading the corrosion of the entire VIM Ni-based alloy. The category of the oxide cannot be accurately determined at the moment, but its presence effectively lowers the corrosion rate and can preserve itself for long periods of time. This result confirms the fact that this oxide has a very high resistance towards corrosion in a 60% H2SO4 solution. Compared with such a significant phenomenon, pitting corrosion plays an inconspicuous and balanced role in corrosion. The pits provide infiltrating paths for acid solution and/or SO4 2 to cause further attack, and balance the decreasing corrosion rate caused by the oxide films. Acknowledgement The authors gratefully acknowledge support of the China Steel Corporation of Taiwan. Fig. 11. The XRD diffraction patterns of the VIMed Ni-based alloy immersed in a solution of 60% H2SO4 for various times at 25 °C. Phases such as c-Ni, Ni3B, Ni3Si, Cr7C3, and CrB are omitted, and only the possible oxides are evident.
Ni-based alloys immersed in 60% H2SO4 for different periods of time, the possible oxides are Ni2O3, NiO, SiO2, and Ni2SiO4. Many researchers also focus on oxides formed by Ni3Si [20–22] and have indicated that the resistance of corrosion and oxidation come from the formation of NiO and SiO2, while some researchers [21] credit that resistance to the formation of nickel silicate. In order to confirm the identity of the oxide formed by Ni3Si in this experiment, other techniques, such as X-ray photoelectron spectroscopy (XPS), are needed. However, we still suggest that oxides formed by Ni3Si have a high resistance towards corrosion and oxidation. Therefore, the corrosion rate of VIM Ni-based alloys immersed in 60% H2SO4 is most likely attributed to oxides of Ni3Si.
3.6. Corrosion behaviour The above discussions confirm that a passive film improves the corrosion resistance. However, it is only conjecture to say that the VIM Ni-based alloy reaches a passive state due to the steady state corrosion rates of 0.55 ± 0.10 mm/y at 25 °C and 2.98 mm/ y ± 0.13 mm/y at 50 °C. Therefore, other factors must affect the corrosion rate. When comparing Fig. 4 with Fig. 8, a few significant pits occur at the interfaces between the zone of the c-Ni solid solution + Ni3Si particles and other phases, i.e., dendrite Ni3Si, Ni3B, Cr7C3, and CrB, after a short immersion time. When comparing Fig. 8 with Fig. 10, it can be seen that the other corrosion mechanism is pitting corrosion due to the amount of pit increasing with the increased immersion time. A previous work [15] indicated that acid solution and/or SO4 2 would infiltrate into these pits and cause an increase of corrosion depth. Other previous reports [7,14] also had similar results concerning the infiltration of acid solution. Therefore, the corrosion rate stabilised at such a high state should be due to the effect between pitting corrosion and oxide films reaching a balance.
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Corrosion Science journal homepage: www.elsevier.com/locate/corsci
Corrosion behaviour of vacuum induction-melted Ni-based alloy in sulphuric acid J.H. Chang a, J.M. Chou a,*, R.I. Hsieh b, J.L. Lee c a
Department of Materials Science and Engineering, University of I-SHOU, No. 1, Sec. 1, Syuecheng Rd., Dashu Township, Kaohsiung County 84001, Taiwan, ROC Steel and Aluminum R&D Department, China Steel Corporation, Taiwan, ROC c New Materials R&D Department, China Steel Corporation, Taiwan, ROC b
a r t i c l e
i n f o
Article history: Received 1 November 2009 Accepted 26 March 2010 Available online 31 March 2010 Keywords: A. Nickel B. SEM B. EPMA C. Acid corrosion C. Passive film
