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Corrosion resistance of Fe-based amorphous alloys
Journal of Alloys and Compounds xxx (2013) xxx–xxx
Contents lists available at SciVerse ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Corrosion resistance of Fe-based amorphous alloys W.J. Botta a,b,⇑, J.E. Berger b, C.S. Kiminami b, V. Roche a, R.P. Nogueira a, C. Bolfarini b a b
LEPMI, UMR5279 CNRS, Grenoble INP, Université de Savoie, Université Joseph Fourier, 1130, Rue de la piscine, BP 75, 38402 Saint Martin d’Hères, France Departamento de Engenharia de Materiais, Universidade Federal de São Carlos, Rod. Washington Luiz, Km 235, 13565-905 São Carlos, SP, Brazil
a r t i c l e
i n f o
Article history: Available online xxxx Keywords: Corrosion Amorphous alloys Fe-based alloys Steels
a b s t r a c t Fe-based amorphous alloys can be designed to present an attractive combination of properties with high corrosion resistance and high mechanical strength. Such properties are clearly adequate for their technological use as coatings, for example, in steel pipes. In this work, we studied the corrosion properties of amorphous ribbons of the following Fe-based compositions: Fe66B30Nb4, [(Fe0.6Co0.4)0.75B0.2Si0.05]96Nb4, [(Fe0.7Co0.3)0.75B0.2Si0.05]96Nb4, Fe56Cr23Ni5.7B16, Fe53Cr22Ni5.6B19 and Fe50Cr22Ni5.4B23. The ribbons were obtained by rapid solidification using the melt-spinning process, and were characterized by X-ray diffraction (XRD), differential scanning calorimetry (DSC) and optical (OM) and scanning electron microscopy (SEM). The corrosion properties were evaluated by corrosion potential survey and potentiodynamic polarization. The Cr containing alloys, that is the FeCrNiB type of alloys, showed the best corrosion resistance properties with the formation of a stable passive film that ensured a very large passivation plateau. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction Amorphous alloys are remarkable for a number of physical and chemical properties, quite different from the conventional crystalline alloys. Among these novel properties, amorphous alloys often show high corrosion resistance [1–3], which can be attributed to the chemical homogeneity and the absence of crystallographic imperfections, such as, grains, grain boundaries, second phase elements, dislocations or segregations, which are more susceptible to chemical attack [4,5]. Combined to adequate chemical composition; for instance Cr presence in the case of ferrous-based alloys; such structural characteristic results in a surface passive film which is more uniform and stable than the films formed on the crystalline alloys surface [6,7]. Many different amorphous alloys have been studied so far in respect to corrosion behavior, including Fe-based alloys [8–13], Zr-based alloys [14,15], Mg-based alloys [16] and Cu-based alloys [17]. In particular, the Fe-based amorphous alloys are considered to be excellent candidates for protecting steel surfaces because they have high crystallization temperature, high corrosion and wear resistance, good magnetic properties and relatively low material cost [18–20]. With such properties, these type of alloys are
⇑ Corresponding author at: Departamento de Engenharia de Materiais, Universidade Federal de São Carlos, Rod. Washington Luiz, Km 235, 13565-905 São Carlos, SP, Brazil. Tel.: +55 1633519479. E-mail address: [email protected] (W.J. Botta).
adequate to resist extremely aggressive environments [21,22], such as chemical industries and oil refineries. The effect of the compositional changes in the corrosion resistance of Fe-based amorphous alloys has been intensively studied, and additions of metals such as Cr, Nb, Mo or Ni improve greatly the corrosion properties in particular with the spontaneous passivation of the alloys [8,12,23]. In the present work we report on the corrosion properties of different Fe-based amorphous alloys, which were chosen due to their good glass forming abilities and possibility to be used as protective coatings in steels. Two of the alloys are from the composition family [(Fe1 x Cox)0.75 B0.2Si0.05]96Nb4 which, as indicated in the literature [24,25], present excellent mechanical properties in addition to the good glass forming ability. A third alloy, Fe66B30Nb4, has been previously evaluated in our group [26] to be used in spray forming processing. Finally, the last group of alloys, of the system FeCrNiB, was developed in this work, with compositions based in modifications of available commercial coatings alloys (ARMACOR) and superduplex stainless steel. The corrosion resistance was evaluated in different corrosive media, especially simulating the marine environment.
2. Experimental procedure The following Fe-based compositions have been chosen for the evaluation of corrosion properties: Fe66B30Nb4, [(Fe0.6Co0.4)0.75B0.2Si0.05]96Nb4, [(Fe0.7Co0.3)0.75B0.2Si0.05]96Nb4, Fe56Cr23Ni5.7B16, Fe53Cr22Ni5.6B19 and Fe50Cr22Ni5.4B23. The alloy ingots were prepared by arc-melting high-purity constituent elements in a Ti-gettered high-purity argon atmosphere; each ingot was re-melted several times for chemical homogenization. The alloy ingot was then induction-melted
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W.J. Botta et al. / Journal of Alloys and Compounds xxx (2013) xxx–xxx
Table 1 Chemical composition of stainless steel 316L.
