The open circuit corrosion potential of the tungsten electrode decreases by 59 ... Impedance measurements of the corroding tungsten show that the reaction.
On the Electrochemical Behavior of Tungsten: Corrosion Behavior of Tungsten in Buffer Solutions as Revealed by Potential and Impedance Measurements at Open Circuit* M. S. EL-BASIOUNY, F. EL-TAIB HEAKEL, and M. M. HEFNY *
Abstract This paper discusses systematically the potential and impedance behavior of the tungsten electrode in buffer solutions. The open circuit corrosion potential of the tungsten electrode decreases by 59 mV/unit increase of pH. Impedance measurements of the corroding tungsten show that the reaction film on the metal deteriorates in acidic buffers, whereas in alkaline ones, the surface gets is polished.
Introduction
04
There have been numerous studies on the electrochemical behavior of tungsten and tungsten oxide. These have been concerned recently with the phenomenon of electrochromism in W03 film which is un-
0.3
der active investigation due to its potential for passive alphanumeric displays. 1-10 In a previous work, 11 the formation and dissolution of the anodic oxide film of tungsten in sulfuric acid was
0 .2
investigated. The results indicated a first order mechanism for the dissolution process. Despite the importance of the above subject, it is noted that very little is known about the thin, naturally formed passive film on tungsten. However, the electropolishing of tungsten and the corrosion rate of the metal are governed mainly by the dissolution of this film. The importance of this corrosion film becomes obvious from the fact that any reaction at the metal surface which proceeds by migration of ions and/or electrons through these films depends mainly on their permeability and thickness.' 2 In this work, the potential and impedance of mechanically polished tungsten electrode were studied in buffer solutions. The
0.1
e",
w
0.0
z E-0•1
-0.2
results thus obtained give a sound basis to clarify the behavior of tungsten surface in relation to oxide film formation in aqueous
- 0.3
electrolytes. -0-4
Experimental The electrodes used were of specpure tungsten rod obtained from Johnson and Matthey—London. A copper wire was wound around the tungsten electrode placed in a glass tube, leaving an exposed surface area of 0.125 cm 2 to contact the test solution. The electrode was mechanically polished with 3/0 emery paper in a manner similar to that used previously. 13-15 It was then rinsed several times with triply distilled water and inserted into the capacitance cell in such a way that the electrode surface faced directly a square of a platinized Pt of area 5.0 cm 2 , which served as the opposite lead to the ac bridge. Clark-Lub's and Ringer's buffers 16 of pH range 1 to 12 were prepared from Analar grade chemicals using triply distilled water. Before measurements, the test solutions were either deoxygenated by a stream of purified hydrogen bubbles or saturated with oxygen by bubbling oxygen gas through the test solution. Open circuit potential measurements were always made while the leads to the bridge were disconnected. Potentials were measured *
Submitted for publication March 1980; revised August, 1980. *Chemistry Department, Faculty of Science, Cairo University, Cairo, Giza, Egypt.
Vol. 37, No. 3, March, 1981
0
2-0
4-0
6.0
So
10.0
12.0
pH
FIGURE 1 — Initial (after one minute) -0-0 and final (after 6 hours) -A-A- potential during open circuit corrosion of tungsten in deaerated buffer solutions.
against the SCE, but all quoted values are referred to NHE. All measurements were carried out at 30 C. Impedance measurements were carried out using the symmetrical high precision Wien bridge, the known arm of which is a capacitor, C m in series with a resistor, R m . The bridge was standardized using standard capacitance and resistance and the divergence was much less than 2%, being within the tolerance of the standards used. This bridge was previously used for impedance measurements on tantalum, 17 hafnium, 18,19 niobium, 20 and lately 11 on tungsten. The input ac voltage to the bridge was 10 mV. The frequency used, unless otherwise stated, was 1000 Hz.
0010-9312/81/000057/53.00/0 © 1981, National Association of Corrosion Engineers
175
Results and Discussions pH 1.0
Electrode Potential and Corrosion Kinetics After insertion of the mechanically polished tungsten in the test
8. 0
solution, the potential was followed with time until steady values were obtained. In the acidic range of pH 1 to 3, the initial potential of tungsten drifts with time to more active steady potential values.
