Ajit Kumar Mishra, R. Balasubramaniam *

lanthanum and cerium on cathodic intermetallic sites, which reduced the overall corrosion rate.

corrosion resistance. Research eorts in the aeronautical industry have focused on the

* Corresponding author.E-mail address: bala@iitk.ac.in (R. Balasubramaniam).

Corrosion Science 49 (2007) 10271044

www.elsevier.com/locate/corsci0010-938X/$ - see front matter 2006 Elsevier Ltd. All rights reserved. 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 theirlow density, favorable mechanical properties, good surface nish and relatively goodDepartment of Materials and Metallurgical Engineering, Indian Institute of Technology, Kanpur 208 016, India

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

Abstract

The eect of LaCl3 and CeCl3 inhibitor additions in 3.5% NaCl solution on the corrosion behav-iour of aluminum alloy AA2014 has been investigated. Four dierent concentrations (250, 500, 750and 1000 ppm) of LaCl3 and CeCl3 were studied. The polarization resistance increased signicantlyand the corrosion rate decreased by an order of magnitude with the addition of 1000 ppm of LaCl3and CeCl3, with maximum decrease noticed for CeCl3. EIS studies showed that there was a signif-icant increase in overall resistance after addition of 1000 ppm LaCl3 and CeCl3, when comparedto the case without inhibitor. The double layer resistance and lm resistance increased after inhibitoraddition. Scanning electron microscopy conrmed formation of precipitates of oxide/hydroxide ofCorrosion inhibition of aluminum alloy AA 2014by rare earth chloridesdoi:10.1016/j.corsci.2006.06.026

study of AlCu and AlZn alloys. The electrochemical behaviour of Al and its alloys hasattracted the attention of many investigators. The natural oxide lm on aluminum doesnot oer sucient protection against aggressive anions. In this context, inhibitors are usedto improve protective features of the surface. Currently, chromates are widely used in anti-corrosive pre-treatments of aluminum alloys [15]. However, because of their high toxic-ity, an intense research eort is underway for their replacement. Rare earth chlorides havebeen tested as corrosion inhibitors for Al alloys like AA5083 [6], AA7075 [7], AA8090 [8],AA6061 [9] and AA2024 [10]. These rare earth chlorides act as cathodic inhibitors[6,11,12].

Bethencourt et al. [6] observed, from weight loss and polarization results, that lantha-num, cerium and samarium chlorides are eective uniform corrosion inhibitors of AA5083in aerated 3.5% NaCl solution. As no pits were observed in samples immersed in solutionscontaining inhibitors, they concluded that these rare earth salts also act as pitting corro-sion inhibitors. Arnott et al. [7] investigated the corrosion inhibition behaviour of1000 ppm concentration of dierent 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 the eect of dierent concentrations of LaCl3 and CeCl3 on the corrosion ofAA 8090 in 3.56% NaCl solution. Maximum inhibition was obtained after addition of1000 ppm CeCl3 and 250 ppm LaCl3. Cerium chloride inhibited intergranular corrosionmore eectively than lanthanum chloride. Neil and Garrard [9] studied the eect of ceriumpre-treatments on AA6061 prior to immersion in 3.5% NaCl solution. They immersed thesamples in 0.1 M NaCl/1000 ppm CeCl3 solution for one week and then transferred themto the test solution. They concluded that cerium pre-treatment decreased the corrosionsusceptibility, but the eect of the treatment was short lived. Aldykewicz et al. [10]reported that the corrosion inhibition of cerium chloride addition in NaCl solution wasrelated to the development of a cerium-rich lm over the cathodic copper surface in thecase of AA2024. Aballe et al. [11] analyzed the eect of CeCl3, LaCl3 and mixture of bothCeCl3 and LaCl3 on the corrosion of AA 5083 alloy in NaCl solution. They found thatcorrosion resistance oered by these rare earth metal chlorides was of same order as thosefound with classical Cr-based compounds. They further observed that there was a two-foldincrease in polarization resistance after addition of 500 ppm LaCl3, four-fold after500 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 bet-ter performance than solely LaCl3/CeCl3 inhibitor.

The aim of the present work was to study the eect of LaCl3 and CeCl3 inhibitors atdierent 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 cathodicCuAl2 precipitates.

