- Email: [email protected]
HF solutions
Corrosion Science 42 (2000) 1169±1183
The behaviour of permalloy in NH4F/HF solutions J.E.A.M. van den Meerakker*, P.C. Baarslag Philips Research Laboratories, Prof. Holstlaan 4, 5656 AA Eindhoven, The Netherlands Received 12 March 1999; accepted 20 November 1999
Abstract The oxidative behaviour of Ni80Fe20 (Permalloy) in an NH4F/HF solution was investigated. The in¯uence of O2 concentration and the eects of short-circuiting Permalloy with a large area Pt or Alsimag electrode on the corrosion were studied. Current±voltage curves were measured to unravel the mechanism of corrosion. The corrosion started with the preferential dissolution of Fe. Possible corrosion prevention was examined by in¯uencing the anodic and cathodic partial currents of the corrosion process. 7 2000 Elsevier Science Ltd. All rights reserved. Keywords: Alloy; Iron; Nickel; Electrodeposited ®lms; Oxygen reduction
1. Introduction Permalloy is an alloy of 80 at% Ni and 20 at% Fe with excellent magnetic properties. This alloy is frequently used in thin ®lm heads for magnetic recording systems. In the production process of these heads, Permalloy is electrodeposited as a thin ®lm (1±10 mm) on a substrate wafer and subsequently etched to de®ne a pattern of magnetic material on a non-magnetic, mechanically hard substrate. Alsimag, a mixture of TiC and Al2O3, is often used as the substrate material. After etching the Permalloy, the wafer is covered with an insulating quartz layer. * Corresponding author. Tel.: +31-40-27-42145; fax: +31-40-27-43352. E-mail address: [email protected] (J.E.A.M. van den Meerakker). 0010-938X/00/$ - see front matter 7 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 0 1 0 - 9 3 8 X ( 0 0 ) 0 0 0 0 4 - 4
1170
J.E.A.M. van den Meerakker, P.C. Baarslag / Corrosion Science 42 (2000) 1169±1183
In this quartz layer, holes are etched to be able to make contact to the Permalloy. The etchant for quartz is a mixture of HF and NH4F, the so-called Buered Oxide Etchant (BOE). At the end of the etching process, bare Permalloy is in contact with the BOE. During this stage of the process, unwanted corrosion of Permalloy is often observed. An electrochemical investigation into the corrosion of Permalloy in BOE was started to unravel the mechanism of corrosion and to identify methods to prevent this process. The corrosion and passivation of Permalloy in solutions other than BOE has been investigated by various authors [1±5]. It is frequently found that bulk Ni/Fe alloys or ®lms deposited by sputtering are much more corrosion-resistant than ®lms deposited by electrodeposition. Marcus et al. [1,2] proved that sulphurcontaining compounds are codeposited in the alloy during electrodeposition. In the very ®rst stage of corrosion, the surface becomes enriched in sulphur. The adsorbed sulphur layer prevents the adsorption of hydroxyl ions, thus preventing the formation of oxide ®lms, and the alloy cannot become passivated. The same eect is observed with sputtered ®lms which were treated in an H2S ambient before the corrosion experiment, or when H2S was present in the corrosion solution. Recently, Ando et al. [6] performed comparable experiments with pure Ni and studied the electrode surface with in situ scanning tunnelling microscopy. They found that at Ni also the formation of oxide ®lms was completely suppressed by the presence of an adsorbed sulphur layer. This indicates that the corrosion of Permalloy is extremely sensitive to S-containing contamination. 2. Experimental Electrodeposition of Ni/Fe alloys was performed on an Alsimag1 substrate on which 100 nm Permalloy was sputter-deposited. A rotating disc electrode (RDE) with an area of 7.1 mm2 was made of this substrate and placed as a cathode in an electrochemical cell with a Ni sheet as anode. Before deposition, the electrode was cleaned in a solution of 0.5 M H2SO4 + 20 g/l sodium dodecylsulphate for 1 min. Unless otherwise stated, corrosion experiments were performed with Permalloy ®lms deposited under the following standard conditions: . Bath composition: 0.46 M NiCl2 0.046 M FeCl2 0.40 M H3BO3 7 10ÿ4 M Na-dodecylsulphate 3.3 10ÿ3 M Na-saccharine This bath was ®ltered through a 0.2 mm ®lter and had a pH of 2.95. . The deposition temperature was 24.08C.
1
Alsimag is a trade name, the supplier is Sumitomo.
J.E.A.M. van den Meerakker, P.C. Baarslag / Corrosion Science 42 (2000) 1169±1183
1171
. During deposition, the electrode was not rotated. . The current density was 7.0 mA/cm2 for 30 min, yielding ®lms of about 3 mm thickness. This resulted in smooth Ni/Fe ®lms with an Fe content between 19 and 21 at%. Preliminary experiments showed that storing the alloys under laboratory conditions had a negative eect on the reproducibility of corrosion. Therefore, after deposition, the Permalloy electrodes were rinsed in deionised water and immediately placed in the BOE, to study the corrosion behaviour. BOE is an aqueous solution of NH4F and HF in a mass ratio of 87.5/12.5 (Merck 1.01171). The pH of this solution, as measured with pH paper, was between 4 and 5. During corrosion, the electrodes were rotated at 1000 rpm and the potential was measured with a Hg/Hg2SO4 reference electrode. In this report, all potentials are given with respect to the Standard Hydrogen Electrode (SHE). After the experiments, the surfaces were investigated using light- or scanning electron microscopy (SEM). Analyses of the deposited ®lms and of the BOE after corrosion were performed using Optical Coupled Plasma±Optical Emission Spectroscopy (OCP±OES). Current±voltage curves were measured with a EG & G 362 scanning potentiostat in a conventional electrochemical cell with an RDE as the working electrode, a large area Pt sheet as the counter electrode and a Hg/Hg2SO4 reference electrode in combination with a Luggin capillary. The scans were in anodic direction with a rate of 20 mV/s. Current±voltage curves were measured at Permalloy, bulk Fe, Ni and Pt electrodes (99.99% pure) and at bare Alsimag. All solutions were prepared with reagent grade chemicals and deionised water. The measurements were performed at room temperature.
