1-s2.0-S0010938X06002320-main

Corrosion Science 49 (2007) 1027–1044 www.elsevier.com/locate/corsci

Corrosion inhibition of aluminum alloy AA 2014 by rare earth chlorides Ajit Kumar Mishra, R. Balasubramaniam

*

Department of Materials and Metallurgical Engineering, Indian Institute of Technology, Kanpur 208 016, India

Received 16 August 2005; accepted 6 June 2006 Available online 20 September 2006

Abstract

The effect of LaCl 3 and CeCl3 inhibitor additions in 3.5% NaCl solution on the corrosion behaviour of aluminum alloy AA2014 has been investigated. Four different concentrations (250, 500, 750 and 1000 ppm) of LaCl3 and CeCl3 were studied. The polarization resistance increased significantly and the corrosion rate decreased by an order of magnitude with the addition of 1000 ppm of LaCl3 and CeCl3, with maximum decrease noticed for CeCl3. EIS studies showed that there was a significant increase in overall resistance after addition of 1000 ppm LaCl3 and CeCl3, when compared to the case without inhibitor. The double layer resistance and film resistance increased after inhibitor addition. Scanning electron microscopy confirmed formation of precipitates of oxide/hydroxide of lanthanum and cerium on cathodic intermetallic sites, which reduced the overall corrosion rate.  2006

Elsevier Ltd. All rights reserved.

Keywords: A. Aluminum alloy; A. Lanthanum chloride; A. Cerium chloride; B. Polarization; B. EIS; B. SEM;

C. Inhibitors

1. Introduction

Aluminum and its alloys are widely used in engineering applications because of their low densit density, y, fav favorab orable le mechani mechanical cal propert properties ies,, good good surface surface finish finish and relativ relatively ely good good corrosion resistance. Research efforts in the aeronautical industry have focused on the

*

Corresponding Corresponding author. E-mail address: [email protected] [email protected] c.in (R. (R. Balasubramaniam).

0010-938X/$ - see front matter doi:10.1016/j.corsci.2006.06.026

 2006

Elsevier Ltd. All rights reserved.

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A.K. Mishra, R. Balasubramaniam Balasubramaniam / Corrosion Science 49 (2007) 1027–1044

study of Al–Cu and Al–Zn alloys. The electrochemical behaviour of Al and its alloys has attracted the attention of many investigators. The natural oxide film on aluminum does not offer sufficient protection against aggressive anions. In this context, inhibitors are used to improve protective features of the surface. Currently, chromates are widely used in anticorrosive pre-treatments of aluminum alloys [1–5] alloys [1–5].. However, because of their high toxicity, an intense research effort is underway for their replacement. Rare earth chlorides have been tested as corrosion inhibitors for Al alloys like AA5083 [6] AA5083 [6],, AA7075 [7] AA7075 [7],, AA8090 [8] AA8090 [8],, AA6061 [9] and AA2024 AA2024 [10]. [10]. Thes Thesee rare rare eart earth h chlo chlori rides des act act as cath cathod odic ic inhi inhibit bitors ors [6,11,12].. [6,11,12] Bethencourt et al. [6] [6] observed, observed, from weight loss and polarization results, that lanthanum, cerium and samarium chlorides are effective uniform corrosion inhibitors of AA5083 in aerated 3.5% NaCl solution. As no pits were observed in samples immersed in solutions containing inhibitors, they concluded that these rare earth salts also act as pitting corrosion sion inhi inhibit bitor ors. s. Arno Arnott tt et al. al. [7] inves investig tigated ated the corros corrosion ion inhibit inhibition ion behavio behaviour ur of 1000 ppm concentration of different rare earth chlorides (YCl3, PrCl3, LaCl3 and CeCl3) and other salts such as FeCl2, CoCl2 and NiCl2, on AA7075 in 3.5% NaCl solution. The best degree of inhibition was achieved by CeCl3 addition. Davo´ and Damborenea [8] studied [8] studied the effect of different concentrations of LaCl3 and CeCl3 on the corrosion of AA 8090 in 3.56% NaCl solution. Maximum inhibition was obtained after addition of 1000 ppm CeCl3 and 250 ppm LaCl3. Cerium chloride inhibited intergranular corrosion more effectively than lanthanum chloride. Neil and Garrard [9] Garrard [9] studied studied the effect of cerium pre-treatments on AA6061 prior to immersion in 3.5% NaCl solution. They immersed the samples in 0.1 M NaCl/1000 ppm CeCl3 solution for one week and then transferred them to the test solution. They concluded that cerium pre-treatment decreased the corrosion susc suscep epti tibi bilit lity, y, but but the the effec effectt of the the trea treatm tmen entt was was shor shortt live lived. d. Aldyk Aldykew ewic iczz et al. al. [10] reported that the corrosion inhibition of cerium chloride addition in NaCl solution was related to the development of a cerium-rich film over the cathodic copper surface in the case of AA2024. Aballe et al. [11] al. [11] analyzed analyzed the effect of CeCl3, LaCl3 and mixture of both CeCl3 and LaCl3 on the corrosion of AA 5083 alloy in NaCl solution. They found that corrosion resistance offered by these rare earth metal chlorides was of same order as those found with classical Cr-based compounds. They further observed that there was a two-fold incr increa ease se in polar polariz izat atio ion n resi resist stan ance ce afte afterr addi additi tion on of 500 ppm ppm La LaCl Cl3, four four-f -fol old d afte afterr 500 ppm CeCl3 and nearly six-fold after addition of 250 ppm each of LaCl3 and CeCl3. They concluded that mixed solutions of LaCl3 and CeCl3 in optimum ratio showed a better performance than solely LaCl3/CeCl3 inhibitor. The aim of the present work was to study the effect of LaCl3 and CeCl3 inhibitors at different concentration levels on the corrosion behaviour of AA2014 in NaCl solution. AA2014 is one of the most corrosion prone Al alloys because of the presence of cathodic CuAl2 precipitates. 2. Experimental procedure

