2.1 Corrosion of aluminum in alkaline solutions

Aluminum and many of its alloys are susceptible to corrosion when exposed to alkaline solutions. This is frequently reported for sodium and potassium hydroxide. On the other hand, the low solubility of lime and calcium hydroxide solutions limits their corrosive action on aluminum materials. High corrosion resistance is observed in ammonium hydroxide solutions for most aluminum alloys. The presence of approx. 4 wt.% Mg in aluminum alloys increases their corrosion resistance in alkaline solutions, even more so in ammonium hydroxide solutions (Davis, 2000).

Aluminum is subjected to alkaline solutions (mainly sodium or potassium hydroxide) in several applications, among which aluminum-air batteries and alkaline etching are the most important (Egan et al., 2013; Ma et al., 2013; Golru et al., 2015a). The performance of aluminum-air batteries is dependent on the mitigation of aluminum corrosion in alkaline solution, which otherwise can lead to self-discharge (Egan et al., 2013).

Pyun and Moon studied the corrosion mechanism of pure aluminum in alkaline solution, using electrochemical techniques (Pyun and Moon, 2000). They reported that aluminum dissolution proceeds first through hydroxide film formation and then its dissolution as a partial anodic reaction. The same authors (Moon and Pyun, 1999) previously concluded that this film is electrochemically formed due to the migration of the hydroxide ions through the oxide layer on the aluminum surface, as shown in Eq. (1):

The aluminum hydroxide film is then chemically dissolved by the attack of the hydroxide ions to form soluble aluminate ions (Eq. (2)):

The combination of Eq. (1) and Eq. (2) gives the partial anodic dissolution reaction of pure aluminum in alkaline solutions, as shown in Eq. (3):

There are two possible partial cathodic reactions, the reduction of oxygen and/or the reduction of water, represented by Eqs. (4) and (5):

The corrosion reaction can be summarized by combining Eqs. (3) and (4):

Experimental work shows that during the corrosion of pure aluminum in alkaline solution gas evolves, which confirms that the aluminum dissolution process goes mainly through the reduction of water (Eq. (7)).

2.2 Corrosion of aluminum in chloride solution

Pitting corrosion is the most dangerous type of aluminum corrosion. It occurs as holes and pits of irregular shapes on the surface of the metal. The diameter and depth of the pits are dependent on the type of material, the corrosive medium, and the properties of the environment that the aluminum and its alloys are exposed to. Pitting corrosion of aluminum takes place frequently in aerated chloride solutions.

Chloride ions attack the natural oxide layer, damaging it in the weakest parts. On the anodic sites two main reactions occur:

Eq. (9) shows that at anodic sites a more acidic (pH = 3–4) environment is created. The chloride ions facilitate the anodic dissolution of aluminum, forming aluminum chloride. The later hydrolyzes to form the hydroxide and acid, which shifts the pH to acidic values. Eqs. (10)–(12) show the possible reactions at the cathodic sites:

As seen in Eq. (12), the cathodic sites are more alkaline due to the local hydroxide formation. The presence of oxygen is crucial for pitting. Cone-shaped accumulations of corrosion products are formed at the mouth of the pits due to the precipitation of aluminum hydroxide outside the pits (Winston Revie, 2011), as seen in Fig. 1.

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Figure 1

Pitting corrosion of a Cu- and Fe-containing aluminum alloy (Winston Revie, 2011).

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2.3 The influence of alloy composition on corrosion resistance

Based on the strengthening method used, commercial wrought aluminum alloys can be divided into two groups. Series 1xxx, 3xxx, 4xxx, and 5xxx alloys are non-heat treatable, while series 2xxx, 6xxx, and 7xxx alloys are heat-treatable. Table 1 presents the alloy composition of these series.

General, galvanic, pitting, stress-corrosion cracking, intergranular, and exfoliation are among the most common types of corrosion for aluminum, independent of the corrosive environment (Davis, 2000; Winston Revie, 2011). The non-heat treatable alloys have a higher corrosion resistance toward general corrosion compared to the heat treatable alloys. However, series 6xxx alloys containing the Al-Mg₂Si system also show considerable resistance to general corrosion (Winston Revie, 2011). The same behavior is observed for series 7xxx alloys that do not contain copper (Al-Zn-Mg). The alloys’ resistance to pitting corrosion increases significantly with increasing purity. The pitting corrosion resistance decreases in the order series 1xxx > series 5xxx (especially when containing less than 3 wt.% Mg) > series 3xxx > series 6xxx > series 7xxx > series 2xxx (Davis, 2000). Alloys containing more than 0.15 wt.% copper are more susceptible to pitting corrosion, especially in environments containing chlorides. An increase in iron content will lead to the promotion of pitting corrosion. A similar effect is observed in series 6xxx alloys (Davis, 1999). Aluminum alloys can be susceptible to intergranular corrosion if second-phase microconstituents are formed at grain boundaries. A corrosion potential of the alloy different from that of the matrix will also cause intergranular corrosion. A balanced content ratio of the magnesium and silicon present in the 6xxx series will lead to higher resistance to intergranular corrosion. The presence of appreciable amounts of soluble alloying elements, such as copper, magnesium, silicon, and zinc, will make these alloys susceptible to stress-corrosion cracking. The same is observed for the heat treatable alloys (series 2xxx and 7xxx, as well as series 5xxx containing more than 3 wt.% Mg) (Davis, 2000).