3. Results and discussion 3.1. Corrosion experiments The in¯uence of the O2 concentration on the corrosion behaviour of Permalloy was studied by saturation of BOE with N2, air or pure O2. In these soutions, freshly prepared Permalloy RDEs were rotated at 1000 rpm, for 25 min. During the experiments, the open-circuit potential was measured. The samples from the N2-saturated BOE showed no corrosion, while those from the air- and O2saturated solutions were slightly attacked. The potentials of all samples showed a small shift in cathodic direction during the ®rst 2 min and were more or less constant during the rest of the experiments. The results are given in the ®rst row of Table 1. It shows that both the start potential t 0 and the end potential t 25 min) shifted in anodic direction when the O2 concentration in the BOE increased. In the production process of magnetic heads, it was observed that Permalloy patterns, which were in electrical contact with the conductive substrate material
1172
J.E.A.M. van den Meerakker, P.C. Baarslag / Corrosion Science 42 (2000) 1169±1183
(Alsimag), showed more often corrosion than Permalloy patterns, which were isolated from the substrate [7]. This could be caused by the fact that in the ®rst case, a galvanic element is formed between Permalloy and Alsimag: a reduction process proceeds at the large back area of the substrate, which enhances the oxidation rate of the small Permalloy patterns. In that case, the potential of Permalloy should shift in anodic direction. For that reason, we repeated the corrosion experiments with the RDEs of Permalloy short-circuited with a 100 times larger Alsimag area. The results are given in the second row of Table 1. The trends with respect to the potentials were the same as in row 1: a slight shift in anodic direction with increasing O2 concentration. In all the three cases, the Permalloy electrodes short-circuited with Alsimag indeed attained a potential more anodic than the isolated Permalloy samples. This resulted in a small increase in corrosion rate. The experiments were repeated with a 350 times larger Pt area instead of the Alsimag. Table 1 shows that the corrosion was now clearly increased. The potentials were substantially shifted in anodic direction with respect to the isolated samples. During the experiments, a clear anodic shift was also observed, especially in BOE saturated with O2. In the latter case, the corrosion was severe. The attack occurred preferentially at crystal boundaries (see Fig. 1). It can be concluded that corrosion of Permalloy does not occur during 25 min of rotating in O2-free BOE. The corrosion process can be substantially enhanced both by increasing the O2 content of BOE and by short-circuiting Permalloy with a large Pt area. Short-circuiting with a large Alsimag area has a much smaller eect. 3.2. Analyses of BOE after corrosion To investigate whether the corrosion process results in corrosion products of the same composition as the Permalloy ®lm itself, chemical analyses of BOE were Table 1 The in¯uence of O2 concentration and short-circuiting Permalloy with a 100 times larger Alsimag or a 350 times larger Pt area, on the open-circuit potential and the corrosion of Permalloy Short-circuited with
Alsimag Pt
a
BOE saturated with
Estart (V/SHE)
Eend (V/SHE)
Corrosiona
N2 Air O2 N2 Air O2 N2 Air O2
ÿ0.14 ÿ0.09 ÿ0.06 ÿ0.10 ÿ0.06 ÿ0.03 ÿ0.09 ÿ0.01 +0.02
ÿ0.15 ÿ0.11 ÿ0.10 ÿ0.10 ÿ0.09 ÿ0.07 ÿ0.06 0.00 +0.40
ÿ +ÿ +ÿ +ÿ +ÿ + + ++ ++
ÿ: No corrosion; +ÿ: slight corrosion; +: corrosion; ++: severe corrosion.
J.E.A.M. van den Meerakker, P.C. Baarslag / Corrosion Science 42 (2000) 1169±1183
1173
Fig. 1. Optical micrograph of the surface of a Permalloy ®lm after 25 min corrosion in O2-saturated BOE. The Permalloy ®lm was short-circuited to a 350 times larger Pt area. The white bar represents 25 mm.
performed after corrosion. Rotating Permalloy electrodes (1000 rpm) were shortcircuited with a six times larger Pt area in O2-saturated BOE. These experiments were done for 2, 6, and 10 min. After 10 min, about 14% of the Permalloy ®lm was dissolved. The results are given in Table 2. It is obvious that for the ®rst 2 min, Fe is preferentially dissolved. This could be expected, as the redox potential of the oxidation of Fe to Fe2+ (ÿ0.44 V/SHE) is 0.19 V more negative than that of Ni to Ni2+ (ÿ0.25 V/SHE). This implies that the surface of the Permalloy became enriched in Ni. During the next 8 min, the dissolution of Fe proceeded at a very slow rate, while the rate of Ni dissolution was only slightly decreased. Fig. 2 shows the potential as a function of time. It is obvious that the preferential Fe dissolution took place at potentials between ÿ0.05 and about +0.05 V/SHE. It is not inconceivable that the resulting Ni enrichment of the surface caused the shift in anodic direction. At +0.34 V/SHE, Ni was predominantly dissolved. Then the Ni surface content decreased again until the stoichiometric composition of Table 2 Results of analyses of O2-saturated BOE after corrosion of Permalloy, which was short-circuited with a six times larger Pt area Corrosion time (min)
mg Ni/ml BOE
mg Fe/ml BOE
Atomic ratio in BOE
2 6 10
0.28 0.62 1.07
0.23 0.25 0.26
Ni54Fe46 Ni70Fe30 Ni80Fe20
1174
J.E.A.M. van den Meerakker, P.C. Baarslag / Corrosion Science 42 (2000) 1169±1183
Permalloy was reached, resulting in a slow decrease of the potential. Similar eects have been observed during the electroless etching of Permalloy in NH4F/ (NH4)2S2O8 solutions [3]. To investigate the dissolution stoichiometry of Permalloy as a function of potential, we performed potentiostatic dissolution experiments at +0.04 and +0.34 V/SHE. Rotating Permalloy electrodes were kept at these potentials in N2saturated BOE for 500 s. Then the BOE was analysed, and the same Permalloy electrode was rotated in a fresh BOE at the same potential for 500 s. This was repeated third time. The results (Table 3) con®rmed the assumptions made in the previous paragraph: preferential Fe dissolution at +0.04 V/SHE and a more or less stoichiometric dissolution of Permalloy at +0.34 V/SHE. In conclusion, it can be stated that the corrosion of Permalloy in BOE starts with a preferential Fe dissolution. This is accompanied by a potential shift in anodic direction to a potential where mainly Ni is dissolved until the surface composition is restored, followed by a more or less stoichiometric dissolution of Permalloy.
Fig. 2. The potential (E ) as a function of time (t ) for a Permalloy electrode, short-circuited to a six times larger Pt area, in O2-saturated BOE.