2. Experimental procedure

The 2014 alloy was obtained from HINDALCO, Renukoot, India in the form of acylindrical rod of diameter 4.2 cm. The composition of the alloy 2014 was (in wt.%)3.95.0 Cu, 0.20.8 Mg, 0.41.2 Mn, 0.51.2 Si,

A.K. Mishra, R. Balasubramaniam / Corrosion Science 49 (2007) 10271044 1029experiments. All the surfaces of the specimens were mechanically polished down to neemery paper (starting from grit number 220 to 1000, ANSI), de-greased with acetoneand then rinsed in distilled water before being used for each electrochemical experiment.All this procedure was followed prior to each electrochemical experiment. All experimentsreported in this paper were repeated a minimum of two times.

Corrosion tests were carried out in freely aerated 3.5% NaCl solution. Lanthanum andcerium chlorides were added at concentrations of 250, 500, 750 and 1000 ppm. Freshlyprepared NaCl solution was used in all the experiments. All the experiments were con-ducted at room temperature (25 C).

Separate samples were immersed for 4 and 168 h in solutions containing dierent con-centration of inhibitors, and later their surfaces were observed in a scanning electronmicroscope (SEM) (FEI QUANTA 200). Local compositions were studied with energydispersive analysis using X-ray (EDAX) unit attached to the SEM.

Electrochemical measurements were performed in a at cell (Amtek, USA) using a 2263PARSTAT (Amtek, USA) potentiostat controlled through a personal computer. In theat cell, the area of exposure of the sample was 1 cm2. An Ag/AgCl electrode wasemployed as the reference electrode. The potential of this electrode with respect to stan-dard hydrogen electrode is +0.197 V. All the potential data presented in this paper arereferred to this Ag/AgCl reference electrode potential.

All electrochemical experiments were performed after stabilization of free corrosionpotential (FCP). In conducting linear polarization experiments, the potential was scannedfrom the cathodic to anodic direction at a rate of 0.166 mV/s. The potential range for lin-ear polarization experiments were 20 mV from FCP. Potentiodynamic polarizationcurves were obtained from 250 mV to +1600 mV from FCP using a scan rate of1 mV/s.

Electrochemical impedance spectroscopy (EIS) measurements were performed byapplying a sinusoidal potential perturbation of 10 mV at FCP. The impedance spectrawere 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). Theimpedance data were tted to appropriate equivalent electrical circuit using a complexnonlinear least-squares tting routine, using both the real and imaginary components ofthe data. Dierent parameters obtained from the best t equivalent circuit were tabulatedand analyzed.

3. Results and discussion

3.1. Polarization

All the experiments were conducted after stabilization of free corrosion potential(FCP). In all the cases, the FCP increased in the noble direction and stabilized. The linearpolarization plots obtained after immersion in 3.5% NaCl solution, with and withoutLaCl3 additions are shown in Fig. 1, while those after CeCl3 additions in Fig. 2. Generally,the slope (i.e., the polarization resistance, Rp) increased after inhibitor additions. A higherRp value indicates lower corrosion rate. The estimated Rp values are tabulated in Table 1.The polarization resistance increased with increase in CeCl3 concentration (Table 1). Forthe case of LaCl3 addition, a similar feature was observed but for the anomalous data at

750 ppm LaCl3 addition.

-5 -4 -3 -2 -1 0 1 2 3 4 5-0.52

-0.51

-0.50

-0.49

-0.48

-0.47

-0.46

-0.45

-0.44

-0.43

-0.42

-0.41

5

4

3

2E

(V vs

SSC

)

I (*10-6) (A/cm2)

1

Fig. 1. Linear polarization curves in 3.5% NaCl solution, with and without LaCl3 additions. (10 ppm,2250 ppm, 3500 ppm, 4750 ppm, 51000 ppm).

-5 -4 -3 -2 -1 0 1 2 3 4 5-0.52

-0.51

-0.50

-0.49

-0.48

-0.47

-0.46

-0.45

-0.44

-0.43

-0.42

-0.41

5

4

3

2

E (V

vs S

SC)

I (*10-6) (A/cm2)

1

Fig. 2. Linear polarization curves in 3.5% NaCl solution, with and without CeCl3 additions. (10 ppm,2250 ppm, 3500 ppm, 4750 ppm, 51000 ppm).

