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Corrosion behavior of niobium, tantalum and their alloys in hot hydrochloric and phosphoric acid solutions
International Journal of Refractory Metals & Hard Materials 18 (2000) 13±21
Corrosion behavior of niobium, tantalum and their alloys in hot hydrochloric and phosphoric acid solutions Alain Robin *, Jorge Luiz Rosa Departamento de Engenharia de Materiais, Faculdade de Engenharia Quõmica de Lorena, Polo Urbo-Industrial, Gleba AI-6 s/n, 12600-000 Lorena SP, Brazil Received 21 September 1999; accepted 16 November 1999
Abstract The corrosion resistance of niobium, tantalum and Nb±20, 40, 60 and 80 wt% Ta alloys in 5, 10, 15 and 20 wt% HCl and 20, 40, 60 and 80 wt% H3 PO4 solutions at boiling point, 150°C and 200°C is evaluated using the mass-loss technique. The corrosion rates of all materials increase with acid concentration and temperature. At boiling point the corrosion rates diminish with time but for long exposure time, they tend to stabilize due to the formation of super®cial oxides. The corrosion resistance increases with tantalum content. The iso-corrosion curves of niobium, tantalum and Nb±Ta alloys in 5±20 wt% HCl and 20±80 wt% H3 PO4 up to 200°C are drawn from interpolation of the measured corrosion rates. Ó 2000 Elsevier Science Ltd. All rights reserved. Keywords: Corrosion; Niobium; Tantalum; Alloys; Hydrochloric acid; Phosphoric acid
1. Introduction Niobium has an excellent corrosion resistance in mineral acids [1±7], except in hydro¯uoric acid [6], but in extremely corrosive media, like concentrated acids and/ or at high temperatures [4], niobium is appreciably attacked. On the contrary, tantalum is practically unaffected in all acidic solutions, even concentrated and hot [1,9±12]. Hence, tantalum, even if it is more expensive [9], is preferred to niobium for use in highly corrosive solutions. About 4% of the tantalum production is used in chemical industry for corrosion resistant components, like heaters, heat exchangers, reaction vessels, impellers for pumps etc. Some niobium±tantalum alloys were developed in order to obtain a material with better corrosion resistance than niobium and a lower price than tantalum [6,8,10,11,13]. The advantages of these alloys are: lower price, lower density, intermediary corrosion resistance between niobium and tantalum [6,8,10,11], ease to obtain as niobium and tantalum form a solid solution for all concentrations [14] ease of fabrication
*
Corresponding author. Fax: +55-12-553-3006.
of components due to the excellent ductility and ease of welding by Tungsten Inert Gas welding. In spite of the importance of both niobium and tantalum and the niobium±tantalum alloys, data on corrosion behavior of these materials in mineral acids, particularly of the alloys, are limited and dispersed except in sulfuric acid. In this work we present our results about the corrosion resistance of niobium, tantalum, Nb±20, 40, 60 and 80 wt% Ta alloys in two other important mineral acids, hydrochloric acid and phosphoric acid up to 200°C. The corrosion rates were measured by means of weight-loss technique, varying the acid concentration, the exposure time and the temperature. Measurements on all materials were conducted under the same conditions and the results were compared. From our corrosion data, we drew the iso-corrosion curves of niobium, tantalum and their alloys in hydrochloric and phosphoric acid solutions. 2. Experimental 2.1. Material preparation Niobium and tantalum ingots were obtained by melting in a 300 kW continuous casting electron-beam
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A. Robin, J.L. Rosa / International Journal of Refractory Metals & Hard Materials 18 (2000) 13±21
furnace under dynamic high-vacuum conditions (<10ÿ5 mbar). Afterwards, the ingots were cold-swaged, then cold-rolled to a thickness of 0.5 mm. Part of the shapes was used to prepare the test specimens of pure niobium and tantalum. The other part was used to produce the Nb±20, 40, 60 and 80 wt% Ta alloys. Niobium and tantalum sheets in the required proportion were melted in a laboratory arc-furnace under an ultrapure argon atmosphere. Successive meltings of the same buttons were operated to homogenize the alloys. Afterwards, the buttons were cold-rolled to a thickness of 0.5 mm. The chemical composition of niobium, tantalum and their alloys, determined using ion exchange technique for metal determination and by means of the technique of melting and inert gas carrier for gas analysis, is reported in Table 1. Despite the use of low-oxygen niobium and tantalum shapes to prepare the specimens and the previous melting of a titanium getter to catch residual oxygen contained in the furnace, materials with high-oxygen content (up to 0.1 wt%) were obtained. 2.2. Corrosion test procedure For corrosion tests in HCl and H3 PO4 solutions up to the boiling point, 1 l glass ¯asks and glass condensers to re¯ux vapors back into the solutions were used. The apparatus were heated by hemispherical heating mantles, whose temperature was controlled by a thermostat. For corrosion tests in HCl and H3 PO4 solutions above the boiling point, pressure vessels with internal PTFE (Te¯on) clad were used. The vessels were placed inside a resistive furnace and the temperature was controlled by a thermostat and a chromel±alumel thermocouple. Test solutions were prepared from HCl and H3 PO4 reagents (pro analysis) and deionized water. The corrosion tests were performed without stirring or aeration, except the natural. Rectangular test specimens (typical dimensions 35 10 0:5 mm3 ) were cut from the cold-rolled sheets of niobium, tantalum and Nb±Ta alloys, degreased, rinsed with distilled water and dried with acetone. No thermal treatment of the cold-rolled specimen was made. Surface areas were measured within 1% and
Table 2 Density of niobium, Nb±Ta alloys and tantalum Material
Density (g cmÿ3 )
Nb Nb±20Ta Nb±40Ta Nb±60Ta Nb±80Ta Ta
8.57 9.48 10.62 12.07 13.98 16.6
weights were determined with a precision of 0.1 mg. No polishing was performed. The specimens were placed vertically on a PTFE holder at the bottom of the ¯asks or the pressure vessels. Corrosion test samples were exposed to 5, 10, 15 and 20 wt% HCl and 20, 40, 60 and 80 wt% H3 PO4 solutions at dierent temperatures: boiling point, 150°C and 200°C. At the boiling point, the exposure times varied from 3 to 14 days and above the boiling point, the exposure duration was 14 days. After exposure, the samples were cleaned to remove any possible corrosion product and dried with acetone. The corrosion rates (in lm/yr) were calculated from the material weight-loss, the exposed area, the exposure time and the material density (Table 2). For every new set of test conditions (concentration, exposure time and temperature), unexposed samples were used. In order to obtain the iso-corrosion curves for 25, 125, 250 and 500 lm/yr corrosion rates with better accuracy, some additional tests at other temperatures and acid concentrations were necessary and the corresponding results will be presented too. These iso-corrosion curves were drawn by interpolation of our corrosion data.
