Impedance studies of corrosion resistance of aluminium in chloride media

Surface and Coatings Technology, 29 (1986) 335 - 345

335

IMPEDANCE STUDIES OF CORROSION RESISTANCE OF ALUMINIUM IN CHLORIDE MEDIA A. A. MAZHAR, W. A. BADAWY* and M. M. ABOU-ROMIA Department of Chemistry, Faculty of Science, University of Cairo, Cairo (Egypt) (Received June 28, 1985)

Summary The electrochemical behaviour of aluminium in chloride solutions has been studied. Open-circuit impedance measurements reveal that in both acidic and neutral media the dissolution of the oxide formed on aluminium is governed by an empirical relation of the form Cm’ =a—Bt’~’2

where a and B are constants. The rate of the oxide film dissolution in the chloride-containing solutions is found to be markedly lower than that in other halide media, especially in fluoride solutions. The behaviour of the oxide is determined by the pH of the dissolution medium rather than its chloride ion content. Complex plane analysis of the anodic oxide film formed on aluminium indicates that the corrosion resistance is very high in neutral chloride solutions in comparison with acid solutions containing the same amount of chloride ions. Both the charge transfer resistance 0 and the Warburg impedance caused by diffusional mass transfer attain markedly higher values in the neutral media which reflect the high passivation properties of the oxide film. It is suggested that many constructions could be coated with aluminium oxide films to protect them from corrosion, especially in marine media.

I. Introduction The electrochemical behaviour of aluminium has attracted the attention of several investigators because of its widespread use in different industries. Thus, the corrosion and passivation of aluminium have been studied under a variety of conditions [1- 8]. A comparison of the relative passivity and pitting resistance [9] of a group of metals, including Al, in aqueous and non-aqueous *Author to whom correspondence should be addressed. Present address: Freie Universitàt Berlin, Fach Bereich Chemie, Institut für Physikalische Chemie, Takustrasse 3, 1000 Berlin 33, F.R.G. 0376-4583/86/$3.50

© Elsevier Sequoia/Printed in The Netherlands

336

chloride solutions showed that Al dissolves actively in both media containing 0.4% HCI. A discussion included the action of water as a passivating agent. According to the complex ion theory of corrosion [101 anions function in a specific manner forming chemical species in the solution, and the subsequent accelerated dissolution or repassivation of the metal depends on the stability of these species. It has been emphasized by Lorking [1, 2] that the corrosion of Al depends to a great extent on the nature of the anion. Studies on the dissolution behaviour of Al in phosphate [11] and fluoride [12] media indicated the duplex nature of the oxide; the dissolution of the oxide followed a zero-order mechanism. In halide solutions [8] the films formed on Al were stable during cathodic polarization but broke down during anodic polarization. This was attributed to either the inability of the halide ion to promote anodic oxide repair owing to the formation of soluble salts of Al [1], or to the adsorption of chloride ions upon the anodic areas once the metal had been bared which prevented the repair process [13]. In neutral chloride solutions aluminium [14] has been found to exhibit a pitting potential which can be expressed as a function of chloride ion concentration in the medium. In aqueous solutions a protective film is formed only in the pH range 4 9 [15]. The problem of the breakdown of passive films has been discussed in a number of publications [16 19] in relation to pitting, and a classification for pit initiation [20] has been given. The electrochemical behaviour of Al has been investigated to clarify the conditions which lead to the passivation of Al in chloride media. The importance of such a study is rendered necessary by the wide utility of Al in different types of construction and industries. -