3.1 Organic corrosion inhibitors in alkaline solutions

3.2 Organic corrosion inhibitors in chloride solution

Sherif and Park evaluated the inhibition effectiveness of 1,4-naphtoquinone (NQ) as a corrosion inhibitor of 99.99% aluminum in aerated and de-aerated 0.5 M NaCl solutions at 25 °C, using the PDP and EIS techniques (Sherif and Park, 2006). The inhibition effectiveness in both aerated and de-aerated solutions increased with increasing NQ concentration. The highest inhibition effectiveness was obtained for the addition of 1 mM NQ. The addition of the inhibitor shifted the corrosion and breakdown potentials to more positive potentials. The shifts are larger in de-aerated than aerated solution. Based on the scanning electron microscope (SEM) images and quartz crystal analyser measurements, the authors concluded that NQ molecules strongly adsorb on the aluminum surface, protecting it from further anodic dissolution. Sherif and Park also investigated the ability of 1,5-naphthalenediol to protect 99.99% aluminum in 0.5 M NaCl solution, using different electrochemical techniques (Sherif and Park, 2005). The inhibition effectiveness increased with increasing compound concentration. The compound decreased both the cathodic and anodic current, and consequently the corrosion current and shifted the corrosion and breakdown potentials to more positive potentials. The authors concluded that the 1,5-naphthalenediol molecules are adsorbed on the aluminum surface, covering up the preformed pits and protecting them from further anodic dissolution. In addition, it was suggested that the aluminum oxides/hydroxides are blocked by the corrosion inhibitor from forming complexes with chloride ions. Sherif reported on the effects of 3-amino-1,2,4-triazole-5-thiol (ATAT) on the corrosion inhibition of 99.99% aluminum in freely aerated stagnant 3.5 wt.% NaCl solution, using the cyclic polarization (CP) and EIS techniques (Sherif, 2012). The inhibition effectiveness increased with increasing concentration of ATAT. The presence of this compound increased the polarization resistance of aluminum in chloride solution. The cathodic and anodic corrosion and passive currents decreased upon the addition of ATAT, while the corrosion and pitting potential were shifted to more negative potentials. According to the authors, the ATAT molecules not only repaired the flawed areas on the oxide film on the aluminum surface, but also prevented the formation of the soluble aluminum chloride and oxychloride complexes.

El-Shafei et al. reported on the use of indole, tryptamine and tryptophane as corrosion inhibitors for pure aluminum in 0.1 M NaCl solution, using the PDP technique (El-Shafei et al., 2004). The authors reported that the inhibitive action was dependent on the number of active centers in the corrosion inhibitor molecule, molecular size, and the mode of adsorption. The inhibition effectiveness was found to increase with increasing compound concentration and followed the order indole < tryptophane < tryptamine.

Garrigues et al. studied the role of 8-HQ as a corrosion inhibitor for aluminum with purity higher than 99% in neutral (0.5 M NaCl) and acidic solutions (pH = 3), using the PDP and EIS techniques (Garrigues et al., 1996). The authors showed that the passive aluminum layer is strengthened, particularly for long immersion times, due to the formation of an aluminum-chelate complex. They concluded that the adsorption of 8-HQ prevented the adsorption of chloride ions and the destruction of the oxide layer on the aluminum surface in acidic solutions (pH = 3). The authors found that 8-HQ does not modify the corrosion mechanism of aluminum, due to the solubility of the aluminum chelate in acidic media.

Zor and Saǧdinç reported on the inhibition effectiveness of sulfathiazole in the corrosion of 99.9% aluminum in 0.1 M NaCl solution at 22–60 °C, using the PDP technique (Zor and Saǧdinç, 2014). The inhibition effectiveness increased with increasing inhibitor concentration. The PDP measurements showed that sulfathiazole acted as a mixed-type inhibitor. Based on the SEM analysis, the authors suggested that the corrosion is inhibited by the formation of a protective layer on the aluminum surface. Quantum chemical studies showed that this adsorption occurs through the oxygen and nitrogen atoms. The protonated form of sulfathiazole showed a considerable tendency to adsorb on the metal surface. The authors reported that the adsorption process has a mixed nature (physisorption and chemisorption).

Solmaz et al. investigated the inhibition effectiveness of citric acid as a corrosion inhibitor for 99.9% aluminum in 2 M NaCl (pH = 2) solution, using the PDP, polarization of resistance (R_p), and EIS techniques (Solmaz et al., 2008). The inhibition effectiveness increased with increasing citric acid concentration up to 1 · 10⁻⁵ M of inhibitor added and then decreased for higher concentrations. However, the results presented showed a poor correlation between the η calculated values from the PDP and EIS, on one hand, and R_p measurements, on the other, especially at low concentrations of citric acid. The lowest values reported, for the same concentration of citric acid added, are 37.5% and 70.2%, from the PDP and EIS measurements, respectively. The PDP measurements showed that citric acid acted as a mixed-type inhibitor.

Jevremović and Misković-Stanković studied the inhibition effectiveness of monoethanolamine (MEA) in the corrosion of 99.7% aluminum in 3 wt.% NaCl solution saturated with CO₂ at 20 °C, using the EIS, linear sweep voltammetry (LSV), and WL techniques (Jevremović and Stanković, 2012). The inhibition effectiveness of MEA increased with increasing concentration up to 5 mM, and then decreased for higher concentration values. The polarization measurements showed that upon the addition of MEA a shift in the corrosion potential toward more positive values occurred without a significant change in the anodic and cathodic Tafel slopes. Based on the thermodynamic calculations, the authors suggested that MEA physisorbed on the aluminum surface.