J.E.A.M. van den Meerakker, P.C. Baarslag / Corrosion Science 42 (2000) 1169±1183
1175
3.3. Current±voltage measurements Fig. 3 shows current±voltage curves measured in N2-saturated BOE at stationary Fe, Ni and Permalloy electrodes. It is obvious that the anodic dissolution of Fe started at more negative potentials than that of Ni, as expected. The Fe dissolution increased monotonically with increasing potential, while Ni was passivated at more anodic potentials. The Ni curve showed two peaks, one at about +0.2 V/SHE and other at about +0.55 V/SHE. It could be possible that the peak at +0.55 V/SHE was due to the oxidation of Ni(II) species into Ni(III) species [8,9]. This is not relevant for the present study, as this potential was not within the range in which Permalloy corrosion was studied. The dissolution of Permalloy started at about the same potential as that of Ni, but the current density was much higher. This illustrates that Permalloy was much more sensitive to corrosion than to Ni or Fe. Most probably, this was due to the incorporation of S-containing compounds in the Permalloy [1,2,6]. At about +0.4 V/SHE, the current at Permalloy decreased drastically to a very low value. A completely passivated surface (I < 10ÿ3 mA/cm2) was obtained at about the same potential where Ni was passivated. It should be noted that the current±voltage curves of Permalloy were not very reproducible; the peak current, the peak potential and the potential of the sudden decrease varied from sample to sample. This indicates that small dierences in the surface composition (Ni/Fe ratio and S amount) aected the corrosion rate. However, the qualitative picture was always the same: a much higher peak current than the Fe and Ni currents and a sharp current decrease at potentials more negative than the second peak potential at Ni. Comparison of the potentials measured during the corrosion experiments (Table 1) with the current±voltage curve of Permalloy shows that at ÿ0.14 V/SHE, no corrosion can occur. Between ÿ0.14 and about ÿ0.06 V/SHE, a slight corrosion can occur, dependent on the surface composition of the alloy. As the potential shifts in anodic direction, corrosion will start becoming severe at potentials positive with respect to SHE. This is in complete agreement with the observations. To investigate whether the amount of Fe in the alloy in¯uenced the dissolution behaviour, we measured current±voltage curves of two dierent alloys. To obtain Table 3 Results of analyses of BOE after potentiostatic dissolution of Permalloy at dierent potentials E (V/SHE) +0.04 First 500 s Second 500 s Third 500 s +0.34 First 500 s Second 500 s Third 500 s
Atomic ratio in BOE Ni31Fe69 Ni40Fe60 Ni70Fe30 Ni73Fe27 Ni83Fe17 Ni75Fe25
1176
J.E.A.M. van den Meerakker, P.C. Baarslag / Corrosion Science 42 (2000) 1169±1183
an alloy with higher Fe content, the Fe2+ concentration in the electrodeposition solution was increased. Fig. 4 shows the dierence between Ni80Fe20 and Ni58Fe42. It is obvious that the anodic dissolution of the Fe-richer alloy started at more negative potentials, and that two peaks are observed, indicating two dierent oxidation processes. It has been found by others [10,11], for both thin ®lm and bulk samples, that the atmospheric corrosion of Ni/Fe alloys is less for Permalloy than for every other composition. Also the crystallographic properties of Ni/Fe alloys depend on the composition [12]. It is possible that Permalloy behaves like a real alloy, while other compositions behave like a mixture of the separate metals. In that case, the left peak of the Fe-rich alloy could be due to Fe oxidation, while the other one represents the oxidation of Ni. Although this peak separation was
Fig. 3. Current±voltage curves of stationary Ni, Fe and Permalloy electrodes in N2-saturated BOE.
J.E.A.M. van den Meerakker, P.C. Baarslag / Corrosion Science 42 (2000) 1169±1183
1177
not observed for Permalloy, these results are in agreement with the results of the former section: preferential Fe dissolution around 0 V/SHE and mainly Ni dissolution at more positive potentials. The Ni surface enrichment, due to the preferential Fe dissolution, is nicely illustrated in Fig. 5. It shows the current±voltage curve of a Permalloy electrode after 5 min corrosion in O2-saturated BOE. The shape of the curve closely resembles that of pure Ni (dotted curve of Fig. 3). Even the second oxidation peak was present, indicating that after 5 min the surface consists, almost completely, of Ni. The fact that the current densities are higher in the case of Fig. 3 is due to the presence of sulphur compounds in the case of Fig. 5. From the corrosion experiments, it was concluded that reduction processes at
Fig. 4. Current±voltage curves of Ni80Fe20 and Ni58Fe42 electrodes at 1000 rpm in N2-saturated BOE.
1178
J.E.A.M. van den Meerakker, P.C. Baarslag / Corrosion Science 42 (2000) 1169±1183
Pt, and to a much smaller extent at Alsimag, enhanced the corrosion of Permalloy when Permalloy was in electrical contact with one of these materials. So, we measured the current±voltage curves for both a Pt and an Alsimag electrode in O2-saturated BOE. Fig. 6 shows the results at Pt. The reduction of O2 started at about +0.5 V/SHE and became diusion controlled at about 0.0 V/SHE. The increase in current density at ÿ0.25 V/SHE was due to H2 evolution. Comparison of Fig. 6 with Fig. 3 reveals that Fe dissolution can take place due to both the hydrogen evolution reaction and the O2 reduction, while Ni dissolution can only take place due to O2 reduction. These measurements prove that the enhanced corrosion of Permalloy, which was short-circuited, with Pt was caused by reduction processes at the large Pt surface. The results for Alsimag are given in Fig. 7. The solid curve represents the ®rst scan with a fresh Alsimag electrode. An oxidation process took place at potentials positive from ÿ0.2 V/SHE, while a reduction current was observed at more negative potentials. It is not clear what reactions occurred. However, it is clear that reduction of O2 did not take place at a considerable rate at this electrode. Comparison of Fig. 3 with the solid curve of Fig. 7 reveals that at potentials where Permalloy was oxidised, no reduction at Alsimag took place, indicating that
Fig. 5. Current±voltage curve of a Permalloy RDE at 1000 rpm in N2-saturated BOE, after corrosion during 5 min in O2-saturated BOE.
J.E.A.M. van den Meerakker, P.C. Baarslag / Corrosion Science 42 (2000) 1169±1183
1179
the corrosion of Permalloy should not be enhanced by short-circuiting with Alsimag. However, the dashed curve of Fig. 7 was obtained after immersion in BOE for about 30 min. The oxidation process was diminished, while the reduction process was enhanced and started at much more anodic potentials. Now the corrosion of Permalloy could be enhanced by a reduction reaction at Alsimag. As the reduction currents at Alsimag were very small, the enhancement can only be minor, as observed. 3.4. Corrosion prevention As corrosion is a summation of two partial processes, the anodic oxidation of Permalloy and the cathodic reduction of H+ and/or O2, both processes can be in¯uenced to diminish corrosion. We started to investigate whether the addition of Fe2+ and Ni2+ salts to BOE aected corrosion by in¯uencing the anodic partial process. Table 4 gives the results of the corrosion experiments. These results clearly show that NiCl2 had hardly any eect on the corrosion behaviour, while FeCl2 suppressed it completely. As the addition of NaCl also had no eect, the eect of FeCl2 was due to Fe2+. This is in agreement with the conclusion of earlier sections that the corrosion starts with the dissolution of Fe. Whether Fe2+ ions indeed in¯uenced the anodic process, was investigated by measuring the current±voltage curve of Permalloy in FeCl2-saturated BOE. Fig. 8 shows this
Fig. 6. Current±voltage curves of a Pt electrode in O2-saturated BOE at 0, 500, 1000 and 3000 rpm.
