Passivity and passivity breakdown of Al and Al–Zn alloys in SCN⁻ solutions and the effect of some inorganic inhibitors – Galvanostatic, potentiostatic and ICP-AES studies

3.1 Galvanostatic measurements

Our previous study (Amin et al., 2009), based on polarization and impedance studies, showed that Al and Al–Zn alloys pit in KSCN solutions. SCN⁻ anions tend to rupture the passive film and initiate pitting (Amin et al., 2009). In the present work, Al and its two Al–Zn alloys were anodized galvanostatically in KSCN solutions under the effect of various experimental conditions with the aim to study the growth kinetics of the passive oxide films formed on Al and its two Al–Zn alloys, and their breakdown.

Fig. 1 presents the dependence of the anodic potential on time for anodization of the three Al samples in 0.04 M KSCN solution at a current density of 50 μA cm⁻² at 25 °C. 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 1

E/t curves recorded for the three Al samples in 0.04 M KSCN solution at an applied anodic current density (j_a) of 50 μA cm⁻² at 25 °C.

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It follows from the data of Fig. 1 that the value and location of potential maximum depend on percentage of alloyed Zn in Al. The value of the potential maximum (i.e., E_pit) decreases and its location shifts towards shorter times (i.e., shorter t_i values) with alloyed Zn. These findings confirm results obtained in our previous study that alloyed Zn activates pitting corrosion of Al (Amin et al., 2009).

The acceleration influence of alloyed Zn towards the aggressive attack of SCN⁻ anions was explained in our previous study (Amin et al., 2009) based on EDX examinations of the electrode surface. The obtained EDX spectra revealed increased adsorption of SCN⁻ anions in presence of alloyed Zn. This alloyed Zn is thought to form ZnO in the Al₂O₃ matrix producing a more defective passive film, which enabled an easier ingress of the aggressive SCN⁻ anions (Amin et al., 2009). These findings are in agreement with the work of Brillas et al. (1998) during studying the susceptibility of Al–Mg alloys towards localized corrosion in NaCl solutions.

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). Under galvanostatic conditions, E_rp is the only potential at which the ac1tive area remains constant and independent of time (Galvele, 1978). It obvious that the values of E_rp obtained from cyclic voltammetry (Amin et al., 2009) and galvanostatic measurements are in good agreements. These findings confirm that the potential maximum observed in the E/t plots corresponds to E_pit.

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 pit initiation, pit growth takes place immediately as a result of increasing the concentration of the aggressive anions by migration. The increased concentration of metal cations (Al³⁺ ions) inside the pit leads to changes in localized solution chemistry. First, following Galvele (1978), hydrolysis occurs to lower the pH. A second effect arises from the well-recognized requirement that electrical neutrality must be maintained throughout the electrolyte.

Therefore, aggressive ions (SCN⁻ in the present study) must migrate into the pit to compensate the local increase in metal cations concentration, as shown by the theoretical analysis of Ateya and Pickering (Szklarska-Smialowska, 1986). The solution inside pits in this way becomes aggressive and allows active dissolution of Al and formation of soluble corrosion products that could be removed from pits by diffusion. This reflects the auto-catalytic nature of the pitting corrosion process. Consequently, the pit once initiated continues to propagate and enlarge (i.e., enhances in size and depth). These events reflect the decline in potential after E_pit till reaching E_rp (Amin et al., 2009).

The effect of various experimental conditions, including SCN⁻ concentration, anodic current density and solution temperature, on the E/t responses of Al and its two Al–Zn alloys was studied. Results obtained for Al are depicted in Figs. 2–4. Similar results were obtained from the two Al–Zn alloys. It follows from Figs. 2–4 that the values of maximum (i.e., E_pit) are dependent upon the SCN⁻ concentration, constant current density applied, j_a, and solution temperature. The repassivation potentials, E_rp, are, however, unaffected by these parameters, reaching almost identical values after the potential drop.

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Figure 2

E/t curves recorded for Al in KSCN solutions of various concentrations at j_a = 50 μA cm⁻² at 25 °C. (1) 0.01 M KSCN; (2) 0.02 M KSCN; (3) 0.03 M KSCN; (4) 0.04 M KSCN; (5) 0.05 M KSCN; (6) 0.06 M KSCN.