-

2. Experimental details The electrical circuit and the electrolytic cell were the same as used previously [21]. The electrode was made from Specpure Al rod (Johnson Matthey, London). A stout copper wire was used for the electrical contact at one end. The electrode was fixed into appropriate glass tubing by an epoxy resin, leaving an exposed area of 0.08 cm2. The electrode was mechanically polished with 4/0 emery paper until mirrorbright. HC1 and H 3P04 solutions were prepared from AR grade concentrated solutions by appropriate dilution using triply distilled water. NaC1 was AR grade (BDH). Open-circuit anodic oxide formation and dissolution were carried out in naturally aerated solutions. The measured Cm and Rm of the impedance of the Al electrode under open-circuit conditions (naturally formed oxide) were traced at 1000 Hz. Cm and Rm for the electrode with an anodic oxide on its surface were recorded at different frequencies ranging from 200 Hz to 20 KHz. All measurements were carried out in a double-walled electrically controlled air thermostat at 25 ±0.1 °C.

337

The pH of solutions was measured using a Beckman pH meter. The electrode potential reported, eh, was referred to the normal hydrogen electrode (NHE). 3. Results and discussion A comparison of the extent of dissolution of Al in different halide solutions is shown in Fig. 1. It is clear that the dissolution in chloride medium is comparatively small with respect to other halides; Cm’ is taken as a relative measure [22] of the oxide thickness. This observation is in accord with the dependence of the corrosion of Al on the nature of the anion present in the solution [1, 2], and the complex ion theory of corrosion [10]. 13.0

110~J~

10.0

—

9.0

—

8.0

—

7fl C)

60

—

50

—

4.0

—

3.0

—

S S

2.0—

S S

0.0 2

I

I

I

I

I

1.

6

8 1 /2

10

12

14

~[T,mm

Fig. 1. Variation of Cm1 with t’~2during the dissolution of aluminium oxide in different halide media at the same concentration (0.1 M): 0, KF; 0, KBr; ~, KI; A, KC1.

338 440

360

-

0;

Fig. 2. Effect on the variation of Cm’ with 0, 0.4;A, 1.15;?, 2.35;•, 3.25.

tU2

at constant Cl content (1 M) of pH:

12.0

10.0—

0

iIi~i

16

,flE, mm.1 /2

Fig. 3. Effect on the variation of Rm with t”2 at constant Cl— content (1 M) of pH: 0, 0.4;A, 1.15;~,2.35;0, 3.25.

The effect of pH on the electrochemical behaviour of Al in chloride medium is shown in Figs. 2 and 3. The chloride ion content was kept at a constant value of 1 g ion 11, while the components Cm and Rm of the electrode impedance were recorded as a function of time for each pH value. In the chloride medium, as previously noted for phosphate and fluoride media [11, 121, the dissolution process is represented by two stages. This is taken as an indication of the presence of two oxide layers of different structures, i.e. a duplex nature of the formed oxide [23, 24]. The predominance of dissolution in both stages over oxide thickening may be due to formation of soluble chloride salts of Al [1], or adsorption of chloride ions upon the anodic areas once the metal has been bared, which prevents the repair process [13]. -

339

The results presented in Fig. 2 can be formulated as Cm~aBt’~’2

(1)

where a and B are constants. It is apparent that the values of a and B for the first stage are nearly independent of pH, which may indicate that the outer oxide layer is of definite thickness and has a dielectric constant independent ofthe ambient electrolyte. Apparently, the outer oxide layeris more resistant to dissolution than the inner layer. Also, the outer layer appears to be very thin [8], for although its rate of dissolution is slow, it dissolves in all the solutions studied in about 30 mm. These results do not exclude the possibility of oxide building, but rather indicate the predominance of dissolution. In Fig. 2 the second part, after the inflection point, represents the dissolution of the inner layer, which apparently occurs more freely. This is indicated by the increase in the value of the dissolution coefficient B for the second layer. The dissolution increases with decrease in pH; this is in accord with the fact that Al is unlikely to form a stable passive film at such pH values [15]. After the induction period, during which the outer layer dissolves, the inner layer is subjected to an increased rate of dissolution [1] at preferred sites. This increased rate of dissolution of the inner layer should be related to the possible existence of imperfections in the film. Once the anodic areas are bared and chloride ions adsorbed upon them, the repair process is prevented [3, 13]. It has been suggested that a soluble chloride complex is then formed and diffuses into the solution [3]. It should be observed that the lowest value of Cm’ is obtained in the most conducting solutions; this may be a result of the changes in the oxide film and the dielectric properties [25]. The effect of pH is considered to influence qualitatively the intermediate formed during the process of pitting of Al [20]. The stoichiometry of the Al complex is understood to be pH dependent. At low pH values the following mechanism was proposed Cl(bu& solution)

