Pitting of Al and Al–Si alloys in KSCN solutions and the effect of light

2.1 Solutions

The electrolyte employed in this study was potassium thiocyanate, KSCN, (the pitting corrosion agent). This electrolyte was purchased from Merck (Darmstadt, Germany). The electrolyte was prepared using analytical grade reagents (Merck) and triply distilled water. Potassium thiocyanate was dissolved in the triply distilled water at a concentration of 0.04 M. The solution temperature was adjusted to the required values (±1 °C), using water thermostat.

2.2 Electrodes and apparatus

Four working electrodes were used in this work; very pure Al rod (99.99 % Koch Light Laboratories, Colnbrook Buckes, UK) and three (Al-xSi) alloys; x = 6, 12 and 18%. For electrochemical measurements, the investigated materials were cut as cylindrical rods, welded with Cu-wire for electrical connection and mounted in glass tubes of appropriate diameter using Araldite to offer an active flat disc-shaped surface of (0.50 cm²) geometric area, to contact the test solution. Prior to each experiment, the working electrodes were mechanically polished with aluminum powders of different grain sizes down to 0.05 μm, using waters lubricant, and were then repeatedly rinsed with water, dried, and then etched in a 0.10 M NaOH solution for 30 s. The etched electrode was rinsed with deionized water rapidly, followed by immediate rinsing with absolute ethanol. The aim of the NaOH etching method is to produce a fresh and active (oxide-free) electrode surface as much as possible.

The electrochemical set-up consisted of a classical three electrode arrangement, in a transparent polystyrene cell, with a Pt counter electrode and a saturated calomel reference electrode. The SCE was connected via a Luggin capillary, the tip of which was very close to the surface of the working electrode to minimize the IR drop. All potentials given in this paper are referred to this reference electrode. Electrochemical measurements were performed using An Autolab Potentiostat/Galvanostat (PGSTAT30) connected with a personal computer with GPES and FRA (ver. 4.9) software provided by Autolab. The stabilization period prior to collecting data was 12 h.

Cyclic polarization measurements were carried out by sweeping linearly the potential from the starting potential, −2000 mV (SCE), into the positive direction at a scan rate of 0.50 mV s⁻¹ till the ending potential, 2000 mV (SCE), and then reversed with the same scan rate till forming a well-defined hysteresis loop. The potentiostatic j/t transient measurements were carried out after a two step procedure, namely: the working electrode was first held at the starting potential for 60 s to attain a reproducible electroreduced electrode surface. Then the electrode was held at constant anodic step potential (E_s.a) where the anodic current was recorded with time. In galvanostatic measurements, the potential is monitored as a function of time at an anodic current density of 60 μA cm⁻² to establish a E/t curve for the tested Al samples in 0.04 M KSCN solution. The stabilization period prior to collecting data was 12 h. The open circuit potential of the working electrode was measured as a function of time during this stabilization time. This time was quite sufficient to reach a quasi-stationary value for the open circuit potential.

The working electrode was illuminated through the bottom of the cell with a 250 W tungsten-halogen lamp fed with a variator. A relative intensity scale was established by measuring the photocurrent in a silicon photodiode as a function of the variator setting. In the following, the relative intensities represent the ratio of the photocurrent obtained on the photodiode relative to that when the lamp is fed with its nominal voltage. The latter conditions (100% relative intensity) correspond to a useful photon flux on the order of 10¹⁷ s⁻¹ cm⁻² (Abdel-Samad et al., 2004). In illumination experiments, the electrodes were illuminated continuously throughout the potential scan. In current–time studies and incubation time measurements, the electrode was initially polarized at E_corr, for a 15 min period, and then stepped to an appropriate point where passivity could be observed for the non-illuminated specimens, and the current transients were recorded as a function of time.

3.1 Cyclic polarization plots (estimation of the pitting corrosion parameters)

In order to evaluate the pitting corrosion process of the tested Al samples in KSCN solutions, pitting potentials and other pitting corrosion parameters were estimated from cyclic potentiodynamic curves as a function of sample composition (Fig. 1). These samples in this aggressive medium show a cathodic region (exhibiting Tafel behavior), a free corrosion potential (E_corr), an active–passive transition, a passive region and the pitting potential (E_pit) indicated by a permanent rise in current in the passive region. At E_pit, the aggressive SCN⁻ anions displace the adsorbed passivating species at some locations and accelerate local anodic dissolution. This mechanism is accelerated when SCN⁻ concentration increases (Amin et al., 2009). Pitting growth occurs as a result of an increase in SCN⁻ concentration resulting from its migration inside pits and hydrolysis of Al³⁺ ions, since the high acidity required for pitting corrosion site growth must be achieved by hydrolysis of Al³⁺ ions inside pits; see more details in our previous study (Amin et al., 2008; Amin et al., 2009).