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Figure 3

E/t curves recorded for Al in 0.04 M KSCN solutions at different applied anodic current densities at 25 °C.

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Figure 4

E/t curves recorded for Al in 0.04 M KSCN solutions at j_a = 50 μA cm⁻² at different temperatures. (1) 15 °C; (2) 25 °C; (3) 35 °C; (4) 45 °C; (5) 55 °C; (6) 65 °C.

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Fig. 3 reveals that the slope of the linear part of the potential against time curves, and therefore the rate of oxide film growth, increases with increase in current density. This behaviour is due to the increase in the rate of ${\text{O}}_2^{-}$ ion transport across the pre-existing oxide film toward the metal/oxide interface, where the growth of the oxide film takes place (Albella et al., 1987). An increase in current density enhances the electric field across the oxide film, and therefore increases the rate of ion transport. Thus, the higher the current density used, the higher the rate of the oxide growth. This may explain the positive shift of the pitting potential with increasing current density (see again Fig. 3).

Referring again to Figs. 2–4 it is obvious that the incubation time, t_i, decreases with increasing j_a, SCN⁻ concentration and temperature. These observations can be confirmed by plotting the rate of pit nucleation (i.e., the number of events per unit time), defined as ( $t_{\text{i}}^{-1}$ ), for the three samples vs. j_a,[SCN⁻] or temperature, as shown in Figs. 5a–c, respectively. These relations denote that the rate of pit nucleation increases with these three parameters. The rate enhances with increase in Zn content in the Al samples, confirming the accelerating influence of the alloyed Zn (Amin et al., 2009).

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Figure 5

Dependence of the rate of pit nucleation, $t_{\text{i}}^{-1}$ , for the three samples in KSCN solutions, and (a) j_a, (b)[SCN⁻] or (c) temperature.

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3.2 Potentiostatic measurements

Potentiostatic measurements were also performed alongside galvanostatic ones to gain more information regarding the kinetics of passivity and passivity breakdown of the three tested Al in SCN⁻ solutions. Fig. 6 shows j/t transients recorded for Al in 0.04 M KSCN solution under the influence of the applied step anodic potential, E_s,a, (before and after E_pit, see polarization measurements in Amin et al., 2009) at 25 °C. Similar results were obtained for the two Al–Zn alloys (data not included here). When E_s,a < E_pit, the anodic current density initially decreases and then tends to attain steady-state current density, j_ss. The decrease in the current density is due to the formation and growth of Al₂O₃ on the anode surface. The steady-state current density corresponds to the current density passing through the passive layer (i.e., the passive current density, j_pass) that is previously discussed in the polarization measurements of Al/SCN⁻ system (Amin et al., 2009).

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Figure 6

Potentiostatic j/t transients recorded for Al in 0.04 M KSCN solution under the influence of the applied step anodic potential, E_s,a, at 25 °C.

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The constancy of the steady-state current may be explained on the basis that the rate of oxide formation equals the rate of its dissolution so that the oxide film hardly grows. It is clear that this steady-state current density increases with increase in E_s,a, reflecting thinning of the passive layer and formation of a less protective (thin and/or porous) passive film. The value of j_ss, at any given time, is found to be greater in presence of alloyed Zn, corresponding to the formation of a less protective passive film susceptible to the aggressive attack of SCN⁻ anions. These data agree well with the potentiodynamic polarization results (Amin et al., 2009).

On the other hand, when E_s,a > E_pit, the anodic current density decreases initially to a minimum value determining the characteristic pitting parameter, namely the incubation time (t_i). The magnitude of t_i reflects, as previously shown, the susceptibility of sample to pitting corrosion. Beyond t_i (i.e., after pit initiation), the current density increases rapidly due to propagation and growth of pits. 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 then attained between the metal dissolution and oxide film formation including a blockade by pitting corrosion product. Within this steady sate, 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 blockage by pitting corrosion product, leading to nearly constant current density.