The 2014 alloy was obtained from HINDALCO, Renukoot, India in the form of a cylindrical rod of diameter 4.2 cm. The composition of the alloy 2014 was (in wt.%) 3.9–5.0 Cu, 0.2–0.8 Mg, 0.4–1.2 Mn, 0.5–1.2 Si, <0.7 Fe, <0.1 Cr and remainder Al. Specime Specimens ns for electro electroche chemica micall tests tests (of thickn thickness ess 6.5 mm) were were sectio sectioned ned from from the as-received rod. The cross section of the rod was exposed to the electrolyte in all the

A.K. Mishra, R. Balasubramaniam / Corrosion Science 49 (2007) 1027–1044

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experiments. All the surfaces of the specimens were mechanically polished down to fine emery paper (starting from grit number 220 to 1000, ANSI), de-greased with acetone and then rinsed in distilled water before being used for each electrochemical experiment. All this procedure was followed prior to each electrochemical experiment. All experiments reported in this paper were repeated a minimum of two times. Corrosion tests were carried out in freely aerated 3.5% NaCl solution. Lanthanum and cerium chlorides were added at concentrations of 250, 500, 750 and 1000 ppm. Freshly prepared NaCl solution was used in all the experiments. All the experiments were conducted at room temperature (25  C). Separate samples were immersed for 4 and 168 h in solutions containing different concentration of inhibitors, and later their surfaces were observed in a scanning electron microscope (SEM) (FEI QUANTA 200). Local compositions were studied with energy dispersive analysis using X-ray (EDAX) unit attached to the SEM. Electrochemical measurements were performed in a flat cell (Amtek, USA) using a 2263 PARSTAT (Amtek, USA) potentiostat controlled through a personal computer. In the flat cell, the area of exposure of the sample was 1 cm2. An Ag/A Ag/AgC gCll elec electr trod odee was was employed as the reference electrode. The potential of this electrode with respect to standard hydrogen electrode is +0.197 V. All the potential data presented in this paper are referred to this Ag/AgCl reference electrode potential. All electrochemical experiments were performed after stabilization of free corrosion potential potential (FCP). In conducting linear polarization experiments, experiments, the potential potential was scanned from the cathodic to anodic direction at a rate of 0.166 mV/s. The potential range for linear polariz polarizati ation on experi experimen ments ts were ±20 mV from from FCP. FCP. Potent Potentiody iodynami namicc polariz polarizati ation on curves curves were were obtain obtained ed from from 25 2500 mV to +1 +160 6000 mV from from FCP FCP usin usingg a scan scan rate rate of 1 mV/s. Electro Electroche chemica micall impedan impedance ce spectr spectrosc oscopy opy (EIS) (EIS) measure measuremen ments ts were were perform performed ed by applying a sinusoidal potential perturbation of 10 mV at FCP. The impedance spectra were measured with a frequency sweep from 100 kHz to 10 mHz in logarithmic increment. The impedance data was analyzed using the ZSimpWin 3.00 software (Amtek, USA). The impedance data were fitted to appropriate equivalent electrical circuit using a complex nonlinear least-squares fitting routine, using both the real and imaginary components of the data. Different parameters obtained from the best fit equivalent circuit were tabulated and analyzed. 3. Results and discussion

