This article appeared in a journal published by Elsevier. The attachedcopy is furnished to the author for internal non-commercial researchand education use, including for instruction at the authors institution

and sharing with colleagues.

Other uses, including reproduction and distribution, or selling orlicensing copies, or posting to personal, institutional or third party

websites are prohibited.

In most cases authors are permitted to post their version of thearticle (e.g. in Word or Tex form) to their personal website orinstitutional repository. Authors requiring further information

regarding Elsevier’s archiving and manuscript policies areencouraged to visit:

http://www.elsevier.com/authorsrights

Author's personal copy

Electrochimica Acta 124 (2014) 109–118

Contents lists available at ScienceDirect

Electrochimica Acta

j ourna l ho me page: www.elsev ier .com/ locate /e lec tac ta

Encapsulation of dodecylamine corrosion inhibitor on silicananoparticles

J.M. Falcóna, F.F. Batistab, I.V. Aokia,∗

a Chemical Engineering Department, Polytechnic School, University of São Paulo, Av. Prof. Luciano Gualberto, Travessa 3, no. 380, CEP 05508 010 São Paulo,Brazilb Mettler Toledo Ind. e Com. Ltd., Al. Araguaia, 200, Alphaville CEP 06455 000, Brazil

a r t i c l e i n f o

Article history:Received 8 April 2013Received in revised form 19 June 2013Accepted 20 June 2013Available online 4 July 2013

Keywords:NanocontainersLayer-by-layerSelf-healingCorrosion inhibitorsEISIR probe

a b s t r a c t

The use of nanocontainers or nanoreservoirs containing corrosion inhibitors as active corrosion protectionsystems is one of the issues of great interest in the corrosion protection scientific literature. The aim of thispaper is to study the use of nanocontainers with dodecylamine encapsulated for active corrosion protec-tion of carbon steel. To produce the nanocontainers the LbL deposition procedure was used where severalpolyelectrolytes (PEI and PSS) and inhibitor (dodecylamine) layers were alternatively deposited on SiO2

nanoparticles surface for obtaining a final structure of SiO2/PEI/PSS/dodecylamine/PSS/dodecylaminetype. The tests for indirectly evaluating the amount of released inhibitor from nanocontainers wasperformed evaluating the corrosion resistance of carbon steel samples by electrochemical impedance(EIS) measurements in aerated 0.1 mol/L NaCl solution containing (1 wt.%) nanocontainers at differentpH values (2.0, 6.2 and 9.0). The anticorrosive performance of an alkyd primer loaded with 10 wt.% ofnanocontainers containing entrapped dodecylamine was tested on coated carbon steel panels with aprovoked defect, by immersion of the coated samples in 0.01 mol/L NaCl, by EIS and scanning vibratingelectrode technique (SVET). The results of EIS were treated and kinetics curves showed that for pH = 2condition the release of inhibitor was more rapid and efficient in comparison to other pH conditions (9and 6.2) where the release was slower allowing to prove the nanocontainers permeability properties foran external stimulus of pH change. The release of dodecylamine was directly monitored by a IR probein situ. On the other hand, the self-healing effect of the alkyd primer coating doped with nanocontainersloaded with dodecylamine was demonstrated in situ by SVET and EIS measurements.

© 2013 Elsevier Ltd. All rights reserved.

1. Introduction

Corrosion of metals is an issue of great importance for manyindustries due to huge spending costs associated with metalsdegradation [1]. One of the common strategies used by corrosionengineers is the use of polymeric coating systems applied to ametal surface, which provides a dense barrier against the ingressof aggressive species and avoid their contact with metallic surface[2]. The effectiveness of coating systems is based on strong barrierproperties which inhibit the diffusion of the electrolyte, oxygenand water to the substrate [3]. However, if this barrier is partiallydisrupted, the coating itself cannot stop propagation of corrosionprocess [4]. For this reason, the development of active systems toprotect metallic substrates against corrosion is a chalenge of greatimportance for many industrial applications [5]. This developmentis important for replacement of protection systems that use

∗ Corresponding author. Tel.: +55 1130912274; fax: +55 1130913038.E-mail address: idavaoki@usp.br (I.V. Aoki).

chromates, which are known as the most effective inhibitors butare strongly carcinogenic. This known carcinogenic nature andtoxicity of chromates and other corrosion inhibitors is coming toits complete elimination in most industrial applications [6,7]. Oneof the solutions for developing these systems of active protectionis to introduce an environmentally friendly corrosion inhibitordirectly into a coating, as primary amines, for example, whichhave the ability to adsorb strongly on metal’s surface to createa passive barrier between the metal and aggressive media [8].Expected undesirable reactions between inhibitor molecule andconstituents of the coating polymeric matrix are the main reasonsfor avoiding the direct introduction of inhibitors into coatings andtrying to encapsulate it [19]. With advances in nanotechnology,these barriers are having dual functionality, namely, a physical bar-rier against corrosive species and other active response at changesin the local environment (pH, temperature, ionic strength, etc.).

