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Cathodic breakdown of anodic oxide film on Al and Al–Sn alloys in NaCl solution
Electrochimica Acta 50 (2005) 5624–5632
Cathodic breakdown of anodic oxide film on Al and Al–Sn alloys in NaCl solution S. Gudi´c ∗ , J. Radoˇsevi´c 1 , I. Smoljko, M. Kliˇski´c 1 Department of Electrochemistry and Materials Protection, Faculty of Chemical Technology, Teslina 10/V, 21000 Split, Croatia Received 26 August 2004; received in revised form 22 December 2004; accepted 11 March 2005 Available online 24 May 2005
Abstract The effect of cathodic polarisation on stability of defined oxide films on Al and Al–Sn alloys (with up to 0.40% Sn) has been investigated in a 0.5 M NaCl solution using the potentiostatic pulse method. The dependence of the cathodic current on time (in the period of 1, 10 and 100 s) was recorded on Al and Al–Sn alloys when subjected to a potential pulse from EOCP to different negative values (up to −2.0 V). Anodic current responses to the return to the EOCP were also recorded at three different time scales (1, 10 and 100 s). It has been established that the cathodic polarisation of passivated Al and Al–Sn alloys in a chloride solution is characterized by two regions of potentials with distinctly different phenomena: the range of low and high cathodic potentials (LCP and HCP). In the LCP range, the oxide film retains its properties, while in the HCP range cathodic breakdown and hydration of the oxide take place. The boundary between these two potential ranges shifts towards more negative potential values when the percentage of Sn in the alloy increases. The longer the duration of the cathodic pulse, the more positive the potentials at which the oxide film breakdown takes place. This shift is more marked with alloys containing higher percentage of Sn. Cathodic polarisation (duration of 100 s) activates alloys with 0.20% and 0.40% Sn for anodic dissolution. © 2005 Elsevier Ltd. All rights reserved. Keywords: Aluminium; Al–Sn alloy; Aluminium activation; Anodic oxide film; Cathodic polarisation
1. Introduction Aluminium and its alloys are of great technological significance due to their exceptional mechanical properties and good corrosion resistance both in the atmosphere and in the aqueous solution. The main reason for the corrosion resistance of aluminium is the formation of a thin, compact oxide film on the surface of the metal that may be tens of angstroms thick. Properties of oxide film can be additionally improved by the anodising process [1–10]. On the other hand, the standard reversible potential o of aluminium (EAl/Al 3+ = −1.66 V versus NHE [11]) and its high energetic capacity (2980 Ah kg−1 ) are very
∗ 1
Corresponding author. E-mail address: [email protected] (S. Gudi´c). ISE member.
0013-4686/$ – see front matter © 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2005.03.041
attractive properties when aluminium is used as active anodic material in chemical power sources or in sacrificial anodes in cathodic protection of steel in seawater. However, pure aluminium is practically impossible to use because a protective oxide film forms on it, making the potential positive, which in turn makes aluminium unattractive as energetic material. If aluminium is alloyed with small quantities of elements such as In, Ga, Hg, Sn, Zn and others [12–21], its anodic activity is improved as the structure of the aluminium/aluminium oxide/chloride electrolyte system is changed. Activation of aluminium can be also achieved by adding small quantities of suitable metal cations, such In3+ , Ga3+ , Hg2+ , Sn4+ or Sn2+ to the electrolyte [15,20,22– 28]. It has been reported that Sn accelerates the anodic dissolution of Al when present in solid solutions and that the potential range in which the current output increases extends between the SnH4(g) /Sn0 and Sn0 /SnO3 2− reversible potentials [29].
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Al–Sn alloys have the open circuit potential more negative than pure Al and exhibit increased anodic dissolution rates and reduced wasteful corrosion rates. Furthermore, it has been found that cathodic polarisation also activates the Al–Sn alloy for anodic dissolution [14,18]. In this potential region, once the hydrogen evolution has started by electron tunnelling, diffusion or ionic exchange through the protective Al oxide, the pH under the growing bubble increases and oxide dissolution takes place by local alkalisation. This results in local breakdown of the oxide film and formation of micro-pits within the film. Under these conditions, an active area of the Al–Sn alloy could be produced favouring Cl− ion adsorption at more negative potentials than aluminium, leading to its activation. This study has examined the stability of defined oxide films on Al and Al–Sn alloys in a 0.5 M NaCl solution at different cathodic potentials and durations of cathodic pulse.
2. Experimental The experiments were performed on samples of high purity Al (99.999%) and Al–Sn binary alloys with varying tin contents (0.02%, 0.09%, 0.20% and 0.40%), obtained by courtesy of Alcan International. The Al–Sn alloys had been prepared with high purity aluminium as the primary component, and tin as the alloying component. After alloying the metal was homogenised at 600 ◦ C for more than 1 h. At this temperature, Sn has its maximum solubility in Al of approximately 0.1% Sn [30]. The alloy was quenched in cold water. The Al and Al–Sn samples were made into electrodes by inserting insulated copper wires and protecting all sides but one with epoxy resin. Before each measurement, the surface was pre-treated by mechanical polishing of the electrode surface up to a mirror finish, followed by the alkali attack by immersing the electrode for 1 min in 0.1 M NaOH heated to 40 ◦ C. The electrode was then rinsed in doubly distilled water. The exposed geometric area was 0.57 cm2 . A conventional three-electrode glass cell was used with a platinum sheet and a saturated calomel electrode (SCE) serving as the counter and the reference electrodes, respectively. Electrochemical measurements were performed using a PAR M273A potentiostat driven by a computer. The potentiostatic pulse method was used to examine the cathodic reduction and anodic re-oxidation of defined oxide films on Al and Al–Sn alloys in a 0.5 M NaCl solution at 25 ◦ C. To that purpose, oxide films were formed potentiostatically on Al and Al–Sn alloys in the borate buffer solution (0.5 M H3 BO3 + 0.05 M Na2 B4 O7 ·10H2 O, pH 7.8) under the defined conditions (E = 0.8 V, t = 1 h, T = 25 ◦ C). The electrode was then transferred to the NaCl solution, in which, after attaining a stable value of the open circuit potential, EOCP (values are shown in Table 1), the dependence of current on time was monitored at different potentials in the cathodic range (from −1.6 to −2.0 V). The time of staying at the given potential in the cathodic range varied: 1, 10, and 100 s.
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Table 1 Open circuit potential, EOCP , values for Al and Al–Sn alloys in 0.5 M NaCl solution EOCP (V) Al Al–0.02% Sn Al–0.09% Sn Al–0.20% Sn Al–0.40% Sn
−0.80 −0.85 −1.02 −1.30 −1.40
After the cathodic pulse ended, the electrode potential was returned to EOCP , and the anodic current—time responses were recorded in the period of 1, 10, and 100 s.