3. Results 3.1. Corrosion behavior in HCl The corrosion behavior of niobium and Nb±20 wt% Ta alloy in boiling HCl as a function of acid concen-
Table 1 Chemical composition of niobium, Nb±Ta alloys and tantalum Material
Nb (wt%)
Ta (wt%)
N (wt ppm)
O (wt ppm)
Nb Nb±20Ta Nb±40Ta Nb±60Ta Nb±80Ta Ta
98.6 77.5 59.5 38.8 18.3 ±
± 20.4 39.4 60.9 80.8 99
25 2 42 12 33 6 29 5 20 4 13 2
156 15 599 41 643 42 653 24 966 15 204 3
A. Robin, J.L. Rosa / International Journal of Refractory Metals & Hard Materials 18 (2000) 13±21
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Fig. 1. Corrosion rates of niobium in boiling HCl solutions vs exposure time.
Fig. 3. Corrosion rates of niobium and Nb±20 wt% Ta alloy in boiling HCl solutions vs acid concentration.
Fig. 2. Corrosion rates of Nb±20 wt% Ta alloy in boiling HCl solutions vs exposure time.
Fig. 4. Corrosion rates of niobium, tantalum and their alloys in HCl solutions at 150°C vs acid concentration.
tration and exposure time is shown in Figs. 1 and 2. The corrosion rates diminish with test duration. The corrosion rates of both materials measured for the longest test duration are presented in Fig. 3. For niobium and Nb± 20 wt% Ta alloy, the corrosion rate increases with acid concentration. We note that the Nb±20 wt% Ta alloy is more corrosion resistant than niobium. Two complementary corrosion tests of niobium were made in order to have additional data for drawing iso-corrosion curves. In boiling 17.5 wt% HCl and in 20 wt% HCl at 75°C, the corrosion rates of niobium are 97 and 33 lm/yr, respectively after a 14 days exposure time. For the other Nb±Ta alloys and tantalum, no weight changes were measured in all boiling solutions for 14 days exposure times. At 150°C and 200°C, the same observations can be made: increase of corrosion rates of all materials with acid concentration; increase of corrosion resistance with tantalum content (Figs. 4 and 5). A great increase of corrosion rates is noted between 15 and 20 wt% HCl at these high temperatures.
Fig. 5. Corrosion rates of niobium, tantalum and their alloys in HCl solutions at 200°C vs acid concentration.
The corrosion rates increase with temperature for all materials and all acid concentrations (Figs. 3±5). For all the experiments, the attack occurred uniformly and no localized corrosion was noted. After
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corrosion tests, the samples were generally etched and had a silvery appearance. From all the data, the iso-corrosion curves for niobium, Nb±20, 40 and 60 wt% Ta were obtained (Figs. 6±9). For the other alloy and tantalum, the curves were not drawn since these materials did not present corrosion rates higher than 25 lm/yr. We note that each iso-corrosion curve shifts to the direction of higher acid concentration and higher temperature when the tantalum content increases. 3.2. Corrosion behavior in H3 PO4 The corrosion rates of niobium, tantalum and their alloys in boiling H3 PO4 solutions are presented in Figs. 10±15 as a function of acid concentration and exposure time. As observed in boiling HCl solutions, the corrosion rates decrease with time and tend to stabilize for long exposure times. No weight loss was measured for the Nb±80 wt% Ta alloy and tantalum in boiling 20
Fig. 8. Iso-corrosion curves of Nb±40 wt% Ta alloy in HCl solutions.
Fig. 9. Iso-corrosion curves of Nb±60 wt% Ta alloy in HCl solutions. Fig. 6. Iso-corrosion curves of niobium in HCl solutions.
Fig. 7. Iso-corrosion curves of Nb±20 wt% Ta alloy in HCl solutions.
Fig. 10. Corrosion rates of niobium in boiling H3 PO4 solutions vs exposure time.
A. Robin, J.L. Rosa / International Journal of Refractory Metals & Hard Materials 18 (2000) 13±21
Fig. 11. Corrosion rates of Nb±20 wt% Ta alloy in boiling H3 PO4 solutions vs exposure time.
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Fig. 14. Corrosion rates of Nb±80 wt% Ta alloy in boiling H3 PO4 solutions vs exposure time.
Fig. 15. Corrosion rates of tantalum in boiling H3 PO4 solutions vs exposure time. Fig. 12. Corrosion rates of Nb±40 wt% Ta alloy in boiling H3 PO4 solutions vs exposure time.
Fig. 16. Corrosion rates of niobium, tantalum and their alloys in boiling H3 PO4 solutions vs acid concentration.
Fig. 13. Corrosion rates of Nb±60 wt% Ta alloy in boiling H3 PO4 solutions vs exposure time.
wt% H3 PO4 and for tantalum in boiling 40 wt% H3 PO4 after 14 days exposure. The corrosion rates obtained for the longest exposure time (Fig. 16) increase with acid concentration and decrease with tantalum content.