Active protection systems point to recover the initial propertiesof the coated substrate when this is attacked by the corrosion pro-cess [9]. This process is called self-healing, where the material haveability to repair damage caused by mechanical challenge over time

0013-4686/$ – see front matter © 2013 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.electacta.2013.06.114

Author's personal copy

110 J.M. Falcón et al. / Electrochimica Acta 124 (2014) 109–118

without human intervention [10]. For providing self-healing abilitythe use of microcapsules or nanocontainers is necessary what canserve as host reservoirs for corrosion inhibiting species [11,12]. Animportant property of these nanocontainers is the slow and pro-longed release of encapsulated active material, which is triggeredby specific changes in the environment surrounding the nanocon-tainer or directly in the nanocontainer shell [13]. Currently, someencapsulation technologies for corrosion inhibitors have been suc-cessfully applied to the self-healing anticorrosion coatings whereinhibiting molecules are loaded into microcapsules or nanocontain-ers and the release process is mainly diffusion-controlled [14]. Thelayer-by-layer (LbL) technique involves an assembly of alternat-ing layers of oppositely charged polyelectrolytes and inhibitor ontothe surface of the substrate of variable shape [9]. This technique isextremely versatile due to the fact that is possible to form thin filmswith a wide range of properties onto various substrates, includingcolloidal particles and porous membranes [15].

One of these important properties is the possibility to controlthe nanocontainer shell properties, which open perspectives forfurther applications of these nanocontainers in many research andindustrial fields such as medicine, pharmaceutical, cosmetic andfood industries [16].

The polyelectrolyte shell, especially composed of oppositelyweakly charged polymers, is pH sensitive, and provides the control-lable release of corrosion inhibitors by regulating its permeabilityproperties [17].

Usually, the polyelectrolyte shell is permeable for macro-molecules at low pH (<3) or high ionic strength, whereas it isin a closed state at high pH (>8) for avoiding the output of theinhibitor [18]. There are different types of nanocontainers as forexample: (1) polymeric capsules containing active compounds orcorrosion inhibitors; (2) inert nanoparticles (silica nanoparticles,titanium oxide nanoparticles and clay minerals) with encapsulatedcorrosion inhibitors by LbL method [19]. For proving the release ofthe corrosion inhibitor from the nanocontainers and for evaluatingself-healing ability, some electrochemical techniques were usedas: electrochemical impedance spectroscopy (EIS) and scanningvibrating electrode technique (MEV). EIS is a powerful, rapid andaccurate non-destructive method for the evaluation of a wide rangeof materials and which provide detailed information of the systemsunder examination as electrochemical mechanisms and reactionkinetics [20]. Furthermore, this technique allows to understandthe corrosion behavior and to assess the effect of the nanocontain-ers when they are added on smart coatings applied as a primer[21–28]. On the other hand, the SVET technique provides detailedinformation about the type of corrosion and also shows the self-healing effect in the provoked defects on primer coating embeddedwith nanocontainers loaded with corrosion inhibitors [9,29–31].The vibrating probe measures potential differences in the elec-trolytic phase in contact with the corroding metal arising fromthe fluxes of ionic species that participate in the electrochemicalreactions occurring at the metal/solution interface [32]. The anti-corrosive performance of an alkyd primer loaded with 10 wt.% ofnanocontainers containing entrapped dodecylamine was tested oncoated carbon steel panels, by immersion of the coated samples in0.01 mol/L NaCl, by a IR probe, EIS and scanning vibrating electrodetechnique (SVET).

2. Experimental

Plates of carbon steel AISI 1020 were used in this study whichwere previously treated with emery papers from 120 to 600 meshgrade, sequentially, and then rinsed with distilled water, alcoholand acetone. To produce the nanocontainers 20 mL of SiO2 (15 wt.%)colloidal solution from Sigma–Aldrich were mixed with 3 mL of

2 mg/mL PEI (polyethyleneimine) solution for 15 min. Then, theSiO2/PEI sample was washed three times with distilled water andseparated by centrifugation. The deposition of the negative PSSlayer (poly-styrene sulphonate) was carried out mixing for 15 min3 ml of 2 mg/mL PSS solution with the centrifuged SiO2/PEI sam-ple. Then, the SiO2/PEI/PSS sample was washed three times indistilled water and separated by centrifugation. Deposition of thethird layer (dodecylamine corrosion inhibitor) was accomplishedmixing for 15 min 30 ml of an aqueous acidic solution of the dode-cylamine inhibitor (pH 3) in a concentration of 1 mg/ml with thecentrifuged SiO2/PEI/PSS sample. Then, the SiO2/PEI/PSS/inh sam-ple was washed three times in distilled water and separated bycentrifugation. The last two deposition steps (PSS and inhibitor)were repeated once more to ensure the highest inhibitor loading inthe final LbL structure, where the resulting system has a final struc-ture of the type SiO2/PEI/PSS/inhib/PSS/inhib. Dispersion in waterand sonication for 30 min in ultrasonic bath between different steps(after each centrifugation step) and drying the final particles in airwas accomplished.

