Passivity and passivity breakdown on a β-FeAl intermetallic compound in sulphate and chloride containing solutions

Corrosion Science, Vol. 38, No. 2, pp. 307-3 15, 1996 Copyright c 1996 Elsevier Science Ltd Printed in Great Britain. All rights reserved 001(r938X/96$15.00+0.00

0010-938X(96)00125-5

PASSIVITY AND PASSIVITY BREAKDOWN ON A fi-FeAl INTERMETALLIC COMPOUND IN SULPHATE AND CHLORIDE CONTAINING SOLUTIONS N. DE CRISTOFARO,+

S. FRANGINI$

and A. MIGNONEt

’ CSM, Materiali e Acciai Speciali, Rome-EUR, Italy z ENEA-CRE Casaccia, Corrosion Laboratory, CP 2400,00060 Rome, Italy Abstract-The passivity and passivity breakdown of a P-FeAl-based iron aluminide containing 24 wt% Al has been studied in borate buffer solution (pH 8.4) containing sulphates, chlorides and mixtures of both using potentiodynamic techniques complemented with SEM analysis. The voltammetric response in the borate solution shows the peak characteristic of iron oxidation/reduction processes. Sulphate addition to the buffer solution modifies the voltammetric profile and makes the passive film more stable. It is suggested that a mixed iron (IIHIII) oxide film is responsible for passivation of FeAl aluminide in sulphate-borate solution in contrast to a rich iron (III) oxide film forming in absence of sulphate. The addition of aluminium to iron enables the formation of a passivating layer. While iron suffers from pitting, there was no sign of pitting on the aluminide in these conditions. The addition of NaCl to the borate solution at concentrations higher than 6 x 10p4M produces nucleation of pits on FeAl aluminide. The pitting corrosion potential increases linearly with log Cc,_ with a slope of 0.40 V/decade. The pitting potentials are shifted in a noble direction with the addition of 0.5 M Na2S04, indicating that the latter behaves as pitting inhibitor. Keywords: A. intermetallics, B. cyclic voltammetry, B. polarization. C. passive films, C. pitting corrosion

INTRODUCTION Intermetallic alloys are one of the most interesting new classes of metallic materials by virtue of their superior characteristics. Along with excellent mechanical properties, some of them also possess a high corrosion resistance. For example, iron aluminides having an ordered B2 FeAl structure (hereafter indicated simply as FeAl aluminides) are attractive mainly because of their better high temperature corrosion resistance in comparison with pure iron and, in some circumstances, with stainless aIloys1.2 and superalloys. The corrosion behaviour of such iron aluminides in aqueous media has been the subject of very few works, although this topic should be of interest for a reliable development of this class of intermetallics. In previous work4 the passivation characteristics of an FeAl aluminide containing 24 wt% Al have been investigated in acid sulphate solutions. The voltammograms showed the characteristic peaks of the oxidation/reduction process of iron indicating that the anodic behaviour of FeAl aluminides is controlled by iron dissolution in the active region and by iron oxidation processes in the passive one. A comparison of the anodic behaviour of the aluminide with those of the pure elements revealed its greater tendency to passivation.

Manuscript received 15 January 1993; in amended form 19 September 1994. 307

N. De Cristohro

308

VI cd.

Buchanan and Kim’ have studied the aqueous corrosion behaviour of iron aluminides containing 28 at% Al (Fe3Al based aluminides with a DO3 structure). They reported that the cyclic anodic polarization testing performed in a sulphuric acid solution (pH 4) containing 200ppm Cl- indicated passivation, but with a relatively low breakdown potential for pitting corrosion and a protection potential lower than the open-circuit corrosion potential. The aim of the present work is therefore to gain further insight into formation and breakdown of passivity of FeAl based aluminides in borate buffer solution (pH 8.4) containing sulphates. chlorides and mixtures of both. using open circuit potential measurements and potentiodynamic techniques complemented with scanning electron microscopy @EM).

EXPERIMENTAL

METHOD

Specimens of FeAl aluminide (wt% 24.4 Al; 0. I Zr; 0.05 Mn; 0.05 MO; 0.05 Ni; 0.03 Cr, others < 0.04 wt%), produced as described in Ref. 4, were used as working electrodes. The samples were axially embedded in Araldite holders to obtain a circular exposed area of 1 cm’. Armco iron and aluminium of commercial purity were also used for comparison purposes. The metal surfaces were ground to 1000 grit and rinsed with triply distilled water. The standard polarization cell (ASTM G-5) was completed by a platinum counter electrode and a saturated calomel electrode (SCE) provided with a Luggin-Haber capillary tip. The reference electrode contained a KNOs salt bridge to avoid the presence of chloride in the electrolyte. All the measurements were carried out at 30°C (f 1“C) in solutions saturated with purified nitrogen. The electrolyte prepared from AR chemicals and triply distilled water consisted of boric-borate buffer (0.11 M HsBOs + 0.022 M Na2B704) pH 8.4 andNaC1(10-3M
EXPERIMENTAL