SS 316LN
C
Si
Mn
Ni
Cr
Mo
N
Al
0.028
0.571
1.018
11.995
18.003
2.989
0.029
0.0036
alloy in the solution. The potentiodynamic polarization curves were obtained by sweeping the potential from 30 mV below the corrosion potential to a maximum potential corresponding to a current of 1 mA/cm2 at a scan rate of 1 mV/s. For comparison purposes, electrochemical analysis were also carried out under the same conditions on an austenitic stainless steel 316L, which is known to present good corrosion properties [27,28]. The chemical composition of the 316L steel that was used is shown in Table 1. Topography analysis of the amorphous ribbons were performed before and after the electrochemical tests, in an optical microscope Leitz Laborlux 12ME S, Leica.
3. Results and discussion
Fig. 1. XRD patterns of the Fe-based ribbons; corresponding compositions are indicated in the figure.
Fig. 1 shows the XRD patterns corresponding to all the ribbons, with the corresponding compositions indicated in each pattern. All ribbons showed similar XRD patterns, with one main halo, typical of an amorphous structure. Fig. 2 shows the XRD patterns of the internal surface (which solidified in contact with the copper wheel) and the external surface of the ribbon Fe66B30Nb4, as a typical example for all the alloys. Again, in each case, there is only one main halo corresponding to the amorphous phase, ensuring uniformity of the ribbons relative to their thickness. Topography observations were made before and after the electrochemical analysis (maximum current density of 1 mA/cm2). All
Fig. 2. Typical XRD patterns of the external and internal surfaces of the amorphous ribbons; in this case for the Fe66B30Nb4 alloy.
under a high-purity argon atmosphere in a quartz tube and injected through a nozzle onto a Cu wheel to produce rapidly solidified ribbons by melt spinning. The amorphous ribbons were characterized by X-ray diffraction (XRD) and differential scanning calorimetry (DSC). The electrochemical analysis were carried out in the following three conditions to simulate an environment of sea water: (i) neutral solution using deionized water, 0.6 mol/L of NaCl; (ii) acid solution using deionized water, 0.6 mol/L of NaCl and addition of H2SO4 until pH = 1.0 and (iii) alkaline solution using deionized water, 0.6 mol/L of NaCl and addition of NaOH until pH = 10.0. Controlling of pH was performed using a pHmeter instrument. Electrochemical analyzes were performed using a conventional three electrodes set-up immersed in solution. The working electrode was the amorphous metal ribbon, with an immersed average area of 1 cm2, the counter electrode was a platinum sheet and the reference was a saturated calomel electrode (sce). Before the analysis, the solutions were subjected to air bubbling for 30 min to complete saturation of atmospheric gases. Open circuit analysis were carried out in a potentiostat, model 1287 Electrochemical Interface IS (potentiostat/galvanostat SOLARTRON, Schlumberger), and the software CORRWARE 2. The duration of each analysis was 30 min in order to reach steady corrosion potential conditions of the
Fig. 3. Optical microscopy of the Fe53Cr22Ni5.6B19 ribbon before the electrochemical analysis. In (a) the external surface, and in (b) the internal surface.
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W.J. Botta et al. / Journal of Alloys and Compounds xxx (2013) xxx–xxx
Fig. 4. Optical microscopy of the [(Fe0.7Co0.3)0.75B0.2Si0.05]96Nb4 ribbon, after analysis in acid media (pH = 1.0). In (a) the external surface and in (b) the internal surface.
the original alloys (‘‘fresh samples’’), that is, before electrochemical analysis, exhibited good surface homogeneity. Fig. 3a corresponds to the external surface of the Fe53Cr22Ni5.6B19 ribbon, the surface is homogenous, smooth, with small reliefs. Fig. 3b corresponds to the internal surface of the ribbon, showing some roughness and defects which tend to align in the direction of copper wheel rotation. Samples were electrochemically as prepared without any previous polishing treatment. Both the behaviors of the external and internal surfaces were recorded since the ribbons were completely immersed. The electrochemical response corresponds then to an overall behavior of the tested electrode. The optical microscopy observations after the electrochemical measurements indicated different behavior between alloys of different compositions. Fig. 4a and b corresponds to the surfaces images of the internal and external [(Fe0.7Co0.3)0.75B0.2Si0.05]96Nb4 ribbon surfaces, respectively, showing severe corrosion damage in these Cr-free alloys with a huge number of pits of different depths and diameters, probably related to the absence of passive film as it will clearly be seen in the electrochemical tests. Fig. 5a and b shows the general internal and external surface appearance of the Fe50Cr22Ni5.4B23 ribbon, respectively; even in the highly aggressive media (pH = 1) the cleaner surfaces (very similar to the ‘‘fresh’’ surfaces) indicate excellent corrosion resistance for the Cr containing alloys. For these alloys, the increase in current density up to 1 mA/cm2 must be attributed to the release of oxygen and not to corrosion, which was confirmed by polarization curves, as will be discussed later.