a b s t r a c t A vacuum induction-melted (VIM) Ni-based alloy was immersed in 60% H2SO4 solution to investigate its corrosion behaviour and resistance. The results indicate that the microstructure contains a c-Ni solid solution + Ni3Si particles, dendrite Ni3Si, Ni3B, Cr7C3, and CrB. The corrosion started at the zones of the c-Ni solid solution + Ni3Si particles and dendrite Ni3Si. These zones transformed to oxide films and protected the alloy from significant attack. However, the pitting corrosion created paths for acid solution and/or SO4 2 to further attack. Therefore, the corrosion rate decreased and then stabilised at a high value as the immersion time increased. Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction NiCrBSi alloy is frequently used in making hardfacing alloys and is utilised in industries requiring materials resistant to corrosion and wear. The addition of B and Si lowers the melting point of Ni-based alloys, and the addition of Cr can form chromium boride or chromium carbide, both of which increase the alloy’s resistance towards wear. Furthermore, the addition of Cr can also increase resistance towards corrosion [1,2]. Methods frequently used to construct Ni-based hardfacing alloys include laser cladding and high velocity oxygen fuel (HVOF). Researchers have begun to investigate NiCrBSi hardfacing alloys formed using these two methods and have discovered that structures like an c-Ni solid solution, c-Ni + Ni3(B,Si) interdendritic eutectic, borides, and carbides are included [1–5]. Due to the lack of discussion about the corrosion behaviour of NiCrBSi alloy, Nevill and Hodgkiess [6] conducted electrochemical and immersion tests on vacuum furnace-fused NiCrBSi coatings using seawater. These experimental results indicated that the predominant mechanism was crevice corrosion due to attack on the Ni-based matrix. Zhao et al. [7] investigated the electrochemical behaviour of a NiCrBSi alloy crafted using HVOF. The results revealed that the alloy is highly resistant to corrosion in 1 N NaOH. Comparisons made with results from experiments using 1 N H2SO4, 1 N HCl, and 3.5% NaCl showed that corrosion from by Cl is more severe than corrosion from SO4 2 . Porosity and inclusions play important roles in the corrosion behaviour. A large porosity causes the penetration of solu* Corresponding author. Tel.: +886 7 6577711x2201; fax: +886 7 6577469. E-mail address: [email protected] (J.M. Chou). 0010-938X/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.corsci.2010.03.026
tion, while large inclusions lead to local corrosion. The corrosion behaviour of NiCrBSi in 3.5% NaCl solution was further discussed, and the mechanism of corrosion was determined [8]. Many researchers have already conducted research on the corrosion resistance of non-NiCrBSi Ni-based alloys in H2SO4 [9–14]. Liu et al. [10] discovered that passive films form on the surface of Ni-based superalloys when reacting with 0.5 M NaCl + 0.05 M H2SO4. This effect decreases the adhesion of Cl and consequently reduces pitting corrosion. Scherer et al. [12] used scanning tunnelling microscopy (STM), surface X-ray scattering using synchrotron radiation, and electrochemical technology to research the passivation effect of Ni (1 1 1) in 0.05 M H2SO4 (pH 1.0). Lee and Min [13] investigated the corrosion properties of HVOF-sprayed Ni–Cr–W–Mo–B alloy coatings by using electrochemical and immersion tests in H2SO4 solutions. The results showed that the compositional difference led to a galvanic effect and localised corrosion occurred at the Ni matrix. The mechanism changed to general corrosion as the compositional difference decreased. Cho et al. [14] not only agreed with the above discussion, but also considered that the microcracks were an important factor influencing the corrosion resistance. In previous work [15], the corrosion behaviour of a VIM Co-based alloy that was immersed in 72% H2SO4 solution was discussed. The galvanic effect would improve the occurrence of pitting corrosion at the cobalt matrix/chromium-rich phase and cobalt matrix/tungsten-rich phase interfaces. Although the formation of a silicon oxide film reduced the corrosion rate, the acid solution and/or SO4 2 could still permeate where the silicon oxide film was not dense enough. The corrosion behaviour of nickel or its alloys should be extensively discussed. However, the microstructure of the VIM NiCrBSi
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Table 1 Chemical composition (in wt.%) of the NiCrBSi alloy powder. C 0.75
Si 4.3
Cr 14.8
B 3.1
Fe 3.7
Ni Bal.
