CORROSION

RESISTANCE

TANTALUM

ALLOYS

VARIOUS

OF

TITANIUM-MOLYBDENUM-

IN SULFURIC

CONCENTRATIONS

AT

ACID

SOLUTIONS

OF

20 ~

N . M. S h m a k o v , V . S. M i k h e e v , L. I. Lishcheta, R. F. Sabynina, and E. G. Turyanskaya

UDC 669.295.5'28'294.018.8:546.226-325

Sulfuric acid of v a r i o u s c o n c e n t r a t i o n s , p o s s e s s i n g e x t r e m e l y high c h e m i c a l activity, is finding wide use in c h e m i c a l industry 9 T h e r e f o r e , the c r e a t i o n of c o r r o s i o n - r e s i s t a n t m e t a l l i c alloys for c h e m i c a l app a r a t u s is of g r e a t p r a c t i c a l significance. Stainless s t e e l h a s now achieved w i d e s p r e a d use in industry; h o w e v e r , it does not withstand the prolonged influence of sulfuric acid of v a r i o u s c o n c e n t r a t i o n s . Special highly alloyed s t a i n l e s s s t e e l s r e q u i r e a special technology for t h e i r p r e p a r a t i o n . V a r i o u s t i t a n i u m - b a s e d alloys are finding wide use in the p r e p a r a t i o n of c h e m i c a l a p p a r a t u s . T h e r e are studies in the l i t e r a t u r e devoted to the c o r r o s i o n r e s i s t a n c e of industrial titanium alloys, b r a n d s AT [1,2] and VT [3,4]. Reduced c o r r o s i o n r e s i s t a n c e of these alloys has been established, e s p e c i a l ly in 40 and 80% solutions of sulfuric acid, in view of which t h e i r use in c h e m i c o p h a r m a c e u t i c a l production in this m e d i u m is limited, since this m a y lead to contamination of drug p r e p a r a t i o n s . Recently new alloys have been developed, such as an alloy of titanium with tantalum (6-10% by weight) [5, 6]. The titanium alloy 4201 (T32M) containing 32% m o l y b d e n u m is also known [7]. These alloys a r e c o r r o s i o n - r e s i s t a n t in sulfuric acid solutions, but on account of the high c o s t of tantalum and the low technological p r o p e r t i e s and t h e r m a l instability of the titanium alloy. 4201, they are finding b

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Fig. 1. Dependence of the c o r r o s i o n r a t e of the t i t a n i u m - m o l y b d e n u m - t a n t a l u m s y s t e m on the duration of testing, a) Mo + T a = 4.8%; b) Mo + T a = 8%; 1) 20%; 2) 40%; 3) 50% ; 4) 80% s o l u t i o n of sulfuric acid. Akrikhin C h e m i c o p h a r m a c e u t i c a l F a c t o r y , Kupavna, Moscow Region. T r a n s l a t e d f r o m KhimikoF a r m a t s e v t i c h e s k i i Zhurnal, Vol. 8, No. 2, pp. 41-46, F e b r u a r y , 1974. Original article submitted" October 24, 1972. 9 1974 Consultants Bureau, a division of Plenum Publishing Corporation, 227 West 17th Street, New York, N. Y. 10011. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, microfilming, recording or otherwise, without written permission of the publisher. A copy of this article is available from the publisher for $15.00.

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F i g . 2. D e p e n d e n c e of the r a t e of c o r r o s i o n of the t i t a n i u m - m o l y b d e n u m - t a n t a l u m s y s t e m on the s u l f u r i c a c i d c o n c e n t r a t i o n at 20 ~ . a) Mo + T a = 4.8%; b) Mo + T a = 8%; 1) 24 h; 2) 54 h; 3) 120 h; 4) 240 h . -O,t 8 U,O O,Z

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Fig. 3. Anodic polarization curves of the titanium-molybdenumtantalum system at 20 ~ a) 20%; b) 40%; c) 50% solution of sulfuric acid; I) Mo + Ta = 1.6%; 2) 2.8%; 3) 4%; 4) 4,8%; 5) 8%. Along the x axis: current density; along the y axis: potential relative to a saturated calomel electrode.

