Langmuir 1096,11, 4605-4608
4605
In-Situ Radiotracer and Electrochemical Study of Sulfate Accumulation on 4 2 0 2 4 Alloy A. Kolics, A. E.Thomas, and A. Wieckowski" Department of Chemistry and Frederick Seitz Materials Research Laboratory, University of Illinois at Urbana-Champaign, 600 South Mathews Avenue, Urbana, Illinois 61801 Received August 22, 1995. In Final Form: September 29, 1995@ We have applied radiotracer, electrochemical and ultrahigh vacuum techniques to study sulfate accumulation in passive films on pure aluminum and A2024 alloy in 0.1 M NaC104 containing 0.1 mM Na2S04. We have found that the sulfate coverageis pH and electrode potential dependent and that sulfate is bonded to the surface in two distinctively different ways. While the breakdown ofthe passive film results in sulfate removal, the subsequent repassivation reintroduces the sulfate anion into the passive film. There is a strong tendency of sulfate to remain in the passive film which explains the inhibitive properties of sulfate in aluminum corrosion. Our data reveal that the anomalous sulfate accumulation during the negative-going polarization can be attributed to the copper content of the alloy surface. "he formation of copper-containing nodules determines the electrode potential threshold below which sulfate anions desorb.
Introduction Diagnostics of corrosion of aluminum-based alloys and prevention of occurrence of corrosion in aqueous or moist gaseous environments are of paramount importance in naval, aircraft, and related industries.' In this type of corrosion, adsorption of ionic species is a key mechanistic step which may either initiate passive film degradation (pitting corrosion) or contribute to corrosion i n h i b i t i ~ n . ~ - ~ Quantitative determination ofthe ions adsorbed on, or in, surface alloy films is difficult. Traditional tools utilized to provide quantitative surface composition data have included in-situ radiotracer labeling t e ~ h n i q u e s , ~ for -l~ instance, to study ion accumulation on iron5, n i ~ k e l , ~ J aluminum,8and stainless steel^.^-^^ However, in most of these methods, the electrode configuration was limited to rough electrodeposits or thin films/foil~.~-~ Such electrode geometry constraints made those methods inapplicable to solving all and, perhaps, key corrosion queries. To remedy the situation, we have developed a type of radiotracer t e ~ h n i q u e 'that ~ is well suited for studies of adsorption using smooth, as-obtained electrodes, including those cut out of commercial substrates.lOJ1 By using this method, we have obtained experimentalresults on sulfate accumulation on A12024 alloy, a widely used material in the aircraft industry. Our measurements simulate, to some extent, what may be happeningwhen an unprotected surface of the alloy interacts with wet atmosphere containing sulfate anion, a common atmospheric compoAbstract published in Advance ACS Abstracts, November 1, 1995. (1)Jones, D.Principles and Prevention of Corrosion; MacMillan @
Publishing Company: New York, 1992. (2)Nguyen, T. H.; Foley, R. T. J.Electrochem. SOC. 1980,127,2563. (3) Mansfeld, F.; Kendig, M. W.; Lorenz, W. J. J. Electrochem. Soc. 1986,132,290. (4)McCafferty, E.J. Electrochem. SOC. l f M , 137,3731. (5)Jovancicevic, V.; Bockris, J. OM.; Carbajal, J. L.; Zelenay, P.; Mizuno, T.J. Electrochem. Soc. 1986,133, 2219. (6)Herbelin, J.-M.;Barbouth, N.; Marcus, P. J. Electrochem. SOC. 1990,137, 3410. (7)Marcus, P.; Herbelin, J. M. Corros. Sci. 1993,34,1123. (8)Tomcsdnyi, L.; Varga, K.; Bartik, I.; Maleczki, E.; Horbyi, G. Electrochim. Acta 1989,34,855. (9)Varga, K.; Maleczki, E.; Horhyi, G. Electrochim. Acta 1988,33, 25. (10)momas, A. E.; Sung, Y.-E.; Gamboa-AldBco, M.; Franaszczuk, K.; Wieckowski, A. J. Electrochem. Soc. 1995, 142, 476. (11)Thomas, A. E.; Kolics, A.; Wieckowski, A. Submitted for publication in J. Electrochem. SOC. (12)Kolics, A.;Hordnyi, G. Electrochim. Acta 1995,40,2465. (13)Krauskopf, E.It;Chan, K.; Wieckowski, A. J.Phys. Chem. 1987, 91,2327.
nent which gives rise to acid rains. For a fundamental reason, we provide comparison of the data with those obtained with pure Al substrate.