3. Results 3.1. Effect of cathodic potential Fig. 1 shows the pulse signal as the characteristic signal of response obtained for the Al electrode during and after the cathodic pulse of −1.6 V lasting for 1 s. The temporal response of cathodic current decays exponentially. When the cathodic pulse ends and the electrode potential returns to EOCP , anodic current appears, also decaying exponentially with time. 3.1.1. The cathodic part of the current–time response Fig. 2 shows the characteristic series of temporal current responses for the increasing series of cathodic pulses of 0.1 V (in the range from −1.6 to −2.0 V) lasting for 1 s for Al and Al–Sn alloys. It is evident that such cathodic polarisation of the passive Al electrode (Fig. 2a) creates two forms of the current–time response. At lower cathodic potentials (to −1.8 V), temporal responses are obtained in which the current shows the characteristic exponential decay, while at higher cathodic potentials the dependence of current on time is much more complex. For a short duration (to 0.1 s) of the
Fig. 1. Current–time responses for Al in a 0.5 M NaCl during and after termination of the cathodic pulse of −1.6 V. Pulse duration: 1 s; detail: pulse signal.
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Fig. 2. Sequences of cathodic current–time responses to the potential pulse from EOCP to different negative values in 0.5 M NaCl for: (a) Al, (b) Al–0.02% Sn, (c) Al–0.09% Sn, (d) Al–0.20% Sn and (e) Al–0.40% Sn. Pulse duration: 1 s.
−1.9 V cathodic pulse, the exponential decay is observed. After that there is a minimum or a stop, but then the current increases if the pulse lasts longer. At the cathodic pulse of −2.0 V, the current increases without the occurrence of minimum. As shown in Figs. 2b–e, a similar series of cathodic current–time responses has been obtained for Al–Sn alloys. However, when the Sn content in the alloy increases, the cathodic current–time responses become increasingly more complex at more negative potentials. In the case of the alloy with the maximum content of Sn, the current decays exponentially with time in the whole range of cathodic potentials examined. The surface below the cathodic current–time response represents the quantity of charge spent on reduction, QC , and the values obtained for each sample within 1 s are shown in Table 2. The values of QC increase with the increase of the cathodic potential and mainly decrease with the increase of
the tin content in the alloy. Based on the results obtained, it is possible to define for all the samples the potential range in which cathodic breakdown of the oxide film occurs. The exponential decay of current with time, as well as small values of QC (from ≈10 to 150 C cm−2 ), indicate that in the defined potential range (range of low cathodic potentials, LCP) for each individual sample, the oxide layer retains its properties. As indicated in Table 2, the LCP range for Al, Al–0.02% Sn and Al–0.09% Sn extends from −1.6 to −1.8 V; this range is somewhat wider for the Al–0.20% Sn alloy, i.e., from −1.6 to −1.9 V. However, to define correctly the LCP range for the alloy with 0.40% Sn, this sample should be subjected to cathodic pulses more negative than −2.0 V. Potentials more negative than the values indicated previously, for each individual sample, determine the range of high cathodic potentials (HCP). In the HCP range, the shape of the current–time response as well as the values obtained for QC (from ≈320 to 1400 C cm−2 ) indicate that the cathodic
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Table 2 Values for charge, QC , obtained for Al and Al–Sn alloys at different cathodic potentials and at cathodic pulse duration of 1 s
breakdown occurs and the oxide film hydrates. Hydration of the oxide film takes place as a consequence of the local pH increase at the oxide/solution interface by hydrogen splitting away from water. The hydrated part of the oxide film is assumed to penetrate towards the metal/oxide interface. This results in the increase in the ionic conductance inside this portion of the film and in an increased rate of electron transfer (tunnelling) through the remaining anhydrous part. Finally, hydrogen is being evolved at the bare metal at an anomalous yield due to a simultaneous and equivalent dissolution of the metal [31]. However, according to the classic Wagner–Traud theory on electrochemical corrosion of metal [32], the partial anodic current, corresponding to the metal dissolution rate, should decrease as the potential shifts to the negative direction. Instead, with aluminium an opposite situation was observed. Many authors have found that, at a certain critical negative potential, a sudden cathodic current rise takes place not only at the hydrogen evolution rate, but also at that of metal dissolution [33–36]. Besides, one should bear in mind the possibility of chemical dissolution of metal. According to some authors, the chemical attack by OH− ions and water, in a reaction of the Al + OH− + H2 O ↔ AlO2 − + 3/2H2 type, seems to be dominant in the dissolution of metal [33,37]. Therefore, when the potential is shifted in the negative direction, substances may form on the surface of Al and Al–Sn alloys that can form only in the direct contact of the metal and released hydrogen, such as AlH3 , AlH2 + , AlH2+ [38] and SnH4 [11].
3.1.2. The anodic part of the current–time response The return of the potential to the EOCP value results in the appearance of anodic currents decaying with time. It may be assumed that this function reflects both charging of the interfacial capacitance due to a sudden change of potential, and the oxidation of substances (other than gaseous hydrogen)
formed during the cathodic process which did not diffuse away, but which are unstable at the more positive potentials. By integrating the surface under the anodic current–time responses, the quantity of charge, QA , was evaluated for each sample within 1 s, and is shown in Table 3. It is obvious that with the increased Sn content in the alloy, the QA value decreases, which means that a significantly smaller quantity of substance remaining at the surface of the metal is formed in the cathodic process. This is probably due to the fact that the beginning of hydration of the oxide film, for the alloys with a larger Sn content, is shifted towards the more negative potentials, so that during the shorter time the cathodic process takes place at the bare metal, a considerably smaller amount of the readily oxidizing substance is created. As can be seen from Table 3 the charge QA in the two negative potentials regions essentially differs for Al, Al–0.02% Sn and Al–0.09% Sn alloys from those for the Al–0.20% Sn and Al–0.40% Sn alloys. The charge quantities on the latter two, obtained in two negative potential regions, are smaller and similar. Radoˇsevi´c et al. [37] recorded accumulation of substances other than the gaseous hydrogen; both in the oxide before hydration and in the Al surface after hydration took place. Those authors ascribed the fact that the anodic current decay is a complex exponential with several time constants to different kinetics of anodic oxidation of substances present at the surface. Besides these, there are other literature data that suggest that more than one substance is formed during the cathodic pulse, some being more easily oxidized than others. Therefore, during cathodic polarisation different substances form and quickly diffuse in the solution mass [37]. Besides, substances remaining at the surface of the electrode have different oxidation potentials [39], so that only some of them oxidize when the electrode potential returns to EOCP . This might explain the great differences in values of QA and QC .
Table 3 Charge, QA , for Al and Al–Sn alloys, determined after termination of cathodic polarisation lasting 1 s and after the return of the electrode potential to the EOCP t (s)
1
E(V)
−1.6 −1.7 −1.8 −1.9 −2.0
QA (C cm−2 ) Al
Al–0.02% Sn
Al–0.09% Sn
Al–0.20% Sn
Al–0.40% Sn
4 4 4 17 37
3 4 5 17 36
3 3 4 15 30
2 3 4 5 7
2 3 3 4 4
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Fig. 3. Sequences of cathodic current–time responses to the potential pulse from EOCP to different negative values in 0.5 M NaCl for: (a) Al and (b) Al–0.40% Sn. Pulse duration: 100 s.