Electrochemical impedance spectroscopy measurements (EIS)were employed to evaluate two different types of systems: at firstfor determining indirectly the release of corrosion inhibitor fromnanocontainers by monitoring of the corrosion behavior of car-bon steel samples in 0.1 mol/L NaCl solution at different pHs (2,6.2 and 9) and containing 1 wt.% of nanocontainers loaded withdodecylamine as corrosion inhibitor and at second for evaluatingthe corrosion protection performance of coated samples with alkydpaint loaded with corrosion inhibitor during 2 days of immersion in0.01 mol/L NaCl solution. In order to accelerate the corrosion pro-cess of these samples a small defect on the coated sample was madeby an indenter just before starting the experiment.

EIS measurements were performed at open circuit potentialafter different contact times of nanocontainers with the aggressivesolution using a EG&G/PAR Model 273A potentiostat–galvanostatcoupled to a Solartron model 1255B frequency analyzer and con-trolled by Softcorr M352 software. A frequency range from 50 kHzto 5 mHz with a sinusoidal potential amplitude perturbation of10 mV rms was adopted.

For the IR measurements an infrared probe ReactIR from Met-tler Toledo was immersed directly in the aerated 0.1 mol/L NaClsolution at pH 2 containing the nanocontainers loaded with dode-cylamine inhibitor during a period of 6 h and under constantstirring.

SVET measurements were performed by using the ApplicableElectronics equipment controlled by ASET (Sciencewares) software.Samples were prepared for SVET measurements by cutting into1 × 1 cm2 area plates. The probe was located 100 �m above the sur-face and vibrated in the perpendicular direction to the surface (Z)with 20 �m amplitude. The frequency of vibration of the probe was164 Hz. The scanned area was around 4 × 5 mm2. In order to accel-erate the corrosion process and evaluate the corrosion resistance ofthese samples small scratches on the coated sample using a sharptool were made of approximately 3–4 mm long and about 0.2 �mwide. Periodical measurements were taken during of the course ofimmersion of the coated samples in a 0.01 mol/L NaCl solution usedas electrolyte.

For obtaining the coated samples, the paint was applied withthe help of a brush on the samples (previously cleaned with alcoholand acetone) until getting two layers of approximately 100 �m totaldry thickness. Low solids (65 wt.%) alkyd paint containing nanocon-tainers was prepared to coat (100 mm × 150 mm) panels as a primer(first layer of about 50 �m dry thickness) with 10 wt.% nanocontain-ers and a second layer without nanocontainers was also applied.Thickness measurements were made in a Fisher Model DualScope®

MP40 with a probe based on attenuation of the magneticfield.

Author's personal copy

J.M. Falcón et al. / Electrochimica Acta 124 (2014) 109–118 111

0 500 10 00 150 0 2000 2500

0

500

100 0

150 0

200 0

250 0

1h

2h

3h

5h

16h

Z' / cm2

- Z

" /

cm

2

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

2,0

2,2

2,4

2,6

2,8

3,0

3,2

3,4

3,6

3,8

4,0

1h

2h

3h

5h

16h

log (f / Hz)

log

(|Z

| /

cm

2)

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

0

10

20

30

40

50

601h

2h

3h

5h

16h

log (f / Hz )

- /

de

gre

e

Fig. 1. Impedance diagrams: Nyquist (a) and Bode ((b) and (c)) plots of carbon steel after different immersion times in 0.1 mol/L NaCl at pH 2 containing 1 wt.% of nanocontainerswith encapsulated dodecylamine.

Salt spray experiments were performed by using the equipmentBass Model USC-ISSO-(PLUS), following the prescriptions of ASTMB 117-11 Standard [33]. In order to accelerate the corrosion processlong scratches were made on the coated samples using a sharp tool.

3. Results and discussion

3.1. ElS measurements

3.1.1. Indirect determination of the amount of inhibitor releasedfrom nanocontainers

Fig. 1 shows the impedance diagrams for carbon steel afterdifferent immersion times in a 0.1 mol/L NaCl solution at pH = 2 con-taining 1 wt.% of nanocontainers loaded with dodecylamine, whereit can be observed that for initial immersion times (1 h, 2 h, 3 h)there was a small increase in the capacitive arcs diameter shown inNyquist diagrams whose values oscillate around 750 � cm2, but forlong contact period (16 h), this diameter increases significantly upto a value of 2500 � cm2. These results demonstrate the efficientrelease of the inhibitor with time and the permeability propertiesof polyelectrolyte walls which are open at pH values < 3.