~

CF(adsorbed on A1 20,.nH2O)

3~(mA1 Al

2O3~nH2O) +

4C1

~

A1CI4

(2) (3)

followed by oxide thinning. The formation of the aluminium chloride complex predominates over that of the oxide, as may be inferred from the present results. It is possible that the solubiity of the complex formed is pH dependent, which would agree with the results of Fig. 2. Variation of the Rm component with time is plotted in Fig. 3; it is clear that the same trends are observed with a two-stage dissolution process, the decrease in Em with time indicating the predominance of dissolution. Table 1 gives a record of the open-circuit potential eh with time as a function of pH. In agreement with Figs. 2 and 3 the results indicate a shift to less noble potentials with time for each pH value. It is clear from the previous results that the inner oxide layer suffers dissolution in acidic chloride media with a rate dependent on the pH value of the solution. To emphasize the role of pH in the electrochemistry of Al in

340 TABLE 1 Variation of open-circuit potential with immersion time and pH pH

eh(V)

0.4 1.15 2.35 3.25

After 8 mm immersion

After 196 mm immersion

—0.565 —0.555 —0.522 —0.500

—0.695 —0.672 —0.640 —0.615

chloride solutions, the effect of chloride ion concentration was studied at a constant pH value of 0.72. Figure 4 shows the results of variation of eh with time for the two extreme concentrations studied. Actual chloride ion concentrations investigated were 0.01, 0.1, 0.5 and 1 M chloride. For the variation of Cm_i and Em with time, plots similar to Figs. 2 and 3 were obtained. It was noticed that the induction period is not affected by chloride ion concentration; the results showed that a hundred-fold increase in concentration does not alter the dissolution coefficient. This agrees with the prevalence of influence of pH over chloride ion concentration [2]. This may be evidence that the dissolution process which occurs via formation of a soluble Al chloride complex is retarded chiefly by water, which plays the main role in passivation [9]. The competitive adsorption of Cl, or OH- and H20 result in either dissolution or passivation, respectively [20]. Results for Em were also recorded, the initial value of the resistance depending on the solution resistance, but after one interval reaching the same value for all concentrations and remaining within the same range. The two-stage dissolution process was also observed. 0.5r

0.55-

0

0

~

0

0

0 0

0 >

0.60

0

C,

0.55

9

.

0

0.70

—

I 2

1.

6

8

101”2

I

I

12

11.

mm. t”2 at constant pH (0.72) of C1 content: Fig. 4. Effect on the variation of eh with 1.0 M; 0,0.01 M.

•,

341

3.1. Complex plane analysis

For further analysis, an oxide film was formed on the Al electrode by anodic polarization in 1 M H3P04 at a current density of 3 X iO—~A cm2 till 25 V(SCE). After interrupting the current, the oxide-covered electrode was rinsed with a few drops of distilled water and immediately transferred to the impedance measurement cell. Measurements of the two impedance components (Cm and Em) were carried out as a function of time in both 0.1 N HC1 and 1 M NaCl at different frequencies. Figure 5 shows the impedance spectra in both acidic and neutral media. Comparison of the results shows flattened diagrams in acid media, which are related to increased dissolution of the metal [26]. This is expected, since Al forms a protective film in aqueous solutions only at pH 4 9 [15]. At neutral pH values, the following chloride complex [20] has been suggested -