3.1. Polarization Polarization

All the experim experiment entss were were conduct conducted ed aft after er stabil stabiliza ization tion of free free corros corrosion ion potent potential ial (FCP). In all the cases, the FCP increased in the noble direction and stabilized. The linear polarization plots obtained after immersion in 3.5% NaCl solution, with and without LaCl3 additions are shown in Fig. in Fig. 1, 1, while those after CeCl3 additions in Fig. in Fig. 2. 2. Generally, the slope (i.e., the polarization resistance, R p) increased increased after inhibitor inhibitor additions. A higher in Table 1. 1. Rp value indicates lower corrosion rate. The estimated R p values are tabulated in Table The polarization resistance increased with increase in CeCl3 concentration (Table (Table 1). 1). For the case of LaCl3 addition, a similar feature was observed but for the anomalous data at 750 ppm LaCl3 addition.

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A.K. Mishra, R. Balasubramaniam Balasubramaniam / Corrosion Science 49 (2007) 1027–1044 -0.41 -0.42

2

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Fig. Fig. 1. Linear Linear polari polarizat zatio ion n curves curves in 3.5% 3.5% Na NaCl Cl soluti solution, on, with with and witho without ut LaCl LaCl 3 additions. (1–0 ppm, 2–250 ppm, 3–500 ppm, 4–750 ppm, 5–1000 ppm).

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Fig. Fig. 2. Linear Linear polari polarizat zation ion curves curves in 3.5% 3.5% Na NaCl Cl soluti solution, on, with with and witho without ut CeCl CeCl 3 additions. (1–0 ppm, 2–250 ppm, 3–500 ppm, 4–750 ppm, 5–1000 ppm).


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Table 1 Variation in polarization resistance after LaCl 3 and CeCl 3 additions Polarization resistance Rp (k X cm2)

Solution

LaCl3 3.5% 3.5% 3.5% 3.5% 3.5%

NaCl + 0 ppm NaCl + 250 ppm NaCl + 500 ppm NaCl + 750 ppm NaCl + 1000 ppm

CeCl3

2.19 7.12 20.52 6.98 104.75

2.19 18.83 53.82 103.62 205.21

The Rp values were higher for CeCl3 addition at all the concentrations compared to LaCl3 additions (Table (Table 1), 1), indicating that CeCl3 is a better corrosion inhibitor. This was in agreement with the results obtained by other authors for different Al alloys, like AA5083 [6] [6],, AA7075 [7] [7] and and AA8090 [8] [8].. The potentiodynamic polarization curves obtained after LaCl3 and CeCl3 additions are presented in Figs. in Figs. 3 and 4, 4, respectively. In both cases of inhibitor addition, the anodic portion remained similar whereas the cathodic portion shifted towards the left (i.e. to lower current densities), indicating decrease in overall corrosion rate. The polarization curves confirmed the cathodic nature of rare earth chloride inhibitors. The results of the present study are in conformity with earlier studies of LaCl3 and CeCl3 inhibitors where similar shifts in polarization curves were observed for AA5083 [6] [6] and and AA8090 [8] [8]..

0.0

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log (I) log (A/cm ) Fig. 3. Potentiodynamic Potentiodynamic polarization polarization curves in 3.5% NaCl solution, with and without LaCl 3 additions. (1–0 ppm, 2–250 ppm, 3–500 ppm, 4–750 ppm, 5–1000 ppm).

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log (I) log (A/cm ) Fig. 4. Potentiodynamic Potentiodynamic polarization curves in 3.5% NaCl solution, with and without CeCl 3 additions. (1–0 ppm, 2–250 ppm, 3–500 ppm, 4–750 ppm, 5–1000 ppm).