7.0
In alkaline buffers of pH 8 to 12, the reverse takes place, where the potential drifts with time towards more stabilized nobler values. In
acquired by tungsten depends markedly on the pH range, whether acidic, neutral, or alkaline. It should be mentioned that both the initial and final potential values show sensitive variation with pH and do not represent a true thermodynamic potential of a definite electrode reaction. 31 The dependence of the open circuit corrosion potential of tungsten on the pH of the solution should have a significance in this respect and may be interpreted in terms of Wagner-Traud 22 and Petrocelli 23 representation of open circuit mixed corrosion potential. Thus, at open circuit, the cathodic and anodic partial c.ds i c and i s are equal at E = E m , where E m is the mixed potential. Under this condition, assuming Tafel relations for both the anodic and cathodic reactions, for a given concentration of dissolved oxygen and pH of the solution, thus, ic = io.c [ HI • exp [- be (E - Ea ,c )]
(1)
6.0 adventskerk te 's-gravenhage-1oosdiiineis. cliché katte. bouwblad
neutral ones, the behavior of tungsten is intermediate and no significant potential change was recorded. Typical data of the initial and final corrosion potential of mechanically polished tungsten are shown in Figure 1. From this potential-pH diagram, it is noted that the potential behavior
40
NN 8-0 u_
E
5. 0
3.0
ZO Where i o are the exchange c.ds, b, the inverse Tafel slope, (zF/2.303 RT), E a , the equilibrium potential of both cathodic and anodic reactions. At the condition when i s = i c , then E = E m . From the above equations we obtain:
[H + ) exp [-Is c (E m - Ee,c)l = i o,a exp [b a (E m - E a ,a )]
o) De-aerated solution •
1.0 0
2
oxygenated solution 4
3
Time , (hours)
(0 c
I) [H]= b a (E m - E a,a ) + b e (E m - Ee,c)
In (
pH 12.0
• 4.0
(2)
is = jo.a exp [ba (E Ee,a)]
PH 7.0
5.0
(3)
(4)
FIGURE 2 — Capacitance vs time during spontaneous corrosion of tungsten in deaerated, -00, and oxygenated -4141- buffer solutions.
( o,a
j o,c In [H I + b a E a ,a + b e E a ,c
Em =
Equation (8) indicates that the potential of tungsten electrode decreases by 59 mV per unit increase of pH which is in accord with the observed experimental results.
ba + bc
,off
According to the previous investigations reported by Butler, et a/, 24 on the corrosion potential of some common metals, the cathodic conjugate reaction, namely the reduction of oxygen determine predominantly the corrosion potential. This means that b a has very small magnitude as compared to b e and is assumed that b a Putting this approximation in Equation (5) gives: jo.c
In Em==— — b pH + E Ee c ec ( o.a'
(6)
The second and third terms of Equation (6) are considered to be constant for this corrosion environment, so that Equation (6) may be written as:
Em = K —
pH b c
(7)
and replacing the value of b, Equation (7) becomes:
E 176
m = K' — 0.059 pH
Impedance Measurements of Tungsten in Buffer Solutions In this series of experiments, the capacitance C m and resistance R m of the corroding tungsten in buffer solutions was followed with time until more or less steady values were recorded. The capacitance C m varies continuously with time and depends markedly on the pH of the solution. Figure 2 shows the variation of the capacitance C m with time for the tungsten electrode in hydrogen deaerated and oxygen saturated buffer solutions of pH 1, 7, and 12. In buffer solutions of pH 1, the continous increase of C m with time should indicate the surface reactivity of tungsten electrode, which seems to be related to progressive film thinning and chemical dissolution of the surface film by the acid. 228 This behavior reflects the gradual drift of the open circuit potential of the tungsten electrode in acidic buffers to less noble potentials. Reichman and Bard 29 reported that both the single and polycrystalline tungsten oxide showed high stability toward dissolution in aqueous solutions under open circuit. The amorphous film electrode on the other hand, was found to dissolve slowly. In a recent study in sulfuric acid (0.05 to 5.0 N), we found that the tungsten electrode
(8)
covered with an anodically formed oxide film, showed an active
CORROSION—NACE
9-0 30 0
••
7.0 4
4
• •
250
6.04'
a
\..
5.0
200
E
cc
o conductivity cell, Jones and Pollinger type • Impedance cell
4-0 15 0 3-0 100 2.0
50
1.0
0 0
2
4
68 pH
10
12
FIGURE 3 — Variation with pH of stabilized surface capacitance, C m and resistance, R m of tungsten electrode in oxygenated buffer solutions.
behavior when left at open circuit. The kinetics of the dissolution were studied and a first order mechanism was suggested. 11 It should be mentioned that the susceptibility of tungsten oxide
FIGURE 4 — Relative conductance of Clark-Lub's and Ringer's buffer solutions.