1030 A.K. Mishra, R. Balasubramaniam / Corrosion Science 49 (2007) 10271044

The Rp values were higher for CeCl3 addition at all the concentrations compared toLaCl3 additions (Table 1), indicating that CeCl3 is a better corrosion inhibitor. Thiswas in agreement with the results obtained by other authors for dierent Al alloys, likeAA5083 [6], AA7075 [7] and AA8090 [8].

The potentiodynamic polarization curves obtained after LaCl3 and CeCl3 additions arepresented in Figs. 3 and 4, respectively. In both cases of inhibitor addition, the anodic por-tion remained similar whereas the cathodic portion shifted towards the left (i.e. to lowercurrent densities), indicating decrease in overall corrosion rate. The polarization curvesconrmed the cathodic nature of rare earth chloride inhibitors. The results of the presentstudy are in conformity with earlier studies of LaCl3 and CeCl3 inhibitors where similarshifts in polarization curves were observed for AA5083 [6] and AA8090 [8].

Table 1Variation in polarization resistance after LaCl3 and CeCl3 additions

Solution Polarization resistance Rp (kX cm2)

LaCl3 CeCl3

3.5% NaCl + 0 ppm 2.19 2.193.5% NaCl + 250 ppm 7.12 18.833.5% NaCl + 500 ppm 20.52 53.823.5% NaCl + 750 ppm 6.98 103.623.5% NaCl + 1000 ppm 104.75 205.21

A.K. Mishra, R. Balasubramaniam / Corrosion Science 49 (2007) 10271044 1031-0.4

-0.2

0.0

V vs

SSC)-9 -8 -7 -6 -5 -4 -3 -2 -1 0-0.8

-0.6

5

4

3

21

E (

log (I) log (A/cm2)Fig. 3. Potentiodynamic polarization curves in 3.5% NaCl solution, with and without LaCl3 additions. (10 ppm,2250 ppm, 3500 ppm, 4750 ppm, 51000 ppm).

1032 A.K. Mishra, R. Balasubramaniam / Corrosion Science 49 (2007) 10271044-9 -8 -7 -6 -5 -4 -3 -2 -1 0-0.8

-0.6

-0.4

-0.2

0.0

5

4

32

1E ( V

vs SS

C)

log (I) log (A/cm2)Fig. 4. Potentiodynamic polarization curves in 3.5% NaCl solution, with and without CeCl3 additions. (10 ppm,2250 ppm, 3500 ppm, 4750 ppm, 51000 ppm).3.2. Electrochemical impedance spectroscopy

The EIS data (Bode phase plots and Bode magnitude plots) for 2014 alloy in 3.5% NaClsolution, with and without LaCl3, obtained at FCP, are presented in Fig. 5.

The Bode phase plot indicates that two time constants could be identied in the EISdata for case without the addition of LaCl3, but only one time constant was exhibitedfor the data obtained with addition of inhibitor, as seen in Fig. 5a. It is also apparent thatthe phase angle maxima are quite broad after LaCl3 addition. The Bode magnitude plot(Fig. 5b) also indicates two slopes without inhibitor solution, whereas only one wasobtained after addition of LaCl3. This kind of behaviour has been attributed to a dual pro-tection mechanism of the passive lm, which behaves both as a barrier to corrosion andoers increased resistance to charge transfer processes [13].

The surface of aluminium and its alloys is covered by a ne layer of oxide (Al2O3) gen-erated during its handling. In aqueous solution, the surface oxide lm is composed ofAl2O3, Al(OH)3 and AlO(OH) phases [14]. Lee and Pyun [15] observed that the AlO(OH)phase enhances the passivity of the Al surface, and subsequently the corrosion resistancein chloride solution. One of the characteristics of this lm is the discontinuities present inthe zone occupied by the particles of intermetallic compounds in the alloy [16]. In view ofthis, two physical aspects of the corroding surface are the lm and the metalsolutioninterface.

It is anticipated that the locations where the intermetallics are present, the surfacewould be exposed to the environment and therefore, it is the double layer connected withintermetallicsolution interface that is really addressed while referring to metalsolutioninterface.