3.2. Effect of cathodic pulse duration Results of measurements shown in Section 3.1 indicate that the duration of staying of 1 s on each cathodic potential is too short for a more significant breakdown of the oxide film. Therefore, a series of measurements were made to determine how a longer stay (10 and 100 s) at the cathodic potential of a certain value affects the stability of the previously formed oxide layer. Fig. 3 shows some of the results. We see that in the case of treatment of the passive Al electrode (Fig. 3a) by cathodic potentials of different values and duration of 100 s, two forms of current–time response are obtained. At cathodic potentials of −1.6 and −1.7 V, the time responses obtained show the exponential decay of current, while at higher potentials the current increases with
pulse duration. Similar series of cathodic current–time responses were obtained for Al–Sn alloys as well, and Fig. 3b shows the cathodic current–time responses for the alloy with the highest percentage of Sn. It can be seen that at the cathodic polarisation of ≤−1.8 V the cathodic current–time responses decay exponentially, while for higher cathodic pulses the current–time dependence is a more complex one. By integrating the surface under the cathodic current–time responses, the quantity of charge used for reduction, QC , was determined, and Table 4 shows the values obtained for the case of cathodic polarisation of different values and durations. In both cases, i.e., when cathodic polarisation lasts 10 and 100 s, the value of QC increases with the value of the cathodic potential, and decreases with the increase of the Sn content in the alloy. Furthermore, it is important to observe that when the cathodic pulse duration increases, the boundary between LCP and HCP shifts towards more positive potentials. This is especially evident for alloys with the increased Sn content. If the duration of staying at the cathodic potential is prolonged from 1 to 100 s, the boundary between LCP and HCP for the Al electrode shifts by 100 mV in the positive direction, while the shift for the Al–0.40% Sn alloy is higher, i.e., the boundary between LCP and HCP is shifted by 200 mV in the positive direction. The interruption of the cathodic pulse and the return of the electrode potential to the EOCP result in anodic currents whose decay with time up to 10 s indicates the exponential dependence in all cases. Quantities of charge used up for oxidation of the substance, QA , are shown in Table 5. For all samples, the increase of QA was observed with the increase of cathodic potential. The increase of the Sn content in the alloy decreases the QA , except in case of Al–0.40% Sn alloy where the increase of QA has been observed. However, after prolonged staying (100 s) at a specific potential in the cathodic range, and after subsequent return to the EOCP value, anodic currents appear that do not observe the usual pattern of exponential decay with time in all the samples. Fig. 4 shows the current–time responses for pure Al and Al–0.40% Sn alloy obtained during the −1.9 V pulse and after its termination. The duration of the pulse was 100 s. As seen in Fig. 4a, after the cathodic treatment of Al, anodic current decays exponentially with time. The Al–Sn alloys with lower Sn contents, i.e. 0.02% and 0.09% Sn, behave in a similar manner. However, Fig. 4b shows that this is not true for the alloy with the maximum Sn content, i.e., 0.40% (and for the 0.20% Sn alloy, which is not shown in the figure) where, after termination of cathodic polarisation and the return of the potential to EOCP , the anodic current appears and increases with time. The greater the value of the previous cathodic pulse, the greater the value of the anodic current (Fig. 5). This undoubtedly indicates that cathodic polarisation activates alloys with 0.20% and 0.40% Sn for anodic dissolution. This is confirmed by values of QA shown in Table 5.
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Table 4 Values for charge, QC , obtained for Al and Al–Sn alloys at different cathodic potentials and at cathodic pulse duration of 10 and 100 s
Table 5 Charge, QA , for Al and Al–Sn alloys, determined after termination of cathodic polarisation lasting 10 and 100 s and after the return of the electrode potential to the EOCP QA (C cm−2 )
t (s)
E (V)
10
−1.6 −1.7 −1.8 −1.9 −2.0
30 36 38 152 577
31 34 38 146 569
29 31 36 122 288
18 26 34 96 134
18 27 38 123 603
−1.6 −1.7 −1.8 −1.9 −2.0
476 734 2655 5140 8714
485 726 2103 5232 8678
325 567 1868 3741 6171
497 1114 2842 28670 227600
7788 7010 8514 45330 671600
Al
100
Al–0.02% Sn
4. Discussion At complex interfaces such as that of Al covered with a thin, compact oxide film, in contact with aqueous phase, several processes can take place upon cathodic polarisation. By alloying Al with small quantities of Sn, the structure of the Al/Al2 O3 /electrolyte system changes, which additionally complicates the matter. Therefore, in order to obtain a clearer insight into the results of this study it is first necessary to consider the effects of Sn on passivation of Al as well as on properties of anodically formed oxide films. The earlier studies, performed on Al and Al–Sn alloys in borate buffer solution, have shown that (i) the presence of Sn in the alloy improves the oxide film properties (increased thickness and resistance) [40,41] and (ii) the presence of Sn increases the current efficiency for the oxide film formation [40]. The results obtained were explained in terms of the point defect model for passive film growth proposed by Macdonald and co-workers [42–46]. In this model, the film growth is due to a flux of cation vacancies from the film/solution interface to the metal/film interface and to a flux of oxygen vacancies in the opposite direction. Both interfaces are assumed to be under electrochemical equilibrium.
Al–0.09% Sn
Al–0.20% Sn
Al–0.40% Sn
During anodic oxidation of the Al–Sn alloy, Sn may be incorporated into the Al2 O3 lattice. As Sn has two oxidation states, Sn2+ and Sn4+ , both forms can be incorporated, which has been established experimentally [17]. Incorporation of Sn2+ and Sn4+ ions into the Al2 O3 inevitably leads to changes in concentration of cation and anion vacancies. Incorporation of Sn2+ into the Al2 O3 film leads to formation of anion vacancies, while Sn4+ causes cation vacancies. The vacancies formed help the migration of O2− to the metal/oxide phase boundary or the migration of metal ions to the metal/solution phase boundary. In solutions like the borate buffer, in which the barrier film is formed, this may increase its thickness and resistance. As shown in Table 6, at identical passivation conditions (E = 0.8 V, t = 1 h, T = 25 ◦ C) the oxide films formed in alloys with higher Sn contents have better properties (the data have been taken from Table 6 Characteristic sizes of oxide films on Al [9] and Al–Sn alloys [40,41] obtained at passivation in the borate buffer solution (E = 0.8 V, t = 1 h, T = 25 ◦ C)
Al Al–0.02% Sn Al–0.09% Sn Al–0.20% Sn Al–0.40% Sn
d (nm)
R (k cm2 )
1.31 1.52 1.80 1.92 2.08
57.39 60.28 61.45 70.75 76.01
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Fig. 4. Current–time responses for (a) Al and (b) Al–0.40% Sn alloy in 0.5 M NaCl during and after termination of the cathodic pulse of −1.9 V. Pulse duration: 100 s.