3
Fig. 5. Optical microscopy of the Fe50Cr22Ni5.4B23 ribbon, after analysis in acid media (pH = 1.0). In (a) the external surface and in (b) the internal surface.
The electrochemical results are presented comparing the data from all alloys in each specific condition. Figs. 6–8 show the polarization curves for all the alloys tested in this work, for neutral (pH = 5.5), acid (pH = 1.0) and alkaline (pH = 10.0) solutions, respectively. In each figure Pt electrochemical behavior was also recorded as a reference for the measurement of the potential of oxygen evolution. The polarization curve for the crystalline stainless steel 316L is also represented. Tables 2–4 show the main information obtained from the polarization curves; corrosion potential (Ecorr), corrosion current (icorr), critical potential (Ecrit) and passivation width (DE = Ecrit Ecorr). The values of Ecrit can be associated either with the disruption of the passive layer or with oxygen evolution, as discussed below. The critical potential or the breaking-off of the passive layer was defined as the potential where the current density reaches the level of 10 lA/cm2. In all three media (neutral, acidic or alkaline) the results followed the same trends, indicating that the differences in the alloy´s compositions are more important than the exposed environment. The three alloys Fe66B30Nb4, [(Fe0.6Co0.4)0.75B0.2Si0.05]96Nb4 and [(Fe0.7Co0.3)0.75B0.2Si0.05]96Nb4, show low corrosion resistance whatever the media. In fact the results, which suggest no effect of the different Co content in the FeCo-based alloys, indicate low corrosion potential and high corrosion current density; in addition, these alloys did not show a passive region. Inversely, the Cr-containing alloys, Fe56Cr23Ni5.7B16, Fe53Cr22Ni5.4B23 and Fe50Cr22Ni5.6B19 exhibit excellent anticorrosive
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Fig. 6. Polarization curves in neutral solution (pH = 5.5) for the alloys indicated in the figure.
properties, characterized by high corrosion potential and low corrosion current density, consistently lower than the values given by the alloy SS 316 LN. Moreover, a well defined passivation state, with a large passivation plateau has always been clearly observed. The three compositions presented a highly stable passivation layer as clearly shown by the comparison of the polarization curves of the FeCrNiB amorphous alloys and Pt. Indeed, the increase in current at high potential values always took place at roughly the same values as for the Pt electrode. This is a clear evidence that this increase must be ascribed to oxygen evolution and not to the corrosion of the alloys, which, in fact, resisted to all the potentials applied. On the other hand, the alloy used as reference, 316 LN, showed a small passive plateau soon interrupted by disruption of the passive film by pitting in all media analyzed. These results
indicate that for both the general and localized corrosion behavior, the FeCrNiB amorphous alloys showed superior performance to the 316L. This point deserves an important remark: the Cr content was 18% for the SS and 22% or 23% for the amorphous alloys. It is however worth noticing that even if the higher Cr content can be effectively expected to enhance and improve passivation, on the other hand the amorphous alloys contained less Mo than the SS, which is well-known to greatly improve corrosion resistance. Indeed, the weight of Mo in the improvement of localized corrosion resistance is considered to be 3.3 times greater than the one of Cr as it appears in the calculation of the widespread PREn (pitting resistance equivalent number, see for instance [29]). This can thus be seen as an indirect evidence of the beneficial role of the amorphous structure of the tested alloys in the overall corrosion resistant.
Fig. 7. Polarization curves in acid solution (pH = 1.0) for the alloys indicated in the figure.
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Fig. 8. Polarization curves in alkaline solution (pH = 10.0) for the alloys indicated in the figure.