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Fig. 17. Corrosion rates of niobium, tantalum and their alloys in H3 PO4 solutions at 150°C vs acid concentration.
materials (Figs. 16±18). Niobium and Nb±20 wt% Ta alloy were not tested in 60 and 80 wt% H3 PO4 at 200°C since these materials presented high corrosion rates in less concentrated solutions at the same temperature. Some tests at other temperatures and H3 PO4 concentrations were performed during 14 days in order to obtain data for drawing the iso-corrosion curves with better accuracy. These test conditions and the corresponding results are presented in Table 3. We observe that below the boiling point, the corrosion rates also increase with temperature and acid concentration and decrease with tantalum content. Uniformity of attack and etched and silvery appearance of the corroded samples were also observed after the corrosion tests in H3 PO4 solutions. The iso-corrosion curves for niobium, Nb±Ta alloys and tantalum are shown in Figs. 19±24. As observed in HCl solutions, each iso-corrosion curve shifts to the direction of higher acid concentration and higher temperature when the tantalum content increases.
4. Discussion
Fig. 18. Corrosion rates of niobium, tantalum and their alloys in H3 PO4 solutions at 200°C vs acid concentration.
At 150°C and 200°C, the corrosion rates also increase with acid concentration and decrease with tantalum content (Figs. 17 and 18). The corrosion rates increase with temperature for all acid concentrations and all
The corrosion resistance of niobium, tantalum and Nb±Ta alloys in HCl and H3 PO4 solutions depends on acid concentration, temperature and tantalum content. An increase of acid concentration and temperature results in an increase of the corrosion rate, whereas an increase of the latter parameter leads to the decrease of the corrosion rate. Most of the data on niobium behavior in HCl solutions reports that its corrosion rates in boiling 10 and 20 wt% HCl are respectively 5±7 lm/yr [3,8,15] and 205± 375 lm/yr [3,8,13,15] which is in accordance with our values, 8 and 220 lm/yr, respectively. Bishop [4] found higher corrosion rates, 37.5, 100, 450 and 1000 lm/yr in
Table 3 Corrosion rates of niobium and Nb±Ta alloys in H3 PO4 solutions at dierent temperatures H3 PO4 concentration (wt%)
a
Temperature (°C)
Corrosion rate (lm/yr) Nb
Nb±20Ta
Nb±40Ta
Nb±60Ta
60
50 80
4.3 42.6
NTa NT
NT NT
NT NT
70
50 100 BP
4.4 101.3 NT
0.3 67.1 NT
NT NT 49.6
NT NT NT
80
50 70 100
5.9 22.7 107.2
0.4 3.4 100.7
NT NT 18.1
NT NT 2.0
NT ± not tested.
A. Robin, J.L. Rosa / International Journal of Refractory Metals & Hard Materials 18 (2000) 13±21
Fig. 19. Iso-corrosion curves of niobium in H3 PO4 solutions.
Fig. 20. Iso-corrosion curves of Nb±20 wt% Ta alloy in H3 PO4 solutions.
Fig. 21. Iso-corrosion curves of Nb±40 wt% Ta alloy in H3 PO4 solutions.
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Fig. 22. Iso-corrosion curves of Nb±60 wt% Ta alloy in H3 PO4 solutions.
Fig. 23. Iso-corrosion curves of Nb±80 wt% Ta alloy in H3 PO4 solutions.
Fig. 24. Iso-corrosion curves of tantalum in H3 PO4 solutions.
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A. Robin, J.L. Rosa / International Journal of Refractory Metals & Hard Materials 18 (2000) 13±21
boiling 5, 10, 15 and 20 wt% HCl solutions. This author did not specify the exact chemical composition of niobium and the procedure for preparation of the test coupons. It is known that such parameters can in¯uence signi®cantly the corrosion behavior of metals [16]. The comparison of the 125 lm/yr iso-corrosion curve of niobium proposed by Hunkeler [9], drawn from a compilation and interpolation of data of several sources, with our corresponding data (Fig. 6) show that our niobium specimens are a little more corrosion resistant. Indeed, above the boiling point, the corrosion rate of niobium is reported to be higher than 125 lm/yr for acid concentration higher than 12.5 wt% [9] against nearly 17 wt% HCl in this work. For tantalum, we measured very low corrosion rates up to 20 wt% HCl and up to 200°C, according to the data reported in the literature [4,8±10,12]. For the Nb±20 wt% Ta alloy, corrosion rates lower than 125 lm/yr were measured for HCl concentrations lower than 20 wt% up to 150°C and lower than 15 wt% up to 200°C [8,10]. In this work, our Nb±20 wt% Ta alloy resists with corrosion rates lower than 125 lm/yr in 5±20 wt% HCl up to 125°C and in 5±15 wt% HCl up to 200°C (Fig. 7) according to the data of literature. For the Nb±40, 60 and 80 wt% Ta alloys, the corrosion rates are reported to be lower than 125 lm/yr up to 200°C and up to 20 wt% HCl concentration [2,8±10,13,16]. Our results on Nb±60 and 80 wt% Ta alloys are also in agreement with these data (Figs. 5 and 9). Nevertheless, our Nb±40 wt% Ta alloy is less resistant since its corrosion rates are higher than 125 lm/yr in 17.5±20 wt% HCl at 200°C and in 20 wt% HCl above nearly 160°C (Fig. 8). The few data available on the corrosion behavior of niobium in H3 PO4 solutions show that in 85 wt% H3 PO4 at room temperature [1], 88°C [1], 100°C [1], boiling point [1] and 205°C [15], the corrosion rates are 2.5, 50, 125, 3750 and 31,000 lm/yr, respectively. Though our experiments were made in slightly dierent conditions, our results are not far from these data on niobium as we found for 80 wt% H3 PO4 at 50°C, 75°C, 100°C, boiling point and 150°C, 5.9, 23, 107, 477 and 1,518 lm/yr, respectively. For tantalum, the corrosion rates reported in the literature are lower than 25 lm/yr for all H3 PO4 concentrations up to 190°C [1,4]. In our work, tantalum is less resistant (Fig. 18). Its highest corrosion rate (nearly 125 lm/yr) was measured in 80 wt% H3 PO4 at 200°C. For the Nb±Ta alloys, the unique data we found in literature are the corrosion rates of Nb±33% Ta (Nb± 48.9 wt% Ta) and Nb±40% Ta (Nb±56 wt% Ta) in 85 wt% H3 PO4 at 205°C, respectively 6400 and 3800 lm/yr [15]. Our corrosion rate values for the Nb±40 and 60 wt% Ta alloys in 80 wt% H3 PO4 at 200°C, 11,552 and 3056 lm/yr, respectively, are congruent with these data.