A.K. Mishra, R. Balasubramaniam / Corrosion Science 49 (2007) 10271044 103310

20

30

40

50

60

70

80

90 0 ppm 250 ppm 500 ppm 750 ppm 1000 ppm

hase

of Z

(deg

rees)Surface lms formed on 2014 without and after LaCl3 and CeCl3 addition was studiedusing scanning electron microscopy. Intermetallic particles in 2014 alloy are mainly spher-ical CuAl2 (h-phase) and irregularly shaped AlCuFeMnSi phase [17]. In the NaClelectrolyte, these intermetallics tend to be cathodic to the matrix [18]. Pits are likely to ini-tiate in the copper-depleted zone around these particles and grow around the periphery ofthe particles [18]. The surface of 2014 after exposure to 3.5% NaCl solution for 4 h hasbeen presented in Fig. 6a. Two particles A and B of dierent nature were indicated by theirdierent contrast in the back-scattered electron image. Local compositional analysis usingEDAX of region A in Fig. 6a conrmed the presence of CuAl2 intermetallic (Fig. 6b). TheAlCuFeMnSi intermetallic was identied with particle B (Fig. 6c). It can be seen fromFig. 6a that these intermetallic particles are responsible for discontinuities in the surface

0.01 0.1 1 10 100 1000 10000 100000-30

-20

-10

0P

frequency (Hz)

0.01 0.1 1 10 100 1000 10000 1000001

10

100

1000

10000

100000 0 ppm250 ppm500 ppm750 ppm1000 ppm

|Z| (oh

ms cm

2 )

frequency (Hz)Fig. 5. EIS plots in 3.5% NaCl solution, with and without LaCl3 additions, at free corrosion potential: (a) Bodephase plots; and (b) Bode magnitude plots.

1034 A.K. Mishra, R. Balasubramaniam / Corrosion Science 49 (2007) 10271044lm formed in aqueous solution. Moreover, these particles, being cathodic in nature, areconverted into permanent cathodes with mainly the reduction of oxygen to OH ions tak-ing place [19]. This causes a local increase of the pH, further resulting in the dissolution ofthe oxide layer surrounding the particles. Once this layer has dissolved, the local increasein 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 with500 ppm LaCl3 is shown in Fig. 7a. It was observed that oxides/hydroxides of lanthanumwere precipitated on or adjacent to the cathodic CuAl2 intermetallic. This was conrmedby EDAX analysis (Fig. 7c and d). These precipitates on cathodic intermetallic sites wereresponsible for a decrease in the overall corrosion rate. An SEM image after immersion for168 h in 3.5% NaCl solution with 500 ppm LaCl3 addition is shown in Fig. 7b. EDAX

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

A.K. Mishra, R. Balasubramaniam / Corrosion Science 49 (2007) 10271044 1035analysis of the white product in this gure conrmed that these were lanthanum oxide/hydroxide. Two dierent morphologies, large hexagonal shape plates and small leaf shapeswere noticed on the surface. It was noted that with increased times of immersion, most ofthe 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 lm nature was proposed as shown in Fig. 8a. The protectionmechanism after addition of rare earth chloride is schematically shown in Fig. 8b. Inthe absence of inhibitor, the solution is in contact with the metal surface (mainly cathodeintermetallic location) and the porous surface lm. In the presence of inhibitor (eitherLaCl3 or CeCl3), the open locations in the porous layer (i.e. the exposed cathodic siteson surface) are blocked due to precipitation of La(OH)3 or Ce(OH)3. Utilizing the above

Fig. 7. SEMmicrograph after immersion in 3.5% NaCl solution with 500 ppm LaCl3 addition: (a) 4 h immersion;(b) 168 h immersion; (c) EDAX of point A in (a); and (d) EDAX of point B in (a).

Rs

R1

R2

Q1

Q2

Porous Al2O3 film

Al alloy

La(OH)3 product Al alloy

c

1036 A.K. Mishra, R. Balasubramaniam / Corrosion Science 49 (2007) 10271044surface model, the equivalent circuit shown in Fig. 8c was used to model the experimentalimpedance data. This equivalent model can be represented as Rs(Q1(R1(Q2R2))), where Rsis the solution resistance, Q1 is the constant phase element (CPE) of the lm, R1 the resis-tance of the lm, Q2 the CPE of the double layer and R2 the resistance of the double layer.The impedance of a constant phase element is dened as [22]

ZCPE Qjxn1; 1where Q and n are frequency independent parameters, and 1 6 n 6 1 [22]. CPE describesan ideal capacitor for n = 1, an ideal inductor for n = 1 and an ideal resistor for n = 0[22].

