Corrosion study of an Al-La alloy manufactured by directional solidification
The Authors
L. Dzib-Pérez, Programa de Corrosión del Golfo de México, Universidad Autónoma de Campeche, Campeche, México
J. González-Sánchez, Programa de Corrosión del Golfo de México, Universidad Autónoma de Campeche, Campeche, México
T. Pérez, Programa de Corrosión del Golfo de México, Universidad Autónoma de Campeche, Campeche, México
A. Juárez, CIATEQ, Calzada del Retablo, Querétaro, México
P. Bartolo-Pérez, Programa de Corrosión del Golfo de México, Universidad Autónoma de Campeche, Campeche, México Applied Physics Department, CINVESTAV-Mérida, Mérida, Yucatán, México
Acknowledgements
The authors would like to acknowledge Dr L. Veleva for her valuable help on discussing some aspects of this work. Also they would like to acknowledge CONACyT (Mexican Government) for supporting the studentship of LDP.
Abstract
Purpose – To study the corrosion resistance of the novel alloy Al-12.6La (wt%) manufactured using directional solidification.
Design/methodology/approach – Samples fabricated using the Bridgman growth technique at three different withdrawal velocities were subjected to total immersion tests in distilled water and in 3.5 per cent NaCl solution and to DC polarisation tests in distilled water. XPS analyses conducted on samples after polarisation indicated the presence of an La compound in the non passive corrosion products film formed.
Findings – Anodic polarisation induced dissolution of the alloy with the formation of a non passive corrosion product film. During potentiodynamic polarisation, a sudden current increment occurred at a potential value that was more positive for samples solidified at higher rates. The corrosion resistance of this Al-12.6%La alloy decreased as the solidification rate increased.
Originality/value – The results presented in this work are an insight to the understanding of the corrosion resistance and electrochemical behaviour of this alloy for future engineering applications and development.
Article Type:
Research paper
Keyword(s):
Alloys; Corrosion resistance; Corrosion products; Films (states of matter).
Journal:
Anti-Corrosion Methods and Materials
Volume:
53
Number:
3
Year:
2006
pp:
153-160
Copyright ©
Emerald Group Publishing Limited
ISSN:
0003-5599
Introduction
Aluminium is certainly the most important non-ferrous structural metal of commerce around the world. This metal and its alloys offer interesting properties such as low density (2.7 g/cm3), which represents a density one-third that of steels, high specific strength and excellent thermal and electric conductivity. Although pure aluminium presents a relatively low strength, aluminium alloys provide a high strength to weight ratio which makes these alloys very suitable for automotive and aerospace applications. The stability in the terrestrial atmosphere is an important attribute of aluminium alloys which is observed through their satisfactory resistance to atmospheric corrosion and aqueous corrosion in chloride free neutral solutions. The main reason for the long life of aluminium in neutral electrolytes is its self forming surface layer which is a microscopically thin aluminium passive layer. The film has a two-part structure as suggested by Bockris and Minevsky (1993); an inner barrier layer which is very thin and a thick outer barrier. The corrosion resistance characteristics of aluminium alloys is strongly improved by the anodizing process, which induces the formation of a closed-packed array of porous columnar hexagonal cells normal to the substrate surface and separated from it by a barrier layer. Pure Al and aluminium alloys have a wide spectrum of technological applications although they are susceptible to localised attack such as pitting and crevice corrosion in chloride containing aqueous electrolytes. For this reason, the corrosion resistance of these metallic materials has attracted the attention of several investigators which has originated intense research work on this topic (Caig, 1991; Francis and Cheung, 1994; Pyun and Moon, 1999; Sayed et al., 2004; Lampeas and Koutsoukos, 1994). For the case of pure aluminium in chloride containing solutions, McCafferty (2003) showed that corrosion pit initiation occurs by chloride-assisted localised dissolution at the oxide/metal interface. However, on aluminium alloys, localised corrosion is typically associated with the existence of interfaces formed between the aluminium metal matrix and intermetallic compounds or with non metallic particles (Díaz-Ballote et al., 2004; Frankel, 1998).
There is an increasing demand around the world for new materials and metallic alloys with high mechanical resistance, low weight and high chemical stability, i.e. corrosion resistance. Novel aluminium alloys containing rare earth metals (REM), fabricated by directional solidification, show increased strength due to the introduction of a hard and stable intermetallic compound (Al11REM3) into the aluminium metal matrix (Ruder and Eliécer, 1989; Matera et al., 1973).