[9,40,41]). However, if a sample of the Al–Sn alloy (with the oxide film on its surface) is immersed in an aggressive solution containing halogen ions (Cl− ), increased concentration of point defects may cause rapid corrosion of the base metal. According to Venugopal et al. [17], in this case, anion vacancies help migration not only of O2− ions but Cl− ions as well, resulting in deeper penetration of Cl− ions into the oxide film on Al–Sn
Fig. 5. A series of anodic current–time responses for the Al–0.40% Sn alloy in 0.5 M NaCl obtained at EOCP after termination of cathodic pulses of different values with duration of 100 s.
alloys than on pure Al. This would provide for accelerated corrosion of the base alloy. Two regions of potential (LCP and HCP) with distinctly different phenomena can be found at cathodic polarisation of passivated aluminium and Al–Sn alloys in the chloride solution. In the LCP region (for each sample separately), the oxide film appears to preserve its properties as the experimental curves show that the cathodic current decays exponentially with time. Cathodic currents are very low (in the A range) and there is no increase in the current after prolonged cathodic polarisation. It may be assumed that those curves reflect both charging of the interfacial capacitance due to a sudden change of potential, and the reduction of water by electron tunnelling through the oxide film at the metal surface [37]. As shown in Table 2, the LCP area for Al, Al–0.02% Sn and Al–0.09% Sn lies in the range from −1.6 to −1.8 V; for the Al–0.20% Sn alloy this range is somewhat wider, i.e., from −1.6 to −1.9 V, while for the Al–0.40% Sn alloy the whole range of cathodic potentials observed represents the LCP area. A dramatic change takes place when going more negative than this potential limit, i.e., in the HCP region. In this potential region, after the double layer charging, the current was found to increase again. This behaviour is mainly attributed to some dissolution and/or some hydration of the oxide film due to local pH increase produced by hydrogen evolution. This results in local breakdown of the oxide film and formation of micro-pits within the film, which increases the active area. Our previous works have established that the oxide film was damaged locally after cathodic treatment of pure aluminium and Al–Sn alloys [18]. Namely, as hydrogen evolution results in equivalent production of OH− ions from water at an increasing rate, at this critical potential they start causing hydration of the oxide film, making it ionically conducting or less resistive (or even partially dissolving in the form of AlO2 − ions). Thus, the interface at which the reduction takes place is shifted all the way to the metal surface at the bottom of micro-pits. This is a self-accelerating process and the current density increases rapidly. Finally, hydrogen is being evolved at the bare metal at an anomalous yield due to a simultaneous and equivalent dissolution of the metal [31]. Therefore, in this potential range substances may form on the surface of Al and Al–Sn alloys that can form only in the direct contact of the metal and released hydrogen, such as aluminium hydrides [38], but also tin hydrides [11]. When the potential returns to the EOCP value, these substances oxidise which leads to renewal of the oxide film. The obtained results (Fig. 2 and Table 2) show that with the increase of the Sn content in the alloy the cathodic breakdown and hydration of oxide film shift towards more negative potentials. This means that the presence of Sn tends to stabilize the oxide film on Al, which is particularly expressed in the case of cathodic pulse with 1 s duration. This can be understood if data shown in Table 6 are taken into account, showing that the resistance and thickness of the oxide film increase with the increase of the Sn content in the alloy. One second at the cathodic potential is too short a time for a greater
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breakdown of oxide film to take place in all the samples, especially in those with better properties. However, if the time of cathodic treatment is prolonged, major changes take place. The longer the time of the cathodic pulse, the more positive the potentials at which the oxide film breakdown takes place. This shift is more marked with alloys containing 0.20% and 0.40% of Sn. Besides, in these samples after the cathodic polarisation lasting for 100 s, the anodic current that appears at EOCP increases with time (opposite to all other similar cases). The higher the cathodic polarisation and the Sn content in the alloy, the higher the anodic current and charge quantity QA (Table 5). This undoubtedly indicates that the cathodic polarisation lasting for 100 s activates the Al–0.20% Sn and Al–0.40% Sn alloys for anodic dissolution. It is well known that the Al activation is reached only when a true Al–Sn metallic contact is produced in the presence of Cl− ions [13,20]. During the cathodic polarisation, hydrogen evolution may occur at an enhanced rate at the bottom of micro-pits, forming due to local breakdown of oxide layer. As can be seen, the cathodic breakdown of the oxide film depends on the value of the cathodic potential and its duration (as shown in Tables 2 and 4). The exposed Al–Sn surface at the places where the oxide film is damaged serves as the preferential site for Cl− ion adsorption. The formation of such active sites is responsible for Cl− adsorption at more electronegative potentials than Al [13,20], leading to its activation. The results have shown that only cathodic polarisation lasting for 100 s leads to the activation of the Al–0.20% Sn and Al–0.40% Sn alloys. Hence, this time would be related to the time necessary to accumulate a minimum amount of the active Al–Sn surface, or active sites. Furthermore, it is interesting to consider the situation where Al–0.20% Sn and Al–0.40% Sn alloys are subjected to cathodic treatment at the potentials ≤−1.8 V, i.e., in the LCP range. It is in this potential range that the oxide film retains its properties. However, upon the return of the potential to the EOCP value the activation effect is observed. It is assumed that under the effect of the electrical field, Sn+2 and Sn+4 ions help Cl− ions migrate through the oxide layer, mainly through defect sites and pores in the oxide. The depth of penetration of Cl− ions into the oxide film depends on time. During prolonged cathodic polarisation, local alkalising may produce film thinning making it ionically conducting or less resistive and favouring its activation when the potential returned to the rest value. In these conditions, Cl− ions penetrate deeply into the oxide layer and reach the metal/oxide phase boundary where they get into the direct contact with the active surface, thereby activating Al. It is evident (Fig. 5) that the activation attained in this way is lower (anodic currents are lower) in comparison with the cases when the sample was subjected to cathodic pulses in the HCP range. The fact that the activation has been observed only in alloys with higher tin contents can be explained by the limit of solubility of Sn in Al. Due to very low solubility of Sn in Al, which is 0.1%, Sn-enriched phases are expected (which has been actually established [18]). It has been seen that
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the surface condition promotes and maintains the activation of Al. Therefore, it is expected that the attack will initiate and propagate in those zones enriched in Sn during the solidification process, i.e., grain boundaries and intermetallic zones. In our previous works [18], the surface of pure Al and Al–0.40% Sn alloy was analysed by means of an electron micro-analyser applying the scanning method prior to and after the cathodic pulse. The elemental composition of surfaces of the same samples was examined by means of EDAX analysis. The data obtained show that the morphology as well as the composition of the alloy differs from that of pure aluminium. Specifically, a separate tin phase was observed with the alloy containing 0.40% tin. The EDAX analysis of that part of the surface has confirmed the presence of tin. After the cathodic treatment of the Al–0.40% Sn alloy, pits were observed in places of surface where tin existed as a separate phase.
5. Conclusion Based on results obtained, the potential range in which cathodic breakdown of the oxide film occurs has been defined for all the samples. In the range of lower cathodic potentials, the oxide film retains its properties, while in the range of higher cathodic potentials cathodic breakdown and hydration of the oxide layer take place. The boundary between these two potential ranges shifts towards more negative potential values when the Sn content in the alloy increases. The longer the time of cathodic polarisation, the more positive the potentials at which the oxide film breakdown takes place. This shift is more marked with alloys with higher Sn contents. The shape of anodic current–time responses and the values of QA indicate that a longer stay at the cathodic potential (100 s) activates alloys with 0.20% and 0.40% Sn for anodic dissolution.