Experimental Section The working electrodewas made of technological A12024 alloy (Si, 0.50%;Fe, 0.50%;Cu, 3.8-4.9%; Mn, 0.30-0.9%; Mg, 1.21.8%; Cr, 0.10%;Zn, 0.25%; Ti,0.15%; Al, balance) or of pure (99.99%)aluminum. The geometric surface area was 0.6 cm2. Prior to the adsorption measurements, the electrodes were polished down to -0.5pm with Sic emerypaper, No. 4000. After polishing,the electrodeswere degreased in acetone,washed with Millipore water, and introduced into the measuring cell. The counter electrodewas a thin gold wire and the reference was the AgIAgC1 ([Cl-] = 3 M)electrode. The reference electrode, with its own C1- containing solution compartment, was held closed in a vacuum-quality ground glassjoint filled up with supporting electrolyte. This eliminated any conceivable leakage of chloride from the reference electrodecompartmentto the electrochemical cell. All experiments begun with attaining an open circuit potential (OCP) which required approximately 2 h of waiting. The OCP did not significantly change from an experiment to experiment and was in the range of -0.68 to -0.72 V. To conduct radiotracer experiments, sodium sulfate labeled with 36S(ICN) was used. The molar activity of the radioactive stock solutionwas 1.11GBq/mol. A glass scintillatorwas applied to convert the ,!?--radiationto photons that are transferred through a light pipe to an EG&G Ortec photomultiplier tube (9893b). The radiotracer technique, the calculation of surface excess concentrationvalues, and other experimental conditions are as previouslydescribed.10Jl The deviation ofI' values during experiments was &ll% from the mean value. Results and Discussion Accumulation of sulfate anions on the A12024 surface under open circuit conditions is relatively fast and a constant surface concentration is attained in a matter of minutes (Figure 1). How much of the anion per em2of the surface is to be given depends on the roughness factor,lOJ1 which usually cannot be accurately determined for aluminum or aluminum-based electrodes. Considering the argument put forward in our previous work,l0J1 we may assume that the surface roughness ranges from 1to 2, which is consistent with the observation of a mirrortype surface after electrode polishing. In this respect, surface concentrations presented in Figure 1correspond to a multilayer (ML)coverage of the anion. In the entire electrode potential range examined, the electrode is covered by a protective aluminum oxide/ hydroxide film.2-4 Since a multilayer anion coverage is
0743-7463/95/2411-4605$09.00/00 1995 American Chemical Society
Letters
4606 Langmuir, Vol. 11, No. 12, 1995
a
0.4
-
% Oe3 -
5B
-
2.
0.2
-
0.1
-
0.0
-
. . I
v1 Q) U
04 0
e
t
2000
1000
U
4000
3000
h Q)
s
Time (seconds)
Figure 1. Accumulation of sulfate as a function of time on Al 2024in 0.1 M NaC104 at pH = 3.0 (csulfate= M; open circuit potential, OCP, conditions).
-0.7 -0.6 -0.5 -0.4 -0.3 -0.2 -0.1 0.0 0.1
E / V vs. Ag/AgCI
/I
I *
I v
8
48
1-
U
s"
i o . 5 -:
04 -0.8
IY
o y 0
I
io00
I
2000
I
3000
-0.4
-0.2
0
EN vs. AgIAgCl
I
4000
I
-0.6
5000
Time(seconds) Figure 2. Exchange of labeled sulfate accumulatedon AI 2024 in 0.1 M NaC104 at pH = 3.0 at OCP: 1, addition of M nonlabeled sulfate; 2,addition of M nonlabeled sulfate; 3, addition of NaOH (pH is shifted to 7). The initial concentration of (labeled)sulfatewas 10-4M.The arrows denote the beginning of the exchange (see text).Irel is the ratio of count rates before and after the addition of the non-labeled substances.
found, we may conclude that the accumulation process reflects a penetration of sulfate anions into the passive film. The high surface concentration and the speed of the accumulationprocess imply that the passive film is rather porous. To obtain information on the mobility of accumulated sulfate, we studied an exchange of the labeled sulfate adsorbate with unlabeled sulfate present in the bulk of solution, that is, on the solution side of the interface. M Addition of the nonlabeled sulfate a t and causes a rapid reduction of the surface counts and, within 10 min of the experiment, ca. 80% of the labeled sulfate is exchanged (Figure 2 curves 1 and 2). However, irrespective of the bulk concentration of the nonlabeled reagent, there remains approximately 20% of the radioactive species which is not accessible to the exchange. This implies that sulfate binds to the electrode in a t least two different modes (Figure2 curves 1and 2). This observation is in agreement with previous data which show that the interaction of sulfate with passive films causes the formation of an inner (stronglybonded) and outer (weakly bonded) sulfate layer.14 The formation of the inner layer indicates that the sulfate anion is chemicallyincorporated (clathrated) into the oxide film, rather than simply (physically)saturates the porous substrate. (14) Ginsberg, H.;Wefers, K. Metalloberflache1963,17, 202.