Banerjee et al. tested phthalic acid, o-phenylenediamine, and anthranilic acid as corrosion inhibitors for 99.15% aluminum alloy in 0.6 M NaCl solution at 30 °C, using the PDP technique (Banerjee et al., 2011). The authors reported that the inhibition effectiveness of all three compounds first increased with increasing compound concentration and then at a certain concentration decreased. The inhibition effectiveness followed the order phthalic acid < o-phenylenediamine < anthranilic. However, this was true only for the optimum concentration (4 · 10⁻⁵ M), but not for the lowest and highest concentration values. The authors attributed the inhibition order of the compounds to the existence of the N—O donor atom combinations in the molecules.

Popoola et al. investigated the inhibition effectiveness of ferrous gluconate as a corrosion inhibitor for 98.99% aluminum alloy in 0.05 M NaCl solution at 28 °C, using the WL, R_p, and PDP techniques (Popoola et al., 2013). The inhibition effectiveness of ferrous gluconate first increased with increasing compound concentration, up to 1.0 g/mL, and then decreased for higher concentrations. The PDP measurements showed that ferrous gluconate acted as a mixed-type inhibitor.

Lakshmi et al. reported on the inhibition effectiveness of diisopropyl thiourea (DISOTU) in the corrosion of 98.25% aluminum in 3.5 wt.% NaCl solution, using the WL, HE, PDP, and EIS techniques (Lakshmi et al., 2013). The inhibition effectiveness increased with increasing DISOTU concentration. The PDP measurements showed that DISOTU acted as a cathodic-type inhibitor.

Hakeem et al. studied the inhibition effectiveness of calcium gluconate as a corrosion inhibitor for 95% aluminum in aqueous solution containing 60 ppm Cl⁻ (pH = 11) with and without the addition of Zn²⁺, using the WL, PDP, and AC impedance techniques (Hakeem et al., 2014). The inhibition effectiveness increased with increasing calcium gluconate concentration. A further increase in the inhibition effectiveness was observed upon the addition of Zn²⁺, as shown by the WL measurements. However, there is a poor correlation between the inhibition effectiveness values calculated with the two electrochemical techniques under the same conditions (the η calculated from PDP was 55% while the AC impedance resulted in η = 25%). The authors attributed the inhibition action of calcium gluconate to the formation of a protective film on the aluminum surface consisting of Al³⁺-calcium gluconate complex and Zn(OH)₂. The existence of this film was confirmed by SEM and AFM analyses. The same was previously reported, which would suggest that this is the possible mechanism involving organic corrosion inhibitors and Zn²⁺ (Kalaivani et al., 2013a).

Ho et al. investigated cerium dibutylphosphate (Ce(dbp)₃) as a corrosion inhibitor for AA2024-T3 aluminum alloy in 0.001 M NaCl solution, using the WL, PDP, and CV techniques (Ho et al., 2006). The inhibition effectiveness increased significantly with increasing exposure time and with increasing compound concentration. The authors suggested that Ce(dbp)₃ is a better corrosion inhibitor for aluminum at higher chloride concentrations due to easier deposition of the inhibiting film when some corrosion has already taken place. Based on the focused ion beam secondary-ion mass spectroscopy analysis, the authors reported the presence of a 300–500 nm thick cerium-containing layer on the aluminum surface after 10 days of immersion. Next, Ce(dbp)₃ was also tested as corrosion inhibitor for AA2024-T3 in 0.05 M NaCl solution, using the EIS and SEM- energy dispersive X-ray spectroscopy (EDS) techniques (Garcia et al., 2013). The authors concluded that the inhibition action of Ce(dbp)₃ is due to the formation of cerium oxide on Cu-rich areas, combined with the deposition of the dibutylphosphate on the surface. As a result, a stable organic layer was formed that isolated the aluminum surface from the Cl⁻ ions and enhanced the Ce deposition on Cu-rich sites. In addition, García et al. investigated the influence of pH on the inhibition effectiveness of Ce(dbp)₃ as an inhibitor in the corrosion of AA2024-T3 aluminum alloy in 0.1 M NaCl solution, using the PDP and high-throughput (multielectrode) technique (García et al., 2010). The inhibition effectiveness of Ce(dbp)₃ was studied as a function of pH, at pH values 2, 4, 5.5, 7, 9, and 11. The solution’s pH was adjusted by the addition of NaOH and HCl. The inhibition effectiveness of Ce(dbp)₃ at 10⁻⁴ M concentration is highly affected by the pH of the solution. The authors reported the highest anticorrosive properties in the neutral pH range (5.5–7). According to them, both techniques presented similar trends as regards protection throughout nearly the full pH range. However, it is clear from the data that there is a poor correlation between the protection efficiencies calculated from the two techniques. This is true for the whole pH range studied. In two cases (pH = 2 and 9) the two techniques yield different conclusions as regards the performance of the inhibitor (one suggested protection and the other the acceleration of corrosion). The authors attribute these changes to solution changes due to speciation and precipitation in solution rather than to the multielectrode setup. Moreover, the highest inhibition effectiveness values are not obtained only for the neutral range (pH = 5.5–7), as claimed by the authors. The inhibition value at pH = 9 is high as well (η = 88.22% as reported from the PDP measurements). Moreover, in our opinion, the addition of HCl is not the best way to adjust pH as one introduces additional source of chlorides in the solution that additionally increases the corrosion rate. This will lead to an overestimation of the pH influence on the corrosion rate.