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Fig. 4. Dependence of the c o r r o s i o n r a t e of the t i t a n i u m - m o l y d e n u m t a n t a l u m s y s t e m on the c o m p o s i t i o n of the alloying e l e m e n t s , a) D u r a tion of testing 24 h; b) to 40 h; 1) 20~c; 2) 40%; 3) 50%; 4) 80% solution of sulfuric acid. limited u s e . T h e r e f o r e , the s e a r c h for new t i t a n i u m - b a s e d alloys, which would contain molybdenum and t a n t a l u m in s m a l l e r a m o u n t s , was of g r e a t i n t e r e s t . The s e l e c t i o n of t h e s e e l e m e n t s was b a s e d on the fact that when t a n t a l u m and molybdenum i n t e r a c t with titanium, they do not f o r m m e t a l l i c Compounds and, like titanium, they belong to the group of r e a d i l y p a s s i v a t e d e l e m e n t s . It was expected that t e r n a r y alloys in the r e g i o n of c o n c e n t r a t e d alpha and alpha + b e t a solid solutions would be c o r r o s i o n - r e s i s t a n t . The t e r n a r y s y s t e m t i t a n i u m - t a n t a l u m - m o l y b d e n u m was studied in [8]. The e x i s t e n c e of a b r o a d region of alpha and alpha + b e t a solid solutions has been e s t a b l i s h e d in this s y s t e m . We s e l e c t e d titanium alloys alloyed with m o l y b d e n u m and t a n t a l u m at a 3 : 1 ratio, i.e., alloys with a p r e d o m i n a n t content of molybdenum, but t o g e t h e r with tantalum up to 8%. The alloys w e r e p r e p a r e d a c c o r d ing to the technology of melting and t r e a t m e n t of titanium and o t h e r b r a n d s of titanium alloys d e s c r i b e d in [9]. The alloys w e r e t e s t e d for c o r r o s i o n by g r a v i m e t r i c [10, 11, 12] and potentiostatic methods [13]. The c o r r o s i o n of the alloys was studied in 20, 40, 50, and 80% solutions of sulfuric acid at 20~ . The total t i m e of testing was 240 h. The highest l o s s e s a r e o b s e r v e d in alloys containing up to 4% of the s u m of m o l y b denum and t a n t a l u m . When the s u m of the alloying e l e m e n t s is doubled, i.e., to 8%, the c o r r o s i o n l o s s e s in 50% sulfuric acid solution a r e reduced a l m o s t 160-fold in 240 h of testing. This alloy is m o r e c o r r o s i o n r e s i s t a n t in 40 and 80% solutions of sulfuric acid. F o r two alloys with contents of the s u m of molybdenum and tantalum 4.8 and 8%, Fig. 1 depicts diag r a m s c h a r a c t e r i z i n g the change in the c o r r o s i o n r a t e as a function of the t i m e of testing. Both alloys are the m o s t c o r r o s i o n - r e s i s t a n t in sulfuric acid solutions. T h e s e alloys a r e c h a r a c t e r i z e d by the fact that in i n t e r a c t i o n with sulfuric acid solutions, they show a s h a r p i n c r e a s e in c o r r o s i o n l o s s e s in the initial period of t e s t i n g . Then the alloys a r e p a s s i v a t e d , and as can be s e e n f r o m the d i a g r a m , the c o r r o s i o n l o s s e s a r e substantially r e d u c e d . The i n c r e a s e in the r e s i s t a n c e of these alloys to the 'influence of sulfuric acid is due to the f o r m a t i o n of a p r o t e c t i v e oxide film, f i r m l y bound to the m e t a l . Only the c u r v e s showing the change in the r a t e of c o r r o s i o n of an alloy with 4.8% of the s u m of m o l y b denum and t a n t a l u m in an 80% solution of sulfuric acid and the alloy with 8% of the sum of molybdenum and t a n t a l u m in 50 and 80% solutions of sulfuric acid a r e of a different n a t u r e . No s h a r p i n c r e a s e in the c o r r o sion l o s s e s is o b s e r v e d on these c u r v e s ; they a r e smooth, and with a negligible r i s e , they change with i n c r e a s i n g duration of testing and show high c o r r o s i o n r e s i s t a n c e of the alloys in these m e d i a . The change in the c o r r o s i o n r a t e of alloys containing 4.8 and 8% of the s u m of alloying e l e m e n t s as a function of the sulfuric acid c o n c e n t r a t i o n with v a r i o u s durations of testing is i l l u s t r a t e d b y the d i a g r a m s in Fig. 2. An alloy with a 4.8% content of the s u m of m o l y b d e n u m and tantalum is c h a r a c t e r i z e d by the f o r m a t i o n of a pronounced m a x i m u m on the c u r v e s of the c o r r o s i o n r a t e for v a r i o u s durations of testing (at the t r a n s i t i o n to a 50% s u l f u r i c acid solution), and one m i n i m u m (at the t r a n s i t i o n to an80% solution of 112