surface film to such dissolution may be in accord with the idea that the bond in WOx is more ionic than the typical valve meta1. 30 In alkaline buffers such as in pH 12, the initial C m values decrease continuously with time to reach a more stabilized value with a rate which is greater in oxygen saturated solutions. This lowering of Cm could be explained by taking into consideration the nature of the surface film on tungsten. The potentials relating the oxides of tungsten are: 31
W+2 H20=14102 + 4 H+ +4‘,E 0 =-0.119 — 0.059pH 2 WO2 + H20 = W203 + 2
H+
+
E 0 = -0.031 - 0.059 pH
WzOs + H20 = 2 W03 + 2 H+ + 2 E 0 -0.029-0.059 pH
The E 0 values for the latter two reactions are very similar and should lead to an equilibria involving the metal and other oxides in the presence of hydrogen ions. Tungsten metal is not attacked by alkaline solutions in absence of oxygen or oxidizing agents. 32 Thus, in presence of oxygen in alkaline solution, the lower oxides are oxidized to W03 which is freely soluble in alkaline solutions. 21 However, the lowering of C m in alkaline solutions in the case of tungsten is more likely related to chemical polishing 33-36 of its surface which is different from other valve metals. 18 In neutral buffers as in pH 7, an intermediate behavior was obtained. At first, Cm increases with time as in acidic buffer then decrease to lower Cm values. The above results shown in Figure 2 show clearly the influence of pH and prevailing gas on the magnitude of C m of the corroding 4ungsten. The variation with the pH of the electrolyte of C was . 98 ascribed by Bardina and Lukovtsev 37 and Lehovec and Armco to possible changes in the film and its dielectric properties. This placed some doubt on the presence of a well defined oxide film acting as a
Vol. 37, No. 3, March, 1981
perfect dielectric and hence, calculation of film thickness on tungsten electrode seems to be doubtful. The resistive component showed greater constancy with time. A consideration of the resistive component R m at its variation with the pH may be significant in relation to C m . Figure 3 represents the final steady values of C m and R m and shows that the lowest R m values are associated with the more conducting solutions, with a reverse trend to C m . Since the impedance cell used in this work is of fixed geometry and the tungsten electrodes have the same apparent area, R m seems to be a relative measure of the solution resistance. This is confirmed from measurements of the relative conductance of these buffer solutions in the conductivity cell of Jones and Pollinger 39 type, shown in Figure 4. The reciprocal of R m measured in the impedance cell using the tungsten electrode with the same buffer solutions is shown also in Figure 4. It is clear that the measured R m of the tungsten electrode provides a relative measure of the solution resistance. Similar observation concerning the relation between C m and R m has been reported by Bardina and Lu kovtsev. 3 7 From the above observed reactivity of the metal towards its environments, it should be mentioned that the measured impedance of the tungsten electrode is the sum of the electrochemical reaction in a double layer and the impedance of the surface film both are represented in Cm and R m values.
References 1. B. Reichman and A. J. Bard. J. Electrochem. Soc., Vol. 126, p. 583 119791. 2. A. Di Paola, F. Di Quarto, and C. Sunseri. J. Electrochem. Soc., Vol. 125, p. 1344 11978 ► . 3. 0. F. Schirmer, V. Wittwer, G. Baur, and G. Brandt. J. Electrochem. Soc., Vol. 124, p. 749 (19771.
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4. W. C. Dautremont, M. Green, and K. S. Kang. Electrochem. Acta., Vol. 22, p. 751 (1977). 5. H. R. Zeller and H. U. Beyeler. Appl. Phys., Vol. 13, p. 231 (1977). 6. R. S. Cranall and B. W. Faughnan. Appl. Phys. Lett., Vol. 28, p. 95 (1976). 7. G. Hollinger, T. M. Duc and A. Deneuville. Phys. Rev. Lett., Vol. 37, p. 1564 (1976). 8. B. W. Faughnan, R. S. Crandall and P. M. Heyman. RCA. Rev., Vol. 36, p. 177 (1975). 9. I. F. Change, B. L. Gilbert, and T. I. Sun. J. Electrochem. Soc., Vol. 122, p. 955 (1975). 10. S. K. Deb. Appl. Opt. Suppl., Vol. 3, p. 192 (1969). 11. M. S. El-Basiouny, S. A. Hassan, and M. Hefny. Corrosion, Sci., In Press (1980). 12. L. Young, Anodic Oxide Films, Academic Press, London and New York, p. 1 (1961). 13. M. S. El-Basiouny and S. Haruyama. Corrosion Sc., Vol. 16, p. 529 (1976). 14. M. S. El-Basiouny and S. Haruyama. ibid., Vol. 17, p. 405 (1977). 15. M. S. EI-Basiouny. Br. Corros. J., Vol. 12, p. 89 (1977). 16. H. T. S. Britton Hydrogen Ions, London: Chapman & Hall. p. 353 (1955). 17. M. S. EI-Basiouny. Thesis, Cairo University (1972). 18. M. S. El-Basiouny, A. M. El-Kot, and M. M. Hefny. Br. Corros. J., Vol. 14, p. 51 (1979). 19. M. S. El-Basiouny, A. M. Bekheet, and M. M. Hefny. Corros. NACE., Vol. 35, p. 566 (1979). 20. M. S. El-Basiouny and A. M. Bekheet. Br. Corros. J., in press. (1980). 21. W. M. Latimer. The oxidation states of the elements and their potentials in aqueous solutions (Prentice. Hall. Inc. Englewood Cliffs, N. J.) p. 255 (1957).