alloy and its corrosion behaviour are rarely investigated. In this study, 60% H2SO4 was employed as an acid solution for immersion tests due to the corrosion product improved the corrosion resistance and its special growth orientation. By examining the microstructure of the VIM Ni-based alloy, the corroded surface, and the solution after the immersion test, the corrosion behaviour of the VIM Ni-based alloy immersed in 60% H2SO4 was defined. 2. Experimental procedures 2.1. Materials and vacuum induction melting process The NiCrBSi alloy chosen in this experiment had a melting point of 970.1 °C and was a spherical powder with an average diameter of roughly 70 lm. The chemical composition is shown in Table 1. According to analytical results from scanning electron microscopy (SEM), X-ray diffraction (XRD), and transmission electron microscopy (TEM), although there were two kinds of morphology after etching, both of them contained a layer structure containing a c-Ni solid solution + Ni31Si12, polygonal Cr2B and dendritic Cr7C3 (shown in Fig. 1). It was determined that the difference between forms was a result of the varied rate of cooling during fabrication of the powder. AISI 4140 steel was used as a substrate and was fashioned into a crucible to receive the Ni-based alloy powder (shown in Fig. 2). The crucible receiving the alloy powder was then placed into chambers with a vacuum degree of 102 Torr, and was heated using a high frequency induction device. When the internal temperature reached 1100 °C, the sample was cooled to 500 °C in vacuum chambers to avoid oxidation. The sample was then air cooled to accelerate the rate of cooling, and the VIM Ni-based alloy fused with AISI 4140 steel was formed.
boride, chromium carbide, and nickel silicide by SEM. An energy dispersive spectrometer (EDS) was used to determine the chemical compositions of each of the phases. In addition, TEM was employed to confirm the structure of the phases observed using EPMA and SEM, as well as to observe smaller extracts that were not previously seen. 2.3. Immersion test To understand the corrosion behaviour of the VIM Ni-based alloy in 60% H2SO4 solution, the AISI 4140 steel substrate was removed before the experiment. The VIM Ni-based alloy was then ground with diamond paste, polished with a suspension of 1 lm Al2O3, and then dehydrated at 80 °C for 24 h. After completing these steps, the mass (W1) and density (D) of the sample were measured at 25 and 50 °C. 60% H2SO4 solution was chosen as the acid, and the immersion time was set between 3 and 72 h. After testing, the corroded sample was dehydrated at 80 °C for 24 h, and the mass of the sample was measured (W2). An inductively coupled plasma-optical emission spectrometer (ICP-OES) was used to analyse the ions that were dissolved in the acid solution. The corrosion rate was calculated using the following equation [16].
corrosion rateðmm=yÞ ¼
87:6 DW tAD
ð1Þ
mm/y (mm per year): corrosion rate; DW (mg): W1 W2; t (h): immersion time; A (cm2): total surface area of sample; D (g/cm3): density of sample. 2.4. Characterisation of the VIM Ni-based alloy after immersion In order to identify possible changes in the microstructure and the presence of corrosion products, the sample was examined using XRD after corrosion. The working conditions were identical to those used before the immersion test. The corrosion surface as observed using SEM, and the composition was identified using EDS. 3. Results and discussion
2.2. Characterisation of the VIM Ni-based alloy before immersion
3.1. Microstructure of VIM Ni-based alloy before immersion
XRD was used to identify the phases present in the VIM Ni-based alloys with Cu Ka radiation. The scanning speed of the XRD was 1°/min, the scanning range was 20–90°, and the working voltage and current were 45 kV and 40 mA, respectively. X-ray mapping of the electron probe microanalyser (EPMA) was used to observe the distribution of elements in the microstructures. The microstructure etched by the etching solution (nitric acid:hydrochloric acid = 1:4) was employed to observe chromium
Fig. 3 shows the results of X-ray mapping of the VIM Ni-based alloys. The BEI image (Fig. 3a) shows that the microstructure of the VIM Ni-based alloys was separated into two bright gray zones, one gray zone, one dark gray zone, and one black zone. Looking at the spread of each element (shown in Fig. 3b–g), the dendrite bright gray zone contained primarily silicon, iron, and nickel; the other bright gray zone contained chromium, boron, and iron. The gray zone contained nickel, silicon, chromium, and iron. The dark
Fig. 1. The Ni-based alloy powder selected in this study shows different morphology after etching by a solution of weak aqua regia.
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Fig. 2. Schematic illustrations showing the standard AISI 4140 steel crucible and the states between the Ni-based alloy and AISI 4140 steel before and after the induction heating process.