Al-REM alloys represent an interesting group of nonferrous materials with which important research has been carried out regarding the preferred microstructure formed as a function of solidification condition and material composition (Hawksworth et al., 1999; Dill et al., 1994; Liu and Jones, 1992).
For example, during the Bridgman-type directional solidification process, a crucible containing a liquid sample is drawn downwards, at a uniform velocity V, through a constant temperature gradient G. Using the Bridgman method (Dill et al., 1994) observed an increase of the solubility of La in the aluminium phase with increasing solidification rate of the Al-La alloy which can modify the chemical and mechanical properties of these types of alloys.
It has been found that Mg-Zn-Al-Ca-REM alloys are promising materials for substituting aluminium alloys used in the automobile industry. The addition of La results in the crystallization of a large amount of the acicular intermetallic compound (Al11La3) along and across the grain boundaries (Anyanwu et al., 2004).
Owing to the recent development of the Al-12.6%La alloy there are not any documented studies concerning its resistance against corrosion. However, RE metals have been added to metals as minor alloying elements to improve the corrosion resistance of the alloys (Singh et al., 1989).
Crossland et al. (1998), found that during the anodic oxidation of Al-Ce alloys in ammonium pentaborate and sodium hydroxide electrolytes Ce3+ and Al3+ ions are incorporated into the mainly amorphous anodic oxide films at the alloy/film interface in proportion to the concentration of the respective elements in the bulk alloys. These authors reported that cerium ions migrate outward in the films more rapidly than do Al3+ ions by factors that increase with the reduction of cerium content in the alloy. They concluded that the faster migration of Ce3+ leads to the formation of a layer of relatively pure cerium oxide, or for dilute alloys a cerium-enriched layer of alumina adjacent to the alloy/film interface.
On the other hand, lanthanum as an alloying element, which is another REM next to cerium in the periodic table, does not induce an upgrading of the corrosion resistance of the Al-rich alloy as found in the present work. However, the microstructural development and behaviour of the Al-La alloy as a function of solidification rate and alloy composition (wt%), is very similar to that for Ce at concentrations between 11.0 and 14.5 wt% (Hawksworth et al., 1999; Juarez-Hernández, 1999).
In the present work the alloy Al-12.6%La showed electrochemical stability when samples were in contact with distilled water at open circuit potential (OCP) but presented active corrosion when the alloy was subjected to anodic polarisation at room temperature.
It is worth mentioning that lanthanum and cerium chloride and their binary mixtures have been investigated as corrosion inhibitors of AA5083 Al-Mg alloy in aerated 3.5 per cent NaCl aqueous solution. The highest protection degree was found for the binary solution doped with 250 ppm CeCl3 and 250 ppm LaCl3 (Aballe et al., 2001).
Methodology
Materials
The aluminium alloy of composition Al-12.6La (wt%) was prepared as rectangular chill-cast ingots of dimensions 15×15×120 mm from melts of 99.99 per cent Al with 99.9%La. Samples of the melt were solidified into open-ended alumina tubes of 3 mm diameter, 0.5 mm wall-thickness and 120 mm length. These tubed samples were subjected to remelting and re-solidification by the Bridgman growth technique at withdrawal velocities of 0.1, 1.03 and 3.02 mm/s and an applied temperature gradient at 600°C of ∼12 K/min. Detailed information about the solidification process has been described by Juarez-Hernández (1999). The specimens used consisted of cylindrical ingots of 0.02 m length and 0.00295 m diameter (6.83 × 10−6 m2 surface area). The microstructure was a parallel eutectic lamellae formed by αAl and the intermetallic compound Al11La3 as shown by the SEM image in Figure 1.
Total immersion Tests
Specimens of 0.5 cm length were mechanically cleaned, degreased with acetone and weighed. After cleaning, the samples were weighed and immersed in the test solutions over a period of 48 h. At the end of the immersion period the specimens were cleaned with distilled water, dried and weighed to evaluate the weight loss or gain. The tests solutions consisted of: 3.5 per cent sodium chloride solution (pH=5.6) and distilled water (pH=6.0). The NaCl solution was prepared with distilled water and analytical grade purity sodium chloride.