1180
J.E.A.M. van den Meerakker, P.C. Baarslag / Corrosion Science 42 (2000) 1169±1183
Fig. 7. Current±voltage curves of an Alsimag electrode in O2-saturated BOE at 1000 rpm. The solid curve is the ®rst curve measured at a fresh Alsimag surface. The dashed curve is measured after about 30 min at the rest potential in BOE.
curve. Comparison with the solid curve of Fig. 3 reveals that addition of Fe2+ ions decreased the anodic dissolution of Permalloy considerably. Apparently, the passivation of Permalloy by the formation of an iron(II) oxide is enhanced when the concentration of Fe2+ in solution is high. The current±voltage curve measured after 5 min in Oÿ 2 and FeCl2-saturated BOE was exactly the same as that of Fig. 8, proving that the corrosion of Permalloy was indeed completely suppressed. Two reduction processes could be responsible for the corrosion: the hydrogen evolution reaction and the reduction of O2. Expelling O2 from the solution by N2 Table 4 The in¯uence of the addition of Ni2+ and Fe2+ salts on the open-circuit potential and the corrosion of Permalloy Short-circuited with
Six times larger pt area
a
BOE saturated with
Estart (V/SHE)
Eend (V/SHE)
Corrosiona
O2 O2 O2 O2 O2 O2
ÿ0.06 ÿ0.10 ÿ0.18 ÿ0.05 ÿ0.10 ÿ0.21
ÿ0.10 ÿ0.12 ÿ0.24 +0.23 +0.15 ÿ0.25
+ÿ +ÿ ÿ + + ±
+ NiCl2 + FeCl2 + NiCl2 + FeCl2
ÿ: No corrosion; +ÿ: slight corrosion; +: corrosion.
J.E.A.M. van den Meerakker, P.C. Baarslag / Corrosion Science 42 (2000) 1169±1183
1181
purging suppressed the corrosion, as can be seen in Table 1. Under technological conditions, however, purging a gas through a HF-containing solution is unwanted. For that reason, we tried to in¯uence the hydrogen evolution reaction. Fig. 6 showed that this reaction starts at about ÿ0.25 V/SHE, in agreement with the redox potential for that reaction at the pH of BOE (4 to 5). The standard redox potential of Fe dissolution is ÿ0.44 V/SHE. In fresh BOE, the equilibrium potential for Fe will be even more negative. If the pH of BOE could be increased to a value where the reduction of H+ would be more cathodic than the Fe oxidation, the Fe dissolution would be suppressed. We added NaOH to BOE up to a concentration of 1 M. At that concentration, a white precipitate was observed, while the pH, as measured with pH paper, was only slightly increased. This was due to the buering capacity of the concentrated HF/NH4F solution. Corrosion experiments in this solution revealed that the corrosion was not suppressed, as expected. There is a third potential way to suppress corrosion: passivation of the Permalloy surface. The current±voltage curves predicted that in that case the potential of Permalloy in BOE should be more positive than about +0.5 V/SHE. A possible way to reach this goal is through the addition of a high concentration of a strong oxidising agent [13]. If the reduction current density of the oxidising agent is higher than the peak current density of Permalloy, the Permalloy can only
Fig. 8. Current±voltage curve of a Permalloy electrode in N2- and FeCl2-saturated BOE at 1000 rpm.
1182
J.E.A.M. van den Meerakker, P.C. Baarslag / Corrosion Science 42 (2000) 1169±1183
attain a potential in the passivation region. We added H2O2 to BOE up to a concentration of 2 M. In that case, the potential indeed shifted to a more anodic value (+0.11 V/SHE). However, as this potential was in the active dissolution range, the corrosion was considerably enhanced. Attempts to passivate the Permalloy surface before the immersion in BOE had the same eect: a pretreatment of Permalloy in a 30% H2O2 solution for 15 min resulted in a start potential of +0.35 V/SHE, resulting in enhanced corrosion. It can be concluded that corrosion of Permalloy can be suppressed by expelling O2 from the solution and by the addition of Fe2+ ions to BOE. Raising the pH of BOE and attempts to passivate the Permalloy surface failed as possible corrosion prevention.
4. Conclusions . Corrosion of Permalloy in BOE starts with the preferential dissolution of Fe. This results in an increasing surface content of Ni and a potential shift to more anodic values. . The corrosion of Permalloy is enhanced by increasing the O2 concentration in the solution and by short-circuiting of Permalloy with a large area of a material at which the reduction of O2 and of H+ can proceed. . At Pt, reduction of both H+ and O2 is observed at potentials where Permalloy oxidation takes place. So, if Permalloy is in electrical contact with Pt, the corrosion is considerably enhanced. . At a fresh Alsimag surface, reduction of O2 is not observed. However, a reduction process is found after a 30 min stay of Alsimag in BOE. The chemical nature of this process is unknown, but it could be the reason for the slight increase in corrosion of Permalloy when it is short-circuited with a large Alsimag area. . Attempts to suppress the corrosion of Permalloy through passivation in H2O2 solutions or increasing the pH of BOE failed. . Expelling O2 from BOE and saturating BOE with FeCl2 suppresses the corrosion.
Acknowledgements The authors are very grateful to Hendrik-Jan Dreuning and Inge Vorstenbosch for their skilful experimental assistance. They also thank Amanda Troost-de Jong for the accurate and quick analyses of BOE. Peter Meeuws is acknowledged for his interest and helpful discussions.
J.E.A.M. van den Meerakker, P.C. Baarslag / Corrosion Science 42 (2000) 1169±1183
1183
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13]
P. Marcus, A. Teissier, J. Oudar, Corr. Sci. 24 (1984) 259. P. Marcus, I. Olefjord, J. Oudar, Corr. Sci. 24 (1984) 269. M. KoÈhler, A. Wiegand, A. Lerm, J. Electrochem. Soc. 132 (1985) 2293. G. Dagan, M. Tomkiewicz, J. Electrochem. Soc. 139 (1992) 461. G.S. Frankel, J.O. Dukovic, V. Brusic, B.M. Rush, C.V. Jahnes, J. Electrochem. Soc. 139 (1992) 2196. S. Ando, T. Suzuki, K. Itaya, J. Electroanal. Chem. 412 (1996) 139. P.M. Meeuws, private communications. R.S. Schrebler Guzman, J.R. Vilche, A.J. Arvia, J. Electrochem. Soc. 125 (1978) 1578. F. Hahn, D. Floner, B. Beden, C. Lamy, Electrochim. Acta 32 (1987) 1631. R. Landau, C.W. Oldach, Trans. Electrochem. Soc. 81 (1942) 521. D.W. Rice, J. Suits, D. Nepela, R. Tremoureux, J. Appl. Phys. 50 (1979) 7089. D.L. Grimmett, M. Schwartz, K. Nobe, Magnetic Materials, Processes and Devices, in: L.T. Romankiw, D.A. Herman (Eds.), Electrochem. Soc. Proc., vol. 92-10, 1992, pp. 303±314. J.E.A.M. van den Meerakker, M.H.M. van der Straaten, J. Electrochem. Soc. 37 (1990) 1239.