3.2. Electrochemical impedance spectroscopy

The EIS data (Bode phase plots and Bode magnitude plots) for 2014 alloy in 3.5% NaCl solution, with and without LaCl3, obtained at FCP, are presented in Fig. in Fig. 5. 5. The Bode phase plot indicates that two time constants could be identified in the EIS data for case without the addition of LaCl3, but only one time constant was exhibited for the data obtained with addition of inhibitor, as seen in Fig. in Fig. 5a. 5a. It is also apparent that the phase angle maxima are quite broad after LaCl3 addition. The Bode magnitude plot (Fig. 5b) 5b) also also indi indicat cates es two two slop slopes es witho without ut inhi inhibi bito torr solu solutio tion, n, where whereas as only only one one was was obtained after addition of LaCl3. This kind of behaviour behaviour has been attributed to a dual protection mechanism of the passive film, which behaves both as a barrier to corrosion and offers increased resistance to charge transfer processes [13] processes [13].. The surface of aluminium and its alloys is covered by a fine layer of oxide (Al2O3) generated during its handling. In aqueous solution, the surface oxide film is composed of Al2O3, Al(OH)3 and AlO(OH) phases [14] phases [14].. Lee and Pyun [15] Pyun [15] observed observed that the AlO(OH) phase enhances the passivity of the Al surface, and subsequently the corrosion resistance in chloride solution. One of the characteristics of this film is the discontinuities present in the zone occupied by the particles of intermetallic compounds in the alloy [16] alloy [16].. In view of this, two physical aspects of the corroding surface are the film and the metal–solution interface. It is anticipated that the locations where the intermetallics are present, the surface would be exposed to the environment and therefore, it is the double layer connected with intermetallic–solution interface that is really addressed while referring to metal–solution interface.


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0 ppm 250 ppm 500 ppm 750 ppm 1000 ppm

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Fig. 5. EIS plots in 3.5% NaCl solution, solution, with and without without LaCl 3 additions, at free corrosion potential: (a) Bode phase plots; and (b) Bode magnitude plots.

Surface films formed on 2014 without and after LaCl3 and CeCl3 addition was studied using scanning electron microscopy. microscopy. Intermetallic Intermetallic particles in 2014 alloy are mainly spherical CuAl2 (h-phase) and irregularly shaped Al–Cu–Fe–Mn–Si phase phase [17]. [17]. In the NaCl electrolyte, these intermetallics tend to be cathodic to the matrix [18] matrix [18].. Pits are likely to initiate in the copper-depleted zone around these particles and grow around the periphery of the particles [18] particles [18].. The surface of 2014 after exposure to 3.5% NaCl solution for 4 h has been presented in Fig. in Fig. 6a. 6a. Two particles A and B of different nature nature were indicated by their different contrast in the back-scattered electron image. Local compositional analysis using EDAX of region A in Fig. in Fig. 6a 6a confirmed the presence of CuAl2 intermetallic (Fig. (Fig. 6b). 6b). The Al–Cu–Fe–Mn–Si intermetallic was identified with particle B (Fig. (Fig. 6c). 6c). It can be seen from Fig. 6a 6a that these intermetallic particles are responsible for discontinuities in the surface

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Fig. 6. (a) SEM micrograph of surface after immersion immersion for 4 h in 3.5% NaCl solution without inhibitor addition; addition; (b) EDAX of point A in (a); and (c) EDAX of point B in (a), indicating different type of intermetallics.

film formed in aqueous solution. Moreover, these particles, being cathodic in nature, are converted into permanent cathodes with mainly the reduction of oxygen to OH ions taking place [19] place [19].. This causes a local increase of the pH, further resulting in the dissolution of the oxide layer surrounding the particles. Once this layer has dissolved, the local increase in alkalinity can cause an intense attack on the matrix. The SEM image of 2014 surface after immersion for 4 h in 3.5% NaCl solution with 500 ppm LaCl3 is shown in Fig. in Fig. 7a. 7a. It was observed that oxides/hydroxides of lanthanum were precipitated on or adjacent to the cathodic CuAl2 intermetallic. This was confirmed by EDAX analysis (Fig. (Fig. 7c 7c and d). These precipitates on cathodic intermetallic sites were responsible for a decrease in the overall corrosion rate. An SEM image after immersion for 168 h in 3.5% NaCl solution with 500 ppm LaCl3 addition is shown in in Fig. 7b. 7b. EDAX


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Fig. 7. SEM micrograph micrograph after immersion immersion in 3.5% NaCl solution with 500 ppm LaCl LaCl 3 addition: (a) 4 h immersion; (b) 168 h immersion; (c) EDAX of point A in (a); and (d) EDAX of point B in (a).