Table 2 Electrochemical properties obtained from the polarization curves in neutral media (pH = 5.5). Ecorr (mV) Ecrit (mV) DE (mV) Icorr (A/cm2) Platinum Fe66B30Nb4 [(Fe0.6Co0.4)0.75B0.2Si0.05]96Nb4 [(Fe0.7Co0.3)0.75B0.2Si0.05]96Nb4 Fe56Cr23Ni5.7B16 Fe53Cr22Ni5.6B19 Fe50Cr22Ni5.4B23 SS 316LN
131 700 550 630 128 200 167 203
1096 689 512 611 1125 1116 1122 330
965 11 38 19 1253 1316 1289 533
2.0 10 1.5 10 2.0 10 7.0 10 1.5 10 6.0 10 2.0 10 2.0 10
8 5 6 6 8 8 8 7
A remarkable feature is that these Cr-containing amorphous Fe based alloys showed a corrosion resistance level close to that of the inert Pt in all studied media, ranging from very acidic to very alkaline chloride-rich solutions. These results were obtained notwithstanding the fact that both faces were immerged. This indicates that one can neglect for these samples the scenario of an intrinsic and deleterious galvanic coupling between the internal and the external faces that could deteriorate the corrosion performance. This constitutes a very wide range of electrochemical stability that is certainly of much concern for several industrial domains exposed to highly aggressive conditions. 4. Conclusions
Table 3 Electrochemical properties obtained from the polarization curves in acid media (pH = 1.0). Ecorr (mV) Ecrit (mV) DE (mV) Icorr (A/cm2) Platinum Fe66B30Nb4 [(Fe0.6Co0.4)0.75B0.2Si0.05]96Nb4 [(Fe0.7Co0.3)0.75B0.2Si0.05]96Nb4 Fe56Cr23Ni5.7B16 Fe53Cr22Ni5.6B19 Fe50Cr22Ni5.4B23 SS 316LN
47 458 381 389 192 190 246 219
866 453 373 380 910 929 918 15
913 5 8 9 1102 1119 1164 204
9.0 10 1.5 10 1.5 10 1.5 10 6.0 10 9.5 10 8.0 10 9.0 10
8 5 5 5 8 8 8 7
Table 4 Electrochemical properties obtained from the polarization curves in alkaline media (pH = 10.0). Ecorr (mV) Ecrit (mV) DE (mV) Icorr (A/cm2) Platinum Fe66B30Nb4 [(Fe0.6Co0.4)0.75B0.2Si0.05]96Nb4 [(Fe0.7 Co0.3)0.75B0.2Si0.05]96Nb4 Fe56Cr23Ni5.7B16 Fe53Cr22Ni5.6B19 Fe50Cr22Ni5.4B23 SS 316LN
119 637 509 520 209 247 320 377
1072 574 470 498 1028 1146 1016 99
953 63 39 22 1237 1393 1336 476
3.0 10 7.0 10 1.5 10 6.0 10 2.0 10 9.5 10 9.0 10 4.5 10
The corrosion resistance of Fe-based amorphous alloys was evaluated in comparison with a commercial stainless steel. In contrast with the crystalline stainless steel 316L, the electrochemical corrosion properties of the amorphous alloys did not vary significantly when measured in neutral, acid or alkaline solutions. However, the corrosion properties of the Fe-based amorphous alloys were strongly dependent on the alloy composition; the presence of Cr in this family of alloys had a great positive influence on the corrosion resistance with the formation of a stable passive film ensuring an extended passivation plateau.
7 6 6 6 8 8 8 7
References [1] N. Sorensen, R. Diegle, Corrosion of amorphous metals, Metals Handbook: corrosion, vol. 13, ninth ed., ASME, 1987, pp. 864–870. [2] D. Polk, B. Giessen, Overview of Principles and Applications: Metallic Glasses, ASME, 1978, pp. 2–35. [3] M. Telford, Materials Today 7 (2004) 36–43. [4] K. Hashimoto, M. Froment, Passivity of Metals and Semiconductors, Elsevier Science Publishers, Amsterdam, 1983. [5] S. Virtanen, H. Bohni, Corrosion Science 31 (1990) 333–342. [6] K. Leinartas, M. Samuleviciene, A. Bagdonas, R. Juskenas, E. Juzeliunas, Surface and Coating Technology 168 (2003) 70–77. [7] K. Asami, K. Hashimoto, T. Masumoto, S. Shimodaira, Corrosion Science 16 (1976) 909–914. [8] C.S. Kiminami, C.A.C. Souza, L.F. Bonavina, C. Bolfarini, W.J. Botta, Journal of Non-Crystalline Solids 356 (2010) 2651–2657. [9] S. Pang, T. Zhang, K. Asami, A. Inoue, Materials Transactions 42 (2001) 376– 379. [10] N.A. Mariano, C.A.C. Souza, J.E. May, S.E. Kuri, Materials Science and Engineering A 354 (2003) 1–5.