The effect of various experimental parameters including, applied anodic potential, SCN⁻ concentration and solution temperature on the j/t transients of Al and its two Al–Zn alloys has been studied. Figs. 6–8 depict results obtained for Al, as a representative example. Similar results were obtained for the two Al–Zn alloys. The data reveal that j_pit (the rising part of the transient) increases and t_i decreases with increase in E_s,a, SCN⁻ concentration or solution temperature corresponding to increased susceptibility towards pitting. The relationships between the rate of pitting initiation, $t_{\text{i}}^{-1}$ , and these three parameters for the three Al samples in 0.04 M KSCN solutions have been constructed (not included here). The obtained plots showed that, for the three samples, the rate of pit initiation increases with increase in E_s,a, SCN⁻ concentration or temperature. The rate of pitting is always greater in presence of alloyed Zn. These findings confirm polarization and impedance results previously obtained (Amin et al., 2009).

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Figure 7

Potentiostatic j/t transients recorded for Al in KSCN solutions of various concentrations at E_s,a = 1300 mV at 25 °C.

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Figure 8

Potentiostatic j/t transients recorded for Al in 0.04 M KSCN solutions at E_s,a = 1300 mV at different temperatures.

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The increase in the rate of pitting initiation with SCN⁻ concentration may be due to the adsorption of more aggressive SCN⁻ anions on the available active sites. On the other hand, the increase of this rate with temperature could be explained on the basis that the increase in temperature accelerates the rates of migration and diffusion of the reactant and product species into and from pits. In addition, an increase in temperature enhances the solubility of the passive oxide film (Amin et al., 2006). All these events facilitate the mission of the aggressive SCN⁻ anion to destruct passivity of the oxide film, resulting in the occurrence of intense pitting corrosion (Amin et al., 2009). EDX spectra presented in our previous study (Amin et al., 2009) revealed increased adsorption of SCN⁻ at high temperatures probably as a result of increased porosity of the passive film. These findings are in accordance with Augustynski et al. (1978), who used the XPS method to confirm the increased adsorption of Cl⁻ at high temperatures.

Finally, the dependence of the rate of pit nucleation on the applied potential suggests that there is a distribution of nucleation sites of different energies which nucleate at distinct potential (Metikos-Hukovic, 1992). In other words, the more positive is the applied potential, the more will be the active sites available for pit nucleation. This in turn enhances the adsorption of the aggressive SCN⁻ anions on the passive electrode surface. It has been shown in our previous studies, based on XPS and EDX examinations, that the adsorption of SCN⁻ (Amin et al., 2009) and ${\text{ClO}}_4^{-}$ and ${\text{ClO}}_3^{-}$ anions (Amin, 2009) on aluminium oxide surface increases with increasing anodic polarization even above the pitting potential. Kolics et al. (1998) came to the same conclusion regarding the adsorption of Cl⁻ on passivated Al.

In the present work, the increase in the rate of pitting corrosion of Al with the increase of applied anodic potential and alloyed Zn is further confirmed chemically using inductively coupled plasma-atomic emission spectroscopy (ICP-AES). This method of chemical analysis involved the determination of the dissolved Al³⁺ ions as a function of applied anodic potential (>E_pit) and alloy composition. Obtained data are collected in Table 1. It is obvious that the concentration of Al³⁺ in solution, due to pitting corrosion, increases as the potential is made more positive. The Al³⁺ concentrations are always greater in presence of increasing amounts of alloyed Zn, reflecting the ability of the alloying element (Zn) to activate pitting corrosion process.

3.3 Role of some inorganic inhibitors

The polarization responses of Al have been studied in 0.04 M KSCN solutions in the absence and presence of various concentrations of Na₂MoO₄, Na₂WO₄, Na₂SiO₃ or K₂CrO₄ as inorganic inhibitors, see Figs. 9a–d.

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Figure 9

Potentiodynamic anodic polarization curves recorded for Al in 0.04 M KSCN solutions in the absence and presence of various concentrations of (a) Na₂MoO₄, (b) Na₂WO₄, (c) Na₂SiO₃ or (d) K₂CrO₄ at a scan rate of 0.5 mV s⁻¹ at 25 °C. (1) Blank; (2) 10⁻⁴ M Inhib.; (3) 2 × 10⁻⁴ M Inhib.; (4) 5 × 10⁻⁴ M Inhib.; (5) 10⁻³ M Inhib.; (6) 2 × 10⁻³ M Inhib.