analysis of the white product in this figure confirmed that these were lanthanum oxide/ hydroxide. Two different morphologies, large hexagonal shape plates and small leaf shapes were noticed on the surface. It was noted that with increased times of immersion, most of the imperfections on the surface were covered. Based on the surface observation in the current study and review of literature to date [10,11,20,21],, the surface film nature was proposed as shown in Fig. [10,11,20,21] in Fig. 8a. 8a. The protection mechanism after addition of rare earth chloride is schematically shown in in Fig. 8b. 8b. In the absence of inhibitor, the solution is in contact with the metal surface (mainly cathode intermetallic location) and the porous surface film. In the presence of inhibitor (either LaCl3 or CeCl3), the open locations in the porous layer (i.e. the exposed cathodic sites on surface) are blocked due to precipitation of La(OH)3 or Ce(OH)3. Utilizing the above

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Al alloy Porous Al2O3 film

c

Q1 Rs R1

Q2

R2

Al alloy

La(OH)3 product

Fig. 8. Surface film model in NaCl solution: (a) without addition addition of rare earth chloride; (b) after addition of rare earth chloride addition; and (c) equivalent circuit model based on surface film model.

surface surface model, the equivalent equivalent circuit shown in Fig. in Fig. 8c 8c was used to model the experimental impedance data. This equivalent model can be represented as R s(Q1(R1(Q2R2))), where R s is the solution resistance, Q 1 is the constant phase element (CPE) of the film, R1 the resistance of the film, Q 2 the CPE of the double layer and R 2 the resistance of the double layer. The impedance of a constant phase element is defined as [22] as [22] n 1

Z CPE CPE ¼ ½Qð jxÞ 

;

ð1Þ

where Q and n are frequency independent parameters, and  and 11 6 n 6 1 [22] [22].. CPE describes describes an ideal capacitor for n = 1, an ideal inductor for n =  = 11 and an ideal resistor for n = 0 [22].. [22] The overall response of the system is obtained as a result of the superposition of the responses due to the film (R1 – Q1) and to the metal–solution interface (R2 – Q2). It is important to note that the elements (R1 – Q1) encompass all the information information related to the surface layer and the possible defects that may be present within it. On the other hand, all the processes related to charge transfer at the electrical double layer and also diffusional transport from the metal–solution interface would be included in the (R2 – Q2) loop. Based on the fit of R(Q(R(QR))) model to the experimental EIS data, the values of Rs, in Table 2. 2. The solution resisQ1, n1, R1, Q2, n2 and R2 were obtained and are tabulated in Table tance did not vary notably, with and without LaCl3 addition (Table (Table 2). 2). It can be seen that the value of n n 1 were very close to 1 (n > 0.9). This indicated a near capacitive behaviour of the surface film formed on AA2014 sample. It was observed that constant phase element (Q1) decreased with increase in LaCl3 concentration and was minimum for 1000 ppm

Table 2 Results of modelling of experimental impedance spectra obtained for AA 2014 with and without LaCl 3 and CeCl3 inhibitor to the Rs(R1(Q1(R2Q2))) model Circ Circui uitt elem elemen ents ts

Para Parame mete ters rs 0 ppm

250 ppm LaCl3

2

Rs ( X cm ) Q1 ( X1 s cm2) n1 R1 ( X cm2) Q2 ( X1 s cm2) n2 R2 ( X cm2) n

n

9.7 1.58 E4 0.90 826 1.70 E3 1.00 1703

9.7 1.58 E4 0.90 826 1.70 E3 1.00 1703

10.8 2.84 E5 0.88 949 7.85 E5 0.74 4187

500 ppm

CeCl 3 10.8 8.45 E6 0.90 8164 7.58 E5 0.81 7977

LaCl3 13.1 6.80 E6 0.96 1442 8.13 E6 0.60 18210

750 ppm

CeCl3 12.5 7.35 E6 0.95 18800 1.41 E6 1.00 23390

LaCl3 11.1 2.48 E5 0.90 3586 1.28 E4 0.96 3570

1000 ppm

CeCl3 11.3 7.57 E6 0.93 16020 2.81 E5 0.60 137600

LaCl3 12.8 7.62 E6 0.94 17690 9.00 E7 1.00 33380

CeCl3 14.3 7.53 E6 0.94 158400 1.76 E5 0.76 56590

A .K . M i s h r a , R . B a l a s u b r a m a n i a m / C or or s i o n S c i e n c e 4 9 ( 2 0 0 7 ) 1 0 2 7 – 1 0 4 4