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[11] C.A.C. Souza, C.S. Kiminami, Journal of Non-Crystalline Solids 219 (1997) 155– 159. [12] C.A.C. Souza, M.F. Oliveira, J.E. May, W.J. Botta, N.A. Mariano, S.E. Kuri, C.S. Kiminami, Journal of Non-Crystalline Solids 273 (2000) 282–288. [13] S. Pang, T. Zhang, K. Asami, A. Inoue, Materials Transactions 41 (2000) 1490– 1494. [14] C.A.C. Souza, F.S. Politi, C.S. Kiminami, Scripta Materialia 39 (1998) 329–334. [15] V.R. Raw, U. Kuhn, U. Wolff, F. Schneider, J. Eckert, R. Reiche, A. Gebert, Materials Letters 57 (2002) 173–177. [16] Y. Li, S.C. Ng, C.H. Kamam, Materials Letters 36 (1998) 214–217. [17] S. Pang, T. Zhang, K. Asami, A. Moue, Materials Transactions 43 (2002) 2137– 2151. [18] B.L. Shen, A. Inoue, C.T. Chang, Applied Physics Letters 85 (2004) 4911–4913. [19] S.J. Pang, T. Zhang, K. Asami, Corrosion Science 44 (2002) 1847–1856. [20] A. Inoue, A. Makino, T. Mizushima, Journal of Magnetism and Magnetic Materials 215–216 (2000) 246–252.
[21] A.L. Greer, K.L. Rutherford, I.M. Hutchings, International Materials Reviews 47 (2002) 87–112. [22] D. Zander, U. Koster, Materials Science and Engineering A 375–377 (2004) 53–59. [23] M.F. Lopez, M.L. Escudero, E. Vida, A.R. Piernab, Electrochimica Acta 42 (1997) 659–665. [24] M. Stoica, K. Hajlaoui, A. le Moulec, A.R. Yavari, Philosophical Magazine Letters 86 (2006) 267–275. [25] A. Inoue, B.L. Shen, C.T. Chang, Acta Materialia 52 (2004) 4093–4099. [26] L.F. Bonavina, C. Bolfarini, W.J. Botta, E.R. d’Almeida, C.S. Kiminami, Journal of Alloys and Compounds 495 (2010) 417–419. [27] Information Handbook: Stainless Steel for Coastal and Salt Corrosion. The Stainless Steel Information Centre, 2012. www.ssina.com. [28] Copper Development Association. Materials for Seawater Pipelines. CDA, Publication TN38, 1986. [29] R.N. Gunn, Duplex Stainless Steels—Microstructure, Properties and Applications, Abingtion Publishing, Cambridge, England, 1999.
Please cite this article in press as: W.J. Botta et al., J. Alloys Comp. (2013), http://dx.doi.org/10.1016/j.jallcom.2012.12.130
Contents lists available at SciVerse ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Corrosion resistance of Fe-based amorphous alloys W.J. Botta a,b,⇑, J.E. Berger b, C.S. Kiminami b, V. Roche a, R.P. Nogueira a, C. Bolfarini b a b
LEPMI, UMR5279 CNRS, Grenoble INP, Université de Savoie, Université Joseph Fourier, 1130, Rue de la piscine, BP 75, 38402 Saint Martin d’Hères, France Departamento de Engenharia de Materiais, Universidade Federal de São Carlos, Rod. Washington Luiz, Km 235, 13565-905 São Carlos, SP, Brazil
a r t i c l e
i n f o
Article history: Available online xxxx Keywords: Corrosion Amorphous alloys Fe-based alloys Steels
a b s t r a c t Fe-based amorphous alloys can be designed to present an attractive combination of properties with high corrosion resistance and high mechanical strength. Such properties are clearly adequate for their technological use as coatings, for example, in steel pipes. In this work, we studied the corrosion properties of amorphous ribbons of the following Fe-based compositions: Fe66B30Nb4, [(Fe0.6Co0.4)0.75B0.2Si0.05]96Nb4, [(Fe0.7Co0.3)0.75B0.2Si0.05]96Nb4, Fe56Cr23Ni5.7B16, Fe53Cr22Ni5.6B19 and Fe50Cr22Ni5.4B23. The ribbons were obtained by rapid solidification using the melt-spinning process, and were characterized by X-ray diffraction (XRD), differential scanning calorimetry (DSC) and optical (OM) and scanning electron microscopy (SEM). The corrosion properties were evaluated by corrosion potential survey and potentiodynamic polarization. The Cr containing alloys, that is the FeCrNiB type of alloys, showed the best corrosion resistance properties with the formation of a stable passive film that ensured a very large passivation plateau. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction Amorphous alloys are remarkable for a number of physical and chemical properties, quite different from the conventional crystalline alloys. Among these novel properties, amorphous alloys often show high corrosion resistance [1–3], which can be attributed to the chemical homogeneity and the absence of crystallographic imperfections, such as, grains, grain boundaries, second phase elements, dislocations or segregations, which are more susceptible to chemical attack [4,5]. Combined to adequate chemical composition; for instance Cr presence in the case of ferrous-based alloys; such structural characteristic results in a surface passive film which is more uniform and stable than the films formed on the crystalline alloys surface [6,7]. Many different amorphous alloys have been studied so far in respect to corrosion behavior, including Fe-based alloys [8–13], Zr-based alloys [14,15], Mg-based alloys [16] and Cu-based alloys [17]. In particular, the Fe-based amorphous alloys are considered to be excellent candidates for protecting steel surfaces because they have high crystallization temperature, high corrosion and wear resistance, good magnetic properties and relatively low material cost [18–20]. With such properties, these type of alloys are
⇑ Corresponding author at: Departamento de Engenharia de Materiais, Universidade Federal de São Carlos, Rod. Washington Luiz, Km 235, 13565-905 São Carlos, SP, Brazil. Tel.: +55 1633519479. E-mail address: [email protected] (W.J. Botta).