Fig. 2 shows the impedance diagrams for carbon steel for differ-ent immersion times in a 0.1 mol/L NaCl solution at pH = 6.2 loadedwith 1 wt.% of nanocontainers. In this case, it can be seen that foran immersion time of 5 h (300 min) the value of the capacitive arcdiameter was around 2000 � cm2, where the corresponding valuefor initial immersion times was 1500 � cm2). For long periods

of contact (16 h), it can be observed only a slight increase in thecapacitive arc diameter (3000 � cm2), which indicates that forthis condition of neutral pH, the release of the inhibitor is slowerand prolonged. Polyelectrolyte walls are less permeable for thispH condition avoiding a significant release of the inhibitor fromthe nanocontainers. Therefore the corrosion inhibitor is storeduntil local pH changes occur due to the initiation of the corrosionprocess at defect areas.

Fig. 3 shows the impedance diagrams for carbon steel for dif-ferent immersion times in a 0.1 mol/L NaCl solution at pH = 9additivated with 1 wt.% of nanocontainers loaded with dodecy-lamine. In this case, one can observe almost the same responseobtained for pH = 6.2, where the values of capacitive arc diametersfor long immersion times (16 h) do not present a significant increaseas shown for pH = 2 condition.

3.1.2. Kinetics curves of inhibitor release from nanocontainersFig. 4 shows the linear kinetics for release of encapsulated

inhibitor from nanocontainers for different immersion times in0.1 mol/L NaCl solution and different pHs. It can be seen that atpH 2 the ratio between the impedance modulus for the conditionwith and without nanocontainers increase quickly for longer times(slope of 0.0039) in comparison with the values obtained for shortertimes. However, at pH 6.2 and pH 9 this increasing is more grad-ual (slope of 0.0010 for both pH condition), the release of inhibitoris slower due to the resistance of nanocontainer walls which arequite closed in these pH conditions. According to the slopes of the

Author's personal copy

112 J.M. Falcón et al. / Electrochimica Acta 124 (2014) 109–118

0 50 0 10 00 1500 2000 2500 3000 3500

0

500

1000

1500

2000

2500

3000

3500

- Z

" /

cm

21h

2h

3h

5h

16 h

Z' / cm2

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

2,0

2,2

2,4

2,6

2,8

3,0

3,2

3,4

3,6

3,8

4,0

4,2

1h

2h

3h

5h

16h

log (f / Hz)

log

(|Z

| /

cm

2)

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

0

10

20

30

40

50

60

70

1h

2h

3h

5h

16 h

log (f / Hz )

- / d

eg

ree

Fig. 2. Impedance diagrams: Nyquist (a) and Bode ((b) and (c)) plots of carbon steel after different immersion times in 0.1 mol/L NaCl at pH 6.2 containing 1 wt.% ofnanocontainers with encapsulated dodecylamine.

kinetics curves it is possible to affirm that at pH = 2 the inhibitorrelease rate is 3.9 times higher compared with the values obtainedat pH = 6.2 and pH = 9.

To determine the total amount of inhibitor released fromnanocontainers, these were added in a 0.1 mol/L NaCl solution atpH = 2 until has obtained the greatest diameter of capacitive arcin the Nyquist diagram, which was around 5000 � cm2 (Fig. 5)for an immersion time of 89 h, where it can be assumed thatafter such a long time the amount of inhibitor released (dodecy-lamine) was maximum. Furthermore, an additional study was doneusing known concentrations of dodecylamine inhibitor (100 ppm,200 ppm, 300 ppm, 400 ppm and 500 ppm) which were added to0.1 mol/L NaCl solution at pH = 2 and whose results are shown inFig. 6.

Thus, comparing impedance values from Figs. 5 and 6 it canbe inferred that the total achieved concentration of dodecylaminereleased from the nanocontainers after 89 h was approximately400 ppm, considering that only dodecylamine acts as corrosioninhibitor. One possibility is that the polyelectrolytes can adsorb onthe carbon steel surface and act also as inhibitors. In this case, theestimation of inhibitor dodecylamine could be overestimated. Cal-culating the amount of released inhibitor by mass balance fromthe nanocontainers before and after the release process, a concen-tration of about 500 ppm was determined. This calculated valuewas higher than the estimated by EIS (400 ppm) and showed thatthere was also a release of polyelectrolytes (PEI and PSS) that form

the nanocontainer walls. Infrared spectroscopy results by in situprobe measurements (see Section 3.1.3) using an optic fiber probeconfirmed the release of the dodecylamine inhibitor and also ofthe polyelectrolytes (PEI and PSS). According to the mass balancethe maximum possible amount of encapsulated inhibitor was esti-mated to be around 6 wt.%, what means 60 mg of dodecylamine pereach 1 g of nanocontainers. This result is in agreement with otherpublished works, where the maximum loaded inhibitor is about 8%for LbL encapsulating method [4,5]

3.1.3. Direct monitoring the release of the encapsulateddodecylamine inhibitor on silica nanoparticles using infraredspectroscopy

Fig. 7 represents the superimposed IR spectra of the dodecy-lamine, polyethyleneimine and polystyrenesulfonate, where onecan select and observe an intense peak at 1596 cm−1 correspond-ing to pure dodecylamine. This peak will be monitored during theexperiment to prove the release of the dodecylamine from thenanocontainers.