The behaviour of permalloy in NH4F/HF solutions J.E.A.M. van den Meerakker*, P.C. Baarslag Philips Research Laboratories, Prof. Holstlaan 4, 5656 AA Eindhoven, The Netherlands Received 12 March 1999; accepted 20 November 1999
Abstract The oxidative behaviour of Ni80Fe20 (Permalloy) in an NH4F/HF solution was investigated. The in¯uence of O2 concentration and the eects of short-circuiting Permalloy with a large area Pt or Alsimag electrode on the corrosion were studied. Current±voltage curves were measured to unravel the mechanism of corrosion. The corrosion started with the preferential dissolution of Fe. Possible corrosion prevention was examined by in¯uencing the anodic and cathodic partial currents of the corrosion process. 7 2000 Elsevier Science Ltd. All rights reserved. Keywords: Alloy; Iron; Nickel; Electrodeposited ®lms; Oxygen reduction
1. Introduction Permalloy is an alloy of 80 at% Ni and 20 at% Fe with excellent magnetic properties. This alloy is frequently used in thin ®lm heads for magnetic recording systems. In the production process of these heads, Permalloy is electrodeposited as a thin ®lm (1±10 mm) on a substrate wafer and subsequently etched to de®ne a pattern of magnetic material on a non-magnetic, mechanically hard substrate. Alsimag, a mixture of TiC and Al2O3, is often used as the substrate material. After etching the Permalloy, the wafer is covered with an insulating quartz layer. * Corresponding author. Tel.: +31-40-27-42145; fax: +31-40-27-43352. E-mail address: [email protected] (J.E.A.M. van den Meerakker). 0010-938X/00/$ - see front matter 7 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 0 1 0 - 9 3 8 X ( 0 0 ) 0 0 0 0 4 - 4
1170
J.E.A.M. van den Meerakker, P.C. Baarslag / Corrosion Science 42 (2000) 1169±1183
In this quartz layer, holes are etched to be able to make contact to the Permalloy. The etchant for quartz is a mixture of HF and NH4F, the so-called Buered Oxide Etchant (BOE). At the end of the etching process, bare Permalloy is in contact with the BOE. During this stage of the process, unwanted corrosion of Permalloy is often observed. An electrochemical investigation into the corrosion of Permalloy in BOE was started to unravel the mechanism of corrosion and to identify methods to prevent this process. The corrosion and passivation of Permalloy in solutions other than BOE has been investigated by various authors [1±5]. It is frequently found that bulk Ni/Fe alloys or ®lms deposited by sputtering are much more corrosion-resistant than ®lms deposited by electrodeposition. Marcus et al. [1,2] proved that sulphurcontaining compounds are codeposited in the alloy during electrodeposition. In the very ®rst stage of corrosion, the surface becomes enriched in sulphur. The adsorbed sulphur layer prevents the adsorption of hydroxyl ions, thus preventing the formation of oxide ®lms, and the alloy cannot become passivated. The same eect is observed with sputtered ®lms which were treated in an H2S ambient before the corrosion experiment, or when H2S was present in the corrosion solution. Recently, Ando et al. [6] performed comparable experiments with pure Ni and studied the electrode surface with in situ scanning tunnelling microscopy. They found that at Ni also the formation of oxide ®lms was completely suppressed by the presence of an adsorbed sulphur layer. This indicates that the corrosion of Permalloy is extremely sensitive to S-containing contamination. 2. Experimental Electrodeposition of Ni/Fe alloys was performed on an Alsimag1 substrate on which 100 nm Permalloy was sputter-deposited. A rotating disc electrode (RDE) with an area of 7.1 mm2 was made of this substrate and placed as a cathode in an electrochemical cell with a Ni sheet as anode. Before deposition, the electrode was cleaned in a solution of 0.5 M H2SO4 + 20 g/l sodium dodecylsulphate for 1 min. Unless otherwise stated, corrosion experiments were performed with Permalloy ®lms deposited under the following standard conditions: . Bath composition: 0.46 M NiCl2 0.046 M FeCl2 0.40 M H3BO3 7 10ÿ4 M Na-dodecylsulphate 3.3 10ÿ3 M Na-saccharine This bath was ®ltered through a 0.2 mm ®lter and had a pH of 2.95. . The deposition temperature was 24.08C.
1
Alsimag is a trade name, the supplier is Sumitomo.
J.E.A.M. van den Meerakker, P.C. Baarslag / Corrosion Science 42 (2000) 1169±1183
1171
. During deposition, the electrode was not rotated. . The current density was 7.0 mA/cm2 for 30 min, yielding ®lms of about 3 mm thickness. This resulted in smooth Ni/Fe ®lms with an Fe content between 19 and 21 at%. Preliminary experiments showed that storing the alloys under laboratory conditions had a negative eect on the reproducibility of corrosion. Therefore, after deposition, the Permalloy electrodes were rinsed in deionised water and immediately placed in the BOE, to study the corrosion behaviour. BOE is an aqueous solution of NH4F and HF in a mass ratio of 87.5/12.5 (Merck 1.01171). The pH of this solution, as measured with pH paper, was between 4 and 5. During corrosion, the electrodes were rotated at 1000 rpm and the potential was measured with a Hg/Hg2SO4 reference electrode. In this report, all potentials are given with respect to the Standard Hydrogen Electrode (SHE). After the experiments, the surfaces were investigated using light- or scanning electron microscopy (SEM). Analyses of the deposited ®lms and of the BOE after corrosion were performed using Optical Coupled Plasma±Optical Emission Spectroscopy (OCP±OES). Current±voltage curves were measured with a EG & G 362 scanning potentiostat in a conventional electrochemical cell with an RDE as the working electrode, a large area Pt sheet as the counter electrode and a Hg/Hg2SO4 reference electrode in combination with a Luggin capillary. The scans were in anodic direction with a rate of 20 mV/s. Current±voltage curves were measured at Permalloy, bulk Fe, Ni and Pt electrodes (99.99% pure) and at bare Alsimag. All solutions were prepared with reagent grade chemicals and deionised water. The measurements were performed at room temperature.