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Figure 1

Cyclic polarization plots recorded for Al and its three selected Al–Si alloys in 0.04 M KSCN solution at a scan rate of 0.50 mV s⁻¹ at 25 °C. (1) Al; (2) Al–6%Si alloy; (3) Al–12% Si alloy; (4) Al–18% Si alloy.

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The current rise continues even after scan reversal. A current hysteresis loop, characteristic of localized corrosion phenomena, appears. The obtained hysteresis loop allows the repassivation (nucleation) potentials, E_rp (E_n) to be determined (Szklarska-Smialowska, 1986). When the critical repassivation or nucleation potential E_rp = E_n is reached, the anodic current decreases rapidly. Repassivation potentials correspond to the potential values below which no pitting occurs and above which pit nucleation begins (Amin et al., 2009). Once the cyclic polarization plot is established, pitting potentials, repassivation potentials, nucleation potentials as well as passivation current density (j_pass) were calculated as characteristic parameters for defining the behavior of a material in a corrosive medium.

The existence of a hysteresis loop in a cyclic potentiodynamic polarization curve indicates a delay in repassivation of an existing pit when the potential is scanned towards negative direction. The larger the hysteresis loop, the more difficult it becomes to repassivate the pit (Szklarska-Smialowska, 1986). It is obvious that the passivation current diminishes with the alloyed Si content of the sample. At the same time, both E_pit and E_rp shifted towards more positive values. These results indicate that alloyed Si improves the pitting corrosion resistance of Al to an extent depending on its content in the sample.

The role played by Si in enhancing the pitting corrosion resistance of Al in these solutions may be explained on the basis that any localized dissolution will preferentially dissolve Al and leave the surface enriched in unreactive Si atoms. Enrichment of alloy surface in unreactive Si atoms during dissolution blocks the active sites available for Al dissolution. At this stage, dissolution is retarded and pitting ceases. In order for pitting to recommence, the potential must be raised even higher[to (E_pit)_alloy] to activate dissolution at the less favourable sites. As the Si content is increased, Si atoms will appear with greater frequency. Therefore, dissolution processes will be retarded until increasingly higher potentials are reached. Therefore, (E_pit)_alloy must increase with Si content, as shown in the insert of Fig. 1.

Other research workers attributed the high corrosion resistance of Al-Si alloys to the incorporation of Si atoms in the Al₂O₃ passive film (Wood and Brock, 1966). This incorporation repairs the film defects and precludes significant dissolution of the oxide film (Strehblow and Doherty, 1978). This makes it more difficult for the aggressive SCN⁻ anions to migrate through the oxide film. Si would as a result increase the difficulty of soluble film formation required for film rupture to occur (Foley, 1986). Si may also slow down the rate of metal dissolution by reducing the amount of free metal ions in the pit solution resulting in a decrease in diffusion of SCN⁻ anions into the pit and in diffusion of metal cations out of the pit (Bohni, 1987). It is also probable that the presence of Si in Al assists the rapid repassivation of the bare metal so that further metal dissolution and, therefore, pit formation is prevented.

3.2 Transient techniques

3.2.1

3.2.1 Potentiostatic j/t measurements

Fig. 2 illustrates potentiostatic current transients recorded for Al and its three Al–Si alloys subjected to a constant step anodic potential, E_s,a, of 1600 mV (SCE) in 0.04 M KSCN solution at 25° C. In Fig. 2, it can be seen that the pitting process is categorized into the three stages, i.e. the first passivation stage, the second pit formation and growth stage and the final steady-state stage. The anodic current density of the first stage descended abruptly with time and then reached a current minimum at a certain incubation time, t₁.

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Figure 2

Potentiostatic current-time plots recorded for Al and its three selected Al–Si alloys in 0.04 M KSCN solution at a an applied anodic potential of 1600 mV (SCE) at 25 °C. (1) Al; (2) Al–6% Si alloy; (3) Al–12% Si alloy; (4) Al–18% Si alloy.

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The fall of current density indicates thickening of oxide film on the surface. In the second stage, the value of current density ascended from the moment just after t₁ to another time t₂, which is attributed to passivity breakdown induced by the aggressive attack of SCN⁻ anions and subsequent formation and growth of pits (Amin et al., 2009). It is obvious that t₁ and t₂ vary with sample composition, being longer, corresponding to high resistance against pitting, with increasing alloyed Si content in Al samples, see curves 2–4 in Fig. 2.