adequate to resist extremely aggressive environments [21,22], such as chemical industries and oil refineries. The effect of the compositional changes in the corrosion resistance of Fe-based amorphous alloys has been intensively studied, and additions of metals such as Cr, Nb, Mo or Ni improve greatly the corrosion properties in particular with the spontaneous passivation of the alloys [8,12,23]. In the present work we report on the corrosion properties of different Fe-based amorphous alloys, which were chosen due to their good glass forming abilities and possibility to be used as protective coatings in steels. Two of the alloys are from the composition family [(Fe1 x Cox)0.75 B0.2Si0.05]96Nb4 which, as indicated in the literature [24,25], present excellent mechanical properties in addition to the good glass forming ability. A third alloy, Fe66B30Nb4, has been previously evaluated in our group [26] to be used in spray forming processing. Finally, the last group of alloys, of the system FeCrNiB, was developed in this work, with compositions based in modifications of available commercial coatings alloys (ARMACOR) and superduplex stainless steel. The corrosion resistance was evaluated in different corrosive media, especially simulating the marine environment.
2. Experimental procedure The following Fe-based compositions have been chosen for the evaluation of corrosion properties: Fe66B30Nb4, [(Fe0.6Co0.4)0.75B0.2Si0.05]96Nb4, [(Fe0.7Co0.3)0.75B0.2Si0.05]96Nb4, Fe56Cr23Ni5.7B16, Fe53Cr22Ni5.6B19 and Fe50Cr22Ni5.4B23. The alloy ingots were prepared by arc-melting high-purity constituent elements in a Ti-gettered high-purity argon atmosphere; each ingot was re-melted several times for chemical homogenization. The alloy ingot was then induction-melted
0925-8388/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2012.12.130
Please cite this article in press as: W.J. Botta et al., J. Alloys Comp. (2013), http://dx.doi.org/10.1016/j.jallcom.2012.12.130
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Table 1 Chemical composition of stainless steel 316L.
SS 316LN
C
Si
Mn
Ni
Cr
Mo
N
Al
0.028
0.571
1.018
11.995
18.003
2.989
0.029
0.0036
alloy in the solution. The potentiodynamic polarization curves were obtained by sweeping the potential from 30 mV below the corrosion potential to a maximum potential corresponding to a current of 1 mA/cm2 at a scan rate of 1 mV/s. For comparison purposes, electrochemical analysis were also carried out under the same conditions on an austenitic stainless steel 316L, which is known to present good corrosion properties [27,28]. The chemical composition of the 316L steel that was used is shown in Table 1. Topography analysis of the amorphous ribbons were performed before and after the electrochemical tests, in an optical microscope Leitz Laborlux 12ME S, Leica.
3. Results and discussion
Fig. 1. XRD patterns of the Fe-based ribbons; corresponding compositions are indicated in the figure.
Fig. 1 shows the XRD patterns corresponding to all the ribbons, with the corresponding compositions indicated in each pattern. All ribbons showed similar XRD patterns, with one main halo, typical of an amorphous structure. Fig. 2 shows the XRD patterns of the internal surface (which solidified in contact with the copper wheel) and the external surface of the ribbon Fe66B30Nb4, as a typical example for all the alloys. Again, in each case, there is only one main halo corresponding to the amorphous phase, ensuring uniformity of the ribbons relative to their thickness. Topography observations were made before and after the electrochemical analysis (maximum current density of 1 mA/cm2). All
Fig. 2. Typical XRD patterns of the external and internal surfaces of the amorphous ribbons; in this case for the Fe66B30Nb4 alloy.