In both HCl and H3 PO4 solutions, the corrosion attack was uniform and the specimen surface had a etched and silvery appearance. The decrease of the corrosion rates with exposure time has ever been observed for niobium in HCl [5] and H2 SO4 [17] solutions and for niobium, Nb±Ta alloys and tantalum in H2 SO4 solutions [11]. Such a decrease is typical of the formation of a ®lm on the metal or alloy that provides better corrosion resistance of the surface to the corrosive environment. Films of Nb2 O5 and Ta2 O5 amorphous oxides (usually a few nanometers thick) are formed on niobium and tantalum [11,16±18] according to: 2M 5H2 O ! M2 O5 10H 10e
M Nb or Ta
and the metal mass loss is due to the following reactions: M ! M V 5e M2 O5 10H ! 2M V 5H2 O The composition of the oxides formed on Nb±Ta alloys is not well-known. Lupton and Aldinger [19] showed that all the binary alloys develop amorphous oxide ®lms containing both niobium and tantalum during exposure in boiling 70 wt% H2 SO4 solutions. There is evidence that these ®lms are composed of a-Nb2 O5 and b-Ta2 O5 according to the crystalline oxides phase diagram. In the tantalum content range where a-Nb2 O5 develops, high corrosion rates were measured in boiling 20 wt% HCl solutions, whereas for higher tantalum content where b-Ta2 O5 forms, no notable corrosion was observed [16]. Similar behavior was noted in this work as we also found that an increase in tantalum content improved the corrosion resistance. No analysis of oxygen was made on corroded specimen. Nevertheless, since the oxides formed spontaneously on these metals and alloys in acid solutions are usually few nanometers-thick and their densities are expected to be between 4.35 (Nb2 O5 ) and 7.93 g.cmÿ3 (Ta2 O5 ), the weight of O incorporated in a 10 nm thick (for example) oxide layer on a 7.5 cm2 specimen area should be nearly 10 lg. This low value has little in¯uence on the mass-loss measurements and consequently should not aect signi®cantly the thickness changes calculated from mass changes and materials densities. Comparing the corrosion data and the iso-corrosion curves of the Nb±Ta alloys in both HCl and H3 PO4 solutions, the alloys become very corrosion resistant from 60 wt% Ta content. Lupton [19] and Robin [11] in H2 SO4 and Mosolov [10] in HCl solutions found nearly the same value. This behavior explains why the usual substitute for pure tantalum is an alloy with high tantalum content (Nb±60 wt% Ta or KBI40 alloy) [13].
A. Robin, J.L. Rosa / International Journal of Refractory Metals & Hard Materials 18 (2000) 13±21
5. Conclusions The study of corrosion behavior of niobium, tantalum and Nb±Ta alloys in HCl and H3 PO4 solutions from boiling point to 200°C shows that:
[7]
· an increase of acid concentration results in an increase of corrosion rates for all materials and temperatures, · the corrosion rates increase with an increase of temperature for all materials and acid concentrations, · the addition of tantalum improves the corrosion resistance of niobium, · the Nb±Ta alloys become very corrosion resistant from 60 wt% Ta content, and · the corrosion rates decrease with exposure time due to the formation of super®cial oxides.
[9]
[8]
[10]
[11] [12]
[13] [14]
References [1] Metals handbook. Corrosion. Metals Park, OH: ASM International, 1987;13:722±39. [2] Burns RH, Shuker FS, Manning PE. Industrial applications of corrosion resistant tantalum, niobium and their alloys. In: Smallwood RE, editor. ASTM Special Technical Publication 849, ASTM, 1984. p. 50±69. [3] Mc Leary DL. Testing of columbium and columbium alloys. Corrosion 1962;18:67t±9t. [4] Bishop CR. Corrosion tests at elevated temperatures and pressures. Corrosion 1963;19:308t±14t. [5] Covino Jr BS, Carter JP, Cramer SD. The corrosion behavior of niobium in hydrochloric acid solutions. Corrosion 1980;36:554±8. [6] Lupton D, Schimann W, Schreiber F, Heitz E. Corrosion behaviour of tantalum and possible substitutes materials under
[15] [16] [17]
[18] [19]
21
extreme conditions. In: Proceedings of the Eighth International Congress on Metallic Corrosion, Mainz, 1981;2:1441±6. Uehara I, Sakai T, Ishikawa H, Ishii E, Nakane M. The corrosion behavior of niobium and tantalum in hydrobromic acid solutions. Corrosion 1986;42:492±8. Kieer R, Bach H, Stempkowski I. Korrosionuntersuchungen an Niob-Tantal Legierungen. Werkst Korros 1967;18:782±4. Hunkeler FJ. Properties of tantalum for applications in the chemical process industry. In: Smallwood RE, editor. ASTM Special Technical Publication 849, ASTM, 1984. p. 28±49. Mosolov AV, Nefedova ID, Klinov IYa. Corrosion and electrochemical behaviour of niobium±tantalum alloys in HCl at elevated temperatures and pressures. Zashch Met 1968;4: 248±51. Robin A. Corrosion behavior of niobium, tantalum and their alloys in boiling sulfuric acid solutions. Int J Refr Metals and Hard Mater 1997;15:317±23. Eichner P, Ferat A, Mazille H, Tamagne C. Reactivite du Tantale en Milieu Acide Concentre et Chaud: Corrosion et Fragilisation. In: Barret P, Dufour LC, editors. Reactivity of metals. Amsterdam: Elsevier, 1985. p. 201±3. Alloy digest. March 1984:Ta 7:2. Massalski TB. Niobium±tantalum phase diagram. In: Binary alloy phase diagrams, vol. 2. ASM, OH, 1986. Lupton D, Aldinger F, Schulze K. Niobium in corrosive environments. In: Proceedings of the International Symposium NiobiumÕ81, San Francisco, 1981. p. 533±60. Krehl M, Schulze K, Olzi E, Petzow G. Korrosionverhalten von Gegluhten Niob-Tantal-Legierungen. Z Metallkde 1983;74: 358±63. Bulh~ oes LOS, Joanni E. Characteristics of corrosion of niobium and anodized niobium in sulfuric acid solutions. In: Proceedings of the 11th International Congress on Metallic Corrosion, Ottawa, 1984;3:437±39. Uehara I, Sakai T, Ishikawa H, Takenaka H. Corrosion behavior of tantalum and niobium in hydrobromic acid solutions (II) on passive ®lms and hydrogen absorption. Corrosion 1989;45:548±53. Lupton D, Aldinger F. Possible substitutes for tantalum in chemical plant handling mineral acids. In: Proceedings of the 10th Plansee Seminar, Reutte, 1981;1:101±30.