Lamaka et al. studied the inhibition effectiveness of eleven organic compounds, i.e., salicylaldoxime (SAL), 2-mercaptobenzothiazole, 8-HQ, thioacetamide, quinaldic acid, α-benzoionoxime, 2-(2-hydroxyphenyl) benzoxazole, dithiooxamide, cuprizone, and cupferron, in the corrosion of AA 2024-T3 aluminum alloy in 0.05 M NaCl solution (Lamaka et al., 2007). The highest inhibition effectiveness was achieved for salicylaldoxime, 8-HQ, and quinaldic acid. The authors attributed the inhibition effect to the passivation of active intermetallic zones, as well as to the formation of a chelate layer on the alloy surface. According to the authors, the chemisorption and precipitation of complexes occur on the alloy surface, including active S-phases (these are intermetallic zones, present in about 60% of all intermetallic inclusions, and are composed of Al₂MgCu). They showed that these compounds acted as mixed-type inhibitors. On the other hand, Snihirova et al. (2015) also studied the inhibition effectiveness of 8-HQ, SAL and 2,5-dimercapto-1,3,4-thiadiazolate (DMTD) and the combinations of these compounds as well as with Ce³⁺, as corrosion inhibitors for AA2024-T3 aluminum alloy in 0.05 M NaCl solution, using the EIS technique. Based on attenuated total reflectance FTIR Kretschman spectroscopy the authors concluded that 8-HQ and SAL adsorb on the aluminum surface forming a thin film. In case of DMTD the SEM-EDS analysis showed formation of precipitates or adsorbates on the Cu-rich intermetallic particles of the alloy. The inhibition effectiveness of these compounds increased when combined. Based on the EIS measurements the authors reported that for short immersion times the highest inhibition effectiveness was observed for the SAL-8-HQ mixture. The mixture Ce-SAL performed better at long immersion times.

Zheludkevich et al. investigated the inhibition effectiveness of four triazole and thiazole derivatives, i.e., 1,2,4-triazole (TA), 3-amino-1,2,4-triazole (ATA), benzotriazole (BTA), and 2-mercaptobenzothiazole (2-MBT), in the corrosion of AA 2024 alloy in neutral chloride solutions (0.05 M NaCl), using the direct current (DC) polarization and EIS techniques (Zheludkevich et al., 2005). The data showed that 2-MBT and BTA are the most effective inhibitors for long-term protection of the 2024 aluminum alloy under these conditions. As a result of the usage of 2-MBT precipitation, a deposit occurs on the copper-rich particles, reducing the anodic and cathodic reactions of the corrosion couple.

Hu et al. synthesized cerium tartrate and tested it as a corrosion inhibitor for AA2024-T3 aluminum alloy in 0.05 M NaCl solution (Hu et al., 2015). The authors reported that this compound affected both anodic and cathodic corrosion processes, rapidly forming a protective film on the alloy surface. Surface characterization revealed that this protective film inhibited the dealloying of the Al₂CuMg phase in the initial stage and then cerium ions transformed into cerium oxide/hydroxides and appeared at the Al₂CuMg phase, blocking further corrosion at those corrosion sites.

Shi et al. synthesized cerium cinnamate and tested it as an inhibitor in the corrosion of AA2024-T3 aluminum alloy in 0.05 M NaCl solution, using electrochemical and surface analytical techniques (Shi et al., 2011). The authors reported that this compound significantly inhibits corrosion during the initial 72 h of immersion and that it acts as an anodic-type inhibitor. However, the PDP measurements showed that this ability became less effective after 168 h of immersion. The electrochemical techniques (PDP and EIS) indicated a change from anodic inhibition through film deposition, to mixed-type inhibition by the cinnamate group and cerium ions. The XPS measurements showed that the carboxylic group in the cinnamate bonded with the Al matrix, while Ce³⁺ transformed into cerium oxide/hydroxide with valences of Ce(III) and Ce(IV). The PDP measurements showed that cerium cinnamate acted as a mixed-type inhibitor retarding the onset of corrosion from the second phase particles and surrounding Al matrix.

Markley et al. tested cerium diphenyl phosphate (Ce(dpp)₃) and mischmetal diphenyl phosphate (Mm(dpp)₃) as inhibitors in the corrosion of AA2024 aluminum alloy in 0.1 M NaCl solution, using the CP technique (Markley et al., 2007). Ce(dpp)₃ inhibited the cathodic reaction, while Mm(dpp)₃ acted as a mixed-type inhibitor. Mm(dpp)₃ gave higher inhibition effectiveness under the studied conditions (after 30 min and 24 h of immersion) compared to Ce(dpp)₃. The authors employed Raman mapping and confirmed the presence of the inhibitors on the alloy surface.

Catubig et al. investigated the inhibition effectiveness of cerium mercaptoacetate (Ce(Macet)₃) and praseodymium mercaptoacetate (Pr(Macet)₃) as corrosion inhibitors for AA2024-T3 aluminum alloy in 0.1 M NaCl solution (pH = 6), using the PDP and SEM techniques (Catubig et al., 2014). Based on the PDP measurements, the authors reported that the increase in concentration of these compounds led to the dominance of the inhibition of the cathodic reaction, while inhibition of the anodic reaction dominated with increasing immersion time. They concluded that Pr(Macet)₃ is a better inhibitor than Ce(Macet)₃.

Zhou et al. studied the inhibition effectiveness of a mixture of sodium dodecylbenzenesulfonate (SDBS) and lanthanum chloride in the corrosion of AA2024-T3 in 0.58 g/L NaCl solution (pH = 10), using the electrochemical and surface analysis methods (Zhou et al., 2015). The authors reported a reduction in the corrosion current density with increasing SDBS concentration. The inhibition effectiveness of the two compounds after 24 h was lower than that of their mixture; therefore, no synergistic effect was observed. According to the authors, in the earlier stage the DBS (an ion from SDBS) adsorbed on the aluminum alloy surface, promoting Al dissolution by the formation of Al(OH)₃, which induces passivation. On the other hand, the oxides or hydroxides of lanthanum and La(DBS)₃ deposited on the surface, forming a layer that mitigates corrosion. The passive layer was damaged with time due to the formation of pits around the Cu-rich phases from the galvanic effect between the inclusion and the aluminum substrate.