RESULTS

AND

DISCUSSION

Potentioclvnumic measurements Figure 1 shows typical cyclic voltammograms of the FeAl aluminide in borate buffer in the presence of 0.5 M Na2S04 (full line) and without it (dashed line). In the latter case three anodic current peaks at -0.75 V (peak Al), -0.65V (peak Al’) and -0.32 V (peak A2) that overlap to a great extent, have been detected. The reverse scan shows one cathodic current peak centred at -0.65 V (peak Cl). The results are quite similar to those found for pure iron. mild steel and stainless steel in phosphate-borate and borate buffersG9 The voltammetric response of the FeAl aluminide can be interpreted taking into account the

Passivity

and passivity

breakdown

309

0.4

0.2

- 0.2

-0.4

-1.2

’

I

I

I

:

:

I

I

I

-08

1

1

1

-0,4

1

00

1

1

OL,

I

I

0.0

E(SCE)IV Fig.

1. Voltammograms

of FeAl-based aluminide in borate buffer solution both in absence (---) and in presence (-) of 0.5 M Na2S04. (pH 8.4, 3O”C, 40 mV/s.)

contribution of the iron component considering that aluminium is known to not present any current peak in the studied potential range. Thus, peak Al can be assigned to the electroformation of a pre-passive Fe(OH)z layer. The peak Al’ occurs at -0.65 V, where only the formation of Fe(I1) species can be postulated. Hence, this peak should be ascribed to dissolution of iron through the pre-passive layer. The Fe(OH)* layer is later electrooxidized to FeOOH in the potential range of peak A2. Conjugated peaks A2 and Cl are related to the electroformation and electroreduction of the hydrous FeOOH/Fe(OH)_, films. With the addition of Na2S04 to the buffer solution the current density of the pre-passive peak is increased, indicating an increased importance of the process of metal dissolution over film formation. Peaks Al and Al’ merge into a single peak located at -0.70 V (SCE). The decrease of current peak A2, related to the formation of ferric oxide, has also been reported for the iron/carbonate-bicarbonate system, although the authors did not give much interpretation of it.” This fact may signify that sulphate retards the oxidation to iron(II1) or, better, that the adsorption of sulphate anions causes less iron(I1) species to be available on the electrode surface for oxidation. It is known that the composition of the passive layer differs according to the different anions present in solutions when it is formed. For example, in neutral phosphate solutions the oxide layer is reported to be an iron (II)-(III) mixed oxide film in contrast to a prevalent iron (III) oxide of the film forming in borate solutions.18 Thus, from the foregoing observation that sulphate slows down the oxidation of iron (II) species to higher valence states, it is supposed that a film with characteristics similar to an iron (II)(III) mixed oxide should also be formed in this case. Concerning the cathodic branch, no significant differences have been recorded in the magnitude of the peak Cl. The presence of

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et ml.

a second cathodic peak, C2, positioned at - 1.10 V (SCE) is to be related to electroreduction of Fe(I1) species to lower valence intermediate species or simply to metallic iron. Polarization curves recorded for Fe Armco. Al and FeAl in the borate buffer solution without and with 0.5 M Na2S04 addition are shown in Fig. 2. The passivating role of aluminium, when this element is added to iron, can be easily understood comparing the lower passive current densities of FeAl aluminide with those of the pure elements. As already noted, the sulphate added to the borate buffer solution contributes to an increase in the metal dissolution during the early stage of passivation of FeAl aluminide (Figs 2(e) and 2(f)). This is not observed on iron, whose active-passive transition is practically unaffected by the addition of sulphate. Concerning aluminium, no significant differences are found in the borate solution with and without sulphate addition and no pitting attack has been detected. Curves (a) and (b) of Fig. 2 indicate that Al is passivated under the conditions of the present experiment, although the passive current appears to be higher than that of iron and FeAl aluminide. This means that anodic dissolution is controlled by transfer of Al(II1) ions at the metal/oxide interface.” An important difference between iron and FeAl aluminide lies in the fact that sulphate promotes the initiation of pitting on iron, as evidenced from the abrupt current increase above -0.2 V (SCE). A subsequent microscopic examination of the aluminide surfaces did not reveal any sign of pitting in these conditions. The improvement of corrosion resistance of iron by alloying with aluminium cannot be interpreted in terms of higher passivating capabilities of aluminium with respect to iron. This is evident considering the anodic response of Al, which shows anodic current densities higher than both iron and FeAl aluminide. In previous work4 the XPS analysis showed clearly the coexistence of iron and aluminium oxides on the FeAl surfaces passivated in sulphuric acid or exposed to air. Although it was not possible to conclude whether a mixed oxide or an iron oxide film stabilized by the presence of aluminium was responsible for the improvement in corrosion resistance of FeAl aluminide. it seems reasonable to find some analogies with the passivating effects ascribed to other alloying elements commonly used in iron-based alloys, such as vanadium, niobium, molybdenum and tungsten.’ ’ According to this interpretation, the beneficial effects of alloying elements that do not form their own