Electrochemical tests
Samples of 1 cm long were cut from the ingots obtained at different solidification rates and were embedded in polyester resin to be used as working electrodes in a conventional three electrode electrochemical cell. The free-surface area of the specimens was grinded to a surface finish of grit 800, washed with distilled water and degreased with acetone. The OCP of samples solidified at different rates immersed in the two electrolytes was measured using a saturated calomel electrode (SCE) as the reference electrode. For the potentiodynamic and potentiostatic studies a graphite bar was used as auxiliary electrode and the SCE as the reference electrode. An EG&G Potentiostat (model 273A) was used for the polarisation tests in distilled water. The specimens were immersed in distilled water for 24 h before the polarisation tests were carried out. The potentiodynamic polarisation consisted of the application of a cathodic overpotential of 250 mV followed by a potential scan to more positive values at a rate of 10 mV/min up to reach a potential of 1,500 mV vs SCE. During the polarisation tests in distilled water, a sudden anodic current increase was observed which took place at a potential value characteristic of each solidification rate. This potential value (the potential of sudden current rise E scr) was used to establish the potential value at which the subsequent study of the anodic behaviour of the alloy in distilled water was carried out.
An anodic potentiostatic polarisation was applied to the specimens during 1 h at two different potential levels: 50 mV below and 100 mV above the potential of sudden current rise (E scr). The resultant anodic current was recorded as a function of time.
XPS analysis
In order to determine the components of the corrosion product film formed on the surface of the samples after the potentiostatic polarisation in distilled water XPS analysis was performed using a Perkin-Elmer PHI 560 ESCA-SAM system. Argon ion sputtering was performed with 4 keV energy ions and 0.36 μA current beam, yielding an approximate 1 nm/min sputtering rate. For the XPS analysis, samples were excited with 1,486.6 eV energy AlKα X-rays source operated at a power of 300 W (15 kV, 20 mA). XPS spectra were obtained under two different conditions:
- a survey spectrum mode of 0-1,000 eV; and
- a multiplex repetitive scan mode.
The spectrometer was calibrated using the Cu 2p3/2 (932.4 eV) and Cu 3p3/2 (74.9 eV) lines. Binding energy calibration was based on C 1 s at 284.6 eV.
Results
Immersion test
It was found that the specimens of the Al-12.6%La alloy immersed in the NaCl solution and in distilled water increased their weight after 48 h of immersion. The results of weight gain in the two electrolytes at OCP are presented in Table I.
Electrochemical study
The value of the OCP as a function of time of samples solidified at the three different rates immersed during 48 h in distilled water and in 3.5 per cent NaCl solution is shown in Figure 2(a) and (b), respectively.
After 24 h of immersion in distilled water the OCP of the specimens solidified at 3.02, 1.03 and 0.1 mm/s was in the range from −500 to −800 mV. The specimens solidified at 3.02 and 1.03 mm/s showed a similar behaviour with the most positive values of OCP, whereas the specimen solidified at 0.1 mm/s presented the most negative.
The OCP in the 3.5 per cent NaCl solution (pH=5.6) for samples of the alloy solidified at the different rates were in the range from −650 to −980 mV vs SCE (Figure 2(b)). The alloy presented moderated corrosion under OCP conditions in this solution. However, an intense dissolution was observed when the samples were subjected to potentiodynamic polarisation.
Potentiodynamic polarisation
Anodic polarisation induced intense dissolution of the alloy in the 3.5 per cent NaCl solution, even at very low anodic overpotentials. The sample dissolved completely before the polarisation was completed. Localised attack patterns characteristic of Al alloys in chloride containing solutions was observed, in this case with a high density of corrosion pits nucleation.
In distilled water, the alloy presented more corrosion resistance compared with the behaviour obtained in the NaCl solution. The applied potentiodynamic polarisation induced charge transfer controlled dissolution of the alloy on samples of the three different solidification rates as shown in Figure 3.
Chronoamperometries
The chronoamperometries at a potential level 50 mV below the E scr indicated that the samples solidified at 3.02 presented anodic current densities two orders of magnitude higher than samples solidified at 0.1 and 1.03, as shown in Figure 4. In general, the Al-12.6%La alloy presented dissolution under anodic polarisation with anodic current densities registered in the range from 0.5 × 10−3 to 0.35 mA/cm2.
These results support the fact that the corrosion products layer formed on the surface of the alloy did not induce a passive condition as the anodic current density increased as the polarisation continued except for samples solidified at 0.1 mm/s.