Balaskas et al. evaluated the performance of 2-MBT, BTA, and 8-HQ as corrosion inhibitors for AA2024-T3 aluminum alloy in 3.5 wt.% NaCl solution, using the EIS, PDP, and image-assisted electrochemical noise techniques (Balaskas et al., 2015). However, from the experimental data it can be noticed that there is a poor correlation between the data obtained with the different techniques. PDP showed that only 2-MBT significantly decreased both the anodic and cathodic reaction rates. 2-MBT protected the second phase particles, preventing dealloying, trenching, and the initiation of corrosion. BTA retarded only the anodic reaction. By comparing PDP curves for the 8-HQ-inhibited and non-inhibited solutions, a similar behavior was observed. On the other hand, 8-HQ was previously studied as a corrosion inhibitor for AA2024 aluminum alloy, but in lower concentrations of the corrosive media (0.05 M NaCl) by Lamaka et al. (2007). They found that saturated 8-HQ is an effective inhibitor and that it decreased the corrosion rates of both the anodic and cathodic reactions. In addition, Li et al. reported on the corrosion behavior of AA2024-T3 aluminum alloy in 3.5 wt.% NaCl solution in the presence of 8-HQ and 8-hydroxy-quinoline-5-sulfonic acid (HQS), using the PDP and EIS techniques (Li et al., 2007). The PDP measurements showed that 8-HQ acted as a mixed-type inhibitor by blocking the active sites of the metal surface, while HQS promoted corrosion by activating the cathodic reaction. The authors reported that 8-HQ absorbed on the metal surface, leading to the formation of a protective layer. They concluded that the inhibition action of 8-HQ is due to the insoluble aluminum-chelate Al(HQ)₃, which prevents the adsorption of the chloride ion. According to the authors, HQS breaks down the oxide film.

Li et al. studied the efficiency of tryptophan as an inhibitor in the corrosion of AA2024 aluminum alloy in 3.5 wt.% NaCl and 20 wt.% CaCl₂ solutions using the WL, polarization, and EIS techniques (Li et al., 2011). The inhibition effectiveness increased with increasing tryptophan concentration. The compound acted as an anodic inhibitor in both 20 wt.% CaCl₂ and 3.5 wt.% NaCl solutions. Based on the quantum chemical calculations, the authors reported that interactions between the metal surface and tryptophan occur mainly over the indole ring plane.

Balbo et al. evaluated the inhibition effectiveness of four anionic surfactants, i.e., sodium salts of N–lauroylsarcosine (NLS), N–lauroyl–N–methyltaurine (NLT), dodecybenzensulfonic acid (DBS), and sodium lauryl sulfate (LS), in the corrosion of AA2198 aluminum alloy in 0.01 M NaCl solutions, 1–168 h of immersion at 25 °C, using the PDP, LSV, and EIS techniques (Balbo et al., 2013). These surfactants affected both the cathodic and anodic corrosion reactions, shifting the breakdown potential to more positive potentials. NLS and DBS were the most efficient inhibitors. The addition of these surfactants rendered the natural aluminum passive layer hydrophobic. The Al-surfactant salt precipitation plugged the pores in the passive layer, further increasing in this manner the inhibition effectiveness.

Shahrabi et al. tested 2-butine-1,4-diol and potassium sodium tartrate as corrosion inhibitors for AA3003 aluminum alloy in 0.5 wt.% NaCl solution at room temperature, using the PDP and EIS techniques (Shahrabi et al., 2008). The inhibition effectiveness of both compounds increased with increasing compound concentration. The Tafel curve measurements indicated that these compounds acted as mixed-type inhibitors. Thermodynamic calculations showed that these compounds physisorbed on the aluminum surface. The inhibition effectiveness increased further when the two compounds were combined. The authors reported that the optimum concentration ratio of potassium sodium tartrate to 2-butine-1,4-diol is 2:1.

Yazdzad et al. studied the inhibition effectiveness of propargyl alcohol and potassium sodium tartrate, as well as their mixture in the corrosion of AA3003 aluminum alloy in 0.5 wt.% NaCl solution, using the Tafel extrapolation and EIS techniques (Yazdzad et al., 2008). The inhibition effectiveness increased with increasing compound concentration. Both inhibitors showed optimum protection at concentrations of 1–1.5 mM when used separately. The inhibition effectiveness of potassium sodium tartrate was previously investigated by the same group (Shahrabi et al., 2008) and the values presented here are in accordance with those. The polarization measurements showed that these compounds acted as mixed-type inhibitors. Thermodynamic calculations indicated that both compounds physisorbed on the aluminum surface. When combined together, a synergistic effect was observed, resulting in higher inhibition effectiveness. The authors reported that the optimum concentration ratio of potassium sodium tartrate to propargyl alcohol is 5:2.

Wang et al. reported on the use of 8-aminoquinoline (8-AQ) and 8-nitroquinoline (8-NQ) as effective corrosion inhibitors for AA5052 aluminum alloy in 3 wt.% NaCl solution, using the WL and electrochemical techniques (Wang et al., 2015). 8-NQ was found to be a better inhibitor for the alloy compared to the 8-AQ. The authors concluded that both inhibitors form a protective film on the alloy surface, retarding the anodic reaction. Based on density functional theory calculation it was reported that a strong hybridization occurs between the p-orbital of reactive sites in the inhibitor molecules and the sp-orbital of the Al atom.

