Mechanism of Corrosion Inhibition of AA2024 by Rare-Earth Compounds Kiryl A. Yasakau,�� Mikhail L. Zheludkevich,*,�� Sviatlana V. Lamaka,�� and Mario G. S. Ferreira��,��� Department of Ceramics and Glass Engineering, UniVersity of AVeiro, CICECO, 3810-193 AVeiro, Portugal, and Department of Chemical Engineering, Instituto Superior Tecnico,�� ICEMS, AV. RoVisco Pais 1049-001 Lisboa, Portugal ReceiVed: October 23, 2005 In Final Form: January 22, 2006 The mechanism of corrosion protection of the widely used 2024-T3 aluminum alloy by cerium and lanthanum inhibitors in chloride media is described in detail in the present work. The corrosion process was investigated by means of scanning Kelvin probe force microscopy (SKPFM), in situ atomic force microscopy, and scanning electron microscopy coupled with energy dispersive spectroscopy. Employment of the high-resolution and in situ techniques results in a deep understanding of the details of the physical chemistry and mechanisms of the corrosion processes. The applicability of the SKPFM for mechanistic analysis of the effect of different corrosion inhibitors is demonstrated for the first time. The inhibitors under study show sufficient hindering of the localized corrosion processes especially in the case of pitting formation located around the intermetallic S-phase particles. The main role of Ce3+ and La3+ in the corrosion protection is formation of hydroxide deposits on S-phase inclusions buffering the local increase of pH, which is responsible for the acceleration of the intermetallics dealloying. The formed hydroxide precipitates can also act as a diffusion barrier hindering the corrosion processes in active zones. Cerium nitrate exhibits higher inhibition efficiency in comparison with lanthanum nitrate. The higher effect in the case of cerium is obtained due to lower solubility of the respective hydroxide. A detailed mechanism of the corrosion process and its inhibition is proposed based on thermodynamic analysis. 1. Introduction Localized corrosion is a very complex process involving many heterogeneous and homogeneous reactions. The information on the mechanism and the physicochemistry of localized corrosion is an issue of prime importance, since this knowledge can lead to the development of novel effective corrosion protection systems. The important role of intermetallic particles for the initiation and propagation of localized corrosion on the alumi- num alloys is well-known and was discussed in a great number of works.1-10 The highest attention was justly paid to the localized corrosion of 2024 aluminum alloy, which is extensively used in the aerospace industry due to the excellent weight-to- strength ratio.3 The intermetallic particles segregated in the grain boundaries confer enhanced mechanical properties, but at the same time they increase the susceptibility of the alloy to a localized corrosion attack. Thus, AA2024 is one of the alloys in commercial applications most prone to localized corrosion attack. Nowadays many groups worldwide attempt to develop effective corrosion protection systems for this alloy. Two main types of localized corrosion occur on AA2024 in the neutral chloride solutions, namely, pitting corrosion and corrosion along the grain boundaries. Therefore the anodic polarization plot for this alloy exhibits two breakdown potentials. The first current increase, at more negative potential, was assigned to the dissolution of the intermetallic particles, while the nobler one is a result of intergranular corrosion.11,12 Thus, the first place where corrosion starts is the zone of the intermetallic particles. Different kinds of intermetallic particles were found in the structure of AA2024. The most predominant type (above 60%) corresponds to the S-phase precipitates with Al2CuMg composition. These intermetallics cover almost 3% of the geometrical surface area of the alloy.13 A16(Cu, Fe, Mn) intermetallics were revealed as the second largest type constitut- ing about 12% of all the precipitates. Minor concentrations of Al20Mn3Cu2, Al2Cu, Al7Cu2Fe, and (Al, Cu)6Mn are present in the alloy as well.11,13,14 All kinds of intermetallics excepting S-phase are composed of metals nobler than aluminum, thus showing cathodic character. However, the S-phase