Figure 3. (a)Anodic polarization of Al 2024in 0.1 M NaC104 at pH = 3.0 in absence and presence of sulfate (lo-*M). Sweep rate was 0.2mV s-l. (b)Accumulation of sulfate on A12024 as a function of potential during anodic polarization in 0.1 M NaC104 at pH = 3.0 (csulfate = low4M). Empty arrows show the change of sulfate accumulation at the given potential after 40 min.
As shown in Figure 2, curve 3, increasing solution pH by addition of a base to the neutral supporting electrolyte dramatically decreases the sulfate coverage. This is as expected under the assumption that sulfate interacts with the oxide-covered surface, rather than with clean metal sites present on the surface. In this respect, recall that the pH of potential of zero charge (PZC) for aluminum oxideshydroxides lies between 6 and 9,15and the passive surfacefilm possesses a net positive charge. This promotes the accumulation of negatively charged ions. Higher solution pH results in a less positive surface charge which lowers the sulfate coverage, as seen. That is, the accumulation of sulfate on A12024 is both pH and electrode potential dependent (see below). The current-potential curves obtained on the positivegoing polarization in the presence and absence of lov4M S042- in 0.1 M NaC104 exhibit a sharp current increase below -0.05 V, evidence for breakdown potentials in these two solutions(Figure 3a). In the sulfate-containingmedia, the breakdown potential is more positive, and the dissolution current is smaller than that measured in pure NaC104, demonstratingthe inhibitiveproperties of sulfate in aluminum corrosion.l6 Judging from the negative-going curve, the repassivation potential is ca. -0.20 V (Figure 3a). Data from the radiotracer experiments correlate well with the electrochemical results, in the sense that the (15) Parks, G. A. Chem. Rev. 1965,65, 177. (16) h r i , F.;Tomcshnyi, L.; Tlirmezey,T. Electrochim.Acta 1988,
33, 323. .
Letters
Langmuir, Vol. 11, No. 12, 1995 4607
change in the surface concentration (or in the surface counts) responds to the change in the electrode potential only when the dissolution current is generated; cf. data in Figure 3. Here, the plot in Figure 3b shows the relationship between the surface concentration and the electrode potential, with the r averaged from 10 measurements, 10 min long, per each potential. One should indeed see that the positive-goingpotential excursion from the OCP (EOCP -0.7 V) up to -0.10 V does not cause any measurable change in the sulfate coverage. However, as soon as the substrate dissolution begins, the sulfate coverage is reduced, and the drop in the sulfate accumulation continues even on the negative-going scan until the value of -0.20 V, that is a t the repassivation potential, is reached (the coverage reaches a minimum at -0.20 V). The decreasing surface concentration can be attributed to dissolution of the passive film into which sulfate is incorporated. A longer time ofwaiting (than 10 min) a t the most positive potentials tried in this study resulted in a massive dissolution of the alloy and a concomitant significant removal of the surface sulfate; see the open symbols in Figure 3b. On the negative-going polarization, below -0.2 V, the sulfate accumulation increases showing a n efficient restoration of the passive film (see Figure 3b). The detailed examination of the radiochemical data measured with A12024 on the negative side of the OCP (without polarizing the electrode first to the more positive potentials than OCP) demonstrates that there are three potential regions which exhibit different surface concentration-electrode potential characteristics (Figure 4b). Down to -1.0 V, the surface concentration decreases, but slightly. At -1.1 V, the sulfate accumulation increases and the increase continues until -1.2 V, below which a rapid decay in sulfate coverage is observed. (During 2 h ofthe experiment a t - 1.2V, the sulfate coverage doubles.). At -1.4 V after 10 min of the experiment, only 17%of the originally accumulated sulfate is present on the surface. Since a t such negative potentials hydrogen evolution occurs (Figure 4a), a part of the desorption may be accounted for by a competition with hydrogen adsorption (which is a hydrogen evolution precursor). Even more probably, the hydrogen evolution process may increase the local pH and cause a partial dissolution of the outer, sulfate entrapping oxide layer to form, for instance, Al(OH)4-.17 The occurrence of the latter mechanism of sulfate removal is supported by the fact that at -1.4 V even a part of the nonmobile sulfate is removed in a longtime experiment (6.Figure 2 and 4b). The accumulation of sulfate on pure aluminum is higher than on A12024 (Figure 4b) and, in contrast to the A12024 data, the surface concentration continuously decreases with the negatively increasing potentials below -1.1 V (Figure 4b). The first observation is most likely related to a different roughness factor of the two electrodes, although a lower sorption capacity of sulfate by copperrich nodules (see below) cannot be ruled out. The difference in the r vs E profile is, in part, due to the hydrogen evolution that starts a t a significantly more negative potential on pure aluminum than on Al 2024 (the difference in the evolution thresholds is ca. 0.5 V, Figure 4a). This is obviouslybecause some Cu-containing phases, like (CuAlZ), (CuFeMn)AlG,or even Cu particles, are present in the alloy,18and the overpotential ofhydrogen evolution is much lower on copper than on oxidized
-
(17) Vedder, W.;Vermilyea, D.A. Trans. Faraday SOC. 1969,65,561. (18) Saito,A.;Latanision, R. M. Inlnternational Congress on Metallic Corrosion. Toronto, Canada. June 3-7 (1984). Proceedings. Vol. 3, pp 122-129.