The overall response of the system is obtained as a result of the superposition of theresponses due to the lm (R1Q1) and to the metalsolution interface (R2Q2). It is impor-tant to note that the elements (R1Q1) encompass all the information related to the surfacelayer and the possible defects that may be present within it. On the other hand, all the pro-cesses related to charge transfer at the electrical double layer and also diusional transportfrom the metalsolution interface would be included in the (R2Q2) loop.

Based on the t of R(Q(R(QR))) model to the experimental EIS data, the values of Rs,Q1, n1, R1, Q2, n2 and R2 were obtained and are tabulated in Table 2. The solution resis-tance did not vary notably, with and without LaCl3 addition (Table 2). It can be seen thatthe value of n1 were very close to 1 (n > 0.9). This indicated a near capacitive behaviour ofthe surface lm formed on AA2014 sample. It was observed that constant phase element(Q1) decreased with increase in LaCl3 concentration and was minimum for 1000 ppm

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

Table 2Results of modelling of experimental impedance spectra obtained for AA 2014 with and without LaCl3 and CeCl3 inhibitor to the Rs(R1(Q1(R2Q2))) model

Circuit elements Parameters

0 ppm 250 ppm 500 ppm 750 ppm 1000 ppm

LaCl3 CeCl3 LaCl3 CeCl3 LaCl3 CeCl3 LaCl3 CeCl3

Rs (X cm2) 9.7 9.7 10.8 10.8 13.1 12.5 11.1 11.3 12.8 14.3

Q1 (X1 sn cm2) 1.58E4 1.58E4 2.84E5 8.45E6 6.80E6 7.35E6 2.48E5 7.57E6 7.62E6 7.53E6

n1 0.90 0.90 0.88 0.90 0.96 0.95 0.90 0.93 0.94 0.94R1 (X cm

2) 826 826 949 8164 1442 18800 3586 16020 17690 158400Q2 (X

1 sn cm2) 1.70E3 1.70E3 7.85E5 7.58E5 8.13E6 1.41E6 1.28E4 2.81E5 9.00E7 1.76E5n2 1.00 1.00 0.74 0.81 0.60 1.00 0.96 0.60 1.00 0.76R2 (X cm

2) 1703 1703 4187 7977 18210 23390 3570 137600 33380 56590

A.K.Mish

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/Corro

sionScien

ce49(2007)10271044

1037

1038 A.K. Mishra, R. Balasubramaniam / Corrosion Science 49 (2007) 10271044LaCl3. The 750 ppm data was anomalous, as noted from other techniques too. As Q1 isinversely proportional to thickness of the lm, a decrease in Q1 implies probably anincrease in thickness of the lm. This increase in thickness may be related to the formationof a reaction product due to the presence of lanthanum ions in the solution (i.e. for exam-ple, lanthanide oxide/hydroxide both on and in the surface lm). The formation ofreaction products can increase the thickness of the lm, thereby resulting in a decreasein Q1. It was observed that R1 increased with increase in concentration of LaCl3, as shownin Table 2.

The constant phase element Q2 decreased drastically after LaCl3 addition, taking noteof the anomalous data for 750 ppm additions. At cathodic intermetallic positions, rareearth ions precipitate their oxide/hydroxide [8]. This appears to have aected the natureof double layer at solution metal interface. This change in Q2 must be considered in asso-ciation with n2. The parameter, n2, before inhibitor addition was 1, showing purely capac-itive behaviour of the EDL. After LaCl3 addition, n2 value decreases until the 500 ppminhibitor level where n2 was 0.6, as shown in Table 2. This may be indicative of diu-sion-related processes becoming important with inhibitor addition, due to lling up ofhydroxides at the exposed solutionmetal interface. At the highest concentration of LaCl3,n2 value increased to 1, showing again pure capacitive behaviour. The reason for thisincrease in n2 value can be attributed to the fact that, at high concentration, lanthanumions form a thicker lm of their oxides/hydroxides, which was clearly evident from thedecrease in Q2 value too (Table 2), taking note of the anomalous data for the 750 ppmaddition. The value of R2 is related to the charge transfer resistance of the electrical doublelayer on local cathodic sites, as per the proposed model. R2 increased with increase inLaCl3 concentration with the anomalous data for the 750 ppm LaCl3 addition. Theincrease is related to the formation of oxides/hydroxides of lanthanum on the cathodicintermetallic 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% NaClsolution, with and without CeCl3, obtained at FCP, are presented in Fig. 9. As both Ceand La belong to the lanthanide group, their chlorides behave similarly with respect totheir chemical nature. Therefore, the basic inhibition mechanism must be similar. Thesame model and the circuit (Fig. 8) proposed for LaCl3 addition was also used for CeCl3.