1 0 3 7

1038


LaCl3. The 750 ppm data was anomalous, as noted from other techniques too. As Q1 is inversely proportional to thickness of the film, a decrease in Q1 implies probably an increase increase in thickness thickness of the film. This increase increase in thickness thickness may be related to the formation of a reaction product due to the presence of lanthanum ions in the solution (i.e. for example, ple, lant lanthan hanid idee oxid oxide/ e/hy hydr droxi oxide de both both on and and in the the surf surfac acee film) film).. The The form format atio ion n of reaction products can increase the thickness of the film, thereby resulting in a decrease in Q1. It was observed that R1 increased with increase in concentration of LaCl3, as shown in Table in Table 2. 2. The constant phase element Q2 decreased drastically after LaCl3 addition, taking note of the anomalous data for 750 ppm additions. At cathodic intermetallic positions, rare earth ions precipitate their oxide/hydroxide [8] [8].. This appears to have affected the nature of double layer at solution metal interface. This change in Q 2 must be considered in association ciation with n 2. The parameter, n 2, before inhibitor addition was 1, showing purely capacitive behaviour of the EDL. After LaCl3 addition, n2 value decreases until the 500 ppm inhibitor level where n2 was 0.6, as shown in in Table 2. 2. This may be indicative of diffusion-related processes becoming important with inhibitor addition, due to filling up of hydroxides at the exposed solution–metal interface. At the highest concentration of LaCl3, n2 value increased to 1, showing again pure capacitive behaviour. The reason for this increase in n2 value can be attributed to the fact that, at high concentration, lanthanum ions form a thicker film of their oxides/hydroxides, which was clearly evident from the decrease in Q2 value too (Table (Table 2), 2), taking note of the anomalous data for the 750 ppm addition. The value of R R2 is related to the charge transfer resistance of the electrical double layer on local cathodic sites, as per the proposed model. R2 increased with increase in LaCl3 concen concentr trat atio ion n with with the the anom anomal alou ouss data data for for the the 75 7500 ppm ppm La LaCl Cl3 addition. addition. The

1038


LaCl3. The 750 ppm data was anomalous, as noted from other techniques too. As Q1 is inversely proportional to thickness of the film, a decrease in Q1 implies probably an increase increase in thickness thickness of the film. This increase increase in thickness thickness may be related to the formation of a reaction product due to the presence of lanthanum ions in the solution (i.e. for example, ple, lant lanthan hanid idee oxid oxide/ e/hy hydr droxi oxide de both both on and and in the the surf surfac acee film) film).. The The form format atio ion n of reaction products can increase the thickness of the film, thereby resulting in a decrease in Q1. It was observed that R1 increased with increase in concentration of LaCl3, as shown in Table in Table 2. 2. The constant phase element Q2 decreased drastically after LaCl3 addition, taking note of the anomalous data for 750 ppm additions. At cathodic intermetallic positions, rare earth ions precipitate their oxide/hydroxide [8] [8].. This appears to have affected the nature of double layer at solution metal interface. This change in Q 2 must be considered in association ciation with n 2. The parameter, n 2, before inhibitor addition was 1, showing purely capacitive behaviour of the EDL. After LaCl3 addition, n2 value decreases until the 500 ppm inhibitor level where n2 was 0.6, as shown in in Table 2. 2. This may be indicative of diffusion-related processes becoming important with inhibitor addition, due to filling up of hydroxides at the exposed solution–metal interface. At the highest concentration of LaCl3, n2 value increased to 1, showing again pure capacitive behaviour. The reason for this increase in n2 value can be attributed to the fact that, at high concentration, lanthanum ions form a thicker film of their oxides/hydroxides, which was clearly evident from the decrease in Q2 value too (Table (Table 2), 2), taking note of the anomalous data for the 750 ppm addition. The value of R R2 is related to the charge transfer resistance of the electrical double layer on local cathodic sites, as per the proposed model. R2 increased with increase in LaCl3 concen concentr trat atio ion n with with the the anom anomal alou ouss data data for for the the 75 7500 ppm ppm La LaCl Cl3 addition. addition. The increase is related to the formation of oxides/hydroxides of lanthanum on the cathodic intermetallic positions which, in the without inhibitor case, were exposed to solution. The EIS data (Bode phase plots and Bode magnitude plots) for 2014 alloy in 3.5% NaCl solution, with and without CeCl3, obtained at FCP, are presented in Fig. in Fig. 9. 9. As both Ce and La belong to the lanthanide group, their chlorides behave similarly with respect to their chemical nature. Therefore, the basic inhibition mechanism must be similar. The same model and the circuit (Fig. (Fig. 8) 8) proposed for LaCl3 addition was also used for CeCl3. The EIS results have been modelled and the results of the analyses are presented in Table 2. 2. It was observed that, the solution resistance (Rs) was almost similar for both with and without CeCl3. Q 1 values decreased after CeCl3 addition. Further, R 1 values increased with increase in concentration of CeCl3 and were maximum for 1000 ppm CeCl3 addition. This increase in resistance can be attributed to the fact that growth of a protective cerium oxide/hydroxide film blocks both anodic and cathodic active surface areas [23] areas [23].. Q2 values decreased after CeCl3 addition, indicating the electrical double layer was affected by formation of oxides/hydroxides of cerium on cathodic intermetallic regions. Further it was observ observed ed from from Tabl Tablee 2 that n2 value valuess show showed ed near near capa capaci citi tive ve beha behavi viou ourr unti untill the the 500 ppm CeCl3 inhibitor level, after that the value changed from 1 to 0.6 for 750 ppm and 0.76 for 1000 ppm CeCl3. The reason for this can be attributed to the fact that, at higher CeCl3 concentration, large clusters of oxides/hydroxides of cerium were formed, as shown in Fig. in Fig. 10a. 10a. It was observed that after immersion for 168 h in solution containing 500 ppm CeCl3, precipitates of oxide/hydroxide of cerium possessed a plate-like morphology (Fig. (Fig. 10b). 10b). The cluster-nature was also noted after immersion for 168 h in solution contai containing ning 100 10000 ppm ppm CeCl CeCl3 (Fig. 10c). 10c). It was noticed that 1000 ppm CeCl3 addition