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In all cases, the polarization curves show a passive region. This passive region is followed by pitting potential, E_pit, indicated by a permanent rise in current in the passive region. Fig. 10 depicts the relationship between E_pit and the logarithmic concentration of the inhibitors[log (C_inhib/M)]. It follows that E_pit values are shifted in the noble direction and the values of j_pass are decreased (see, for example, the insert of Fig. 9d) to an extent depending on the type and concentration of the introduced inhibitor. A retardation in the pitting events therefore results. At a fixed concentration of additive, E_pit was found to increase in the order: ${\text{MoO}}_4^{2-}$  <  ${\text{WO}}_4^{2-}$ $\ll$ ${\text{SiO}}_3^{2-}$  ≈  ${\text{CrO}}_4^{2-}$ . These results indicate that ${\text{SiO}}_3^{2-}$ and ${\text{CrO}}_4^{2-}$ inhibit the aggressive attack caused by SCN⁻ anions more effectively than ${\text{MoO}}_4^{2-}$ and ${\text{WO}}_4^{2-}$ .

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Figure 10

Relationship between E_pit and the logarithmic concentration of the inhibitors[log (C_inhib/M)] for Al in 0.04 M KSCN solutions at 25 °C.

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The effect of these inhibitors on the incubation time of the pitting corrosion was also studied, see for example Figs. 11a and b. The samples were first passivated at −600 mV (SCE) in 1.0 M Na₂SO₄ solution for 15 min, in order to get a stable Al₂O₃ film on the electrode surface (Amin et al., 2008), and then SCN⁻ was added to the sulphate solution to establish a 0.04 M thiocyanate concentration. Exactly the same procedure was followed for Al in 1.0 M Na₂SO₄ solution containing 0.04 M SCN⁻ + 0.002 M of the inorganic inhibitor. Similar results were obtained for the other tested inhibitors. Within this study, the incubation time was defined as the time period form thiocyanate addition until significant rise in the current transient results, denoting the occurrence of pitting corrosion.

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Figure 11

Potentiostatic current/time transients recorded for Al in 1.0 M Na₂SO₄ solution at 25 °C and at a step anodic potential (E_s,a) of −600 mV(SCE), followed by addition (after 15.0 min) of KSCN to establish a concentration of 0.04 M SCN⁻ in the ${\text{SO}}_4^{2-}$ solution (a) without and (b) with 0.002 M ${\text{CrO}}_4^{2-}$ .

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Addition of ${\text{CrO}}_4^{2-}$ anions to thiocyanate-sulphate containing solutions significantly decreases j_pit and extends t_i. These events were found to be significant in the presence of ${\text{SiO}}_3^{2-}$ and ${\text{CrO}}_4^{2-}$ (see for example Fig. 11b), indicating a significant increase in resistance of the passive film towards pitting attack in presence of these two anions. These findings can be confirmed by plotting the relation between the rate of pitting initiation, $t_{\text{i}}^{-1}$ , for Al and log C_inhib (Fig. 12). It follows from the data of Fig. 12 that, for all inhibitors, the rate of pitting initiation decreases with increasing their concentrations, and this decrease in the rate of pit initiation enhances following the order: ${\text{MoO}}_4^{2-}$  <  ${\text{WO}}_4^{2-}$ $\ll$ ${\text{SiO}}_3^{2-}$  ≈  ${\text{CrO}}_4^2$ . It seems that pits, as will be discussed, find it more difficult to develop in the presence of the inhibitors. The transition from passive state to pitting has therefore been made more difficult by the inhibitors. The inhibition mechanism of these oxyanions will be discussed later, after surface analysis has been presented.

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Figure 12

Relation between the rate of pitting initiation, $t_{\text{i}}^{-1}$ , and log C_inhib for Al in 0.04 M KSCN solutions at 25 °C.