After pit initiation, see the current rise between t₁ and t₂, it is possible that pitting corrosion products precipitate inside the pits. The corrosion products block up the pits, and therefore, hinder the current flow through the pits. Thus, a steady-state was attained between the metal dissolution and oxide film formation including a blockade by pitting corrosion products in the third stage of current transient after t₂. In the third stage, the increment in current density caused due to the metal dissolution just equals the sum of the decrement in current density due to oxide film formation and the decrement of current density due to the blockade by pitting corrosion products, leading to nearly constant current density.

The current transients presented in Fig. 2 also revealed that there is a growing tendency that the rate of passivation dominates over the rate of oxide dissolution as alloyed Si increased, resulting in the increase in t₁. From the experimental findings that the slope of current transient in the range from t₁ to t₂ is drastically diminished with increasing alloyed Si content, it is deduced that the rate of pit growth suppressed with Si, confirming polarization results.

3.2.2

3.2.2 Galvanostatic E/t measurements

Figs. 3a–d represent the dependence of the anodic potential on time for anodization of Al and Al-6%Si, Al-12%Si and Al-18%Si alloys in 0.04 M KSCN solution at a current density of 60 μA cm⁻² at 25 °C, respectively. The linear increase in potential at the commencement of anodization accompanies the initial formation and growth of the oxide film. A potential maximum appears at a certain time, assigned as the incubation time, t_i, which is necessary before pit growth to occur. This maximum is thought to correspond, as will be shown, to the pitting potential (E_pit). The appearance of which is the correspondence of competition between two processes, namely, further oxide film growth and its breakdown.

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Figure 3

Galvanostatic potential-time plots recorded for (a) Al, (b) Al–6% Si alloy, (c) Al–12% Si alloy, and (d) Al–18% Si alloy in 0.04 M KSCN solution at a an applied anodic current of 60 μA cm⁻² at 25 °C.

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The decline in potential following the maximum corresponds to breakdown of passive layer followed by the nucleation and formation of pits as a result of the aggressive attack of SCN⁻ anions (Amin et al., 2009). After prevalence of the latter reaction at E_pit, or in other words, after t_i, the potential drops to a steady value, corresponding to the repassivation potential (E_rp). It obvious that the values of E_rp obtained from cyclic polarization (Fig. 1) and galvanostatic measurements (Fig. 3) are in good agreements. These findings confirm that the potential maximium observed in the E/t plots corresponds to E_pit.

It follows from the data of Fig. 3 that the rate of potential increase (i.e., the rate of oxide growth) and the value and location of potential maximum depend on the sample composition. The value of the potential maximum (i.e., E_pit) increases and its location shifts towards more positive values (i.e., longer t_i values) when the alloyed Si increases. These findings confirm results of E/j (Fig. 1) and j/t (Fig. 2) measurements that alloyed Si retarded pitting corrosion of Al in KSCN solutions.

3.3 Photo-inhibition of localized corrosion

In this section, the photo-inhibition of pitting on Al in aggressive KSCN solutions and the factors affecting the degree of photo-inhibition are reported. Fig. 4 shows typical cyclic polarization curves recorded for Al in 0.04 M KSCN solution under conditions of illumination (increasing light intensity) and non-illumination. It is evident that an increase in the pitting resistance occurs on illumination of the electrode. Both the pitting and the repassivation potentials are shifted in the noble direction with increase in the light intensity, reflecting an increase in the pitting corrosion resistance. This increased resistance to pitting attack is explained in terms of the semiconducting nature of the passive film. It is proposed that generation of electron-hole pairs leads to a quenching of the electric field strength and consequent modification of the vacancy structure, leading to a decrease in the flux of cation vacancies across the barrier layer.

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Figure 4

Cyclic polarization curves recorded for Al in 0.04 M KSCN solution as a function of light (arbitrary relative units) intensity at a scan rate of 0.50 mV s⁻¹ at 25 °C. (1) Non-illuminated electrode; (2) 20% light; (3) 40% light; (4) 60% light.

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Comparing Fig. 4 with Fig. 1, it seems that the pitting potentials of the illuminated Al specimens approach those of the (Al-Si) alloys, as measured in the dark. These results indicate perhaps a comparable degree of passivity enhancement between alloying and illumination under these conditions.

Further evidence for photo-inhibition of pitting attack was obtained from j/t measurements, where current-decay transients were monitored as a function of time for illuminated (40% light) and non-illuminated conditions. The obtained plots (not shown here) are very similar to those presented in Fig. 2. A significant increase in the induction periods and a decrease in the rate of pit nucleation were observed for specimens pre-passivated under illumination conditions, indicating that illumination leads to a modification of the passive film that persists even after irradiation is removed.