3. Results and discussion 3.1. Corrosion experiments The in¯uence of the O2 concentration on the corrosion behaviour of Permalloy was studied by saturation of BOE with N2, air or pure O2. In these soutions, freshly prepared Permalloy RDEs were rotated at 1000 rpm, for 25 min. During the experiments, the open-circuit potential was measured. The samples from the N2-saturated BOE showed no corrosion, while those from the air- and O2saturated solutions were slightly attacked. The potentials of all samples showed a small shift in cathodic direction during the ®rst 2 min and were more or less constant during the rest of the experiments. The results are given in the ®rst row of Table 1. It shows that both the start potential t 0 and the end potential t 25 min) shifted in anodic direction when the O2 concentration in the BOE increased. In the production process of magnetic heads, it was observed that Permalloy patterns, which were in electrical contact with the conductive substrate material
1172
J.E.A.M. van den Meerakker, P.C. Baarslag / Corrosion Science 42 (2000) 1169±1183
(Alsimag), showed more often corrosion than Permalloy patterns, which were isolated from the substrate [7]. This could be caused by the fact that in the ®rst case, a galvanic element is formed between Permalloy and Alsimag: a reduction process proceeds at the large back area of the substrate, which enhances the oxidation rate of the small Permalloy patterns. In that case, the potential of Permalloy should shift in anodic direction. For that reason, we repeated the corrosion experiments with the RDEs of Permalloy short-circuited with a 100 times larger Alsimag area. The results are given in the second row of Table 1. The trends with respect to the potentials were the same as in row 1: a slight shift in anodic direction with increasing O2 concentration. In all the three cases, the Permalloy electrodes short-circuited with Alsimag indeed attained a potential more anodic than the isolated Permalloy samples. This resulted in a small increase in corrosion rate. The experiments were repeated with a 350 times larger Pt area instead of the Alsimag. Table 1 shows that the corrosion was now clearly increased. The potentials were substantially shifted in anodic direction with respect to the isolated samples. During the experiments, a clear anodic shift was also observed, especially in BOE saturated with O2. In the latter case, the corrosion was severe. The attack occurred preferentially at crystal boundaries (see Fig. 1). It can be concluded that corrosion of Permalloy does not occur during 25 min of rotating in O2-free BOE. The corrosion process can be substantially enhanced both by increasing the O2 content of BOE and by short-circuiting Permalloy with a large Pt area. Short-circuiting with a large Alsimag area has a much smaller eect. 3.2. Analyses of BOE after corrosion To investigate whether the corrosion process results in corrosion products of the same composition as the Permalloy ®lm itself, chemical analyses of BOE were Table 1 The in¯uence of O2 concentration and short-circuiting Permalloy with a 100 times larger Alsimag or a 350 times larger Pt area, on the open-circuit potential and the corrosion of Permalloy Short-circuited with
Alsimag Pt
a
BOE saturated with
Estart (V/SHE)
Eend (V/SHE)
Corrosiona
N2 Air O2 N2 Air O2 N2 Air O2
ÿ0.14 ÿ0.09 ÿ0.06 ÿ0.10 ÿ0.06 ÿ0.03 ÿ0.09 ÿ0.01 +0.02
ÿ0.15 ÿ0.11 ÿ0.10 ÿ0.10 ÿ0.09 ÿ0.07 ÿ0.06 0.00 +0.40
ÿ +ÿ +ÿ +ÿ +ÿ + + ++ ++
ÿ: No corrosion; +ÿ: slight corrosion; +: corrosion; ++: severe corrosion.
J.E.A.M. van den Meerakker, P.C. Baarslag / Corrosion Science 42 (2000) 1169±1183
1173
Fig. 1. Optical micrograph of the surface of a Permalloy ®lm after 25 min corrosion in O2-saturated BOE. The Permalloy ®lm was short-circuited to a 350 times larger Pt area. The white bar represents 25 mm.
performed after corrosion. Rotating Permalloy electrodes (1000 rpm) were shortcircuited with a six times larger Pt area in O2-saturated BOE. These experiments were done for 2, 6, and 10 min. After 10 min, about 14% of the Permalloy ®lm was dissolved. The results are given in Table 2. It is obvious that for the ®rst 2 min, Fe is preferentially dissolved. This could be expected, as the redox potential of the oxidation of Fe to Fe2+ (ÿ0.44 V/SHE) is 0.19 V more negative than that of Ni to Ni2+ (ÿ0.25 V/SHE). This implies that the surface of the Permalloy became enriched in Ni. During the next 8 min, the dissolution of Fe proceeded at a very slow rate, while the rate of Ni dissolution was only slightly decreased. Fig. 2 shows the potential as a function of time. It is obvious that the preferential Fe dissolution took place at potentials between ÿ0.05 and about +0.05 V/SHE. It is not inconceivable that the resulting Ni enrichment of the surface caused the shift in anodic direction. At +0.34 V/SHE, Ni was predominantly dissolved. Then the Ni surface content decreased again until the stoichiometric composition of Table 2 Results of analyses of O2-saturated BOE after corrosion of Permalloy, which was short-circuited with a six times larger Pt area Corrosion time (min)
mg Ni/ml BOE
mg Fe/ml BOE
Atomic ratio in BOE
2 6 10
0.28 0.62 1.07
0.23 0.25 0.26
Ni54Fe46 Ni70Fe30 Ni80Fe20
1174
J.E.A.M. van den Meerakker, P.C. Baarslag / Corrosion Science 42 (2000) 1169±1183
Permalloy was reached, resulting in a slow decrease of the potential. Similar eects have been observed during the electroless etching of Permalloy in NH4F/ (NH4)2S2O8 solutions [3]. To investigate the dissolution stoichiometry of Permalloy as a function of potential, we performed potentiostatic dissolution experiments at +0.04 and +0.34 V/SHE. Rotating Permalloy electrodes were kept at these potentials in N2saturated BOE for 500 s. Then the BOE was analysed, and the same Permalloy electrode was rotated in a fresh BOE at the same potential for 500 s. This was repeated third time. The results (Table 3) con®rmed the assumptions made in the previous paragraph: preferential Fe dissolution at +0.04 V/SHE and a more or less stoichiometric dissolution of Permalloy at +0.34 V/SHE. In conclusion, it can be stated that the corrosion of Permalloy in BOE starts with a preferential Fe dissolution. This is accompanied by a potential shift in anodic direction to a potential where mainly Ni is dissolved until the surface composition is restored, followed by a more or less stoichiometric dissolution of Permalloy.
Fig. 2. The potential (E ) as a function of time (t ) for a Permalloy electrode, short-circuited to a six times larger Pt area, in O2-saturated BOE.