-0.1 MNaCIO,, AI 2024
a
+ lo4 MNap04, AI 2024 0.1 MNaCIO,, Pure Aluminum c ____-_
....... .. 0.1 MNaClO,
/'
I'
,
-1.0
,
Q -2.0
&
-2.5
E
-3.0-
u3
-3.5
'c)
E
,
-1.8
-1.6
-1.4
-1.2
-1.0
-0.8
-0.6
E N vs. AgIAgC1 4-
-2
-1.5
-1
-0.5
EN vs. Ag/AgCl Figure 4. (a)Cathodic polarization ofAl2024 in absence and M) and of pure Al in absence of presence of sulfate ions ( sulfate ions in 0.1 M NaC104 at pH = 3.0. Sweep rate: 0.2 mV s-l. (b) Accumulation of sulfate ions on A1 2024 (1)and pure AI (2) as a function of potential during cathodic polarization in 0.1 M NaC104 at pH = 3.0 (csulfate= loT4M).
a 1 ~ m i n u m . lBecause ~ of the difference in the overpotential, and a related wide range of the ideal polarizability of the oxidized pure aluminum, the sulfate adsorbate is less affected by the hydrogen evolution, and survives more negative potential than on A12024. In fact, our A E 3 data (not shown) indicate that the amount of copper on the AI 2024 surface increases below -1.2 V, providing evidence that copper segregates to the surface under hydrogen evolution conditions. If this is so, it is important to recall that no sulfate is adsorbed on copper a t - 1.2 V under the present measuring conditions.20The two effects combined, the copper segregation and the hydrogen evolution effect, account for the difference in the two r vs E curves shown in Figure 4b. Interestingly, there is a short plateau on the current-potential curve, from -1.05 to -1.2 V measured with A12024 (Figure 4a) that coincides with a rapidly increasing sulfate accumulation in this potential range (Figure 4b). Clearly, the electrode is undergoing a n important structural transformation, most likely with respect to its copper content to which sulfate adsorption actively responds. The nature of this transformation is being investigated. In summary, we have obtained a n insight into a reversible and a n irreversible component of the sulfate (19) Conway, B.E. EZectrochemicaZData;Elsevier: New York, 1952. (20) Rice-Jackson, L.M.; Horhyi, G.;Wieckowski, A. EZectrochim. Acta 1991,36, 753.
Letters
4608 Langmuir, Vol. 11, No. 12, 1995
adsorption on the A12024 electrode and compared some of the data obtained with the alloy to those obtained with a pure Al electrode. Copper-containing nodules in the alloy determine the electrode potential threshold below which sulfate anions desorb. During the breakdown of passivity, the sulfate anions leave the surface but are readsorbed when conditions for the passive filmrestoration are met (on the negative polarization). That is, there is a strong tendency of sulfate to remain in the passive film. This explains the well-known inhibitive properties of sulfate in aluminum corrosion16which, in this work, are reflected by the comparison of the polarization curve of
the Al 2024 substrate in solution containing and not containing the sulfate additive (Figure 3a).
Acknowledgment. This work was financially supported by the Department of Energy Grant DE-ACOB76ER01198,administered by the Frederick Seitz Materials Research Laboratory at the University of Illinois. A.E.T. acknowledges the fellowship support from the American Chemical Society, Analytical Chemistry Division, which is sponsored by the DuPont Company. LA950705S