under a high-purity argon atmosphere in a quartz tube and injected through a nozzle onto a Cu wheel to produce rapidly solidified ribbons by melt spinning. The amorphous ribbons were characterized by X-ray diffraction (XRD) and differential scanning calorimetry (DSC). The electrochemical analysis were carried out in the following three conditions to simulate an environment of sea water: (i) neutral solution using deionized water, 0.6 mol/L of NaCl; (ii) acid solution using deionized water, 0.6 mol/L of NaCl and addition of H2SO4 until pH = 1.0 and (iii) alkaline solution using deionized water, 0.6 mol/L of NaCl and addition of NaOH until pH = 10.0. Controlling of pH was performed using a pHmeter instrument. Electrochemical analyzes were performed using a conventional three electrodes set-up immersed in solution. The working electrode was the amorphous metal ribbon, with an immersed average area of 1 cm2, the counter electrode was a platinum sheet and the reference was a saturated calomel electrode (sce). Before the analysis, the solutions were subjected to air bubbling for 30 min to complete saturation of atmospheric gases. Open circuit analysis were carried out in a potentiostat, model 1287 Electrochemical Interface IS (potentiostat/galvanostat SOLARTRON, Schlumberger), and the software CORRWARE 2. The duration of each analysis was 30 min in order to reach steady corrosion potential conditions of the
Fig. 3. Optical microscopy of the Fe53Cr22Ni5.6B19 ribbon before the electrochemical analysis. In (a) the external surface, and in (b) the internal surface.
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Fig. 4. Optical microscopy of the [(Fe0.7Co0.3)0.75B0.2Si0.05]96Nb4 ribbon, after analysis in acid media (pH = 1.0). In (a) the external surface and in (b) the internal surface.
the original alloys (‘‘fresh samples’’), that is, before electrochemical analysis, exhibited good surface homogeneity. Fig. 3a corresponds to the external surface of the Fe53Cr22Ni5.6B19 ribbon, the surface is homogenous, smooth, with small reliefs. Fig. 3b corresponds to the internal surface of the ribbon, showing some roughness and defects which tend to align in the direction of copper wheel rotation. Samples were electrochemically as prepared without any previous polishing treatment. Both the behaviors of the external and internal surfaces were recorded since the ribbons were completely immersed. The electrochemical response corresponds then to an overall behavior of the tested electrode. The optical microscopy observations after the electrochemical measurements indicated different behavior between alloys of different compositions. Fig. 4a and b corresponds to the surfaces images of the internal and external [(Fe0.7Co0.3)0.75B0.2Si0.05]96Nb4 ribbon surfaces, respectively, showing severe corrosion damage in these Cr-free alloys with a huge number of pits of different depths and diameters, probably related to the absence of passive film as it will clearly be seen in the electrochemical tests. Fig. 5a and b shows the general internal and external surface appearance of the Fe50Cr22Ni5.4B23 ribbon, respectively; even in the highly aggressive media (pH = 1) the cleaner surfaces (very similar to the ‘‘fresh’’ surfaces) indicate excellent corrosion resistance for the Cr containing alloys. For these alloys, the increase in current density up to 1 mA/cm2 must be attributed to the release of oxygen and not to corrosion, which was confirmed by polarization curves, as will be discussed later.
3
Fig. 5. Optical microscopy of the Fe50Cr22Ni5.4B23 ribbon, after analysis in acid media (pH = 1.0). In (a) the external surface and in (b) the internal surface.
The electrochemical results are presented comparing the data from all alloys in each specific condition. Figs. 6–8 show the polarization curves for all the alloys tested in this work, for neutral (pH = 5.5), acid (pH = 1.0) and alkaline (pH = 10.0) solutions, respectively. In each figure Pt electrochemical behavior was also recorded as a reference for the measurement of the potential of oxygen evolution. The polarization curve for the crystalline stainless steel 316L is also represented. Tables 2–4 show the main information obtained from the polarization curves; corrosion potential (Ecorr), corrosion current (icorr), critical potential (Ecrit) and passivation width (DE = Ecrit Ecorr). The values of Ecrit can be associated either with the disruption of the passive layer or with oxygen evolution, as discussed below. The critical potential or the breaking-off of the passive layer was defined as the potential where the current density reaches the level of 10 lA/cm2. In all three media (neutral, acidic or alkaline) the results followed the same trends, indicating that the differences in the alloy´s compositions are more important than the exposed environment. The three alloys Fe66B30Nb4, [(Fe0.6Co0.4)0.75B0.2Si0.05]96Nb4 and [(Fe0.7Co0.3)0.75B0.2Si0.05]96Nb4, show low corrosion resistance whatever the media. In fact the results, which suggest no effect of the different Co content in the FeCo-based alloys, indicate low corrosion potential and high corrosion current density; in addition, these alloys did not show a passive region. Inversely, the Cr-containing alloys, Fe56Cr23Ni5.7B16, Fe53Cr22Ni5.4B23 and Fe50Cr22Ni5.6B19 exhibit excellent anticorrosive
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Fig. 6. Polarization curves in neutral solution (pH = 5.5) for the alloys indicated in the figure.