J.E.A.M. van den Meerakker, P.C. Baarslag / Corrosion Science 42 (2000) 1169±1183
1175
3.3. Current±voltage measurements Fig. 3 shows current±voltage curves measured in N2-saturated BOE at stationary Fe, Ni and Permalloy electrodes. It is obvious that the anodic dissolution of Fe started at more negative potentials than that of Ni, as expected. The Fe dissolution increased monotonically with increasing potential, while Ni was passivated at more anodic potentials. The Ni curve showed two peaks, one at about +0.2 V/SHE and other at about +0.55 V/SHE. It could be possible that the peak at +0.55 V/SHE was due to the oxidation of Ni(II) species into Ni(III) species [8,9]. This is not relevant for the present study, as this potential was not within the range in which Permalloy corrosion was studied. The dissolution of Permalloy started at about the same potential as that of Ni, but the current density was much higher. This illustrates that Permalloy was much more sensitive to corrosion than to Ni or Fe. Most probably, this was due to the incorporation of S-containing compounds in the Permalloy [1,2,6]. At about +0.4 V/SHE, the current at Permalloy decreased drastically to a very low value. A completely passivated surface (I < 10ÿ3 mA/cm2) was obtained at about the same potential where Ni was passivated. It should be noted that the current±voltage curves of Permalloy were not very reproducible; the peak current, the peak potential and the potential of the sudden decrease varied from sample to sample. This indicates that small dierences in the surface composition (Ni/Fe ratio and S amount) aected the corrosion rate. However, the qualitative picture was always the same: a much higher peak current than the Fe and Ni currents and a sharp current decrease at potentials more negative than the second peak potential at Ni. Comparison of the potentials measured during the corrosion experiments (Table 1) with the current±voltage curve of Permalloy shows that at ÿ0.14 V/SHE, no corrosion can occur. Between ÿ0.14 and about ÿ0.06 V/SHE, a slight corrosion can occur, dependent on the surface composition of the alloy. As the potential shifts in anodic direction, corrosion will start becoming severe at potentials positive with respect to SHE. This is in complete agreement with the observations. To investigate whether the amount of Fe in the alloy in¯uenced the dissolution behaviour, we measured current±voltage curves of two dierent alloys. To obtain Table 3 Results of analyses of BOE after potentiostatic dissolution of Permalloy at dierent potentials E (V/SHE) +0.04 First 500 s Second 500 s Third 500 s +0.34 First 500 s Second 500 s Third 500 s
Atomic ratio in BOE Ni31Fe69 Ni40Fe60 Ni70Fe30 Ni73Fe27 Ni83Fe17 Ni75Fe25
1176
J.E.A.M. van den Meerakker, P.C. Baarslag / Corrosion Science 42 (2000) 1169±1183
an alloy with higher Fe content, the Fe2+ concentration in the electrodeposition solution was increased. Fig. 4 shows the dierence between Ni80Fe20 and Ni58Fe42. It is obvious that the anodic dissolution of the Fe-richer alloy started at more negative potentials, and that two peaks are observed, indicating two dierent oxidation processes. It has been found by others [10,11], for both thin ®lm and bulk samples, that the atmospheric corrosion of Ni/Fe alloys is less for Permalloy than for every other composition. Also the crystallographic properties of Ni/Fe alloys depend on the composition [12]. It is possible that Permalloy behaves like a real alloy, while other compositions behave like a mixture of the separate metals. In that case, the left peak of the Fe-rich alloy could be due to Fe oxidation, while the other one represents the oxidation of Ni. Although this peak separation was
Fig. 3. Current±voltage curves of stationary Ni, Fe and Permalloy electrodes in N2-saturated BOE.
J.E.A.M. van den Meerakker, P.C. Baarslag / Corrosion Science 42 (2000) 1169±1183
1177
not observed for Permalloy, these results are in agreement with the results of the former section: preferential Fe dissolution around 0 V/SHE and mainly Ni dissolution at more positive potentials. The Ni surface enrichment, due to the preferential Fe dissolution, is nicely illustrated in Fig. 5. It shows the current±voltage curve of a Permalloy electrode after 5 min corrosion in O2-saturated BOE. The shape of the curve closely resembles that of pure Ni (dotted curve of Fig. 3). Even the second oxidation peak was present, indicating that after 5 min the surface consists, almost completely, of Ni. The fact that the current densities are higher in the case of Fig. 3 is due to the presence of sulphur compounds in the case of Fig. 5. From the corrosion experiments, it was concluded that reduction processes at
Fig. 4. Current±voltage curves of Ni80Fe20 and Ni58Fe42 electrodes at 1000 rpm in N2-saturated BOE.
1178
J.E.A.M. van den Meerakker, P.C. Baarslag / Corrosion Science 42 (2000) 1169±1183
Pt, and to a much smaller extent at Alsimag, enhanced the corrosion of Permalloy when Permalloy was in electrical contact with one of these materials. So, we measured the current±voltage curves for both a Pt and an Alsimag electrode in O2-saturated BOE. Fig. 6 shows the results at Pt. The reduction of O2 started at about +0.5 V/SHE and became diusion controlled at about 0.0 V/SHE. The increase in current density at ÿ0.25 V/SHE was due to H2 evolution. Comparison of Fig. 6 with Fig. 3 reveals that Fe dissolution can take place due to both the hydrogen evolution reaction and the O2 reduction, while Ni dissolution can only take place due to O2 reduction. These measurements prove that the enhanced corrosion of Permalloy, which was short-circuited, with Pt was caused by reduction processes at the large Pt surface. The results for Alsimag are given in Fig. 7. The solid curve represents the ®rst scan with a fresh Alsimag electrode. An oxidation process took place at potentials positive from ÿ0.2 V/SHE, while a reduction current was observed at more negative potentials. It is not clear what reactions occurred. However, it is clear that reduction of O2 did not take place at a considerable rate at this electrode. Comparison of Fig. 3 with the solid curve of Fig. 7 reveals that at potentials where Permalloy was oxidised, no reduction at Alsimag took place, indicating that
Fig. 5. Current±voltage curve of a Permalloy RDE at 1000 rpm in N2-saturated BOE, after corrosion during 5 min in O2-saturated BOE.
J.E.A.M. van den Meerakker, P.C. Baarslag / Corrosion Science 42 (2000) 1169±1183
1179
the corrosion of Permalloy should not be enhanced by short-circuiting with Alsimag. However, the dashed curve of Fig. 7 was obtained after immersion in BOE for about 30 min. The oxidation process was diminished, while the reduction process was enhanced and started at much more anodic potentials. Now the corrosion of Permalloy could be enhanced by a reduction reaction at Alsimag. As the reduction currents at Alsimag were very small, the enhancement can only be minor, as observed. 3.4. Corrosion prevention As corrosion is a summation of two partial processes, the anodic oxidation of Permalloy and the cathodic reduction of H+ and/or O2, both processes can be in¯uenced to diminish corrosion. We started to investigate whether the addition of Fe2+ and Ni2+ salts to BOE aected corrosion by in¯uencing the anodic partial process. Table 4 gives the results of the corrosion experiments. These results clearly show that NiCl2 had hardly any eect on the corrosion behaviour, while FeCl2 suppressed it completely. As the addition of NaCl also had no eect, the eect of FeCl2 was due to Fe2+. This is in agreement with the conclusion of earlier sections that the corrosion starts with the dissolution of Fe. Whether Fe2+ ions indeed in¯uenced the anodic process, was investigated by measuring the current±voltage curve of Permalloy in FeCl2-saturated BOE. Fig. 8 shows this
Fig. 6. Current±voltage curves of a Pt electrode in O2-saturated BOE at 0, 500, 1000 and 3000 rpm.