-5.6

-5.0

-5k

- 4.6

Lcgi /Ami

Fig. 2. Anodic polarization curves for (a) Al, (c) Fe Armco and (e) FeAl aluminide in borate buffer solution, and (b) Al, (d) Fe Armco and (f) FeAl aluminide in borate buffer plus 0.5 M Na2S04. (pH 8.4, 30°C. I mV/s.)

312

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et ul.

1.6

0.8

00

0.1

0.2

Ok 0.3 E(SCE)IV

0.5

0.6

0.7

Fig. 4. Reactivation times of FeAl aluminide passtvated for 30 s at Increasing anodic potentials in the borate buffer solution with different concentrations of Na2SOa: (0) no sulphate; (0) 0.1 M; and (A) 0.5 M. (pH 8.4. 30-C.)

0.8 >

0.L .-

a

Li g 0.0 -

w-0.4

-

-0.8

-

I -9

I

I -7

t

I

-5

I ,

I -3

Log i /AcI?I~

Fig. 5.

Anodic

polarization

curves

of FeAl aluminide

in borate

buffer solution

(pH 8.4. 3O’C.

I mV;s): (a) with 0.003 M NaCI: (b) 0.003 M NaCl plus 0.5 M NazSOd.

are shown in Fig. 5. In both the cases, when the applied potential exceeds a critical value, passivity breakdown takes place as seen through the sudden increase in the anodic current. Film breakdown occurs at about 0.5 V (SCE) in the chloride-borate solution, while the addition of sulphate shifts the onset of pitting to the transpassive region.

Passivity and passivity breakdown

313

-0.2

-4.0

-3.0

-2.0 Lag

Fig. 6.

-1.0

0.0

[NaCl]

Pitting potentials of FeAl aluminide vs chloride concentration in (0) borate buffer solution. (0) buffer solution with 0.5 M Na2S04, ( x ) iron in borate solution at pH 8.0.14

Figure 6 summarizes the results obtained from the polarization tests. The pitting potentials of FeAl aluminide in both the presence and absence of sulphate are reported. For comparison, the results found by Strehblow and Titze for iron in borate buffer (pH 8.0) are also plotted. l4 From observation of Fig. 6 it follows that in borate buffer, at a given concentration of Cl-, the pitting potentials of FeAl aluminide are higher than the corresponding ones of Fe, revealing the greater pitting resistance of the former. The pitting potentials of aluminide decrease with increasing Cl- concentration, fitting a linear relationship, whose slope is close to 0.40 V/decade. The corresponding value for Fe is about 0.13 V/decade. This indicates a greater influence of Cl- concentration on the pitting potentials of aluminide. The minimum Cl- concentration necessary for pitting to occur has been found to be 6 x low4 M. This value is not far away from that reported for iron (3 x lo-’ M in the same supporting electrolyte or 3 x lop4 M in NazS04).1s9’6 The addition of 0.5 M Na2S04 shifts the pitting potentials of FeAl aluminide in the noble direction by about 0.2 V, corresponding to improved pitting resistance. The slope of Er, vs Cl- concentration is practically unaffected by the presence of sulphate. Immunity to pitting was evident at chloride levels below 3 x 10e3 M. Figures 7 and 8 show typical SEM micrographs of the pits produced by anodic polarization in borate buffer containing 0.1 M NaCl, in both the absence and presence of sulphate, respectively. In the former case a large number of small irregular shaped pits have been detected. In the lower part of Fig. 7 the internal surface of a pit shows clearly the evidence of the grains encircled by grooves that appear similar to the tunnels characteristic of crystallographic pits. On the other hand, both pit density and morphology observed in sulphate borate media are greatly modified. The number of pits per unit area is greatly reduced and it is accompanied by a transformation from irregular to hemispherical morphology. Figure 8 shows a typical hemispherical macroscopic pit. It is generally accepted that the pit morphology depends on whether the process is activation or diffusioncontrolled.” Therefore, it is believed that the change in shape reflects the higher current densities inside the pits on specimens with low pit density such as those generated in

314

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e/ c/l

F:tg. 7.