Fig. 3. X-ray mapping of the VIMed Ni-based alloy, which was obtained at 1100 °C, showing (a) BEI morphology, and the dispersion of (b) B; (c) C; (d) Si; (e) Cr; (f) Fe; (g) Ni.
gray zone was formed by chromium and carbon, and the black zone was formed by mostly chromium and boron. The etched microstructure of the VIM Ni-based alloy shown in Fig. 4 shows
that the numerous particles were observed in the gray zone. Results from EDS measurements indicate that these particles are rich in nickel and silicon. According to the XRD results, the
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Fig. 4. The etched microstructure of the VIMed Ni-based alloy.
microstructure of the VIM Ni-based alloy is primarily composed of c-Ni, Ni3B, Ni3Si, Cr7C3, and CrB. Therefore, the above results suggest that the dendrite bright gray zone is Ni3Si and that the gray zone consists of c-Ni solid solution and Ni3Si particles. In addition, the dark gray zone is a Cr7C3 type of chromium carbide, and the black zone is a CrB type of chromium boride.
The bright field TEM images and the results of the selected-area diffraction shown in Figs. 5 and 6 were carried out in many areas of the specimen and compared with the SEM observations. Fig. 5a shows that Ni3Si particles are distributed in a c-Ni solid solution. In addition, both the Ni3Si particles and c-Ni solid solutions are face-centred cubes and have similar lattice constants. High-resolution imaging (Fig. 5b) confirmed that, although in SEM there appeared to be no Ni3Si particles with the c-Ni solid solution, smaller Ni3Si particles exist with diameters under 20 nm. In addition, in some areas, there were either blurry lattice lines or no apparent lattice lines, but this result does not prove that these areas are not crystalline. It is highly probable that these images are influenced by the magnetism of nickel. The selected-area diffraction of c-Ni and Ni3Si in Fig. 5b is a good example of magnetic influence. Fig. 6 shows the bright field image and selected-area diffraction of Cr7C3 and CrB. The experimental results indicate that the structures of Cr7C3 and CrB are both orthorhombic. The lattice constants of Cr7C3 are a = 4.526 Å, b = 7.010 Å, and c = 12.142 Å, and the lattice constants of CrB are a = 2.966 Å, b = 7.866 Å, and c = 2.932 Å. Therefore, the results obtained from TEM further indicate that the results from the SEM and XRD are accurate. On the other hand, Takasugi et al. [17,18] discovered that Ni3Si and Ni3Al have similar structures. In addition, because Ni3Si can easily form an oxidation layer that is resistant to corrosion and oxidation, it can provide these qualities to materials to which Ni3Si is applied. Grosdider et al. [19] showed the transition mechanism of
Fig. 5. The (a) bright field and (b) high resolution images of c-Ni + Ni3Si.
Fig. 6. The bright field images of (a) Cr7C3 and (b) CrB.
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the corrosion rate started out at 5.97 mm/y, decreased to 2.98 mm and then stabilised at 2.988 ± 0.13 mm/y. Combining the above results, it was deduced that the VIM Ni-based alloy immersed in 25 °C H2SO4 is more resistant to corrosion than when immersed in 50 °C H2SO4. 3.3. Corroded surface
Fig. 7. The corrosion rate of the VIMed Ni-based alloy immersed in 60% H2SO4 at different temperatures for various immersion times.