1180
J.E.A.M. van den Meerakker, P.C. Baarslag / Corrosion Science 42 (2000) 1169±1183
Fig. 7. Current±voltage curves of an Alsimag electrode in O2-saturated BOE at 1000 rpm. The solid curve is the ®rst curve measured at a fresh Alsimag surface. The dashed curve is measured after about 30 min at the rest potential in BOE.
curve. Comparison with the solid curve of Fig. 3 reveals that addition of Fe2+ ions decreased the anodic dissolution of Permalloy considerably. Apparently, the passivation of Permalloy by the formation of an iron(II) oxide is enhanced when the concentration of Fe2+ in solution is high. The current±voltage curve measured after 5 min in Oÿ 2 and FeCl2-saturated BOE was exactly the same as that of Fig. 8, proving that the corrosion of Permalloy was indeed completely suppressed. Two reduction processes could be responsible for the corrosion: the hydrogen evolution reaction and the reduction of O2. Expelling O2 from the solution by N2 Table 4 The in¯uence of the addition of Ni2+ and Fe2+ salts on the open-circuit potential and the corrosion of Permalloy Short-circuited with
Six times larger pt area
a
BOE saturated with
Estart (V/SHE)
Eend (V/SHE)
Corrosiona
O2 O2 O2 O2 O2 O2
ÿ0.06 ÿ0.10 ÿ0.18 ÿ0.05 ÿ0.10 ÿ0.21
ÿ0.10 ÿ0.12 ÿ0.24 +0.23 +0.15 ÿ0.25
+ÿ +ÿ ÿ + + ±
+ NiCl2 + FeCl2 + NiCl2 + FeCl2
ÿ: No corrosion; +ÿ: slight corrosion; +: corrosion.
J.E.A.M. van den Meerakker, P.C. Baarslag / Corrosion Science 42 (2000) 1169±1183
1181
purging suppressed the corrosion, as can be seen in Table 1. Under technological conditions, however, purging a gas through a HF-containing solution is unwanted. For that reason, we tried to in¯uence the hydrogen evolution reaction. Fig. 6 showed that this reaction starts at about ÿ0.25 V/SHE, in agreement with the redox potential for that reaction at the pH of BOE (4 to 5). The standard redox potential of Fe dissolution is ÿ0.44 V/SHE. In fresh BOE, the equilibrium potential for Fe will be even more negative. If the pH of BOE could be increased to a value where the reduction of H+ would be more cathodic than the Fe oxidation, the Fe dissolution would be suppressed. We added NaOH to BOE up to a concentration of 1 M. At that concentration, a white precipitate was observed, while the pH, as measured with pH paper, was only slightly increased. This was due to the buering capacity of the concentrated HF/NH4F solution. Corrosion experiments in this solution revealed that the corrosion was not suppressed, as expected. There is a third potential way to suppress corrosion: passivation of the Permalloy surface. The current±voltage curves predicted that in that case the potential of Permalloy in BOE should be more positive than about +0.5 V/SHE. A possible way to reach this goal is through the addition of a high concentration of a strong oxidising agent [13]. If the reduction current density of the oxidising agent is higher than the peak current density of Permalloy, the Permalloy can only
Fig. 8. Current±voltage curve of a Permalloy electrode in N2- and FeCl2-saturated BOE at 1000 rpm.
1182
J.E.A.M. van den Meerakker, P.C. Baarslag / Corrosion Science 42 (2000) 1169±1183
attain a potential in the passivation region. We added H2O2 to BOE up to a concentration of 2 M. In that case, the potential indeed shifted to a more anodic value (+0.11 V/SHE). However, as this potential was in the active dissolution range, the corrosion was considerably enhanced. Attempts to passivate the Permalloy surface before the immersion in BOE had the same eect: a pretreatment of Permalloy in a 30% H2O2 solution for 15 min resulted in a start potential of +0.35 V/SHE, resulting in enhanced corrosion. It can be concluded that corrosion of Permalloy can be suppressed by expelling O2 from the solution and by the addition of Fe2+ ions to BOE. Raising the pH of BOE and attempts to passivate the Permalloy surface failed as possible corrosion prevention.
4. Conclusions . Corrosion of Permalloy in BOE starts with the preferential dissolution of Fe. This results in an increasing surface content of Ni and a potential shift to more anodic values. . The corrosion of Permalloy is enhanced by increasing the O2 concentration in the solution and by short-circuiting of Permalloy with a large area of a material at which the reduction of O2 and of H+ can proceed. . At Pt, reduction of both H+ and O2 is observed at potentials where Permalloy oxidation takes place. So, if Permalloy is in electrical contact with Pt, the corrosion is considerably enhanced. . At a fresh Alsimag surface, reduction of O2 is not observed. However, a reduction process is found after a 30 min stay of Alsimag in BOE. The chemical nature of this process is unknown, but it could be the reason for the slight increase in corrosion of Permalloy when it is short-circuited with a large Alsimag area. . Attempts to suppress the corrosion of Permalloy through passivation in H2O2 solutions or increasing the pH of BOE failed. . Expelling O2 from BOE and saturating BOE with FeCl2 suppresses the corrosion.
Acknowledgements The authors are very grateful to Hendrik-Jan Dreuning and Inge Vorstenbosch for their skilful experimental assistance. They also thank Amanda Troost-de Jong for the accurate and quick analyses of BOE. Peter Meeuws is acknowledged for his interest and helpful discussions.
J.E.A.M. van den Meerakker, P.C. Baarslag / Corrosion Science 42 (2000) 1169±1183
1183
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13]
P. Marcus, A. Teissier, J. Oudar, Corr. Sci. 24 (1984) 259. P. Marcus, I. Olefjord, J. Oudar, Corr. Sci. 24 (1984) 269. M. KoÈhler, A. Wiegand, A. Lerm, J. Electrochem. Soc. 132 (1985) 2293. G. Dagan, M. Tomkiewicz, J. Electrochem. Soc. 139 (1992) 461. G.S. Frankel, J.O. Dukovic, V. Brusic, B.M. Rush, C.V. Jahnes, J. Electrochem. Soc. 139 (1992) 2196. S. Ando, T. Suzuki, K. Itaya, J. Electroanal. Chem. 412 (1996) 139. P.M. Meeuws, private communications. R.S. Schrebler Guzman, J.R. Vilche, A.J. Arvia, J. Electrochem. Soc. 125 (1978) 1578. F. Hahn, D. Floner, B. Beden, C. Lamy, Electrochim. Acta 32 (1987) 1631. R. Landau, C.W. Oldach, Trans. Electrochem. Soc. 81 (1942) 521. D.W. Rice, J. Suits, D. Nepela, R. Tremoureux, J. Appl. Phys. 50 (1979) 7089. D.L. Grimmett, M. Schwartz, K. Nobe, Magnetic Materials, Processes and Devices, in: L.T. Romankiw, D.A. Herman (Eds.), Electrochem. Soc. Proc., vol. 92-10, 1992, pp. 303±314. J.E.A.M. van den Meerakker, M.H.M. van der Straaten, J. Electrochem. Soc. 37 (1990) 1239.