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Cathodic breakdown of anodic oxide film on Al and Al–Sn alloys in NaCl solution S. Gudi´c ∗ , J. Radoˇsevi´c 1 , I. Smoljko, M. Kliˇski´c 1 Department of Electrochemistry and Materials Protection, Faculty of Chemical Technology, Teslina 10/V, 21000 Split, Croatia Received 26 August 2004; received in revised form 22 December 2004; accepted 11 March 2005 Available online 24 May 2005
Abstract The effect of cathodic polarisation on stability of defined oxide films on Al and Al–Sn alloys (with up to 0.40% Sn) has been investigated in a 0.5 M NaCl solution using the potentiostatic pulse method. The dependence of the cathodic current on time (in the period of 1, 10 and 100 s) was recorded on Al and Al–Sn alloys when subjected to a potential pulse from EOCP to different negative values (up to −2.0 V). Anodic current responses to the return to the EOCP were also recorded at three different time scales (1, 10 and 100 s). It has been established that the cathodic polarisation of passivated Al and Al–Sn alloys in a chloride solution is characterized by two regions of potentials with distinctly different phenomena: the range of low and high cathodic potentials (LCP and HCP). In the LCP range, the oxide film retains its properties, while in the HCP range cathodic breakdown and hydration of the oxide take place. The boundary between these two potential ranges shifts towards more negative potential values when the percentage of Sn in the alloy increases. The longer the duration of the cathodic pulse, the more positive the potentials at which the oxide film breakdown takes place. This shift is more marked with alloys containing higher percentage of Sn. Cathodic polarisation (duration of 100 s) activates alloys with 0.20% and 0.40% Sn for anodic dissolution. © 2005 Elsevier Ltd. All rights reserved. Keywords: Aluminium; Al–Sn alloy; Aluminium activation; Anodic oxide film; Cathodic polarisation
1. Introduction Aluminium and its alloys are of great technological significance due to their exceptional mechanical properties and good corrosion resistance both in the atmosphere and in the aqueous solution. The main reason for the corrosion resistance of aluminium is the formation of a thin, compact oxide film on the surface of the metal that may be tens of angstroms thick. Properties of oxide film can be additionally improved by the anodising process [1–10]. On the other hand, the standard reversible potential o of aluminium (EAl/Al 3+ = −1.66 V versus NHE [11]) and its high energetic capacity (2980 Ah kg−1 ) are very
∗ 1
Corresponding author. E-mail address: [email protected] (S. Gudi´c). ISE member.
0013-4686/$ – see front matter © 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2005.03.041
attractive properties when aluminium is used as active anodic material in chemical power sources or in sacrificial anodes in cathodic protection of steel in seawater. However, pure aluminium is practically impossible to use because a protective oxide film forms on it, making the potential positive, which in turn makes aluminium unattractive as energetic material. If aluminium is alloyed with small quantities of elements such as In, Ga, Hg, Sn, Zn and others [12–21], its anodic activity is improved as the structure of the aluminium/aluminium oxide/chloride electrolyte system is changed. Activation of aluminium can be also achieved by adding small quantities of suitable metal cations, such In3+ , Ga3+ , Hg2+ , Sn4+ or Sn2+ to the electrolyte [15,20,22– 28]. It has been reported that Sn accelerates the anodic dissolution of Al when present in solid solutions and that the potential range in which the current output increases extends between the SnH4(g) /Sn0 and Sn0 /SnO3 2− reversible potentials [29].
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Al–Sn alloys have the open circuit potential more negative than pure Al and exhibit increased anodic dissolution rates and reduced wasteful corrosion rates. Furthermore, it has been found that cathodic polarisation also activates the Al–Sn alloy for anodic dissolution [14,18]. In this potential region, once the hydrogen evolution has started by electron tunnelling, diffusion or ionic exchange through the protective Al oxide, the pH under the growing bubble increases and oxide dissolution takes place by local alkalisation. This results in local breakdown of the oxide film and formation of micro-pits within the film. Under these conditions, an active area of the Al–Sn alloy could be produced favouring Cl− ion adsorption at more negative potentials than aluminium, leading to its activation. This study has examined the stability of defined oxide films on Al and Al–Sn alloys in a 0.5 M NaCl solution at different cathodic potentials and durations of cathodic pulse.
2. Experimental The experiments were performed on samples of high purity Al (99.999%) and Al–Sn binary alloys with varying tin contents (0.02%, 0.09%, 0.20% and 0.40%), obtained by courtesy of Alcan International. The Al–Sn alloys had been prepared with high purity aluminium as the primary component, and tin as the alloying component. After alloying the metal was homogenised at 600 ◦ C for more than 1 h. At this temperature, Sn has its maximum solubility in Al of approximately 0.1% Sn [30]. The alloy was quenched in cold water. The Al and Al–Sn samples were made into electrodes by inserting insulated copper wires and protecting all sides but one with epoxy resin. Before each measurement, the surface was pre-treated by mechanical polishing of the electrode surface up to a mirror finish, followed by the alkali attack by immersing the electrode for 1 min in 0.1 M NaOH heated to 40 ◦ C. The electrode was then rinsed in doubly distilled water. The exposed geometric area was 0.57 cm2 . A conventional three-electrode glass cell was used with a platinum sheet and a saturated calomel electrode (SCE) serving as the counter and the reference electrodes, respectively. Electrochemical measurements were performed using a PAR M273A potentiostat driven by a computer. The potentiostatic pulse method was used to examine the cathodic reduction and anodic re-oxidation of defined oxide films on Al and Al–Sn alloys in a 0.5 M NaCl solution at 25 ◦ C. To that purpose, oxide films were formed potentiostatically on Al and Al–Sn alloys in the borate buffer solution (0.5 M H3 BO3 + 0.05 M Na2 B4 O7 ·10H2 O, pH 7.8) under the defined conditions (E = 0.8 V, t = 1 h, T = 25 ◦ C). The electrode was then transferred to the NaCl solution, in which, after attaining a stable value of the open circuit potential, EOCP (values are shown in Table 1), the dependence of current on time was monitored at different potentials in the cathodic range (from −1.6 to −2.0 V). The time of staying at the given potential in the cathodic range varied: 1, 10, and 100 s.
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Table 1 Open circuit potential, EOCP , values for Al and Al–Sn alloys in 0.5 M NaCl solution EOCP (V) Al Al–0.02% Sn Al–0.09% Sn Al–0.20% Sn Al–0.40% Sn
−0.80 −0.85 −1.02 −1.30 −1.40
After the cathodic pulse ended, the electrode potential was returned to EOCP , and the anodic current—time responses were recorded in the period of 1, 10, and 100 s.