Fig. 8 shows a 3D graph of IR spectra generated by therelease of the dodecylamine with time, where the appearance ofthree peaks during the experiment can be seen; the first peaklocated at 1100 cm−1 and second peak located at 1640 cm−1 areattributed to the overlaid peaks of the polystyrenesulfonate (PSS)and polyethyleneimine (PEI). The third peak located at 1596 cm−1 isrelated to dodecylamine. Thus it can be concluded that, besides the

Author's personal copy

J.M. Falcón et al. / Electrochimica Acta 124 (2014) 109–118 113

-3 -2 -1 0 1 2 3 42,2

2,4

2,6

2,8

3,0

3,2

3,4

3,6

3,8

4,0

log (f / Hz)

log

(|Z

| /

cm

2)

1h

2h

3h

5h

16h

0 500 100 0 150 0 2000 250 0 300 0

0

500

1000

1500

2000

2500

3000

Z' / cm2

- Z

" /

cm

2

1 h

2 h

3 h

5h

16 h

-3 -2 -1 0 1 2 3 4

0

5

10

15

20

25

30

35

40

45

50

log (f / Hz)

- /

de

gre

e

1 h

2 h

3 h

5 h

16 h

(a) (b)

(c)

Fig. 3. Impedance diagrams: Nyquist (a) and Bode ((b) and (c)) plots of carbon steel after different immersion times in 0.1 mol/L NaCl at pH 9 containing 1 wt.% of nanocontainerswith encapsulated dodecylamine.

release of dodecylamine there was also the release of the polylelec-trolytes that form the nanocontainer walls. These results confirmthe mass balance results obtained from the capsules before andafter the release process, described earlier.

Fig. 9 shows more clearly the release of dodecylamine inhibitorfrom the nanocontainers in function of time for the condition of pH2 with increasing intensities of absorption peak at 1596 cm−1 withtime. Hence, this system is sensitive to the pH shift in acidic regionreleasing the inhibitor on demand when the anodic process starts.

Fig. 4. Kinetics curves for the release of encapsulated dodecylamine for differentimmersion times in 0.1 mol/L NaCl solution and different pHs.

3.2. Self-healing effect by EIS and SVET for carbon steel samplescoated with alkyd primers dopped with silica nanoparticlesloaded with dodecylamine inhibitor

3.2.1. EIS measurementsFigs. 10 and 11 show the Nyquist diagrams for carbon steel

coated with alkyd paint formulated or not with 10 wt.% nanocon-tainers in 0.01 mol/L NaCl where a defect was provoked with anindenter.

500040003000200010000

0

1000

2000

3000

4000

5000

89 hours

Z' / cm2

- Z

" /

cm

2

Fig. 5. Nyquist diagram of carbon steel obtained after 89 h of immersion in 0.1 mol/LNaCl at pH 2 containing 1 wt.% of nanocontainers with encapsulated dodecylamine.

Author's personal copy

114 J.M. Falcón et al. / Electrochimica Acta 124 (2014) 109–118

0 6000450030001500

0

1500

3000

4500

6000100 pp m

200 pp m

300 pp m

400 pp m

500 pp m

- Z

" /

cm

2

Z' / cm2

Fig. 6. Nyquist diagrams of carbon steel obtained for different concentrations ofdodecylamine intentionally added to 0.1 mol/L NaCl solution at pH 2 after 1 h ofimmersion.

Fig. 7. Spectra of dodecylamine (DDA), polyethyleneimine (PEI) and poly sodium4-styrenesulfonate (PSS) corrected by subtracting the spectrum of the solvent.

Nyquist diagrams in Fig. 10 show that the values of the capac-itive arcs diameter decrease with time, in other words, the metalbecomes less resistant to corrosion due to the direct ingress of cor-rosive species to defective zone caused by the indenter. On theother hand, the Nyquist diagrams in Fig. 11 show an increase incapacitive arcs diameter during the first 8 h of immersion. Afterthis time, the arcs begin to decrease gradually until reaching low

Fig. 8. 3D graph of spectra generated by the release of the dodecylamine and thepolyelectrolytes (PEI and PSS) with time. Spectra were corrected subtracting thespectrum of the solvent.

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0.45

0.5

0:15:52

0:45:52

1:15:52

1: 45:52

2:15:55

2:45:52

3: 15:53

3: 45:52

4:15:52

4: 45:53

5: 15:52

Ab

so

rban

ce /a.u

.

Time / h

Dodecylamine Released Over Time

Dode cylamine

Fig. 9. Absorbance peak intensity at 1596 cm−1 (characteristic of dodecylamine)with time showing the release of dodecylamine corrosion inhibitor from thenanocontainers at pH 2.