Corrosion behavior of niobium, tantalum and their alloys in hot hydrochloric and phosphoric acid solutions Alain Robin *, Jorge Luiz Rosa Departamento de Engenharia de Materiais, Faculdade de Engenharia Quõmica de Lorena, Polo Urbo-Industrial, Gleba AI-6 s/n, 12600-000 Lorena SP, Brazil Received 21 September 1999; accepted 16 November 1999
Abstract The corrosion resistance of niobium, tantalum and Nb±20, 40, 60 and 80 wt% Ta alloys in 5, 10, 15 and 20 wt% HCl and 20, 40, 60 and 80 wt% H3 PO4 solutions at boiling point, 150°C and 200°C is evaluated using the mass-loss technique. The corrosion rates of all materials increase with acid concentration and temperature. At boiling point the corrosion rates diminish with time but for long exposure time, they tend to stabilize due to the formation of super®cial oxides. The corrosion resistance increases with tantalum content. The iso-corrosion curves of niobium, tantalum and Nb±Ta alloys in 5±20 wt% HCl and 20±80 wt% H3 PO4 up to 200°C are drawn from interpolation of the measured corrosion rates. Ó 2000 Elsevier Science Ltd. All rights reserved. Keywords: Corrosion; Niobium; Tantalum; Alloys; Hydrochloric acid; Phosphoric acid
1. Introduction Niobium has an excellent corrosion resistance in mineral acids [1±7], except in hydro¯uoric acid [6], but in extremely corrosive media, like concentrated acids and/ or at high temperatures [4], niobium is appreciably attacked. On the contrary, tantalum is practically unaffected in all acidic solutions, even concentrated and hot [1,9±12]. Hence, tantalum, even if it is more expensive [9], is preferred to niobium for use in highly corrosive solutions. About 4% of the tantalum production is used in chemical industry for corrosion resistant components, like heaters, heat exchangers, reaction vessels, impellers for pumps etc. Some niobium±tantalum alloys were developed in order to obtain a material with better corrosion resistance than niobium and a lower price than tantalum [6,8,10,11,13]. The advantages of these alloys are: lower price, lower density, intermediary corrosion resistance between niobium and tantalum [6,8,10,11], ease to obtain as niobium and tantalum form a solid solution for all concentrations [14] ease of fabrication
*
Corresponding author. Fax: +55-12-553-3006.
of components due to the excellent ductility and ease of welding by Tungsten Inert Gas welding. In spite of the importance of both niobium and tantalum and the niobium±tantalum alloys, data on corrosion behavior of these materials in mineral acids, particularly of the alloys, are limited and dispersed except in sulfuric acid. In this work we present our results about the corrosion resistance of niobium, tantalum, Nb±20, 40, 60 and 80 wt% Ta alloys in two other important mineral acids, hydrochloric acid and phosphoric acid up to 200°C. The corrosion rates were measured by means of weight-loss technique, varying the acid concentration, the exposure time and the temperature. Measurements on all materials were conducted under the same conditions and the results were compared. From our corrosion data, we drew the iso-corrosion curves of niobium, tantalum and their alloys in hydrochloric and phosphoric acid solutions. 2. Experimental 2.1. Material preparation Niobium and tantalum ingots were obtained by melting in a 300 kW continuous casting electron-beam
0263-4368/00/$ - see front matter Ó 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 2 6 3 - 4 3 6 8 ( 9 9 ) 0 0 0 3 4 - 7
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A. Robin, J.L. Rosa / International Journal of Refractory Metals & Hard Materials 18 (2000) 13±21
furnace under dynamic high-vacuum conditions (<10ÿ5 mbar). Afterwards, the ingots were cold-swaged, then cold-rolled to a thickness of 0.5 mm. Part of the shapes was used to prepare the test specimens of pure niobium and tantalum. The other part was used to produce the Nb±20, 40, 60 and 80 wt% Ta alloys. Niobium and tantalum sheets in the required proportion were melted in a laboratory arc-furnace under an ultrapure argon atmosphere. Successive meltings of the same buttons were operated to homogenize the alloys. Afterwards, the buttons were cold-rolled to a thickness of 0.5 mm. The chemical composition of niobium, tantalum and their alloys, determined using ion exchange technique for metal determination and by means of the technique of melting and inert gas carrier for gas analysis, is reported in Table 1. Despite the use of low-oxygen niobium and tantalum shapes to prepare the specimens and the previous melting of a titanium getter to catch residual oxygen contained in the furnace, materials with high-oxygen content (up to 0.1 wt%) were obtained. 2.2. Corrosion test procedure For corrosion tests in HCl and H3 PO4 solutions up to the boiling point, 1 l glass ¯asks and glass condensers to re¯ux vapors back into the solutions were used. The apparatus were heated by hemispherical heating mantles, whose temperature was controlled by a thermostat. For corrosion tests in HCl and H3 PO4 solutions above the boiling point, pressure vessels with internal PTFE (Te¯on) clad were used. The vessels were placed inside a resistive furnace and the temperature was controlled by a thermostat and a chromel±alumel thermocouple. Test solutions were prepared from HCl and H3 PO4 reagents (pro analysis) and deionized water. The corrosion tests were performed without stirring or aeration, except the natural. Rectangular test specimens (typical dimensions 35 10 0:5 mm3 ) were cut from the cold-rolled sheets of niobium, tantalum and Nb±Ta alloys, degreased, rinsed with distilled water and dried with acetone. No thermal treatment of the cold-rolled specimen was made. Surface areas were measured within 1% and