3. Results 3.1. Effect of cathodic potential Fig. 1 shows the pulse signal as the characteristic signal of response obtained for the Al electrode during and after the cathodic pulse of −1.6 V lasting for 1 s. The temporal response of cathodic current decays exponentially. When the cathodic pulse ends and the electrode potential returns to EOCP , anodic current appears, also decaying exponentially with time. 3.1.1. The cathodic part of the current–time response Fig. 2 shows the characteristic series of temporal current responses for the increasing series of cathodic pulses of 0.1 V (in the range from −1.6 to −2.0 V) lasting for 1 s for Al and Al–Sn alloys. It is evident that such cathodic polarisation of the passive Al electrode (Fig. 2a) creates two forms of the current–time response. At lower cathodic potentials (to −1.8 V), temporal responses are obtained in which the current shows the characteristic exponential decay, while at higher cathodic potentials the dependence of current on time is much more complex. For a short duration (to 0.1 s) of the
Fig. 1. Current–time responses for Al in a 0.5 M NaCl during and after termination of the cathodic pulse of −1.6 V. Pulse duration: 1 s; detail: pulse signal.
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Fig. 2. Sequences of cathodic current–time responses to the potential pulse from EOCP to different negative values in 0.5 M NaCl for: (a) Al, (b) Al–0.02% Sn, (c) Al–0.09% Sn, (d) Al–0.20% Sn and (e) Al–0.40% Sn. Pulse duration: 1 s.
−1.9 V cathodic pulse, the exponential decay is observed. After that there is a minimum or a stop, but then the current increases if the pulse lasts longer. At the cathodic pulse of −2.0 V, the current increases without the occurrence of minimum. As shown in Figs. 2b–e, a similar series of cathodic current–time responses has been obtained for Al–Sn alloys. However, when the Sn content in the alloy increases, the cathodic current–time responses become increasingly more complex at more negative potentials. In the case of the alloy with the maximum content of Sn, the current decays exponentially with time in the whole range of cathodic potentials examined. The surface below the cathodic current–time response represents the quantity of charge spent on reduction, QC , and the values obtained for each sample within 1 s are shown in Table 2. The values of QC increase with the increase of the cathodic potential and mainly decrease with the increase of
the tin content in the alloy. Based on the results obtained, it is possible to define for all the samples the potential range in which cathodic breakdown of the oxide film occurs. The exponential decay of current with time, as well as small values of QC (from ≈10 to 150 C cm−2 ), indicate that in the defined potential range (range of low cathodic potentials, LCP) for each individual sample, the oxide layer retains its properties. As indicated in Table 2, the LCP range for Al, Al–0.02% Sn and Al–0.09% Sn extends from −1.6 to −1.8 V; this range is somewhat wider for the Al–0.20% Sn alloy, i.e., from −1.6 to −1.9 V. However, to define correctly the LCP range for the alloy with 0.40% Sn, this sample should be subjected to cathodic pulses more negative than −2.0 V. Potentials more negative than the values indicated previously, for each individual sample, determine the range of high cathodic potentials (HCP). In the HCP range, the shape of the current–time response as well as the values obtained for QC (from ≈320 to 1400 C cm−2 ) indicate that the cathodic
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Table 2 Values for charge, QC , obtained for Al and Al–Sn alloys at different cathodic potentials and at cathodic pulse duration of 1 s
breakdown occurs and the oxide film hydrates. Hydration of the oxide film takes place as a consequence of the local pH increase at the oxide/solution interface by hydrogen splitting away from water. The hydrated part of the oxide film is assumed to penetrate towards the metal/oxide interface. This results in the increase in the ionic conductance inside this portion of the film and in an increased rate of electron transfer (tunnelling) through the remaining anhydrous part. Finally, hydrogen is being evolved at the bare metal at an anomalous yield due to a simultaneous and equivalent dissolution of the metal [31]. However, according to the classic Wagner–Traud theory on electrochemical corrosion of metal [32], the partial anodic current, corresponding to the metal dissolution rate, should decrease as the potential shifts to the negative direction. Instead, with aluminium an opposite situation was observed. Many authors have found that, at a certain critical negative potential, a sudden cathodic current rise takes place not only at the hydrogen evolution rate, but also at that of metal dissolution [33–36]. Besides, one should bear in mind the possibility of chemical dissolution of metal. According to some authors, the chemical attack by OH− ions and water, in a reaction of the Al + OH− + H2 O ↔ AlO2 − + 3/2H2 type, seems to be dominant in the dissolution of metal [33,37]. Therefore, when the potential is shifted in the negative direction, substances may form on the surface of Al and Al–Sn alloys that can form only in the direct contact of the metal and released hydrogen, such as AlH3 , AlH2 + , AlH2+ [38] and SnH4 [11].
3.1.2. The anodic part of the current–time response The return of the potential to the EOCP value results in the appearance of anodic currents decaying with time. It may be assumed that this function reflects both charging of the interfacial capacitance due to a sudden change of potential, and the oxidation of substances (other than gaseous hydrogen)
formed during the cathodic process which did not diffuse away, but which are unstable at the more positive potentials. By integrating the surface under the anodic current–time responses, the quantity of charge, QA , was evaluated for each sample within 1 s, and is shown in Table 3. It is obvious that with the increased Sn content in the alloy, the QA value decreases, which means that a significantly smaller quantity of substance remaining at the surface of the metal is formed in the cathodic process. This is probably due to the fact that the beginning of hydration of the oxide film, for the alloys with a larger Sn content, is shifted towards the more negative potentials, so that during the shorter time the cathodic process takes place at the bare metal, a considerably smaller amount of the readily oxidizing substance is created. As can be seen from Table 3 the charge QA in the two negative potentials regions essentially differs for Al, Al–0.02% Sn and Al–0.09% Sn alloys from those for the Al–0.20% Sn and Al–0.40% Sn alloys. The charge quantities on the latter two, obtained in two negative potential regions, are smaller and similar. Radoˇsevi´c et al. [37] recorded accumulation of substances other than the gaseous hydrogen; both in the oxide before hydration and in the Al surface after hydration took place. Those authors ascribed the fact that the anodic current decay is a complex exponential with several time constants to different kinetics of anodic oxidation of substances present at the surface. Besides these, there are other literature data that suggest that more than one substance is formed during the cathodic pulse, some being more easily oxidized than others. Therefore, during cathodic polarisation different substances form and quickly diffuse in the solution mass [37]. Besides, substances remaining at the surface of the electrode have different oxidation potentials [39], so that only some of them oxidize when the electrode potential returns to EOCP . This might explain the great differences in values of QA and QC .
Table 3 Charge, QA , for Al and Al–Sn alloys, determined after termination of cathodic polarisation lasting 1 s and after the return of the electrode potential to the EOCP t (s)
1
E(V)
−1.6 −1.7 −1.8 −1.9 −2.0
QA (C cm−2 ) Al
Al–0.02% Sn
Al–0.09% Sn
Al–0.20% Sn
Al–0.40% Sn
4 4 4 17 37
3 4 5 17 36
3 3 4 15 30
2 3 4 5 7
2 3 3 4 4
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Fig. 3. Sequences of cathodic current–time responses to the potential pulse from EOCP to different negative values in 0.5 M NaCl for: (a) Al and (b) Al–0.40% Sn. Pulse duration: 100 s.