the shape of c0 (Ni3Al) and confirmed the shape of the c0 (Ni3Al) transition from sphere, cube, ogdoadically diced cube and then to dendrite. A higher rate of cooling gives a higher chance of forming spherical c0 (Ni3Al), and slower cooling often results in dendrites. The above discussion confirms that different shapes formed by Ni3Si are caused by different rates of cooling. 3.2. Corrosion rate Fig. 7 shows the relationship between the immersion time and the corrosion rate for the VIM Ni-based alloys immersed in 60% H2SO4 at different temperatures. It was observed that the corrosion rate was roughly 1.46 mm/y for the sample immersed in 25 °C H2SO4. When the immersion time reached 12 h, the corrosion rate decreased to 0.25 mm/y. When the immersion time reached 24 h and then 72 h, the corrosion rate first increased to 0.60 mm/y and then eventually remained constant at 0.55 ± 0.10 mm/y. On the other hand, when the alloy was immersed in 50 °C H2SO4,
a
Fig. 8 shows the corroded surfaces and the EDS analysis results of the VIM Ni-based alloys immersed in 25 °C 60% H2SO4 solution for 3 h (Fig. 8a) and in 50 °C 60% H2SO4 solution for 0.5 h (Fig. 8b). As shown in Fig. 8a, only the c-Ni solid solution + Ni3Si particles and dendrite Ni3Si are corroded in the 25 °C H2SO4 solution. By combining the above result with the EDS results from point 1 and point 2, it is confirmed that both of these zones have traces of oxidation. Based on all of these results, it is concluded that both of these zones are not resistant to corrosion in a 60% H2SO4 solution. However, literature sources [20–22] mention that oxides of Ni3Si suppress both corrosion and further oxidation. Based on the EDS results, it is shown that the corrosion rate decrease is due to the formation of oxides. A corroded surface similar to the surface indicated above (Fig. 8b) also displays those properties. According to the four EDS analysis results, it can be suggested that after being immersed in 50 °C 60% H2SO4 for 0.5 h, the c-Ni solid solution + Ni3Si particles and dendrite Ni3Si have higher degrees of oxidation than when they are immersed in 25 °C 60% H2SO4 for 3 h. This result is especially apparent in the dendrite Ni3Si, which transformed to oxide films. A comparison of Fig. 8a and b indicate that increasing the temperature of the solutions increases the corrosion rate, which matches the result in Fig. 7. Fig. 9a indicates that after immersing the VIM Ni-based alloys in 60% H2SO4 solution for 12 h at 25 °C, the c-Ni solid solution + Ni3Si particles and dendrite Ni3Si found on the corroded surfaces had all converted into oxide films. However, the transformation behaviours were different. For the c-Ni solid solution + Ni3Si particles, Ni3Si particles first oxidise and then convert to oxide films following certain orientations (Fig. 9b). There are no specific rules indicating how dendrite Ni3Si oxidises (Fig. 8). As the duration of immersion increased, the density of the oxide films also increased. Using 60% H2SO4 solution at 25 °C as an example, the oxide film
b 4 1 3 2
Ni3Si particles O
Si
Cr
Fe
Ni
1 2
2.84 7.15
3.68 9.82
3.75 -
7.35 1.25
49.84 45.63
3 4
4.84 11.99
7.62 14.19
2.72 -
5.49 -
67.26 58.25
Fig. 8. The corroded surface morphology of the VIM Ni-based alloy immersed in a solution of 60% H2SO4 for (a) 3 h at 25 °C and (b) 0.5 h at 50 °C.
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Fig. 9. The corroded surface morphology of the VIM Ni-based alloy immersed in a solution of 60% H2SO4 for (a) 12 h at 25 °C and (b) the magnification of (a).
Fig. 10. The corroded surface morphology of the VIM Ni-based alloy immersed in a solution of 60% H2SO4 for (a) 72 h at 25 °C and (b) 12 h at 50 °C.
formed after immersing for 72 h (Fig. 10a) was denser than films formed after immersing for 24 and 48 h. For films immersed in 60% H2SO4 solution at 50 °C, it only took 12 h to form oxide films (Fig. 10b) similar to the ones found in Fig. 10a. Therefore, these results support the idea that an increase in solution temperature will decrease the time required for corrosion to take place. In addition, the corrosion rate decreased before 12 h and remained stable after 12 h, due to possible influence from the oxide films.