is composed of nobler copper as well as of active magnesium. Several assumptions were described in the literature concerning the electrochemical character of the S-phase. Many authors assert that the Al2CuMg particles have an anodic potential respective to the alloy matrix based on the fact that the dissolution of the S-phase occurs at the first stage of pitting corrosion.2,15-17 How- ever, the Volta potential measurements clearly show cathodic character of Al2CuMg particles when compared to the alloy matrix.4,5,9,18 This contradiction raises the question: why do the cathodic intermetallics exhibit high anodic dissolution activity? Many works in the literature were devoted to the mechanism of pitting corrosion of AA2024 in chloride-containing media and clarifying details of different stages of this process. At the first stage, chloride ions attack a passive oxide film6 causing its breakdown in the places of the intermetallic precipitates. The dissolution of magnesium and aluminum from the copper-rich particle leads to the formation of copper remnants with ���Swiss cheese���-like morphology. The fast dealloying of the S-phase * Corresponding author: tel, +351-234-378146 fax, +351-234-425300 e-mail, mzheludkevich@cv.ua.pt (M. L. Zheludkevich). �� University of Aveiro. ��� Instituto Superior Te ��cnico. 5515 J. Phys. Chem. B 2006, 110, 5515-5528 10.1021/jp0560664 CCC: $33.50 �� 2006 American Chemical Society Published on Web 02/17/2006

starts immediately after contact of solution with the surface of the intermetallic.5,7,8,13,14 Simultaneously the deposition of a thin copper layer occurs around the pits. The redeposited copper layer and the porous copper remnants become a good cathode for oxygen reduction that promotes the dissolution of the surround- ing alloy matrix and the further propagation of pits. A balance between metal dissolution and mass transfer in the electrolyte control the kinetics of pit propagation.7 However, several details of the pitting corrosion mechanism are not yet completely clear and do not reveal a clear picture of the full process. Taking into account the high susceptibility of AA2024 to localized corrosion attack the effective corrosion protection appears as an issue of prime importance. The carcinogenic chromate pretreatments are currently used to hinder the localized corrosion of the AA2024. However, stricter environmental regulations and the needs of industry stimulated an intense research effort to develop novel environmental-friendly pre- treatments and inhibitors. The salts of rare earth (RE) elements were found to provide an effective corrosion inhibition effect to the aluminum alloys.19-21 They control the cathodic reaction by precipitating metal hydroxide (Ln(OH)3) at local regions, which are associated with increase of pH due to oxygen reduction.21-23 Cerium shows maximum corrosion protection efficiency as compared with other RE compounds. An important role in superior efficiency of cerium can be played by Ce4+, which can be formed at high pH values in aerated chloride environments.24,25 RE compounds can be introduced in corrosion protection systems for the aluminum alloy using different strategies. Formation of a conversion coating composed by hydrated oxide layer on top of the aluminum alloy confers an enhanced corrosion protection.4,26 Another approach is use of the cerium conversion coating technique to seal the porous film of anodized aluminum alloy.27 The cerium-based inhibitors can be also introduced in the thin hybrid coatings used as pretreatment for aluminum alloys and exhibit promising results.28-30 However, introduction of cerium compounds can decrease the stability of the hybrid polymer matrix with decrease of the barrier proper- ties. Introduction of zirconia nanoparticles doped with cerium ions into hybrid sol-gel matrix was found to avoid the negative effect of the cerium cations on the film and to provide a prolonged release of cerium inhibitor in the places of localized corrosion attack.30,31 While many works are dedicated to the investigation of the corrosion inhibition with RE-based inhibi- tors, there are still many contradictions and ambiguities concerning the mechanism of inhibition. The present work is devoted to the investigation of the mechanism