Al3~(tflAl 2 0,•nH2O) +

-

2CF + 20H

—f

Al(OH)2C12

(4)

followed by oxide thinning. The appearance of the Warburg impedance [27] at low frequencies indicates that diffusion is predominant. The absence of the inductive feature, which would have appeared at low frequency as part of a hemicircie at negative values (WCm)~ [28] is in. favour of the absence of adsorption at both pH values (1 and 7) studied. When such a feature is observed, it is an indication of the presence of a solution-soluble or adsorbed intermediate in the charge transfer reaction. The presence of adsorbed intermediates was reported previously [29, 30] for other electrodes, e.g. (N~OH)adsand (COOH)ads. Actually diffusion of the chloride complex given by eqns. (3) and (4) is predominant. At high frequencies an incomplete hemicircle is formed, which is characteristic for a simple eletrochemical reaction. From these figures the corrosion resistance was obtained [31] and is listed in Table 2. In agreement with the previous results, the corrosion resistance is very high in neutral media as compared with acid media. Further, the anodically-formed oxide is more resistant to corrosion than the naturally-formed oxide, in both media. The most important feature is that the anodically-formed oxide possesses the same corrosion resistance value in the neutral medium after 24 h as after

TABLE 2 Corrosion resistance of Al oxide under different conditions Medium

Treatment

Immersion time

Rcorr (~2)

0.1 N HC1 0.1 N HC1 3 N NaC1 3 N NaC1 3 N NaC1

Mechanically polished Anodically polarized Mechanically polished Anodically polarized Anodically polarized

15 24 15 15 24

192 224 1150 2180 2180

mm h mm mm h

120-

LO

.

0

0

0

0

cl

Rm ,Ohm

I

lLOO-

1600

( b)

Fig. 5. Impedance spectra. .. (a) 0.1 N HCl (0, open circuit; 0, anodized); dized for 24 h).

60

20

.

.

11

(a)

Y’

ao-

' loo-

5 3 -

6

E

lLO_

160-

loo-

200-

.Ohm

(b) 3 N NaCl(0, open circuit; 0, anodized for 1 5 min; A, ano-

R,

343

15 mm, which can be taken as an indication of the passivity of the Al oxide under the stated conditions. Figures 6 and 7 show the components of the faradaic impedance where 0 represents the charge transfer resistance and Z is the Warburg impedance caused by diffusional mass transfer. In general, the highest resistance is possessed by the anodically formed oxide (neutral medium). The results show 100 90

-

io2 Fig. 6. Variation of 0 with frequency under different conditions: 0.1 N HU (0, open circuit; •, anodized (24 h)); 3.0 N NaCl (A, open circuit (24 h); V, anodized (15 mm); 0, anodized (24 h)).

102

~ Hz

Fig. 7. Variation of Z with frequency (same symbols as Fig. 6).

344

some deviation from the impedance plots characteristic for a diffusion process; this may be due to the inability to extend the measurements to very low frequencies. This includes the departure of the reported values of 0 from 90°,which indicates a non-ideal capacitor. In neutral solutions, the formation of the complex given by eqn. (4) is retarded leading to passivation of the Al electrode. Possibly this occurs by retarding the diffusion of the Cl from the bulk solution to the electrode surface.

4. Concluding remarks (1) Under open-circuit conditions, the oxide formed is of duplex nature; the outer layer is thin while the inner possesses a non-continuous (imperfect) structure. (2) The dissolution process of Al depends on pH but not on C1 concentration. Since the cathodic reaction of Al in chloride solutions is H2 reduction, its retardation inhibits the dissolution process. (3) The electrochemical reaction occurring when the Al electrode is immersed in chloride solutions is simple; no intermediates are adsorbed and the diffusion process predominates (possibility of retardation of the diffusion of~Cl(bU~ soin.) ~ Al203.nH2O). (4) High impedance indicates a stable oxide film in neutral chloride solutions.

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