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3.4 Surface analysis

A further insight into the processes occurring at Al in 0.04 M KSCN solutions without and with 0.002 M ${\text{MoO}}_4^{2-}$ , ${\text{WO}}_4^{2-}$ , ${\text{SiO}}_3^{2-}$ or ${\text{CrO}}_4^{2-}$ anions was enabled by EDX data (Fig. 13). EDX was first performed in water (Fig. 13a), only Al and O were detected, which indicated that the passive film contained only alumina, Al₂O₃. However, in KSCN solution (Fig. 13b), the EDX spectra showed additional lines characteristic for the existence of S, C and N. This means that the oxide film formed in KSCN solution contains a certain amount of incorporated S, C and N, presumably as SCN⁻ anions. This indicates a deep penetration of this anion into the bulk of the passive oxide film. These results confirm that SCN⁻ anions generate passivity breakdown by their adsorption and incorporation into the passive film (Amin et al., 2009).

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Figure 13

EDX spectra recorded for Al in (a) water and in 0.04 M KSCN solution in the absence (b) and presence of 0.002 M (c) ${\text{MoO}}_4^{2-}$ , (d) ${\text{WO}}_4^{2-}$ (e) ${\text{SiO}}_3^{2-}$ , or (f) ${\text{CrO}}_4^{2-}$ anions at 25 °C.

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On the other hand, the EDX spectra recorded for Al in 0.04 M KSCN solution containing 0.002 M ${\text{MoO}}_4^{2-}$ , ${\text{WO}}_4^{2-}$ , ${\text{SiO}}_3^{2-}$ or ${\text{CrO}}_4^{2-}$ anions (Fig. 13c–f) showed additional lines characteristic for the existence of Mo, W, Si or Cr, respectively. In addition, the O signal is enhanced due to oxygen atoms present in these oxy-anions. Moreover, the spectra of Fig. 13c–f show that the S, C and N signals are considerably suppressed relative to the samples prepared in bare KSCN solution (Fig. 13b). These findings confirm that these anions compete with SCN⁻ anions for surface adsorption sites, and consequently suppress the adsorption and incorporation of the aggressive SCN⁻ anions into the passive film, thereby reducing the pitting susceptibility. It seems that these elements or their reduced forms, as will be seen, become part of Al₂O₃ passive film and tend to plug its pores and flaws, thereby imparting to it better protective properties.

Further inspection of Fig. 13 reveals that signals due to SCN⁻ are significantly suppressed in presence of ${\text{SiO}}_3^{2-}$ and ${\text{CrO}}_4^{2-}$ (Fig. 13e and f) as compared with ${\text{WO}}_4^{2-}$ and ${\text{MoO}}_4^{2-}$ anions (Fig. 13c and d). These results may confirm the results obtained from electrochemical measurements that ${\text{SiO}}_3^{2-}$ and ${\text{CrO}}_4^{2-}$ anions are more effective than ${\text{WO}}_4^{2-}$ and ${\text{MoO}}_4^{2-}$ anions in the inhibition of pitting corrosion of Al.

The adsorption and inhibition action of these oxyanions was further confirmed by SEM examinations of the electrode surface before and after exposure to the inhibitor solution (Fig. 14). The morphology of specimen surface presented in Fig. 14a reveals that, in the absence of inhibitor, a well-defined irregular shaped pit is developed on the electrode surface. However, in presence of the four tested inorganic inhibitors (images b–e), the rate of corrosion is suppressed, as can be seen from the observed improvement in surface morphology.

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Figure 14

SEM examinations of Al surface in 0.04 M KSCN solution (a) without and with 0.002 M (b) ${\text{MoO}}_4^{2-}$ , (c) ${\text{WO}}_4^{2-}$ (d) ${\text{SiO}}_3^{2-}$ , or (e) ${\text{CrO}}_4^{2-}$ anions at 25 °C. In each solution, the Al specimen was potentiostatically held for 5.0 min at E_s,a = 1300 mV.