of localized corrosion on AA2024 and to the inhibition of corrosion with cerium and lanthanum inhibitors in chloride media. The experimental techniques used were scanning Kelvin probe force microscopy (SKPFM), in situ atomic force microscopy (AFM), and scanning electron mi- croscopy (SEM) coupled with energy dispersive spectroscopy (EDS). dc polarization was used as a supplemental method as well. 2. Experimental Section 2.1. Substrate Preparation. The 2024-T3 aluminum alloy with elemental composition as shown in Table 1 was used under study. Aluminum plates used for dc polarization tests were treated using the following procedure. Cleanup was with acetone and then immersion in a water solution of the alkaline cleaner TURCO 4215, 50 g/L, for 35 min at 65 ��C, followed by rinsing with distillated water and then immersion in a 20% solution of nitric acid for 10 min at 35 ��C following rinsing with distilled water and drying. The specimens used for AFM, SKPFM, and SEM analysis after treatment according to the above-mentioned procedure were polished with nonaqueous diamond paste down to 2 ��m and then cleaned in acetone. 2.2. Testing Solutions. High grade reagents Ce(NO3)3���6H2O and La(NO3)3���6H2O were used as inhibitors. The aluminum substrates were immersed in 0.05 M sodium chloride testing solutions doped with different concentrations of the inhibitors for 10 min, 1 h, and 2 h prior to the dc polarization tests. Additionally, the polished aluminum samples were immersed in 0.005 M NaCl solution with different concentrations of Ce or La inhibitors for 1 or 2 h. After immersion, the samples were rinsed with distillated water and dried in a desiccator. For in situ AFM measurements, testing solutions 0.005 M and 0.5 NaCl with 0.5 wt % of Ce and La inhibitors were used. 2.3. Techniques. Potentiodinamic polarization curves were obtained in chloride solutions using a VoltaLab PGZ 100 potentiostat. A three-electrode cell was used, consisting of a saturated calomel reference electrode (SCE), a platinum foil counter electrode, and the AA2024 specimen as a working electrode with a surface area of 3.4 cm2. Polarization measure- ments were performed in the anodic direction from -200 mV vs SCE to 250-300 mV vs SCE versus the open circuit potential, at a sweep rate of 1 mV/s. Prior to the polarization measurements, the samples were immersed in NaCl solution for 10, 60, and 120 min. A commercial AFM Digital Instruments NanoScope III system with an Extender electronic module was used for scanning Kelvin probe force microscopy (SKPFM). The AFM operated in lift mode with two pass scans. Lift scan height was 100 nm. Drive amplitude in interleave control for the second pass scan was 800 mV. For all measurements silicon probes covered with PtIr5 were used. The in situ AFM measurements of the alloy topography evolution in a fluid cell containing NaCl solution were conducted in contact mode with silicon nitride tips. 3. Results 3.1. Corrosion of the AA2024 in Chloride Solution. In view of the fact that the main goal of the present work is to study the details of corrosion inhibition of 2024 aluminum alloy by cerium and lanthanum compounds, knowledge of the intimate details of the corrosion mechanism of this alloy appears as an issue of prime importance. The different scanning methods were employed to study the corrosion mechanism of alloy in the chloride solution. A typical electron micrograph of the AA2024 surface immersed in aerated 0.005 M NaCl solution for 2 h is depicted in Figure 1. Corrosion can already be seen even after relatively short immersion in such diluted electrolyte. At the beginning the localized corrosion preferentially starts in the places of intermetallic particles as was discussed above. The strongest corrosion attack appears in the region of the bright round inclusions. Analysis of the chemical composition (Table 2) shows that such zones (zones 1 and 4) are enriched in copper and magnesium in comparison with the surrounding alloy matrix (zone 5). Thus these particles can be surely ascribed to the TABLE 1: Composition of 2024-T3 Aluminum Alloy, wt % Cu Cr Fe Mg Mn Si Ti Zn other Al concn 3.8-4.9 0.1 0.5 1.2-1.8 0.3-0.9 0.5 0.15 0.25 0.15 balance 5516 J. Phys. Chem. B, Vol. 110, No. 11, 2006 Yasakau et al.