Table 2 Density of niobium, Nb±Ta alloys and tantalum Material
Density (g cmÿ3 )
Nb Nb±20Ta Nb±40Ta Nb±60Ta Nb±80Ta Ta
8.57 9.48 10.62 12.07 13.98 16.6
weights were determined with a precision of 0.1 mg. No polishing was performed. The specimens were placed vertically on a PTFE holder at the bottom of the ¯asks or the pressure vessels. Corrosion test samples were exposed to 5, 10, 15 and 20 wt% HCl and 20, 40, 60 and 80 wt% H3 PO4 solutions at dierent temperatures: boiling point, 150°C and 200°C. At the boiling point, the exposure times varied from 3 to 14 days and above the boiling point, the exposure duration was 14 days. After exposure, the samples were cleaned to remove any possible corrosion product and dried with acetone. The corrosion rates (in lm/yr) were calculated from the material weight-loss, the exposed area, the exposure time and the material density (Table 2). For every new set of test conditions (concentration, exposure time and temperature), unexposed samples were used. In order to obtain the iso-corrosion curves for 25, 125, 250 and 500 lm/yr corrosion rates with better accuracy, some additional tests at other temperatures and acid concentrations were necessary and the corresponding results will be presented too. These iso-corrosion curves were drawn by interpolation of our corrosion data.
3. Results 3.1. Corrosion behavior in HCl The corrosion behavior of niobium and Nb±20 wt% Ta alloy in boiling HCl as a function of acid concen-
Table 1 Chemical composition of niobium, Nb±Ta alloys and tantalum Material
Nb (wt%)
Ta (wt%)
N (wt ppm)
O (wt ppm)
Nb Nb±20Ta Nb±40Ta Nb±60Ta Nb±80Ta Ta
98.6 77.5 59.5 38.8 18.3 ±
± 20.4 39.4 60.9 80.8 99
25 2 42 12 33 6 29 5 20 4 13 2
156 15 599 41 643 42 653 24 966 15 204 3
A. Robin, J.L. Rosa / International Journal of Refractory Metals & Hard Materials 18 (2000) 13±21
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Fig. 1. Corrosion rates of niobium in boiling HCl solutions vs exposure time.
Fig. 3. Corrosion rates of niobium and Nb±20 wt% Ta alloy in boiling HCl solutions vs acid concentration.
Fig. 2. Corrosion rates of Nb±20 wt% Ta alloy in boiling HCl solutions vs exposure time.
Fig. 4. Corrosion rates of niobium, tantalum and their alloys in HCl solutions at 150°C vs acid concentration.
tration and exposure time is shown in Figs. 1 and 2. The corrosion rates diminish with test duration. The corrosion rates of both materials measured for the longest test duration are presented in Fig. 3. For niobium and Nb± 20 wt% Ta alloy, the corrosion rate increases with acid concentration. We note that the Nb±20 wt% Ta alloy is more corrosion resistant than niobium. Two complementary corrosion tests of niobium were made in order to have additional data for drawing iso-corrosion curves. In boiling 17.5 wt% HCl and in 20 wt% HCl at 75°C, the corrosion rates of niobium are 97 and 33 lm/yr, respectively after a 14 days exposure time. For the other Nb±Ta alloys and tantalum, no weight changes were measured in all boiling solutions for 14 days exposure times. At 150°C and 200°C, the same observations can be made: increase of corrosion rates of all materials with acid concentration; increase of corrosion resistance with tantalum content (Figs. 4 and 5). A great increase of corrosion rates is noted between 15 and 20 wt% HCl at these high temperatures.
Fig. 5. Corrosion rates of niobium, tantalum and their alloys in HCl solutions at 200°C vs acid concentration.
The corrosion rates increase with temperature for all materials and all acid concentrations (Figs. 3±5). For all the experiments, the attack occurred uniformly and no localized corrosion was noted. After
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A. Robin, J.L. Rosa / International Journal of Refractory Metals & Hard Materials 18 (2000) 13±21
corrosion tests, the samples were generally etched and had a silvery appearance. From all the data, the iso-corrosion curves for niobium, Nb±20, 40 and 60 wt% Ta were obtained (Figs. 6±9). For the other alloy and tantalum, the curves were not drawn since these materials did not present corrosion rates higher than 25 lm/yr. We note that each iso-corrosion curve shifts to the direction of higher acid concentration and higher temperature when the tantalum content increases. 3.2. Corrosion behavior in H3 PO4 The corrosion rates of niobium, tantalum and their alloys in boiling H3 PO4 solutions are presented in Figs. 10±15 as a function of acid concentration and exposure time. As observed in boiling HCl solutions, the corrosion rates decrease with time and tend to stabilize for long exposure times. No weight loss was measured for the Nb±80 wt% Ta alloy and tantalum in boiling 20
Fig. 8. Iso-corrosion curves of Nb±40 wt% Ta alloy in HCl solutions.
Fig. 9. Iso-corrosion curves of Nb±60 wt% Ta alloy in HCl solutions. Fig. 6. Iso-corrosion curves of niobium in HCl solutions.
Fig. 7. Iso-corrosion curves of Nb±20 wt% Ta alloy in HCl solutions.
Fig. 10. Corrosion rates of niobium in boiling H3 PO4 solutions vs exposure time.