Hill et al. reported on the corrosion resistance of 7000 series aluminum alloys, i.e., AA7022, AA7050, and AA7075, in 0.1 M NaCl solution in the presence of ceryl diphenyl phosphate, using the EIS technique (Hill et al., 2011). There is no significant change in the inhibition effectiveness values calculated from the EIS measurements taken at 30 min, 3 days, and 6 days. The inhibition effectiveness for the same concentration of ceryl diphenyl phosphate slightly increased in the order AA7075 > AA7022 > AA7050. ToF-SIMS data indicated the presence of a complex Ce/Al organophosphate film, not necessarily homogeneous on the surface. The authors proposed that this film consisted of hydrolysis products of the inhibitor and aluminum oxide.

Liu et al. tested 8-HQ as a corrosion inhibitor for AA7075 aluminum alloy in 3.5 wt.% NaCl solution, using the EIS and PDP techniques. 8-HQ was tested in aqueous 3.5 wt.% NaCl and ethanol 3.5 wt.% NaCl solutions (Liu et al., 2014). The inhibition effectiveness increased with increasing compound concentration. However, it has to be noted that there is a poor correlation between the data obtained from the two different techniques, especially for lower concentrations. Based on the PDP measurements, the authors concluded that 8-HQ inhibited the corrosion process by reducing the rate of the anodic and cathodic reactions on the alloy surface. This is in agreement with what was found when 8-HQ was used as a corrosion inhibitor for AA2024 alloy in 0.05 M NaCl and in 3.5 wt.% NaCl solutions (Lamaka et al., 2007; Li et al., 2007).

Yurt et al. performed quantum chemical studies on the inhibition effectiveness of six amino acids, i.e., glycine, aspartic acid, valine, alanine, phenylalanine, and glutamic acid at pH = 5, and six hydroxy carboxylic acids, i.e., glucolic acid, malic acid, lactic acid, mandelic acid, benzylic acid, and citric acid at pH = 8, in the corrosion of AA7075 aluminum alloy in 0.05 M NaCl solution (Yurt et al., 2005). The authors concluded that both physisorption and chemisorption played an important role in the inhibition of pitting corrosion.

Qafsaoui et al. investigated the role of 1-pyrrolidine dithiocarbamate (PDTC) as a corrosion inhibitor for pure aluminum and AA2024-T3 aluminum alloy in 0.2 g/L NaCl solution, using the WL and electrochemical techniques (Qafsaoui et al., 2015). They concluded that PDTC has only a small influence on the corrosion of pure aluminum, but strongly inhibited the alloy corrosion. According to the authors, this is due to the presence of copper-rich particles in the alloy. PDTC increased the galvanic coupling resistance through strong interaction with the Cu-rich cathodic sites associated with the intermetallics through its sulfur atoms. The active cathodic surface sites are reduced as a result of a stable and adherent Cu(I)-PDTC complex formed by the interaction between PDTC and these particles. SEM-EDX analysis showed that PDTC adsorbed preferentially on the Al-Cu-Mg particles.

Vrsalović et al. reported on the inhibition effectiveness of sinapinic acid as a corrosion inhibitor for Al-2.5 Mg (97.5% aluminum) alloy in 0.5 M NaCl solution, using a rotating disk electrode at 20–50 °C and the PDP, R_p, and EIS techniques (Vrsalovic et al., 2007). The inhibition effectiveness increased with increasing concentration of sinapinic acid, but decreased with increasing temperature. The PDP measurements showed that this compound acted as a cathodic-type inhibitor. Based on the EIS measurements, the authors concluded that a thicker protective film formed on the aluminum surface as a result of the sinapinic acid adsorption. Physisorption was the proposed adsorption mechanism for this compound. Vrsalović et al. investigated two phenolic acids, i.e., sinapinic and gentisic acid, as corrosion inhibitors for Al-0.8 Mg aluminum alloy (98.9% aluminum) in 0.5 M NaCl solution at 20 °C, using the PDP, R_p, and EIS techniques and a rotating disk electrode (Vrsalović et al., 2009). The inhibition effectiveness increased with increasing acid concentration, but decreased with increasing electrode rotation rate. Sinapinic acid was found to be a better inhibitor than gentisic acid under the same conditions. The PDP measurements showed that both phenolic acids acted as cathodic-type inhibitors. The authors explained the inhibition action of these acids with the formation of complex molecules with aluminum ions, which protected the metal through deposition in places where the oxide film was destroyed. They proposed physisorption as the possible adsorption mechanism for both acids. The same conclusions were previously reported for the corrosion inhibition of Al-2.5 Mg (97.5% aluminum) in 0.5 M NaCl solution by sinapinic acid using a rotating disk electrode (Vrsalovic et al., 2007).

Önal and Aksüt studied the inhibition effectiveness of tolyltriazole (TTA) as a corrosion inhibitor for four aluminum alloys, i.e., Al-8%Si-3%Cu, Al-4%Cu, Al-12%Cu, and Al-22%Cu-4%Fe, in 1 M NaCl solution (pH = 6 and 11), at 15–35 °C, using the PDP and R_p techniques (Önal and Aksüt, 2000). The inhibition effectiveness increased with increasing TTA concentration, but decreased with increasing temperature and pH. The authors tested TTA in 1 M HCl solution (pH = 0.5) as well, in which TTA performed better than in solutions at pH 6 and 11. Moreover, it was shown that TTA is a more effective inhibitor at pH = 6 than at pH = 11. The PDP measurements showed that TTA acted as a mixed-type inhibitor in all solutions. The authors concluded that the inhibition effect of TTA is due to its adsorption on Cu particles and the formation of a Cu(I)-TTA film. They suggested that this film is formed from the interaction of CuCl₂⁻ complex with TTA (TTAH₂⁺, TTAH or TTA⁻ form).