3.2. Effect of cathodic pulse duration Results of measurements shown in Section 3.1 indicate that the duration of staying of 1 s on each cathodic potential is too short for a more significant breakdown of the oxide film. Therefore, a series of measurements were made to determine how a longer stay (10 and 100 s) at the cathodic potential of a certain value affects the stability of the previously formed oxide layer. Fig. 3 shows some of the results. We see that in the case of treatment of the passive Al electrode (Fig. 3a) by cathodic potentials of different values and duration of 100 s, two forms of current–time response are obtained. At cathodic potentials of −1.6 and −1.7 V, the time responses obtained show the exponential decay of current, while at higher potentials the current increases with
pulse duration. Similar series of cathodic current–time responses were obtained for Al–Sn alloys as well, and Fig. 3b shows the cathodic current–time responses for the alloy with the highest percentage of Sn. It can be seen that at the cathodic polarisation of ≤−1.8 V the cathodic current–time responses decay exponentially, while for higher cathodic pulses the current–time dependence is a more complex one. By integrating the surface under the cathodic current–time responses, the quantity of charge used for reduction, QC , was determined, and Table 4 shows the values obtained for the case of cathodic polarisation of different values and durations. In both cases, i.e., when cathodic polarisation lasts 10 and 100 s, the value of QC increases with the value of the cathodic potential, and decreases with the increase of the Sn content in the alloy. Furthermore, it is important to observe that when the cathodic pulse duration increases, the boundary between LCP and HCP shifts towards more positive potentials. This is especially evident for alloys with the increased Sn content. If the duration of staying at the cathodic potential is prolonged from 1 to 100 s, the boundary between LCP and HCP for the Al electrode shifts by 100 mV in the positive direction, while the shift for the Al–0.40% Sn alloy is higher, i.e., the boundary between LCP and HCP is shifted by 200 mV in the positive direction. The interruption of the cathodic pulse and the return of the electrode potential to the EOCP result in anodic currents whose decay with time up to 10 s indicates the exponential dependence in all cases. Quantities of charge used up for oxidation of the substance, QA , are shown in Table 5. For all samples, the increase of QA was observed with the increase of cathodic potential. The increase of the Sn content in the alloy decreases the QA , except in case of Al–0.40% Sn alloy where the increase of QA has been observed. However, after prolonged staying (100 s) at a specific potential in the cathodic range, and after subsequent return to the EOCP value, anodic currents appear that do not observe the usual pattern of exponential decay with time in all the samples. Fig. 4 shows the current–time responses for pure Al and Al–0.40% Sn alloy obtained during the −1.9 V pulse and after its termination. The duration of the pulse was 100 s. As seen in Fig. 4a, after the cathodic treatment of Al, anodic current decays exponentially with time. The Al–Sn alloys with lower Sn contents, i.e. 0.02% and 0.09% Sn, behave in a similar manner. However, Fig. 4b shows that this is not true for the alloy with the maximum Sn content, i.e., 0.40% (and for the 0.20% Sn alloy, which is not shown in the figure) where, after termination of cathodic polarisation and the return of the potential to EOCP , the anodic current appears and increases with time. The greater the value of the previous cathodic pulse, the greater the value of the anodic current (Fig. 5). This undoubtedly indicates that cathodic polarisation activates alloys with 0.20% and 0.40% Sn for anodic dissolution. This is confirmed by values of QA shown in Table 5.
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Table 4 Values for charge, QC , obtained for Al and Al–Sn alloys at different cathodic potentials and at cathodic pulse duration of 10 and 100 s
Table 5 Charge, QA , for Al and Al–Sn alloys, determined after termination of cathodic polarisation lasting 10 and 100 s and after the return of the electrode potential to the EOCP QA (C cm−2 )
t (s)
E (V)
10
−1.6 −1.7 −1.8 −1.9 −2.0
30 36 38 152 577
31 34 38 146 569
29 31 36 122 288
18 26 34 96 134
18 27 38 123 603
−1.6 −1.7 −1.8 −1.9 −2.0
476 734 2655 5140 8714
485 726 2103 5232 8678
325 567 1868 3741 6171
497 1114 2842 28670 227600
7788 7010 8514 45330 671600
Al
100
Al–0.02% Sn
4. Discussion At complex interfaces such as that of Al covered with a thin, compact oxide film, in contact with aqueous phase, several processes can take place upon cathodic polarisation. By alloying Al with small quantities of Sn, the structure of the Al/Al2 O3 /electrolyte system changes, which additionally complicates the matter. Therefore, in order to obtain a clearer insight into the results of this study it is first necessary to consider the effects of Sn on passivation of Al as well as on properties of anodically formed oxide films. The earlier studies, performed on Al and Al–Sn alloys in borate buffer solution, have shown that (i) the presence of Sn in the alloy improves the oxide film properties (increased thickness and resistance) [40,41] and (ii) the presence of Sn increases the current efficiency for the oxide film formation [40]. The results obtained were explained in terms of the point defect model for passive film growth proposed by Macdonald and co-workers [42–46]. In this model, the film growth is due to a flux of cation vacancies from the film/solution interface to the metal/film interface and to a flux of oxygen vacancies in the opposite direction. Both interfaces are assumed to be under electrochemical equilibrium.
Al–0.09% Sn
Al–0.20% Sn
Al–0.40% Sn
During anodic oxidation of the Al–Sn alloy, Sn may be incorporated into the Al2 O3 lattice. As Sn has two oxidation states, Sn2+ and Sn4+ , both forms can be incorporated, which has been established experimentally [17]. Incorporation of Sn2+ and Sn4+ ions into the Al2 O3 inevitably leads to changes in concentration of cation and anion vacancies. Incorporation of Sn2+ into the Al2 O3 film leads to formation of anion vacancies, while Sn4+ causes cation vacancies. The vacancies formed help the migration of O2− to the metal/oxide phase boundary or the migration of metal ions to the metal/solution phase boundary. In solutions like the borate buffer, in which the barrier film is formed, this may increase its thickness and resistance. As shown in Table 6, at identical passivation conditions (E = 0.8 V, t = 1 h, T = 25 ◦ C) the oxide films formed in alloys with higher Sn contents have better properties (the data have been taken from Table 6 Characteristic sizes of oxide films on Al [9] and Al–Sn alloys [40,41] obtained at passivation in the borate buffer solution (E = 0.8 V, t = 1 h, T = 25 ◦ C)
Al Al–0.02% Sn Al–0.09% Sn Al–0.20% Sn Al–0.40% Sn
d (nm)
R (k cm2 )
1.31 1.52 1.80 1.92 2.08
57.39 60.28 61.45 70.75 76.01
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Fig. 4. Current–time responses for (a) Al and (b) Al–0.40% Sn alloy in 0.5 M NaCl during and after termination of the cathodic pulse of −1.9 V. Pulse duration: 100 s.