The anodic polarisation of the Al-12.6%La alloy at a potential level 100 mV above E scr induced a similar behaviour of the alloy in terms of the dissolution process of the samples solidified at the three rates. However, the anodic current density of the samples solidified at 0.1 and 1.03 mm/s was one order of magnitude higher than that for the polarisation at 50 mV below the E scr. Figure 5 shows the chronoamperometries at a potential level of 100 mV above the E scr.
XPS analyses
Evidence of the presence of La(OH)3 was found. In Figure 6, XPS survey spectra of the Al-12%La alloy are presented. From Figure 6, the La 3d5/2 (842 eV), O 1 s (534 eV), N 1 s (401 eV), C 1 s (287 eV) Si 2p (103 eV) and Al 2p (77 eV) core level principal peaks can be observed. Also, O(A) (980 eV) and Al 2 s (122 eV) secondary peaks were detected.
Discussion
It was observed that at OCP conditions, the Al-12.6%La alloy developed a corrosion product film which poorly protected the alloy against corrosion in distilled water and in the 3.5 per cent NaCl solution. The results indicate that if the dissolution rate does not vary with the exposure time, in quiescent distilled water and in 3.5 per cent NaCl solution, the alloy solidified at 0.1 mm/s will dissolve 8 per cent per year. The alloy solidified at 3.02 mm/s will dissolve 18 per cent per year in distilled water and it will dissolve completely after just five months in 3.5 per cent NaCl solution. Considering that aluminium has a substantial pH range (4-9) in which protective oxide film exist, it can be suggested that the thermodynamic behaviour of aluminium predicted by the E-pH diagram is affected by the presence of lanthanum which can dissolve in neutral pH solutions to form La3+ ions (Pourbaix, 1974).
Thermodynamics indicate that the measured values of OCP for the alloy in distilled water correspond to the formation of the hydrated Al2O3 for pure Al (Pourbaix, 1974). The presence of La modified the characteristic of the formed corrosion products film and made it non-passive. On the other hand, under these potential values and pH conditions the stable species of lanthanum is La3+ as indicated also by the E-pH diagram for this metal.
Anodic polarisation in distilled water did not induce the passivation of the alloy regardless of the solidification rate. On the other hand, the potential range at which the activation controlled dissolution was found to be dependant on the solidification rate of the alloy. Samples solidified at 3.02 mm/s presented the highest potential range and the highest potential value at which the sudden current rise was observed (E scr), followed by samples solidified at 1.03 mm/s and finally by samples solidified at 0.1 mm/s. The potential range for the charge transfer controlled dissolution was very similar for samples solidified at 1.03 and 0.1 mm/s. The less polarised anodic reaction corresponded to samples solidified at 0.1 mm/s, which was expected considering that these samples presented the most negative corrosion potential. The behaviour observed can be explained considering that the main microstructure of the alloy in the range of solidification rates used is the eutectic (e), constituted by αAl and the intermetallic compound Al11La3 (Hawksworth et al., 1999; Juarez-Hernández, 1999). A guide of microstructures in Al-REM alloys as a function of the REM content and solidification rate is presented in Table II (Juarez-Hernández, 1999).
The fast solidification process promoted the presence of lanthanum dissolved in the αAl phase as the solidification rate increased, which limited the diffusion of La in the Al matrix to form the Al11La3, leaving a La oversaturated αAl phase (Juarez-Hernández, 1999). Also it has been found that a variation of the αAl – Al11X3 (X-REM) interlamellar spacing (λ) which decreases as the growth velocity increases (Street et al., 1967). Thus, the samples solidified at higher rates should be more active due to the presence of La in the saturated αAl continuous phase and because the separation between the Al11La3 lamellas is reduced.
The sudden current rise potential, E scr, was more positive for higher solidification rate of the alloy as can be seen in Figure 3. At this potential level, the anodic current remarkably increased for approximately two orders of magnitude except for the specimens solidified at 0.1 mm/s. As the anodic polarisation continued above the E scr the slope of the curve log|i| vs potential increased. This behaviour cannot be regarded as a passive condition of the alloy due to the high values of the current density (0.1-1.0 mA/cm2), which was corroborated by the high dissolution observed in the samples. At potentials above the E scr, the behaviour of the alloy is the same independently of the solidification rate.