22. C. Wagner and W. Traud. Z. Elektrochem., Vol. 44, p. 391 (1938). 23. J. V. Petrocel I i. J. Electrochem. Soc., Vol. 97, p. 10 (1950). 24. G. Buttler, P. E. Francis, and A. S. McKie. Corros. Sci., p. 715 (1969). 25. I. A. Ammar and R. Salim. Corrosion Sci., Vol. 11, p. 591 (1971). 26. M. R. Arora and R. Kelly. J. Electrochem. Soc., Vol. 124, p. 1493 (1977). 27. A. Dipole, F. DiQuarto, and G. Sevarravalle. J. Less-Common Met., Vol. 42, p. 315 (1975). 28. A. T. Vas'ko and V. V. Tobolich. Akad. Nauk. Ukr. SSRS. Ser. B., Vol. 33, p. 706 (1971) (Ukrain). 29. B. Reichman and A. J. Bard. J. Electrochem. Soc., Vol. 126, p. 583 (1979). 30. H. C. Gatos. The surface chemistry of Metal and Semiconductor, John Wiley, London, p. 382 (1965). 31. M. Pourbaix. Atlas of Electrochemical Equilibria in Aqueous Solution, Pergamon Press, Oxford, p. 280 (1966). 32. C. L. Roll inson. The Chemistry of Chromium Molybdenum and Tungsten. Pergamon Texts in Inorganic, Vol. 21, p. 742 (1975). 33. J. Neugebauer and A. Kiss. Acta Chim. Acad. Sci. Hung., Vol. 44, p. 241 (1965). 34. T. H. Heumann and N. Stolica. Electrochim. Acta., Vol. 16, p. 1635 (1971). 35. K. Nagai and K. Mano. Science Repts. Research Inst. Tohoku Univ. Ser. B., Vol. 42, p. 391 (1951). N. G. Bardina and P. D. Lukovtsev. zh. Fiz. Khim. 36. J. R. Stephen. Met. Soc. Conf., Vol. 18, p. 125 (1961). 37. N. G. Bardina and P. D. Lukovtsev. zh. Fiz. Khim., Vol. 37, p. 57 (1963). 38. K. Lehovec and J. D'Amico, Vol. 114, p. 363 (1967). 39. J. Jones and G. M. Pollinger. J. Am. Chem. Soc., Vol. 53, p. 411 (1931).
Crevice Corrosion of Stainless Steels in Sea Water: Correlation of Field Data with Laboratory Ferric Chloride Tests * A. GARNER * (1) Abstract Two year sea water immersion tests were carried out at four North American sites on a wide range of austenitic stainless steels. Some of these steels were found to be completely free from crevice corrosion at all exposure sites. Immersion of the alloys in FeCl3 in the laboratory provided a more severe test of their crevice corrosion resistance, which could be used to make conservative predictions of their performance in sea water.
Introduction In a mildly oxidizing neutral chloride environment such as aerated sea water, stainless steels are prone to crevice corrosion. 1-1 2 For this reason, stainless steels have traditionally seen limited use in sea water service despite their excellent resistance to general corrosion. Over the last decade, a number of new stainless steels have become commercially available, with markedly better resistance to crevice corrosion in sea water than that of 304, 316, and 317. The present work was designed to evaluate this improved crevice corrosion resistance for a range of austenitic stainless steels and at the same
* Submitted for publication March, 1980; revised September,
1980. *Placer Development Limited, Endako Mines Division, Molybdenum Research, Ottawa, Canada. (1) The Pulp and Paper Research Institute of Canada, Pointe Claire,
P. Q., Canada.
time to assess the reliability of a FeCI3 laboratory test 13 ' 14 in predicting immunity to crevice corrosion in sea water. Much of the earlier work in this field concentrated on determining the amount of crevice and pitting corrosion to be expected under given exposure conditions (e.g., sea water velocity, cathodic protection). 1-4 More recently, increased understanding and skill have been applied to the formulation of stainless steels and new steelmaking techniques (principally AOD, VIM) have extended the range of commercially feasible stainless steels. As a result, at least three types of stainless steels which should be essentially immune to crevice and pitting corrosion in ambient sea water are now commercially feasible, namely, (1) conventional austenitic stainless steels with 18-24% Ni and more than 6% Mo, (2) Mn-substituted austenitic stainless steels with more than about 4.5% Mo, and (3) low interstitial ferritic stainless steels with more than 25% Cr and over 3% Mo. The new ferritic steels have commanded great attention since their recent introduction and have been themselves the subject
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© 1981, National Association of Corrosion Engineers
CORROSION-NACE