0 100 200 300 400 500

0

100

200

300

400

Z' / KΩ cm2

20 mHz20 mHz

Without

nanocontainers

Immersion times

1 h

4 h

6 h

8 h

24 h

28 h

32 h

48 h

- Z

" / KΩ

cm

2

20 mHz

Fig. 10. Nyquist diagrams of carbon steel coated with alkyd primer withoutnanocontainers obtained after different immersion times in 0.01 mol/L NaCl.

values of impedance (35 k� cm2). This behavior can be explainedby the action of the inhibitor during the first hours of immersion,when it is released from the nanocontainers due to lowering of pHin the anodic zones caused by the corrosion process around inden-ter defect area. After a longer time, the amount of inhibitor in thesolution is constant and in the absence of its replenishment themetal is more attacked by the aggressive solution, reaching lowimpedance values.

0 50 100 150 200 250 300

0

50

100

150

200

250

300

Z' / KΩ cm2

20 mHz 20 mHz

20 mHz20 mHz

20 mHz

Immersion times

1 h

4 h

6 h

8 h

24 h

28 h

32 h

48 h

- Z

" / KΩ

cm

2

10 wt % of

nanocontainers

20 mHz

Fig. 11. Nyquist diagrams of carbon steel coated with alkyd primer doped with 10%of nanocontainers obtained after different immersion times in 0.01 mol/L NaCl.

Author's personal copy

J.M. Falcón et al. / Electrochimica Acta 124 (2014) 109–118 115

Fig. 12. SVET maps of ionic currents measured above the surface of carbon steel coated with alkyd primer without nanocontainers obtained after different immersion timesin 0.01 mol/L NaCl.

Author's personal copy

116 J.M. Falcón et al. / Electrochimica Acta 124 (2014) 109–118

Fig. 13. SVET maps of ionic currents measured above the surface of the carbon steel coated with an alkyd primer doped with 10 wt.% of nanocontainers obtained afterdifferent immersion times in 0.01 mol/L NaCl.

Author's personal copy

J.M. Falcón et al. / Electrochimica Acta 124 (2014) 109–118 117

Fig. 14. Aspect of coated samples with two layers of alkyd primer containing 0 wt.%and 10 wt.% of nanocontainers after 0 h and 96 h of exposure in the salt spray cham-ber.

3.2.2. Scanning vibrating electrode technique (SVET)measurements

SVET measurements for carbon steel coated with two layers ofalkyd primer without nanocontainers and a provoked defect areshown in Fig. 12.

The results showed the presence of an anodic region around thedefect after 1.5 h of immersion, which increases with time until itreaches a maximum of anodic current density after 10.5 h of immer-sion. After 14.5 h of immersion there was a small decrease in theanodic current due to the large formation of corrosion products onthe defect area. The corrosion products act as a physical barrier andreduce the intensity of the corrosion process. The presence of cor-rosion products on the defect region during the whole experimentwas observed.

Fig. 13 shows SVET ionic currents map results obtained for car-bon steel panels coated with two layers of alkyd primer with thefirst layer doped with 10 wt.% of nanocontainers where a defectwas provoked, for different immersion times (0.5 h, 9.5 h, 17.5 hand 20.5 h) in 0.01 mol/L NaCl.

For these conditions, the results of SVET in Fig. 13 show thatafter 0.5 h of immersion, the anodic activity around the defect isnot well established, since anodic current density reached valuesaround 63 �A/cm2 and there was no formation of any corrosionproducts on the defect area. After 9.5 h of immersion, the anodicactivity starts increasing until reaching a maximum value of anodiccurrent density of 142 �A/cm2, and after this period (9.5 h) thevalues of anodic current density begin to decrease to 52 �A/cm2

(after 17.5 h of immersion), showing that the corrosion processwas stopped due to the action of dodecylamine inhibitor that wasreleased from the nanocontainers on the provoked defect area aftera low pH was achieved. After this time (20.5 h immersion) it is possi-ble to see that the values of current density begin to increase again,

which indicate depletion in the amount of inhibitor released fromthe nanocontainers.

Small amounts of corrosion products appear during the firsthours of immersion around the defect and remain the same until20.5 h of immersion. These results show a behavior similar to thatobserved by several authors [9,30,31,34], indicating the release ofthe inhibitor from nanocontainers caused by a pH change to lowervalues, with the adsorption of the inhibitor on the metal’s surfaceinhibiting the kinetics of corrosion process. The results obtained bySVET confirm the results obtained by EIS.

3.3. Salt spray measurements

Fig. 14 presents the results obtained for the salt spray tests forexposure times of 0 h and 96 h, where it is possible to observethat after 96 h a significant corrosion process is observed especiallyfor the coated samples without nanocontainers, which exhibitan intense blistering under the coating, indicating permeation ofthe aggressive solution toward metal surface through the defectprovoked by the sharp tool, which forms preferential paths foraggressive species in the coating layer. According to these resultsit can be concluded that the addition of 10 wt.% of nanocontainersinto the primer coating provided an additional protection againstcorrosion of carbon steel, and after 96 h of exposure there was lesscorrosion products and blistering around defect area.

4. Conclusions

EIS technique allowed to follow indirectly the kinetics of releaseof the dodecylamine inhibitor from nanocontainers, and it wasobserved that for the pH 2 condition the release rate of the inhibitorwas four times larger than for other pH conditions (6.2 and 9).