The EIS results have been modelled and the results of the analyses are presented inTable 2. It was observed that, the solution resistance (Rs) was almost similar for both withand without CeCl3. Q1 values decreased after CeCl3 addition. Further, R1 values increasedwith 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 ceriumoxide/hydroxide lm blocks both anodic and cathodic active surface areas [23]. Q2 valuesdecreased after CeCl3 addition, indicating the electrical double layer was aected by for-mation of oxides/hydroxides of cerium on cathodic intermetallic regions. Further it wasobserved from Table 2 that n2 values showed near capacitive behaviour until the500 ppm CeCl3 inhibitor level, after that the value changed from 1 to 0.6 for 750 ppmand 0.76 for 1000 ppm CeCl3. The reason for this can be attributed to the fact that, athigher CeCl3 concentration, large clusters of oxides/hydroxides of cerium were formed,as shown in Fig. 10a. It was observed that after immersion for 168 h in solution containing500 ppm CeCl3, precipitates of oxide/hydroxide of cerium possessed a plate-like morphol-ogy (Fig. 10b). The cluster-nature was also noted after immersion for 168 h in solution

containing 1000 ppm CeCl3 (Fig. 10c). It was noticed that 1000 ppm CeCl3 addition

A.K. Mishra, R. Balasubramaniam / Corrosion Science 49 (2007) 10271044 103930

40

50

60

70

80

90 0 ppm 250 ppm 500 ppm 750 ppm 1000 ppm

e of

Z (d

egree

s)resulted in better surface coverage. Moreover, two dierent morphologies, large hexagonalshaped plates and small leaf shapes were observed, like those noted with LaCl3 addition.EDAX analysis conrmed that the oxide/hydroxide precipitates were formed on, or adja-cent to, cathodic intermetallic sites and thus reduced the overall corrosion rate by sup-pressing the cathodic reaction. Formation of these big clusters resulted in an increase inroughness and this could explain the lower value of n2 at high CeCl3 concentrations. Ear-lier, authors reported that n is a measure of surface roughness and that when it approaches0.5, it represents a rough surface and when it moves towards 1, a smooth one [24].

R2 values increased after CeCl3 addition. This is because of the formation of a protec-tive lm of oxides/hydroxides of cerium on cathodic intermetallic positions. It was noticed

0.01 0.1 1 10 100 1000 10000 100000-20

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frequency (Hz)

0.01 0.1 10 100 1000 10000 1000001

10

100

1000

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100000

1000000 0 ppm 250 ppm 500 ppm 750 ppm 1000 ppm

|Z| (o

hms

cm2 )

frequency (Hz)1

Fig. 9. EIS plots in 3.5% NaCl solution, with and without CeCl3 additions, at free corrosion potential: (a) Bodephase plots; and (b) Bode magnitude plots.

Fig. 10. SEM micrographs of surface after immersion in 3.5% NaCl solution with (a) 1000 ppm CeCl3 for 4 himmersion, (b) 500 ppm CeCl3 for 168 h immersion, and (c) 1000 ppm CeCl3 for 168 h immersion.

1040 A.K. Mishra, R. Balasubramaniam / Corrosion Science 49 (2007) 10271044

that at 750 ppm CeCl3 addition, R2 increased signicantly and then decreased for1000 ppm, as shown in Table 2, although the R1 value was relatively high for 1000 ppmCeCl3 addition. The EIS data is useful because it indicates that the increasing resistancefor the 1000 ppm CeCl3 addition may be primarily due to the eect of Ce2O3 or Ce(OH)3modifying the surface lm properties more than just blocking the cathodic sites and aect-ing double layer. This aect 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 the750 ppm concentration level.

As observed from Table 2, the resistance (R2) oered by oxides/hydroxides of cerium atcathodic intermetallic region was much higher than that of lanthanum. Also, the overallsurface resistance (R1 + R2) after cerium addition was larger compared to lanthanumadditions. This showed cerium chloride as a superior inhibitor to lanthanum for 2014 alloyin NaCl solution.