Harvey et al. reported on the inhibition effectiveness of Na-mercaptopropionic acid, BTA, Na-(diethyl(dithiocarbamate)), Na-4-mercaptobenzoate, Na-(6-mercaptonicotinate), Na-mercaptoacetic acid, 2-mercaptobenzothiazole, thiophenol, 2-mercaptobenzimidazole, 2-mercaptopyrimidine, 6-amino-2-mercaptobenzothiazole, and Na-2-mercaptobenzoate as corrosion inhibitors for AA2024 aluminum alloy in 0.1 M NaCl at 21 °C, using the WL technique (Harvey et al., 2011). The same technique was used to study the corrosion resistance of AA7075 aluminum alloy in 0.1 M NaCl solution in the presence of Na-(diethyl(dithiocarbamate)), 6-amino-2-mercaptobenzothiazole, BTA, 2-mercaptobenzothiazole, Na-(6-mercaptonicotinate), 2-mercaptobenzimidazole, 4,5-diaminopyrimidine, Na-mercaptoacetic acid, 4,5-diamino-2,6-dimercaptopyrimidine, Na-2-mercaptobenzoate, Na-4-mercaptobenzoate, and Na-2-mercaptonicotinate. The authors reported that the corrosion of AA2024 aluminum alloy was promoted and not inhibited in the presence of pyrimidine, Na-benzoate, pyridine, Na-(4-phenylbenzoate), Na-4-hydroxybenzoate, Na-nicotinate, Na-isonicotinate, Na-salicylate, and Na-acetate. In the case of AA7075 aluminum alloy, corrosion was promoted by pyrimidine, Na-benzoate, pyridine, Na-(4-phenylbenzoate), Na4hydroxybenzoate, Nanicotinate, Na-isonicotinate, Na-3-mercaptobenzoate, Na-salicylate, and 2,5-dimercapto-1,3,4-thiadiazole. It was found that several functional groups or the presence of certain atoms significantly improved the corrosion inhibition ability of the corrosion inhibitor. According to the authors, the presence of a thiol group, para- and ortho-positions of carboxylate on a monoaromatic ring, and the substitution of N for C in an aromatic ring where it may form a coordinating site with the carbonyl or nitrogen, play an important role therein. They concluded that the presence of a hydroxyl group in the corrosion inhibitor structure was slightly inhibitive, while the carboxylate group provided little or no corrosion inhibition on its own. Thiol-containing compounds were more effective on AA2024 alloy than on AA7075 alloy, and the authors attributed that to the higher amount of Cu in the former. Two compounds, i.e., 6-amino-2-mercaptobenzothiazole and 4,5-diamino-2,6-dimercaptopyrimidine, show good corrosion inhibition for both alloys at lower concentration (approx. 0.1 and 0.5 mM, respectively, according to their maximum solubility). Compounds having the (HS—C⚌S) group were also found to be very effective corrosion inhibitors.

Qafsaoui et al. studied the inhibition effectiveness of BTA in the corrosion of 99.999% aluminum and AA2024-T3 aluminum alloy in a 0.1 M Na₂SO₄ + (0.001–0.5) M NaCl mixture, using the electrochemical noise technique (Qafsaoui et al., 2009). The authors reported that at high chloride concentrations (0.5 M NaCl) the addition of BTA inhibited uniform corrosion, resulting in a decrease in the amplitude of the potential and current fluctuations. Moreover, SEM analysis confirmed that neither the Al matrix nor the Cu-rich particles were attacked. The authors attributed the inhibition effect of BTA to its ability to form a polymeric film with Cu(I), which hinders the oxygen reduction reaction. However, BTA was unable to prevent 99.999% aluminum corrosion. At low chloride concentrations (0.001 and 0.1 M NaCl), the authors reported that BTA protected the AA2024 aluminum alloy from pitting corrosion, even after one week of immersion (lower potential and current fluctuations were observed in the inhibited solutions).

Rodič and Milošev tested cerium(III) acetate as a corrosion inhibitor for 99% aluminum and AA2024-T3 and AA7075-T6 alloys in 0.1 M NaCl solution at 25 °C, using the PDP technique (Rodič and Milošev, 2016). The authors attributed the inhibitory action of this compound to the type of substrate and the type and concentration of the anion. The inhibition effectiveness was found to decrease with increasing Ce(Ac)₃ concentration (from 1 to 4 mM, in the case of 99% aluminum, and from 1 to 5 mM, in the case of the AA7075-T6 aluminum alloy). The authors also tested two other inorganic cerium(III) salts, i.e., cerium(III) nitrate and chloride. They reported that Ce(Ac)₃ was the most effective inhibitor for all three materials studied. The authors also showed that the corrosion inhibition effectiveness of Ce(Ac)₃ followed the order AA7075-T6 > 99% Al > AA2024-T3. The changes in pH with the addition of Ce(Ac)₃ not only affect the stability of the intermetallic particles, but also that of the surrounding aluminum oxide matrix.

Sherif studied the corrosion of 99.99% aluminum in aerated Arabian Gulf Seawater (AGS) and 3.5 wt.% NaCl solution in the presence of 3-amino-5-mercapto-1,2,4-triazole (AMTA), using the CP and EIS techniques (Sherif, 2011). The measurements showed that AGS is more corrosive than the 3.5 wt.% NaCl solution. However, AMTA protects aluminum better in AGS than in 3.5 wt.% NaCl solution. The inhibition effectiveness increased with increasing AMTA concentration. The corrosion and pitting potentials of aluminum were shifted toward less negative values when AMTA is added to the corrosion environment. The compound reduced the corrosion rate by preventing the formation of soluble chloride and oxychloride complexes on the aluminum surface.