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frequency (Hz)

Fig. 9. EIS plots in 3.5% NaCl solution, solution, with and without without CeCl 3 additions, at free corrosion potential: (a) Bode phase plots; and (b) Bode magnitude plots.

resulted in better surface coverage. Moreover, two different morphologies, large hexagonal shaped plates and small leaf shapes were observed, like those noted with LaCl3 addition. EDAX analysis confirmed that the oxide/hydroxide precipitates were formed on, or adjacent to, cathodic intermetallic sites and thus reduced the overall corrosion rate by suppressing the cathodic reaction. Formation of these big clusters resulted in an increase in roughness and this could explain the lower value of n n 2 at high CeCl3 concentrations. Earlier, authors reported that n is a measure of surface roughness roughness and that when it approaches approaches 0.5, it represents a rough surface and when it moves towards 1, a smooth one [24] one [24].. R2 values increased after CeCl3 addition. This is because of the formation of a protective film of oxides/hydroxides of cerium on cathodic intermetallic positions. It was noticed

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Fig. 10. SEM micrograph micrographss of surface after immersion immersion in 3.5% NaCl solution solution with (a) 1000 ppm ppm CeCl 3 for 4 h immersion, (b) 500 ppm CeCl 3 for 168 h immersion, and (c) 1000 ppm CeCl 3 for 168 h immersion.


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that that at 75 7500 ppm ppm CeCl CeCl3 addition, R2 incre increase ased d sign signifi ifican cantl tlyy and and then then decr decrea eased sed for for 1000 ppm, as shown in Table in Table 2, 2, although the R1 value was relatively high for 1000 ppm CeCl3 addition. The EIS data is useful because it indicates that the increasing resistance for the 1000 ppm CeCl3 addition may be primarily due to the effect of Ce2O3 or Ce(OH)3 modifying modifying the surface surface film properties properties more than just blocking the cathodic cathodic sites and affecting double layer. This affect on the double layer appears to peak at the 750 ppm addition. Further experiments will be needed to clarify the role of both LaCl3 and CeCl3 at the 750 ppm concentration level. As observed from Table from Table 2, 2, the resistance (R2) offered by oxides/hydrox oxides/hydroxides ides of cerium at cathodic intermetallic region was much higher than that of lanthanum. Also, the overall surface resistance (R1 + R 2) after cerium addition was larger compared to lanthanum additions. This showed cerium chloride as a superior inhibitor to lanthanum for 2014 alloy in NaCl solution. 3.3. Comparison of corrosion resistance

The corrosion data obtained by several different techniques was compared. The total surface resistance, i.e, R1 + R2 (calculated from EIS) was plotted against polarization resistance Rp (calculated from linear polarization). This is shown in in Fig. 11 11 for LaCl3 and CeCl3 addit additio ions ns.. In the the idea ideall case case,, ther theree shou should ld be a dire directl ctlyy line linear ar rela relatio tions nshi hip p between R1 + R2 and Rp. Such a relationship was generally noticed in the present study. Exceptions were noted for 1000 ppm LaCl3 and 750 ppm CeCl3 additions, where there was deviation from the line drawn with a slope of 45 (see Fig. (see Fig. 11). 11).