properties, characterized by high corrosion potential and low corrosion current density, consistently lower than the values given by the alloy SS 316 LN. Moreover, a well defined passivation state, with a large passivation plateau has always been clearly observed. The three compositions presented a highly stable passivation layer as clearly shown by the comparison of the polarization curves of the FeCrNiB amorphous alloys and Pt. Indeed, the increase in current at high potential values always took place at roughly the same values as for the Pt electrode. This is a clear evidence that this increase must be ascribed to oxygen evolution and not to the corrosion of the alloys, which, in fact, resisted to all the potentials applied. On the other hand, the alloy used as reference, 316 LN, showed a small passive plateau soon interrupted by disruption of the passive film by pitting in all media analyzed. These results
indicate that for both the general and localized corrosion behavior, the FeCrNiB amorphous alloys showed superior performance to the 316L. This point deserves an important remark: the Cr content was 18% for the SS and 22% or 23% for the amorphous alloys. It is however worth noticing that even if the higher Cr content can be effectively expected to enhance and improve passivation, on the other hand the amorphous alloys contained less Mo than the SS, which is well-known to greatly improve corrosion resistance. Indeed, the weight of Mo in the improvement of localized corrosion resistance is considered to be 3.3 times greater than the one of Cr as it appears in the calculation of the widespread PREn (pitting resistance equivalent number, see for instance [29]). This can thus be seen as an indirect evidence of the beneficial role of the amorphous structure of the tested alloys in the overall corrosion resistant.
Fig. 7. Polarization curves in acid solution (pH = 1.0) for the alloys indicated in the figure.
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Fig. 8. Polarization curves in alkaline solution (pH = 10.0) for the alloys indicated in the figure.
Table 2 Electrochemical properties obtained from the polarization curves in neutral media (pH = 5.5). Ecorr (mV) Ecrit (mV) DE (mV) Icorr (A/cm2) Platinum Fe66B30Nb4 [(Fe0.6Co0.4)0.75B0.2Si0.05]96Nb4 [(Fe0.7Co0.3)0.75B0.2Si0.05]96Nb4 Fe56Cr23Ni5.7B16 Fe53Cr22Ni5.6B19 Fe50Cr22Ni5.4B23 SS 316LN
131 700 550 630 128 200 167 203
1096 689 512 611 1125 1116 1122 330
965 11 38 19 1253 1316 1289 533
2.0 10 1.5 10 2.0 10 7.0 10 1.5 10 6.0 10 2.0 10 2.0 10
8 5 6 6 8 8 8 7
A remarkable feature is that these Cr-containing amorphous Fe based alloys showed a corrosion resistance level close to that of the inert Pt in all studied media, ranging from very acidic to very alkaline chloride-rich solutions. These results were obtained notwithstanding the fact that both faces were immerged. This indicates that one can neglect for these samples the scenario of an intrinsic and deleterious galvanic coupling between the internal and the external faces that could deteriorate the corrosion performance. This constitutes a very wide range of electrochemical stability that is certainly of much concern for several industrial domains exposed to highly aggressive conditions. 4. Conclusions
Table 3 Electrochemical properties obtained from the polarization curves in acid media (pH = 1.0). Ecorr (mV) Ecrit (mV) DE (mV) Icorr (A/cm2) Platinum Fe66B30Nb4 [(Fe0.6Co0.4)0.75B0.2Si0.05]96Nb4 [(Fe0.7Co0.3)0.75B0.2Si0.05]96Nb4 Fe56Cr23Ni5.7B16 Fe53Cr22Ni5.6B19 Fe50Cr22Ni5.4B23 SS 316LN
47 458 381 389 192 190 246 219
866 453 373 380 910 929 918 15
913 5 8 9 1102 1119 1164 204
9.0 10 1.5 10 1.5 10 1.5 10 6.0 10 9.5 10 8.0 10 9.0 10
8 5 5 5 8 8 8 7
Table 4 Electrochemical properties obtained from the polarization curves in alkaline media (pH = 10.0). Ecorr (mV) Ecrit (mV) DE (mV) Icorr (A/cm2) Platinum Fe66B30Nb4 [(Fe0.6Co0.4)0.75B0.2Si0.05]96Nb4 [(Fe0.7 Co0.3)0.75B0.2Si0.05]96Nb4 Fe56Cr23Ni5.7B16 Fe53Cr22Ni5.6B19 Fe50Cr22Ni5.4B23 SS 316LN
119 637 509 520 209 247 320 377
1072 574 470 498 1028 1146 1016 99
953 63 39 22 1237 1393 1336 476
3.0 10 7.0 10 1.5 10 6.0 10 2.0 10 9.5 10 9.0 10 4.5 10
The corrosion resistance of Fe-based amorphous alloys was evaluated in comparison with a commercial stainless steel. In contrast with the crystalline stainless steel 316L, the electrochemical corrosion properties of the amorphous alloys did not vary significantly when measured in neutral, acid or alkaline solutions. However, the corrosion properties of the Fe-based amorphous alloys were strongly dependent on the alloy composition; the presence of Cr in this family of alloys had a great positive influence on the corrosion resistance with the formation of a stable passive film ensuring an extended passivation plateau.
7 6 6 6 8 8 8 7
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