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3.5 Inhibition mechanism

The decrease of the rate of pitting initiation and growth of Al in SCN⁻ solutions due to the presence of these oxyanions may be explained on the basis that the inhibitors block the active sites available for a pit initiation. The inhibitor species penetrate the oxide film in parallel with SCN⁻ anions under the influence of the electric field (Ilevbare and Burstein, 2001). Penetration of aggressive (and inhibiting) anions is thus expected to occur preferentially at active sites. The inhibitors would have either to hinder or to prevent completely the migration of the aggressive SCN⁻ anions through the oxide film. This could be achieved by improving the oxide film. These inhibitors may improve the oxide film by incorporation into the defective film sites, and subsequent reduction into stable oxides, following Eqs. (1)–(3):

The reduced forms of the inhibitors, namely Cr₂O₃, WO₂ and MoO₂ may be incorporated in the passive film and tend to repair any flawed regions on the surface. This may either change the electronic properties for instance by reversing the anion selectivity (Baszkiewicz et al., 1992; Sakashita and Sato, 1979; Craig, 1991) of the film making it more difficult for SCN⁻ anions to migrate through it, or take part in enhancing the oxide film by increasing its thickness (Sugimoto and Sawada, 1977).

The possibility of any of these occurring is still being debated. These inhibitors as a result increase the difficulty of soluble film formation required for film rupture to occur (Foley, 1986). This difficulty in soluble film formation prevents direct contact of the bare metal surface with aggressive SCN⁻ anions so that further metal dissolution and, therefore, pit formation is prevented or at least retarded.

A further explanation for the inhibiting ability of K₂CrO₄, Na₂MoO₄ and Na₂WO₄ is that they reduce the local acidity at the sites where they react. Eq. (1), as an example, shows that for 2 moles of ${\text{CrO}}_4^{2-}$ reduced, 5 mol of H₂O are consumed and 10 mol of OH⁻ ions are produced which increase local pH. In addition, it has been shown that the dissolution of Na₂SiO₃ in water generally increases the pH since silicic acid, H₂SiO₃, is a weak acid with pK₁ = 9.5 and pK₂ = 12.7 (Armstrong et al., 1994). This means that when Na₂SiO₃ is dissolved in water the following reaction occurs:

For example, in one of the experiments of the present Na₂SiO₃ is added to 0.04 M KSCN solution to establish a concentration of 0.002 M Na₂SiO₃. The resultant solution actually contains: 0.04 M SCN⁻, 0.001 M ${\text{SiO}}_3^{2-}$ , 0.001 M ${\text{HSiO}}_3^{-}$ and 0.001 M OH⁻, giving a pH of 11. This increase in pH would affect the transition from passive state to pitting since a low pH is important to maintain the aggressiveness necessary for pit development.

Another factor, which might affect the strength of an inhibitor, is the size of the inhibiting species. It is important to determine whether the inhibitor species are able to penetrate or be pulled through the oxide film under the influence of the electric field. It has been shown that the radii and volumes (assuming perfect spheres) of SCN⁻ and polyatomic anions of the tested inhibitors are comparable (Weast, 1976). It is therefore possible for these polyanions to be pulled through the oxide film under the influence of an electric field since they are not much larger than the SCN⁻ anion.

Once these inhibitor species migrate through the oxide film, they reduce the amount of SCN⁻ anions, which eventually migrate to a particular site to react and cause a rupture. This might increase the strength of the passive layer, corresponding to low passive current, as observed in Fig. 9, since the large amounts of SCN⁻ anions required for the formation of soluble species to activate pitting would not be readily accumulated.

Because these ions carry a negative charge as the aggressive SCN⁻ anion it also suggests that SCN⁻ anions would be repelled from these sites. Thus the rate at which SCN⁻ anions migrate through the oxide film will be reduced by the parallel migration of other species. This would cause a reduction in the probability of generating pits, and therefore the rate of pit growth ceases. It is also possible that the presence of these inhibitor species assists in repairing defects (may be via repassivation) of the oxide film so that further metal dissolution and, therefore, pit formation is prevented. Repassivation may be enhanced because the amount of SCN⁻ anions present at these sites in the presence of the inhibitors is reduced. This would also make the transition from passivity to its breakdown (pit initiation and growth) more difficult because the necessary amount of SCN⁻ anions required for transition to take place is no longer available.