250000

LaCl 3 CeCl 3 1000

200000

)

2

m c s m h o (

150000

R

100000

p

1000 750

500

50000

0

250 500 250 750

0

50000

100000

150000

200000

250000

R1 + R 2 (ohms-cm2) Fig. 11. Relation Relationship ship between between Rp and R1 + R2 in 3.5% NaCl solution, with and without LaCl 3 and CeCl 3 additions, understood by plotting each on same figure. Data close to the line with 45  slope indicate good match.

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A.K. Mishra, R. Balasubramaniam Balasubramaniam / Corrosion Science 49 (2007) 1027–1044 100

80 y c n e 60 i c i f f E r o t i b i 40 h n I %

R2 R 1 + R 2

20

Rp

0 0

250

500

750

1000

1250

LaCl3 (ppm)

Fig. 12. Variatio Variation n of % inhibitor inhibitor efficiency as function function of LaCl 3 concentrations in 3.5% NaCl solution using different parameters. parameters.

100

80 y c n e 60 i c i f f E r o t i b i h 40 n I %

R2 R1 + R2 Rp

20

0 0

250

500

750

1000

1250

CeCl3 (ppm)

Fig. 13. Variatio Variation n of % inhibito inhibitorr efficiency as function function of CeCl 3 concentrations in 3.5% NaCl solution using different parameters. parameters.


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3.4. Inhibitor efficiency

The variation of inhibitor efficiency with rare earth chloride addition was also understood. Different parameters were used to measure inhibitor efficiency: R1, R1 + R 2 (both calculated from EIS) and R p (from linear polarization). The percentage inhibitor efficiency (IE%) was calculated using the equations:



IEð IEð%Þ ¼ 1 

R WO RWI



 100

;

ð2Þ

where R WO is the resistance without inhibitor addition and R WI the resistance after inhibitor addition. The variation in inhibitor efficiency as a function of LaCl3 and CeCl3 additions are shown in Figs. in Figs. 12 and 13, 13, respectively. All these parameters showed a similar trend in case of LaCl3 addition (Fig. (Fig. 12). 12). It was observed that the inhibitor efficiency increased with increase in LaCl3 concentration and was maximum for 1000 ppm, with the anomalous behaviour at 750 ppm. After CeCl3 addition (Fig. (Fig. 13), 13), there was a significant increase in inhibitor efficiency. Even for 250 ppm CeCl3, there was significant increase in inhibitor efficiency, when compared to the case without inhibitor. Further, the inhibitor efficiency for 500 ppm and 1000 ppm CeCl3 addition was nearly similar and on the higher side, which confirmed the superiority of CeCl3 as a protective inhibitor compared to LaCl3. 4. Conclusions

Electrochemical techniques have been applied to evaluate the inhibitive effects of lanthanum and cerium chloride additions in NaCl solution on the corrosion of AA 2014 alloy. Polarization resistance measurements indicated a decrease in corrosion rate after addition of lanthanum and cerium chlorides to 3.5% NaCl solution, with a maximum decrease in corrosion rate observed for 1000 ppm addition for the case of both inhibitors. EIS studies showed that there was a significant increase in overall resistances after addition of 1000 ppm LaCl3 and CeCl3. The corrosion resistance generally increased with increasing inhibitor addition. At all concentrations, CeCl3 was a better corrosion inhibitor compare to LaCl3. The formation of precipitates of oxides/hydroxides of lanthanum and cerium on cathodic intermetallic sites resulted in improved corrosion resistance. Acknowledgement

The authors thank HINDALCO, Renukoot, India for providing the alloy used in the study. References [1] J. Zhao, L. Xia, A. Sehgal, D. Lu, R.L. McCreery, G.S. Frankel, Effects of chromate and chromate conversion coatings on corrosion of aluminum alloy 2024-T3, Surf. Coat. Technol. 140 (2001) 51–57. [2] L. Xia, E. Akiyama, G. Frankel, R. McCreery, Storage and release of soluble hexavalent chromium from chromate conversion coatings. Equilibrium aspects of Cr VI concentration, J. Electrochem. Soc. 147 (2000) 2556. [3] J. Zhao, G. Frankel, R.L. McCreery, Corrosion protection of untreated AA-2024-T3 in chloride solution by a chromate conversion coating monitored with Raman spectroscopy, J. Electrochem. Soc. 145 (1998) 2258.

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