[9,40,41]). However, if a sample of the Al–Sn alloy (with the oxide film on its surface) is immersed in an aggressive solution containing halogen ions (Cl− ), increased concentration of point defects may cause rapid corrosion of the base metal. According to Venugopal et al. [17], in this case, anion vacancies help migration not only of O2− ions but Cl− ions as well, resulting in deeper penetration of Cl− ions into the oxide film on Al–Sn
Fig. 5. A series of anodic current–time responses for the Al–0.40% Sn alloy in 0.5 M NaCl obtained at EOCP after termination of cathodic pulses of different values with duration of 100 s.
alloys than on pure Al. This would provide for accelerated corrosion of the base alloy. Two regions of potential (LCP and HCP) with distinctly different phenomena can be found at cathodic polarisation of passivated aluminium and Al–Sn alloys in the chloride solution. In the LCP region (for each sample separately), the oxide film appears to preserve its properties as the experimental curves show that the cathodic current decays exponentially with time. Cathodic currents are very low (in the A range) and there is no increase in the current after prolonged cathodic polarisation. It may be assumed that those curves reflect both charging of the interfacial capacitance due to a sudden change of potential, and the reduction of water by electron tunnelling through the oxide film at the metal surface [37]. As shown in Table 2, the LCP area for Al, Al–0.02% Sn and Al–0.09% Sn lies in the range from −1.6 to −1.8 V; for the Al–0.20% Sn alloy this range is somewhat wider, i.e., from −1.6 to −1.9 V, while for the Al–0.40% Sn alloy the whole range of cathodic potentials observed represents the LCP area. A dramatic change takes place when going more negative than this potential limit, i.e., in the HCP region. In this potential region, after the double layer charging, the current was found to increase again. This behaviour is mainly attributed to some dissolution and/or some hydration of the oxide film due to local pH increase produced by hydrogen evolution. This results in local breakdown of the oxide film and formation of micro-pits within the film, which increases the active area. Our previous works have established that the oxide film was damaged locally after cathodic treatment of pure aluminium and Al–Sn alloys [18]. Namely, as hydrogen evolution results in equivalent production of OH− ions from water at an increasing rate, at this critical potential they start causing hydration of the oxide film, making it ionically conducting or less resistive (or even partially dissolving in the form of AlO2 − ions). Thus, the interface at which the reduction takes place is shifted all the way to the metal surface at the bottom of micro-pits. This is a self-accelerating process and the current density increases rapidly. Finally, hydrogen is being evolved at the bare metal at an anomalous yield due to a simultaneous and equivalent dissolution of the metal [31]. Therefore, in this potential range substances may form on the surface of Al and Al–Sn alloys that can form only in the direct contact of the metal and released hydrogen, such as aluminium hydrides [38], but also tin hydrides [11]. When the potential returns to the EOCP value, these substances oxidise which leads to renewal of the oxide film. The obtained results (Fig. 2 and Table 2) show that with the increase of the Sn content in the alloy the cathodic breakdown and hydration of oxide film shift towards more negative potentials. This means that the presence of Sn tends to stabilize the oxide film on Al, which is particularly expressed in the case of cathodic pulse with 1 s duration. This can be understood if data shown in Table 6 are taken into account, showing that the resistance and thickness of the oxide film increase with the increase of the Sn content in the alloy. One second at the cathodic potential is too short a time for a greater
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breakdown of oxide film to take place in all the samples, especially in those with better properties. However, if the time of cathodic treatment is prolonged, major changes take place. The longer the time of the cathodic pulse, the more positive the potentials at which the oxide film breakdown takes place. This shift is more marked with alloys containing 0.20% and 0.40% of Sn. Besides, in these samples after the cathodic polarisation lasting for 100 s, the anodic current that appears at EOCP increases with time (opposite to all other similar cases). The higher the cathodic polarisation and the Sn content in the alloy, the higher the anodic current and charge quantity QA (Table 5). This undoubtedly indicates that the cathodic polarisation lasting for 100 s activates the Al–0.20% Sn and Al–0.40% Sn alloys for anodic dissolution. It is well known that the Al activation is reached only when a true Al–Sn metallic contact is produced in the presence of Cl− ions [13,20]. During the cathodic polarisation, hydrogen evolution may occur at an enhanced rate at the bottom of micro-pits, forming due to local breakdown of oxide layer. As can be seen, the cathodic breakdown of the oxide film depends on the value of the cathodic potential and its duration (as shown in Tables 2 and 4). The exposed Al–Sn surface at the places where the oxide film is damaged serves as the preferential site for Cl− ion adsorption. The formation of such active sites is responsible for Cl− adsorption at more electronegative potentials than Al [13,20], leading to its activation. The results have shown that only cathodic polarisation lasting for 100 s leads to the activation of the Al–0.20% Sn and Al–0.40% Sn alloys. Hence, this time would be related to the time necessary to accumulate a minimum amount of the active Al–Sn surface, or active sites. Furthermore, it is interesting to consider the situation where Al–0.20% Sn and Al–0.40% Sn alloys are subjected to cathodic treatment at the potentials ≤−1.8 V, i.e., in the LCP range. It is in this potential range that the oxide film retains its properties. However, upon the return of the potential to the EOCP value the activation effect is observed. It is assumed that under the effect of the electrical field, Sn+2 and Sn+4 ions help Cl− ions migrate through the oxide layer, mainly through defect sites and pores in the oxide. The depth of penetration of Cl− ions into the oxide film depends on time. During prolonged cathodic polarisation, local alkalising may produce film thinning making it ionically conducting or less resistive and favouring its activation when the potential returned to the rest value. In these conditions, Cl− ions penetrate deeply into the oxide layer and reach the metal/oxide phase boundary where they get into the direct contact with the active surface, thereby activating Al. It is evident (Fig. 5) that the activation attained in this way is lower (anodic currents are lower) in comparison with the cases when the sample was subjected to cathodic pulses in the HCP range. The fact that the activation has been observed only in alloys with higher tin contents can be explained by the limit of solubility of Sn in Al. Due to very low solubility of Sn in Al, which is 0.1%, Sn-enriched phases are expected (which has been actually established [18]). It has been seen that
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the surface condition promotes and maintains the activation of Al. Therefore, it is expected that the attack will initiate and propagate in those zones enriched in Sn during the solidification process, i.e., grain boundaries and intermetallic zones. In our previous works [18], the surface of pure Al and Al–0.40% Sn alloy was analysed by means of an electron micro-analyser applying the scanning method prior to and after the cathodic pulse. The elemental composition of surfaces of the same samples was examined by means of EDAX analysis. The data obtained show that the morphology as well as the composition of the alloy differs from that of pure aluminium. Specifically, a separate tin phase was observed with the alloy containing 0.40% tin. The EDAX analysis of that part of the surface has confirmed the presence of tin. After the cathodic treatment of the Al–0.40% Sn alloy, pits were observed in places of surface where tin existed as a separate phase.
5. Conclusion Based on results obtained, the potential range in which cathodic breakdown of the oxide film occurs has been defined for all the samples. In the range of lower cathodic potentials, the oxide film retains its properties, while in the range of higher cathodic potentials cathodic breakdown and hydration of the oxide layer take place. The boundary between these two potential ranges shifts towards more negative potential values when the Sn content in the alloy increases. The longer the time of cathodic polarisation, the more positive the potentials at which the oxide film breakdown takes place. This shift is more marked with alloys with higher Sn contents. The shape of anodic current–time responses and the values of QA indicate that a longer stay at the cathodic potential (100 s) activates alloys with 0.20% and 0.40% Sn for anodic dissolution.
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