3.4. ICP-OES analysis Table 2 shows the results of ICP-OES analysis of each of the solutions after the immersion experiments. Every element contained in the VIM Ni-based alloys aside from carbon was detected and measured in the solutions. However, the concentration of silicon ions did not increase with immersion time. This result confirms two ideas. First of all, although the corrosion of Ni3B and CrB did not seem to take place when observing the corroded surfaces, both of these two phases were still attacked. Based on the results from X-ray mapping (Fig. 3) and under the premise that only Ni3B and CrB contain boron, the presence of boron ions in solution affirms that at least one of these two phases had been attacked by the solutions. In addition, because the wavelength of carbon ions is close to visible light, they cannot be measured by the instruments used here. As a result, we could not determine whether Cr7C3 had been corroded. Based on Figs. 3 and 8, only the c-Ni solid solution + Ni3Si particles and dendrite Ni3Si contain silicon. After a short period of time, both of these zones were readily corroded. However, after the oxide films were formed, the corrosion rate
Table 2 The ion concentrations (mg/L) of the solutions after testing. Si
Cr
B
Fe
Ni
25 °C 12 24 48 72
0.1 0.1 0.1 0.1
0.8 2.0 4.0 6.0
1.3 1.5 1.9 2.1
0.7 1.2 1.8 2.3
6.9 13.0 20.3 27.0
50 °C 12 24 48 72
0.1 0.2 0.2 0.2
7.0 11.4 26.1 43.4
2.2 2.8 5.2 8.0
2.6 3.3 7.8 12.0
28.3 39.9 92.4 143.1
did not increase as the immersion time increased. These observations indicate that when these two zones are corroded, they enter the solution with chromium. However, after the oxides are formed and silicon is deposited on the corroded surface and because the oxide cannot be further corroded, silicon did not return to the solution. This result explains why the concentration of silicon did not increase when the alloy was immersed for extended periods of time. 3.5. Oxides The results described above show that the oxides formed by Ni3Si play important roles in affecting the corrosion of VIM Ni-based alloys in a 60% H2SO4 solution. Based on the JCPDS Card database and the XRD diffraction figure (Fig. 11) of the VIM
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4. Conclusions It was confirmed through XRD, X-ray mapping, and TEM that VIM Ni-based alloys contain microstructures such as c-Ni solid solution + Ni3Si particles and dendrite Ni3Si, Ni3B, Cr7C3, and CrB. When immersed in 60% H2SO4, the alloy displayed a corrosion rate that decreases with time, but eventually stabilises. However, the resistance to corrosion was higher in a 25 °C solution than in a 50 °C solution. The c-Ni solid solution + Ni3Si particles and dendrite Ni3Si were the first zones to be corroded in the microstructure. At the same time, the oxides formed by these two zones play very important roles in leading the corrosion of the entire VIM Ni-based alloy. The category of the oxide cannot be accurately determined at the moment, but its presence effectively lowers the corrosion rate and can preserve itself for long periods of time. This result confirms the fact that this oxide has a very high resistance towards corrosion in a 60% H2SO4 solution. Compared with such a significant phenomenon, pitting corrosion plays an inconspicuous and balanced role in corrosion. The pits provide infiltrating paths for acid solution and/or SO4 2 to cause further attack, and balance the decreasing corrosion rate caused by the oxide films. Acknowledgement The authors gratefully acknowledge support of the China Steel Corporation of Taiwan. Fig. 11. The XRD diffraction patterns of the VIMed Ni-based alloy immersed in a solution of 60% H2SO4 for various times at 25 °C. Phases such as c-Ni, Ni3B, Ni3Si, Cr7C3, and CrB are omitted, and only the possible oxides are evident.
Ni-based alloys immersed in 60% H2SO4 for different periods of time, the possible oxides are Ni2O3, NiO, SiO2, and Ni2SiO4. Many researchers also focus on oxides formed by Ni3Si [20–22] and have indicated that the resistance of corrosion and oxidation come from the formation of NiO and SiO2, while some researchers [21] credit that resistance to the formation of nickel silicate. In order to confirm the identity of the oxide formed by Ni3Si in this experiment, other techniques, such as X-ray photoelectron spectroscopy (XPS), are needed. However, we still suggest that oxides formed by Ni3Si have a high resistance towards corrosion and oxidation. Therefore, the corrosion rate of VIM Ni-based alloys immersed in 60% H2SO4 is most likely attributed to oxides of Ni3Si.
3.6. Corrosion behaviour The above discussions confirm that a passive film improves the corrosion resistance. However, it is only conjecture to say that the VIM Ni-based alloy reaches a passive state due to the steady state corrosion rates of 0.55 ± 0.10 mm/y at 25 °C and 2.98 mm/ y ± 0.13 mm/y at 50 °C. Therefore, other factors must affect the corrosion rate. When comparing Fig. 4 with Fig. 8, a few significant pits occur at the interfaces between the zone of the c-Ni solid solution + Ni3Si particles and other phases, i.e., dendrite Ni3Si, Ni3B, Cr7C3, and CrB, after a short immersion time. When comparing Fig. 8 with Fig. 10, it can be seen that the other corrosion mechanism is pitting corrosion due to the amount of pit increasing with the increased immersion time. A previous work [15] indicated that acid solution and/or SO4 2 would infiltrate into these pits and cause an increase of corrosion depth. Other previous reports [7,14] also had similar results concerning the infiltration of acid solution. Therefore, the corrosion rate stabilised at such a high state should be due to the effect between pitting corrosion and oxide films reaching a balance.
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