Marcelin and Pébère showed that 8-HQ and BTA are two effective corrosion inhibitors for AA2024 aluminum alloy in a mixture of 0.1 M Na₂SO₄ + 0.05 M NaCl (Marcelin and Pébère, 2015). The mixture of these two inhibitors showed a synergistic effect, with BTA acting mainly on the Cu-rich intermetallic particles, while 8-HQ acted on the aluminum matrix due to the chelating properties. The galvanic coupling between the aluminum matrix and the intermetallic particles is strongly limited when the inhibitor mixture is used compared to the inhibitors used separately.

Boisier et al. tested sodium decanoate as a corrosion inhibitor for AA2024 aluminum alloy in a mixture of 0.1 M Na₂SO₄ + NaCl at pH values of 4, 6, 8, and 10, using different electrochemical techniques (Boisier et al., 2010). This compound formed a hydrophobic layer on the aluminum surface (as shown from the high contact angle values at pH = 6 solution), preventing the attack on the passive layer by the chloride ions. Moreover, the addition of sodium decanoate limited the galvanic coupling between the intermetallic particles and the surrounding matrix. The EIS data showed that at pH 6 and 8, the pH range where the alloy was in the passive state, the inhibitor efficiency was constant and higher than for the lowest (pH = 4) and highest (pH = 10) pH values.

Sherif investigated the corrosion resistance of 99.99% aluminum in naturally aerated stagnant Arabian Gulf seawater with and without the addition of 3-amino-1,2,4-triazole-5-thiol (ATAT), using the EIS, CP, and potentiostatic current-time techniques (Sherif, 2013). The authors reported an increase in both the solution and polarization resistances, as well as a shift in the corrosion and pitting potentials to more negative potentials when ATAT was added, indicating a decrease in the uniform corrosion rate. These findings are in accordance with what was reported previously when the same compound (ATAT) was studied in the corrosion of 99.99% aluminum in 3.5 wt.% NaCl solution (Sherif, 2012). The inhibition effectiveness increased with increasing ATAT concentration. There is a good correlation between the inhibition effectiveness reported by the author in aerated stagnant Arabian Gulf seawater (Sherif, 2013) and in aerated stagnant 3.5 wt.% NaCl solution (Sherif, 2012), for the same aluminum sample (99.99% Al) and the same ATAT concentrations used (1 mM and 5 mM). The authors concluded that the ATAT molecules adsorbed on the aluminum surface, increasing in this manner the stability of aluminum oxide.

Rosliza et al. (2008) studied the corrosion of AA6061 aluminum alloy in tropical seawater (no composition given) in the presence of sodium benzoate (SB), in static and air circulation conditions, using the PDP, EIS, and SEM techniques. SB acts as a cathodic-type inhibitor and no change occurred in the corrosion process due to an increase in either the immersion time or the inhibitor concentration. Under static conditions, the highest inhibition effectiveness was achieved after 180 days of exposure to the corrosive environment, while under air circulation conditions, after 60 days. The EIS measurements showed that the corrosion process was mainly kinetically-controlled. The charge transfer resistance increased with the addition of the inhibitor, while the capacitance decreased, indicating the formation of a surface film.

Rosliza et al. reported on the effect of vanillin on the corrosion inhibition of AA6061 aluminum alloy in seawater at 25 °C, using the PDP, R_p, and EIS techniques (Rosliza et al., 2010). The inhibition effectiveness increased with increasing compound concentration. The PDP measurements showed that vanillin acted as a mixed-type inhibitor. The EIS measurements showed that the corrosion process in the presence of vanillin was mainly kinetically-controlled, without changing the corrosion mechanism compared to the non-inhibited solutions. The authors concluded that vanillin reduced the overall corrosion rates by the formation of a thin film of precipitate on the aluminum surface.

Wan Nik et al. studied the corrosion of AA6063 aluminum alloy in seawater (no composition given) at room temperature in the presence of SB, using the WL, PDP, and EIS techniques (Wan Nik et al., 2013). The authors concluded that the inhibition effectiveness of SB decreased with increasing immersion time (30–180 days). However this was valid only up to 120 days of immersion. After 120 days the inhibitor efficiency increased again (to values close to those of 30 days) and then decreased again after 180 days. PDP data showed that SB acted as a cathodic-type inhibitor. The authors concluded that the corrosion process was mainly kinetically-controlled and no change in the corrosion mechanism occurred due to either the immersion time or the addition of inhibitor.

Zor and Özkazanç tested benzamide (BA), 4-aminobenzenesulfonamide (ABSA), and thioacetamide (TAA) as inhibitors in the corrosion of aluminum in 0.1 M NaCl solution, using the PDP and EIS techniques (Zor and Özkazanç, 2010). The inhibition effectiveness increased with increasing compound concentration and followed the order TAA > ABSA > BA. The EIS measurements showed that the charge transfer resistance increased with increasing inhibitor concentration, while the double layer capacitance decreased. The authors reported that these compounds adsorbed on the aluminum surface, forming a protective layer.

Ren et al. reported on the inhibition effectiveness of triisopropanolamine (TIPA) as a corrosion inhibitor for ADC12 aluminum alloy (no composition given) in 3 wt.% NaCl solution, at 25 °C, using the WL, PDP, and EIS techniques (Ren et al., 2015). The inhibition effectiveness increased with increasing TIPA concentration. The PDP measurements showed that TIPA acted as a mixed-type inhibitor. Based on the thermodynamic and kinetic parameters calculations, the authors suggested a mixed-type (both physisorption and chemisorption) adsorption mechanism for TIPA. The authors used quantum chemical calculations to conclude that TIPA adsorbed on the aluminum surface through the oxygen atom.