3.3. Comparison of corrosion resistance

The corrosion data obtained by several dierent techniques was compared. The totalsurface resistance, i.e, R1 + R2 (calculated from EIS) was plotted against polarizationresistance Rp (calculated from linear polarization). This is shown in Fig. 11 for LaCl3and CeCl3 additions. In the ideal case, there should be a directly linear relationshipbetween 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

A.K. Mishra, R. Balasubramaniam / Corrosion Science 49 (2007) 10271044 1041was deviation from the line drawn with a slope of 45 (see Fig. 11).

0 50000 100000 150000 200000 2500000

50000

100000

150000

200000

250000

750

1000

500

250

1000

500750

250

Rp (oh

ms-

cm2 )

R1 + R2 (ohms-cm2)

LaCl3 CeCl3

Fig. 11. Relationship between Rp and R1 + R2 in 3.5% NaCl solution, with and without LaCl3 and CeCl3

additions, understood by plotting each on same gure. Data close to the line with 45 slope indicate good match.

0 250 500 750 1000 12500

20

40

60

80

100

% In

hibi

tor E

fficie

ncy

CeCl3 (ppm)

R2 R1 + R2 Rp

Fig. 13. Variation of % inhibitor eciency as function of CeCl3 concentrations in 3.5% NaCl solution usingdierent parameters.

0 250 500 750 1000 12500

20

40

60

80

100

% In

hibi

tor E

fficie

ncy

LaCl3 (ppm)

R2

R1 + R2

Rp

Fig. 12. Variation of % inhibitor eciency as function of LaCl3 concentrations in 3.5% NaCl solution usingdierent parameters.

1042 A.K. Mishra, R. Balasubramaniam / Corrosion Science 49 (2007) 10271044

A.K. Mishra, R. Balasubramaniam / Corrosion Science 49 (2007) 10271044 10433.4. Inhibitor eciency

The variation of inhibitor eciency with rare earth chloride addition was also under-stood. Dierent parameters were used to measure inhibitor eciency: R1, R1 + R2 (bothcalculated from EIS) and Rp (from linear polarization). The percentage inhibitor eciency(IE%) was calculated using the equations:

IE% 1 RWORWI

100; 2

where RWO is the resistance without inhibitor addition and RWI the resistance after inhib-itor addition.

The variation in inhibitor eciency as a function of LaCl3 and CeCl3 additions areshown in Figs. 12 and 13, respectively. All these parameters showed a similar trend in caseof LaCl3 addition (Fig. 12). It was observed that the inhibitor eciency increased withincrease in LaCl3 concentration and was maximum for 1000 ppm, with the anomalousbehaviour at 750 ppm. After CeCl3 addition (Fig. 13), there was a signicant increase ininhibitor eciency. Even for 250 ppm CeCl3, there was signicant increase in inhibitor e-ciency, when compared to the case without inhibitor. Further, the inhibitor eciency for500 ppm and 1000 ppm CeCl3 addition was nearly similar and on the higher side, whichconrmed the superiority of CeCl3 as a protective inhibitor compared to LaCl3.

4. Conclusions

Electrochemical techniques have been applied to evaluate the inhibitive eects of lan-thanum and cerium chloride additions in NaCl solution on the corrosion of AA 2014alloy. Polarization resistance measurements indicated a decrease in corrosion rate afteraddition of lanthanum and cerium chlorides to 3.5% NaCl solution, with a maximumdecrease in corrosion rate observed for 1000 ppm addition for the case of both inhibitors.EIS studies showed that there was a signicant increase in overall resistances after additionof 1000 ppm LaCl3 and CeCl3. The corrosion resistance generally increased with increas-ing inhibitor addition. At all concentrations, CeCl3 was a better corrosion inhibitor com-pare to LaCl3. The formation of precipitates of oxides/hydroxides of lanthanum andcerium on cathodic intermetallic sites resulted in improved corrosion resistance.

Acknowledgement

The authors thank HINDALCO, Renukoot, India for providing the alloy used in thestudy.

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Corrosion inhibition of aluminum alloy AA 2014 by rare earth chloridesIntroductionExperimental procedureResults and discussionPolarizationElectrochemical impedance spectroscopyComparison of corrosion resistanceInhibitor efficiency

ConclusionsAcknowledgementReferences