IR probe results in situ allowed us to monitor directly the kineticsof release of the inhibitor dodecylamine through the absorptionpeaks intensity obtained for different immersion times, where theintensity of each peak will depend on the concentration of eachcompound within the solution. Therefore, one can conclude that,besides the release of the dodecylamine there was also a release ofthe polyelectrolyte layers that constitute the imprisoning layers ofthe inhibitor.

EIS results for the carbon steel coated with primer pretreatmentdoped with 10 wt.% of nanocontainers showed self-healing abilityby release of the inhibitor encapsulated and it was also confirmedby the results obtained with SVET measurements.

The salt spray tests showed that the addition of nanocontain-ers on the coated samples provided self-healing properties, wherethe inhibitor is released from the nanocontainer shells inhibitingthe kinetics of corrosion process what implies in minor corrosionproducts and blistering in the defect area.

Acknowledgement

Authors would like to express sincere gratitude to CNPq forthe financial support for developing this research (Process nr.141051/2010-08).

References

[1] H.H. Uhlig, R.W. Revie, Corrosion and Corrosion Control, An introduction toCorrosion Science and Engineering, 3rd ed., John Wiley Sons, New York, 1985.

[2] V. Sauvant-Moynot, S. Gonzalez, J. Kittel, Self-healing coatings: an alternativeroute for anticorrosion protection, Progress in Organic Coatings 63 (2008) 307.

[3] E.D. Mekeridis, I.A. Kartsonakis, G.C. Kordas, Multilayer organic-inorganic coat-ing incorporating loaded with inhibitors for corrosion protection of AA2024-T3,Progress in Organic Coatings 73 (2012) 142.

[4] S.V. Lamaka, D. Shchukin, D.V. Andreeva, M.L. Zheludkevich, H. Möhwald, M.G.S.Ferreira, Sol–gel/polyelectrolyte active corrosion protection system, AdvancedFunctional Materials 18 (2008) 3137.

Author's personal copy

118 J.M. Falcón et al. / Electrochimica Acta 124 (2014) 109–118

[5] E.V. Skorb, D. Fix, D.V. Andreeva, H. Möhwald, D. Shchukin, Surface-modifiedmesoporous SiO2 containers for corrosion protection, Advanced FunctionalMaterials 19 (2009) 2373.

[6] D.V. Andreeva, D. Fix, H. Möhwald, T. Hack, D. Shchukin, Self-healing anticor-rosion coatings based oh pH-sensitive polyelectrolyte/inhibitor sandwichlikenanostructures, Advanced Materials 20 (2008) 2789.

[7] J. Tedim, S.K. Poznyak, A. Kuznetsova, D. Raps, T. Hack, M.L. Zheludkevich,M.G.S. Ferreira, Enhancement of active corrosion protection via combinationof inhibitor-loaded nanocontainers, Applied Materials and Interfaces 2 (2010)1528.

[8] T. Saeed, A. Ali, S.U. Rahman, The cyclic hydroxylamines: a new class of corro-sion inhibitors of carbon steel in acidic medium, Anticorrosion Methods andMaterials 50 (2003) 201.

[9] D.V. Andreeva, E.V. Skorb, D. Shchukin, Layer-by-layer polyelectrolyte/inhibitornanostructures for metal corrosion protection, Applied Materials and Interfaces2 (2010) 1954.

[10] N. Selvakumar, K. Jeyasubramanian, R. Sharmila, Smart coating for corrosionprotection by adopting nanoparticles, Progress in Organic Coatings 74 (2012)461.

[11] S.A.S. Dias, S.V. Lamaka, C.A. Nogueira, T.C. Diamantino, M.G.S. Ferreira, Sol–gelcoatings modified with fillers active corrosion protection, Corrosion Science 62(2012) 153.

[12] W. Meier, Polymer nanocontainers, Nanoscience and Nanotechnology 56(2002) 490.

[13] D. Shchukin, H. Möhwald, Surface-engineered nanocontainers for entrapmentof corrosion inhibitors, Advanced Functional Materials 17 (2007) 1451.

[14] T. Chen, J.J. Fu, pH-responsive nanovalves based on hollow mesoporous silicaspheres for controlled release of corrosin inhibitor, Nanotechnology 23 (2012)1.

[15] D. Lee, Z. Gemici, M.F. Rubner, R.E. Cohen, Multilayers of oppositely chargedSiO2 nanoparticles: effect of surface charge on multilayer assembly, Lagmuir23 (2007) 8833.

[16] D.O. Grigoriev, T. Bukreeva, H. Möhwald, D. Shchukin, New method for fab-rication of loaded micro- and nanocontainers: emulsion encapsulation bypolyelectrolyte layer-by-layer deposition on the liquid core, Langmuir 24(2008) 999.

[17] T. Chen, J.J. Fu, An intelligent anticorrosion coating based on pH-responsivesupramolecular nanocontainers, Nanotechnology 23 (2012) 1.