A. Robin, J.L. Rosa / International Journal of Refractory Metals & Hard Materials 18 (2000) 13±21
Fig. 11. Corrosion rates of Nb±20 wt% Ta alloy in boiling H3 PO4 solutions vs exposure time.
17
Fig. 14. Corrosion rates of Nb±80 wt% Ta alloy in boiling H3 PO4 solutions vs exposure time.
Fig. 15. Corrosion rates of tantalum in boiling H3 PO4 solutions vs exposure time. Fig. 12. Corrosion rates of Nb±40 wt% Ta alloy in boiling H3 PO4 solutions vs exposure time.
Fig. 16. Corrosion rates of niobium, tantalum and their alloys in boiling H3 PO4 solutions vs acid concentration.
Fig. 13. Corrosion rates of Nb±60 wt% Ta alloy in boiling H3 PO4 solutions vs exposure time.
wt% H3 PO4 and for tantalum in boiling 40 wt% H3 PO4 after 14 days exposure. The corrosion rates obtained for the longest exposure time (Fig. 16) increase with acid concentration and decrease with tantalum content.
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A. Robin, J.L. Rosa / International Journal of Refractory Metals & Hard Materials 18 (2000) 13±21
Fig. 17. Corrosion rates of niobium, tantalum and their alloys in H3 PO4 solutions at 150°C vs acid concentration.
materials (Figs. 16±18). Niobium and Nb±20 wt% Ta alloy were not tested in 60 and 80 wt% H3 PO4 at 200°C since these materials presented high corrosion rates in less concentrated solutions at the same temperature. Some tests at other temperatures and H3 PO4 concentrations were performed during 14 days in order to obtain data for drawing the iso-corrosion curves with better accuracy. These test conditions and the corresponding results are presented in Table 3. We observe that below the boiling point, the corrosion rates also increase with temperature and acid concentration and decrease with tantalum content. Uniformity of attack and etched and silvery appearance of the corroded samples were also observed after the corrosion tests in H3 PO4 solutions. The iso-corrosion curves for niobium, Nb±Ta alloys and tantalum are shown in Figs. 19±24. As observed in HCl solutions, each iso-corrosion curve shifts to the direction of higher acid concentration and higher temperature when the tantalum content increases.
4. Discussion
Fig. 18. Corrosion rates of niobium, tantalum and their alloys in H3 PO4 solutions at 200°C vs acid concentration.
At 150°C and 200°C, the corrosion rates also increase with acid concentration and decrease with tantalum content (Figs. 17 and 18). The corrosion rates increase with temperature for all acid concentrations and all
The corrosion resistance of niobium, tantalum and Nb±Ta alloys in HCl and H3 PO4 solutions depends on acid concentration, temperature and tantalum content. An increase of acid concentration and temperature results in an increase of the corrosion rate, whereas an increase of the latter parameter leads to the decrease of the corrosion rate. Most of the data on niobium behavior in HCl solutions reports that its corrosion rates in boiling 10 and 20 wt% HCl are respectively 5±7 lm/yr [3,8,15] and 205± 375 lm/yr [3,8,13,15] which is in accordance with our values, 8 and 220 lm/yr, respectively. Bishop [4] found higher corrosion rates, 37.5, 100, 450 and 1000 lm/yr in
Table 3 Corrosion rates of niobium and Nb±Ta alloys in H3 PO4 solutions at dierent temperatures H3 PO4 concentration (wt%)
a
Temperature (°C)
Corrosion rate (lm/yr) Nb
Nb±20Ta
Nb±40Ta
Nb±60Ta
60
50 80
4.3 42.6
NTa NT
NT NT
NT NT
70
50 100 BP
4.4 101.3 NT
0.3 67.1 NT
NT NT 49.6
NT NT NT
80
50 70 100
5.9 22.7 107.2
0.4 3.4 100.7
NT NT 18.1
NT NT 2.0
NT ± not tested.
A. Robin, J.L. Rosa / International Journal of Refractory Metals & Hard Materials 18 (2000) 13±21
Fig. 19. Iso-corrosion curves of niobium in H3 PO4 solutions.
Fig. 20. Iso-corrosion curves of Nb±20 wt% Ta alloy in H3 PO4 solutions.
Fig. 21. Iso-corrosion curves of Nb±40 wt% Ta alloy in H3 PO4 solutions.
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Fig. 22. Iso-corrosion curves of Nb±60 wt% Ta alloy in H3 PO4 solutions.
Fig. 23. Iso-corrosion curves of Nb±80 wt% Ta alloy in H3 PO4 solutions.
Fig. 24. Iso-corrosion curves of tantalum in H3 PO4 solutions.