This tendency can be the result of the primary dissolution of the La saturated phase with the formation of a non-passive corrosion products film containing a La compound. Once the E scr is reached, selective dissolution is initiated on sites of high La concentration. The solidification process could induce segregation and formation of La saturated αAl sites.
Specimens solidified at 1.03 and 0.1 mm/s did not show active dissolution during almost 40 min of anodic polarisation, after that period the anodic current density increased rapidly. From the electrochemical behaviour observed of the alloy Al-12.6%La, it may be suggested that some La compounds could induce discontinuities on the Al corrosion products film. It is known that all trivalent REM cations undergo progressive hydrolysis in aqueous solution as solution pH is increased with the consequent precipitation of solid ternary hydroxides (Pourbaix, 1974). During anodising, the elements added to the Al may be transferred from the metal substrate into the anodic oxide films as either oxides or they may be dissolved into the solution by the selective dissolution (Skeldon et al., 1999). As discussed in the next section of XPS analyses, evidence was found of the presence of La(OH)3. Böhm et al. (2000), have proposed that it is possible the deposition of La(OH)3 over metal surfaces immersed in LaCl3 aqueous solution at pH 5.8. It is suggested that the anodic polarisation could induce the formation of La3+ that was introduced in the corrosion products film and transformed to hydroxide.
To date, there has been no documented information about the structure and chemical composition of a native oxide film formed on the alloy Al-12.6%La that supports the suggestion proposed in this work.
In order to determine oxidation states of the Al and La elements, the lines were recorded with a narrow sweep in the range of 8-15 eV. The Al 1 s (Figure 7(a)) peak at a binding energy of 75.6 eV corresponds to Al2O3 and AlOOH compounds (Barr, 1978).
Formation of these compounds is confirmed by the O 1 s peak that appears in 531.8 eV, this is shown in Figure 7 (b). The shoulder that appears in approximately 835 eV as shown in Figure 7(c) corresponds to La2O3 and LaOOH compounds. It has been reported that oxygen markedly alters the La 3D lineshapes (Perkins et al., 1999).
Conclusions
Contrary to the behaviour of cerium in aluminium alloys, which inhibits the corrosion process of the Al-Ce alloy, lanthanum did not inhibit the corrosion process of the Al-12.6%La alloy in aqueous solutions.
The solidification rate of the Al-12.6 wt%La alloy influences its electrochemical behaviour being the samples solidified at 3.02 mm/s the less resistant to corrosion.
Anodic potentiodynamic polarisation in distilled water induced charge transfer-controlled dissolution of the alloy followed by a sudden increase of current density for which a sudden current rise potential was determined E scr.
The potential of sudden current rise (E scr) was more positive as the solidification rate was higher.
In distilled water, the alloy forms a corrosion products layer that limits the corrosion of the alloy at OCP conditions, however, this is not a passive film.
In the 3.5 per cent NaCl solution the alloy presented less resistance to corrosion and under anodic polarisation it dissolves severely.
The dissolution process of this alloy involves the formation of a corrosion products layer constituted by Al2O3 with some content of a La compound.
Figure 1Microstructure of the Al-12.6%La alloy manufactured by directional solidification
Figure 2OCP vs time of samples immersed in: (a) distilled water and (b) 3.5 per centw NaCl solution
Figure 3Potentiodynamic polarisation curves of Al-12.6%La alloy samples solidified at different rates in distilled water
Figure 4Chronoamperometries for samples of the three solidification rates in distilled water at an applied potential of 50 mV below the E
scr of specimens
Figure 5Chronoamperometries for samples solidified at the three different rates in distilled water at an applied potential of +100 above the E
scr
Figure 6XPS survey spectra of Al-12%La alloy
Figure 7Narrow sweep lines in the range of 8-15 eV recorded to determine oxidation states of the Al and La elements
Table IWeight gain (per cent) after 48 h of immersion in the two different electrolytes
![ImageMicrostructure of binary Al-REM alloys, as a function of alloy composition (wt%) and growth velocity (mm/s). Al dendrites+eutectic (+), fully eutectic () and primary AlREM+eutectic (+) [20]](../../../fig/1280530302009.png)
Table IIMicrostructure of binary Al-REM alloys, as a function of alloy composition C
o (wt%) and growth velocity V (mm/s). αAl dendrites+eutectic (α+e), fully eutectic (e) and primary βAl11REM3+eutectic (β+e) [20]
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