Anodic polarization and corrosion of vanadium, and VTa, VW, VCr and VCrW alloys in 10% H2SO4 and 3% NaCl in water

Journal of the Less-Common

Ll

Metals, 132 (1987) Ll - L4

Letter Anodic polarization and corrosion of vanadium, and V-Ta, V-W, V-Cr and V-G-W alloys in 10% HzS04 and 3% NaCl in water W. J. TOMLINSON, R. RUSHTON, R. CINDERY and S. PALMER Department

of Materials, Coventry (Lanchester)

Polytechnic,

Coventry, CVI 5FB (U.K.)

(Received December 2, 1986)

Vanadium has many properties desirable in a structural material [ 11. It has a high melting point (1900 “C), relatively low density (6.1 Mg mp3) and an excellent fabricability at room temperature; but it suffers from a low tensile strength and a poor resistance to oxidation. Alloying additions have, however, substantially improved the mechanical properties and oxidation behaviour of vanadium. Selected vanadium-base alloys offer potentially significant advantages over other candidates as structural materials for fusion reactor first wall and blanket applications, and it is clear that a major concern in the use of such alloys is the limited available database [2]. Information on the electrochemical and corrosion behaviour of vanadium and its alloys is scarce. Vanadium is a very base metal and has a simulated nobility due to passivation of the surface [3]. In a broad technical assessment of the corrosion of vanadium and some of its alloys, the overall weight loss was measured in a wide variety of environments [4], and the electrochemical behaviour of vanadium and a V-Ti alloy in 10% HzS04 and 3% NaCl solution was also briefly investigated [ 51. Corrosion of a metal in service depends on many factors. In addition to the general dissolution rate, it is important to acknowledge the possible occurrence of pitting (and crevice) corrosion and the various stress-enhanced processes. Anodic polarisation is an accelerated test used to assess the corrosion behaviour of a metal and its susceptibility to pitting, and the present work determines the anodic polarisation and corrosion behaviour in 10% H,S04 and 3% NaCl solution of vanadium and some of its alloys. The alloys were melted and cast (by J. Leader of Cambridge University) using a non-consumable argon-arc process in an argon atmosphere of about 250 Torr. The 50 g charge was remelted and turned three times before being given a final pass to produce a uniform finger ingot with a smooth finish. The analysed composition of the alloys is given in Table 1. A small rod of 5 mm diameter was machined from the ingot, mounted in a PTFE holder, and then polarised using a standardised ramp procedure. In outline, a specimen polished to a 600 grit finish was positioned about 1 mm from a Luggin capillary in a cell with a saturated calomel reference electrode (SCE) @ Elsevier Sequoia/Printed in The Netherlands

L2 TABLE

1

Composition

Alloy Alloy

of the vanadium

(wt.%)a (at.%)

Oxygen (wt.ppm)b Nitrogen (wt.ppm) Carbon (wt.ppm)

and vanadium

alloys

V

V-Ta

v-w

V-C?

V-Cr-

-

22.OTa 7.3Ta

7.1w 2.1w

13.6Cr 13.4Cr

12.6Cr-7.5W 13.1Cr-2.2W

(3OO)C (100) (400)

240 75 300

575 5 46

990 5 1200

1300

W

6 1600

aFrom SEM EDAX analyses. bAnalysed by H. Wiggin Alloys Limited, Hereford, U.K. CParentheses indicate the values estimated from other sources.

and a cleaned platinum gauze auxillary electrode. After cathodic cleaning by evolution of hydrogen, the specimen was then anodically polarised at 25 mV min-’ , The solutions were naturally aerated and stirred with a magnetic stirrer at room temperature. Anodic polarisation curves for the metals in sulphuric acid and in salt water are shown in Fig. 1, and some corrosion parameters from these data are collected in Table 2. Anodic dissolution in sulphuric acid typically occurs with a large limiting current density. A voluminous black reaction product formed with vanadium and the surface was highly polished. Chromium is the only alloying element to raise the potential at which active dissolution starts significantly, and it appears that the active corrosion loop has been eliminated by the formation of a Crz03 film and the corrosion for both the V-Cr and V-Cr-W alloys corresponds to the region of transpassivity for chromium [6]. Above 1.6 V the presence of tantalum resulted in the formation of a golden brown film with a relatively low passivation current density. How-

.

b

a 0

I

0

250

1

500

t

I

0

10

I

0

10

t

20

1

30

l/mA

Fig. 1. The anodic polarisation behaviour of pure vanadium (O), V-7.1%W V-13.6%Cr (o), V-12.6%Cr-7.5%W (m), and V-22.O%Ta (A) in (a) 10% HaS04, 10% HzS04 and (c) 3% NaCl aqueous solution; specimen area 20 mm2.

(O), (b)

L3 TABLE 2 Corrosion parametersa of vanadium and vanadium alloys in naturally aerated 10% H&XI4 and in 3% NaCl solution Metal

V

1150

20

0.46

1500 (2500

6.60 4.95)

1400 (2500

0.30 0.25)

1

250

V-22wt.%Ta

975

V-7.lwt.%W

1200

16

1450 (2300

4.7 3.0)

1

300

V-13.6wt.%Cr

1900b

17.5

-

-

3

950

V-12.6wt.%Cr-7.5wt.%W

1850b

29.0

-

-

1

1050

30

No pitting

and i,, refer to the peak values of the active corrosion loop (see Fig. l), Epass refers to the passivation potential, and the passivating current density ip,,, is also shown (in parentheses) for a high value of the passivating potential; all potentials are with respect to the SCE. bFor secondary passivation (see text). aEPP

ever, in practical corrosion situations such high potentials are unlikely and the passivating effect of chromium would be more beneficial. In salt water, judging from the low passive current densities, a relatively protective passive film forms at the natural corrosion potential. Both vanadium and the V-W alloy have a relatively low pitting potential and pits were observed microscopically to have occurred on the anodically polarised specimens. Chromium as an alloying addition is clearly effective in raising the pitting potential and we may assume that at the 14% chromium level the passivating film consists of Cr203_ Tantalum readily forms a passivating film which has an inherent resistance to attack by sea water [ 71, and the presence of 7.3 at.% tantalum in vanadium is clearly very effective in resisting attack in the 3% NaCl solution. The relatively high passivation current density of 30 A rnA2(Table 2) suggests that the film does not consist wholly of Taz05 and that a slightly higher tantalum content in the alloy would provide a more complete protection of the vanadium alloy against corrosion and pitting. The authors wish to thank the United Kingdom Atomic Energy Authority’s Culbam laboratory for providing financial assistance for the purchase of the metals and the melting and casting of the ingots. 1 W. Rostoker, The Metallurgy of Van~iu~, Wiley, New York, 1958. 2 D. L. Smith, B. A. Loomis and D. R. Diercks, J. Nuel. Mater., 135 (1985) 125. 3 M. Pourbaix, Atlas of Potentia~/~H Diagrams, Pergamon, Oxford, 1962, p. 234.

L4 4 W. L. Acherman, J. P. Carter, C. B. Kenahan and D. Schlain, Report No. 6715, 1966, United States Dept. Interior, Bureau of Mines, Washington, D.C. 5 L. L. Migai, E. G. Mal’chevskii, V. I. Arons, I. P. Druzhimina and L. P. Voroob’eva, Prof. Met., 7 (1971) 610. 6 J. M. West, Basic Corrosion and Oxidation, Ellis Horwood, Chichester, 2nd edn., 1986, p. 126. 7 M. Pourbaix, Lectures on Electrochemical Corrosion, Plenum, New York, 1973, p. 164.