A SEM micrograph

of irregular-shaped pits on FeAl aluminide borate buffer containing 0. I M NaCI.

after anodic polarization

Fig. 8.

A SEM micrograph of a typical hemispherical pit grown on FeAl surface during polarization test in borate buffer containing 0.1 M NaCl and 0.5 M Na2S04.

in

a.nodic

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and passivity

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315

suiphate-borate media. In these conditions, diffusion predominates, favouring formation of hemispherical pits. CONCLUSIONS 1. Aluminium plays a beneficial role on the passivation of FeAl-based aluminides as can be seen clearly by comparing the anodic response of the aluminides with those of the base metals in borate buffer solution. 2. The addition of sulphate to the borate solution greatly modifies the voltammetric response of FeAl aluminides resulting in a more stable passive layer. It is probable that an iron((III) oxide film would be responsible for passivation of FeAl aluminides in sulphate-borate solution in contrast to an essentially iron (III) oxide film forming in absence of sulphates. 3. While iron is known to suffer from pitting in sulphate-borate solutions, this phenomenon has not been observed on FeAl aluminides. 4. In weakly alkaline chloride solution buffered with borate the passivity breaks down by pitting. The minimum critical concentration of Cl- for pitting to occur on FeAl aluminides has been found as 6 x low4 M, which is comparable to iron in the same supporting electrolyte. At a given concentration of Cl-, the pitting potentials of FeAl aluminides are always more noble than iron. Sulphate addition to the borate solution containing chloride ions shifts the pitting potentials to more positive values. Ackno\l,ledgemenfs-The authors are pleased to acknowledge the support of this research by the Centro Sviluppo Materiali, Rome. Italy, who also supplied the intermetallic specimens. The authors also acknowledge the assistence of Mr Masci for the scanning electron microscopy work. One of the authors (NDC) undertook this work with the support of the ICTP Programme for Training and Research in Italian Laboratories, Trieste, Italy.

REFERENCES I. C.G. McKamey, J.H. De Van, P.F. Tortorelli and V.K. Sikka. J. Mater. Res. 6, 1779 (1991). 2. J.H. De Van, Oxidation qf High Temperature Intermetallics (ed. T. Grobstein and J. Doychak), p. 107. THS, Warrendale, PA (1989). 3. N. Stoloff and C.T. Sims, Superalloy II (ed. C.T. Sims, N. Stoloff and W. C. Hagel), p. 519. Wiley, New York (1987). 4. S. Frangini, N.B. De Cristofaro, J. Lascovich and A. Mignone, Corros. Sci. 35, 153 (1993). 5. R.A. Buchanan and J.C. Kim, Proc. 4th Co& of High Temperature Ordered Intermetallic Alloys, 27-30 November, Boston, MA (1990). 6. C.A. Acosta, R.C. Salvarezza. H.A. Videla and A.J. Arvia. Corros. Sci. 25, 291 (1985). 7. K. Ogura and K. Sato. Electrochim. Acfa 25, 857 (1980). 8. R.C. Salvarezza, N.B. De Cristofaro. C.D. Pallotta and A.J. Arvia, Electrochim. Acta. 1049 (1987). 9. M.E. Vela, J.R. Vilche and A.J. Arvia. Passivir.v qf Metah. p. 59. The Electrochemical Society (1978). 10. C.R. Valentini, C.A. Moina, J.R. Vilche and A.J. Arvia, Corros. Sci. 26, 781 (1986). 1 I. K. Hashimoto. K. Asami, M. Naka and T. Masumoto, Corros. Sci. 19, 857 (1979). 12. K. Ogura and M. Kaneko, Corros. Sci. 23, 1229 (1983). 13. A. Cakir and J.C. Scully. Passivity of Mefals. p. 385. The Electrochemical Society (1978). 14. H.H. Strehblow and B. Titze, Corros. Sci. 23, 461 (1977). 15. N.D. Stolica, Corros. Sri. 9, 461 (1969). 16. K.J. Vetter and H.H. Streblow, Eer. Bunsen Ges. Physik Chem. 74, 1024 (1970). 17. Z. Szklarska-Smialowska, Pitting Corrosion of Merals. p. 138. Nate, Houston, TX (1986). 18. R. Nishimura and N. Sato, Eoshoku Gijutsrt (Corrosion Engineering, Japan) 26,305 (1977). 19. T. Hurlen, H. Lian, O.S. Odegard and T. Valand, Elecfrochim. Acta, 29, 579 (1984).