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A. Robin, J.L. Rosa / International Journal of Refractory Metals & Hard Materials 18 (2000) 13±21
boiling 5, 10, 15 and 20 wt% HCl solutions. This author did not specify the exact chemical composition of niobium and the procedure for preparation of the test coupons. It is known that such parameters can in¯uence signi®cantly the corrosion behavior of metals [16]. The comparison of the 125 lm/yr iso-corrosion curve of niobium proposed by Hunkeler [9], drawn from a compilation and interpolation of data of several sources, with our corresponding data (Fig. 6) show that our niobium specimens are a little more corrosion resistant. Indeed, above the boiling point, the corrosion rate of niobium is reported to be higher than 125 lm/yr for acid concentration higher than 12.5 wt% [9] against nearly 17 wt% HCl in this work. For tantalum, we measured very low corrosion rates up to 20 wt% HCl and up to 200°C, according to the data reported in the literature [4,8±10,12]. For the Nb±20 wt% Ta alloy, corrosion rates lower than 125 lm/yr were measured for HCl concentrations lower than 20 wt% up to 150°C and lower than 15 wt% up to 200°C [8,10]. In this work, our Nb±20 wt% Ta alloy resists with corrosion rates lower than 125 lm/yr in 5±20 wt% HCl up to 125°C and in 5±15 wt% HCl up to 200°C (Fig. 7) according to the data of literature. For the Nb±40, 60 and 80 wt% Ta alloys, the corrosion rates are reported to be lower than 125 lm/yr up to 200°C and up to 20 wt% HCl concentration [2,8±10,13,16]. Our results on Nb±60 and 80 wt% Ta alloys are also in agreement with these data (Figs. 5 and 9). Nevertheless, our Nb±40 wt% Ta alloy is less resistant since its corrosion rates are higher than 125 lm/yr in 17.5±20 wt% HCl at 200°C and in 20 wt% HCl above nearly 160°C (Fig. 8). The few data available on the corrosion behavior of niobium in H3 PO4 solutions show that in 85 wt% H3 PO4 at room temperature [1], 88°C [1], 100°C [1], boiling point [1] and 205°C [15], the corrosion rates are 2.5, 50, 125, 3750 and 31,000 lm/yr, respectively. Though our experiments were made in slightly dierent conditions, our results are not far from these data on niobium as we found for 80 wt% H3 PO4 at 50°C, 75°C, 100°C, boiling point and 150°C, 5.9, 23, 107, 477 and 1,518 lm/yr, respectively. For tantalum, the corrosion rates reported in the literature are lower than 25 lm/yr for all H3 PO4 concentrations up to 190°C [1,4]. In our work, tantalum is less resistant (Fig. 18). Its highest corrosion rate (nearly 125 lm/yr) was measured in 80 wt% H3 PO4 at 200°C. For the Nb±Ta alloys, the unique data we found in literature are the corrosion rates of Nb±33% Ta (Nb± 48.9 wt% Ta) and Nb±40% Ta (Nb±56 wt% Ta) in 85 wt% H3 PO4 at 205°C, respectively 6400 and 3800 lm/yr [15]. Our corrosion rate values for the Nb±40 and 60 wt% Ta alloys in 80 wt% H3 PO4 at 200°C, 11,552 and 3056 lm/yr, respectively, are congruent with these data.
In both HCl and H3 PO4 solutions, the corrosion attack was uniform and the specimen surface had a etched and silvery appearance. The decrease of the corrosion rates with exposure time has ever been observed for niobium in HCl [5] and H2 SO4 [17] solutions and for niobium, Nb±Ta alloys and tantalum in H2 SO4 solutions [11]. Such a decrease is typical of the formation of a ®lm on the metal or alloy that provides better corrosion resistance of the surface to the corrosive environment. Films of Nb2 O5 and Ta2 O5 amorphous oxides (usually a few nanometers thick) are formed on niobium and tantalum [11,16±18] according to: 2M 5H2 O ! M2 O5 10H 10e
M Nb or Ta
and the metal mass loss is due to the following reactions: M ! M V 5e M2 O5 10H ! 2M V 5H2 O The composition of the oxides formed on Nb±Ta alloys is not well-known. Lupton and Aldinger [19] showed that all the binary alloys develop amorphous oxide ®lms containing both niobium and tantalum during exposure in boiling 70 wt% H2 SO4 solutions. There is evidence that these ®lms are composed of a-Nb2 O5 and b-Ta2 O5 according to the crystalline oxides phase diagram. In the tantalum content range where a-Nb2 O5 develops, high corrosion rates were measured in boiling 20 wt% HCl solutions, whereas for higher tantalum content where b-Ta2 O5 forms, no notable corrosion was observed [16]. Similar behavior was noted in this work as we also found that an increase in tantalum content improved the corrosion resistance. No analysis of oxygen was made on corroded specimen. Nevertheless, since the oxides formed spontaneously on these metals and alloys in acid solutions are usually few nanometers-thick and their densities are expected to be between 4.35 (Nb2 O5 ) and 7.93 g.cmÿ3 (Ta2 O5 ), the weight of O incorporated in a 10 nm thick (for example) oxide layer on a 7.5 cm2 specimen area should be nearly 10 lg. This low value has little in¯uence on the mass-loss measurements and consequently should not aect signi®cantly the thickness changes calculated from mass changes and materials densities. Comparing the corrosion data and the iso-corrosion curves of the Nb±Ta alloys in both HCl and H3 PO4 solutions, the alloys become very corrosion resistant from 60 wt% Ta content. Lupton [19] and Robin [11] in H2 SO4 and Mosolov [10] in HCl solutions found nearly the same value. This behavior explains why the usual substitute for pure tantalum is an alloy with high tantalum content (Nb±60 wt% Ta or KBI40 alloy) [13].
A. Robin, J.L. Rosa / International Journal of Refractory Metals & Hard Materials 18 (2000) 13±21
5. Conclusions The study of corrosion behavior of niobium, tantalum and Nb±Ta alloys in HCl and H3 PO4 solutions from boiling point to 200°C shows that:
[7]
· an increase of acid concentration results in an increase of corrosion rates for all materials and temperatures, · the corrosion rates increase with an increase of temperature for all materials and acid concentrations, · the addition of tantalum improves the corrosion resistance of niobium, · the Nb±Ta alloys become very corrosion resistant from 60 wt% Ta content, and · the corrosion rates decrease with exposure time due to the formation of super®cial oxides.
[9]
[8]
[10]
[11] [12]
[13] [14]
References [1] Metals handbook. Corrosion. Metals Park, OH: ASM International, 1987;13:722±39. [2] Burns RH, Shuker FS, Manning PE. Industrial applications of corrosion resistant tantalum, niobium and their alloys. In: Smallwood RE, editor. ASTM Special Technical Publication 849, ASTM, 1984. p. 50±69. [3] Mc Leary DL. Testing of columbium and columbium alloys. Corrosion 1962;18:67t±9t. [4] Bishop CR. Corrosion tests at elevated temperatures and pressures. Corrosion 1963;19:308t±14t. [5] Covino Jr BS, Carter JP, Cramer SD. The corrosion behavior of niobium in hydrochloric acid solutions. Corrosion 1980;36:554±8. [6] Lupton D, Schimann W, Schreiber F, Heitz E. Corrosion behaviour of tantalum and possible substitutes materials under
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