[18] G.B. Sukhorukov, S. Donath, A.S. Susha, A. Voigt, J. Hartmann, H. Möhwald,Microencapsulation by means of step-wise adsorption of polyelectrolytes, Jour-nal of Microencapsulation 17 (2000) 177.

[19] M.L. Zheludkevich, D. Shchukin, K.A. Yasakau, H. Möhwald, M.G.S. Ferreira,Anticorrosion coatings with self-healing effect based on nanocontainersimpregnated with corrosion inhibitor, Chemical Materials 19 (2007) 402.

[20] A.S. Hamdy, E.E. Shenawy, T.E. Bitar, Electrochemical impedance spectroscopystudy of the corrosion behaviour of some niobium bearing stainless steels in3.5% NaCl, International Journal of Electrochemical Science 1 (2006) 171.

[21] M.F. Montemor, D.V. Snihirova, M.G. Taryba, S.V. Lamaka, I.A. Kartsonakis, A.C.Balaskas, G.C. Kordas, J. Tedim, A. Kuznetsova, M.L. Zheludkevich, M.G.S. Fer-reira, Evaluation of self-healing ability in protective coatings modified withcombinations of layered double hydroxides and cerium molibdate nanocon-tainers filled with corrosion inhibitors, Electrochimica Acta 60 (2012) 31.

[22] A.C. Balaskas, I.A. Kartsonakis, L.A. Tziveleka, G.C. Kordas, Improvement of anti-corrosive properties of epoxy-coated AA 2024-T3 with TiO2 nanocontainersloaded with 8-hydroxyquinoline, Progress in Organic Coatings 74 (2012) 418.

[23] F. Kuang, T. Shi, J. Wang, F. Jia, Microencapsulation technology for thioureacorrosion inhibitor, Journal Solid State Electrochemical 13 (2009) 1729.

[24] S.K. Poznyak, J. Tedim, L.M. Rodrigues, A.N. Salak, M.L. Zheludkevich, L.F.P. Dick,M.G.S. Ferreira, Novel inorganic host layered double hydroxides intercalatedwith guest organic inhibitors for anticorrosion applications, Applied Materialsand Interfaces 1 (2009) 2353.

[25] M.L. Zheludkevich, S.K. Poznyak, L.M. Rodrigues, D. Raps, T. Hack, L.F. Dick, T.Nunes, M.G.S. Ferreira, Active protection coatings with layered double hydrox-ides nanocontainers of corrosion inhibitor, Corrosion Science 52 (2010) 602.

[26] I.A. Kartsonakis, E.P. Koumoulos, A.C. Balaskas, G.S. Pappas, C.A. Charitidis, G.C.Kordas, Incorporation of ceramic nanocontainers into epoxy coatings for thecorrosion protection of hop dip galvanized steel, Corrosion Science 57 (2012)56.

[27] N.P. Tavandashti, S. Sanjabi, Corrosion study of hibrid sol-gel coatings con-taining boehmite nanoparticles loaded with cerium nitrate corrosion inhibitor,Process in Organic Coatings 69 (2010) 384.

[28] E.D. Mekeridis, I.A. Kartsonakis, G.S. Pappas, G.C. Kordas, Release studiesof corrosion inhibitors from cerium titanium oxide nanocontainers, JournalNanoparticules Research 13 (2011) 541.

[29] S.V. Lamaka, M.L. Zheludkevich, K.A. Yasaku, R. Serra, S.K. Poznyak, M.G.S. Fer-reira, Nanopouros titiana interlayer as reservoir of corrosion inhibitors forcoatings with self-healing ability, Process in Organic Coatings 58 (2007) 127.

[30] D. Fix, D.V. Andreeva, Y.M. Shchukin, H. Mohwald, Application of inhibitor-loaded halloysite nanotubes in active-corrosive coatings, Advanced FunctionalMaterials 19 (2009) 1720.

[31] D. Borisova, H. Mohwald, D.G. Shchukin, Mesoporous sílica nanoparticles foractive corrosion protection, Journal of Materials Chemistry 5 (2011) 1939.

[32] J. Izquierdo, L. Nagy, J.J. Santana, G. Nagy, R.M. Souto, A novel microlectro-chemical strategy for the study of corrosion inhibitors employing the scanningvibrating electrode technique and dual potentiometric/amperometric opera-tion in scanning electrochemical microscopy: application to the study of thecathodic inhibition by benzotriazole of the galvanic corrosion of cooper coupledto iron, Electrochimica Acta 58 (2011) 707.

[33] ASTM B117 – 11 Standard Practice for Operating Salt Spray (Fog) Apparatus,G01.05, Book of Standards Volume: 03.02, 2011.

[34] E. Abdullayev, R. Price, D.G. Shchukin, Y.M. Lvov, Halloysite tubes as nanocon-tainers for anticorrosion coating with benzotriazole, Applied Materials andInterfaces 1 (2009) 1443.