sulfuric acid). The corrosion rate of alloys containing 1.6, 2.8, and 4% of the sum of molybdenum and tantalum varies analogously for different durations of testing in sulfuric acid solutions of the same concentration, but with the formation of a more pronounced maximum in the transition to a 50% sulfuric acid solution~ The high rate of dissolution of these alloys is determined by their structure, consisting of a supersaturated alpha solid solution and beta phase, and the influence of residual stresses. The corrosion resistance of alloys containing 4.8 and 8~c of the sum of molybdenum and tantalum is graphically characterized by the an0die polarization curves of potential vs current density, depicted in Fig. 3 for 20, 40, and 50% solutions of sulfuric acid. In Fig. 3 it is also evident that alloys containing 1.6, 2.8, and 4~c of the sum of alloying elements have the most substantial changes in the current density with increasing potential, especially in a 50% solution of sulfuric acid, which is evidence of a more intensive dissolution of these alloys. Low-alloyed alloys are passivated in 20 and 40% solutions of sulfuric acid in the positive region of potentials or in the tr~msition to the positive region, while in a 50% sulfuric acid solution, passivation is shifted into the negative potential region. Superpassivation is observed on the anodic polarization curves in the positive potential region; moreover, the more concentrated the sulfuric acid solution, the lower the potential at which superpassivation occurs. Superpassivation is an indication of low strength of the oxide film and a weak bond of it to the base metal, which promotes its breakdown under the action of the applied voltage. Alloys containing 4.8 and 8% of the sum of molybdenum and tantalum are characterized by a low current density and a negligible increase in it in the positive region of potentials, beginning with 0 V in 20 and 40% sulfuric acid solutions, This is an indication of the presence of a high-strength oxide film, formed on the surface of the alloys in the interaction with air. An alloy containing 8% of the sum of molybdenum and tantalum is the mostcorrosion-resistant; during its testing, no changes in the current density with increasing potential are observed, while in the testing of an alloy containing 4.8~c of the sum of alloying elements, }here is a negligible increase in the current density with increasing potential. The experimental data obtained on the change in the current density as a function of the potential in sulfuric acid solutions agree with the curves of the corrosion rate vs the composition for various durations of testing. Figure 4 presents curves characterizing the change in the corrosion rate of the alloys as a function of the composition in 24 and 240 h of Lesting in sulfuric acid solutions of various concentrations. From a comparison of these curves it is evident that all the alloys are characterized by small losses and a negligible change in the corrosion rate in an 80~c solution of sulfuric acid in 24 and 240 h of testing in comparison with the losses and corrosion rate of the alloys in sulf~tric acid solutions of other concentrations. The nature of the change in the corrosion rate as a function of the composition of the alloys is the same with durations of testing 24 and 240 h. In the interval of variation of the content of the sum of the alloying elements from 4 to 4.8~c, there is a substantial decrease in the corrosion rate, while when the sum of the alloying elements is increased to 8~c, the corrosion rate changes by a very small amount with a duration of testing 240 h. On the basis of the results of the tests, highly corrosion-resistant alloys of the titanium-molybdenum-tantalum system containing 4.8-8~c of the sum of alloying elements can be recommended for industrial use in sulfuric acid solutions at 20 ~ . LITERATURE

1. 2. 3. 4. 5. 6.

CITED

Z. I. Konopkina, in: Titanium and Its Alloys, No. 7 [in Russian], Moscow (1962), p. 274. F. N. Tavadze, S. N. Mandzhgaladze, I. N. Lopdkipanidze, et al., in: Titanium and Its Alloys, No. 7 [in Russian], Moscow (1962), p. 253. N. D. Tomashev, R. M. Al'tovskii, A. V. Prosvirin, et al., in: Corrosion and Protection of Construction Materials. Collection of Articles [in Russian] (N. D. Tomashev, editor), Moscow (1961), p. 151. N. D. Tomashev and R. M. #l'tovskii, Corrosion and Protection of Titanium [in Russian], Moscow (1963), pp. 26, 40. T. A. Tumanova, V. V. Andreeva, et al. USSR Patent No. 322,389; Otkrytiya, Izobreteniya, No. 36 (1971), p. 66. T. A. Tumanova.~ Corrosion and Electrochemical Behavior of Titanium and Its Alloys in Solutions of Mineral Acids in the Presence of an Oxidizing Agent, Candidate's Dissertation [in Russian], Moscow (1970).

113

7. 8. 9. 10. 11. 12. 13.

114

N . P . Shamban and S. A. Belyaev, Titanium and Its Alloys and Their Use in Chemical Industry, No. 2 [in Russian], Moscow (1971). P . N . Nikitin and V. S. Mikheev, Izv. Akad. Nauk SSSR, Metally, No. 1, 211 (1971). N . M . Shmakov, V. S. Mikheev, L. I. Lisheheta, et al., K hi m . -Farm at s. Zh., No. 8, 33 (1973). A . G . Natradze, Yu. P. Aronson, I. F. Rozen, et al., Protection of Chemical Apparatus from C o r r o sion in the Chemicopharmaceutical Industry [in Russian], Moscow (1958), p. 16. N . P . Zhuk, Course in the Corrosion and Protection of Metals [in Russian], Moscow (1968), p. 361. I. Ya. Kornilov, P. G. Udyma, A. V. Molokanov, e t al., Chemical Equipment Designed for Corrosion Resistance, Collection [in Russian], Moscow (1970), p. 38. L . I . Freiman, V. A. Makarov, and I. E. Bryskii, Potentiostatic Methods in Corrosion Investigations and Electrochemical Protection [in Russian], Leningrad (1972).